The pyrolysis of 4-alkylidene-3,3,5,5-tetramethyf-1-pyrazolines

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Abstract
The pyrolysis of 4-alkylidene-3,3,5,5-tetramethyf-1-pyrazolines [e.g. compound (1)] normally gives alkylidene2,2,3,3-tetramethylcyclopropanes [e.g. compound (2)] and at higher temperatures or longer reaction times conjugated dienes [e.g. compound (3)]. In some cases interesting variations occur. For example 3,3,5,5-tetramethyl1-pyrazolin-4-ylidenemalonitrile (22) gives (eventually) 3-cyano-4-isopropenyl-5-methylpyridine (27) and in the dichloro-substituted series a rearrangement from 2,2-dichloro-3,3-dimethylisopropylidenecyclopropane (30) to 3,4-dichloro-3,5-dimethylhexa-2,4-diene (31) and a double HCI elimination to give 2,5-dimethylhexa-1,5-dien-3yne (32) are observed. The kinetics of nitrogen elimination have been determined for ten differently substituted pyrazolines (33). The reactions have positive entropies of activation and hence seem to pass through a singlet rather than (as suggested in analogous systems) a triplet intermediate but it is difficult to be sure whether this is a TMM biradical (simultaneous cleavage of both C–N bonds) or a diazenyl biradical (sequential cleavage of the C–N bonds). It is, however, interesting to note that there is no correlation between the reaction rate and the radicalstabilising ability of the substituents X and Y on the alkylidene group. This, in turn, suggests little or no rotation of the CMe2 groups at the transition state and possibly the formation of a bis-orthogonal TMM (47) or a monoorthogonal diazenyl biradical (48).

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