Synthetic Utility of the Captodative Effect

Substituent effects, whether of electronic or steric origin, have a major influence on the efficiency and selectivity of chemical reactions - heterolytic, homolytic, and concerted. The quantitative study of these effects has often relied on linear free energy relationships, and has led to much valuable information on reaction mechanisms 

It is this fact which leads to the captodative effect, that is the stabilization of captodative radicals 1, bearing simultaneous captor and donor substituents. In turn, this means that compounds such as 2 and 3 tend to react by radical pathways [3], via radical abstraction (C-X homolysis of 2) or radical addition (to the n system of 3). 

Although the precise quantitative importance of the captodative effect is still being debated, it has been shown to exist by many experiments, and, as a guiding 

In another example, Riichardt has reviewed the factors influencing C-C bond energies [14]. This analysis led to the recognition of captodative synergistic stabilizations of respectively 10.8 and 6.7 kcal mol for the a-amino-ot-methylcarbonyl radical 5 [15] and the alanine-derived radical 6 [16]. 

In synthesis, the captodative effect can be used to weaken a- and r-bonds thus creating a bias towards radical-like reactivity and enhancing selectivity. Reactions 

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