Relative Rates of Bond Rotation and Ring Closure in the Photocycloaddition Intermediates from Fullerene-C60 and the Isomeric 2,4-Hexadienes

Photocycloaddition of fullerene-C₆₀ to the isomeric 2,4-hexadienesreported by Orfanopoulos and co-workers gave a series of 2 + 2 cycloadducts in which bond rotation had occurred in some products. Relative rates of bond rotation and ring closure in the biradical intermediates were calculated from the product distributions using equations developed by Bartlett and co-workers. These data are reinterpreted to show that the ratio of the rate of bond rotation in the intermediate to the rate of ring closure to the cyclobutane product is ~70 for the trans-1-propenyl intermediate formed in the initial cis conformation. Data are inconsistent for the analogous cis-1-propenyl intermediate. The relative rate for the trans-1-propenyl intermediate formed in the initial trans conformation is ~9. Accurate relative rate determinations are highly dependent on very accurate product distribution determinations.