Titanium-mediated reductive coupling of chiral formylphosphaferrocenes: Formation of bis(phosphaferrocenyl)-substituted ethylenes and pinacols

Treatment of 3,4-dimethylphosphaferrocen-2-carbaldehyde (1) with the standard McMurry reagent [TiCL3.(DME)(1.5)] and Zn(Cu) gives good yields of either the 1,2-bis(phosphaferrocenyl)-ethenes 2 as a mixture of E- and Z-isomers or the respective pinacol coupling products, depending on the reaction conditions. The stereochemical course of the pinacol coupling was investigated. Starting from enantiomerically pure aldehyde 1, only one isomeric diol 3 is obtained, while racemic starting material leads to the formation of two diastereomeric products. Configurations of all coupling products were determined by a combination of NMR spectroscopy and X-ray diffraction. The distribution of stereoisomers can be rationalized assuming the formation of a titanaoxirane as an intermediate. The configuration of the oxirane C atom is determined by the chirality of the adjacent heterometallocene unit. Insertion of another molecule of aldehyde into the Ti-C bond yields a titanapinacolate as the coupling product. The chirality of the inserting aldehyde seems to be unimportant for the stereochemical course of this step. Diol 3 and its bis(methyl)ether derivative were employed as P,P chelate ligands in the formation of metal complexes with Cp*RuCl and MO(CO)(4) fragments, respectively, and the latter complex was characterized by X-ray diffraction.