6604
J . Am. Chem. SOC.1985, 107, 6604-6609
Photocyclization of 2-Vinylbiphenyls and Related Compounds under Triplet Sensitization: An Adiabatic Reaction on the Triplet Surface S. Lazare, R. Lapouyade, and R. Bonneau* Contribution from U. A . 348 du CNRS, Laboratoire de Chimie- Physique A , UniversitZ de Bordeaux I , 33405- Talence, France. Received December 28, 1984
Abstract: The mechanism of the cyclization of 2-vinylbiphenyl derivatives under sensitization by the triplet state of xanthone has been studied by laser-flash photolysis. The first step of the mechanism is an adiabatic reaction giving a cyclized polyene in the triplet state which decays to the ground state with a lifetime in the range 550-750 ns. From the effect of oxygen on the yield of formation of the polyene, the rate constant of cyclization of the planar triplet is found to be around 3 X lo8 SKI. When a 90’ twist of the vinylic double bond is allowed by the molecular structure, the resulting perpendicular triplet gives a nonstereospecific cyclization which occurs, most probably, after distortion of the biphenyl moiety. In some cases, a transient absorption assigned to the triplet state of an unreactive rotamer of the arylethylenic compounds has been observed, and the effect of rotamers on the reaction quantum yield is briefly discussed.
Upon direct excitation, 2-vinylbiphenyls and structurally related compounds undergo a cyclization reaction to give dihydrophenanthrene derivatives in a quantitative chemical yield. This photochemical reaction has been used for the synthesis of several aromatic compounds.’” A flash-photolysis study revealed that the first step of the photocyclization is the formation of a cyclic p ~ l y e n e .This ~ primary photoproduct may then undergo a 1,s hydrogen shift to give the dihydrophenanthrene derivative or may be dehydrogenated in a substituted phenanthrene by reaction with a n oxidizing agent such as iodine. The quantum yield of the reaction greatly depends on the structure of the molecule and can be rather large: for instance 4 = 0.24 for 1, 0.61 for 2, and 0.33 for 12.’ Unlike the photocyclization of cis-stilbene and other related 1,2-diarylethyIenes with an hexatrienic subsystem, which is known to occur only from the excited singlet,6 the cyclization of many molecules containing the 2-vinylbiphenyl moiety can be triplet sensitized with a large quantum yield:7 for instance, 4 = 0.58 for 1 and 0.89 for 2. Similar triplet sensitized photocyclizations have been reported, a long time ago, for 2-biphenylyl isocyanates and 2-biphenylyl imines,* heteroatomic analogues of the 2vinylbiphenyls, but with much lower quantum yields. There are two striking differences between the direct and sensitized photocyclization of 2-vinylbiphenyl derivatives: (a) for some compounds, giving an efficient cyclization under direct excitation, the sensitized reaction is quite ineffi~ient,~ for instance, 4
