Insertion of Acetylenes into Carbon-Hydrogen Bonds Catalyzed by Rhodium-Trimethylphosphine Complexes

The insertion of terminal acetylenes (RC≡CH) into benzene C-H bonds has been found to be pho-tochemically (λ > 350 nm) catalyzed by Rh(PMe₃)₂(CO)Cl. Chemical yields (e.g. R = t-Bu, 58%; R = p-MeOC_βH₄-, 95%) and turnover numbers (>200/Rh) are fairly good, although the quantum efficiency is low (e.g. λ = 366 nm, R = p-MeOC₆H₄, Ø = 3.6 × 10⁻³). Crossover experiments reveal that both the H atom and Ph group added to the triple bond are derived from the same molecule of benzene. The orientation of addition is dependent upon the R group; for R = aryl, exclusively 1,1-substituted (branched) olefins are produced, while for R = n-propyl or tert-butyl a mixture of 1,1- and 1,2-substituted (linear) olefins is formed. The linear olefins are those resulting from exclusively syn addition across the C≡C bond. These results are consistent with a mechanism that involves a concerted addition across a triple bond; in particular addition of the Rh-H bond of a hydridophenylrhodium species is suggested. Additional results consistent with such a pathway include a primary isotope effect (kc₆h₆/c₆d₆ = 1.4) and a reactivity order among alkynes (and olefins) that is inconsistent with a variety of nonconcerted pathways such as transfer of hydrogen atom or hydride. Labeling experiments reveal that the stereochemistry of the branched olefinic products is that resulting from anti addition across the triple bond; possible mechanistic implications are discussed. In a nonphotochemical reaction, [(Rh(PMe₃)₂Cl]₂ efficiently catalyzes the dimerization of terminal acetylenes by insertion into the acetylenic C-H bond. Similar reactivity patterns are found between this reaction and the Rh(PMe₃)₂(CO)Cl-catalyzed reaction with benzene. All products from the dimerization result from syn addition.