PHOTO-CHEMICAL SERIES .116. MOLECULAR CONTROL OF EXCITED CROSS-CONJUGATED TRIENE REARRANGEMENTS - EXPLORATORY AND MECHANISTIC ORGANIC-PHOTOCHEMISTRY

The photochemistry of 1-dicyanomethylene-4, 4-diphenyl-2, 5-cyclohexadiene and 1-diphenylmethylene-4, 4-diphenyl-2, 5-cyclohexadiene was investigated for comparison with 4, 4-diphenylcyclohexadienone, l-methylene-4, 4-diphenyl-2, 5-cyclohexadiene, and l-methylene-4, 4-dimethyl-2, 5-cyclohexadiene. All of the trienes rearranged via their singlet excited states but had unreactive triplets. The diphenyldicyanotriene rearranged with phenyl migration to give αα-dicyano-3, 4-diphenyl-toluene. In contrast, the tetraphenyltriene rearranged with vinyl-vinyl bridging in a Type A rearrangement to give 2-diphenylmethylene-6, 6-diphenylbicyclo[3.1.0]hex-3-ene. cis-and trans-2-dicyanomethylene-5, 6-diphenylbicyclo[3.1.0]hex-3-ene and 2-dicyanomethylene-6, 6-diphenylbicyclo[3.1.0]hex-3-ene were synthesized as potential intermediates in the rearrangement of the diphenyldicyanotriene. Only the first two bicyclic dienes gave solely the same product while the 6, 6-diphenylbicyclic diene led to both αα-dicyano-2, 3-diphenyltoluene and αα-dicyano-3, 4-diphenyltoluene with the former predominating 2:1. The 5, 6-diphenylbicyclic dienes were excluded as intermediates in the rearrangement of the diphenyldicyanotriene using a tracer experiment. This employed the photolysis of a mixture of tritiated diphenyldicyanotriene and nonradioactive 5, 6-di-phenylbicyclic diene; after partial conversion, recovered 5, 6-diphenylbicyclic diene was nonradioactive. Sensitization of the bicyclic dienes gave no reaction in the case of the 6, 6-diphenylbicyclic diene and only cis-trans isomerization from the 5, 6-diphenyl stereoisomers. The rates of excited singlet rearrangement of the diphenyldicyanotriene (1kr= 2.76 X 109 s-1) and the tetraphenyltriene (1kr = 3.50 X 1010 s-1) were determined by single photon counting. A theoretical treatment of the systems studied is offered with SCF-CI calculations along the various reaction coordinates. The effect of phenyl substitution on the exomethylene group of the triene is to alter vinyl-vinyl bridging from an excited state forbidden to an allowed process and the effect is interpreted. The SCF-CI calculations of potential energy surfaces correctly predict the observed photochemical preferences. The intervention of ground-state zwitterions analogous to those of ketone photochemistry is discussed. © 1979, American Chemical Society. All rights reserved.