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Metal-free ring-opening of epoxides with potassium trifluoroborates† Silvia Roscales and Aurelio G. Csa´ky¨*
Received 26th September 2013, Accepted 1st November 2013 DOI: 10.1039/c3cc47360k www.rsc.org/chemcomm
The ring-opening of epoxides with potassium trifluoroborates proceeds smoothly in the presence of trifluoroacetic anhydride under metal-free conditions. The reactions are regioselective and afford a single diastereomer. Both electron-rich and electron-poor aryltrifluoroborates are tolerated.
Epoxides are one of the essential functional groups in organic chemistry, readily accessible from alkenes and from carbonyl compounds. The strained ring system of this class of ethers confers a distinctive type of reactivity which makes them highly versatile as synthetic intermediates.1 Regioselective and stereodefined ring-opening reactions performed using nucleophiles are some of the most characteristic transformations, useful in the synthesis of a wide variety of alcohols. With regard to the use of carbon nucleophiles, reported examples have been limited to the use of stabilized carbanions, strong organometallic reagents, and p-rich aromatics.2 Boronic acids3 and potassium trifluoroborates4 are important reagents for the synthesis of complex molecules due to their low toxicity, thermal stability, and wide compatibility with functional groups. Many of these compounds are commercially available, and an ample variety of synthetic methods have been developed for their preparation. Due to the relatively low nucleophilicity of these aryl- and alkenylboron compounds, most of their reactions are catalyzed by transition metal complexes. Comparatively, synthetic transformations under metal-free conditions remain scarce and have been limited to Mannich-type reactions,5 conjugate additions,6 and reactions with oxonium cations.7 The finding of reaction conditions which could enable the ring-opening of epoxides with these boron-based carbon nucleophiles under metal-free conditions, an unprecedented reaction,8 would be a highly attractive addendum to the synthetic arsenal. Instituto Pluridisciplinar, Universidad Complutense, Campus de Excelencia Internacional Moncloa, 28040-Madrid, Spain. E-mail: [email protected]; Tel: +34 913943280 † Electronic supplementary information (ESI) available: Synthesis of starting materials, experimental details and characterization data. See DOI: 10.1039/ c3cc47360k
454 | Chem. Commun., 2014, 50, 454--456
Herein, we report our initial studies towards the ring-opening of epoxides with potassium aryl- and alkenyltrifluoroborates under metal-free conditions to afford the corresponding 2-arylethanols or homoallylic alcohols, respectively. Under optimum conditions, these reactions are promoted by trifluoroacetic anhydride (TFAA), and they are completely regioselective (eqn (1)), giving rise to the formation of a single diastereomer with retention of the configuration at the reacting centre of the epoxide. (1)
3-Arylglycidates were chosen as initial substrates because of their easy availability9,10 and proven utility as electrophiles in ring-opening reactions with electron-rich arenes.11,12 On the basis of recent studies on conjugate addition reactions of alkenylboronic acids catalyzed by acylating reagents,13 we first examined the reaction of trans ethyl 3-phenylglycidate (1a) and (E)-styrenylboronic acid (2) in the presence of AcCl or TFAA (eqn (2)). However, no reaction was observed.
