Kinetic Studies that Evaluate the Solvolytic Mechanisms of Allyl and Vinyl Chloroformate Esters

Abstract

At 25.0 °C the specific rates of solvolysis for allyl and vinyl chloroformates have been determined in a wide mix of pure and aqueous organic mixtures. In all the solvents studied, vinyl chloroformate was found to react significantly faster than allyl chloroformate. Multiple correlation analyses of these rates are completed using the extended (two-term) Grunwald-Winstein equation with incorporation of literature values for solvent nucleophilicity (NT) and solvent ionizing power (YCl). Both substrates were found to solvolyze by similar dual bimolecular carbonyl-addition and unimolecular ionization channels, each heavily dependent upon the solvents nucleophilicity and ionizing ability.

HFIP in Organic Synthesis

“…The authors proposed that a resonance-stabilized acylium ion intermediate generated via unimolecular ionization would be further stabilized by fluoroalcohol solvents. Similar results were observed for the specific rate of solvolysis of allyl and vinyl chloroformates in which highly nucleophilic, low ionizing solvents such at EtOH favored a bimolecular reaction pathway whereas fluorinated solvents promoted unimolecular ionization . Neighboring π-stabilization was not a requirement for a change in mechanism; a superimposed unimolecular (S N 1) type ionization pathway was suggested for n -alkyl chloroformates such as isopropyl, isobutyl, and neopentyl chloroformates .…”

“…Neighboring π-stabilization was not a requirement for a change in mechanism; a superimposed unimolecular (S N 1) type ionization pathway was suggested for n -alkyl chloroformates such as isopropyl, isobutyl, and neopentyl chloroformates . Chloroformates with more distant π-bonds like 2-butyn-1-yl-chloroformate , also switched to a unimolecular ionization pathway in the presence of fluorinated alcohols, where a strong rear-side nucleophilic solvation of the developing carbocation was proposed.…”