AXIALLY DISSYMMETRIC DIPHOSPHINES IN THE BIPHENYL SERIES - SYNTHESIS OF (6,6'-DIMETHOXYBIPHENYL-2,2'-DIYL)BIS(DIPHENYLPHOSPHINE) (MEO-BIPHEP) AND ANALOGS VIA AN ORTHO-LITHIATION IODINATION ULLMANN-REACTION APPROACH

The new axially dissymmetric diphosphines (R)- and (S)-6,6'-dimethoxybiphenyl-2,2'-diyl)bis(dipheyl phosphine) ((R)- and (S)-5a; 'MeO-BIPHEP') and the analogues (R)- and (S)-5b and 5c have been synthesized in enantiomerically pure form. These ligands have become readily available by a synthetic scheme which employs, as key steps, an ortho-lithiation/iodination reaction of the (m-methoxyphenyl)diphenylphosphine oxides 8 and a subsequent Ullmann reaction of the resulting iodides 9 to provide the racemic bis(phosphine oxides) 10. The bis(phosphine oxides) 10 subsequently are resolved with (-)-(2R,3R)- and (+)-(2S,3S)-O-2,3-dibenzoyltartaric acid and reduced to diphosphines 5. The Ullmann reaction constitutes a new and efficient route to 2,2'-bis(phosphinoyl)-substituted biphenyl systems. Absolute configurations were established for (R)-5a by X-ray analysis of the derived Pd complex (R,R)-17a, and for 5b and 5c by means of H-1-NMR comparisons of the derived Pd complexes 16 or 17, respectively, and by means of CD comparisons. The MeO-BIPHEP diphosphine 5a proved to be as efficient as the previously described BIPHEMP diphosphine ((6,6'-dimethylbiphenyl-2,2'-diyl)bis(diphenylphosphine)) in enantioselective isomerizations and hydrogenations.