Rw. Barnhart et B. Bosnich, HOMOGENEOUS CATALYSIS - MECHANISM OF DIASTEREOSELECTIVE HYDROACYLATION OF 3-SUBSTITUTED-4-PENTENALS USING CHIRAL RHODIUM CATALYSTS, Organometallics, 14(9), 1995, pp. 4343-4348
Catalytic hydroacylation of 4-pentenals using [Rh(diphosphine)](+) cat
alysts leads to the formation of cyclopentanones. Catalytic kinetic re
solution of racemic 3-phenyl-4-pentenal using the chiral catalyst, [Rh
((S)-binap)](+), leads not only to the diastereoselective formation of
beta-phenylcyclopentanone but also to the diastereoselective producti
on of 4-phenyl-4-pentenal. The kinetics of the formation of these two
products, together with deuterium distribution studies, indicates that
hydroacylation proceeds by a variety of reversible steps. The 4-pheny
l-4-pentenal is formed by carbonyl deinsertion-insertion steps. Althou
gh the steps appear to be reversible, equilibrium is not established.
It is concluded that asymmetric hydroacylation is not governed by a si
ngle enantioselective step, but rather that the enantioselection is co
ntrolled by a number of reversible steps involving reaction intermedia
tes.