ALKENE TRIPLETS AS 1,2-BIRADICALS - THE PHOTOADDITION OF P-ACETYLSTYRENE TO STYRENE

The photoaddition of p-acetylstyrene (PAS) triplet to styrene (STY) ha s been studied. Direct irradiation of PAS in the presence of styrene y ielded trans-1-(4-acetylphenyl)-2-phenylcyclobutane, an expected addit ion product, its cis isomer, and 1-(4-acetylphenyl)-1,2,3,4-tetrahydro naphthalene. The rate of addition of (3)PAS to styrene was determined by laser flash photolysis to be 3.2 X 10(6) M(-1) s(-1), and by quantu m yield measurements as 2.0 X 10(6) M(-1) s(-1). The rate is 1 order o f magnitude greater than that of addition of a primary alkyl radical t o styrene and thus in only fair agreement with the 1,2-biradical model . The reaction of 3PAS with p-methoxystyrene is even faster. A rough e stimate of the rate constant for (STY)-S-3 + STY is in better agreemen t with expectation. The possible involvement of a weak exciplex betwee n the perpendicular PAS triplet and the styrene ground state is sugges ted to rationalize the rate enhancement. The validity of the 1,2-birad ical representation of alkene triplets is discussed.