Effects of Conjugation and Aromaticity on the Sulfoxide Bond¹

Abstract

An ab initio computational study on a series of sulfoxides is reported. The SO bond dissociation energy (BDE) of sulfoxides in which the sulfur atom is included in a formally aromatic ring (e.g., thiophene sulfoxide) is found to be decreased by as much as 25 kcal/mol, compared to DMSO. A complementary effect is observed for sulfoxides in which the sulfur is included in a formally antiaromatic ring (e.g., thiirene sulfoxide), in which SO BDEs are increased by as much as 15 kcal/mol. Both effects are attenuated by benzannulation. Examination of calculated geometries and isodesmic reactions with pure hydrocarbons leads to the conclusion that the observed effects are due to a severe disruption of the (anti) aromaticity of the sulfurcontaining ring on oxidation. The cyclic sulfoxides appear to be neither significantly aromatic nor antiaromatic by energetic considerations. No significant SO bond strength effect is observed for simple conjugation. Inversion barriers for several sulfoxides are also calculated. Disciplines Chemistry CommentsReprinted (adapted) An ab initio computational study on a series of sulfoxides is reported. The SO bond dissociation energy (BDE) of sulfoxides in which the sulfur atom is included in a formally aromatic ring (e.g., thiophene sulfoxide) is found to be decreased by as much as 25 kcal/mol, compared to DMSO. A complementary effect is observed for sulfoxides in which the sulfur is included in a formally antiaromatic ring (e.g., thiirene sulfoxide), in which SO BDEs are increased by as much as 15 kcal/mol. Both effects are attenuated by benzannulation. Examination of calculated geometries and isodesmic reactions with pure hydrocarbons leads to the conclusion that the observed effects are due to a severe disruption of the (anti) aromaticity of the sulfur-containing ring on oxidation. The cyclic sulfoxides appear to be neither significantly aromatic nor antiaromatic by energetic considerations. No significant SO bond strength effect is observed for simple conjugation. Inversion barriers for several sulfoxides are also calculated.

Generation and Decay of Aryl Sulfinyl and Sulfenyl Radicals: A Transient Absorption and Computational Study

“…In a previous computational work on sulfoxides in which S-O bond dissociation energies were determined by isodesmic exchange of the oxygen between a test sulfoxide and dimethyl sulfide, it was found that certain structural features led to significant differences in computed bond energies between Hartree-Fock and Møller-Plesset methods. 87 Moreover, Hartree-Fock methods are not generally reliable for predicting transition state energies. Therefore, it is not surprising to see a significant difference in the rotational barriers for PhSO • calculated by ROHF and RMP2 methods, though the magnitude of difference (2 kcal/mol Vs 13 kcal/mol) is large in this case.…”

Sulfur and Selenium Ylide Bond Enthalpies

“…Unlike other sulfur and selenium ylides, detailed thermochemical data allowing calculation of sulfoxide bond strengths are available for representative compounds. [23][24][25][26][27][28][29][30] Standard sulfoxide S-O BDEs are on the order of 87-90 kcal/ mol. Electronegative substituents raise the bond strength; for example, the BDE for F 2 SO is 114 kcal/mol.…”

“…31,32 On the other hand, conjugation of the sulfoxide to phenyl or vinyl substituents does not have a large effect. 29 However, the S-O bond strengths of thiophene derivatives are weakened because of the extra stabilization of the thiophene ring, compared to the nonaromatic sulfoxides. Previous calculations predict a BDE of about 65 kcal/mol for thiophene-Soxide.…”

The nature of the SO bond of chlorinated sulfur–oxygen compounds

“…We use generalized valence bond (GVB) theory to provide insights into the nature of the bonding in these molecules. A prior study of sulfur-oxygen compounds in our group showed that recoupled pair bonding involving the electrons in the p orbitals accounts for the strength of the SO bond in the ground X 3 R -state of diatomic SO, as well as for the large differences in geometry and SO bond strength of theX 2 A 00 states of HSO and SOH [17]. We found that a recoupled pair p bond is formed by the interaction of the electrons in the S3pp 2 lone pair and the O2pp 1 orbital in SO(X 3 R -).…”

“…Of interest here are molecules containing a single sulfinyl (SO) group with two additional ligands, which have the general formula XYSO. The SO bonds in these species are typically shorter and stronger than a standard single SO covalent bond [1], and their bond dissociation energies can vary significantly depending on the identity of the substituents (X, Y) [2]. Both the electronegativity and aromaticity of X and Y affect the SO bond dissociation energy.…”