Kinetics and mechanism of photocycloaddition of deoxyuridines to 2,3-dimethyl-2-butene.

The mechanism of photocycloaddition of 2'-deoxyuridine (1a) and thymidine (1b) to 2,3-dimethyl-2-butene (Bu) in acetonitrile by UV irradiation has been studied. The reciprocal quantum yield for the cycloaddition increased linearly with reciprocal concentrations of Bu in acetonitrile to give limiting quantum yields at infinite concentration of Bu as 0.030 and 0.0096 for 1a and 1b, respectively. This shows that the cycloaddition proceeds in a two-step mechanism between the triplet state of 1 and Bu through biradical intermediates. Addition of cis-1,3-pentadiene quenched the reaction obeying the Stern-Volmer equation. The above quenching experiments and laser transient spectroscopy revealed that the triplet state of 1a reacts with Bu with much larger rate constant (1.3-1.6 x 10(9) M-1 s-1) than that of 1b (4-5 x 10(7) M-1 s-1) reflecting larger steric hindrance exerted in the reaction of 1b than that of 1a.