Synthesis of chiral cyrhetrenes and their application in asymmetric catalysis

Planar-chiral diphosphine (R,S_p)-4, having a η⁵-cyclopentadienylrhenium(I) tricarbonyl backbone was prepared by a sequence involving an enantioselective (CBS) reduction of acetylcyrhetrene 5 to give alcohol (R)-6, several retentive S_N1-type substitution reactions, and a diastereoselective introduction of the phosphino substituent at the cyclopentadienyl ring through directed ortho-lithiation. The application of a palladium complex of (R,S_p)-4 in allylic alkylation reactions afforded a product with up to 88% ee. Use of (R,S_p)-4 in Rh-catalyzed hydrogenations of dimethyl itaconate and acetamidocinnamic acid gave enantio-selectivities of up to 95 and 89% ee, respectively. Thus, in the catalyses cyrhetrenyl diphosphine (R,S_p)-4 compares well with its ferrocene-based analogue, Josiphos 1a. The ligand/metal binding mode of P,N-cyrhetrene (R,S_p)-10 was revealed by the X-ray crystal structure analysis of complex [PdCl₂·(R,S_p)-10]. The results of this study confirmed the expected structural similarities between the planar-chiral cyrhetrene and the analogous ferrocene and suggested a lower basicity of the former due to the electron-withdrawing property of its Re(CO)₃ fragment. This structure-based analysis is in accord with the interpretation of the catalytic properties of metal complexes bearing cyrhetrenyl diphosphine (R,S_p)-4.