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Cite this: Chem. Commun., 2014, 50, 11056
Iridium complexes of new NCP pincer ligands: catalytic alkane dehydrogenation and alkene isomerization†
Received 7th September 2013, Accepted 29th July 2014
Xiangqing Jia, Lei Zhang, Chuan Qin, Xuebing Leng and Zheng Huang*
DOI: 10.1039/c3cc46851h www.rsc.org/chemcomm
Iridium complexes of novel NCP pincer ligands containing pyridine and phosphinite arms have been synthesized. One Ir complex shows good catalytic activity for alkane dehydrogenation, and all complexes are highly active for olefin isomerization. A combination of the Ir complex and a (PNN)Fe pincer complex catalyzes the formation of linear alkylboronates selectively from internal olefins via sequential olefin isomerization–hydroboration.
Transition metal complexes of pincer ligands are currently a subject of great interest.1 One of the potential applications of such complexes is the pincer iridium-catalyzed alkane dehydrogenation.2 This transformation converts low-value alkanes into olefins that are important synthetic intermediates. Despite the fact that a wide range of pincer Ir complexes with various donors are now known,1d,e,3 only bis(phosphine)-ligated (PCP)Ir2b,4,5 and bis(phosphinite)-ligated (POCOP)Ir6 pincer complexes exhibit good catalytic activity for alkane dehydrogenation.7 Several (CCC)Ir complexes of dicarbene pincer ligands have also been investigated, but they are significantly less effective than the (PCP)Ir and (POCOP)Ir catalysts in catalytic alkane dehydrogenation.8 Driven by our interest in developing a new type of pincer complex for catalytic alkane functionalization, herein we report the synthesis and catalytic properties of Ir complexes supported by new NCP pincer ligands with pyridine and phosphinite arms. The dehydrogenation activity of one (NCP)Ir complex is comparable to that of the (PCP)Ir system. More notably, these complexes are exceptionally active precatalysts for olefin isomerization. A dual-catalyst system has been developed for regioselective conversion of internal olefins to linear alkylboronate esters via tandem Ir-catalyzed olefin isomerization and Fe-catalyzed olefin hydroboration. State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Road, Shanghai, 200032, China. E-mail: [email protected]; Fax: +86 5492 5533; Tel: +86 5492 5522 † Electronic supplementary information (ESI) available: Experimental procedure, NMR data, and crystallographic data for 1a(H2O), 1b, and 1c. CCDC 957716– 957718. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc46851h
11056 | Chem. Commun., 2014, 50, 11056--11059
The synthesis of the NCP ligands and (NCP)IrHCl complexes is outlined in Scheme 1. Suzuki coupling between (3-methoxyphenyl)boronic acid and 2-bromopyridines gave 2-(3-methoxyphenyl)pyridines. Deprotection of the methoxy groups with HBr, followed by the reaction with NaH and di-tert-butylchlorophosphine formed the NCP ligands 2a–c. Metalation of 2a–c with [Ir(COD)Cl]2 under a hydrogen atmosphere produced the (RNCP)IrHCl (R = H, 1a; Me, 1b; tBu, 1c) complexes. The Ir complexes were characterized by NMR spectroscopy and single crystal X-ray diffraction. The characteristic hydridic IrH resonances appear at 39.24 ppm for 1a and 33.27 ppm for 1b in the 1H NMR spectra, consistent with that observed for a pentacoordinate Ir hydridochloride species with the hydride trans to a vacant coordination site. In contrast, the IrH resonance for 1c appears at 21.89 ppm, implying that the site trans to the hydride is occupied. The solid structure of 1c reveals an agostic interaction between the tBu group on the pyridine ring and the Ir centre (Ir-H13A, 2.045 Å), and it shows that the hydride is indeed trans to Cl (Fig. 1). The structure of 1b adopts a severely distorted square pyramidal geometry with the hydride being at the apical site. Different to the structures of related Ir(III) hydridochloride pincer complexes,6,9 the Cl atom in 1b significantly deviates from the C–P–Ir–N plane, most likely due to the steric repulsion between the Me group
Scheme 1
Synthesis of the NCP ligands and (NCP)Ir complexes.
