Fragmentation, ring-opening, and addition reactions of oxygen-conjugated alkenyl radicals derived by rapid 1,5-hydrogen shifts in vinyl radicals. An electron spin resonance and kinetic investigation

Abstract

The addition of ether-derived oxygen-conjugated radicals ˙CR¹R²OCHR¹R² to alkynes R³CCR⁴ leads to vinyl radicals, most of which undergo a rapid 1,5-shift (k > 10⁵ s^–1) to give radicals ˙CR¹R²OCR¹R²CR³CHR⁴. The reactions of the latter species, as revealed by e.s.r. spectroscopy, include fragmentation (to give R¹R²CO and ˙CR¹R²CR³CHR⁴), ring-opening (e.g. for radicals from tetrahydrofuran and dioxane and their derivatives), and trapping via reaction with a further molecule of alkyne. A kinetic analysis is presented and results are interpreted in terms of the electronic and steric effects of substituents in the intermediates.