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 RhHκ3-P, O, P-xant (PiPr2) 2 (1; xant (PiPr2) 2 = 9, 9-dimethyl-4, 5-bis (diisopropylphosphino) xanthene) to give Rh (E) –CH═CHPhκ3-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) 2κ3-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) 2κ3-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) Ph2C≡CC (OH) Ph22κ3-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.