Insertion of Diphenylacetylene into Rh–Hydride and Rh–Boryl Bonds: Influence of the Boryl on the Behavior of the β‑Borylalkenyl Ligand

Reactions of complexes RhH­{κ3-P,O,P-[xant­(PiPr2)2]} (1) and Rh­(Bpin)­{κ3-P,O,P-[xant­(PiPr2)2]} (2; Bpin = pinacolboryl, xant­(PiPr2)2 = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) with diphenylacetylene have been studied. Complex 1 reacts with the alkyne to give the E-alkenyl derivative Rh­{(E)-C­(Ph)CHPh}­{κ3-P,O,P-[xant­(PiPr2)2]} (3) as a result of the syn-addition of the Rh–H bond to the C–C triple bond. In benzene, at room temperature, complex 3 is unstable and slowly evolves into its Z-alkenyl isomer Rh­{(Z)-C­(Ph)CHPh}­{κ3-P,O,P-[xant­(PiPr2)2]} (4), which is also unstable and undergoes an alkenyl-to-ortho-alkenylaryl transformation to afford Rh­{C6H4-2-(E-CHCHPh)}­{κ3-P,O,P-[xant­(PiPr2)2]} (5). The latter adds HBpin. The resulting rhodium­(III) species, RhH­(Bpin)­{C6H4-2-(E-CHCHPh)}­{κ3-P,O,P-[xant­(PiPr2)2]} (6), eliminates trans-4,4,5,5-tetramethyl-2-(2-styrylphenyl)-1,3,2-dioxaborolane and regenerates 1, closing a cycle for the hydroboration of the alkyne to the ortho-alkenyl-aryl compound. However, this cycle is not catalytic. The direct reaction of the alkyne with the borane in the presence of 1 leads to Z,E-mixtures of PhCHC­(Ph)­Bpin. Diphenylacetylene also undergoes syn-addition of the Rh–B bond of 2. The Bpin group accelerates the isomerization of the alkenyl ligand. Thus, the resulting E-β-borylalkenyl derivative Rh­{(E)-C­(Ph)C­(Bpin)­Ph}­{κ3-P,O,P-[xant­(PiPr2)2]} (7) rapidly evolves into its Z-isomer Rh­{(Z)-C­(Ph)C­(Bpin)­Ph}}­{κ3-P,O,P-[xant­(PiPr2)2]} (8), which also undergoes alkenyl-to-ortho-alkenylaryl transformation to give Rh­{C6H4-2-[E-CHC­(Bpin)­Ph]}­{κ3-P,O,P-[xant­(PiPr2)2]} (9). However, in contrast to 5, complex 9 is less stable than its precursor 8.