The stereochemistry of thermolysis of 4-alkylidene-1-pyrazolines

Abstract

. Can. J. Chem. 56,998 (1978). The products of thermolysis of 11 4-alkylidene-I-pyrazolines were isolated and identified. It is observed that these products cannot be rationalized in terms of a planar singlet trimethyleneniethane type of intermediate, nor can they be reconciled in terms of the orthogonal triniethylenemethane singlets. Evidence is presented that for 4-ethylidene-I-pyrazolines the carbon utlti t o the methyl has the greatest propensity to become a cyclopropane carbon, and that which is sytl is most likely t o become the exocyclic niethylene.ROBERT J. CRAWFORD, HIROKAZU TOKUNAGA, LOUIS M. H. C. SCHRIJVER et JACKY C. GODARD. Can. J. Chem. 56.998 (1978).O n a isole et identifie les produits de thermolyse de I I alkylidene-4 pyrazolines-I. O n a observe qile ces produits ne peilvent &tre expliques en ternie d'un intermediaire singulet planaire de type triniCthylenemtthane et qu'ils ne peilvent non plus &tre reconciliks en terrne de singulets orthogonaux trimethylenemethane. O n presente des donnees B I'effet que, dans I'ethylidene-4 pyrazoline-I, le carbone ntiti par rapport ail groupe niethyle possede Ic plus grand pouvoir de devenir un carbone du cyclopropane et que c e l i qui se trouve en . S~I I a le plus de chance d e devenir un methylkne exocyclique.[Traduit par le journal]Over the past decade Dowd (1) has demonstrated that planar trimethylenemethane has a triplet ground state and can be generated either by the photolysis of 3-inethylenecyclobutanone or 4-methylene-1-pyrazoline. Structurally modified 4-inethylene-I-pyrazolines such as the bicyclic system (38) studied by Berson and collaborators (2) have been demonstrated to produce both singlet and triplet 1,3-diyls, by both photolysis and thermolysis, as evidenced by trapping experiments and by dimer formation. The kinetics of thermolysis of 4-methylene-1-pyrazoline (I) and its deuterium-and '3C-labelled derivatives have implicated a nitrogen free diyl as a possible intermediate (3). Theoretical studies (4) have suggested that for singlet triinethylenen~ethane the orthogonal diyls are more probable than is the planar species. We have shown that the product proportions from the thermolysis of 4-isopropylidene-1-pyrazoline (7)' and 3,3-dimethyl-4-methylene-1-pyrazoline (6) are easily' rationalized in terms of the intermediates 39a and 39b in Scheme 1. These intermediates were postulated on the basis of the least motion principle and the presumed cleavage o f both carbon nitrogen bonds as implicated from the kinetic studies o n 1 (3). The product proportions from deuterium labelled 1 cannot be.reconciled with a inechanisin analogous to Schei~le 1. Thus while we had kinetic data for 1 and product data for 6 and 7we were unable, because of tautomerism, t o get sound kinetic data for 6 and 7, a n d cannot, because of the possible excess vibrational energy (5), rely upon the product proportions from deuterated 1.This dilemma prevented us from drawing more definitive conclusions a s to the nature o f the intermediates involved. In an attempt to define...