DI-π-METHANE HYPERSURFACES AND REACTIVITY; MULTIPLICITY AND REGIOSELECTIVITY; RELATIONSHIP BETWEEN THE DI-π-METHANE AND BICYCLE REARRANGEMENTS: MECHANISTIC AND EXPLORATORY ORGANIC PHOTOCHEMISTRY12

Abstract

The photochemistry of l,l-dicarbomethoxy-3,3,5,5-tetraphenyl-l,4-pentadiene and 3,3-dicarbomethoxy-l,l,5,5-tetraphenyl-l,4-pentadiene was investigated. Both direct and sensitized photolyses of the 3,3-diester afforded l,l-dicarbomethoxy-2,2-diphenyl-3-(2',2'-diphenylvinyl)cyclopropane with quantum yields of 0.42 and 0.32, respectively. Direct irradiation of the 1,1-dicarbomethoxy-pentadiene gave this same diphenylvinyl-cyclopropane product (Ø = 0.39), while acetophenone sensitization of the 1,1-dicarbomethoxy-pentadiene led to the regioisomeric l,l,2,2-tetraphenyl-3-(2',2'-dicarbomethoxyvinyl) cyclopropane (Ø = 0.92). Thus the unsymmetrically substituted cyclopropyldicarbinyl diradical opened one or the other of two three-ring bonds depending on multiplicity. Direct irradiation of the dicarbomethoxyvinylcyclopropane resulted in a bicycle rearrangement affording the diphenylvinylcyclopropane (Ø = 0.10); additionally, the 1,1-dicarbomethoxy-pentadiene was formed (Ø = 0.069) in this photolysis. The second product can be viewed as arising from a reverse di-π-methane rearrangement or, alternatively, as deriving from partial bicycling of the diphenylcarbon moiety. To confirm that the bicycling process was intramolecular rather then by disengagement of a diphenylcarbene followed by readdition, a crossover experiment was run using l,l-dicarbomethoxy-4,4-di-p-tolylbutadiene. However, no diphenylcarbene transfer was seen. Single photon counting was employed to obtain unimolecular rate constants for the S! excited states undergoing the di-π-methane rearrangements. A plot of the log of the S1 rates for centrally disubstituted l,l,5,5-tetraphenyl-l,4-pentadienes—having central carbomethoxy, cyano or methyl groups—vs Hammett ground state sigma constants proved linear with a rho value of —2.53. The various excited states, intermediate species and the related hypersurfaces were subjected to SCF and SCF-CI calculations. The delta-P treatment showed that excitation is concentrated in the cyclopropyldicarbinyl diradical moiety as the di-7r-methane rearrangement proceeds. The calculations also suggest that for a successful bicycle rearrangement excitation energy needs to be distributed into the three-ring bond not conjugated with the vinyl group. Triptych correlation diagrams were constructed using SCF and SCF-CI energies and are used to interpret the photochemistry. The differing singlet versus triplet regiochemistry was found to follow the "small K vs large K" generalization used previously.