Photochemistry of organic nitrogen compounds. Part IV. The photolysis of 4-alkylidene-1-pyrazolines: a possible route to trismethylenemethyl diradicals

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Abstract
4-Chloromethylene-3,3,5,5-tetramethyl-1-pyrazoline, on irradiation (λ > 290 mµ), photolyses by a singlet mechanism to give 3-chloromethylene-1,1,2,2-tetramethylcyclopropane as the major product and 2-chloro-3-isopropylidene-1,1-dimethylcyclopropane as the minor product. The latter predominates on photosensitisation with sensitisers of high triplet excitation energy. Studies with a range of sensitisers, employing product composition as a criterion of mechanism, show that the triplet excitation energy of the pyrazoline is greater than 55 kcal./mole, and possibly even as high as 65 kcal./mole. The results are interpreted in terms of diradical intermediates including a planar delocalised π-system of the trismethylenemethyl(2-methylenetrimethylene) type, the proportions of the two cyclopropanes being governed by competition between bond-rotation and intramolecular cyclisation. Cyclisation necessitates spin-inversion in the triplet case, and hence photosensitisaton yields a more nearly statistical distribution of products than does direct photolysis. Piperylene has been found to quench singlet excited pyrazoline. Quantitatively, the quenching effects on photolysis and fluorescence of the pyrazoline are identical, and follow the Stern–Volmer mechanism up to 1M-diene. The two methylenecyclopropanes are also produced on pyrolysis (200°) of the 4-alkylidene-1-pyrazoline, but the very rapid thermal interconversion of the products at 200° has so far precluded definite comparison with the photochemical reaction. Some parallel experiments with a related 4-alkylidene-1-pyrazoline are also described.

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