The enantioselectivities arising from a Pd-catalyzed Heck reaction
(>98% ee) and an allylic alkylation
(>90% ee) using a 3,5-di-tert-butyl-MeO−BIPHEP chiral
auxiliary (1) are reported. Higher ee's are observed
with
the 3,5-dialkyl substituents than with the unsubstituted parent
MeO−BIPHEP. It is proposed that the observed
dialkyl “meta-effect”, on enantioselectivity, is the combined
result of a more rigid and slightly larger chiral pocket
and that this effect will have some generality in homogeneous
catalysis. Detailed NMR studies on the allyl complex
[Pd(PhCHCHCHPh)(1)]PF6
(5), and the model hydrogenation catalyst
[RuH(cymene)(1)]BF4
(6), reveal restricted
rotation about several of the P−C(ipso) bonds of the phosphorus
substituents containing the 3,5-di-tert-butyl
groups.
The X-ray structure of 6 reveals that the cymene ligand
is not symmetrically bound to the Ru atom. This
observation
is interpreted as an expression of the chiral pocket of 1.
MM3* calculations on 6 support the NMR findings
and
reproduce the X-ray results.
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