Making a vinyl-trimethylenemethane precursor through the addition of diethyl azodicarboxylate to tropone

Abstract

Diethyl 2-oxo-6,7-diazabicyclo[3.2.2]nona-3,8-diene-6,7-dicarboxylate (11) has been prepared by the addition of diethyl azodicarboxylate to tropone. The 3,4-double bond was protected by reaction with triethyl orthoformate–ethanol–tosic acid (toluene-p-sulphonic acid), the 8,9-double bond reduced, and the enone unit deprotected with water–dioxane–tosic acid. The resultant enone was converted into the conjugated diene by reaction with methyl-lithium and dehydration. Hydrolysis of the carbamate groups (KOH–MeOH) and oxidation (HgO) gave 2-methylene-6,7-diazabicyclo[3.2.2]nona-3,6-diene (9). UV irradiation of a glassy solution of this diazene at 77 K gave the first observable triplet ESR spectrum of a vinyl-trimethylenemethane biradical, 2-methylenecyclohept-3-ene-1,5-diyl (10). Despite the fact that this non-Kekulé polyene has a triplet ground state, the singlet biradical reacts as such to give a mixture of 7-methylenebicyclo[4.1.0]hept-2-ene (28), 2-methylenebicyclo[3.2.0]hept-3-ene (29), and 3-methylenehepta-1,4,6-triene (30) faster than it undergoes intersystem crossing. Several interesting contrasts exist between the chemistry of the bridged vinyl-TMM precursor (9) and the equivalent bridged TMM precursor, 7-isopropylidene-2,3-diazabicyclo[2.2.1]hept-2-ene (4). In the former case, pyrolysis proceeds via the singlet biradical to give mainly monomeric products. There is no CIDNP effect and trapping with alkenes gives low yields of cycloadducts (Scheme 1). In the latter case, pyrolysis gives dimeric products viathe triplet biradical. There is a strong CIDNP effect and in the presence of alkenes high yields of cycloadducts are obtained (Scheme 2).