Kinetic Studies of the Photoreaction of Benzophenone with Furan

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Abstract
The photoreaction of benzophenone with furan was studied kinetically in a benzene solution. A reaction mechanism was proposed, and the rate constants of elementary reactions were determined. Benzophenone gives primarily oxetane with furan through the n-π* lowest triplet state. Benzophenone reacts with the oxetane initially formed to afford a 2 : 1-adduct or to decompose that to the starting materials. The ratio of the addition to the decomposition is 1 : 4. The simplest scheme for the direct cycloaddition of a benzophenone triplet and an alternate scheme involving a biradical intermediate are discussed; the second scheme is more acceptable. The rate of the formation of the intermediate is considerably slower in a benzophenone-furan system than in a benzophenone-ketenimine system (ca. 1/10). The formation of oxetane is greatly dependent on the ratio of the bond-forming to the bond-breaking of the biradical intermediates, which are efficiently formed (4.2×107 m−1 sec−1 for furan and 4.0×107 m−1 sec−1 for oxetane).