The photochemistry of the 4-cyanobenzoic acid esters of trans- and cis-2-phenylcyclohexanol

Irradiation of trans-2-phenylcyclohexyl 4-cyanobenzoate (4) in methanol res ults in formation of the 1-phenylcyclohexene and 4-cyanobenzoic acid as the major products. The mechanism proposed for this example of the Norrish Typ e II-like reaction, normally inefficient for the pi,pi* state of esters, be gins with intramolecular electron transfer in the excited singlet state. Th e next step, intramolecular proton transfer, is highly exergonic as a resul t of the increased acidity of the benzylic hydrogen of the radical cation a nd the increased basicity of the carbonyl oxygen of the radical anion. The final step is cleavage of the 1,4-biradical. This mechanism is proposed on the basis of product studies, absorption spectra, fluorescence spectra, oxi dation and reduction potentials, and nanosecond laser flash photolysis spec tra. The stereochemical requirements for the process have also been examine d using solution NMR, the X-ray structure, and MM3 calculations. The stereo isomer, cis-2-phenylcyclohexyl 4-cyanobenzoate (5), undergoes cis to trans isomerization before photofragmentation. Photolysis of the cis isomer in me thanol-OD demonstrated that the cis to trans isomerization occurred by inve rsion at C1.