Electrocyclic aromatic substitution by the diazo group. Part 2. Ring size effects on the cyclisation of 1-aryl-3-diazoalkenes: a new rearrangement of 3H-pyrazoles to 3H-1,2-benzodiazepines

Abstract

The reactions of the unsaturated diazo compounds derived from the tosylhydrazone salts of 1-acyl-2-arylcycloalkenes have been studied and the results compared with the analogous reactions of 1-diazo-2-diarylmethylenecycloalkanones. In 1-aryl-2-(1-diazoethyl)cyclopentenes (18) the cyclopentyl ring has a less inhibiting effect on the 6π-electron cyclisation than in (2), and compounds (18) are the first 1 -aryl-3-diazo-alkenes to undergo both 6π- and 8π-electron cyclisation to give the 3H-pyrazoles (20) and 3H-1,2-benzodiazepines (21). The strained pyrazoles (20) undergo a novel thermal ring transformation to give the diazepines (21) rather than the usual van Alphen–Huttel rearrangement of 3H-pyrazoles. An increase in the size of the annelated ring results in the usual preference for 6π-electron cyclisation, e.g. 1-phenyl-2-(1-diazoethyl)cyclohexene (24) gives a 3H-pyrazole (25) which on heating does not ring expand to a diazepine but undergoes an unusual van Alphen-Huttel rearrangement involving shift of an alkyl- rather than an aryl-group. 1-Phenyl-2-(1-diazophenylmethyl)cyclopentene (32), 3phenyl-2-(1 -diazoethyl) indene (35), and 1-methyl-2-(1-diazoethyl)cyclopentene (37) do not cyclise but react only via loss of nitrogen to give carbene-derived products.