Photobenzidine Rearrangements. 6. Mechanism of the Photodecomposition of 1,4-Diaryl-1,4-dialkyl-2-tetrazenes

Article abstract of DOI:10.1021/jo01310a036

The photodecomposition of the tetrazene p-XC6H4N(Me)N=N(Me)NC6H4Y-p (1e,X=Y=Me) in dimethoxyethane (DME) gave 47.4percent of p-XC6H4(Me)NN(Me)C6H4Y-p (2e,X=Y=Me) and 39.6 percent of N-methyl-p-toluidine. When irradiation was carried out in the presence of increasing initial concentrations of n-BuSH, the yield of 2e fell and levelled off at 6percent.Similar experiments in cyclohexane showed that the yield of 2e fell from 45.1percent and leveled off at 10percent.The data indicate that 1e decomposes by a radical pathway and that the 2e is formed partly within and partly outside of a solvent cage. Similar studies with 1d(X=Me, Y=CO2Et) in DME gave three hydrazines: 2d(X=Me,Y=CO2Et) in 13percent and 14.7percent yield, 2e in 7.3percent and 8.0percent yield,and 2f(X=Y=CO2Et) in 14.9percent and 21.2percent yield. The formation of three hydrazines again indicates the formation and intermolecular recombination of methylarylamino radicals. Irradiation of 1d in DME solutions containing n-BuSH caused a fall in the yield of 2e to zero and a leveling off in the yield of 2d to 6percent. The yield of 2f also fell but could not be monitored at high concentrations of n-BuSH because of overlapping high-pressure LC peaks. The results with 1d are also consistent with a cage-recombination process (for 2d) and an intermolecular recombination of radicals (for 2d-f). The methylarylamines p-XC6H4NHMe and p-YC6H4NHMe (X=Me; Y=CO2Et) were also formed from 1d. A sixth product was the bis(arylamino)methane p-YC6H4NHCH2NHC6H4Y-p(4f, Y=CO2Et) in 14-32percent yield (three runs). The origin of 4f is believed to be the disproportionation of radicals p-YC6H4NMe, giving p-YC6H4NHMe and p-YC6H4N=CH2(7f). Hydrolysis of 7f (by small amounts of water in the solvent) to p-YC6H4NH2 (6f) and HCHO followed by addition of 6f to 7f would give 4f. HCHO was found as a volatile product after irradiation. The formation of 4f is further evidence for the formation and intermolecular reaction of arylamino radicals in the photodecomposition of 1,4-dialkyl-1,4-diaryl-2-tetrazenes.