Directly Observed Transmetalation from Boron to Rhodium. β-Aryl Elimination from Rh(I) Arylboronates and Diarylborinates

Transmetalation from boron to rhodium in the absence of basic activators is reported to proceed by β-aryl elimination from a series of triethylphospine-ligated rhodium(I) arylboronates and diarylborinates. [Rh(PEt3)3OB(OH)Ar] were prepared by treating {Rh(PEt3)2[N(SiMe3)2} with the corresponding arylboronic acid, ArB(OH)2, in the presence of added PEt3. One example of these complexes was characterized by X-ray diffraction. [Rh(PEt3)3OB(mesityl)2] and [Rh(PEt3)2OB(mesityl)2] were prepared by analogous methods from dimesitylborinic acid in the presence and absence of added PEt3. Heating of the trisphosphine boronate complexes in cyclohexane generated the rhodium aryl complexes, [(PEt3)3RhAr] and boroxin in good to high yields. [Rh(PEt3)3OB(mesityl)2] also underwent aryl migration to form [(PEt3)3Rh(mesityl)] and a cyclic boroxine. Kinetic studies showed that migration of more electron-poor aryl groups from the boronate complexes was slightly faster than migration of more electron-poor aryl groups and that migration of the o-anisyl group was particularly fast. Kinetic results are most consistent with a ligand dissociation pathway with a rate-limiting β-aryl elimination from a 14-electron, bis(phosphine) intermediate.