1,2-shifts of hydrogen atoms in aryl radicals

An energy barrier on the order of 60 kcal/mol is predicted for the 1,2-shift of hydrogen atoms in aryl radicals. Such rearrangements are, therefore, not expected to occur under ordinary laboratory conditions, but they should be prevalent in the aryl radicals formed during combustion, flash vacuum pyrolysis, and other high-temperature gas-phase processes. As a demonstration of this rearrangement, the 2-benzo[c]phenanthryl radical (1) was generated by flash vacuum pyrolysis of the corresponding aryl bromide (9). A 1,2-shift of hydrogen out of the sterically congested cove region of 1, followed by cyclization and rearomatization of the resulting radical (Scheme 1), is proposed to explain the observation of benzo[ghi]fluoranthene (4) as the dominant monomeric product formed. Under the same conditions, [1,3,4,5-H-2(4)]-2-bromobenzo[c]phenanthrene (13) gives [1,2,3,4-H-2(4)]-benzo[ghi]fluoranthene (15) as the dominant monomeric product (Scheme 6), in accord with the expectation of a deuterium atom 1,2-shift.