Intramolecular Phenyl Transfer from Boron to Lithium, Sodium, and Copper: Defining the Limits of Transmetalation

Abstract

Transmetalation is a key elementary step in organometallic chemistry, for which reason there is a keen interest in better understanding the factors governing this reaction. We have previously reported the unusual transfer of a phenyl anion from boron to lithium in the gas-phase dissociation of [LiBr(^tBu)(Ph)Bpin]⁻ (Chem. Eur. J. 2024, 43, e202303653). Here, we use a combination of gas-phase fragmentation experiments and quantum chemical calculations to probe the reactivity of the related adducts [MX(R)(Ph)Bpin]⁻ (R=ⁿBu, ^tBu, Ph; X=Cl, Br, I, BF₄, BPh₄; M=Li, Na, K, Cu). We find the transfer of Ph⁻ to be much favored over that of ⁿBu⁻ and ^tBu⁻. The tendency toward transmetalation is decreased for anions X⁻ that strongly bind to the metal center M. Likewise, it is diminished for more electropositive/less electronegative metals M. According to our theoretical calculations, the coordination of a single molecule of tetrahydrofuran to the metal M also lowers the propensity for transmetalation, thereby approaching the behavior in solution. Thus, our results reveal a subtle interplay of different effects influencing the tendency toward transmetalation.