The transition state of the cyclopropylidene to allene ring opening
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As part of an effort to understand the chemistry of cyclopropylidenes in solution, the effects of substituents on the relative rates of cyclopropylidene ring opening versus intermolecular insertion in methanol have been examined via correlation analysis;The trans-2,3-diarylcyclopropylidenes were generated via the base induced decomposition of ethyl N-nitroso N-(trans-2,3-diarylcyclopropyl)carbamates, in various concentrations of methanol, using toluene as the cosolvent. The partitioning of the disubstituted cyclopropylidenes in methanol solution at room temperature was found to be a linear function of the alcohol concentration;The relative cyclopropylidene to allene ring opening rate was found to increase when electron donors were attached to C[subscript]2 and/or C[subscript]3, and decrease when electron acceptors were similarly attached. This is indicative of decreased electron density at C[subscript]2 and C[subscript]3 at the transition state;The correlation analysis revealed a fairly strong interaction between C[subscript]2, C[subscript]3 and the carbene center of the ring-opening transition structure (rho = -0.72 ± 0.07);Relative to insertion into methanol, the ring opening of cyclopropylidene is favored entropically, while being disfavored enthalpically.