Rhodium-Promoted <i>anti</i>-Hydroboration of Terminal Alkynes via Combined Stoichiometric Reactions

Reactions of RhH{κ3-P,O,P-[xant(PiPr2)2]} (xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) with 2 equiv of tert-butylacetylene and phenylacetylene lead to the acetylide derivatives Rh(C≡CR){κ3-P,O,P-[xant(PiPr2)2]} (R = tBu, Ph). The C–C triple bond of these compounds undergoes the B–H anti-addition of pinacolborane (HBpin) to produce Rh{(E)-C(Bpin)=CHR-Pro-Z}{κ3-P,O,P-[xant(PiPr2)2]} (R = tBu, Ph), which regenerate Rh(C≡CR){κ3-P,O,P-[xant(PiPr2)2]} in the presence of a new alkyne molecule, releasing the respective (Z)-borylolefin. Complex Rh{(E)-C(Bpin)=CHPh-Pro-Z}{κ3-P,O,P-[xant(PiPr2)2]} is unstable in toluene. Initially, the C–C double bond of the borylalkenyl ligand undergoes a E to Z isomerization to produce Rh{(Z)-C(Bpin)═CHPh-Pro-E}{κ3-P,O,P-[xant(PiPr2)2]}, which subsequently evolves to the aryl derivative Rh{C6H4-2-[E-CH═CH(Bpin)]}{κ3-P,O,P-[xant(PiPr2)2]}. The latter reacts with a new phenylacetylene molecule to produce Rh(C≡CPh){κ3-P,O,P-[xant(PiPr2)2]} and the (E)-borylolefin. According to this reactivity, the complex RhH{κ3-P,O,P-[xant(PiPr2)2]} is an effective catalyst precursor for the hydroboration of terminal alkynes to mixtures of (Z)- and (E)-borylolefins. The molar ratio between isomers depends on the substituent of the alkyne; para-substituted aryl substituents with electron-withdrawing groups favor Z-borylolefin.