| Abstract
| - A planar-chiral diphosphine having a η5-cyclopentadienylrhenium(I) tricarbonyl backbone was prepared by asymmetric synthesis, and its use in palladium-catalyzed allylic alkylations and rhodium-catalyzed olefin hydrogenations gave products with up to 95% ee. The ligand/metal binding mode of a P,N-cyrhetrene was revealed by the X-ray crystal structure analysis of a palladium dichloride complex.
- Planar-chiral diphosphine (R,Sp)-4, having a η5-cyclopentadienylrhenium(I) tricarbonylbackbone was prepared by a sequence involving an enantioselective (CBS) reduction ofacetylcyrhetrene 5 to give alcohol (R)-6, several retentive SN1-type substitution reactions,and a diastereoselective introduction of the phosphino substituent at the cyclopentadienylring through directed ortho-lithiation. The application of a palladium complex of (R,Sp)-4 inallylic alkylation reactions afforded a product with up to 88% ee. Use of (R,Sp)-4 in Rh-catalyzed hydrogenations of dimethyl itaconate and acetamidocinnamic acid gave enantioselectivities of up to 95 and 89% ee, respectively. Thus, in the catalyses cyrhetrenyldiphosphine (R,Sp)-4 compares well with its ferrocene-based analogue, Josiphos 1a. Theligand/metal binding mode of P,N-cyrhetrene (R,Sp)-10 was revealed by the X-ray crystalstructure analysis of complex [PdCl2·(R,Sp)-10]. The results of this study confirmed theexpected structural similarities between the planar-chiral cyrhetrene and the analogousferrocene and suggested a lower basicity of the former due to the electron-withdrawingproperty of its Re(CO)3 fragment. This structure-based analysis is in accord with theinterpretation of the catalytic properties of metal complexes bearing cyrhetrenyl diphosphine(R,Sp)-4.
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