Table 1. Hydrosilylation of R-Diketones by
sym-Tetraphenyldisilanea
Table 2. Hydrosilylation of R-Diketones by MePh2SiHa
entry
R
R′
product
meso/dl (anti/syn)
entry
R
R′
product meso/dl (anti/syn)
1
2
3
Ph
Me
Me
Ph
Ph
Me
1
2
3
100:0
(91:9)
86:14
1
2
3
4
5
6
7
a
C6H5
CH3
CH3
C6H5
C6H5
CH3
4
5
6
17:83
(21:79)
19:81
11:89
22:78b
12:88
14:86
a
1
p-C6H4CH3
p-C6H4OCH3 p-C6H4OCH3
p-C6H4F
p-C6H4Br
p-C6H4CH3
7
Product ratios determined by H NMR. All reactions gave complete
conversion to products within 1 h. Results averaged over two to three trials.
8b
9
p-C6H4F
p-C6H4Br
10
1
Product ratios determined by H NMR. Unless otherwise noted, all
reactions gave quantitative conversion to listed products within 1 h. Results
averaged over three to five trials. b For this reaction we observed several
in 70-80% yield with the diastereomer ratios shown in Table
1; susbsequent recrystallization provides 2 as >99% anti and
3 as ∼96% meso. The molecular structure of the major, meso
diastereomer of 3, obtained from a single-crystal X-ray
diffraction study (Figure 1), shows clearly the plane of
7
1
side products by H NMR, giving an overall ∼70% yield of 8.
of benzil catalyzed by B(C6F5)3 (Figure 2). While Me3SiH
gave a very high ratio of the meso product (98:2, same
preferred diastereomer as observed for sym-tetraphenyldisi-
lane), Ph3SiH gave a similarly high proportion of the dl
diastereomer (4:96). The least stereodifferentiating silane was
Et3SiH. This trend in selectivity correlates very well with a
simple change in bulk of the R-siloxy substituent.8,9
The established mechanism for hydrosilylation of carbonyl
groups catalyzed by B(C6F5)3 relies on partial abstraction of
the silane Si-H bond by the borane, as demonstrated by
(1) Examples include the following, and references therein. Reactions
of metal hydride reagents: (a) Li, X.; Zhao, G.; Cao, W. G. Chin. J. Chem.
2006, 24, 1402. (b) Clerici, A.; Pastori, N.; Porta, O. Eur. J. Org. Chem.
2002, 3326. (c) Abernethy, C. D.; Cole, M. L.; Davies, A. J.; Jones, C.
Tetrahedron Lett. 2000, 41, 7567. (d) Ravikumar, K. S.; Sinha, S.;
Chandrasekaran, S. J. Org. Chem. 1999, 64, 5841. Biocatalytic methods:
(e) Edegger, K.; Stampfer, W.; Seisser, B.; Faber, K.; Mayer, S. F.; Oehrlein,
R.; Hafner, A.; Kroutil, W. Eur. J. Org. Chem. 2006, 1904. (f) Demir, A. S.;
Hamamci, H.; Ayhan, P.; Duygu, A. N.; Igdir, A. C.; Capanoglu, D.
Tetrahedron: Asymmetry 2004, 15, 2579. (g) Kihumbu, D.; Stillger, T.;
Hummel, W.; Liese, A. Tetrahedron: Asymmetry 2002, 13, 1069. (h)
Buisson, D.; Elbaba, S.; Azerad, R. Tetrahedron Lett. 1986, 27, 4453.
Organocatalyzed hydroboration: (i) Shimizu, M.; Tsukamoto, K.; Matsutani,
T.; Fujisawa, T. Tetrahedron 1998, 54, 10265. Metal-catalyzed hydrogena-
tion: (j) Toukoniitty, E.; Busygin, I.; Leino, R.; Murzin, D. Y. J. Catal.
2004, 227, 210. (k) Zhou, D. Q.; He, M.; Zhang, Y. H.; Huang, M. Y.;
Jiang, Y. Y. Polym. AdV. Technol. 2003, 14, 287. (l) Li, Y.; Liu, H. F.
Chin. Chem. Lett. 2001, 12, 127. (m) Hattori, K.; Sajiki, H.; Hirota, K.
Tetrahedron 2001, 57, 4817. Metal-catalyzed transfer hydrogenation: (n)
Ikariya, T.; Hashiguchi, S.; Murata, K.; Noyori, R.; Wipf, P.; Amantini, D.
Org. Synth. 2005, 82, 10. (o) Wu, X. F.; Li, X. H.; Zanotti-Gerosa, A.;
Pettman, A.; Liu, J. K.; Mills, A. J.; Xiao, J. L. Chem.sEur. J. 2008, 14,
2209. Catalytic hydrosilylation: (p) Tsuhako, A.; He, J. Q.; Mihara, M.;
Saino, N.; Okamoto, S. Tetrahedron Lett. 2007, 48, 9120. (q) Nadkarni,
D.; Hallissey, J.; Mojica, C. J. Org. Chem. 2003, 68, 594. (r) Kuwano, R.;
Sawamura, M.; Shirai, J.; Takahashi, M.; Ito, Y. Bull. Chem. Soc. Jpn.
2000, 73, 485. (s) Corriu, R. J. P.; Lanneau, G. F.; Yu, Z. F. Tetrahedron
1993, 49, 9019.
Figure 1. Molecular structure of meso-3. Only the ipso carbons of
the phenyl rings are shown. Non-hydrogen atoms are represented
by Gaussian ellipsoids at the 20% probability level. Hydrogen atoms
are shown with arbitrarily small thermal parameters except for
phenyl hydrogens, which are not shown.
symmetry in this reduced diketone, as well as an enforced
eclipsed conformation along the Si-Si bond in this con-
strained, protected diol.
The diastereomer ratios in Table 1 can be rationalized on
the basis of steric constraints associated with ring-closing
(vide infra), but we were surprised to discover that bis-
(hydrosilylation) of benzil by the monosilane substrate
MePh2SiH also proceeds stereoselectively (Table 2, entry
1). Even more interestingly, deprotection of the product, 4,6
showed that the major stereoisomer in this case is dl: an
inversion of preferred stereochemistry relative to that ob-
served for the disilane substrates. The same reaction per-
formed using a variety of different R-diketones (Table 2)
showed minimal impact of the steric and electronic features
of these substrates on the diastereomer ratios. On the other
hand, we observed a profound influence of the structure of
the monosilane on diastereoselectivity in the hydrosilylation
(2) References 1g and 1n stand out in this respect but are limited in
diketone scope.
(3) Mengel, A.; Reiser, O. Chem. ReV. 1999, 99, 1191.
(4) (a) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440. (b)
Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090.
(c) Asao, N.; Ohishi, T.; Sato, K.; Yamamoto, Y. J. Am. Chem. Soc. 2001,
123, 6931. (d) Bach, P.; Albright, A.; Laali, K. K. Eur. J. Org. Chem. 2009,
1961.
Org. Lett., Vol. 12, No. 2, 2010
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