DOI: 10.1002/chem.201402425
Full Paper
& Phosphorus Chemistry
Synthesis of Unsymmetrically Substituted Phosphane Oxides (R1R2P(O)H) and Phosphinous Acids (R1R2POH) Nadine Allefeld,[a] Michael Grasse,[a] Nikolai Ignat’ev,[b] and Berthold Hoge*[a]
Abstract: This paper describes the synthesis of unsymmetrically substituted phosphinous acids and phosphane oxides featuring at least one electron-withdrawing pentafluoroethyl group. The presence of a diethylamino function as a protecting group allows a selective reaction of RClPNEt2 (R = CF3, C6F5, C6H5) with LiC2F5. On treatment with para-toluenesulfonic acid the isolated aminophosphanes R(C2F5)PNEt2 are
readily converted into the corresponding phosphinous acids or phosphane oxides, respectively. Investigation of the tautomeric equilibrium between oxide and acid tautomer revealed (CF3)(C2F5)POH as a stable phosphinous acid, whereas the pentafluorophenyl and phenyl derivatives constitute a solvent dependent equilibrium between the acid and the oxide tautomer.
Introduction
to date, the only literature-known examples of stable phosphinous acids.[2, 3a] All other known diorganyl derivatives of the composition R2P(O)H favor the phosphane oxide form in the pure state. With the less electron-withdrawing C5F4N or 2,4(CF3)2C6H3 groups[4] a solvent-dependent equilibrium was observed, whereby donor solvents, such as DMF or THF, stabilize the acid tautomer. A greater variety of electronic and steric situations can be encountered in unsymmetrically substituted phosphane oxides. In the literature several examples for unsymmetrically substituted alkyl and aryl SPOs are described. Emmick et al. reported on the treatment of monosubstituted phosphinic esters RP(O)(OEt)H with Grignard reagents, yielding in SPOs of the type RR’P(O)H.[7] More recently the synthesis of several alkyland aryl-substituted SPOs starting from a mixed-phenyl-substituted aminochlorophosphane was performed.[8] In this paper we present an efficient synthesis of unsymmetrically substituted secondary phosphane oxides and phosphinous acids featuring one pentafluoroethyl group.
Phosphorus-based ligands are ubiquitous in the synthesis of catalytically active transition-metal complexes. However, the widely used triorganophosphanes suffer from several disadvantages, such as sensitivity towards oxidation or hydrolysis. With regard to this, air and moisture stable secondary phosphane oxides (SPOs), which are easy to handle, are a conceivable alternative. A tautomeric equilibrium between phosphane oxide and phosphinous acid is invoked to explain the role of SPOs as preligands in coordination chemistry.[1] The introduction of electron-withdrawing substituents leads to the generation of strongly p-acidic phosphinous acid ligands, with application in cross-coupling reactions, such as the Suzuki type reaction.[2–6] From a molecular point of view, perfluoroorganyl groups cause a stabilization of the phosphinous acid tautomer with respect to the phosphane oxide form [Eq. (1)].
Results and Discussion In order to synthesize unsymmetric SPOs with different perfluoroorganyl groups, in the first step the aminochlorophosphanes RClPNEt2 (R = CF3, C6F5, C6H5) have to be synthesized as starting materials. Treatment of CF3P(NEt2)2 with two equivalents of hydrogen chloride led to the selective substitution of one amino group by a chlorine atom [Eq. (2)]. The change in the electronic nature of the substituents—an electron-donating amino substituent is substituted by an electron-withdrawing chlorine atom—is reflected in the 31P NMR spectrum, in which the resonance of CF3P(NEt2)2 at 45.6 ppm is shifted to 98.5 ppm for (CF3)P(Cl)NEt2. On the other hand, treatment of the dichlorophosphanes RPCl2 (R = C6F5, C6H5) with two equivalents of diethylamine selectively affords RP(Cl)NEt2 accordingly to Equation (3).
The bis(trifluoromethyl)phosphinous acid ((CF3)2POH) and bis(pentafluoroethyl)phosphinous acid ((C2F5)2POH) represent, [a] Dr. N. Allefeld, M. Grasse, Prof. Dr. B. Hoge Centrum fr Molekulare Materialien Fakultt fr Chemie, Anorganische Chemie Universitt Bielefeld Universittsstraße 25, 33615 Bielefeld (Germany) E-mail: [email protected] [b] Dr. N. Ignat’ev PM-ATI, Merck KGaA Frankfurter Str. 250, 64293 Darmstadt (Germany) Chem. Eur. J. 2014, 20, 1 – 7
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Full Paper ly.[2, 11] Similarly CF3(C2F5)PNEt2 was added to a slurry of excess para-toluenesulfonic acid in 1,6-dibromohexane. The phosphinous acid CF3(C2F5)POH was isolated by means of fractional condensation as the only volatile compound in 90 % yield [Eq. (5)].
The 31P NMR spectrum in [D]chloroform reveals one multiplet at 80.7 ppm (Table 2), which compares well with the resonances of (CF3)2POH (d(31P) = 77.9)[2] and (C2F5)2POH (d(31P) = 93.0).[11] Based on an A3B3MNX spin system (A, B, M, N = 19F; X = 31P) the 31P and 19F NMR spectra were simulated. No spectroscopic evidence for a second isomer could be obtained.
Substitution of an electron-withdrawing chlorine atom by an electron-donating amino group results in an upfield shift of the 31P NMR signal from 162.0 (C6H5PCl2) to 142.1 ppm (C6H5P(Cl)NEt2) and from 137.0 (C6F5PBr2) to 106.0 ppm (C6F5P(Br)NEt2) (Table 1).
Table 2. NMR data of phosphinous acids and phosphane oxides. Table 1. Selected NMR spectroscopical data of the aminophosphanes. 31
[a]
CF3P(Cl)NEt2 CF3P(Br)NEt2[a] C6F5P(Br)NEt2[b] C6H5P(Cl)NEt2[a] CF3(C2F5)PNEt2[a] [a]
C6F5(C2F5)PNEt2 C6H5(C2F5)PNEt2[a]
d( P) [ppm]
2
98.5 97.8 106.0 142.1 44.8
85 82 54[c] – 88[d] 47/59[e] 43/46[e] 71/45[e]
29.9 57.0
d(31P) [ppm]
J(P,F) [Hz]
[a]
CF3(C2F5)POH C6F5(C2F5)POH[c] C6F5(C2F5)P(O)H[a] C6H5(C2F5)POH[c] C6H5(C2F5)P(O)H[a]
The pure compounds were isolated by distillation as colorless liquids in good yields. Bis(trifluoromethyl)phosphinous acid ((CF3)2POH) and bis(pentafluoroethyl)phosphinous acid ((C2F5)2POH) have been prepared from the corresponding aminophosphane R2PNEt2 (R = CF3, C2F5) by treatment with para-toluenesulfonic acid, which results in the replacement of the amino group by a hydroxyl group. The isolation of the phosphinous acid was effected by fractional condensation in 88 and 93 % yield, respectiveChem. Eur. J. 2014, 20, 1 – 7
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– – 572 – 519
2
J(P,F) [Hz]
52/65[b] 65/82[b] n.d.[d] 86 78/76[b]
The experimentally determined (Figure 1 (top)) and the simulated 31P NMR spectra (Figure 1 (bottom)) are in excellent agreement. In addition to the coupling of the phosphorus atom to the CF3 groups (2J(P,F) = 83.0 Hz, 3J(P,F) = 14.1 Hz), two couplings to the diastereotopic fluorine atoms of the CF2 group were detected (2J(P,Fa) = 52.0 Hz, 2J(P,Fb) = 64.9 Hz).
Previous work has shown that pentafluoroethyl lithium (LiC2F5), generated in situ from nBuLi and C2F5H at 78 8C,[9] represents an efficient reagent for the pentafluoroethylation of chlorophosphanes ClnP(NEt2)3 n (n = 1, 2).[10, 11] Treatment of the aminochlorophosphanes RP(X)NEt2 (R = CF3, C6F5, C6H5 ; X = Cl, Br) with LiC2F5 selectively afforded the unsymmetrically substituted aminophosphanes R(C2F5)PNEt2 [R = CF3, C6F5, C6H5,; Eq. (4)].
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J(P,H) [Hz]
[a] [D]chloroform. [b] diastereotopic fluorine atoms of CF2 group. [c] Et2O. [d] Not detected.
[a] [D]chloroform. [b] [D6]benzene. [c] 3J(P,F). [d] Coupling to CF3. [e] Diastereotopic fluorine atoms of CF2 group.
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80.7 81.2 1.9 90.0 17.2
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Figure 1. Experimental (top) and simulated (bottom) 31P NMR spectrum of (CF3)(C2F5)POH.
In accordance to this, the 19F NMR spectrum displays four sets of signals, one for the CF3 group directly bonded to the phosphorus atom at 64.0 ppm, and at 81.9 ppm for the CF3 unit of the C2F5 substituent. The CF2 unit causes two resonances for the diastereotopic fluorine atoms Fa at 126.4 and Fb at 125.8 ppm (Figure 2). 2
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Figure 2. Experimental (top) and simulated (bottom) 19F NMR spectrum of (CF3)(C2F5)POH; I (Fa), II (Fb), III (CF2CF3), IV (CF3).
The phosphinous acid was further studied by IR spectroscopy in the gas phase. Two OH stretching modes were detected at 3669 and 3596 cm 1, respectively, due to the existence of two rotational isomers in the gas phase. The phosphinous acids (CF3)2POH and (C2F5)2POH are also present in two rotamers, in a cis and a trans conformation.[2, 3] Calculations revealed that the cis rotamer is more stable by 7.5 ((CF3)2POH) and 6.3 kJ mol 1 ((C2F5)2POH), respectively (B3LYP/6-311 + G(2df,p)). Correspondingly, the cis rotamer of (CF3)(C2F5)POH is stabilized by 5.1 kJ mol 1 with respect to the trans rotamer (Figure 3). The absence of a P H as well as a P=O stretching mode confirms the exclusive presence of the acid tautomer in the gas phase at room temperature. This agrees with the observations for (CF3)2POH and (C2F5)2POH. Both compounds exist in the solid state as well as in the gas phase solely as the acid
phosphane oxide C6F5(C2F5)P(O)H and the phosphinous acid tautomer, C6F5(C2F5)POH. The 31P NMR spectrum of the neat liquid shows two resonances at 84.3 and 0.7 ppm in an integral ratio of 48:52 that are assigned to the acid and the oxide form, respectively (Figure 4). The resonance of the oxide tautomer reveals a characteristic 1J(P,H) coupling constant of 582 Hz. In solvents with high donor numbers,[12] such as diethyl ether, the tautomeric equilibrium is shifted completely towards the acid tautomer (Table 3). In solvents with low donor numbers, however, the equilibrium is shifted towards the oxide tautomer. In the 31 P NMR spectrum of the chloroform two signals at 80.6 and 1.9 ppm in an integral ratio of 42:58 are detected—the portion of the oxide tautomer increased with regard to the pure sample. A similar behavior was observed for the equilibrium between C6H5(C2F5)POH and C6H5(C2F5)P(O)H.
