The Captodative Effect

Abstract

The concept of captodative substitution implies the simultaneous action of a captor (acceptor) and a donor substituent on a molecule. The chapter analyzes whether the claim of the proponents of the captodative effect has found experimental or theoretical support. The postulate of a synergetic action of a captor and a donor substituent at a radical centre and its chemical consequences has been discussed. Captodative substitution is that chemical consequences should be connected with this substitution pattern. For synthetic purposes, a captodative effect of a few kcal mol- might be helpful for the selection of a lower energy pathway in cases where a radical can react by several reaction paths, discriminated by activation barriers of closely similar energy. Synthetic applications have shown that captodative-substituted radicals generally behave like other stabilized, short-lived radicals. Left alone these radicals usually give dimers in high yield, as, for instance, allylic or benzylic radicals also do. As captodative substitution provides stability to a radical centre it is no surprise that captodative olefins are goodpartners in (2 + 2)-cycloadditions.