Substituent effect on benzylic lithiation of sulfides. Synthesis of diboronic acids derived from aryl–alkyl sulfides

Abstract

The relative benzylic deprotonation rate constants of aryl–benzyl sulfides have been measured and the obtained values were compared with the substituent constants using the Hammett equation and with deprotonation Gibbs energies calculated on B3LYP/aug-cc-pVDZ level. The deprotonation rate depends on the stabilization of the negative charge, which is spread over the benzene ring. The series of brominated alkyl–aryl sulfides was di-lithiated by Br/Li exchange using t-BuLi and the obtained organolithium compounds were converted into the respective diboronic acids. Lithiation of aryl–benzyl sulfides containing a bromine atom in the para or ortho positions occurs selectively, however for the meta derivative, the process is plagued by competitive benzylic deprotonation. This fact can be rationalized on the basis of the obtained relative deprotonation rate constants. The X-ray analysis of bis-(2-dihydroxyborylphenylthio)methane revealed the existence of three different structural motifs, which stabilize the structure by hydrogen bonding formation.