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1.1.6 SOMO and Radical Chemistry in Organocatalysis

DOI: 10.1055/sos-SD-204-00115

MacMillan, D. W. C.; Beeson, T. D.Science of Synthesis: Asymmetric Organocatalysis, (20121271.

General Introduction

One of the great advances that organocatalysis has brought to the field of synthetic chemistry is the introduction of several new modes of chemical activation through which hundreds of new asymmetric reactions have been discovered. For example, two of the earliest described organocatalytic activation modes, enamine catalysis[‌1‌,‌2‌] and iminium catalysis,[‌3‌‌5‌] both activate carbonyl compounds by reversible condensation with an amine catalyst. The former induces a HOMO-raising effect on the α-carbon to the carbonyl and generates a nucleophilic enamine species capable of α-functionalization by a variety of electrophiles. In contrast, iminium catalysis involves LUMO-lowering at the β-carbon of an α,β-unsaturated carbonyl creating an electrophilic iminium species capable of β-functionalization with a spectrum of nucleophiles (Scheme 1). Both of these concepts, while inducing reactivity analogous to that of Lewis acid catalysis,[‌6‌,‌7‌] have enabled many new and even historically unattainable enantioselective catalytic reactions (e.g., the aldehyde cross aldol via enamine catalysis[‌8‌]).

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