Organic Letters
Letter
(7) (a) Williams, D. R.; Plummer, S. V.; Patnaik, S. Tetrahedron 2011,
67, 5083−5097. (b) Williams, D. R.; Plummer, S. V.; Patnaik, S.
Angew. Chem., Int. Ed. 2003, 42, 3934−3938. (c) Williams, D. R.;
Plummer, S. V.; Patnaik, S. Org. Lett. 2003, 5, 5035−5038.
(8) (a) Williams, D. R.; Kiryanov, A. A.; Emde, U.; Clark, M. P.;
Berliner, M. A.; Reeves, J. T. Proc. Nat. Acad. Sci. U.S.A. 2004, 101,
12058−12063. (b) Williams, D. R.; Kiryanov, A. A.; Emde, U.; Clark,
M. P.; Berliner, M. A.; Reeves, J. T. Angew. Chem., Int. Ed. 2003, 42,
1258−1262.
(9) (a) Corey, E. J.; Lee, T. W. Chem. Commun. 2001, 1321−1329.
(b) Castellano, R. K. Curr. Org. Chem. 2004, 8, 845−865.
(10) All computations were carried out using GAUSSIAN09, Rev.
B.01, Gaussian, Inc., Wallingford, CT, 2009. See the Supporting
Information for details.
experiment, calculations (Figure 3) predict low facial selectivity
for formation of methyl ether 35 (R = CH3), since this group
avoids unfavorable steric interactions in both si- and re-face
additions (not shown). Rotamers of the methylene bearing the
ether substituent (OCH3) favor an anti-relationship with
respect to the vicinal ethyl group in 34 as shown in TS-7
(Figure 3, entries 1 and 2).12 However, the TBDPS ether of
aldehyde 14 undergoes selective si-face attack by rotation to
avoid nonbonded interactions with the sulfonyl substituent (g−
position in 34) as shown in TS-8 (entry 3) while the
corresponding re-face TS (entry 4) is substantially destabilized
by additional steric interactions, thereby predicting the
observed diastereoselectivity.13
(11) Computations showed a 1.4−3.2 kcal/mol preference for
arrangement 8 for various model α,β-unsaturated aldehydes. In these
arrangements, the nitrogens are slightly distorted from trigonal planar,
and the nitrogen lone pair is positioned to bisect the O−S−O bond
angle of the sulfonyl group. For our TS arrangements, the nitrogen
lone pair does not appear to provide a stabilizing interaction with the
formyl hydrogen (>3Å). However, favorable interactions between the
sulfonyl oxygens and the aldehydic hydrogen HA may offer
incremental stabilization in TS-1 and TS-3. See the Supporting
Information for further discussions.
(12) Full optimizations in solvent (dichloromethane; CPCM;UFF)
showed similar results.
(13) Full optimizations on the two lowest energy TS structures at the
B3LYP/6-31G(d) level predict a lesser, but still substantial, energetic
difference (∼3 kcal/mol).
In summary, we have shown that asymmetric SE′ reactions of
nonracemic γ-substituted E- and Z-α,β-unsaturated aldehydes
feature matched and mismatched TS structures resulting from
nonbonded steric interactions with chiral 1,3-bis(tolylsulfonyl)-
4,5-diphenyl-1,3-diaza-2-borolidine auxiliaries. The minimiza-
tion of 1,3-allylic strain in the aldehyde is a contributing factor.
Generally, matched cases of nonracemic γ-substituted alde-
hydes and boranes led to excellent diastereoselection in the
production of complex homoallylic alcohols. The choice of
large protecting groups in proximite locations can result in
unanticipated levels of diastereoselectivity in mismatched cases.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental and computational details. This material is
AUTHOR INFORMATION
Corresponding Authors
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation (Grant No. CHE1055441). We acknowl-
edge the UC Davis R. B. Miller Graduate Fellowship (O.G.).
REFERENCES
■
(1) For a review of catalytic enantioselective SE′ reactions, see:
Denmark, S. E.; Fu, J. Chem. Rev. 2003, 103, 2763−2793.
(2) For general reviews of asymmetric SE′ reactions of allylic boranes,
silanes, and stannanes, see: (a) Williams, D. R.; Nag, P. P. In Tin
Chemistry; Fundamentals, Frontiers, and Applications; Davies, A. G.,
Gielen, M., Pannell, K. H., Tiekink, E. R. T., Eds.; Wiley & Sons: New
York, 2008; Chapter 5.2, pp 515−560. (b) Chemler, S. R.; Roush, W.
R. In Modern Carbonyl Chemistry; Otera, J., Ed.; Wiley-VCH: New
York, 2000; Chapter 11, pp 403−490. (c) Denmark, S. E.; Almstead,
N. G. In Modern Carbonyl Chemistry; Otera, J., Ed.; Wiley-VCH: New
York, 2000; Chapter 10, 299−401.
(3) Corey, E. J.; Yu, C. M.; Kim, S. S. J. Am. Chem. Soc. 1989, 111,
5495−5496.
(4) (a) Williams, D. R.; Brooks, D. A.; Meyer, K. G.; Clark, M. P.
Tetrahedron Lett. 1998, 39, 7251−7254. (b) Williams, D. R.; Meyer, K.
G.; Shamim, K.; Patnaik, S. Can. J. Chem. 2004, 82, 120−130.
(5) Williams, D. R.; Brooks, D. A.; Berliner, M. A. J. Am. Chem. Soc.
1999, 121, 4924−4925.
(6) Williams, D. R.; Meyer, K. G. J. Am. Chem. Soc. 2001, 123, 765−
766.
471
dx.doi.org/10.1021/ol403351x | Org. Lett. 2014, 16, 468−471