C O M M U N I C A T I O N S
Supporting Information Available: Experimental procedures,
spectral characterization of catalyst and dihydropyranones, and HPLC
separations of dihydropyranones (PDF, CIF). This material is available
References
(1) (a) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48. (b) An Introduction
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Figure 1. X-ray structure of catalyst 5-PhCHO complex.
enantioselectivity. Electron-rich and electron-poor aldehydes gave
comparable results to benzaldehyde (entries 9-11). In the case of
o-nitrobenzaldehyde, both yield and ee were excellent despite the
steric bulk of an ortho substituent, Lewis basicity of the nitro group,
or competition from background reaction associated with electron
deficient aldehydes. On the other hand, it would appear that sterics
compromised the yield, but not the enantioselectivity, of 1-naph-
thaldehyde (entry 12). The HDA cycloaddition of furfural proceeded
in excellent yield and ee (>99%, entry 13). To the extent that the
furyl group is sterically less demanding than a phenyl group, then
the superior selectivity observed here mirrors the trend seen with
aliphatic aldehydes.
In an effort to shed light on the factors responsible for asymmetric
induction, we have attempted to obtain complexes between the
biaryldiol catalysts and aldehydes. To date, we have succeeded in
obtaining an X-ray structure of an inclusion complex of 2,2′-bis-
(diphenylhydroxymethyl)binaphthylene (5),14 a simple member of
the BAMOL family of catalysts, and benzaldehyde (Figure 1).15
The structure not only shows a 1:1 association between BAMOL
5 and benzaldehyde, but also reveals the presence of an intra-
molecular hydrogen bond between the two hydroxyls and an
intermolecular hydrogen bond to the carbonyl oxygen of benzalde-
hyde.16,17 The above complex suggests that carbonyl activation is
through a single-point hydrogen bond, as was postulated for
TADDOL catalysis.10b,17
The above results illustrate that axially chiral diols of the
BAMOL family are highly effective catalysts for enantioselective
HDA reactions between aminosiloxydiene 1 and a wide variety of
unactivated aldehydes. The reactions proceed in useful yields and
excellent enantioselectivities. The diols function in the same
capacity as Lewis acids, by activating the aldehyde carbonyl group
through hydrogen bonding. The present work also represents the
first successful use of the axially chiral biaryldimethanol scaffold
and points to its potential use in other catalytic asymmetric reactions,
whether through hydrogen bonding or metal-based Lewis acid
catalysis.
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Acad. Sci. U.S.A. 2004, 101, 5846.
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Zhang, X.; Mashima, K.; Koyano, K.; Sayo, N.; Kumobayashi, H.;
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1597.
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(14) BAMOL 5 is modestly effective as a catalyst. Results at -40 °C: 4b,
49% yield, 65% ee; 4i, 33% yield, 59% ee.
(15) Inclusion complexes of racemic bis(diarylhydroxymethyl)biaryl with
carbonyl compounds: (a) Caira, M. R.; Coetzee, A.; Nassimbeni, L. R.;
Weber, E.; Wierig, A. J. Chem. Soc., Perkin Trans. 2 1997, 237. (b)
Ibragimov, B. T.; Beketov, K. M.; Weber, E.; Seidel, J.; Sumarna, O.;
Makhkamov, K. K.; Kohnke, K. J. Phys. Org. Chem. 2001, 14, 697.
(16) The location of the H atoms in the hydrogen bonds is based on the residual
electron density peaks in the difference Fourier map. In the absence of
atoms heavier than O, these small residual peaks become more significant
and are comparable in magnitude to that observed for the aromatic
hydrogens, the positions of which are more certain.
(17) The single-point activation (A) proposed here differs from two-point
activation (B). The latter is the subject of the review in ref 3a.
Acknowledgment. This work is dedicated to Professor Iwao
Ojima, SUNY, Stony Brook, on the occasion of his 60th birthday.
The X-ray structure was solved by Dr. Ian Steele (The University
of Chicago). V.H.R. (CHE-0316803) and H.Y. (CHE-0412060)
thank the National Science Foundation for generous financial
support of this work.
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