A DFT examination of the role of proximal boron functionalities in the S -alkylation of sulfenic acid anions.

Sulfenic acid anions represent an emerging nucleophile for the preparation of sulfoxides. Their S-functionalization chemistry can often be influenced by a nearby group that interacts with the component atoms of the sulfenate through non-bonding interactions. This study uses DFT methods to assess the importance of proximal boron-containing functional groups to direct S -alkylation chemistry of selected sulfenate anions. Several structural variations were modelled at the B3LYP/6-311++G(d,p) level, with the boron species positioned 3 to 5 carbons away from the alkylation site. Transition state free energies of S -alkylation transition states were located with and without sulfenate oxygen precomplexing to the nearby boron atom. The outcomes suggest that an ortho -substituted boronate ester on benzyl bromide can direct and accelerate an alkylation reaction principally due to a reduction of the entropic barrier. It was also determined that an intermolecular precomplex imparts too much stabilization to the sulfenate, thereby reducing its reactivity. The modelling suggests a possible aryl migration of the boronate/sulfenate complex is not competitive with S -alkylation.