Diphosphination of Electron Poor Alkenes

Studies of the reactions between an unsymmetrically substituted 1,1-diaminodiphosphine and electron poor alkenes revealed that, in contrast to the regioselective 1,2-addition of the P−P bond to α,β-unsaturated esters and nitriles with terminal double bonds, ethyl(vinyl)ketone reacted via 1,4-addition and α,β-unsaturated esters or ketones with internal double bonds failed to react at all, presumably owing to the deactivating influence of the alkyl groups. Reaction of 1 with maleic N-phenylimide proceeded stereoselectively under cis-addition but diesters of maleic and fumaric acid gave mixtures of diastereomeric 1,2-bisphosphines. The addition products were characterized by 31P NMR before being converted into palladium complexes that were isolated and comprehensively characterized by spectroscopic data and in most cases by X-ray diffraction studies. Monitoring the reactions of 1 with maleic and fumaric diesters by NMR revealed that both E/Z-isomerization of alkene starting materials and epimerization of stereogenic centers in 1,2-bisphosphines take place and allow isolation from the mixtures of diastereomeric ligands of complexes featuring a uniform stereochemistry of the C2 backbone.