Molecular Radical Chain Initiators for Ambient- to Low-Temperature Applications

An overview of methods for the initiation of radical chain reactions by specific initiator compounds, which generate radicals, is given. These can be utilized to initiate any kind of radical chain reaction by transforming substrates into the desired radical intermediates. Azo initiators, peroxides, nitroxides, trialkylboranes, dialkyl zinc compounds, and type I photoinitiators are discussed, as well as methods of redox- and sonochemical initiation. Methods of direct radical formation from the substrates, such as photoredox catalysis or high-energy irradiation, are not included. The focus of this review lies on rather "low" temperatures in the range of 50 °C down to -78 °C, which can be useful to achieve more selective reactions. Illustrative applications of such radical chain initiators in a variety of reactions are discussed, including stereoselective ones and polymerizations.