2828-00-4Relevant academic research and scientific papers
The Pyrolysis of 4-Alkylidene-3,3,5,5-tetramethyl-1-pyrazolines
Bushby, Richard J.,Jesudason, Malini V.,Pollard, Michael D.
, p. 2655 - 2662 (2007/10/02)
The pyrolysis of 4-alkylidene-3,3,5,5-tetramethyl-1-pyrazolines normally gives alkylidene-2,2,3,3-tetramethylcyclopropanes and at higher temperature or longer reaction times conjugated dienes .In some cases interesting variations occur.Foe example, 3,3,5,5-tetramethyl-1-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 HCl elimination to give 2,5-dimethylhexa-1,5-dien-3-yne (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 seems 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 radical-stabilizing 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).
