Axially Dissymmetric Diphosphines in the Biphenyl Series: Synthesis of (6,6′‐Dimethoxybiphenyl‐2,2′‐diyl)bis(diphenylphosphine)(‘MeO‐BIPHEP’) and Analogues via an ortho‐Lithiation/Iodination Ullmann‐Reaction Approach

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Abstract
The new axially dissymmetric diphosphines (R)‐ and (S)‐(6,6′‐dimethoxybiphenyl‐2,2′‐diyl)bis(diphenyl 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)diprienylphosphine 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 1H‐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.

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