Journal of the American Chemical Society
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(2) For recent examples, see: (a) Gonzalez, A. Z.; Benitez, D.;
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(11) For another example of asymmetric protonation of a silyl enol
ether with a BINAP AgF complex, see: Yanagisawa, A.; Touge, T.; Arai,
3
T. Angew. Chem., Int. Ed. 2005, 44, 1546.
(12) We investigated the possibility that the protonation reaction
was catalyzed by an LBA generated from AgBF4 and alcohol in the
absence of the chiral phosphine ligand. However, under these condi-
tions, the silver-derived LBA did not catalyze the protonation reaction.
(13) See the Supporting Information for additional details.
(14) An enantioselective protonation reaction of acyclic substrates
was reported by Yamamoto; however, the substrates employed could be
only single-isomer silyl enol ethers, such as bis(trimethylsilyl)ketene
acetals. See ref 9d.
(15) Fu has reported an alternative method for the synthesis of
acyclic R-aryl ketones via enantioselective R-arylation of acyclic
ketones. See: (a) Lou, S.; Fu, G. C. J. Am. Chem. Soc. 2010, 132, 1264.
(b) Lundin, P. M.; Esquivias, J.; Fu, G. C. Angew. Chem., Int. Ed. 2009,
48, 154.
(16) The silyl enol ether of 2-phenyl propiophenone was obtained in
different E/Z ratios depending on the reaction conditions. Soft enoliza-
tion (TMSOTf, NEt3) gave the silyl enol ether in a 1:3 ratio, whereas
hard enolization (LDA/TMSCl) gave the silyl enol ether with a 1:2 ratio.
For details, see the Supporting Information.
(17) Under the standard conditions, the gold-catalyzed protonation
reaction of the silyl ketene acetal of methyl 2-phenylpropionate gave
racemic product:
(3) For examples that have been proposed to proceed by mecha-
nisms other than π activation, see: (a) Campbell, M. J.; Toste, F. D. Chem.
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Hayashi, T. J. Am. Chem. Soc. 1986, 108, 6405.
(4) For recent reports of activation of alkenes with gold
complexes, see: (a) LaLonde, R. L.; Brenzovich, W. E., Jr.; Benitez,
D.; Tkatchouk, E.; Kelly, K.; Goddard, W. A., III; Toste, F. D. Chem. Sci.
2010, 1, 226. (b) For enantioselective alkene hydroamination, see:
Zhang, Z.; Lee, S. D.; Widenhoefer, R. A. J. Am. Chem. Soc. 2009,
131, 5372. (c) For a review, see: Widenhoefer, R. A.; Han, X. Eur. J. Org.
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(5) (a) Luzung, M. R.; Mauleon, P.; Toste, F. D. J. Am. Chem. Soc.
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R.; de Fremont, P.; Maseras, F.; Nolan, S. P. Chem.—Eur. J. 2007,
13, 6437.
(18) For an example of formation of gold(I) enolate, see:Nesmeyanov,
A. N.; Grandberg, K. I.; Dyadchenko, V. P.; Lemenovskii, D. A.; Perevalova,
E. G. Izv. Akad. Nauk SSSR, Ser. Khim. 1974, 1206.
(19) For examples of gold-catalyzed reactions of silyl enol ethers that
do not involve formation of gold enolates, see: (a) Staben, S. T.;
Kennedy-Smith, J. J.; Huang, D.; Corkey, B.; LaLonde, R. L.; Toste,
F. D. Angew. Chem., Int. Ed. 2006, 45, 5991. (b) Linghu, X.; Kennedy-Smith,
J. J.; Toste, F. D. Angew. Chem., Int. Ed. 2007, 46, 7671. (c) Lee, K.; Lee, P. H.
Adv. Synth. Catal. 2007, 349, 2092. (d) Ito, H.; Ohmiya, H.; Sawamura, M.
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(6) Kanno, O.; Kuriyama, W.; Wang, Z. J.; Toste, F. D. Angew. Chem.
Int. Ed. 2011 (DOI 10.1002/anie.201104076).
(7) Yamamoto introduced the concept of Lewis acid-assisted
Brønsted acid (LBA) catalysis. For reviews of combined acids, see:
(a) Yamamoto, H.; Futatsugi, K. Angew. Chem., Int. Ed. 2005, 44, 1924.
(b) Ishibashi, H.; Ishihara, K.; Yamamoto, H. Chem. Rec. 2002, 2, 177.
(8) For reviews of enantioselective protonation reactions, see:
(a) Mohr, J. T.; Hong, A. Y.; Stoltz, B. M. Nat. Chem. 2009, 1, 359.
(b) Duhamel, L.; Duhamel, P.; Plaquevent, J. C. Tetrahedron: Asymmetry
2004, 15, 3653. (c) Eames, J.; Weerasooriya, N. Tetrahedron: Asymmetry
2001, 12, 1.(d) Yanagisawa, A.; Yamamoto, H. In Comprehensive
Asymmetric Synthesis III; Jacobsen, E. N., Pfaltz, A., Yamamoto, H.,
Eds.; Springer: Heidelberg, 1999; p 1295. (e) Fehr, C. Angew. Chem., Int.
Ed. Engl. 1996, 35, 2566.
(20) For isomerization between the E and Z isomers of enol ethers in
gold-catalyzed reactions, see: Teles, J. H.; Brode, S.; Chabanas, M.
Angew. Chem., Int. Ed. 1998, 37, 1415.
(9) For LBA-catalyzed enantioselective protonation reactions, see:
(a) Cheon, C. H.; Imahori, T.; Yamamoto, H. Chem. Commun. 2010,
46, 6980. (b) Nakashima, D.; Yamamato, H. Synlett 2006, 150. (c)
Ishihara, K.; Nakashima, D.; Hiraiwa, Y.; Yamamoto, H. J. Am. Chem. Soc.
2003, 125, 24. (d) Nakamura, S.; Kaneeda, M.; Ishihara, K.; Yamamoto,
H. J. Am. Chem. Soc. 2000, 122, 8120. (e) Ishihara, K.; Nakamura, S.;
Yamamoto, H. Croat. Chem. Acta 1998, 69, 513. (f) Ishihara, K.; Ishida,
Y.; Nakamura, S.; Yamamoto, H. Synlett 1997, 758. (g) Ishihara, K.;
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(10) For other examples of enantioselective protonation reactions of
silyl enol ethers, see: (a) With an organocatalyst: Poisson, T.; Dalla, V.;
Marsais, F.; Dupas, G.; Oudeyer, S.; Levacher, V. Angew. Chem., Int. Ed.
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dx.doi.org/10.1021/ja204331w |J. Am. Chem. Soc. 2011, 133, 13248–13251