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ChemComm Table 2 TONs and octene distributions (concentrations in mM) from Ir-catalyzed transfer–dehydrogenation of n-octane and TBE
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Fig. 1 Crystal structures of complexes 1a(H2O), 1b, and 1c. The key bond distances and angles are listed in the ESI.†
on the pyridine ring and the Cl atom. An attempt to grow crystals of 1a in a toluene–pentane solution resulted in the formation of crystals of a H2O-bound six-coordinate Ir species 1a(H2O) (Fig. 1). The least sterically hindered complex 1a appears to be more subjected to water coordination than 1b and 1c.10 We initially examined the catalytic activity of 1a–c for transfer–dehydrogenation of cyclooctane (COA) with tertbutylethylene (TBE) as the hydrogen acceptor. The results are summarized in Table 1. Upon activation with NaOtBu, 1a exhibits good activity, but 1b and 1c are weakly effective. A COA solution containing 1.0 mM [Ir], 2.2 mM NaOtBu and 0.5 M TBE was heated at 150 1C. After 12 h, the process with 1a converted 93% of TBE to tert-butylethane (TBA) (466 TONs), while the processes with 1b and 1c gave only 6 TONs. It is worth noting that reactions with higher concentration of TBE resulted in lower initial turnover frequency. Dehydrogenation with 1a using 0.5, 1.0, 2.0, and 2.5 M TBE gave 168, 123, 101, and 71 TONs after 10 min at 150 1C, respectively (see the ESI† for details). The data indicate that catalysis can be inhibited by TBE at high [TBE], as found for the (PCP)Ir system.4a Next we evaluated the (NCP)Ir complexes for dehydrogenation of linear alkanes. The results are summarized in Table 2. The dehydrogenation of n-octane with 1a as the precatalyst gave 78 TONs after 1 h at 150 1C (entry 1). As observed in reactions with COA, 1b and 1c are less active than 1a in this transformation (entries 2 & 3). However, 1b and 1c showed enhanced activity in dehydrogenation of n-octane compared to the reactions with COA. For example, the dehydrogenation of n-octane with 1b gave 21 TONs after 30 min at 150 1C, while the reaction with COA gave only 3 TONs under the otherwise identical conditions. As shown in Fig. 1, the Me and tBu substituents on the pyridine rings in 1b and 1c create a crowded steric environment around the Ir center. We attribute the higher reactivity of n-octane versus COA in catalysis with 1b and 1c to the reduced steric demand of the linear alkane relative to the cyclic alkane.
Entry
[Ir]
Time (min)
TONsa
1-Octb (%)
trans-2
cis-2
1
1a
5 30 60
27 68 78
2 (7) 3 (4) 3 (4)
6 7 12
7 11 12
2
1b
5 30 60
7 21 28
0.5 (7) 1.3 (6) 1.6 (6)
3 4 5
2 6 7
3
1c
5 30 60
5 14 19
0.5 (10) 0.9 (6) 1.1 (6)
3 5 5
1 4 8
a
TONs were calculated based on conversion of TBE determined by GC. Values in parentheses are the percentage of 1-octene relative to total octenes. b
The selective formation of a-olefin via alkane dehydrogenation is desirable, but remains challenging. Goldman et al. have shown that (PCP)Ir complexes dehydrogenate n-alkane regioselectively to give a-olefin as the major kinetic product, but the a-olefin is eventually isomerized to the internal isomers.4c The kinetic selectivity for a-olefin formation with our (NCP)Ir complexes is significantly lower compared to that reported for the (PCP)Ir system.4c After 5 min, catalysis with 1a, 1b, and 1c gave a small amount of 1-octene, constituting only 7, 7, and 10% of the total octenes, respectively (Table 2). The distribution of octenes in the dehydrogenation reaction is determined by the regioselectivity for dehydrogenation and the rate of olefin isomerization. To assess the isomerization activity of the (NCP)Ir complexes, Ir-catalyzed isomerization of 1-octene was studied. After 30 min at 60 1C, catalysis with only 0.2% of 1a, 1b and 1c converted 73, 90 and 87% of 1-octene into internal octenes. For comparison, the related (PCP)Ir and (POCOP)Ir systems isomerized only 27 and 13% of 1-octene to internal octenes under the same conditions (see the ESI† for details).11 The isomerization of 1-octene over time was monitored. As shown in Fig. 2, the reaction
Table 1 TONs for transfer–dehydrogenation of COA/TBE catalyzed by 1a–c upon activation with NaOtBua
Time
1a
1b
1c
10 min 30 min 12 h
168 257 466
2 3 6
1 2 6
a
TONs were calculated based on conversion of TBE determined by GC.
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Fig. 2 Isomerization of 1-octene catalyzed by 1b/NaOtBu at 60 1C. The reaction was performed with 2.5 mM 1b, 5.5 mM NaOtBu and 1.25 M 1-octene in 1.6 mL mesitylene.
