Asymmetric Hydroformilation of Styrene Catalysed by Platinum-Tin Complexes with Chiral bis-Binaphthophosphole Ligands

A set of novel bidentate ligands 2, 3 and 4, which have two axially chiral binaphthophospholyl (BNP) substituents connected through a carbon chain of different lengths (two to four atoms), has been synthesised by alkylating the binaphthophospholyl anion with the appropriate diiodide or ditosylate. In solution the free diphospholes display fluxional behaviour because of the ready atropisomerisation of the binaphthyl backbone. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. The latter can be converted into the mononuclear species by warming them in the presence of SnCl2. Coordination to platinum enhances the conformational stability of 3 and 4 and separate diastereomeric complexes can be detected in solution up to about 50°C. In the presence of SnCl2, the platinum complexes containing 3 and 4 give rise to catalysts which exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with unprecedently high branched selectivity (80%-5%) and moderate enantioselectivity (up to 45% e.e.).