Synthesis and Reactivity of Ester-Functionalized 5-Membered RhI-k2-C,O-Chelates and their Relevance in Rh(cod)-Mediated Carbene Polymerization

The previous mechanistic studies of the Rh-mediated polymerization of carbenes suggests the involvement of organometallic compounds that have been derived from Rh(diene) species that contain a five-membered chelate ring of the type Rh{κ2-C,O-[–CH(COOR)–CH(Pol)–C(OR)=O–]} (Pol = polymer chain), which are characterized by the coordination of the β-ester group of the growing polymer chain to the metal. Herein we present our efforts to characterize the possibly related RhI(cod){κ2-C,O-[–CH(COOR)–CHR′–C(OR)=O–]} (cod = 1,5-cyclooctadiene) species. These structures can be generated by means of olefin exchange with an allyl complex and with the concomitant release of the corresponding 1,3-diene, which is derived from the allyl ligand. The product is unstable and further reacts by means of a bimolecular C–H activation process to form a related dinuclear complex [(cod)Rh(μ-{–CH(COOR)–CH–C(OR)=O–})2Rh(cod)], in which the carbon dianion of the dimethyl or diethyl succinate bridges two Rh(cod) moieties. The complex was synthesized in an independent way and was structurally characterized. We investigated the importance of these complexes in carbene polymerization.