A Theoretical Study On Rh(I) Catalyzed Enantioselective Conjugate Addition Reactions of Fluoroalkylated Olefins

Abstract

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 (S)-BINAP. Conjugate addition of unsubstituted, o-CH3, p- and o-Cl substituted phenyl groups were considered to explain steric and electronic effects on the reaction mechanism. The activation energy difference between benzene and o-toluene-substituted systems (8.1 kcal/mol for the R 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 o-Cl for R enantiomer). The experimental unreactivity of the o-Cl-substituted system was also confirmed with the calculated high activation energies for both R and S (29.9 and 31.7 kcal/mol for R and S, 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 R 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.