Conclusive Evidence on the Mechanism of the Rhodium-Mediated Decyanative Borylation

The stoichiometric reactions proposed in the mechanism of the rhodium-mediated decyanative borylation have been performed and all relevant intermediates isolated and characterized including their X-ray structures. Complex RhCl{xant((PPr2)-Pr-i)(2)} (1, xant((PPr2)-Pr-i)(2) = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) reacts with bis(pinacolato)diboron (B(2)pin(2)), in benzene, to give the rhodium(III) derivative RhHCl(Bpin){xant((PPr2)-Pr-i)(2)} (4) and PhBpin. The reaction involves the oxidative addition of B(2)pin(2) to 1 to give RhCl(Bpin)(2){xant((PPr2)-Pr-i)(2)}, which eliminates C1Bpin generating Rh(Bpin){xant((PPr2)-Pr-i)(2)} (2). The reaction of the latter with the solvent yields PhBpin and the monohydride RhH{xant((PPr2)-Pr-i)(2)} (6), which adds the eliminated ClBpin. Complex 4 and its catecholboryl counterpart RhHCl(Bcat){xant((PPr2)-Pr-i)(2)} (7) have also been obtained by oxidative addition of HBR2 to 1. Complex 2 is the promoter of the decyanative borylation. Thus, benzonitrile and 4(trifluoromethyl)benzonitrile insert into the Rh-B bond of 2 to form Rh{C(R-C6H4)=NBpin}{xant((PPr2)-Pr-i)(2)} (R = H (8), p-CF3 (9)), which evolve into the aryl derivatives RhPh{xant((PPr2)-Pr-i)(2)} (3) and Rh(p-CF3-C6H4){xant((PPr2)-Pr-i)(2)} (10), as a result of the extrusion of CNBpin. The reactions of 3 and 10 with B(2)pin(2) yield the arylBpin products and regenerate 2.