Mechanism of the [2+2] photocycloaddition of fullerene C-60 with styrenes

Stereochemical studies on [2 + 2] photoaddition of cis-/trans-4-propenylani sole (cis-1 and trans-1) and cis-1-(p-methoxyphenyl)ethylene-2-d(1) (cis-3- d(1)) to C-60 exhibit stereospecificity in favor of the trans-2 cycloadduct in the former case and nonstereoselectivity in the latter. The observed st ereoselectivity in favor of the cis-6-d(3) [2 + 2] diastereomer by 12% in:t he case of the photochemical addition of (E)-1-(p-methoxyphenyl)-2-methyl-p rop-1-ene-3, 3, 3-d(3) (trans-5-d(3)) to C-60 is attributed to a steric kin etic isotope effect (k(H)/k(D) = 0.78). The loss of stereochemistry in the cyclobutane ring excludes a concerted addition and is consistent with a ste pwise mechanism. Intermolecular secondary kinetic isotope effects of the [2 + 2] photocycloaddition of 3-d(0) vs 3-d(1), and 3-d(6) as well as 5-d(0) vs 5-d(1), and 5-d(6) to C-60 were also measured. The intermolecular compet ition due to deuterium substitution of both vinylic hydrogens at the beta - carbon of 3 exhibits a substantial inverse ct-secondary isotope effect k(H) /k(D) = 0.83 (per deuterium). Substitution with deuterium at both vinylic m ethyl groups of 5 yields a small inverse k(H)/k(D) = 0.94. These. results a re consistent with the formation of an open intermediate in the rate-determ ining step.