Recent Advances in Free-Radical-Promoted Selective Activation of Alcohol/Etherα-O-C(sp3)-H Bond

Abstract

Alcohols/ethers are the most common chemicals. And the hydroxyl group in alcohols is believed to be universal functional group in synthetic organic chemistry. It is undeniably attractive to form a new chemical bond through selective cleavage of the alpha-O-C(sp(3))-H bond. Considerable developments in the free-radical-promoted alcohol/ether C(sp(3))-H functionalization have been achieved in recent years. These methods have drawn much attention from synthetic chemists due to the features of alcohol/ether as starting materials, reservation of the hydroxyl group and excellent regioselectivity, etc. This paper summarizes the recent advances in free-radical-initiated selective activation of the alpha-O-C(sp(3))-H bonds in alcohol/ether. For a start, the possible factors such as bond dissociation energy (BDE) and stability of the key radical intermediate that dominates the regioselectivity in radical-initiated C(sp(3))-H bond activation have been analyzed here. Moreover, recent developments in this field have been demonstrated in details from different reaction types as following. (a) Minisci reactions of heterocycles with alcohols/ethers; (b) Radical addition/elimination reactions of activated alkenes with alcohols/ethers; (c) Free-radical addition/cyclization cascade reactions of activated olefins and/or biaryl isonitriles with alcohols/ethers; (d) Free-radical addition/difunctionalization and rearrangement reactions of alcohols/ethers with activated alkenes. Besides, other free radical reactions such as oxidative coupling reactions of aryl boronic acids with ethers, C-O and C-N bond formation reactions have also been exhibited. In addition, the suggested mechanisms for most of these reactions have been depicted and discussed in this review. Finally, the disadvantages of present systems and the possible modifications along with the future studies of this active area have been summarized at the end of this article.