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Entry Olefin
Time (h) Product
1
cis-3-Hexene
2
2
cis-4-Octene
2
3
trans-3-Hexene
2
4
trans-3-Octene
0.5
this system is completely ineffective for hydroboration of internal olefins.14 To our delight, the combination of 1b and 3 efficiently catalyzes the formation of linear alkylboronate esters from internal olefins (Table 3). With NaBHEt3 (11%) as the catalyst activator, the reactions of cis-3-hexene and cis-4octene with HBPin using 0.5% of 1b and 5% of 3 in toluene at 23 1C formed the linear alkylboronate products 4 and 5 in 90 and 92% isolated yield, respectively; no branched boronates were detected (entries 1 and 2).15 The reactions of trans olefins, such as trans-3-hexene, trans-3-octene, and trans-4-octene, also provided the linear products in high yields (entries 3–5). The hydroboration of a mixture of cis-2-hexene and trans-2-hexene afforded 4 cleanly in 82% yield (entry 6). A cis and trans mixture of branched olefin, 5,7,7-trimethyl-2-octene, was converted to 6 selectively in 84% isolated yield (entry 7). Treatment of 1,4-hexadiene with 2 equiv. of HBPin selectively formed the terminal substituted diboronate 7 in 80% yield (entry 8). More interestingly, acetal functionalities are tolerated as shown by the isolation of 8 in 71% yield (entry 9). Lastly, the reaction with (E)-6-chloro-2-hexene afforded the linear hydroboration product 9 in 74% isolated yield (entry 10), albeit with a relatively high loading of the Fe catalyst (20%) (entry 10).16,17 Iridium complexes with new NCP pincer ligands have been prepared. The least sterically hindered complex 1a is substantially more active than 1b and 1c in dehydrogenation of both linear and cyclic alkanes. All complexes are highly active for olefin isomerization. We have developed a tandem system consisting of the (NCP)Ir catalyst for olefin isomerization and a (PNN)Fe catalyst for a-olefin hydroboration. This system provides a novel approach to produce synthetically valuable alkylboronates regioselectively from internal olefin mixtures, which would be more accessible than the pure terminal olefins by means such as alkane dehydrogenation. We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21272255, 21121062) and the Science and Technology Commission of Shanghai Municipality (No. 13QA1404200, 13JC1406900).
5
trans-4-Octene
2
Notes and references
with 1b nearly reached equilibrium in 1 h. A substantial production of 3- and 4-octenes was observed at the early stage of the reaction, suggesting that the rate of isomerization of 2-octenes to 3-octenes is comparable to the rate of isomerization of 1-octenes to 2-octenes. Given the high activity of the (NCP)Ir complexes in olefin isomerization, we envisioned that a combination of the Ir catalyst with a second catalyst that effects selective hydroboration of a-olefin would provide a possibility of converting internal olefin mixtures into linear alkylboronates that are useful synthetic intermediates.12 Such a transformation would be of value because internal olefin mixtures are economically more attractive and more easily available than the pure terminal isomers. Notably, related terminal functionalization of internal olefins via tandem isomerization and hydroformylation has been well established.13 We recently reported that a pincer precatalyst (PNN)FeCl2 (3, see Table 3) is highly active and selective for anti-Markovnikov hydroboration of a-olefins with pinacolborane (HBPin); and Table 3 Isomerization–hydroboration of internal olefins catalyzed by 1b/ 3/NaBHEt3a
6
0.5
7
4
8b
3
9
4
10c
0.5
a
The yields were isolated yields. The reaction conditions were olefin (0.8 mmol) and HBPin (0.8 mmol) with 0.5% of 1b, 5% of 3, and 11% of NaBHEt3 in 1 mL toluene at 23 1C. b Using 2 equiv. of HBPin. c With 0.5% of 1b, 20% of 3, and 40.4% of NaBHEt3.
11058 | Chem. Commun., 2014, 50, 11056--11059
