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perfect chemoselectivity of the reaction, DFT calculations have
been performed. These findings provide a basis for future
research on designing new chiral silyl Lewis acid catalysts
and their use for the asymmetric Mukaiyama aldol reaction.
Notes and references
1 (a) H. F. T. Klare and M. Oestreich, Dalton Trans., 2010, 39, 9176;
(b) A. D. Dilman and S. L. Ioffe, Chem. Rev., 2003, 103, 733.
2 For examples of trialkylsilyl triflimides in catalysis, see: (a) R. B.
Othman, T. Bousquet, M. Othman and V. Dalla, Org. Lett., 2005,
7, 5335; (b) K. Inanaga, K. Takasu and M. Ihara, J. Am. Chem. Soc.,
2005, 127, 3668; (c) B. Mathieu and L. Ghosez, Tetrahedron, 2002,
58, 8219; (d) J. Cossy, F. Lutz, V. Alauze and C. Meyer, Synlett, 2002,
45; (e) B. Mathieu, L. de Fays and L. Ghosez, Tetrahedron Lett., 2000,
41, 9561; ( f ) N. Kuhnert, J. Peverley and J. Robertson, Tetrahedron Lett.,
1998, 39, 3215; (g) B. Mathieu and L. Ghosez, Tetrahedron Lett., 1997,
38, 5497; (h) A. Ishii, O. Kotera, T. Saeki and K. Mikami, Synlett, 1997,
1145. For a review on the super silyl triflimide-catalyzed Mukaiyama
aldol reaction, see: (i) P. B. Brady, B. J. Albert, M. Akakura and
H. Yamamoto, Chem. Sci., 2013, 4, 3223.
Scheme 4 Chemoselective silyl transfer in the (TES)3SiOTf-promoted
Mukaiyama aldol reaction.
To gain mechanistic insight into the perfect chemoselectivity
observed when using super silyl triflates, density functional
theory (DFT) calculations have been performed with M06-2X
functionals (Scheme 2).12 The reaction of benzaldehyde with 1a
using (TMS)3SiOTf as the promoter affords the intermediate A.
A has accessible intramolecular interaction of the alkoxy oxygen
atom with Me3Si+, which triggers the Me3Si+ transfer. The
distance of the newly formed Si–O bond ranges from 3.252 Å
in A to 2.322 Å in TS-1, reflecting the silicon-bridged transition
structure, which leads to the intermediate B. Then, addition of
another benzaldehyde to the bulkier (TMS)3Si+ selectively occurs
(TS-2), providing 2 as the exclusive product. In order to obtain
more accurate information on TS-2, potential energy profiles
for the addition of benzaldehyde to (TMS)3Si+ (TS-3) and Me3Si+
(TS-4) were respectively calculated (Scheme 3). The calculated
activation energy in TS-3 (2.0 kcal molÀ1) was less than that in
TS-4 (2.7 kcal molÀ1), indicating that addition of benzaldehyde
to (TMS)3Si+ would be energetically more favored than that to
Me3Si+.13 Relief of steric congestion around the alkoxy oxygen
atom would provide a lower activation barrier in TS-3.14
3 For
a seminal reference of Mukaiyama aldol reaction, see:
T. Mukaiyama, K. Narasaka and K. Banno, Chem. Lett., 1973, 1011.
4 For references of Mukaiyama aldol reaction and the mechanistic
studies, see: (a) J. Matsuo and M. Murakami, Angew. Chem., Int. Ed.,
2013, 52, 9109; (b) J. M. Lee, P. Helquist and O. Wiest, J. Am. Chem.
Soc., 2012, 134, 14973; (c) K. Ishihara, H. Yamamoto, Modern
Aldol Reactions, ed. R. Mahwald, Wiley-VCH, Weinheim, Germany,
2004, ch. 2, vol. 2, p. 25; (d) R. Mahrwald, Chem. Rev., 1999, 99, 1095;
(e) E. M. Carreira, Comprehensive Asymmetric Catalysis III, ed.