Figure 3. Relative zero-point corrected energies of the two rotational isomers of CF3(C2F5)POH and the oxide tautomer CF3(C2F5)P(O)H calculated at the B3LYP/6-311 + G(2df,p) level of theory. Chem. Eur. J. 2014, 20, 1 – 7
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tautomer. In the liquid phase (C2F5)2POH is rearranged into 20 % of the oxide tautomer, whereas (CF3)2POH and CF3(C2F5)POH are exclusively known in the acid tautomer. The pentafluorophenyl and phenyl derivatives were synthesized analogously by the exposure of the corresponding aminophosphanes R(C2F5)PNEt2 (R = C6F5, C6H5) to para-toluenesulfonic acid and were isolated as colorless liquids by distillation under reduced pressure in a 52 (R = C6F5) and 42 % (R = Ph) yield, respectively [Eq. (6)]. In the neat liquid, the pentafluorophenyl derivative exists in an equilibrium between the
Figure 4. 31P NMR spectrum of C6F5(C2F5)POH/C6F5(C2F5)P(O)H, neat liquid.
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Table 3. Solvent dependent equilibrium between phosphinous acids and their corresponding phosphane oxide tautomers. Solvent
R(C2F5)POH
CHCl3 neat
100 100
0 0
R = C6F5
neat CHCl3 Et2O MeCN DMF
48 42 100 100 100
52 58 0 0 0
R = C6H5
CHCl3 MeCN neat Et2O DMF
0 7 15 30 87
100 93 85 70 13
R = CF3
R(C2F5)P(O)H
Conclusions This work describes an efficient strategy for the synthesis of phosphinous acids and phosphane oxides featuring one pentafluoroethyl group with an additional trifluoromethyl, pentafluorophenyl, or phenyl substituent. The differing stabilization of the phosphinous acid form is evidenced in the tautomeric equilibrium. Whereas the compound CF3(C2F5)POH exists in solution and the gas phase solely as the acid tautomer, the less electron-withdrawing pentafluorophenyl-substituted compounds C6F5(C2F5)POH/C6F5(C2F5)P(O)H are involved in tautomeric equilibria in solution as well as in the neat liquid phase. The even less electron-withdrawing phenyl group, which may be rather regarded as a donor function, clearly favors the oxide tautomer, as observed exclusively in chloroform. In general, electron-withdrawing groups stabilize the acid tautomer. Although the electronic nature of trifluoromethyl and pentafluoroethyl groups are almost comparable, the stabilization of a phosphinous acid is most efficiently realized by a trifluoromethyl substituent.
The phenyl group is less electron-withdrawing than a perfluorophenyl group. Therefore, in chloroform only the oxide tautomer was detected by NMR spectroscopy. In the more donating solvent diethyl ether a tautomeric ratio of acid to oxide of 30:70 was detected. This clearly reflects the different inductive effects of a phenyl and a pentafluorophenyl substituent. For an improved understanding, the energy differences between the acid and the oxide tautomer were calculated on a B3LYP/6-311 + G(2df,p) level of theory. Stabilization of the phosphinous acid tautomer decreases in the series CF3 > C2F5 > C6F5 > C6H5 (Table 4). The phosphinous acid CF3(C2F5)POH is
Experimental Section All chemicals were obtained from commercial sources and used without further purification. The phosphanes CF3P(NEt2)2[14] and C6F5PX2[15] were synthesized according to literature methods. Standard high-vacuum techniques were employed throughout all preparative procedures. Non-volatile compounds were handled in a dry N2 atmosphere using Schlenk techniques. NMR spectra were recorded on a Bruker Model Avance III 300 spectrometer (31P 111.92 MHz; 19F 282.40 MHz; 13C 75.47 MHz, 1H 300.13 MHz) with positive shifts being downfield from the external standards (85 % orthophosphoric acid (31P), CCl3F (19F) and TMS (1H)). IR spectra were recorded on a ALPHA-FT-IR spectrometer (Bruker) using a gas cell with KBr windows.
Table 4. Calculated energy differences of the tautomeric equilibrium between phosphinous acid R1R2POH and phosphane oxide tautomers R1R2P(O)H with CF3, C2F5, C6F5 and C6H5 groups (B3LYP/6-311 + G(2df,p)).[a] R1
R2
DEZP,T [kJ mol 1]
CF3 CF3 C2F5 C6F5 C6F5 C6H5
CF3 C2F5 C2F5 C2F5 C6F5 C2F5
25.7 24.4 22.8 13.3 1.7[b] 8.0
Synthesis of CF3P(X)NEt2 (X=Cl, Br) 1) The addition of diethylamine (2.2 g, 30.8 mmol) to a solution of (CF3)PBr2 (4.0 g, 15.4 mmol) in n-hexane at 78 8C led to the precipitation of a colorless solid. The reaction mixture was slowly warmed to room temperature. After filtration the solvent was removed under reduced pressure. Fractional condensation yielded CF3P(Br)NEt2 (1.8 g, 7.2 mmol, 46 %) in the 40 8C trap as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 0.9 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.2 ppm (m, 2 H; CH2); 31P NMR ([D]chloroform, RT): d = 97.8 ppm (quart/m, 2J(P,F) = 82 Hz, P).
[a] DEZP,T = EZP,T(acid) EZP,T(oxide). [b] B3LYP/6-311G(2d,p).[13]
more stable than the corresponding phosphane oxide by 24.4 kJ mol 1. Thus, this compound is less stabilized than (CF3)2POH (DEZP,T = 25.7 kJ mol 1), but more than (C2F5)2POH (DEZP,T = 22.8 kJ mol 1) with respect to their corresponding phosphane oxide counterparts. The phosphinous acid C6F5(C2F5)POH is by 13.3 kJ mol 1 more stable than the oxide, whereas stabilization of the symmetrically substituted (C6F5)2POH amounts only to 1.7 kJ mol 1. These calculated energy differences are in agreement with the experimental findings. In the neat liquid the ratio of phosphinous acid C6F5(C2F5)POH to phosphane oxide C6F5(C2F5)P(O)H is 48:52, in diethyl ether only the acid tautomer is detected. For comparison, the oxide tautomer (C6F5)2P(O)H is observed to &
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2) A solution of CF3P(NEt2)2 (32.3 g, 132.3 mmol) in n-pentane (300 mL) was treated with gaseous HCl (293.6 mmol) at 78 8C. The reaction mixture was slowly warmed to room temperature and the ammonium salts were removed by filtration. The solvent was removed under reduced pressure. The product CF3P(Cl)NEt2 (16.5 g, 79.6 mmol, 60 %) was separated by vacuum distillation at 20 8C. 1 H NMR ([D]chloroform, RT): d = 0.9 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.3 ppm (m, 2 H; CH2); 13C{1H} NMR ([D]chloroform, RT): d = 13.9 (d, 3 J(P,C) = 5 Hz, CH3), 44.3 (d, 2J(P,C) = 18 Hz, CH2), 122.6 ppm (quart/ d, 1J(C,F) = 324 Hz, 1J(P,C) = 49 Hz, CF3); 19F NMR ([D]chloroform, RT): d = 67.3 ppm (d, 2J(P,F) = 86 Hz, CF3); 31P NMR ([D]chloroform, RT): d = 98.5 ppm (quart, 2J(P,F) = 85 Hz, P).
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Full Paper at 78 8C, the mixture was warmed to room temperature and stirred overnight. The precipitate was filtered off. After evaporation of the solvent, C6F5(C2F5)PNEt2 (7.2 g, 18.4 mmol, 92 %) was obtained by vacuum distillation at 43 8C as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 1.1 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.2 ppm (d/ quart, 3J(P,H) = 11 Hz, 2J(H,H) = 7 Hz, 2 H; CH2); 1H{31P} NMR ([D]chloroform, RT): d = 1.1 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.2 ppm (quart, 2J(H,H) = 7 Hz, 2 H; CH2); 13C{1H} NMR ([D]chloroform, RT): d = 13.7 (d, 3J(P,C) = 4 Hz, CH3), 44.6 ppm (d, 2J(P,C) = 19 Hz, CH2); 13 19 C{ F} NMR ([D]chloroform, RT): d = 117.9 (d, 1J(P,C) = 60 Hz, CF2), 119.8 (d/m, 2J(P,C) = 32 Hz, CF3), 137.6 (s, meta-CF), 143.1 (s, paraCF), 147.5 ppm (d, 2J(P,C) = 13 Hz, ortho-CF); 19F NMR ([D]chloroform, RT): d = 159.9 (m, 2 F; meta-CF), 148.2 (t/t, 3J(F,F) = 31 Hz, 4 J(F,F) = 5 Hz, 1 F; para-CF), 126.8 (m, 2 F; ortho-CF), 118.4 (d/d/t/ quart, 2J(F,F) = 297 Hz, 2J(P,F) = 43 Hz, 5J(F,F) = 15 Hz, 3J(F,F) = 3 Hz, 1 F; CFaFbCF3), 116.4 (d/d/t/quart, 2J(F,F) = 297 Hz, 2J(P,F) = 46 Hz, 5 J(F,F) = 20 Hz, 3J(F,F) = 3 Hz, 1 F; CFaFbCF3), 82.2 ppm (d/t, 3J(P,F) = 23 Hz, 3J(F,F) = 3 Hz, 3 F; CF2CF3); 19F{31P} NMR ([D]chloroform, RT): d = 159.9 (m, 2 F; meta-CF), 148.2 (t/t, 3J(F,F) = 31 Hz, 4J(F,F) = 5 Hz, 1 F; para-CF), 126.8 (m, 2 F; ortho-CF), 118.4 (d/t/quart, 2 J(F,F) = 297 Hz, 5J(F,F) = 15 Hz, 3J(F,F) = 3 Hz, 1 F; CFaFbCF3), 116.4 (d/t/quart, 2J(F,F) = 297 Hz, 5J(F,F) = 20 Hz, 3J(F,F) = 3 Hz, 1 F; 82.2 ppm (t, 3J(F,F) = 3 Hz, 3 F; CF2CF3); 31P NMR CFaFbCF3) ([D]chloroform, RT): d = 29.9 ppm (m, P).
