I. Structure and Reactions of the l-Cyclopropyvinyl Cation. II. Interaction of the Face of a Cyclopropane Ring with Positively Charged Carbon

Citation

Sherrod, Shelby Allen (1971) I. Structure and Reactions of the l-Cyclopropyvinyl Cation. II. Interaction of the Face of a Cyclopropane Ring with Positively Charged Carbon. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9A8S-1W54. https://resolver.caltech.edu/CaltechTHESIS:04182018-155138346

Abstract

I. Structure and Reactions of the 1-Cyclopropyvinyl Cation

The unusual solvolytic reactivity of cyclopropylcar­binyl derivatives suggested that cyclopropyl substitution should stabilize vinyl cations; accordingly, it has been shown that 1-cyclopropylvinyl iodide ionizes readily under solvolytic conditions, with or without silver catalysis. Comparative kinetic measurements show that the cyclopropyl group enhances the uncatalyzed solvolysis rate by a factor greater than 1.2 x 10³, apparently by delocalizing the incipient positive charge. The major products result from reactions of the 1-cyclopropylvinyl cation with solvent, but partial rearrangement of this intermediate to the 2-methyl-enecyclobutyl cation occurs in acetic acid. The enthalpy of activation for this rearrangement exceeds that required for solvent trapping by 2.2 kcal/mol. A small yield of 3,4-pentadien-l-yl acetate may involve rearrangement to the 3,4-pentadien-l-yl cation. Comparative product studies show that 1-cyclopropylvinyl cation is also the major intermediate in the solvolysis of 3,4-pentadien-l-yl derivatives but is not involved in solvolyses of 2-methylenecyclobutyl or 1-cyclobutenylcarbinyl halides. There appears to be a difference between the 2-methylenecyclobutyl cation resulting from rearrangement and that obtained on direct solvolysis.

II. Interaction of the Face of a Cyclopropane Ring with Positively Charged Carbon

In order to evaluate the nature of the title interaction, the solvolytic reactivities of 4-tricyclyl, 1-apocamphyl, and 1-methyl-4-tr icyclo[2.2.2.0^2.6]octyl derivatives were compared. Solvolysis of the trifluoromethanesulfonates (triflates) in 60% aqueous ethanol (buffered) provided good first-order kinetic and Arrhenius data. Extrapolated to 25° the relative rates for these substrates (in the order given above) are 1.0, 2.84 x 10⁴, and 5.45 x 10⁷. In each case, product studies showed that no rearrangement took place, and incorporation of ¹⁸O-in 4-tricyclenol on solvolysis of 4-tricyclyl triflate in isotopically enriched medium showed that even under the most forcing conditions, carbon-oxygen rather than sulfur-oxygen bond cleavage was occurring. Since the rate data are well rationalized by semiempirical calculations that consider only relative strain energies, interaction of an incipient carbonium ion with the face of a cyclopropane ring apparently affords no significant stabilization.

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