(2)
In light of these results, we switched to the more nucleophilic potassium (E)-styrenyltrifluoroborate (3a).14 We were pleased to find that in TFAA-promotion, the ring-opening reaction took place regioselectively by the addition of the styrenyl moiety at the b-position of the glycidic epoxide in a completely stereocontrolled
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Table 1 Ring-opening of trans and cis ethyl 3-phenylglycidates with various trifluoroboratesa
Entry 1 1
1a
R-BF3K (2)
Yieldb (%)
Product (3)
82
(3b) (4b-I)
2
3
4
1a
1a
71
(3c)
(4c-I) 78
(3d)
(4d-I) 76
1a Ph-BF3K (3e) (4e)
5
6
1a
Table 2
Ring-opening of various epoxides with representative trifluoroboratesa
79
(3f)
1a
phenyltrifluoroborate (Table 1, entry 4), an electron-rich aryltrifluoroborate (Table 1, entry 5) and aryltrifluoroborates endowed with electron-withdrawing substituents (Table 1, entries 6 and 7) performed well in the ring-opening reaction. This is in sharp contrast to the Friedel–Crafts type ring opening of epoxides, a process which gives rise to similar products, but is limited only to electron-rich aromatics.11,12 Only one diastereomer (4-I) was obtained in all cases (Table 1, entries 1–7). On the other hand, the reactions of cis ethyl 3-phenylglycidate (1b) (Table 1, entries 8–10) afforded exclusively diastereomers 4-II. Again, this behavior is different to the Friedel–Crafts type ring-opening of epoxides, which is well known to give rise to mixtures of diastereomers.11,12 Secondly, we extended the results to other epoxides and representative trifluoroborates (Table 2). We observed that other 3-arylglycidates with either electron-releasing or electron-withdrawing substituents on the aryl moiety (Table 2, entries 1–4) were also good substrates. Additionally, the reaction allowed the construction of
(4f-I) 81
(3g)
Entry
(4g-I)
Epoxide (5)
3 3f
1 7
8
1a
82
(3h)
1b
6 (% yield)b
(5a)
(4h-I)
(6a, 70) 80
(3a) (4a-II)
3a
2 9
1b
(5a)
83
(3b) (4b-II)
10
1b
(3c)
(6b, 74) 75
(4c-II)
3a
3 (5b)
a Reaction conditions: epoxide (1.0 equiv.), R-BF3K (1.25 equiv.), TFAA (0.5 equiv.), CH2Cl2 (6 mL mmol 1 epoxide), rt, 3 h. b Isolated yield obtained after column chromatography.
fashion to give the (2RS,3SR) product 4a-I exclusively. After some optimization of the reaction parameters (eqn (2)), we found that the use of a slight excess of the boron nucleophile (1.25 equiv.) together with a substoichiometric amount of TFAA (0.5 equiv.) in CH2Cl2 solution at rt were optimal conditions for this transformation.15 On the other hand, no addition was observed with 3 when Ac2O was used as a promoter, and in the presence of BF3OEt2 at 0 1C, the ring-opening reaction took place only with very modest yield. In order to investigate the scope of this transformation, first we examined the ring-opening of trans and cis ethyl 3-phenylglycidates (1a and 1b) with different potassium trifluoroborates (Table 1). We observed that this reaction could be extended to other alkenyltrifluoroborates with preservation of the CQC bond geometry of the nucleophile (Table 1, entries 1 and 2). The reaction also included substitution at the a-carbon of the alkenyltrifluoroborate (Table 1, entry 3). Furthermore, the process was not restricted to alkenyltrifluoroborates, as potassium
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(6c, 78)
4
3a (5c)
5
(5d)c
6
7 8 9
(5e)
(5f) (5g) (5h)
(6d, 80) 3a (6e, 78)d 3a (6f, 65) 3a
—
3a
—
3a
(6g, 52)
a
Reaction conditions: epoxide (1.0 equiv.), R-BF3K (1.25 equiv.), TFAA (0.5 equiv.), CH2Cl2 (6 mL mmol 1 epoxide), rt, 3 h. b Isolated yield obtained after column chromatography. c 1 : 1 mixture of cis and trans epoxides. d 1 : 1 mixture of syn and anti diastereomers.
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tertiary stereocentres (Table 2, entry 5). A similar result to that for esters was obtained with an a,b-epoxyketone (Table 2, entry 6). However, no reaction was observed for a b-unsubstituted-a,bepoxyester (Table 2, entry 7). Apart from a,b-epoxycarbonyl compounds, we tested other types of synthetically relevant epoxides. Thus, we observed that the reaction did not work for styrene oxide (Table 2, entry 8).16 Finally, the reaction with the phenyl ether of glycidol17 afforded the ring-opening reaction products which resulted from the attack to the less hindered position (Table 2, entry 9). The stereochemistry of compounds 4 was determined by transformation of 4b-I and 4b-II respectively, into the cyclic derivatives18 7-I and 7-II (eqn (3)), and by comparison of the NMR data of 6a with those previously reported for the anti diastereomer.12 Assignments for the other products are based on analogy.
