OBSERVATION OF CATIONIC INTERMEDIATES IN THE PHOTOLYSIS OF 1-PHENYLCYCLOHEXENE

The photochemistry of cis-1-phenylcyclohexene and its m-methoxy derivative has been examined in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP). The only photoproduct is the Markovnikov adduct of the solvent, arising from an intermediate 1-arylcyclohexyl cation. This cation is observed as a transient species with laser flash photolysis, for both direct irradiation at 248 nm and benzophenone sensitized irradiation at 308 nm. As suggested in previous studies with phenylcyclohexene in other solvents, the mechanism involves protonation of a strained trans-1-arylcyclohexene formed from both the singlet and triplet excited states. The trans-cyclohexene is not observed in HFIP because of its very rapid protonation by this solvent. Direct protonation of the singlet excited state to give the cyclohexyl cation cannot be ruled out, but it is likely a minor pathway. There is, however, unambiguous evidence for protonation of the singlet, with the protonation occurring on the aromatic ring to give a 2-benzenonium ion. This cation is observed as a second transient in the flash photolysis experiments involving direct 248-nm irradiation while it is not detected with sensitized irradiation. The 2-benzenonium ion intermediate returns to parent cis-1-phenylcyclohexene, and thus results in no overall photoreaction. Its presence, however, is clearly demonstrated in experiments carried out in deuterated HFIP, where exchange of aromatic ring protons in unreacted cyclohexene is observed. In addition to reacting with the solvent, the 2-benzenonium 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 reaction involves the cyclohexene acting as the base, and it results in the formation of additional 1-phenylcyclohexyl cation. The occurrence of a sequence involving aryl ring protonation followed by proton transfer to unreacted phenylcyclohexene is demonstrated through the observation that a portion of the ether product formed in (CF3)2CHOD arises from the formal addition of (CF3)2CHOH.