Investigations into the Mechanism of Solvolysis of 3-aryloxybenzisothiazoles

Abstract

The solvolysis of selected 3-aryloxybenzisothiazoles (6a−c; Figure 1) in alcohols has been theoretically investigated. The geometries of ethers 6a−c were fully optimized at the DFT(O3LYP) level, with the 6-31++G(d,p) and 6-311++G(3df,3pd) basis sets. Calculations including solvation effects were performed with the 6-31++G(d,p) basis set. Overall, theoretical values for bond lengths and angles around the central ether linkage in ethers 6a−c are very close, for the isolated molecule and in methanol, and are also very close to those obtained by X-ray crystallography, revealing that the nature of the substituent on the aryl system has a negligible effect on geometric parameters around the ether linkage. The same applies to charge distributions, predicted using the NPA approach. However, measured rate constants for the solvolysis of the same compounds in alcohols show that the rate is affected by the electron-withdrawing/- donating characteristics of the substituent on the aryl ring and by the polarity of solvent. Two general pathways were considered for the solvolysis of ethers 6: associative (addition-elimination) or dissociative (fragmentation-recombination) mechanisms. Molecular orbital calculations by means of polarized continuum model (PCM) reaction field predicted that solvolysis of ethers 6 prefers an addition-elimination mechanism. Calculations show also that a dissociative mechanism for the solvolysis of ethers 6a−c is energetically much more demanding than its addition-elimination counterpart and is therefore a highly improbable pathway for the solvolysis. In addition, it was found that the putative cation intermediate formed during a dissociative process should easily convert into its 2-cyanobenzenesulfone cation isomer, via cleavage of the S−N bond.