Rhodium-Mediated Dehydrogenative Borylation-Hydroborylation of Bis(alkyl)alkynes: Intermediates and Mechanism

Complex Rh(Bpin){kappa(3)-P,O,P-[xant(PiPr(2))(2)]} (Bpin = pinacolboryl; xant(PiPr(2))(2) = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) catalyzes the addition of B(2)pin(2) to 3-hexyne and 4-octyne to give equimolecular mixtures of conjugated boryldienes and borylolefins, as a result of the addition of the B-B bond of the diborane to different molecules of alkynes and hydride transfer from one to the other. Both the dehydrogenative borylation and hydroborylation reactions form a catalytic cycle that has been deduced on the basis of stoichiometric studies. Complex Rh(Bpin){kappa(3)-P,O,P-[xant(PiPr(2))(2)]} promotes the dehydrogenative borylation of alkynes by means of reactions of insertion of the alkyne into the Rh-B bond, Z-E isomerization of the ss-borylalkenyl ligand of the resulting Rh-alkenyl species, and C-gamma-H bond activation of the alkyl substituent attached to the alkenyl Ca atom. As a consequence of the formation of boryldienes, the monohydride RhH{kappa(3)-P,O,P-[xant(PiPr(2))(2)]} is generated. The latter in a sequential manner reacts with the alkynes and the diborane to give the borylolefin hydroborylation products, via Rh-alkenyl intermediates, and regenerates the initial Rh-boryl compound. The latter also promotes stoichiometric cycles to prepare diboryl-2-olefins via allyl intermediates. In addition, the stoichiometric rhodium-mediated formation of 1-boryl-2-olefins is shown.