1 Selected reviews of pincer metal complexes: (a) M. Albrecht and G. van Koten, Angew. Chem., Int. Ed., 2001, 40, 3750–3781; (b) M. E. van der Boom and D. Milstein, Chem. Rev., 2003, 103, 1759–1792; ´, Chem. Rev., 2010, 111, 2048–2076; (c) N. Selander and K. J. Szabo (d) The Chemistry of Pincer Compounds, ed. D. Morales-Morales and C. Jensen, Elsevier, Amsterdam, 2007; (e) W. Leis, H. A. Mayer and W. C. Kaska, Coord. Chem. Rev., 2008, 252, 1787–1797. 2 (a) J. Choi, A. H. R. MacArthur, M. Brookhart and A. S. Goldman, Chem. Rev., 2011, 111, 1761–1779; (b) C. M. Jensen, Chem. Commun., 1999, 2443–2449. 3 (a) C. I. Lee, J. Zhou and O. V. Ozerov, J. Am. Chem. Soc., 2013, 135, 3560–3566; (b) L. A. Wingard, M. C. Finniss, M. Norris, P. S. White, M. Brookhart and J. L. Templeton, Inorg. Chem., 2012, 52, 515–526; (c) Y. Segawa, M. Yamashita and K. Nozaki, J. Am. Chem. Soc., 2009, 131, 9201–9203; (d) O. V. Ozerov, C. Guo, V. A. Papkov and B. M. Foxman, J. Am. Chem. Soc., 2004, 126, 4792–4793; (e) M. D. Fryzuk, P. A. MacNeil and S. J. Rettig, J. Am. Chem. Soc., 1987, 109, 2803–2812; ( f ) K. M. Schultz, K. I. Goldberg, D. G. Gusev and D. M. Heinekey, Organometallics, 2011, 30, 1429–1437. 4 Selected examples of (PCP)Ir-catalyzed alkane dehydrogenation: (a) M. Gupta, C. Hagen, R. J. Flesher, W. C. Kaska and C. M. Jensen, Chem. Commun., 1996, 2083–2084; (b) W.-w. Xu,
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5
6 7 8
9
G. P. Rosini, K. Krogh-Jespersen, A. S. Goldman, M. Gupta, C. M. Jensen and W. C. Kaska, Chem. Commun., 1997, 2273–2274; (c) F. Liu, E. B. Pak, B. Singh, C. M. Jensen and A. S. Goldman, J. Am. Chem. Soc., 1999, 121, 4086–4087; (d) M. W. Haenel, S. Oevers, K. Angermund, W. C. Kaska, H.-J. Fan and M. B. Hall, Angew. Chem., Int. Ed., 2001, 40, 3596–3600; (e) K. Zhu, P. D. Achord, X. Zhang, K. Krogh-Jespersen and A. S. Goldman, J. Am. Chem. Soc., 2004, 126, 13044–13053; ( f ) A. S. Goldman, A. H. Roy, Z. Huang, R. Ahuja, W. Schinski and M. Brookhart, Science, 2006, 312, 257–261; ( g) R. Ahuja, B. Punji, M. Findlater, C. Supplee, W. Schinski, M. Brookhart and A. S. Goldman, Nat. Chem., 2011, 3, 167–171; (h) J. J. Adams, N. Arulsamy and D. M. Roddick, Organometallics, 2012, 31, 1439–1447. Related (PCP)Ir complexes: (a) S. A. Kuklin, A. M. Sheloumov, F. M. Dolgushin, M. G. Ezernitskaya, A. S. Peregudov, P. V. Petrovskii and A. A. Koridze, Organometallics, 2006, 25, 5466–5476; (b) Y. Shi, T. Suguri, C. Dohi, H. Yamada, S. Kojima and Y. Yamamoto, Chem. – Eur. J., 2013, 19, 10672–10689. ¨ttker-Schnetmann, P. White and M. Brookhart, J. Am. Chem. I. Go Soc., 2004, 126, 1804–1811. Very recently, a highly active (PSCOP)Ir catalyst has been reported for transfer alkane dehydrogenation. W. Yao, Y. Zhang, X. Jia and Z. Huang, Angew. Chem., Int. Ed., 2014, 53, 1390–1394. (a) W. Zuo and P. Braunstein, Organometallics, 2012, 31, 2606–2615; (b) A. R. Chianese, S. E. Shaner, J. A. Tendler, D. M. Pudalov, D. Y. Shopov, D. Kim, S. L. Rogers and A. Mo, Organometallics, 2012, 31, 7359–7367; (c) K. E. Allen, M. D. Heinekey, A. S. Goldman and K. I. Goldberg, Organometallics, 2013, 32, 1579–1582. J. C. Grimm, C. Nachtigal, H. G. Mack, W. C. Kaska and H. A. Mayer, Inorg. Chem. Commun., 2000, 3, 511–514.
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ChemComm 10 The solvent(s) used to grow crystals: toluene–pentane for 1a, toluene–pentane for 1b, and benzene for 1c. All solvents were dried by standard methods prior to use. 11 Also see: S. Biswas, Z. Huang, Y. Choliy, D. Y. Wang, M. Brookhart, K. Krogh-Jespersen and A. S. Goldman, J. Am. Chem. Soc., 2012, 134, 13276–13295. 12 (a) Boronic Acids: Preparation and Applications in Organic Synthesis and Medicine, ed. D. G. Hall, Wiley-VCH, Weinheim, 2005; (b) C. M. Crudden and D. Edwards, Eur. J. Org. Chem., 2003, 4695–4712. ¨rner, ACS Catal., 2014, 4, 13 M. Vilches-Herrera, L. Domke and A. Bo 1706–1724, and references therein. 14 L. Zhang, D. Peng, X. Leng and Z. Huang, Angew. Chem., Int. Ed., 2013, 52, 3676–3680. 15 Control experiments with 1b and NaBHEt3, but with no hydroboration catalyst 3; and 3 and NaBHEt3, but with no isomerization catalyst 1b, were conducted. In neither case was any linear hydroboration product 5 detected. In the reaction with 1b and NaBHEt3, but with no 3, a trace amount (o5%) of branched octylboronate esters were detected by GC and GC/MS. 16 It should be noted that Crudden and Srebnic have reported the selective conversion of internal olefins into linear boronates catalyzed by a Rh catalyst. However, the scope of the reactions was limited to non-functionalized monoenes. See: (a) D. R. Edwards, C. M. Crudden and K. Yam, Adv. Synth. Catal., 2005, 347, 50–54; (b) S. Pereira and M. Srebnik, J. Am. Chem. Soc., 1996, 118, 909–910. 17 During the review of this manuscript, Obligacion and Chirik reported a cobalt-catalyzed tandem olefin isomerization–hydroboration. The system can tolerate ester functionalities. See: J. V. Obligacion and P. J. Chirik, J. Am. Chem. Soc., 2013, 135, 19107–19110.
