Intramolecular Phenyl Transfer from a Boronate to Lithium in the Gas Phase Reveals Crucial Role of Solvation in Transmetalations

Contrary to common belief, transmetalation reactions are not simply controlled by the electronegativity difference of the involved metals/metalloids. A combination of gas-phase and solution experiments, together with quantum chemical calculations, reveals the crucial role of solvation in the transmetalation from lithium to boron. Only in the presence of solvent molecules, the reaction proceeds in the expected direction, whereas the formation of the organolithium compound is favored in the gas phase.

Abstract

In contrast to its behavior in solution, the adduct [(LiBr)( ^t Bu)(Ph)Bpin]⁻ (pin=pinacol) transfers its phenyl anion from boron to lithium upon fragmentation in the gas phase. Quantum chemical calculations predict this exceptional transmetalation to be exothermic relative to the separated reactants, [( ^t Bu)(Ph)Bpin]⁻ and LiBr, which we attribute to the high phenyl-anion affinity of the coordinatively unsaturated LiBr unit. The addition of a single molecule of tetrahydrofuran drastically reduces the phenyl-anion affinity of LiBr and thereby renders the transmetalation from boron to lithium endothermic. Thus, the probed system highlights the importance of solvation and ligation effects in transmetalations. For correctly predicting the direction, in which these reactions proceed, it is not sufficient to consider the electronegativities or partial charges of the involved metals or metalloids. Instead, the individual coordination states and their changes over the course of the reaction must be taken into account.

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