(3)
To account for these results, we suggest the transient formation of an organodifluoroborane19,20 by the reaction of the starting potassium trifluoroborate with TFAA.21 As exemplified for a transepoxide (eqn (4)), coordination of the highly electrophilic boron of this newly generated species to the oxygen of the epoxide would enable the operation of a borderline SNi type mechanism, favoring the transfer of the carbon backbone of the starting trifluoroborate to the most electrophilic position of the epoxide with retention of configuration.22,23 This proposal encompasses both the regio- and stereochemical experimental observations.24
(4)
In summary, we have developed the ring-opening of epoxides with potassium aryl- and alkenyltrifluoroborates in the presence of trifluoroacetic anhydride (TFAA) under metal-free conditions, following a very simple experimental procedure. The reaction is highly regioselective, thus allowing for the synthesis of either syn or anti 2-arylethanols or homoallylic alcohols starting from trans or cis epoxides. In the case of alkenyltrifluoroborates, the E- or Z-stereochemistry of the CQC bond is preserved. In the case of aryltrifluoroborates, the use of either electron-poor or electronrich compounds is well tolerated. Further studies toward enhancing the scope of the reaction to other epoxides will be reported in due course. We thank grant CTQ-2010-16170 from the Spanish government (MICINN). S. Roscales is thankful to the government of Spain and for FPU predoctoral grant No. AP20090051.
456 | Chem. Commun., 2014, 50, 454--456
Notes and references 1 For reviews, see: (a) Aziridines and Epoxides in Organic Synthesis, ed. A. Yudin, Wiley-VCH, Weinheim, 2006; (b) J. B. Johson, Sci. Synth., 2011, 3, 759. 2 For reviews, see: (a) J. G. Smith, Synthesis, 1984, 629; (b) M. Pineschi, Eur. J. Org. Chem., 2006, 4979. 3 Boronic Acids: Preparation and Applications in Organic Synthesis, Medicine and Materials, ed. D. G. Hall, Wiley-VCH, Weinheim, 2011. 4 (a) H. A. Stefani, R. Calla and A. S. Vieira, Tetrahedron, 2007, 63, 3623; (b) S. Darses and J.-P. Genet, Chem. Rev., 2008, 108, 288; (c) G. A. Molander ´rard, Boronic Acids: Preparation and Applications in Organic and L. Jean-Ge Synthesis, Medicine and Materials, ed. D. G. Hall, Wiley-VCH, Weinheim, 2011, vol. 2, p. 507. 5 Review: N. R. Candeias, F. Montalbano, P. M. S. D. Cal and P. M. P. Gois, Chem. Rev., 2010, 110, 6169. 6 (a) S. Hara, S. Hyuga, M. Aoyama, M. Sato and A. Suzuki, Tetrahedron Lett., 1990, 31, 247; (b) S. Hara, H. Shudoh, S. Ishimura and A. Suzuki, Bull. Chem. Soc. Jpn., 1998, 71, 2403; (c) R. S. Paton, J. M. Goodman and S. C. Pellegrinet, J. Org. Chem., 2008, 73, 5078; (d) S.-G. Kim, Tetrahedron Lett., 2008, 49, 6148; (e) S. Lee and D. W. C. MacMillan, J. Am. Chem. Soc., 2007, 129, 15438; ( f ) T. Inokuma, K. Takasu, T. Sakaeda and Y. Takemoto, Org. Lett., 2009, 11, 2425; (g) M. Sugiura, M. Tokudomi and M. Nakajima, Chem. Commun., 2010, 46, 7799; (h) B. J. Lundy, S. Jansone-Popova and J. A. May, Org. Lett., 2011, 13, 4958; (i) H. M. Turner, J. Patel, N. Niljianskul and M. Chong, Org. Lett., 2011, 13, 5796; ( j) Y. Luan and S. E. Schaus, J. Am. Chem. Soc., 2012, 124, 19965. ¨dtke and 7 (a) A. S. Vieira, P. F. Fiorante, T. L. S. Hough, F. P. Ferreira, D. S. Lu H. A. Stefani, Org. Lett., 2008, 10, 5215; (b) T. A. Mitchell and J. W. Bode, J. Am. Chem. Soc., 2009, 131, 18057; (c) P. N. Moquist, T. Kodama and S. E. Schaus, Angew. Chem., Int. Ed., 2010, 49, 7096; (d) C.