Solvent Effects and Steric Course in the Solvolysis of 1,3,3-Trimethyl-2-oxocyclopentyl Mesylate in Comparison with 1,1,3,3-Tetramethyl-2-oxobutyl System

Abstract

The rates of solvolysis in various solvents were determined for 1,1,3,3-tetramethyl-2-oxobutyl tosylate ( 1 OTs) and 1,3,3-trimethyl-2-oxocyclopentyl mesylate ( 4 OMs). The rate data for 1 OTs reinforced that the linear Grunwald − Winstein (GW) relationship of 1 OMs previously reported by Creary for non-aqueous solvents must also hold for aqueous organic solvents. 4 OMs showed a markedly dispersed GW relationship that is, on the other hand, well correlated with an extended GW equation involving a nucleophilicity parameter. Such solvent dependence, such marked effects of added sodium azide on rates, and the 100% inversion of configuration of the solvolysis product showed that the solvolysis of 4 OMs would be categorized to S N 2(intermediate), whereas 1 OMs and 1 OTs solvolyze via limiting S N 1. The negligible susceptibility of 1 OTs toward nucleophilicity of solvent and azide probe indicates that the nucleofuge leaves along the C=O axis in such a manner that the back-strain (B-strain) in the ground state is efficiently relieved in the transition state. Comparison of solvolysis rates of 1 OMs and 4 OMs with those of the corresponding parent substrates suggests that the transition states of these substrates would not be stabilized by carbonyl π conjugation. The origin of the unexpectedly fast rates of solvolysis of 1 OMs has been discussed. #The chemistry of carbocations that are destabilized by a strongly electron-withdrawing substituent has been a subject of considerable interest for the past two decades in the field of physical organic chemistry.1 Some carbocations having a carbonyl or a cyano substituent on the α carbon have been spectroscopically observed under stable ion conditions. 1e The α -carbonyl cation stabilized by p -methoxyphenyl substituents has even been isolated.2 These works appear to have brought about an impression that the α -carbonyl cations were much more stable than had been thought before. 3##The solvolyses of various α -carbonyl substrates have been extensively carried out by Creary and co-workers.1c-e They demonstrated that many α -carbonyl cations can be relatively easily formed as solvolysis intermediates: for example, 1,1,3,3-tetramethyl-2-oxobutyl methanesulfonate (mesylate) ( 1 OMs ) solvolyzes 55 times faster than isopropyl mesylate ( 2 OMs) in 97% 1,1,1,3,3,3-hexafluoro-2-propanol containing 3% water (97HFIP).4 Since 1 OMs had been postulated to solvolyze 10 4 − 10 5 times as slow as 2 OMs, the enormous amount of acceleration was ascribed to possible contribution of the carbonyl substituent to stabilize the incipient cation by the mesomeric (+M) effect 5 3a ↔ 3b (Chart 1). 1,4Previously, we have suggested that the mesomeric stabilization as described by 3a ↔ 3b would be improbable by comparing the solvolysis rates of various 2-oxo bridgehead compounds. 6 We herein report the full account of a previous communication 7 on the solvolysis of 1,3,3-trimethyl-2-oxocy-

Unusual Solvent Effect on a SN2 Reaction. A Quantum-Mechanical and Kinetic Study of the Menshutkin Reaction between 2-Amino-1-methylbenzimidazole and Iodomethane in the Gas Phase and in Acetonitrile

“…Bimolecular nucleophilic substitution (S N 2) reactions are of great importance for both chemical and biochemical processes. These reactions are very common and occur in different environments, such as the gas phase, − liquid solution, ,,,,− enzymes, − zeolites, and micelles. , The thermodynamics and kinetics of these reactions have been studied extensively by different experimental techniques, for example, mass spectrometry, ,,,, UV−visible spectrophotometry, ,,,,,, conductivity, ,,,, potentiometry, , and NMR spectroscopy …”