Chem. Commun., 2014, 50, 11056--11059 | 11059
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Cite this: Chem. Commun., 2014, 50, 11056
Iridium complexes of new NCP pincer ligands: catalytic alkane dehydrogenation and alkene isomerization†
Received 7th September 2013, Accepted 29th July 2014
Xiangqing Jia, Lei Zhang, Chuan Qin, Xuebing Leng and Zheng Huang*
DOI: 10.1039/c3cc46851h www.rsc.org/chemcomm
Iridium complexes of novel NCP pincer ligands containing pyridine and phosphinite arms have been synthesized. One Ir complex shows good catalytic activity for alkane dehydrogenation, and all complexes are highly active for olefin isomerization. A combination of the Ir complex and a (PNN)Fe pincer complex catalyzes the formation of linear alkylboronates selectively from internal olefins via sequential olefin isomerization–hydroboration.
Transition metal complexes of pincer ligands are currently a subject of great interest.1 One of the potential applications of such complexes is the pincer iridium-catalyzed alkane dehydrogenation.2 This transformation converts low-value alkanes into olefins that are important synthetic intermediates. Despite the fact that a wide range of pincer Ir complexes with various donors are now known,1d,e,3 only bis(phosphine)-ligated (PCP)Ir2b,4,5 and bis(phosphinite)-ligated (POCOP)Ir6 pincer complexes exhibit good catalytic activity for alkane dehydrogenation.7 Several (CCC)Ir complexes of dicarbene pincer ligands have also been investigated, but they are significantly less effective than the (PCP)Ir and (POCOP)Ir catalysts in catalytic alkane dehydrogenation.8 Driven by our interest in developing a new type of pincer complex for catalytic alkane functionalization, herein we report the synthesis and catalytic properties of Ir complexes supported by new NCP pincer ligands with pyridine and phosphinite arms. The dehydrogenation activity of one (NCP)Ir complex is comparable to that of the (PCP)Ir system. More notably, these complexes are exceptionally active precatalysts for olefin isomerization. A dual-catalyst system has been developed for regioselective conversion of internal olefins to linear alkylboronate esters via tandem Ir-catalyzed olefin isomerization and Fe-catalyzed olefin hydroboration. State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Road, Shanghai, 200032, China. E-mail: [email protected]; Fax: +86 5492 5533; Tel: +86 5492 5522 † Electronic supplementary information (ESI) available: Experimental procedure, NMR data, and crystallographic data for 1a(H2O), 1b, and 1c. CCDC 957716– 957718. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc46851h
11056 | Chem. Commun., 2014, 50, 11056--11059
The synthesis of the NCP ligands and (NCP)IrHCl complexes is outlined in Scheme 1. Suzuki coupling between (3-methoxyphenyl)boronic acid and 2-bromopyridines gave 2-(3-methoxyphenyl)pyridines. Deprotection of the methoxy groups with HBr, followed by the reaction with NaH and di-tert-butylchlorophosphine formed the NCP ligands 2a–c. Metalation of 2a–c with [Ir(COD)Cl]2 under a hydrogen atmosphere produced the (RNCP)IrHCl (R = H, 1a; Me, 1b; tBu, 1c) complexes. The Ir complexes were characterized by NMR spectroscopy and single crystal X-ray diffraction. The characteristic hydridic IrH resonances appear at 39.24 ppm for 1a and 33.27 ppm for 1b in the 1H NMR spectra, consistent with that observed for a pentacoordinate Ir hydridochloride species with the hydride trans to a vacant coordination site. In contrast, the IrH resonance for 1c appears at 21.89 ppm, implying that the site trans to the hydride is occupied. The solid structure of 1c reveals an agostic interaction between the tBu group on the pyridine ring and the Ir centre (Ir-H13A, 2.045 Å), and it shows that the hydride is indeed trans to Cl (Fig. 1). The structure of 1b adopts a severely distorted square pyramidal geometry with the hydride being at the apical site. Different to the structures of related Ir(III) hydridochloride pincer complexes,6,9 the Cl atom in 1b significantly deviates from the C–P–Ir–N plane, most likely due to the steric repulsion between the Me group
Scheme 1
Synthesis of the NCP ligands and (NCP)Ir complexes.
