BARRELENE TO SEMIBULLVALENE TRANSFORMATION . CORRELATION OF EXCITED-STATE POTENTIAL ENERGY SURFACES WITH REACTIVITY . MECHANISTIC AND EXPLORATORY ORGANIC PHOTOCHEMISTRY .44.

A photochemical transformation of barrelene was observed, wherein a C8H8 isomer is produced as the major product; cyclooctatetraene was obtained as a minor product. The structure of the C8H8 isomer was elucidated as that of bullvalene minus one vinyl bridge, and the compound was named “semibullvalene.” The process was demonstrated to proceed via the triplet of barrelene in a process subsequently shown to be general. The irradiation of semibullvalene afforded cyclooctatetraene. It was found that semibullvalene undergoes degenerate valence tautomerism at a faster rate than any previously studied system. Barrelene was labeled at all vinyl positions with deuterium, leaving only the bridgehead positions still bearing hydrogen. The method developed for deuteration of barrelene involved treatment with lithium N-deuteriocyclohexylamide in N,N-dideuteriocyclohexyl-amine; the method promises to be generally useful for preparing deuterated compounds. The locat on of the hydrogen label in the semibullvalene photolysis product was investigated and found to fit one of two reasonable mechanisms. The reaction mechanism was shown to proceed via a unique bicyclic, allylic, triplet biradical having finite lifetime. The rearrangement was considered from a theoretical viewpoint with the use of three-dimensional Hückel theory, and the results were used to correlate the excited-state potential energy surface with observed photochemical behavior. © 1969, American Chemical Society. All rights reserved.