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)–CHCHPh}{κ3-P,O,P-[xant(PiPr2)2]} (2), which reacts
with a second alkyne molecule to produce Rh(CCPh){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(CCPh)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)–CHCHPh}(CCPh)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)–CHCHC(OH)Ph2}{CCC(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.