Solvolytic Reactions in Fluorinated Alcohols. Role of Nucleophilic and Other Solvation Effects

Abstract

Rate constants and products for solvolyses of chlorodiphenylmethane (Ph 2 CHCl) and p-methoxybenzyl chloride in 2,2,2-trifluoroethanol (TFE)/water and TFE/ethanol are reported, along with additional kinetic data for solvolyses of tert-butyl and other alkyl halides (RX) in 97% w/w TFE/ water and in 97% w/w hexafluoropropan-2-ol/water (HFIP). Results are discussed in terms of the solvent ionizing power (Y) and the solvent nucleophilicity (N), and contributions from other solvation effects are considered. Comparisons with other S N 1 solvolyses show that solvolyses of Ph 2 CHCl in TFE mixtures are unexpectedly fast, but product ratios are unexceptional. An additional solvation effect influences solvolyses leading to delocalized cations, and a delocalized cationic transition state for concerted elimination may explain the recent results of Takeuchi et al., (J. Org. Chem. 1997, 62, 4904) without the need to postulate additional specific solvation effects for adamantyl systems, such as Bronsted-base solvation of Rand -hydrogen atoms; concerted elimination may occur because simple tertiary alkyl cations are too unstable to form in predominantly aqueous media. Iodide/bromide and bromide/chloride rate ratios are very similar for 1-adamantyl halides and corresponding solvolyses of tert-butyl halides; these ratios decrease in the order aq EtOH > TFE > HFIP, as expected for an electrophilic solvation effect (this effect can readily be incorporated into Y values). From kinetic data for a series of tertiary alkyl chlorides in 97% TFE/water, it is shown that the susceptibility of rates of solvolyses of RCl to N decreases with an increase in steric hindrance or with an increase in charge stabilization. Also, the small kinetic solvent isotope effects for typical solvolyses (e.g., methyl tosylate) indicate that nucleophilic attack lags behind heterolysis of the C-X bond. 4654

Correlation of the Rates of Solvolysis of Electron-Rich Benzoyl Chloride Using the Extended Grunwald-Wistein Equation

“…6(b), 23 The k 40%EtOH /k 97%TFE value of 3.88 obtained for the solvolysis of 1 is similar to those values obtained for the eight substrates that are believed to proceed through the S N 1 ionization mechanism (I) (See Table 3). …”

“…In contrast, several k 40%EtOH /k 97%TFE values were reported for the proposed unimolecular S N 1 reaction or ionization pathway: the solvolyses of diphenyl chlorometane, 12,25 p-methoxybenzyl chloride, 12 cinnamyl chloride, 13 1-chloro-1-phenylethane, 23,26 4-chloro-4-methyl-2-pentyne, 22(a) trans-4-chloro-2-pentene, 22(a) tert-butyl chloride, 6(c),23 and 1-piperidinecarbonyl chloride 14 (See Table 3). Small values (k 40%EtOH / k 97%TFE = 0 to ~25) were observed due to the two solvents having the same Y Cl value, i.e., the same degree of solvent assistance for bond cleavage, but different nucleophilicity, Scheme 2 which provides a measure of the minimum extent of nucleophilic assistance.…”

Gas-phase stability of tertiary carbenium ions and rates of solvolysis of tertiary derivatives

“…However, it is known that nucleophilic participation decreases with an increase in steric hindrance and with an increase in charge stabilization, 22 and since the degree of solvent participation in the solvolysis of 17 and the stability of the carbenium ion derived from 17 are unknown, it appears premature to draw conclusions from the single result in MeOH. More extensive stereochemical and kinetic investigations are clearly required.…”

On the Importance of the Aromatic Ring Parameter in Studies of the Solvolyses of Cinnamyl and Cinnamoyl Halides

“…Koo et al have analyzed the solvolytic rate constants and product selectivities ( S ) of cinnamyl chloride [23] and cinnamyl bromide [24] in a number of binary mixtures of water with ethanol, methanol, acetone, and 2,2,2-trifluoroethanol (TFE). The close similarity of solvent kinetic isotope effects, rate-rate profiles of solvent effects on reactivity, and similar selectivity data to p -methoxybenzyl chloride [11, 25] made the authors conclude that the solvolyses of cinnamyl halides can be explained by product formation incorporating a general base-catalyzed nucleophilic attack on a contact ion-pair [23, 24]. Cinnamyl bromide was subject to electrochemical reduction using cyclic voltammetry and controlled-potential electrolysis [26] where it was shown that the substrate could be reduced to a resonance-stabilized cinnamyl radical, which could further be reduced to a carbanion depending upon the selected potential.…”