Thermal ring transformation of 3H-1,2-diazepines to 4-alkenyl-1H-pyrazoles via ring contraction and sigmatropic vinyl group migration

Abstract

3H-1,2-Diazepines (9) undergo a thermally induced ring contraction to 4-alkenyl-1H-pyrazoles via a 1,3-azogroup migration and further rearrangements. The reaction can be rationalised as a multi-step process in which the initial azo-group migration gives an unstable 3-alkenyl-3-methyl-3H-pyrazole which is rapidly converted to a 4H-pyrazole by a [1,5] sigmatropic shift of the alkenyl group and finally to the product via a proton shift to nitrogen. The ring contraction step in which the azo-group migrates is thought to take place by a two-step mechanism involving either a diradical or a dipolar intermediate. Some evidence from product ratios disfavours the former.