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

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.