Orbital Symmetry Control in the Photochemistry of trans-2,3-Diphenyloxirane

We report quantum yields following 266 nm photolysis of trans-2,3-diphenyloxirane (TDPO) for the formation of the trans-ylide (Scheme 1, Phi = 0.099 +/- 0.014), cis-2,3-diphenyloxirane (CDPO, Phi = 0.10 +/- 0.009), benzaldehyde (Phi = 0.47 +/- 0.04), and deoxybenzoin (Phi = 0.077 +/- 0.002). Photolysis of TDPO may lead to both orbital symmetry allowed and forbidden products, but the trans-ylide decays solely via an orbital symmetry predicted pathway to CDPO. Coupling our quantum yields to results of Das and co-workers,(1) we report an extinction coefficient for the trans-ylide of 9.4 x 10(4) M(-)(1) cm(-)(1) at 470 nm. We also report pseudo-first-order rate constants for the reaction between maleic anhydride and the trans-ylide in acetonitrile (k(Q) = 3.32 +/- 0.04 x 10(9) M(-)(1) s(-)(1)) and in cyclohexane (k(Q) = 5.36 +/- 0.07 x 10(9) M(-)(1) s(-)(1)) and between fumaronitrile and the trans-ylide in acetonitrile (k(Q) = 1.57 +/- 0.02 x 10(9) M(-)(1) s(-)(1)) and cyclohexane (k(Q) = 3.69 +/- 0.04 x 10(9) M(-)(1) s(-)(1)). We report the crystal structure of rac-(2R,3R,4R,5R)-3,4-dicyano-2,5-diphenyltetrahydrofuran, the sole product of 1,3-dipolarophilic addition between the trans-ylide and fumaronitrile in cyclohexane. A brief discussion of solvent and steric effects in 1,3-dipolarophilic additions is included.