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

Abstract

Complex Rh(Bpin){κ3-P,O,P-[xant(PiPr2)2]} (Bpin = pinacolboryl; xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) catalyzes the addition of B2pin2 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){κ3-P,O,P-[xant(PiPr2)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 β-borylalkenyl ligand of the resulting Rh–alkenyl species, and Cγ–H bond activation of the alkyl substituent attached to the alkenyl Cα atom. As a consequence of the formation of boryldienes, the monohydride RhH{κ3-P,O,P-[xant(PiPr2)2]} is generated. The latter in a sequential ma...