A computational investigation of the thermal elimination chemistry of β-borylated sulfoxides. Sulfenic acid vs. boryl sulfenate elimination

Abstract

Electronic structure calculations were performed to assess how a β-boryl substituent modulates barriers for the classical Ei elimination of sulfoxides. Four main boron substituents were investigated: H, Me, F and OMe. Across the series, methanesulfenic-acid elimination exhibits reduced activation free energies and enthalpies as the boron functionality accepts electron density from the C_β–H bond, promoting a more asynchronous transition state with advanced C_β–H cleavage and O–H formation and correspondingly less S–C_α bond rupture relative to the benchmark ethyl methyl sulfoxide transition state. Nevertheless, β-boryl substrates of the 1B family access lower-energy minima that lead preferentially to boryl sulfenate elimination: the corresponding ΔG^‡ values are 9.5–15.5 kcal mol⁻¹ lower than for the competing proton-transfer (sulfenic-acid) pathway. Replacing methyl with vinyl or phenyl lowers ΔG^‡ by 1.9–4.9 kcal mol⁻¹ through enhanced stabilization of developing electron density at sulfur. A comparison of common boronic esters (catechol, pinacol, BMIDA) for both proton-transfer and boronic-ester-transfer pathways shows catechol (Bcat) gives the lowest barriers, whereas BMIDA is distinctive in that its methanesulfenic acid elimination resembles that of methyl ethyl sulfoxide, and boryl-sulfenate elimination is disfavoured owing to loss of intramolecular N → B coordination. Collectively, β-boryl substitution lowers Ei barriers via electron-acceptor stabilization and biases reaction manifolds toward boryl sulfenate elimination, with the extent governed by conjugation patterns and ester identity.