Original scientific paper

https://doi.org/10.5562/cca2478

Investigations into the Mechanism of Solvolysis of 3-aryloxybenzisothiazoles

Amin Ismael ; CCMAR and Department of Chemistry and Pharmacy, FCT, University of Algarve, Campus de Gambelas, Faro 8005-039, Portugal;CQC, Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal
David J.P. Gago ; CCMAR and Department of Chemistry and Pharmacy, FCT, University of Algarve, Campus de Gambelas, Faro 8005-039, Portugal
Lília I.L. Cabral ; CCMAR and Department of Chemistry and Pharmacy, FCT, University of Algarve, Campus de Gambelas, Faro 8005-039, Portugal
Rui Fausto ; CQC, Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal
Maria Lurdes S. Cristiano ; CCMAR and Department of Chemistry and Pharmacy, FCT, University of Algarve, Campus de Gambelas, Faro 8005-039, Portugal

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.

Keywords

saccharins; saccharyl ethers; solvolysis; mechanistic investigations; structural and solvent effects

Hrčak ID:

131531

URI

https://hrcak.srce.hr/131531

Publication date:

22.12.2014.

Visits: 1.932 *