Axially Dissymmetric Bis(triaryl)phosphines in the Biphenyl series: Synthesis of (6,6′‐dimethylbiphenyl‐2,2′‐diyl)bis(diphenylphosphine) (‘BIPHEMP’) and analogues, and their use in Rh(I)‐catalyzed asymmetric isomerizations of <i>N,N</i>‐diethylnerylamine

Abstract

Abstract The axially dissymmetric diphosphines (−)‐( R )‐ and (+)‐( S )‐(6‐6′‐dimethylbiphenyl‐2,2′‐diyl)bis(diphenyl‐phosphine) ((−)‐( R )‐ 10 and (+)‐( S )‐ 10 ; ‘BIPHEMP’) have been synthesized, starting from ( R )‐ and ( S )‐6,6′‐dimethylbiphenyl‐2,2′‐diamine (( R )‐ and( S )‐ 16 ), respectively, via Sandmeyer reaction, liathiation, and phosphinylation. Moreover, racemic 4,4′‐ dimethyl‐ and 4,4′‐bis(dimethylamino)‐substituted analogues 11 and 12 respectively, and the 6,6′‐bridged analogues 1,11‐bis(diphenylphosphino)‐5,7‐dihydrodibenz[ c,e ]oxepin (13) were synthesized and resolved into optically pure ( R )‐ and( S )‐enantiomers via complexation with di‐μ‐chlorob is {( R )‐2‐[1‐(dimethylamino)ethyl]pheny‐ C  N }dipalladium(II) (( R )‐ 18 ). The molecular structures of the diphosphines ( S )‐ 10 and ( R )‐ 13 and of two derived cationic Rh(I) complexes,[Rh(( S )‐ 10 )(nbd)]BF 4 and [Rh(( R )‐ 13 )(nbd)]BF 4 were determined by x‐ray analyses. Absolute configurations were established for (+)‐( S )‐ 10 by X‐ray analyses of both the free diphosphine and of the derived Rh(I) complex, and for (−)‐( R )‐ 13 by X‐ray analysis of the derived Rh(I) complex. Configurational assignments for the substituted BIPHEMP analogues 11 12 were achieved by means of 1 H‐NMR comparisons. The BIPHEMP ligand 10 and analogues 11 , 12 and 13 are the first examples of optically active bis(triaylphosphines) containing the axially dissymmetric biphenyl moiety. All these new diphosphines proved to be excellent asymmetry‐inducing ligands in Rh(I)‐catalyzed isomerizations of N,N ‐diethylnerylamine affording citronellat enamine of 98‐99% ee.