Synthesis of C2F5(CF3)P(NEt2): Caution! LiC2F5 is highly reactive and tends to violently decompose above temperatures of 50 8C. A 1.6 m solution of n-butyllithium in n-hexane (59.0 mL, 94.4 mmol) was diluted in diethyl ether (200 mL) and degassed at 78 8C. Then the resulting solution was stirred for 30 min in an atmosphere of pentafluoroethane (123.0 mmol). After the addition of CF3P(Cl)NEt2 (16.5 g, 79.6 mmol) at 78 8C, the mixture was warmed to room temperature and stirred overnight. A precipitate was filtered off. After evaporation of the solvent, C2F5(CF3)PNEt2 (17.1 g, 61.0 mmol, 77 %) was obtained by distillation under reduced pressure at 75 8C as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 1.2 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.2 ppm (m, 2 H; CH2); 13 1 C{ H} NMR ([D6]benzene, RT): d = 14.1 (d, 3J(P,C) = 3 Hz, CH3), 44.2 ppm (d, 2J(P,C) = 21 Hz, CH2); 13C{19F} NMR ([D]chloroform, RT): d = 119.5 (d, 1J(P,C) = 52 Hz, CF2), 120.1 ppm (d, 2J(P,C) = 25 Hz, CF3); 19 F NMR ([D]chloroform, RT): d = 119.0 (d/d/m, 2J(F,F) = 305 Hz, 2 J(P,F) = 59 Hz, 1 F; CFaFbCF3), 118.4 (d/d/m, 2J(F,F) = 304 Hz, 2 a b J(P,F) = 47 Hz, 1 F; CF F CF3), 82.5 (d/m, 3J(P,F) = 18 Hz, 3 F; 2 57.3 ppm (d/m, J(P,F) = 88 Hz, 3 F; CF3); 31P NMR CF2CF3), ([D]chloroform, RT): d = 44.8 ppm (br m, P). Synthesis of C2F5(CF3)POH: para-Toluenesulfonic acid (14.8 g, 86 mmol) was suspended in 1,6-dibromohexane (100 mL). After the addition of CF3(C2F5)PNEt2 (3.9 g, 13.8 mmol) the reaction mixture was stirred for 2 d. Fractional condensation at 78 8C yielded C2F5(CF3)POH (3.0 g, 12.5 mmol, 90 %) as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 4.2 ppm (br s, C2F5(CF3)POH); 13C{19F} NMR ([D]chloroform, RT): d = 116.5 (d, 1J(P,C) = 55 Hz, CF2), 119.0 (d, 2 J(P,C) = 20 Hz, CF2CF3), 127.0 ppm (d, 1J(P,C) = 34 Hz, CF3); 19F NMR ([D]chloroform, RT): d = 126.4 (d/d/quart/quart, 2J(F,F) = 308 Hz, 2 J(P,F) = 52 Hz, 4J(Fa,F) = 8 Hz, 3J(F,F) = 3 Hz, 1 F; CFaFbCF3), 125.9 (d/d/quart/quart, 2J(F,F) = 308 Hz, 2J(P,F) = 65 Hz, 4J(Fb,F) = 10 Hz, 3 J(F,F) = 3 Hz, 1 F; CFaFbCF3), 81.9 (d/d/d/quart, 3J(P,F) = 14 Hz, 3 a 3 b 5 J(F,F ) = 3 Hz, J(F,F ) = 3 Hz, J(F,F) = 2 Hz, 3 F; CF2CF3), 64.0 ppm (d/d/d/quart, 2J(P,F) = 83 Hz, 4J(F,Fb) = 10 Hz, 4J(F,Fa) = 8 Hz, 5J(F,F) = 2 Hz, 3 F; CF3); 31P NMR ([D]chloroform, RT): d = 80.7 ppm (quart/d/ d/quart, 2J(P,F) = 83 Hz, 2J(P,Fb) = 65 Hz, 2J(P,Fa) = 52 Hz, 3J(P,F) = 14 Hz, P); IR (gas phase): n˜ = 398 (vw), 423 (w), 457 (w), 547 (w), 628 (w), 748 (w), 857 (w), 966 (w), 1056 (w), 1151 (m), 1165 (m), 1186 (m), 1227 (s), 1336 (w), 3596 (w), 3669 cm 1 (vw).
Synthesis of C6F5(C2F5)POH/C6F5(C2F5)P(O)H: A slurry of para-toluenesulfonic acid (4.2 g, 22.1 mmol) in diethyl ether (50 mL) was treated with C6F5(C2F5)PNEt2 (2.1 g, 5.6 mmol). The yellow mixture was stirred overnight. Removal of the solvent under reduced pressure and vacuum distillation afforded C6F5(C2F5)POH/C6F5(C2F5)P(O)H (1.0 g, 2.9 mmol, 52 %) as a colorless oil. 1H NMR ([D]chloroform, RT): d = 6.2 (br s, C6F5(C2F5)POH), 8.2 ppm (d, 1 J(P,H) = 572 Hz, C6F5(C2F5)P(O)H); 13C{19F} NMR (Et2O, RT): d = 120.3 (d, 2J(P,C) = 19 Hz, C6F5(CF3CF2)POH), 137.9 (s, C6F5(C2F5)POH (metaCF)), 143.9 (s, C6F5(C2F5)POH (para-CF)), 148.1 ppm (d, 2J(P,C) = 12 Hz, C6F5(C2F5)POH (ortho-CF)); 19F NMR ([D]chloroform, RT): d = 160.3 (br s, meta-CF (acid)), 156.5 (br s, meta-CF (oxide)), 146.9 (br s, para-CF (acid)), 137.8 (br s, para-CF (oxide)), 132.8 (br s, ortho-CF (acid)), 129.5 (br s, ortho-CF (oxide)), 123.9–128.3 (m, CF2), 81.5 (br s, CF3 (acid)), 80.4 ppm (br s, CF3 (oxide)); 19F NMR (Et2O, RT): d = 162.3 (m, 2 F; meta-CF (acid)), 149.5 (m, 1 F; paraCF (acid)), 131.9 (m, 2 F; ortho-CF (acid)), 125.8 (d/d/m, 2J(F,F) = 303 Hz, 2J(P,F) = 65 Hz, 1 F; C6F5(CF3CFaCFb)POH), 124.4 (d/d/m, 2 2 J(F,F) = 303 Hz, J(P,F) = 82 Hz, 1 F; C6F5(CF3CFaFbCFb)POH), 3 81.7 ppm (d, J(P,F) = 14 Hz, 3 F; C6F5(CF3CFaCFb)POH); 31P NMR ([D]chloroform, RT): d = 1.9 (d/t, 1J(P,H) = 572 Hz, 2J(P,F) = 87 Hz, C6F5(C2F5)P(O)H), 81.2 ppm (br m, C6F5(C2F5)POH); 31P NMR (Et2O, RT): d = 87.3 ppm (m, C6F5(C2F5)POH); IR (ATR): n˜ = 417 (m), 478 (m), 515 (m), 546 (w), 589 (w), 640 (m), 730 (w), 747 (m), 760 (w), 836 (w), 864 (m), 875 (m), 898 (m), 962 (s), 980 (s), 1003 (m), 1092 (s), 1130 (m), 1203 (s), 1248 (m), 1313 (m), 1390 (w), 1402 (w), 1475 (s), 1492 (s), 1519 (m), 1644 (w), 3186 cm 1 (w).
Synthesis of C6F5P(Br)NEt2 : A sample of C6F5PBr2 (10.3 g, 28.9 mmol) was dissolved in n-hexane (50 mL) and cooled to 78 8C. Addition of diethylamine (6.2 mL, 58.9 mmol) led to the precipitation of a colorless solid. The reaction mixture was slowly warmed overnight to room temperature and filtered. The solvent was removed under reduced pressure and vacuum distillation at 1 10 3 mbar and 50 8C yielded C6F5P(Br)NEt2 (3.6 g, 10.3 mmol, 36 %) as a colorless oil. 1H NMR ([D6]benzene, RT): d = 0.9 (t, 3 J(H,H) = 7 Hz, 3 H; CH3), 2.9 ppm (m, 2 H; CH2); 13C{1H} NMR ([D6]benzene, RT): d = 9.5 (d, 3J(P,C) = 7 Hz, CH3), 40.7 ppm (d, 2 J(P,C) = 18 Hz, CH2); 13C{19F} NMR ([D6]benzene, RT): d = 133.1 (s, meta-CF), 138.1 (s, para-CF), 142.5 ppm (d, 2J(P,C) = 14 Hz, orthoCF); 19F NMR ([D6]benzene, RT): d = 160.9 (m, 2 F; meta-CF), 150.0 (m, 1 F; para-CF), 131.2 ppm (d/m, 3J(P,F) = 54 Hz, 2 F; ortho-CF); 19F{31P} NMR ([D6]benzene, RT): d = 160.9 (m, 2 F; metaCF), 150.0 (m, 1 F; para-CF), 131.2 ppm (m, 2 F; ortho-CF); 31 P NMR ([D6]benzene, RT): d = 106.0 ppm (t/m, 3J(P,F) = 54 Hz, P); 31 19 P{ F} NMR ([D6]benzene, RT): d = 106.0 ppm (quint, 3J(P,H) = 12 Hz, P).
Synthesis of C6H5P(Cl)NEt2 : Diethylamine (29.3 g, 400.0 mmol) was added to a solution of PhPCl2 (35.8 g, 200.0 mmol) in n-pentane (500 mL) at 78 8C. The reaction mixture was slowly warmed to room temperature and filtered. The solvent was removed under reduced pressure and C6H5P(Cl)NEt2 (22.9 g, 106.5 mmol, 53 %) was obtained by vacuum distillation of the residue as a colorless liquid. 1 H NMR ([D]chloroform, RT): d = 1.2 (t, 3J(H,H) = 7 Hz, 6 H; CH3), 3.2 (m, 4 H; CH2), 7.5 (br m, 3 H; meta/para-CH), 7.8 ppm (br m, 2 H; ortho-CH); 13C{1H} NMR ([D]chloroform, RT): d = 14.2 (d, 3J(P,C) = 6 Hz, CH3), 43.9 (d, 2J(P,C) = 13 Hz, CH2), 128.5 (d, 3J(P,C) = 4 Hz, meta-CH), 129.7 (d, 4J(P,C) = 1 Hz, para-CH), 130.7 (d, 2J(P,C) = 20 Hz,
Synthesis of C6F5(C2F5)PNEt2 : A 1.6 m solution of n-butyllithium in n-hexane (14.4 mL, 23.0 mmol) was diluted in diethyl ether (100 mL) and degassed at 78 8C. Then the resulting solution was stirred for 30 min in an atmosphere of pentafluoroethane (25.0 mmol). After the addition of C6F5P(Br)NEt2 (7.0 g, 20.1 mmol) Chem. Eur. J. 2014, 20, 1 – 7
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Full Paper ortho-CH), 139.6 ppm (d, 1J(P,C) = 29 Hz, ipso-C); 31P NMR ([D]chloroform, RT): d = 142.1 ppm (br s, P).
