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(PEt 3 ) 3 OB(OH)Ar] were prepared by treating {Rh(PEt 3 ) 2 [N(SiMe 3 ) 2 } with the corresponding arylboronic acid, ArB(OH) 2, in the presence of added PEt 3 . One example of these complexes was characterized by X-ray diffraction. [Rh(PEt 3 ) 3 OB(mesityl) 2 ] and [Rh(PEt 3 ) 2 OB(mesityl) 2 ] were prepared by analogous methods from dimesitylborinic acid in the presence and absence of added PEt 3 . Heating of the trisphosphine boronate complexes in cyclohexane generated the rhodium aryl complexes, [(PEt 3 ) 3 RhAr] and boroxin in good to high yields. [Rh(PEt 3 ) 3 OB(mesityl) 2 ] also underwent aryl migration to form [(PEt 3 ) 3 Rh(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.