Photocyclization of 2-vinyldiphenylacetylenes and behavior of the isonaphthalene intermediates

Abstract

The conformation, electronic structure, spectroscopy, and unimolecular photoisomerization of 2-vinyldiphenylacetylene and two derivatives have been investigated. 2-Vinyldiphenylacetylene exists predominantly in a planar anti conformation. Introduction of an α-methyl substituent results in increased phenyl–vinyl dihedral angles for both syn and anti conformers, whereas a cyclic analog is constrained to a syn conformation with a large phenyl–vinyl dihedral angle. All three molecules undergo photocyclization to yield unstable cyclic allene (isonaphthalene) intermediates which undergo further reactions leading to stable products. Both the photocyclization process and behavior of the allene intermediate are dependent upon ground state conformation. The photophysical behavior of the 2-vinyl derivative, namely its short singlet lifetime and low fluorescence quantum yield, is similar to that of diphenylacetylene. It also has a low quantum yield for photocyclization. The 2-isopropenyl derivative and conformationally locked cyclic analog have relatively long singlet lifetimes and large quantum yields for fluorescence and cyclization. The difference in excited state behavior of the planar 2-vinylacetylene and its non-planar analogs is attributed to the effect of the phenyl–vinyl dihedral angle on the barriers for activated decay of the linear singlet state. However, the behavior of the 2-isopropenyl derivative does not appear to be dependent upon ground state conformation (synvs.anti). The cyclic allene intermediates undergo sequential protonation–deprotonation in methanol solution to yield stable products. The 2-vinyl derivative yields only the fully aromatized 2-phenylnaphthalene. However, the 2-isopropenyl and cyclic derivatives yield mixtures of fully and partially aromatized products. Preferential formation of the partially aromatized products is attributed to a stereoelectronic effect on the deprotonation step. In diethyl ether solution only the fully aromatized product is formed via a free radical mechanism.