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

Abstract

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.