Acknowledgements
Synthesis of C6H5(C2F5)PNEt2 : A sample of C6H5P(Cl)NEt2 (5.8 g, 26.9 mmol) was added to a solution of LiC2F5, generated in situ from nBuLi (21.9 mL of 1.6 m in n-hexane) and pentafluoroethane (35.0 mmol) in diethyl ether (150 mL) at 78 8C. The reaction mixture was slowly warmed to room temperature and filtered. The solvent was removed under reduced pressure and vacuum distillation of the residue at 1 10 3 mbar yielded C6H5(C2F5)PNEt2 (5.4 g, 18.2 mmol, 68 %) at 32 8C as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 1.1 (t, 3J(H,H) = 7 Hz, 6 H; CH3), 3.2 (d/quart, 3J(P,H) = 10 Hz, 3J(H,H) = 7 Hz, 4 H; CH2), 7.5 (br m, 3 H; meta/para-CH), 7.7 ppm (br m, 2 H; ortho-CH); 13C{1H} NMR ([D]chloroform, RT): d = 14.1 (d 3J(P,C) = 3 Hz, CH3), 45.0 (d, 2J(P,C) = 17 Hz, CH2), 128.5 (d, 3 J(P,C) = 7 Hz, meta-CH), 129.6 (s, para-CH), 132.0 (d/d, 2J(P,C) = 21 Hz, J = 3, ortho-CH), 133.3 ppm (d/d, 1J(P,C) = 9 Hz, J = 3 Hz, ipsoC); 19F NMR ([D]chloroform, RT): d = 119.0 (d/d/quart, 2J(F,F) = 300 Hz, 2J(P,F) = 45 Hz, 3J(F,F) = 3 Hz, 1J(C,F) = 300 Hz, 1 F; CFaFbCF3), 115.7 (d/d, 2J(F,F) = 300 Hz, 2J(P,F) = 71 Hz, 1J(F,C) = 289 Hz, 1 F; CFaFbCF3), 81.4 ppm (d/t, 3J(P,F) = 17 Hz, 3J(F,F) = 2 Hz, 1J(F,C) = 287 Hz, 3 F; CF3); 31P NMR ([D]chloroform, RT): d = 57.0 ppm (br m, P); 31P{1H} NMR ([D]chloroform, RT): d = 57.0 ppm (d/d/quart, 2 J(P,Fb) = 71 Hz, 2J(P,Fa) = 45 Hz, 3J(P,F) = 17 Hz, P).
This work was supported by Merck KGaA (Darmstadt, Germany) and Solvay (Hannover, Germany) which is gratefully acknowledged. We thank Prof. Dr. Lothar Weber and Dr. Julia Bader for helpful discussions. Keywords: fluorine · phosphane oxides · phosphinous acids · phosphorus · tautomeric equilibrium [1] a) N. V. Dubrovina, A. Bçrner, Angew. Chem. 2004, 116, 6007 – 6010; Angew. Chem. Int. Ed. 2004, 43, 5883 – 5886; b) L. Ackermann, Synthesis 2006, 10, 1557 – 1571. [2] B. Hoge, P. Garcia, H. Willner, H. Oberhammer, Chem. Eur. J. 2006, 12, 3567 – 3574. [3] B. Hoge, J. Bader, H. Beckers, Y. S. Kim, R. Eujen, H. Willner, N. Ignatiev, Chem. Eur. J. 2009, 15, 3567 – 3576; B. Hoge, J. Bader, B. Kurscheid, N. Ignatyev, E. F. Aust, (Merck Patent GmbH, Darmstadt, Germany), WO 2010/009818A1. [4] B. Kurscheid, W. Wiebe, B. Neumann, H.-G. Stammler, B. Hoge, Eur. J. Inorg. Chem. 2011, 5523 – 5529. [5] N. Allefeld, B. Neumann, H.-G. Stammler, N. Ignatiev, B. Hoge, 2014, unpublished results. [6] B. Kurscheid, L. Belkoura, B. Hoge, Organometallics 2012, 31, 1329 – 1334. [7] T. L. Emmick, R. L. Letsinger, J. Am. Chem. Soc. 1968, 90, 3459 – 3465. [8] C. Petit, A. Favre-Reguillon, G. Mignani, M. Lemaire, Green Chem. 2010, 12, 326 – 330. [9] a) M. Henrich, A. Marhold, A. Kolomeitsev, A. Kadyrov, G.-V. Rçschenthaler, J. Barten, (Bayer AG, Leverkusen), DE 10128703, 2001; b) M. F. Ernst, D. M. Roddick, Inorg. Chem. 1989, 28, 1624 – 1627; c) M. H. Kçnigsmann, PhD thesis, University of Bremen, Bremen, 2004. [10] a) R. G. Peters, B. L. Bennett, R. C. Schnabel, D. M. Roddick, Inorg. Chem. 1997, 36, 5962 – 5965; b) M. M. Choate, R. G. Baughman, J. E. Phelps, R. G. Peters, J. Organomet. Chem. 2011, 696, 956 – 962. [11] A. V. Zakharov, Y. V. Vishnevskiy, N. Allefeld, J. Bader, B. Kurscheid, S. Steinhauer, B. Hoge, B. Neumann, H.-G. Stammler, R. J. F. Berger, N. W. Mitzel, Eur. J. Inorg. Chem. 2013, 3392 – 3404. [12] V. Gutmann, Electrochim. Acta 1976, 21, 661 – 670. [13] B. Hoge, S. Neufeind, S. Hettel, W. Wiebe, C. Thçsen, J. Organomet. Chem. 2005, 690, 2382 – 2387. [14] W. Volbach, I. Ruppert, Tetrahedron Lett. 1983, 24, 5509 – 5512. [15] D. D. Magnelli, G. Tesi, J. U. Lowe, W. E. McQuistion, Inorg. Chem. 1966, 5, 457 – 461.
Synthesis of C6H5(C2F5)P(O)H: A sample of C6H5(C2F5)P(NEt2) (1.77 g, 6.18 mmol) was combined with a slurry of para-toluenesulfonic acid (4.7 g, 24.7 mmol) in 1,6-dibromohexane (30 mL). After 12 h the solvent was removed under reduced pressure and C6H5(C2F5)P(O)H (0.63 g, 2.58 mmol, 42 %) was isolated by vacuum distillation at 1 10 3 mbar and 100 8C. 1H NMR ([D]chloroform, RT): d = 7.6 (m, 2 H; meta-CH), 7.7 (m, 1 H; para-CH), 7.7 (d/d, 1J(P,H) = 518 Hz, J = 10 Hz, 1 H; PH), 7.8 ppm (d/m, 3J(P,H) = 15 Hz, 2 H; orthoCH); 13C{1H} NMR ([D]chloroform, RT): d = 129.4 (d, 2J(P,C) = 14 Hz, ortho-C), 131.5 (d, 3J(P,C) = 13 Hz, meta-C), 135.2 (br s, para-C); 13 19 C{ F} NMR ([D]chloroform, RT): d = 111.9 (d, 1J(P,C) = 90 Hz, CF2), 118.6 ppm (d, 2J(P,C) = 15 Hz, CF3); 19F NMR ([D]chloroform, RT): d = 129.7 (d/d, 2J(F,F) = 324 Hz, 2J(P,F) = 76 Hz, 1 F; CFaFbCF3), 125.2 (d/d, 2J(F,F) = 323 Hz, 2J(P,F) = 78 Hz, 1 F; CFaFbCF3), 80.1 ppm (s, 3 F; CF3); 19F{31P} NMR ([D]chloroform, RT): d = 129.7 (d, 2J(F,F) = 324 Hz, 1 F; CFaFbCF3), 125.2 (d, 2J(F,F) = 323 Hz, 1 F; CFaFbCF3), 80.1 ppm (s, 3 F; CF3); 31P NMR ([D]chloroform, RT): d = 17.2 ppm (d/t/t, 1J(P,H) = 519 Hz, 2J(P,F) = 77 Hz, 3J(P,H) = 14 Hz, P); 31P{1H} NMR ([D]chloroform, RT): d = 17.2 ppm (t, 2J(P,F) = 77 Hz, P); 31P{19F} NMR ([D]chloroform, RT): d = 17.2 ppm (d/t, 1J(P,H) = 519 Hz, 3 J(P,H) = 14 Hz, P).
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FULL PAPER & Phosphorus Chemistry N. Allefeld, M. Grasse, N. Ignat’ev, B. Hoge* && – &&
A tautomeric equilibrium between phosphane oxide and phosphinous acid is invoked to explain the role of secondary phosphane oxides (SPOs) as preligands in coordination chemistry. Whereas CF3(C2F5)POH exists in solution
and the gas phase solely as the acid tautomer, the less electron-withdrawing C6F5(C2F5)POH and C6F5(C2F5)P(O)H are involved in equilibria in solution as well as in the neat liquid phase (see figure).
Synthesis of Unsymmetrically Substituted Phosphane Oxides (R1R2P(O)H) and Phosphinous Acids (R1R2POH)
Keeping the balance… ..with electron-withdrawing groups! The unusual form of a phosphinous acid can be stabilized with respect to the corresponding phosphane oxide tautomer by the influence of electron-withdrawing groups. In their Full Paper on page && ff., B. Hoge et al. describe an efficient strategy for the synthesis of phosphinous acids and phosphane oxides featuring one pentafluoroethyl group with an additional trifluoromethyl, pentafluorophenyl, or phenyl substituent. The differing stabilization of the phosphinous acid form is evidenced in the tautomeric equilibrium.
