Abstract
The chiral-at-metal diastereomers (R-Ru,R-C)- and (S-Ru,R-C)-[CpRu(P-P')Hal] (P-P' = (R)-Prophos and (R,R)-Norphos, Hal = Cl, Br, and I) were synthesized, separated, and characterized by X-ray crystallography. Halide exchange and epimerization reactions were studied in 9:1 and 1:1 chloroform/methanol mixtures proceeding at room temperature or slightly above according to first-order. The ...
Abstract
The chiral-at-metal diastereomers (R-Ru,R-C)- and (S-Ru,R-C)-[CpRu(P-P')Hal] (P-P' = (R)-Prophos and (R,R)-Norphos, Hal = Cl, Br, and I) were synthesized, separated, and characterized by X-ray crystallography. Halide exchange and epimerization reactions were studied in 9:1 and 1:1 chloroform/methanol mixtures proceeding at room temperature or slightly above according to first-order. The rate-determining step in the Hal exchange reactions was the dissociation of the Ru-Hal bond, forming the pyramidal 16-electron intermediates (R-Ru,R-C)- and (S-Ru,R-C)-[CpRu(P-P')](+), which maintain the metal configuration. These intermediates can invert their metal configuration or react with nucleophiles with retention of the metal configuration. The measured competition ratios showed that the inversion of the intermediates was slow compared to quenching with nucleophiles, indicating a high pyramidal stability of the 16-electron fragments (R-Ru,R-C)- and (S-Ru,R-C)-[CpRu(P-P')](+) toward inversion in agreement with a basilica-type energy profile. Stereochemically this implies that substitution reactions in (R-Ru,R-C)- and (S-Ru,R-C)-[CpRu(P-P')Hal] occur with predominant retention of configuration, however, accompanied by a well-defined share of inversion, a point overlooked when (R-Ru,R-C)- and (SRu,Rc)-[CpRu(Prophos)Cl] were extensively used as starting materials in the synthesis of a variety of organometallic derivatives. The rates of the approach to the epimerization equilibrium were much smaller than those of the Hal exchange reactions, because in the basilica-type energy profile the intermediates (R-Ru,R-C)-/(S-Ru,R-C)-[CpRu(P-P')](+), formed in the cleavage of the Ru-Cl bond, have to cross another barrier of appreciable height for inversion. Increasing the methanol content of the solvent increased the rates of the Hal exchange and epimerization reactions. Obviously, the pyramidality of the fragments [CpRu(P-P')](+) is enforced by the small P-Ru-P angles (83 degrees in the Prophos derivatives and 86 degrees in the Norphos derivatives of (R-Ru,R-C)- and (S-Ru,R-C)-[CpRu(P-P')Hal]). Due to these small angles, intermediates (R-Ru,R-C)- and (S-Ru,R-C)[CpRu(P-P')](+) resist planarization and thus inversion of the metal configuration.
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