-V. Vo, A. Mitchell and J. W. Bode, J. Am. Chem. Soc., 2011, 133, 14082; (e) J. Zeng, S. Vedachalam, S. Xiang and X.-W. Liu, Org. Lett., 2011, 13, 42; ( f ) T. Kodama, P. N. Moquist and S. E. Schaus, Org. Lett., 2011, 13, 6316. 8 For other reactions of trifluoroborates with epoxides, see: (a) C. Che and Z. Zhang, Synth. Commun., 2004, 34, 4499; (b) M. Lautens, S. G. Ouellet and S. Raeppel, Angew. Chem., Int. Ed., 2007, 39, 4089; (c) L. Wang, M. L. Maddess and M. Lautens, J. Org. Chem., 2007, 72, 1822; (d) D. K. Nielsen and A. G. Doyle, Angew. Chem., Int. Ed., 2011, 50, 6056. 9 T. Rosen, Comprehensive Organic Synthesis, ed. B. M. Trost, I. Fleming and C. H. Heathcock, Pergamon, Oxford, 1991, vol. 2, p. 409. 10 All compounds used were racemic. 11 (a) J. H. van der Westhuizen, D. Ferreira and D. G. Roux, J. Chem. Soc., Perkin Trans. 1, 1980, 2856; (b) R. F. C. Brown, W. R. Jackson, T. D. McCarthy and G. D. Fallon, Aust. J. Chem., 1992, 45, 1833; (c) F. Bertolini, P. Crotti, V. Di Bussolo, F. Macchia and M. Pineschi, J. Org. Chem., 2007, 72, 7761. 12 D. Wilcke and T. Bach, Org. Biomol. Chem., 2012, 10, 6498. ´n, E. Buxaderas and A. G. Csa ´ky¨, Tetrahedron Lett., 13 (a) S. Roscales, A. Rinco ´ky¨, Org. Lett., 2012, 14, 1187. 2012, 53, 4721; (b) S. Roscales and A. G. Csa 14 G. Berionni, V. Morozova, M. Heiniger, P. Mayer, P. Knochel and H. Mayr, J. Am. Chem. Soc., 2013, 135, 6317. 15 Variable ratios of mono- and bis-trifluoroacylated 1,2-diols were obtained as subproducts. 16 A mixture of mono- and bis-trifluoroacylated 1-phenyl-1,2-ethanediols were obtained as the only reaction products. `s, J. Am. 17 (a) R. Marcos, C. Rodrı´guez-Escrich, C. I. Herrerı´as and M. Perica Chem. Soc., 2008, 130, 16838. For a review, see: (b) R. M. Hanson, Chem. Rev., 1991, 91, 437. 18 S. D. Burke, R. A. Ng, J. A. Morrison and M. J. Alberti, J. Org. Chem., 1998, 63, 3160. 19 For the generation of organodifluoroboranes using a variety of Lewis acids, see: R. A. Batey, A. N. Thadani, D. V. Smil and A. J. Lough, Synthesis, 2000, 990. 20 Styrene was detected as a subproduct in the reactions using 3a. This may arise from protodeborylation of the RBF2 intermediate upon TFA formation in work-up. 21 To the best of our knowledge, this is the first example of TFAA being utilized as a fluorophile. 22 R. E. Parker and N. C. Isaacs, Chem. Rev., 1959, 59, 737. 23 For the ring-opening of epoxides by thiols with retention of configuration at the reaction center, see: A. Schwartz, P. B. Madan, E. Mohacsi, J. P. O’Brien, L. J. Todaro and D. L. Coffen, J. Org. Chem., 1992, 57, 851. 24 Independent essays carried out with 3a and the mono- or bistrifluoromethylated derivatives of 2,3-dihydroxy-3-phenyl-propionic acid ethyl ester in the presence or absence of TFAA put forward that these were not intermediates in the formation of 4a-I.
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Metal-free ring-opening of epoxides with potassium trifluoroborates† Silvia Roscales and Aurelio G. Csa´ky¨*
Received 26th September 2013, Accepted 1st November 2013 DOI: 10.1039/c3cc47360k www.rsc.org/chemcomm
The ring-opening of epoxides with potassium trifluoroborates proceeds smoothly in the presence of trifluoroacetic anhydride under metal-free conditions. The reactions are regioselective and afford a single diastereomer. Both electron-rich and electron-poor aryltrifluoroborates are tolerated.