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& Phosphorus Chemistry
Synthesis of Unsymmetrically Substituted Phosphane Oxides (R1R2P(O)H) and Phosphinous Acids (R1R2POH) Nadine Allefeld,[a] Michael Grasse,[a] Nikolai Ignat’ev,[b] and Berthold Hoge*[a]
Abstract: This paper describes the synthesis of unsymmetrically substituted phosphinous acids and phosphane oxides featuring at least one electron-withdrawing pentafluoroethyl group. The presence of a diethylamino function as a protecting group allows a selective reaction of RClPNEt2 (R = CF3, C6F5, C6H5) with LiC2F5. On treatment with para-toluenesulfonic acid the isolated aminophosphanes R(C2F5)PNEt2 are
readily converted into the corresponding phosphinous acids or phosphane oxides, respectively. Investigation of the tautomeric equilibrium between oxide and acid tautomer revealed (CF3)(C2F5)POH as a stable phosphinous acid, whereas the pentafluorophenyl and phenyl derivatives constitute a solvent dependent equilibrium between the acid and the oxide tautomer.
Introduction
to date, the only literature-known examples of stable phosphinous acids.[2, 3a] All other known diorganyl derivatives of the composition R2P(O)H favor the phosphane oxide form in the pure state. With the less electron-withdrawing C5F4N or 2,4(CF3)2C6H3 groups[4] a solvent-dependent equilibrium was observed, whereby donor solvents, such as DMF or THF, stabilize the acid tautomer. A greater variety of electronic and steric situations can be encountered in unsymmetrically substituted phosphane oxides. In the literature several examples for unsymmetrically substituted alkyl and aryl SPOs are described. Emmick et al. reported on the treatment of monosubstituted phosphinic esters RP(O)(OEt)H with Grignard reagents, yielding in SPOs of the type RR’P(O)H.[7] More recently the synthesis of several alkyland aryl-substituted SPOs starting from a mixed-phenyl-substituted aminochlorophosphane was performed.[8] In this paper we present an efficient synthesis of unsymmetrically substituted secondary phosphane oxides and phosphinous acids featuring one pentafluoroethyl group.
Phosphorus-based ligands are ubiquitous in the synthesis of catalytically active transition-metal complexes. However, the widely used triorganophosphanes suffer from several disadvantages, such as sensitivity towards oxidation or hydrolysis. With regard to this, air and moisture stable secondary phosphane oxides (SPOs), which are easy to handle, are a conceivable alternative. A tautomeric equilibrium between phosphane oxide and phosphinous acid is invoked to explain the role of SPOs as preligands in coordination chemistry.[1] The introduction of electron-withdrawing substituents leads to the generation of strongly p-acidic phosphinous acid ligands, with application in cross-coupling reactions, such as the Suzuki type reaction.[2–6] From a molecular point of view, perfluoroorganyl groups cause a stabilization of the phosphinous acid tautomer with respect to the phosphane oxide form [Eq. (1)].
Results and Discussion In order to synthesize unsymmetric SPOs with different perfluoroorganyl groups, in the first step the aminochlorophosphanes RClPNEt2 (R = CF3, C6F5, C6H5) have to be synthesized as starting materials. Treatment of CF3P(NEt2)2 with two equivalents of hydrogen chloride led to the selective substitution of one amino group by a chlorine atom [Eq. (2)]. The change in the electronic nature of the substituents—an electron-donating amino substituent is substituted by an electron-withdrawing chlorine atom—is reflected in the 31P NMR spectrum, in which the resonance of CF3P(NEt2)2 at 45.6 ppm is shifted to 98.5 ppm for (CF3)P(Cl)NEt2. On the other hand, treatment of the dichlorophosphanes RPCl2 (R = C6F5, C6H5) with two equivalents of diethylamine selectively affords RP(Cl)NEt2 accordingly to Equation (3).
The bis(trifluoromethyl)phosphinous acid ((CF3)2POH) and bis(pentafluoroethyl)phosphinous acid ((C2F5)2POH) represent, [a] Dr. N. Allefeld, M. Grasse, Prof. Dr. B. Hoge Centrum fr Molekulare Materialien Fakultt fr Chemie, Anorganische Chemie Universitt Bielefeld Universittsstraße 25, 33615 Bielefeld (Germany) E-mail: [email protected] [b] Dr. N. Ignat’ev PM-ATI, Merck KGaA Frankfurter Str. 250, 64293 Darmstadt (Germany) Chem. Eur. J. 2014, 20, 1 – 7
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Full Paper ly.[2, 11] Similarly CF3(C2F5)PNEt2 was added to a slurry of excess para-toluenesulfonic acid in 1,6-dibromohexane. The phosphinous acid CF3(C2F5)POH was isolated by means of fractional condensation as the only volatile compound in 90 % yield [Eq. (5)].
The 31P NMR spectrum in [D]chloroform reveals one multiplet at 80.7 ppm (Table 2), which compares well with the resonances of (CF3)2POH (d(31P) = 77.9)[2] and (C2F5)2POH (d(31P) = 93.0).[11] Based on an A3B3MNX spin system (A, B, M, N = 19F; X = 31P) the 31P and 19F NMR spectra were simulated. No spectroscopic evidence for a second isomer could be obtained.
Substitution of an electron-withdrawing chlorine atom by an electron-donating amino group results in an upfield shift of the 31P NMR signal from 162.0 (C6H5PCl2) to 142.1 ppm (C6H5P(Cl)NEt2) and from 137.0 (C6F5PBr2) to 106.0 ppm (C6F5P(Br)NEt2) (Table 1).
Table 2. NMR data of phosphinous acids and phosphane oxides. Table 1. Selected NMR spectroscopical data of the aminophosphanes. 31
[a]
CF3P(Cl)NEt2 CF3P(Br)NEt2[a] C6F5P(Br)NEt2[b] C6H5P(Cl)NEt2[a] CF3(C2F5)PNEt2[a] [a]
C6F5(C2F5)PNEt2 C6H5(C2F5)PNEt2[a]
d( P) [ppm]
2
98.5 97.8 106.0 142.1 44.8
85 82 54[c] – 88[d] 47/59[e] 43/46[e] 71/45[e]
29.9 57.0
d(31P) [ppm]
J(P,F) [Hz]
[a]
CF3(C2F5)POH C6F5(C2F5)POH[c] C6F5(C2F5)P(O)H[a] C6H5(C2F5)POH[c] C6H5(C2F5)P(O)H[a]
The pure compounds were isolated by distillation as colorless liquids in good yields. Bis(trifluoromethyl)phosphinous acid ((CF3)2POH) and bis(pentafluoroethyl)phosphinous acid ((C2F5)2POH) have been prepared from the corresponding aminophosphane R2PNEt2 (R = CF3, C2F5) by treatment with para-toluenesulfonic acid, which results in the replacement of the amino group by a hydroxyl group. The isolation of the phosphinous acid was effected by fractional condensation in 88 and 93 % yield, respectiveChem. Eur. J. 2014, 20, 1 – 7
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– – 572 – 519
2
J(P,F) [Hz]
52/65[b] 65/82[b] n.d.[d] 86 78/76[b]
The experimentally determined (Figure 1 (top)) and the simulated 31P NMR spectra (Figure 1 (bottom)) are in excellent agreement. In addition to the coupling of the phosphorus atom to the CF3 groups (2J(P,F) = 83.0 Hz, 3J(P,F) = 14.1 Hz), two couplings to the diastereotopic fluorine atoms of the CF2 group were detected (2J(P,Fa) = 52.0 Hz, 2J(P,Fb) = 64.9 Hz).
Previous work has shown that pentafluoroethyl lithium (LiC2F5), generated in situ from nBuLi and C2F5H at 78 8C,[9] represents an efficient reagent for the pentafluoroethylation of chlorophosphanes ClnP(NEt2)3 n (n = 1, 2).[10, 11] Treatment of the aminochlorophosphanes RP(X)NEt2 (R = CF3, C6F5, C6H5 ; X = Cl, Br) with LiC2F5 selectively afforded the unsymmetrically substituted aminophosphanes R(C2F5)PNEt2 [R = CF3, C6F5, C6H5,; Eq. (4)].
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J(P,H) [Hz]
[a] [D]chloroform. [b] diastereotopic fluorine atoms of CF2 group. [c] Et2O. [d] Not detected.
[a] [D]chloroform. [b] [D6]benzene. [c] 3J(P,F). [d] Coupling to CF3. [e] Diastereotopic fluorine atoms of CF2 group.
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80.7 81.2 1.9 90.0 17.2
1
Figure 1. Experimental (top) and simulated (bottom) 31P NMR spectrum of (CF3)(C2F5)POH.
In accordance to this, the 19F NMR spectrum displays four sets of signals, one for the CF3 group directly bonded to the phosphorus atom at 64.0 ppm, and at 81.9 ppm for the CF3 unit of the C2F5 substituent. The CF2 unit causes two resonances for the diastereotopic fluorine atoms Fa at 126.4 and Fb at 125.8 ppm (Figure 2). 2
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Figure 2. Experimental (top) and simulated (bottom) 19F NMR spectrum of (CF3)(C2F5)POH; I (Fa), II (Fb), III (CF2CF3), IV (CF3).
The phosphinous acid was further studied by IR spectroscopy in the gas phase. Two OH stretching modes were detected at 3669 and 3596 cm 1, respectively, due to the existence of two rotational isomers in the gas phase. The phosphinous acids (CF3)2POH and (C2F5)2POH are also present in two rotamers, in a cis and a trans conformation.[2, 3] Calculations revealed that the cis rotamer is more stable by 7.5 ((CF3)2POH) and 6.3 kJ mol 1 ((C2F5)2POH), respectively (B3LYP/6-311 + G(2df,p)). Correspondingly, the cis rotamer of (CF3)(C2F5)POH is stabilized by 5.1 kJ mol 1 with respect to the trans rotamer (Figure 3). The absence of a P H as well as a P=O stretching mode confirms the exclusive presence of the acid tautomer in the gas phase at room temperature. This agrees with the observations for (CF3)2POH and (C2F5)2POH. Both compounds exist in the solid state as well as in the gas phase solely as the acid
phosphane oxide C6F5(C2F5)P(O)H and the phosphinous acid tautomer, C6F5(C2F5)POH. The 31P NMR spectrum of the neat liquid shows two resonances at 84.3 and 0.7 ppm in an integral ratio of 48:52 that are assigned to the acid and the oxide form, respectively (Figure 4). The resonance of the oxide tautomer reveals a characteristic 1J(P,H) coupling constant of 582 Hz. In solvents with high donor numbers,[12] such as diethyl ether, the tautomeric equilibrium is shifted completely towards the acid tautomer (Table 3). In solvents with low donor numbers, however, the equilibrium is shifted towards the oxide tautomer. In the 31 P NMR spectrum of the chloroform two signals at 80.6 and 1.9 ppm in an integral ratio of 42:58 are detected—the portion of the oxide tautomer increased with regard to the pure sample. A similar behavior was observed for the equilibrium between C6H5(C2F5)POH and C6H5(C2F5)P(O)H.
