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

Abstract

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 (−)‐(2 R ,3 R )‐ and (+)‐(2 S ,3 S )‐ 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 1 H‐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.