E. N. Jacobsen, A. Pfaltz and H. Yamamoto, Springer-Verlag, Berlin,
Heidelberg, 1999, p. 997; ( f ) B. W. Gung, Z. Zhu and R. A. Fouch,
J. Org. Chem., 1995, 60, 2860; (g) K. T. Hollis and J. Bosnich, J. Am.
Chem. Soc., 1995, 117, 4570; (h) S. E. Denmark and W. Lee, J. Org.
Chem., 1994, 59, 707; (i) M. T. Reetz, B. Raguse, C. F. Marth,
H. M. Huegel, T. Bach and D. N. A. Fox, Tetrahedron, 1992, 48, 5731.
5 K. Ishihara, Y. Hiraiwa and H. Yamamoto, Synlett, 2001, 1851.
6 For recent examples of the enantioselective aldol reaction catalyzed
´
by in situ generated silyl Lewis acid, see: (a) L. Ratjen, P. Garcıa-
Garca, M. Edmund Beck and B. List, Angew. Chem., Int. Ed., 2011,
50, 754; (b) C. H. Cheon and H. Yamamoto, Org. Lett., 2010, 12, 2476;
The chemoselective silyl transfer in the super silyl triflate-
promoted Mukaiyama aldol reaction was applied to other
substrates (Scheme 4). In the case of Me3Si silyl enol ether of
cyclohexyl methyl ketone 1b, only the Me3Si aldolate 4 was
produced. We also carried out the reaction of benzaldehyde
´
´
´
´
(c) P. Garcıa-Garcıa, F. Lay, P. Garcıa-Garcıa, C. Rabalakos and
B. List, Angew. Chem., Int. Ed., 2009, 48, 4363.
7 E. M. Carreira and R. A. Singer, Tetrahedron Lett., 1994, 35, 4323.
8 S. E. Denmark and C.-T. Chen, Tetrahedron Lett., 1994, 35, 4327.
9 (a) Y. Hiraiwa, K. Ishihara and H. Yamamoto, Eur. J. Org. Chem.,
2006, 1837; (b) K. Ishihara, Y. Hiraiwa and H. Yamamoto, Chem.
Commun., 2002, 1564.
t
with BuMe2Si-protected silyl enol ether of acetophenone 1c.
The reaction proceeded through intramolecular tBuMe2Si+
transfer, giving 3a as the exclusive product. In both cases, none
of the (TES)3Si aldolate was observed by 1H NMR analysis of the
crude reaction mixture.
In conclusion, we have investigated the influence of the silyl
group of the silyl triflates on the chemoselective silyl transfer in
the Mukaiyama aldol reaction. As a result, we demonstrated
that perfect chemoselective silyl transfer could be realized
10 For the counteranion effect on the catalytic activity of silyl Lewis
acid in the Mukaiyama aldol reaction, see: (a) A. C. S. Reddy,
Z. Chen, T. Hatanaka, T. Minami and Y. Hatanaka, Organometallics,
2013, 32, 3575; (b) A. Takahashi, H. Yanai, M. Zhang, T. Sonoda,
M. Mishima and T. Taguchi, J. Org. Chem., 2010, 75, 1259;
(c) K. Hara, R. Akiyama and M. Sawamura, Org. Lett., 2005, 7, 5621.
11 For recent review articles on the chiral counteranion catalysis, see:
(a) M. Mahlau and B. List, Angew. Chem., Int. Ed., 2013, 52, 518;
(b) K. Brak and E. N. Jacobsen, Angew. Chem., Int. Ed., 2013, 52, 534;
(c) R. J. Phipps, G. L. Hamilton and F. D. Toste, Nat. Chem., 2012,
4, 603; (d) J. Lacour and D. Moraleda, Chem. Commun., 2009, 7073.
when super silyl triflates were used as the promoters; the Me3Si 12 Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215.
13 See the ESI† for the details of the DFT computational analysis.
14 One of the referees has suggested that the silyl transfer might occur
group of Me3Si silyl enol ether chemoselectively transferred to
the aldolate product and the super silyl group of super silyl
in an intermolecular manner. To test this hypothesis, we conducted
triflate did not remain in the product. To gain insight into the
a set of experiments. See the ESI† for the details of this study.
15208 | Chem. Commun., 2014, 50, 15206--15208
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