Figure 3. Relative zero-point corrected energies of the two rotational isomers of CF3(C2F5)POH and the oxide tautomer CF3(C2F5)P(O)H calculated at the B3LYP/6-311 + G(2df,p) level of theory. Chem. Eur. J. 2014, 20, 1 – 7
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tautomer. In the liquid phase (C2F5)2POH is rearranged into 20 % of the oxide tautomer, whereas (CF3)2POH and CF3(C2F5)POH are exclusively known in the acid tautomer. The pentafluorophenyl and phenyl derivatives were synthesized analogously by the exposure of the corresponding aminophosphanes R(C2F5)PNEt2 (R = C6F5, C6H5) to para-toluenesulfonic acid and were isolated as colorless liquids by distillation under reduced pressure in a 52 (R = C6F5) and 42 % (R = Ph) yield, respectively [Eq. (6)]. In the neat liquid, the pentafluorophenyl derivative exists in an equilibrium between the
Figure 4. 31P NMR spectrum of C6F5(C2F5)POH/C6F5(C2F5)P(O)H, neat liquid.
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Table 3. Solvent dependent equilibrium between phosphinous acids and their corresponding phosphane oxide tautomers. Solvent
R(C2F5)POH
CHCl3 neat
100 100
0 0
R = C6F5
neat CHCl3 Et2O MeCN DMF
48 42 100 100 100
52 58 0 0 0
R = C6H5
CHCl3 MeCN neat Et2O DMF
0 7 15 30 87
100 93 85 70 13
R = CF3
R(C2F5)P(O)H
Conclusions This work describes an efficient strategy for the synthesis of phosphinous acids and phosphane oxides featuring one pentafluoroethyl group with an additional trifluoromethyl, pentafluorophenyl, or phenyl substituent. The differing stabilization of the phosphinous acid form is evidenced in the tautomeric equilibrium. Whereas the compound CF3(C2F5)POH exists in solution and the gas phase solely as the acid tautomer, the less electron-withdrawing pentafluorophenyl-substituted compounds C6F5(C2F5)POH/C6F5(C2F5)P(O)H are involved in tautomeric equilibria in solution as well as in the neat liquid phase. The even less electron-withdrawing phenyl group, which may be rather regarded as a donor function, clearly favors the oxide tautomer, as observed exclusively in chloroform. In general, electron-withdrawing groups stabilize the acid tautomer. Although the electronic nature of trifluoromethyl and pentafluoroethyl groups are almost comparable, the stabilization of a phosphinous acid is most efficiently realized by a trifluoromethyl substituent.
The phenyl group is less electron-withdrawing than a perfluorophenyl group. Therefore, in chloroform only the oxide tautomer was detected by NMR spectroscopy. In the more donating solvent diethyl ether a tautomeric ratio of acid to oxide of 30:70 was detected. This clearly reflects the different inductive effects of a phenyl and a pentafluorophenyl substituent. For an improved understanding, the energy differences between the acid and the oxide tautomer were calculated on a B3LYP/6-311 + G(2df,p) level of theory. Stabilization of the phosphinous acid tautomer decreases in the series CF3 > C2F5 > C6F5 > C6H5 (Table 4). The phosphinous acid CF3(C2F5)POH is
Experimental Section All chemicals were obtained from commercial sources and used without further purification. The phosphanes CF3P(NEt2)2[14] and C6F5PX2[15] were synthesized according to literature methods. Standard high-vacuum techniques were employed throughout all preparative procedures. Non-volatile compounds were handled in a dry N2 atmosphere using Schlenk techniques. NMR spectra were recorded on a Bruker Model Avance III 300 spectrometer (31P 111.92 MHz; 19F 282.40 MHz; 13C 75.47 MHz, 1H 300.13 MHz) with positive shifts being downfield from the external standards (85 % orthophosphoric acid (31P), CCl3F (19F) and TMS (1H)). IR spectra were recorded on a ALPHA-FT-IR spectrometer (Bruker) using a gas cell with KBr windows.
Table 4. Calculated energy differences of the tautomeric equilibrium between phosphinous acid R1R2POH and phosphane oxide tautomers R1R2P(O)H with CF3, C2F5, C6F5 and C6H5 groups (B3LYP/6-311 + G(2df,p)).[a] R1
R2
DEZP,T [kJ mol 1]
CF3 CF3 C2F5 C6F5 C6F5 C6H5
CF3 C2F5 C2F5 C2F5 C6F5 C2F5
25.7 24.4 22.8 13.3 1.7[b] 8.0
Synthesis of CF3P(X)NEt2 (X=Cl, Br) 1) The addition of diethylamine (2.2 g, 30.8 mmol) to a solution of (CF3)PBr2 (4.0 g, 15.4 mmol) in n-hexane at 78 8C led to the precipitation of a colorless solid. The reaction mixture was slowly warmed to room temperature. After filtration the solvent was removed under reduced pressure. Fractional condensation yielded CF3P(Br)NEt2 (1.8 g, 7.2 mmol, 46 %) in the 40 8C trap as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 0.9 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.2 ppm (m, 2 H; CH2); 31P NMR ([D]chloroform, RT): d = 97.8 ppm (quart/m, 2J(P,F) = 82 Hz, P).
[a] DEZP,T = EZP,T(acid) EZP,T(oxide). [b] B3LYP/6-311G(2d,p).[13]
more stable than the corresponding phosphane oxide by 24.4 kJ mol 1. Thus, this compound is less stabilized than (CF3)2POH (DEZP,T = 25.7 kJ mol 1), but more than (C2F5)2POH (DEZP,T = 22.8 kJ mol 1) with respect to their corresponding phosphane oxide counterparts. The phosphinous acid C6F5(C2F5)POH is by 13.3 kJ mol 1 more stable than the oxide, whereas stabilization of the symmetrically substituted (C6F5)2POH amounts only to 1.7 kJ mol 1. These calculated energy differences are in agreement with the experimental findings. In the neat liquid the ratio of phosphinous acid C6F5(C2F5)POH to phosphane oxide C6F5(C2F5)P(O)H is 48:52, in diethyl ether only the acid tautomer is detected. For comparison, the oxide tautomer (C6F5)2P(O)H is observed to &
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2) A solution of CF3P(NEt2)2 (32.3 g, 132.3 mmol) in n-pentane (300 mL) was treated with gaseous HCl (293.6 mmol) at 78 8C. The reaction mixture was slowly warmed to room temperature and the ammonium salts were removed by filtration. The solvent was removed under reduced pressure. The product CF3P(Cl)NEt2 (16.5 g, 79.6 mmol, 60 %) was separated by vacuum distillation at 20 8C. 1 H NMR ([D]chloroform, RT): d = 0.9 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.3 ppm (m, 2 H; CH2); 13C{1H} NMR ([D]chloroform, RT): d = 13.9 (d, 3 J(P,C) = 5 Hz, CH3), 44.3 (d, 2J(P,C) = 18 Hz, CH2), 122.6 ppm (quart/ d, 1J(C,F) = 324 Hz, 1J(P,C) = 49 Hz, CF3); 19F NMR ([D]chloroform, RT): d = 67.3 ppm (d, 2J(P,F) = 86 Hz, CF3); 31P NMR ([D]chloroform, RT): d = 98.5 ppm (quart, 2J(P,F) = 85 Hz, P).
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Full Paper at 78 8C, the mixture was warmed to room temperature and stirred overnight. The precipitate was filtered off. After evaporation of the solvent, C6F5(C2F5)PNEt2 (7.2 g, 18.4 mmol, 92 %) was obtained by vacuum distillation at 43 8C as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 1.1 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.2 ppm (d/ quart, 3J(P,H) = 11 Hz, 2J(H,H) = 7 Hz, 2 H; CH2); 1H{31P} NMR ([D]chloroform, RT): d = 1.1 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.2 ppm (quart, 2J(H,H) = 7 Hz, 2 H; CH2); 13C{1H} NMR ([D]chloroform, RT): d = 13.7 (d, 3J(P,C) = 4 Hz, CH3), 44.6 ppm (d, 2J(P,C) = 19 Hz, CH2); 13 19 C{ F} NMR ([D]chloroform, RT): d = 117.9 (d, 1J(P,C) = 60 Hz, CF2), 119.8 (d/m, 2J(P,C) = 32 Hz, CF3), 137.6 (s, meta-CF), 143.1 (s, paraCF), 147.5 ppm (d, 2J(P,C) = 13 Hz, ortho-CF); 19F NMR ([D]chloroform, RT): d = 159.9 (m, 2 F; meta-CF), 148.2 (t/t, 3J(F,F) = 31 Hz, 4 J(F,F) = 5 Hz, 1 F; para-CF), 126.8 (m, 2 F; ortho-CF), 118.4 (d/d/t/ quart, 2J(F,F) = 297 Hz, 2J(P,F) = 43 Hz, 5J(F,F) = 15 Hz, 3J(F,F) = 3 Hz, 1 F; CFaFbCF3), 116.4 (d/d/t/quart, 2J(F,F) = 297 Hz, 2J(P,F) = 46 Hz, 5 J(F,F) = 20 Hz, 3J(F,F) = 3 Hz, 1 F; CFaFbCF3), 82.2 ppm (d/t, 3J(P,F) = 23 Hz, 3J(F,F) = 3 Hz, 3 F; CF2CF3); 19F{31P} NMR ([D]chloroform, RT): d = 159.9 (m, 2 F; meta-CF), 148.2 (t/t, 3J(F,F) = 31 Hz, 4J(F,F) = 5 Hz, 1 F; para-CF), 126.8 (m, 2 F; ortho-CF), 118.4 (d/t/quart, 2 J(F,F) = 297 Hz, 5J(F,F) = 15 Hz, 3J(F,F) = 3 Hz, 1 F; CFaFbCF3), 116.4 (d/t/quart, 2J(F,F) = 297 Hz, 5J(F,F) = 20 Hz, 3J(F,F) = 3 Hz, 1 F; 82.2 ppm (t, 3J(F,F) = 3 Hz, 3 F; CF2CF3); 31P NMR CFaFbCF3) ([D]chloroform, RT): d = 29.9 ppm (m, P).