Epoxides are one of the essential functional groups in organic chemistry, readily accessible from alkenes and from carbonyl compounds. The strained ring system of this class of ethers confers a distinctive type of reactivity which makes them highly versatile as synthetic intermediates.1 Regioselective and stereodefined ring-opening reactions performed using nucleophiles are some of the most characteristic transformations, useful in the synthesis of a wide variety of alcohols. With regard to the use of carbon nucleophiles, reported examples have been limited to the use of stabilized carbanions, strong organometallic reagents, and p-rich aromatics.2 Boronic acids3 and potassium trifluoroborates4 are important reagents for the synthesis of complex molecules due to their low toxicity, thermal stability, and wide compatibility with functional groups. Many of these compounds are commercially available, and an ample variety of synthetic methods have been developed for their preparation. Due to the relatively low nucleophilicity of these aryl- and alkenylboron compounds, most of their reactions are catalyzed by transition metal complexes. Comparatively, synthetic transformations under metal-free conditions remain scarce and have been limited to Mannich-type reactions,5 conjugate additions,6 and reactions with oxonium cations.7 The finding of reaction conditions which could enable the ring-opening of epoxides with these boron-based carbon nucleophiles under metal-free conditions, an unprecedented reaction,8 would be a highly attractive addendum to the synthetic arsenal. Instituto Pluridisciplinar, Universidad Complutense, Campus de Excelencia Internacional Moncloa, 28040-Madrid, Spain. E-mail: [email protected]; Tel: +34 913943280 † Electronic supplementary information (ESI) available: Synthesis of starting materials, experimental details and characterization data. See DOI: 10.1039/ c3cc47360k
454 | Chem. Commun., 2014, 50, 454--456
Herein, we report our initial studies towards the ring-opening of epoxides with potassium aryl- and alkenyltrifluoroborates under metal-free conditions to afford the corresponding 2-arylethanols or homoallylic alcohols, respectively. Under optimum conditions, these reactions are promoted by trifluoroacetic anhydride (TFAA), and they are completely regioselective (eqn (1)), giving rise to the formation of a single diastereomer with retention of the configuration at the reacting centre of the epoxide. (1)
3-Arylglycidates were chosen as initial substrates because of their easy availability9,10 and proven utility as electrophiles in ring-opening reactions with electron-rich arenes.11,12 On the basis of recent studies on conjugate addition reactions of alkenylboronic acids catalyzed by acylating reagents,13 we first examined the reaction of trans ethyl 3-phenylglycidate (1a) and (E)-styrenylboronic acid (2) in the presence of AcCl or TFAA (eqn (2)). However, no reaction was observed.
(2)
In light of these results, we switched to the more nucleophilic potassium (E)-styrenyltrifluoroborate (3a).14 We were pleased to find that in TFAA-promotion, the ring-opening reaction took place regioselectively by the addition of the styrenyl moiety at the b-position of the glycidic epoxide in a completely stereocontrolled
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Table 1 Ring-opening of trans and cis ethyl 3-phenylglycidates with various trifluoroboratesa
Entry 1 1
1a
R-BF3K (2)
Yieldb (%)
Product (3)
82
(3b) (4b-I)
2
3
4
1a
1a
71
(3c)
(4c-I) 78
(3d)
(4d-I) 76
1a Ph-BF3K (3e) (4e)
5
6
1a
Table 2
Ring-opening of various epoxides with representative trifluoroboratesa
79
(3f)
1a
phenyltrifluoroborate (Table 1, entry 4), an electron-rich aryltrifluoroborate (Table 1, entry 5) and aryltrifluoroborates endowed with electron-withdrawing substituents (Table 1, entries 6 and 7) performed well in the ring-opening reaction. This is in sharp contrast to the Friedel–Crafts type ring opening of epoxides, a process which gives rise to similar products, but is limited only to electron-rich aromatics.11,12 Only one diastereomer (4-I) was obtained in all cases (Table 1, entries 1–7). On the other hand, the reactions of cis ethyl 3-phenylglycidate (1b) (Table 1, entries 8–10) afforded exclusively diastereomers 4-II. Again, this behavior is different to the Friedel–Crafts type ring-opening of epoxides, which is well known to give rise to mixtures of diastereomers.11,12 Secondly, we extended the results to other epoxides and representative trifluoroborates (Table 2). We observed that other 3-arylglycidates with either electron-releasing or electron-withdrawing substituents on the aryl moiety (Table 2, entries 1–4) were also good substrates. Additionally, the reaction allowed the construction of
(4f-I) 81
(3g)
Entry
(4g-I)
Epoxide (5)
3 3f
1 7
8
1a
82
(3h)
1b
6 (% yield)b
(5a)
(4h-I)
(6a, 70) 80
(3a) (4a-II)
3a
2 9
1b
(5a)
83
(3b) (4b-II)
10
1b
(3c)
(6b, 74) 75
(4c-II)
3a
3 (5b)
a Reaction conditions: epoxide (1.0 equiv.), R-BF3K (1.25 equiv.), TFAA (0.5 equiv.), CH2Cl2 (6 mL mmol 1 epoxide), rt, 3 h. b Isolated yield obtained after column chromatography.
