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(PiPr2)2} (1, xant­(PiPr2)2 = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene)
reacts with bis­(pinacolato)­diboron (B2pin2),
in benzene, to give the rhodium­(III) derivative RhHCl­(Bpin)­{xant­(PiPr2)2} (4) and PhBpin.
The reaction involves the oxidative addition of B2pin2 to 1 to give RhCl­(Bpin)2{xant­(PiPr2)2}, which eliminates ClBpin generating
Rh­(Bpin)­{xant­(PiPr2)2} (2). The reaction of the latter with the solvent yields PhBpin and
the monohydride RhH­{xant­(PiPr2)2}
(6), which adds the eliminated ClBpin. Complex 4 and its catecholboryl counterpart RhHCl­(Bcat)­{xant­(PiPr2)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(PiPr2)2} (R = H (8), p-CF3 (9)), which evolve into the aryl derivatives RhPh­{xant­(PiPr2)2} (3) and Rh­(p-CF3-C6H4)­{xant­(PiPr2)2} (10), as a result
of the extrusion of CNBpin. The reactions of 3 and 10 with B2pin2 yield the arylBpin products
and regenerate 2.