Synthesis of C2F5(CF3)P(NEt2): Caution! LiC2F5 is highly reactive and tends to violently decompose above temperatures of 50 8C. A 1.6 m solution of n-butyllithium in n-hexane (59.0 mL, 94.4 mmol) was diluted in diethyl ether (200 mL) and degassed at 78 8C. Then the resulting solution was stirred for 30 min in an atmosphere of pentafluoroethane (123.0 mmol). After the addition of CF3P(Cl)NEt2 (16.5 g, 79.6 mmol) at 78 8C, the mixture was warmed to room temperature and stirred overnight. A precipitate was filtered off. After evaporation of the solvent, C2F5(CF3)PNEt2 (17.1 g, 61.0 mmol, 77 %) was obtained by distillation under reduced pressure at 75 8C as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 1.2 (t, 3J(H,H) = 7 Hz, 3 H; CH3), 3.2 ppm (m, 2 H; CH2); 13 1 C{ H} NMR ([D6]benzene, RT): d = 14.1 (d, 3J(P,C) = 3 Hz, CH3), 44.2 ppm (d, 2J(P,C) = 21 Hz, CH2); 13C{19F} NMR ([D]chloroform, RT): d = 119.5 (d, 1J(P,C) = 52 Hz, CF2), 120.1 ppm (d, 2J(P,C) = 25 Hz, CF3); 19 F NMR ([D]chloroform, RT): d = 119.0 (d/d/m, 2J(F,F) = 305 Hz, 2 J(P,F) = 59 Hz, 1 F; CFaFbCF3), 118.4 (d/d/m, 2J(F,F) = 304 Hz, 2 a b J(P,F) = 47 Hz, 1 F; CF F CF3), 82.5 (d/m, 3J(P,F) = 18 Hz, 3 F; 2 57.3 ppm (d/m, J(P,F) = 88 Hz, 3 F; CF3); 31P NMR CF2CF3), ([D]chloroform, RT): d = 44.8 ppm (br m, P). Synthesis of C2F5(CF3)POH: para-Toluenesulfonic acid (14.8 g, 86 mmol) was suspended in 1,6-dibromohexane (100 mL). After the addition of CF3(C2F5)PNEt2 (3.9 g, 13.8 mmol) the reaction mixture was stirred for 2 d. Fractional condensation at 78 8C yielded C2F5(CF3)POH (3.0 g, 12.5 mmol, 90 %) as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 4.2 ppm (br s, C2F5(CF3)POH); 13C{19F} NMR ([D]chloroform, RT): d = 116.5 (d, 1J(P,C) = 55 Hz, CF2), 119.0 (d, 2 J(P,C) = 20 Hz, CF2CF3), 127.0 ppm (d, 1J(P,C) = 34 Hz, CF3); 19F NMR ([D]chloroform, RT): d = 126.4 (d/d/quart/quart, 2J(F,F) = 308 Hz, 2 J(P,F) = 52 Hz, 4J(Fa,F) = 8 Hz, 3J(F,F) = 3 Hz, 1 F; CFaFbCF3), 125.9 (d/d/quart/quart, 2J(F,F) = 308 Hz, 2J(P,F) = 65 Hz, 4J(Fb,F) = 10 Hz, 3 J(F,F) = 3 Hz, 1 F; CFaFbCF3), 81.9 (d/d/d/quart, 3J(P,F) = 14 Hz, 3 a 3 b 5 J(F,F ) = 3 Hz, J(F,F ) = 3 Hz, J(F,F) = 2 Hz, 3 F; CF2CF3), 64.0 ppm (d/d/d/quart, 2J(P,F) = 83 Hz, 4J(F,Fb) = 10 Hz, 4J(F,Fa) = 8 Hz, 5J(F,F) = 2 Hz, 3 F; CF3); 31P NMR ([D]chloroform, RT): d = 80.7 ppm (quart/d/ d/quart, 2J(P,F) = 83 Hz, 2J(P,Fb) = 65 Hz, 2J(P,Fa) = 52 Hz, 3J(P,F) = 14 Hz, P); IR (gas phase): n˜ = 398 (vw), 423 (w), 457 (w), 547 (w), 628 (w), 748 (w), 857 (w), 966 (w), 1056 (w), 1151 (m), 1165 (m), 1186 (m), 1227 (s), 1336 (w), 3596 (w), 3669 cm 1 (vw).
Synthesis of C6F5(C2F5)POH/C6F5(C2F5)P(O)H: A slurry of para-toluenesulfonic acid (4.2 g, 22.1 mmol) in diethyl ether (50 mL) was treated with C6F5(C2F5)PNEt2 (2.1 g, 5.6 mmol). The yellow mixture was stirred overnight. Removal of the solvent under reduced pressure and vacuum distillation afforded C6F5(C2F5)POH/C6F5(C2F5)P(O)H (1.0 g, 2.9 mmol, 52 %) as a colorless oil. 1H NMR ([D]chloroform, RT): d = 6.2 (br s, C6F5(C2F5)POH), 8.2 ppm (d, 1 J(P,H) = 572 Hz, C6F5(C2F5)P(O)H); 13C{19F} NMR (Et2O, RT): d = 120.3 (d, 2J(P,C) = 19 Hz, C6F5(CF3CF2)POH), 137.9 (s, C6F5(C2F5)POH (metaCF)), 143.9 (s, C6F5(C2F5)POH (para-CF)), 148.1 ppm (d, 2J(P,C) = 12 Hz, C6F5(C2F5)POH (ortho-CF)); 19F NMR ([D]chloroform, RT): d = 160.3 (br s, meta-CF (acid)), 156.5 (br s, meta-CF (oxide)), 146.9 (br s, para-CF (acid)), 137.8 (br s, para-CF (oxide)), 132.8 (br s, ortho-CF (acid)), 129.5 (br s, ortho-CF (oxide)), 123.9–128.3 (m, CF2), 81.5 (br s, CF3 (acid)), 80.4 ppm (br s, CF3 (oxide)); 19F NMR (Et2O, RT): d = 162.3 (m, 2 F; meta-CF (acid)), 149.5 (m, 1 F; paraCF (acid)), 131.9 (m, 2 F; ortho-CF (acid)), 125.8 (d/d/m, 2J(F,F) = 303 Hz, 2J(P,F) = 65 Hz, 1 F; C6F5(CF3CFaCFb)POH), 124.4 (d/d/m, 2 2 J(F,F) = 303 Hz, J(P,F) = 82 Hz, 1 F; C6F5(CF3CFaFbCFb)POH), 3 81.7 ppm (d, J(P,F) = 14 Hz, 3 F; C6F5(CF3CFaCFb)POH); 31P NMR ([D]chloroform, RT): d = 1.9 (d/t, 1J(P,H) = 572 Hz, 2J(P,F) = 87 Hz, C6F5(C2F5)P(O)H), 81.2 ppm (br m, C6F5(C2F5)POH); 31P NMR (Et2O, RT): d = 87.3 ppm (m, C6F5(C2F5)POH); IR (ATR): n˜ = 417 (m), 478 (m), 515 (m), 546 (w), 589 (w), 640 (m), 730 (w), 747 (m), 760 (w), 836 (w), 864 (m), 875 (m), 898 (m), 962 (s), 980 (s), 1003 (m), 1092 (s), 1130 (m), 1203 (s), 1248 (m), 1313 (m), 1390 (w), 1402 (w), 1475 (s), 1492 (s), 1519 (m), 1644 (w), 3186 cm 1 (w).
Synthesis of C6F5P(Br)NEt2 : A sample of C6F5PBr2 (10.3 g, 28.9 mmol) was dissolved in n-hexane (50 mL) and cooled to 78 8C. Addition of diethylamine (6.2 mL, 58.9 mmol) led to the precipitation of a colorless solid. The reaction mixture was slowly warmed overnight to room temperature and filtered. The solvent was removed under reduced pressure and vacuum distillation at 1 10 3 mbar and 50 8C yielded C6F5P(Br)NEt2 (3.6 g, 10.3 mmol, 36 %) as a colorless oil. 1H NMR ([D6]benzene, RT): d = 0.9 (t, 3 J(H,H) = 7 Hz, 3 H; CH3), 2.9 ppm (m, 2 H; CH2); 13C{1H} NMR ([D6]benzene, RT): d = 9.5 (d, 3J(P,C) = 7 Hz, CH3), 40.7 ppm (d, 2 J(P,C) = 18 Hz, CH2); 13C{19F} NMR ([D6]benzene, RT): d = 133.1 (s, meta-CF), 138.1 (s, para-CF), 142.5 ppm (d, 2J(P,C) = 14 Hz, orthoCF); 19F NMR ([D6]benzene, RT): d = 160.9 (m, 2 F; meta-CF), 150.0 (m, 1 F; para-CF), 131.2 ppm (d/m, 3J(P,F) = 54 Hz, 2 F; ortho-CF); 19F{31P} NMR ([D6]benzene, RT): d = 160.9 (m, 2 F; metaCF), 150.0 (m, 1 F; para-CF), 131.2 ppm (m, 2 F; ortho-CF); 31 P NMR ([D6]benzene, RT): d = 106.0 ppm (t/m, 3J(P,F) = 54 Hz, P); 31 19 P{ F} NMR ([D6]benzene, RT): d = 106.0 ppm (quint, 3J(P,H) = 12 Hz, P).
Synthesis of C6H5P(Cl)NEt2 : Diethylamine (29.3 g, 400.0 mmol) was added to a solution of PhPCl2 (35.8 g, 200.0 mmol) in n-pentane (500 mL) at 78 8C. The reaction mixture was slowly warmed to room temperature and filtered. The solvent was removed under reduced pressure and C6H5P(Cl)NEt2 (22.9 g, 106.5 mmol, 53 %) was obtained by vacuum distillation of the residue as a colorless liquid. 1 H NMR ([D]chloroform, RT): d = 1.2 (t, 3J(H,H) = 7 Hz, 6 H; CH3), 3.2 (m, 4 H; CH2), 7.5 (br m, 3 H; meta/para-CH), 7.8 ppm (br m, 2 H; ortho-CH); 13C{1H} NMR ([D]chloroform, RT): d = 14.2 (d, 3J(P,C) = 6 Hz, CH3), 43.9 (d, 2J(P,C) = 13 Hz, CH2), 128.5 (d, 3J(P,C) = 4 Hz, meta-CH), 129.7 (d, 4J(P,C) = 1 Hz, para-CH), 130.7 (d, 2J(P,C) = 20 Hz,
Synthesis of C6F5(C2F5)PNEt2 : A 1.6 m solution of n-butyllithium in n-hexane (14.4 mL, 23.0 mmol) was diluted in diethyl ether (100 mL) and degassed at 78 8C. Then the resulting solution was stirred for 30 min in an atmosphere of pentafluoroethane (25.0 mmol). After the addition of C6F5P(Br)NEt2 (7.0 g, 20.1 mmol) Chem. Eur. J. 2014, 20, 1 – 7
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Full Paper ortho-CH), 139.6 ppm (d, 1J(P,C) = 29 Hz, ipso-C); 31P NMR ([D]chloroform, RT): d = 142.1 ppm (br s, P).
