Crystalline Silaspiropentane and Its Rearrangement to Silacyclobutane

The first crystallographically characterized silaspiropentane, a molecule containing 4 carbon atoms and 1 silicon atom in two fused three-membered rings, has been isolated. This species was synthesized from the addition of a well-defined silylene compound to methylidene cyclopropane. The silaspiropentane was shown to undergo a thermal rearrangement to the corresponding methylene silacyclobutane in solution. Kinetics show that this is a first-order reaction with little entropic cost to approaching the transition state (ΔH^⧧ = +26.9 kcal mol^-1 and ΔS^⧧ = -2.2 cal K^-1 mol^-1). As a point of comparison, the addition of cyclic alkyl amino carbene (cAAC) to methylidene cyclopropane allowed isolation of a spiropentane. While this species did not rearrange to the corresponding methylene cyclobutane, attempts to generate analogues containing aryl substituents led directly to the formation of methylene cyclobutane products as E stereoisomers. DFT calculations were used to interrogate the mechanism of these rearrangements and support a concerted process but with distinctly different transition state geometries for the spiropentane and silaspiropentane rearrangement.