Thermal and photochemical decomposition pathways of trinitromethylarenes .1. The conversion of ArC(NO2)(3) to ArNO2 - A rationalization of apparent solvent effects

The thermal and photochemical decomposition of trinitromethylarenes in an inert solvent (dichloromethane or acetonitrile) has been investigated, using 1-methoxy-4-trinitromethylnaphthalene (2) and 2-trinitromethyl-4-chloroanisole (6) as representatives of reactive and unreactive ArC(NO2)(3), respectively. Compound 2 underwent slow thermal decomposition in both solvents to give 4-methoxy-1-naphthoic acid (3) and 1-methoxy-4-nitronaphthalene (5). The reaction was speeded up by additives, such as 1-methoxynaphthalene or nitrous acid. The decomposition of 2 was strongly accelerated by irradiation with light of lambda>430 nm. Both the thermal and photochemical processes were faster in acetonitrile than in dichloromethane. Spin trapping experiments resulted in the trapping of ArCOO. radicals, in both the thermal and photochemical reactions, indicating that the decomposition reaction is at least partially of radical nature. The acid 3 was shown to undergo fast thermal nitrodecarboxylation by treatment with NO2 in dichloromethane. Compound 6 exhibited similar behaviour, except that the rates were much slower, less than 100 times slower than those of 2. The mechanism suggested involves an initial nitro-->nitrito rearrangement of ArC(NO2)(3), followed by homolytic decomposition of the nitritodinitromethylarene formed. The finding that trinitromethylarenes have differing stabilities, depending on the nature of the ring substituent(s) and/or the solvents used, offers a reasonable explanation for the chemoselectivity of tetranitromethane-ArH photolyses.