Homo- and Cross-Coupling of Phenylacetylenes and α‑Hydroxyacetylenes Catalyzed by a Square-Planar Rhodium Monohydride

The C-C triple bond of phenylacetylene undergoes the anti-Markovnikov addition of the Rh-H bond of RhH-{κ³-P,O,P-[xant-(PⁱPr₂)₂]} (1; xant-(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis-(diisopropylphosphino)-xanthene) to give Rh-{(E)-CHCHPh}-{κ³-P,O,P-[xant-(PⁱPr₂)₂]} (2), which reacts with a second alkyne molecule to produce Rh-(CCPh)-{κ³-P,O,P-[xant-(PⁱPr₂)₂]} (3) and styrene before the transformation from 1 to 2 is complete. The metal center of 3 undergoes the oxidative addition of the C-(sp)-H bond of another alkyne molecule to produce RhH-(CCPh)₂{κ³-P,O,P-[xant-(PⁱPr₂)₂]} (4), which also reacts with more phenylacetylene before completing the transformation from 3 to 4. The reaction leads to Rh-{(E)-CHCHPh}-(CCPh)₂{κ³-P,O,P-[xant-(PⁱPr₂)₂]} (5), which reductively eliminates (E)-1,4-diphenyl-1-buten-3-yne to regenerate 3. Complexes 3, 4, and 5 constitute a cycle for head-to-head dimerization of phenylacetylene. Consequently, complex 1 promotes the catalytic homocoupling of terminal alkynes to (E)-enynes, including the dimerization of α-hydroxyacetylenes to (E)-enyne-diols. The rate-determining step of the couplings depends on the nature of the alkyne, being the insertion of the C-C triple bond into the Rh-H bond of a bis-(acetylide)-rhodium-(III)-hydride intermediate for phenylacetylenes and the reductive elimination of the product (E)-enyne-diol for α-hydroxyacetylenes. In support of the latter, complex Rh-{(E)-CHCHC-(OH)-Ph₂}-{CCC-(OH)-Ph₂}₂{κ³-P,O,P-[xant-(PⁱPr₂)₂]} (6) has been isolated and characterized by X-ray diffraction analysis. Complex 1 also effectively promotes the formation of compounds of the type (E)-5-phenyl-2-penten-4-yn-1-ol, by cross-coupling between phenylacetylenes and α-hydroxyacetylenes. These reactions take place through two cycles similar to the cycle that produces the homocouplings, the rate-determining step being the reductive elimination of (E)-enyn-ol for both. The catalytic performance of 1 provides good efficiency in homocoupling and cross-coupling reactions involving progestin-type compounds such as ethisterone.