Photolysis of the carbon–hydrogen bond in pentamethylcyclopentadiene. Properties of the pentamethylcyclopentadienyl radical

Abstract

Irradiation of pentamethylcyclopentadiene (A) in liquid solution with u.v. light results in homolysis of the ring C–H bond to give the pentamethylcyclopentadienyl radical (B)[a(15 H) 6.4, a(¹³C) 3.5 G], and atomic hydrogen which [graphic omitted] abstracts hydrogen from a second molecule of (A) to give molecular hydrogen and a second radical (B). The radicals (B) self-react at a diffusion-controlled rate (2k_t 2 × 10⁹ I mol^–1 s^–1 in hexane at 25°C) by two different routes. The first, which is thermally and photolytically reversible, is the combination to give the dehydrodimer (C), and the second, which is irreversible, is the disproportionation to give the parent cyclopentadiene (A) and the tetramethylfulvene (D).