Homoconjugation and homoaromaticity—XVII: The nature and behavior of the unsubstituted trishomocyclopropenyl cation

Abstract

A more extensive investigation of the solvolysis of the 3-bicyclo-[3.1.0]hexyl tosylates has been carried out using kinetic, NMR and VPC techniques not employed previously. For labeled starting materials, 6,6-dideuterated species have been used. The trishomocyclopropenyl cation is here discussed on the basis of the new information. In line with the presence of a special salt effect in acetolysis of the cis-tosylate, the latter exhibits equilibration of the deuterium label, originally at C 6, over positions 2, 4 and 6 during acetolysis. The sum of the first order equilibration rate constant, k eq, and the titrimetric rate constant, k t, represents a minimum value for the ionization rate constant, k 1. Lithium perchlorate reduces considerably the gap between k t and ( k eq + k t), but it does not eliminate it entirely. Thus, the special salt effect eliminates only part of the ion pair return during the acetolysis. With respect to salt effects, ion pair behavior and lack of common ion rate depression, the cis-3-bicyclo[3.1.0]hexyl system is analogous to the 3-anisyl-2-butyl analog. As regards mechanism of solvolysis of cis-tosylate, the essentially exclusive formation of cis-acetate with retention of configuration and complete equilibration of the label, the absence of elimination, the lack of rearrangement to 2-bicyclo[3.1.0]hexyl and monocyclic structures, and the special salt effect and ion pair return phenomena provide a complete contrast with the behavior of the trans-tosylate. All of the results are explicable on the basis of the trishomocyclopropenyl intermediate formed by anchimerically assisted ionization ( k Δ) of cis-tosylate. Rate constant k Δ evidently exceeds the rate constant for anchimerically unassisted solvolysis, k s, by a factor of ca. 50 in acetolysis and by a somewhat smaller factor in aqueous acetone. The most striking aspect of the solvolysis of trans-tosylate is the complete absence of equilibration of the deuterium label in the cis-acetate part of the acetolysis product. Leakage from the classical cation to the nonclassical trishomocyclopropenyl structure competes unusually poorly with its other reactions. Probably the most important reason for this very inefficient leakage is that the geometry of the classical ion is unfavorable for the change. The reason for this goes back to the preferred boat conformation of 3-bicyclo[3.1.0]hexyl derivatives. This preferred boat conformation also provides part of the reason for the relatively low ( k Δ/ k s) ratios in solvolysis of the cis-tosylate. Corey's alternative suggestion of three rapidly equilibrating “almost classical” nonclassical ions instead of the trishomocyclopropenyl species is discussed. None of the data require this modification, so the application of “Ockham's razor” to the three species and the retention of the simpler interpretation are recommended.