Acknowledgements
Synthesis of C6H5(C2F5)PNEt2 : A sample of C6H5P(Cl)NEt2 (5.8 g, 26.9 mmol) was added to a solution of LiC2F5, generated in situ from nBuLi (21.9 mL of 1.6 m in n-hexane) and pentafluoroethane (35.0 mmol) in diethyl ether (150 mL) at 78 8C. The reaction mixture was slowly warmed to room temperature and filtered. The solvent was removed under reduced pressure and vacuum distillation of the residue at 1 10 3 mbar yielded C6H5(C2F5)PNEt2 (5.4 g, 18.2 mmol, 68 %) at 32 8C as a colorless liquid. 1H NMR ([D]chloroform, RT): d = 1.1 (t, 3J(H,H) = 7 Hz, 6 H; CH3), 3.2 (d/quart, 3J(P,H) = 10 Hz, 3J(H,H) = 7 Hz, 4 H; CH2), 7.5 (br m, 3 H; meta/para-CH), 7.7 ppm (br m, 2 H; ortho-CH); 13C{1H} NMR ([D]chloroform, RT): d = 14.1 (d 3J(P,C) = 3 Hz, CH3), 45.0 (d, 2J(P,C) = 17 Hz, CH2), 128.5 (d, 3 J(P,C) = 7 Hz, meta-CH), 129.6 (s, para-CH), 132.0 (d/d, 2J(P,C) = 21 Hz, J = 3, ortho-CH), 133.3 ppm (d/d, 1J(P,C) = 9 Hz, J = 3 Hz, ipsoC); 19F NMR ([D]chloroform, RT): d = 119.0 (d/d/quart, 2J(F,F) = 300 Hz, 2J(P,F) = 45 Hz, 3J(F,F) = 3 Hz, 1J(C,F) = 300 Hz, 1 F; CFaFbCF3), 115.7 (d/d, 2J(F,F) = 300 Hz, 2J(P,F) = 71 Hz, 1J(F,C) = 289 Hz, 1 F; CFaFbCF3), 81.4 ppm (d/t, 3J(P,F) = 17 Hz, 3J(F,F) = 2 Hz, 1J(F,C) = 287 Hz, 3 F; CF3); 31P NMR ([D]chloroform, RT): d = 57.0 ppm (br m, P); 31P{1H} NMR ([D]chloroform, RT): d = 57.0 ppm (d/d/quart, 2 J(P,Fb) = 71 Hz, 2J(P,Fa) = 45 Hz, 3J(P,F) = 17 Hz, P).
This work was supported by Merck KGaA (Darmstadt, Germany) and Solvay (Hannover, Germany) which is gratefully acknowledged. We thank Prof. Dr. Lothar Weber and Dr. Julia Bader for helpful discussions. Keywords: fluorine · phosphane oxides · phosphinous acids · phosphorus · tautomeric equilibrium [1] a) N. V. Dubrovina, A. Bçrner, Angew. Chem. 2004, 116, 6007 – 6010; Angew. Chem. Int. Ed. 2004, 43, 5883 – 5886; b) L. Ackermann, Synthesis 2006, 10, 1557 – 1571. [2] B. Hoge, P. Garcia, H. Willner, H. Oberhammer, Chem. Eur. J. 2006, 12, 3567 – 3574. [3] B. Hoge, J. Bader, H. Beckers, Y. S. Kim, R. Eujen, H. Willner, N. Ignatiev, Chem. Eur. J. 2009, 15, 3567 – 3576; B. Hoge, J. Bader, B. Kurscheid, N. Ignatyev, E. F. Aust, (Merck Patent GmbH, Darmstadt, Germany), WO 2010/009818A1. [4] B. Kurscheid, W. Wiebe, B. Neumann, H.-G. Stammler, B. Hoge, Eur. J. Inorg. Chem. 2011, 5523 – 5529. [5] N. Allefeld, B. Neumann, H.-G. Stammler, N. Ignatiev, B. Hoge, 2014, unpublished results. [6] B. Kurscheid, L. Belkoura, B. Hoge, Organometallics 2012, 31, 1329 – 1334. [7] T. L. Emmick, R. L. Letsinger, J. Am. Chem. Soc. 1968, 90, 3459 – 3465. [8] C. Petit, A. Favre-Reguillon, G. Mignani, M. Lemaire, Green Chem. 2010, 12, 326 – 330. [9] a) M. Henrich, A. Marhold, A. Kolomeitsev, A. Kadyrov, G.-V. Rçschenthaler, J. Barten, (Bayer AG, Leverkusen), DE 10128703, 2001; b) M. F. Ernst, D. M. Roddick, Inorg. Chem. 1989, 28, 1624 – 1627; c) M. H. Kçnigsmann, PhD thesis, University of Bremen, Bremen, 2004. [10] a) R. G. Peters, B. L. Bennett, R. C. Schnabel, D. M. Roddick, Inorg. Chem. 1997, 36, 5962 – 5965; b) M. M. Choate, R. G. Baughman, J. E. Phelps, R. G. Peters, J. Organomet. Chem. 2011, 696, 956 – 962. [11] A. V. Zakharov, Y. V. Vishnevskiy, N. Allefeld, J. Bader, B. Kurscheid, S. Steinhauer, B. Hoge, B. Neumann, H.-G. Stammler, R. J. F. Berger, N. W. Mitzel, Eur. J. Inorg. Chem. 2013, 3392 – 3404. [12] V. Gutmann, Electrochim. Acta 1976, 21, 661 – 670. [13] B. Hoge, S. Neufeind, S. Hettel, W. Wiebe, C. Thçsen, J. Organomet. Chem. 2005, 690, 2382 – 2387. [14] W. Volbach, I. Ruppert, Tetrahedron Lett. 1983, 24, 5509 – 5512. [15] D. D. Magnelli, G. Tesi, J. U. Lowe, W. E. McQuistion, Inorg. Chem. 1966, 5, 457 – 461.
Synthesis of C6H5(C2F5)P(O)H: A sample of C6H5(C2F5)P(NEt2) (1.77 g, 6.18 mmol) was combined with a slurry of para-toluenesulfonic acid (4.7 g, 24.7 mmol) in 1,6-dibromohexane (30 mL). After 12 h the solvent was removed under reduced pressure and C6H5(C2F5)P(O)H (0.63 g, 2.58 mmol, 42 %) was isolated by vacuum distillation at 1 10 3 mbar and 100 8C. 1H NMR ([D]chloroform, RT): d = 7.6 (m, 2 H; meta-CH), 7.7 (m, 1 H; para-CH), 7.7 (d/d, 1J(P,H) = 518 Hz, J = 10 Hz, 1 H; PH), 7.8 ppm (d/m, 3J(P,H) = 15 Hz, 2 H; orthoCH); 13C{1H} NMR ([D]chloroform, RT): d = 129.4 (d, 2J(P,C) = 14 Hz, ortho-C), 131.5 (d, 3J(P,C) = 13 Hz, meta-C), 135.2 (br s, para-C); 13 19 C{ F} NMR ([D]chloroform, RT): d = 111.9 (d, 1J(P,C) = 90 Hz, CF2), 118.6 ppm (d, 2J(P,C) = 15 Hz, CF3); 19F NMR ([D]chloroform, RT): d = 129.7 (d/d, 2J(F,F) = 324 Hz, 2J(P,F) = 76 Hz, 1 F; CFaFbCF3), 125.2 (d/d, 2J(F,F) = 323 Hz, 2J(P,F) = 78 Hz, 1 F; CFaFbCF3), 80.1 ppm (s, 3 F; CF3); 19F{31P} NMR ([D]chloroform, RT): d = 129.7 (d, 2J(F,F) = 324 Hz, 1 F; CFaFbCF3), 125.2 (d, 2J(F,F) = 323 Hz, 1 F; CFaFbCF3), 80.1 ppm (s, 3 F; CF3); 31P NMR ([D]chloroform, RT): d = 17.2 ppm (d/t/t, 1J(P,H) = 519 Hz, 2J(P,F) = 77 Hz, 3J(P,H) = 14 Hz, P); 31P{1H} NMR ([D]chloroform, RT): d = 17.2 ppm (t, 2J(P,F) = 77 Hz, P); 31P{19F} NMR ([D]chloroform, RT): d = 17.2 ppm (d/t, 1J(P,H) = 519 Hz, 3 J(P,H) = 14 Hz, P).
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Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
Received: February 28, 2014 Published online on && &&, 0000
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Full Paper
FULL PAPER & Phosphorus Chemistry N. Allefeld, M. Grasse, N. Ignat’ev, B. Hoge* && – &&
A tautomeric equilibrium between phosphane oxide and phosphinous acid is invoked to explain the role of secondary phosphane oxides (SPOs) as preligands in coordination chemistry. Whereas CF3(C2F5)POH exists in solution
and the gas phase solely as the acid tautomer, the less electron-withdrawing C6F5(C2F5)POH and C6F5(C2F5)P(O)H are involved in equilibria in solution as well as in the neat liquid phase (see figure).
Synthesis of Unsymmetrically Substituted Phosphane Oxides (R1R2P(O)H) and Phosphinous Acids (R1R2POH)
Keeping the balance… ..with electron-withdrawing groups! The unusual form of a phosphinous acid can be stabilized with respect to the corresponding phosphane oxide tautomer by the influence of electron-withdrawing groups. In their Full Paper on page && ff., B. Hoge et al. describe an efficient strategy for the synthesis of phosphinous acids and phosphane oxides featuring one pentafluoroethyl group with an additional trifluoromethyl, pentafluorophenyl, or phenyl substituent. The differing stabilization of the phosphinous acid form is evidenced in the tautomeric equilibrium.
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
These are not the final page numbers! ÞÞ
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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