Use of Linear Free Energy Relationships (LFERs) to Elucidate the Mechanisms of Reaction of a γ-Methyl-β-alkynyl and an ortho-Substituted Aryl Chloroformate Ester

Abstract

The specific rates of solvolysis of 2-butyn-1-yl-chloroformate (1) and 2-methoxyphenyl chloroformate (2) are studied at 25.0 °C in a series of binary aqueousorganic mixtures. The rates of reaction obtained are then analyzed using the extended Grunwald-Winstein (G-W) equation and the results are compared to previously published G-W analyses for phenyl chloroformate (3), propargyl chloroformate (4), p-methoxyphenyl choroformate (5), and p-nitrophenyl chloroformate (6). For 1, the results indicate that dual side-by-side addition-elimination and ionization pathways are occurring in some highly ionizing solvents due to the presence of the electron-donating γ-methyl group. For 2, the analyses indicate that the dominant mechanism is a bimolecular one where the formation of a tetrahedral intermediate is rate-determining.

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

“…Solvolytic data for a variety of substituted phenyl chloroformates have been documented and thorough Grunwald-Winstein (G-W) analyses of the available experimental results using Equation (2) have been published [9,10,19–31]. The kinetic solvent isotope effects (KSIEs) in methanol and methanol-d ( k MeOH / k MeOD ) for the substituted phenyl chloroformates indicate that general-base catalysis is operating, and its importance decreases with an increase in electron-donating ability of the substituent [21,26,30,32,33].…”

“…For the parent phenyl chloroformate ( 1 ), over a full-range of 49 pure and binary solvents of widely varying nucleophilicity and ionizing power values, the G-W [Equation (2)] analyses resulted in an l value of 1.66 and an m value of 0.56 [27]. It was suggested [10,23,27,30,31] that such l (bond making) and m (bond breaking) values be taken as typical values that are to be expected for substrates that proceed with a rate-determining addition step in a stepwise carbonyl addition-elimination process. Other research groups also proposed homogenous stepwise mechanisms for 1 , where the formation of a zwitterionic tetrahedral intermediate is the rate-determining step [34–38].…”

“…So far the two alkynyl chloroformates to be evaluated using the extended Grunwald-Winstein Equation [Equation (2)] are; propargyl [58] and 2-butyn-1-yl chloroformate [31]. For propargyl chloroformate, the tetrahedral stepwise addition-elimination pathway was confirmed for all 22 solvents studied [58].…”

“…2-butyn-1-yl chloroformate differs from the propargyl substrate by the presence of an adjoining methyl group on the β-triple bond. For this alkynyl containing compound, the addition-elimination pathway again dominates the spectrum of the solvents considered, but the reaction switches over to an ionization channel in the two most strongly hydrogen-bonding solvents studied, 97% and 90% HFIP [31]. …”

Correlation of the rates of solvolysis of tert-butyl chlorothioformate and observations concerning the reaction mechanism

“…Studies involving the application of the G-W equation to chloroformate esters have included the methyl ester [18], examples of primary alkyl esters such as ethyl [6], propyl [19], isobutyl [9], neopentyl [20], and n -octyl [21], examples of secondary alkyl esters such as isopropyl [22,23] and 2-adamantyl [24], phenyl [25], and substituted phenyl esters [26,27]. There has been only one study in terms of the G-W equation of a tertiary alkyl chloroformate.…”