Thermal ring transformation of 3H-1,2-diazepines to 4-alkenyl-1H-pyrazoles via ring contraction and sigmatropic vinyl group migration
- 1 January 1979
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Perkin Transactions 1
- p. 2730-2735
- https://doi.org/10.1039/p19790002730
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.Keywords
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