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ChemComm Table 2 TONs and octene distributions (concentrations in mM) from Ir-catalyzed transfer–dehydrogenation of n-octane and TBE
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Fig. 1 Crystal structures of complexes 1a(H2O), 1b, and 1c. The key bond distances and angles are listed in the ESI.†
on the pyridine ring and the Cl atom. An attempt to grow crystals of 1a in a toluene–pentane solution resulted in the formation of crystals of a H2O-bound six-coordinate Ir species 1a(H2O) (Fig. 1). The least sterically hindered complex 1a appears to be more subjected to water coordination than 1b and 1c.10 We initially examined the catalytic activity of 1a–c for transfer–dehydrogenation of cyclooctane (COA) with tertbutylethylene (TBE) as the hydrogen acceptor. The results are summarized in Table 1. Upon activation with NaOtBu, 1a exhibits good activity, but 1b and 1c are weakly effective. A COA solution containing 1.0 mM [Ir], 2.2 mM NaOtBu and 0.5 M TBE was heated at 150 1C. After 12 h, the process with 1a converted 93% of TBE to tert-butylethane (TBA) (466 TONs), while the processes with 1b and 1c gave only 6 TONs. It is worth noting that reactions with higher concentration of TBE resulted in lower initial turnover frequency. Dehydrogenation with 1a using 0.5, 1.0, 2.0, and 2.5 M TBE gave 168, 123, 101, and 71 TONs after 10 min at 150 1C, respectively (see the ESI† for details). The data indicate that catalysis can be inhibited by TBE at high [TBE], as found for the (PCP)Ir system.4a Next we evaluated the (NCP)Ir complexes for dehydrogenation of linear alkanes. The results are summarized in Table 2. The dehydrogenation of n-octane with 1a as the precatalyst gave 78 TONs after 1 h at 150 1C (entry 1). As observed in reactions with COA, 1b and 1c are less active than 1a in this transformation (entries 2 & 3). However, 1b and 1c showed enhanced activity in dehydrogenation of n-octane compared to the reactions with COA. For example, the dehydrogenation of n-octane with 1b gave 21 TONs after 30 min at 150 1C, while the reaction with COA gave only 3 TONs under the otherwise identical conditions. As shown in Fig. 1, the Me and tBu substituents on the pyridine rings in 1b and 1c create a crowded steric environment around the Ir center. We attribute the higher reactivity of n-octane versus COA in catalysis with 1b and 1c to the reduced steric demand of the linear alkane relative to the cyclic alkane.
Entry
[Ir]
Time (min)
TONsa
1-Octb (%)
trans-2
cis-2
1
1a
5 30 60
27 68 78
2 (7) 3 (4) 3 (4)
6 7 12
7 11 12
2
1b
5 30 60
7 21 28
0.5 (7) 1.3 (6) 1.6 (6)
3 4 5
2 6 7
3
1c
5 30 60
5 14 19
0.5 (10) 0.9 (6) 1.1 (6)
3 5 5
1 4 8
a
TONs were calculated based on conversion of TBE determined by GC. Values in parentheses are the percentage of 1-octene relative to total octenes. b
The selective formation of a-olefin via alkane dehydrogenation is desirable, but remains challenging. Goldman et al. have shown that (PCP)Ir complexes dehydrogenate n-alkane regioselectively to give a-olefin as the major kinetic product, but the a-olefin is eventually isomerized to the internal isomers.4c The kinetic selectivity for a-olefin formation with our (NCP)Ir complexes is significantly lower compared to that reported for the (PCP)Ir system.4c After 5 min, catalysis with 1a, 1b, and 1c gave a small amount of 1-octene, constituting only 7, 7, and 10% of the total octenes, respectively (Table 2). The distribution of octenes in the dehydrogenation reaction is determined by the regioselectivity for dehydrogenation and the rate of olefin isomerization. To assess the isomerization activity of the (NCP)Ir complexes, Ir-catalyzed isomerization of 1-octene was studied. After 30 min at 60 1C, catalysis with only 0.2% of 1a, 1b and 1c converted 73, 90 and 87% of 1-octene into internal octenes. For comparison, the related (PCP)Ir and (POCOP)Ir systems isomerized only 27 and 13% of 1-octene to internal octenes under the same conditions (see the ESI† for details).11 The isomerization of 1-octene over time was monitored. As shown in Fig. 2, the reaction
Table 1 TONs for transfer–dehydrogenation of COA/TBE catalyzed by 1a–c upon activation with NaOtBua
Time
1a
1b
1c
10 min 30 min 12 h
168 257 466
2 3 6
1 2 6
a
TONs were calculated based on conversion of TBE determined by GC.
This journal is © The Royal Society of Chemistry 2014
Fig. 2 Isomerization of 1-octene catalyzed by 1b/NaOtBu at 60 1C. The reaction was performed with 2.5 mM 1b, 5.5 mM NaOtBu and 1.25 M 1-octene in 1.6 mL mesitylene.
