Radical philicity and its role in selective organic transformations

Radical philicity - the ability of a radical to act as a nucleophile or an electrophile - is an important, yet often poorly understood, concept. In this Review, we present a qualitative method to understand and predict radical philicity by classifying the typical reactivity of more than 30 types of radicals into nucleophilic or electrophilic behaviour. Radical intermediates in organic chemistry lack a full octet of electrons and, thus, are commonly said to be electron deficient. By denotation, such a statement is technically correct; however, in modern literature, the term 'electron deficient' carries a connotation of electrophilicity. This lexical quirk leads one to predict that all radicals should behave as electrophiles, when this is not the case. Indeed, practitioners of radical chemistry have known for decades that many radicals behave as nucleophiles, sometimes strongly so. This Review aims to establish guidelines for understanding radical philicity by highlighting examples from recent literature as a demonstration of general reactivity paradigms across a series of different carbon-based and heteroatom-based radicals. We present strategies for predicting the philicity of a given radical on the basis of qualitative features of the radical's structure. Finally, we discuss the implications of radical philicity to selective hydrogen atom transfer.