Gas-phase kinetics / Ring-opening of 3,3-dimethylcyclopropene / Vinylcarbene intermediates The gas-phase pyrolysis of 3.3-dimethylcyclopropene (1) in the temperature range of 185 -225°C gives 3-methyl-I-butyne (2) (91%) and isoprene (3) YO). The gas-phase pyrolysis of 3,3-dimethyl-I -trimethylsilylcyclopropene (4) in the temperature range of 195 -235°C gives 3-methyl-I-trimethylsilyl-1-butyne (5) (99%) and trans-3-methyl-l-trimethylsilyl-1,3-butadiene (6) (1 YO). Both rearrangements are homogeneous, first-order, unimolecular reactions with the following pressure-independent Arrhenius parameters. Reactant log(A/s-') E. CkJ mol-l (kcal mol-')I Gas-Phaseo-Kiaetik der Pyrolpe von &3-Dimetbykydopropea und seinem 1-Trimethylsityl-Derivat. -Der Effekt der Siyl-Substitution a d die C y c l o p r o p e w b d ~g Die Gasphasen-Pyrolyse von 3,3-Dimdthylcyclopropen (1) im Temperaturbereich 185 -225 "C ergibt 3-Methyl-1-butin (2) (91%) und Isopren (3) (9%). Die Gasphasen-Pyrolyse von 3,3-Dimethyl-I-trimethylsilylcyclopropen (41 fiihrt zu 3-Methyl-I -trimethylsilyl-1 -butin (s) (99%) und trans-3-Methyl-I-trimethylsilyl-IJ-butadien (6) (I %). Beide Umlagerunben sind h'omogene, unimolekulare Reaktionen erster Ordnung hit den folgenden druckunabhiingigen Arrhenius-Parametern. 1 13.34 f 0.04 156.1 f 0.3 (37.3 f 0.1) 2 12.81 f 0.16 155.7 f 1.5 (37.2 f 0.4) Rather surprisingly, rhe trimethylsilyl group deactivates cyclopropene with respect to its isomerisation. Possible mechanisms