Elucidation of the Reaction Mechanism of ortho→α Transmetalation Reactions of Alkyl Aryl Sulfone Carbanions

The sulfonyl group, as an electron‐accepting function, displays a strong acidifying effect on neighboring α‐hydrogen atoms, leading to their deprotonation by bases. Recently, reversal of the metalation selectivity of α,γ‐branched alkyl aryl sulfones was described, which instead undergo directed ortho‐metalation with good regioselectivity, despite having a more acidic α‐proton. Upon warming, a transmetalation to the α‐carbanion proceeds; however, nothing is known about the course of this transmetalation. Here, a mechanistic study is presented that sheds light on the facility of the initial metalation and the subsequent transmetalation. Large kinetic isotope effects were observed for both the initial deprotonation and the ortho→α transmetalation, which strongly influence the initial metalation selectivity. The results of a kinetic investigation of the transmetalation and a crossover study indicate that a concerted intermolecular pathway, either from an aggregate or from a monomeric aryllithium, prevails. The potential involvement of a stepwise process via ortho,α‐dilithium intermediates was investigated, but no hint of their involvement was found. This comprehensive mechanistic study sheds light on the facility of the initial metalation of branched alkyl aryl sulfones and their subsequent formal ortho→α rearrangement. A kinetic investigation, combined with studies aimed at the determination of deuterium KIEs, and crossover experiments, support an intermolecular mechanism for the transmetalation.