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Croatian Chemical Society, Croatica Chemica Acta, 4(87), p. 363-373

DOI: 10.5562/cca2478

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Investigations into the Mechanism of Solvolysis of 3-aryloxybenzisothiazoles

This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

The solvolysis of selected 3-aryloxybenzisothiazoles (6a−c; Figure 1) in alcohols has been theoreti-cally 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 metha-nol, 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, meas-ured 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 po-larity 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 energeti-cally much more demanding than its addition-elimination counterpart and is therefore a highly im-probable 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.