LFER Studies Evaluating Solvent Effects on an α-Chloro-and two β,β,β-Trichloro-Ethyl Chloroformate Esters

Classical tosylate/chloride leaving group approach supports a tetrahedral transition state for additions to trigonal carbon

The experimentally measured rates of solvolysis of 2-chloroethoxycarbonyl chloride (2-chloroethyl chloroformate, 3), 2-chloroethoxycarbonyl p-toluenesufonate (5), and phenoxycarbonyl p-toluenesulfonate (6) were followed at 25.0 °C in various pure and binary aqueous-organic solvents with varying degrees of polarity. An analysis of the rate constants for 3, 5, and 6, was carried out using the two-term extended Grunwald-Winstein equation and comparisons are made to the previously published results for ethyl and phenyl chloroformate esters. The k OTS/k Cl rate ratios and the Grunwald-Winstein l/m ratios indicate the probability of a dominant bimolecular carbonyl-addition pathway in the more nucleophilic solvents. Nevertheless in 3 and 5, in the strongly hydrogen-bonding 70% and 50% HFIP mixtures, a side-by-side ionization mechanism is favored.

Correlation analysis of the rates of solvolysis of 4-bromopiperidine: a reaction following a Grob fragmentation pathway

A Grunwald-Winstein treatment of the specific rates of solvolysis of 4-bromopiperidine gives for aqueous ethanol, methanol, acetone, and dioxane a very good logarithmic correlation against the YBr solvent ionizing power values with a slope (m value) of 0.46±0.02, consistent with the operation of a synchronous Grob fragmentation mechanism. When the organic component of the solvent is 2,2,2-trifluoroethanol (TFE), the data points show a negative deviation, consistent with an appreciable deactivating interaction of the acidic TFE component of the solvent with the lone-pair of electrons present on the nitrogen.

Influence of sulfur for oxygen substitution in the solvolytic reactions of chloroformate esters and related compounds

The replacement of oxygen within a chloroformate ester (ROCOCl) by sulfur can lead to a chlorothioformate (RSCOCl), a chlorothionoformate (ROCSCl), or a chlorodithioformate (RSCSCl). Phenyl chloroformate (PhOCOCl) reacts over the full range of solvents usually included in Grunwald-Winstein equation studies of solvolysis by an addition-elimination (A-E) pathway. At the other extreme, phenyl chlorodithioformate (PhSCSCl) reacts across the range by an ionization pathway. The phenyl chlorothioformate (PhSCOCl) and phenyl chlorothionoformate (PhOCSCl) react at remarkably similar rates in a given solvent and there is a dichotomy of behavior with the A-E pathway favored in solvents such as ethanol-water and the ionization mechanism favored in aqueous solvents rich in fluoroalcohol. Alkyl esters behave similarly but with increased tendency to ionization as the alkyl group goes from 1° to 2° to 3°. N,N-Disubstituted carbamoyl halides favor the ionization pathway as do also the considerably faster reacting thiocarbamoyl chlorides. The tendency towards ionization increases as, within the three contributing structures of the resonance hybrid for the formed cation, the atoms carrying positive charge (other than the central carbon) change from oxygen to sulfur to nitrogen, consistent with the relative stabilities of species with positive charge on these atoms.