OBSERVATION OF CATIONIC INTERMEDIATES IN THE PHOTOLYSIS OF 1-PHENYLCYCLOHEXENE

The photochemistry of cis-1-phenylcyclohexene and its m-methoxy deriva tive has been examined in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFI P). The only photoproduct is the Markovnikov adduct of the solvent, ar ising from an intermediate 1-arylcyclohexyl cation. This cation is obs erved as a transient species with laser flash photolysis, for both dir ect irradiation at 248 nm and benzophenone sensitized irradiation at 3 08 nm. As suggested in previous studies with phenylcyclohexene in othe r solvents, the mechanism involves protonation of a strained trans-1-a rylcyclohexene formed from both the singlet and triplet excited states . The trans-cyclohexene is not observed in HFIP because of its very ra pid protonation by this solvent. Direct protonation of the singlet exc ited state to give the cyclohexyl cation cannot be ruled out, but it i s likely a minor pathway. There is, however, unambiguous evidence for protonation of the singlet, with the protonation occurring on the arom atic ring to give a 2-benzenonium ion. This cation is observed as a se cond transient in the flash photolysis experiments involving direct 24 8-nm irradiation while it is not detected with sensitized irradiation. The 2-benzenonium ion intermediate returns to parent cis-1-phenylcycl ohexene, and thus results in no overall photoreaction. Its presence, h owever, is clearly demonstrated in experiments carried out in deuterat ed HFIP, where exchange of aromatic ring protons in unreacted cyclohex ene is observed. In addition to reacting with the solvent, the 2-benze nonium ion is efficiently quenched by unphotolyzed phenylcyclohexene, with a rate constant of 1.4 X 10(9) M-1 s-1 at 20-degrees-C. This reac tion involves the cyclohexene acting as the base, and it results in th e formation of additional 1-phenylcyclohexyl cation. The occurrence of a sequence involving aryl ring protonation followed by proton transfe r to unreacted phenylcyclohexene is demonstrated through the observati on that a portion of the ether product formed in (CF3)2CHOD arises fro m the formal addition of (CF3)2CHOH.