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A study of the pressure dependence of the C, products from the reaction of cis-butene-2 and methylene is reported. Methylene was produced by the photolysis of diazomethane with 4358 A light at 23" or 56", and by photolysis of ketene with 3200 A radiation at 23" or 100". The change with increasing pressure of the relative amounts of the characteristically "triplet products" (trans-1,2-dimethylcyclopropane, trans-pentene-2 (TP2), and 3-methylbutene-1 (3MB 1)) and "singlet products" (cis-1,2-dimethylcyclopropane (CDMC) and cis-pentene-2 (CP2)) are discussed. The behavior is reminiscent of that found in 3CH2cis-butene-2 systems and can be interpreted in terms of the rapid rate of rearrangement of an initial triplet diradical product component, due to 3CH2, relative to the slower rate and readier collisional stabilization of an initial vibrationally-excited dimethyl cyclopropane product component, due to lCH, . Relative rates of reactions of 'CH, with allylic CH:vinyl CH:C =C in the neat liquid were, for diazomethane, 1 : 1.1 : 7.2 and, for ketene, 1 : 1.2 :6.7.

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