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Entry Olefin
Time (h) Product
1
cis-3-Hexene
2
2
cis-4-Octene
2
3
trans-3-Hexene
2
4
trans-3-Octene
0.5
this system is completely ineffective for hydroboration of internal olefins.14 To our delight, the combination of 1b and 3 efficiently catalyzes the formation of linear alkylboronate esters from internal olefins (Table 3). With NaBHEt3 (11%) as the catalyst activator, the reactions of cis-3-hexene and cis-4octene with HBPin using 0.5% of 1b and 5% of 3 in toluene at 23 1C formed the linear alkylboronate products 4 and 5 in 90 and 92% isolated yield, respectively; no branched boronates were detected (entries 1 and 2).15 The reactions of trans olefins, such as trans-3-hexene, trans-3-octene, and trans-4-octene, also provided the linear products in high yields (entries 3–5). The hydroboration of a mixture of cis-2-hexene and trans-2-hexene afforded 4 cleanly in 82% yield (entry 6). A cis and trans mixture of branched olefin, 5,7,7-trimethyl-2-octene, was converted to 6 selectively in 84% isolated yield (entry 7). Treatment of 1,4-hexadiene with 2 equiv. of HBPin selectively formed the terminal substituted diboronate 7 in 80% yield (entry 8). More interestingly, acetal functionalities are tolerated as shown by the isolation of 8 in 71% yield (entry 9). Lastly, the reaction with (E)-6-chloro-2-hexene afforded the linear hydroboration product 9 in 74% isolated yield (entry 10), albeit with a relatively high loading of the Fe catalyst (20%) (entry 10).16,17 Iridium complexes with new NCP pincer ligands have been prepared. The least sterically hindered complex 1a is substantially more active than 1b and 1c in dehydrogenation of both linear and cyclic alkanes. All complexes are highly active for olefin isomerization. We have developed a tandem system consisting of the (NCP)Ir catalyst for olefin isomerization and a (PNN)Fe catalyst for a-olefin hydroboration. This system provides a novel approach to produce synthetically valuable alkylboronates regioselectively from internal olefin mixtures, which would be more accessible than the pure terminal olefins by means such as alkane dehydrogenation. We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21272255, 21121062) and the Science and Technology Commission of Shanghai Municipality (No. 13QA1404200, 13JC1406900).
5
trans-4-Octene
2
Notes and references
with 1b nearly reached equilibrium in 1 h. A substantial production of 3- and 4-octenes was observed at the early stage of the reaction, suggesting that the rate of isomerization of 2-octenes to 3-octenes is comparable to the rate of isomerization of 1-octenes to 2-octenes. Given the high activity of the (NCP)Ir complexes in olefin isomerization, we envisioned that a combination of the Ir catalyst with a second catalyst that effects selective hydroboration of a-olefin would provide a possibility of converting internal olefin mixtures into linear alkylboronates that are useful synthetic intermediates.12 Such a transformation would be of value because internal olefin mixtures are economically more attractive and more easily available than the pure terminal isomers. Notably, related terminal functionalization of internal olefins via tandem isomerization and hydroformylation has been well established.13 We recently reported that a pincer precatalyst (PNN)FeCl2 (3, see Table 3) is highly active and selective for anti-Markovnikov hydroboration of a-olefins with pinacolborane (HBPin); and Table 3 Isomerization–hydroboration of internal olefins catalyzed by 1b/ 3/NaBHEt3a
6
0.5
7
4
8b
3
9
4
10c
0.5
a
The yields were isolated yields. The reaction conditions were olefin (0.8 mmol) and HBPin (0.8 mmol) with 0.5% of 1b, 5% of 3, and 11% of NaBHEt3 in 1 mL toluene at 23 1C. b Using 2 equiv. of HBPin. c With 0.5% of 1b, 20% of 3, and 40.4% of NaBHEt3.
11058 | Chem. Commun., 2014, 50, 11056--11059
