Gas-phase kinetic and mechanistic studies of some interconverting alkylcyclopropene pairs: Involvement of dialkylvinylidene intermediates and their quantitative behaviour

The pyrolyses of two isomeric pairs of alkylcyclopropenes, namely 1,3-dimet hyl- (15) and 1-ethyl-cyclopropene (16), and 1,3,3-trimethyl- (5) and 1-iso propyl-cyclopropene (17), have been studied in the gas phase. Complete prod uct analyses at various conversions up to 95% were obtained for the decompo sition of each compound at five temperatures over a 40 degrees C range. The time-evolution data showed that the isomerisation reactions 15 reversible arrow 16 and 5 reversible arrow 17 were occurring. Kinetic modelling of eac h system allowed the determination of rate constants for these and all othe r decomposition processes. Tests confirmed that all reactions were uni-mole cular and homogeneous. Arrhenius parameters are reported for overall reacti ons and individual product pathways. Further kinetic analysis allowed us to extract the propensities (at 500 K) for 1,3-C-H insertion of the dialkylvi nylidene intermediates involved in the rearrangements as follows: k(prim):k (sec): k(tert)=1:16.5:46.4. Additional experiments with C-13-labelled cyclo propenes yielded alkyl group migration aptitudes for the dialkylvinylidenes (from the pattern of C-13 in the alkyne products) as follows: Me:Et:iPr = 1:3.1:1.5. Explanations for these trends are given. Another important findi ng is that of the dramatic rate enhancements for 1,3-diene product formatio n from the 1-alkylcyclopropenes; this can be explained by either hyperconju gative stabilisation of the vinylcarbene intermediates involved in this pat hway, or their differing propensities to 1,2 H-shift. The observed large va riations in product distribution amongst these four cyclopropenes is interp reted in terms of these specific effects on individual pathways.