Photochemical nitration by tetranitromethane. Part VII. Mode of formation of the nitro substitution products from 1,4-dimethylnaphthalene in dichloromethane and acetonitrile

Photolysis of tetranitromethane and 1,4-dimethylnaphthalene at low temperature in dichloromethane gives predominantly addition products, trans- and cis-1,4-dimethyl-1-nitro-4-trinitromethyl-1,4-dihydronaphthalene (1 and 2, together 90%), in addition to the side-chain nitration product (3, 7%). At higher temperature, 3 becomes the major product [ratio (1 + 2): 3 30 : 65]. With external acid present during the run, partial formation of the 2-nitro substitution product (4) was an additional feature.In acetonitrile, the phenomenology is similar, except that the corresponding nitrito adducts were also present at the beginning of the reaction (maximally 20%) and disappeared toward the end; the 2-nitro substitution product also appeared toward the end of the run at higher temperature.It was shown separately that pure 1 rearranges to an equilibrium mixture with 2 in both acetonitrile and dichloromethane (rate constants 0.11 and 8 × 10–4 min–1, respectively). From 2, the corresponding nitrito adducts are slowly formed in acetonitrile via a, presumably, homolytic nitro/nitrito rearrangement. The latter reaction has the 1-trinitromethylnaphthalenyl radical as the intermediate, capable of existing in equilibrium with both trinitromethanide ion/(1,4-dimethylnaphthalene)˙+ and trinitromethyl radical/1,4-dimethylnaphthalene. Chemical consequences of these equilibria were found in both acid/base promoted reactions and trapping of trinitromethyl radical by the spin trap, α-phenyl-N-tert-butylnitrone.‡ The 2-nitro substitution product 4, in all probability, was formed as a consequence of acid-induced reactions of 2, indicating that in the photochemical experiments 4 is also formed via this pathway.