Developing a Modern Approach To Account for Steric Effects in Hammett-Type Correlations

The
effects of aryl ring ortho-, meta-, and para-substitution on site selectivity and
enantioselectivity were investigated in the following reactions: (1)
enantioselective Pd-catalyzed redox-relay Heck reaction of arylboronic
acids, (2) Pd-catalyzed β-aryl elimination of triarylmethanols,
and (3) benzoylformate decarboxylase-catalyzed enantioselective benzoin
condensation of benzaldehydes. Through these studies, it is demonstrated
that the electronic and steric effects of various substituents on
selectivities obtained in these reactions can be described by NBO
charges, the IR carbonyl stretching frequency, and Sterimol values
of various substituted benzoic acids. An extended compilation of NBO
charges and IR carbonyl stretching frequencies of various substituted
benzoic acids was used as an alternative to Hammett values. These
parameters provide a correlative tool that allows for the analysis
of a much greater range of substituent effects because they can also
account for proximal and remote steric effects.