Study on syn-Dihalogenation of Alkenes via Radical and Pericyclic Mechanisms

Abstract

Stereoselective construction of halogenated carbon center is not an easy task. Whereas a variety of stereoselective halofunctionalizations of alkenes are known, syn-dihalogenation of alkenes is extremely rare and has many limitations. The formation of a bridged halonium intermediate during ionic dihalogenation of alkenes leads to anti-addition exclusively through a backside attack of halide. To promote syn-addition in alkene dihalogenation, new syn-dihalogenating reagents were designed for pericyclic and radical mechanisms. The cyclic transition state of concerted pericyclic reaction is supposed to allow the additions of two halogen atoms in one direction. Hydrazine N,N’-dihalides were proposed as reagents for pericyclic syn-dihalogenation, and N,N’-dichlorination of hydrazines was attempted with a variety of electrophilic halogenating reagents. Unfortunately, dichlorination was not observed, and unexpected radical decomposition products were obtained. The radical syn-dihalogenation involves stepwise additions of two halogen radicals, which must take place prior to the C-C bond rotation. Accordingly, phthalazine N,N’-dihalides were proposed as reagents for radical syn-dihalogenation, and N,N’-dichlorination of phthalazines was attempted. Surprisingly, instead of the desired N,N’-dichloride, N-chlorophthalimide was obtained via an unexpected denitrogenative ring contraction. If the mechanisms of unexpected product formations are elucidated, these reactivities would be applicable to the development of new oxyradical formation and the development of new azetidine synthesis. Several mechanisms were proposed, and studies are underway to examine these proposals.