Reducing Diastereomorphous Bis(phosphane oxide) Atropisomers to One Atropisomerically Pure Diphosphane: A New Ligand and a Novel Ligand‐Preparation Design

1,1′‐Biphenyl‐2,2′‐diphosphanes with an achiral bridge spanning C‐5 and C‐5′ form atropisomers that are enantiomers. Accessing them in an atropisomerically pure form requires resolving a racemic mixture thereof or of a bis(phosphane oxide) precursor. 1,1′‐Biphenyl‐2,2′‐diphosphanes with a homochiral bridge spanning C‐5 and C‐5′ form atropisomers that are diastereomers. We synthesized the first compound of this kind 1) atropselectively and 2) under thermodynamic control—seemingly a first‐time exploit in diphosphane synthesis. The selectivity‐inducing step was a high‐temperature reduction of two non‐interconverting bis(phosphane oxide) atropisomers (60:40 mixture). It furnished the desired diphosphane atropisomerically pure (and atropconvergently because the yield was 67 %). This diphosphane proved worthwhile in Tsuji–Trost allylations, the Hayashi addition of phenylboronic acid to cyclohexenone, and the asymmetric hydrogenation of methyl acetoacetate (up to 95 % yield and 95 % ee). Atropconvergence: A 1,1′‐biaryl‐2,2′‐diphosphane with a “homochiral bridge” between C‐5 and C‐5′ defines a pair of atropisomers that are diastereomers. This suggests that the synthesis of such a compound as a pure atropisomer may be achieved by a unilaterally biased atropisomerization. The first diphosphane synthesis of this kind is reported—the reduction of a 60:40 mixture of atropisomeric bis(phosphane oxides) at 110 °C provided 67 % of a single diphosphane atropisomer.