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A Theoretical Study On Rh(I) Catalyzed Enantioselective Conjugate Addition Reactions of Fluoroalkylated Olefins
In
this study, quantum mechanical calculations have been performed
to elucidate the mechanism and enantioselectivity in rhodium-catalyzed
1,4-conjugate addition reaction of a series of aryl groups to electron-deficient
4,4,4-trifluoro-1-phenyl-2-buten-1-one in the presence of (<i>S</i>)-BINAP. Conjugate addition of unsubstituted, <i>o</i>-CH<sub>3</sub>, <i>p</i>- and <i>o</i>-Cl substituted
phenyl groups were considered to explain steric and electronic effects
on the reaction mechanism. The activation energy difference between
benzene and <i>o</i>-toluene-substituted systems (8.1 kcal/mol
for the <i>R</i> isomer) has shown the impact of steric
effects of substituents at the ortho position. The electronic effect
of a Cl substituent at the ortho position was demonstrated by an even
higher energy barrier (11.9 kcal/mol of energy difference between
benzene and <i>o</i>-Cl for R enantiomer). The experimental
unreactivity of the <i>o</i>-Cl-substituted system was also
confirmed with the calculated high activation energies for both <i>R</i> and <i>S</i> (29.9 and 31.7 kcal/mol for <i>R</i> and <i>S</i>, respectively) product formations.
The system with para-positioned Cl revealed almost the same barriers
for benzene, indicating that substituents at the para position do
not have significant electronic or steric effects in this reaction.
In all the modeled sets, experimental <i>R</i> product predominance
could be reproduced. The quantitative trend was satisfied with the
B3LYP/6-31G* functional, where the LANL2DZ effective core potential
was used for Rh, P, S, and Cl atoms. Benchmark calculations have been
performed to validate the level of theory in this study