Photochemical Transformations of Quintet m-Phenylenedinitrenes

Photochemical transformations of quintet dinitrenes have been studied for the first time by FTIR spectroscopy during the photolysis of 1,3-diazidobenzene and its 2methyl- and 2,4,6-trimethyl-substituted derivatives in solid argon at 13 K. The reactions involve two competitive processes. In the first one, the intramolecular insertion of the nitrene unit into the 1,6-aromatic bond induces ring-opening, which leads to the formation of aminoacetylenes. The second process involves the intramolecular addition of the nitrene unit to the 1,2-aromatic bond followed by ring-opening to form 4(2H-azirin-2-ylidene)-2-butenenitrile derivatives. Methyl groups in ortho-positions to the nitrene units of quintet dinitrenes efficiently protect their aromatic ring from nitrene attacks, substantially increasing the photochemical stability of such species. However, even quintet 2,4,6-trimethylphenylene-1,3-dinitrene decomposes to ring-opened products on extended irradiation. This indicates that quintet dinitrenes are much more photochemically reactive species than triplet nitrenes. Undesirable photochemical rearrangements of the latter at early stages of the photolysis of 1,3-diazidobenzenes are suppressed on introducing the methyl group in position 2 of these azides.