Coupling of two ethyne molecules at rhodium versus coupling of two rhodium atoms at ethyne. 2. Implications for the reactivity. Catalytic and stoichiometric functionalization reactions of ethyne

Article abstract of DOI:10.1021/om00049a024

The rhodacyclopentadiene complex [(triphos)RhCl(η²-C₄H₄)] (3) has been synthesized by treatment of [(triphos)RhCl(C₂H₄)] (1) in CH₂Cl₂ with an excess of ethyne (triphos = MeC(CH₂PPh₂)₃). Complex 3 catalyzes under very mild conditions the cyclotrimerization of ethyne to benzene as well as the cyclocooligomerization of ethyne with acetonitrile to 2-picoline. A plausible catalysis cycle for both reactions is proposed on the basis of multiform experimental evidence. Complex 3 is a potential synthon for the preparation of a variety of heterocyclic compounds containing the C₄H₄ diene moiety. As an example, 3 reacts with dimethyl acetylenedicarboxylate, carbon disulfide, and cyclo-octasulfur, producing dimethyl phthalate, dithiopyrone, and thiophene, respectively. Carbon monoxide reacts with 3 in the presence of TlPF₆, yielding [(triphos)Rh(η⁴-C₄H₄CO)]PF₆ (10), which contains an unsubstituted cyclopentadienone ligand. By treatment with H₂, 3 transforms into the η⁴-butadiene derivative [(triphos)Rh(η⁴-C₄H₆)]BPh₄ (13). The chemistry of 3 has been compared to and contrasted with that of the related perpendicular μ-C₂H₂ complex [(triphos)Rh(μ-Cl)(μ-η²,η²-C ₂H₂)Rh(triphos)]Cl (2). From this study it is concluded that coupling of two or more metal centers at acetylene may lead to inactive complexes for catalytic transformations of acetylene.