1 Selected reviews of pincer metal complexes: (a) M. Albrecht and G. van Koten, Angew. Chem., Int. Ed., 2001, 40, 3750–3781; (b) M. E. van der Boom and D. Milstein, Chem. Rev., 2003, 103, 1759–1792; ´, Chem. Rev., 2010, 111, 2048–2076; (c) N. Selander and K. J. Szabo (d) The Chemistry of Pincer Compounds, ed. D. Morales-Morales and C. Jensen, Elsevier, Amsterdam, 2007; (e) W. Leis, H. A. Mayer and W. C. Kaska, Coord. Chem. Rev., 2008, 252, 1787–1797. 2 (a) J. Choi, A. H. R. MacArthur, M. Brookhart and A. S. Goldman, Chem. Rev., 2011, 111, 1761–1779; (b) C. M. Jensen, Chem. Commun., 1999, 2443–2449. 3 (a) C. I. Lee, J. Zhou and O. V. Ozerov, J. Am. Chem. Soc., 2013, 135, 3560–3566; (b) L. A. Wingard, M. C. Finniss, M. Norris, P. S. White, M. Brookhart and J. L. Templeton, Inorg. Chem., 2012, 52, 515–526; (c) Y. Segawa, M. Yamashita and K. Nozaki, J. Am. Chem. Soc., 2009, 131, 9201–9203; (d) O. V. Ozerov, C. Guo, V. A. Papkov and B. M. Foxman, J. Am. Chem. Soc., 2004, 126, 4792–4793; (e) M. D. Fryzuk, P. A. MacNeil and S. J. Rettig, J. Am. Chem. Soc., 1987, 109, 2803–2812; ( f ) K. M. Schultz, K. I. Goldberg, D. G. Gusev and D. M. Heinekey, Organometallics, 2011, 30, 1429–1437. 4 Selected examples of (PCP)Ir-catalyzed alkane dehydrogenation: (a) M. Gupta, C. Hagen, R. J. Flesher, W. C. Kaska and C. M. Jensen, Chem. Commun., 1996, 2083–2084; (b) W.-w. Xu,
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Communication
5
6 7 8
9
G. P. Rosini, K. Krogh-Jespersen, A. S. Goldman, M. Gupta, C. M. Jensen and W. C. Kaska, Chem. Commun., 1997, 2273–2274; (c) F. Liu, E. B. Pak, B. Singh, C. M. Jensen and A. S. Goldman, J. Am. Chem. Soc., 1999, 121, 4086–4087; (d) M. W. Haenel, S. Oevers, K. Angermund, W. C. Kaska, H.-J. Fan and M. B. Hall, Angew. Chem., Int. Ed., 2001, 40, 3596–3600; (e) K. Zhu, P. D. Achord, X. Zhang, K. Krogh-Jespersen and A. S. Goldman, J. Am. Chem. Soc., 2004, 126, 13044–13053; ( f ) A. S. Goldman, A. H. Roy, Z. Huang, R. Ahuja, W. Schinski and M. Brookhart, Science, 2006, 312, 257–261; ( g) R. Ahuja, B. Punji, M. Findlater, C. Supplee, W. Schinski, M. Brookhart and A. S. Goldman, Nat. Chem., 2011, 3, 167–171; (h) J. J. Adams, N. Arulsamy and D. M. Roddick, Organometallics, 2012, 31, 1439–1447. Related (PCP)Ir complexes: (a) S. A. Kuklin, A. M. Sheloumov, F. M. Dolgushin, M. G. Ezernitskaya, A. S. Peregudov, P. V. Petrovskii and A. A. Koridze, Organometallics, 2006, 25, 5466–5476; (b) Y. Shi, T. Suguri, C. Dohi, H. Yamada, S. Kojima and Y. Yamamoto, Chem. – Eur. J., 2013, 19, 10672–10689. ¨ttker-Schnetmann, P. White and M. Brookhart, J. Am. Chem. I. Go Soc., 2004, 126, 1804–1811. Very recently, a highly active (PSCOP)Ir catalyst has been reported for transfer alkane dehydrogenation. W. Yao, Y. Zhang, X. Jia and Z. Huang, Angew. Chem., Int. Ed., 2014, 53, 1390–1394. (a) W. Zuo and P. Braunstein, Organometallics, 2012, 31, 2606–2615; (b) A. R. Chianese, S. E. Shaner, J. A. Tendler, D. M. Pudalov, D. Y. Shopov, D. Kim, S. L. Rogers and A. Mo, Organometallics, 2012, 31, 7359–7367; (c) K. E. Allen, M. D. Heinekey, A. S. Goldman and K. I. Goldberg, Organometallics, 2013, 32, 1579–1582. J. C. Grimm, C. Nachtigal, H. G. Mack, W. C. Kaska and H. A. Mayer, Inorg. Chem. Commun., 2000, 3, 511–514.
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ChemComm 10 The solvent(s) used to grow crystals: toluene–pentane for 1a, toluene–pentane for 1b, and benzene for 1c. All solvents were dried by standard methods prior to use. 11 Also see: S. Biswas, Z. Huang, Y. Choliy, D. Y. Wang, M. Brookhart, K. Krogh-Jespersen and A. S. Goldman, J. Am. Chem. Soc., 2012, 134, 13276–13295. 12 (a) Boronic Acids: Preparation and Applications in Organic Synthesis and Medicine, ed. D. G. Hall, Wiley-VCH, Weinheim, 2005; (b) C. M. Crudden and D. Edwards, Eur. J. Org. Chem., 2003, 4695–4712. ¨rner, ACS Catal., 2014, 4, 13 M. Vilches-Herrera, L. Domke and A. Bo 1706–1724, and references therein. 14 L. Zhang, D. Peng, X. Leng and Z. Huang, Angew. Chem., Int. Ed., 2013, 52, 3676–3680. 15 Control experiments with 1b and NaBHEt3, but with no hydroboration catalyst 3; and 3 and NaBHEt3, but with no isomerization catalyst 1b, were conducted. In neither case was any linear hydroboration product 5 detected. In the reaction with 1b and NaBHEt3, but with no 3, a trace amount (o5%) of branched octylboronate esters were detected by GC and GC/MS. 16 It should be noted that Crudden and Srebnic have reported the selective conversion of internal olefins into linear boronates catalyzed by a Rh catalyst. However, the scope of the reactions was limited to non-functionalized monoenes. See: (a) D. R. Edwards, C. M. Crudden and K. Yam, Adv. Synth. Catal., 2005, 347, 50–54; (b) S. Pereira and M. Srebnik, J. Am. Chem. Soc., 1996, 118, 909–910. 17 During the review of this manuscript, Obligacion and Chirik reported a cobalt-catalyzed tandem olefin isomerization–hydroboration. The system can tolerate ester functionalities. See: J. V. Obligacion and P. J. Chirik, J. Am. Chem. Soc., 2013, 135, 19107–19110.
Chem. Commun., 2014, 50, 11056--11059 | 11059
