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[13] The Gaussian 03 package was used for the ab initio calculations.
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basis set used was 6-31 + G(d,p) on all atoms other than Zn. The
Zn atom was treated with the LANL2DZ basis set and the
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[14] Initially, we assumed that the presence of Zn ions allowed access
to the Z isomer, which might be expected to undergo reduction
with the opposite sense of selectivity (see Ref. [5b]). While we
can not rule out the possibility that traces of the Z imine are
formed and that this isomer undergoes reduction much faster
with Zn(BH4)2 than does the major E isomer, we feel this is
unlikely for the following reasons: 1) mixtures of E and Z imines
derived from trifluoroacetaldehyde (fluoral) required about
60 minutes at ambient temperature to equilibrate to the
thermodynamically preferred E isomer; 2) mixing the imine
carboxylates with zinc salts such as Zn(OTf)2 or ZnCl2 resulted
in a downfield shift of the proton a to the carboxylate group (the
rest of the spectra remain largely unchanged), presumably as a
result of coordination of the carboxylate with Zn, but analysis of
this complex still shows an NOE interaction between this proton
and the ortho protons of the aromatic ring, which is indicative of
an E imine.
[6] See also Ref. [5c] for an example whereby the addition of a
Grignard reagent to a hemiacetal did not require separation of
the diastereomeric intermediates.
[7] For a related strategy using a-methylbenzylamine as a chiral
controlling agent, see: W. H. Pirkle, J. R. Hauske, J. Org. Chem.
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[9] While the imines could generally be isolated, it was not
necessary to do so and the crude suspensions were carried
directly into the borohydride reductions. Analysis of the the
1
imine carboxylate 1b by H NMR spectroscopy shows an NOE
interaction between the proton a to the carboxylate and the
ortho protons of the aromatic ring, which is consistant with the
proposed E isomer. Ab initio calculations (6-31 + G(d,p)) also
show this isomer to be favored by about 6 kcalmolÀ1
.
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Angew. Chem. Int. Ed. 2007, 46, 1839 –1842