fashion to give the (2RS,3SR) product 4a-I exclusively. After some optimization of the reaction parameters (eqn (2)), we found that the use of a slight excess of the boron nucleophile (1.25 equiv.) together with a substoichiometric amount of TFAA (0.5 equiv.) in CH2Cl2 solution at rt were optimal conditions for this transformation.15 On the other hand, no addition was observed with 3 when Ac2O was used as a promoter, and in the presence of BF3OEt2 at 0 1C, the ring-opening reaction took place only with very modest yield. In order to investigate the scope of this transformation, first we examined the ring-opening of trans and cis ethyl 3-phenylglycidates (1a and 1b) with different potassium trifluoroborates (Table 1). We observed that this reaction could be extended to other alkenyltrifluoroborates with preservation of the CQC bond geometry of the nucleophile (Table 1, entries 1 and 2). The reaction also included substitution at the a-carbon of the alkenyltrifluoroborate (Table 1, entry 3). Furthermore, the process was not restricted to alkenyltrifluoroborates, as potassium
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(6c, 78)
4
3a (5c)
5
(5d)c
6
7 8 9
(5e)
(5f) (5g) (5h)
(6d, 80) 3a (6e, 78)d 3a (6f, 65) 3a
—
3a
—
3a
(6g, 52)
a
Reaction conditions: epoxide (1.0 equiv.), R-BF3K (1.25 equiv.), TFAA (0.5 equiv.), CH2Cl2 (6 mL mmol 1 epoxide), rt, 3 h. b Isolated yield obtained after column chromatography. c 1 : 1 mixture of cis and trans epoxides. d 1 : 1 mixture of syn and anti diastereomers.
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tertiary stereocentres (Table 2, entry 5). A similar result to that for esters was obtained with an a,b-epoxyketone (Table 2, entry 6). However, no reaction was observed for a b-unsubstituted-a,bepoxyester (Table 2, entry 7). Apart from a,b-epoxycarbonyl compounds, we tested other types of synthetically relevant epoxides. Thus, we observed that the reaction did not work for styrene oxide (Table 2, entry 8).16 Finally, the reaction with the phenyl ether of glycidol17 afforded the ring-opening reaction products which resulted from the attack to the less hindered position (Table 2, entry 9). The stereochemistry of compounds 4 was determined by transformation of 4b-I and 4b-II respectively, into the cyclic derivatives18 7-I and 7-II (eqn (3)), and by comparison of the NMR data of 6a with those previously reported for the anti diastereomer.12 Assignments for the other products are based on analogy.
(3)
To account for these results, we suggest the transient formation of an organodifluoroborane19,20 by the reaction of the starting potassium trifluoroborate with TFAA.21 As exemplified for a transepoxide (eqn (4)), coordination of the highly electrophilic boron of this newly generated species to the oxygen of the epoxide would enable the operation of a borderline SNi type mechanism, favoring the transfer of the carbon backbone of the starting trifluoroborate to the most electrophilic position of the epoxide with retention of configuration.22,23 This proposal encompasses both the regio- and stereochemical experimental observations.24
(4)
In summary, we have developed the ring-opening of epoxides with potassium aryl- and alkenyltrifluoroborates in the presence of trifluoroacetic anhydride (TFAA) under metal-free conditions, following a very simple experimental procedure. The reaction is highly regioselective, thus allowing for the synthesis of either syn or anti 2-arylethanols or homoallylic alcohols starting from trans or cis epoxides. In the case of alkenyltrifluoroborates, the E- or Z-stereochemistry of the CQC bond is preserved. In the case of aryltrifluoroborates, the use of either electron-poor or electronrich compounds is well tolerated. Further studies toward enhancing the scope of the reaction to other epoxides will be reported in due course. We thank grant CTQ-2010-16170 from the Spanish government (MICINN). S. Roscales is thankful to the government of Spain and for FPU predoctoral grant No. AP20090051.
456 | Chem. Commun., 2014, 50, 454--456
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