A new frontier in the metal-catalyzed cross-coupling reaction field. The palladium-promoted metal–carbon bond formation. Scope and mechanism of a new tool in organometallic synthesis

The synthetic uses and the investigation of the reaction mechanism of the Pd-catalyzed metal–carbon bond-formation process are accounted. Preparations of simple σ-metal metal acetylides of type M–CC–R (M=Fe, Ru, Mo, W; R=H, Alkyl, Ph), homo and heterobimetallic complexes of type M–CC–Thiop–CC–M′ (Thiop=2,5-thiophene; M, M′=Fe, Ru, Mo, W), polymetallaacetylide tethers of type M–CC–Thiop–CC–M′(η5-Cp)–CC–Thiop–CC–(η5-Cp)M′–CC–Thiop–CC–M (M=Re, Mn; M′=Fe), and metallaacetylide polymers of type [–M–CC–Ar–CC–]n (Ar=2,5-thiophene, 1,4-phenylene) are reported. By the use of properly designed model substrate principal intermediates (i.e. oxidative addition, transmetalation) involved in the catalytic cycle have been isolated and fully characterized. Kinetic and spectroscopic studies of the reaction mechanism have evidenced that, depending from the reaction conditions, the transmetalation step may proceed by two pathways: (i) in concentrated solutions of the complex of oxidative addition (≅10−2 M), transmetalation occurs through the formation of the intermediate species which is an associative complex between the substrate and the organostannane (associative mechanism). (ii) When transmetalation is performed using the complex of oxidative addition in lower concentrations (≅10−4 M), the reaction proceeds through formation of a highly-reactive solvent-coordinate species (dissociative mechanism). These results are showing clear analogies with mechanistic studies of the carbon–carbon coupling process, thus with the use of these model substrates, a unique opportunity to investigate factors affecting the metal–carbon bond formation reaction is offered, as well as the possibility to shed light on unclear aspects of the carbon–carbon coupling itself, a phenomenon of paramount importance in synthetic organic chemistry.