Bimolecular cyclization of di-p-tolylamine to 2,7-dimethylacridine in the presence of bromoform: microsecond flash photolysis study

The UV irradiation of di-p-tolylamine and bromoform results in the formation of 2,7-dimethylacridine (with a quantum yield of 0.20 ± 0.04). The primary transient spectrum (λmax ≈ 700 nm), recorded following microsecond flash lamp excitation, can be attributed to the amine cation radical and, probably, the neutral diarylnitrogen radical, which arises from the former on deprotonation. Both of these radicals can recombine with the dibromomethyl radical to give the intermediate IM1. This is transparent in the visible region of the spectrum and is transformed into the second intermediate IM2 (λmax ≈ 540 nm), which finally gives rise to acridine. The observed first-order rate constant of IM2 formation is dependent on amine concentration, indicating the participation of an additional amine molecule in the transformation of IM1 to IM2. A new reaction scheme is proposed which assumes that the attack of the alkyl radical is predominantly directed towards the nitrogen atom of the amine radical.