2-(1S)-Camphanoyloxy-2'-phosphanylbiphenyl Ligands - Synthesis, Structure and Preliminary Tests in Transition Metal Catalysis

Diastereoisomer separation of the (1S)-camphanic acid 2-isopropylphenylphosphanyl-phenyl ester 1 exemplifies the potential of (1S)-camphanoyl chloride for enantiomer separation of hydroxyl-functional asymmetric phosphanes. Esterification of lithium 2'-phosphanylbiphenyl-2-olates, generated from the respective 2-OH or 2-OSiMe3 precursors 2a(OH) and 2b-f(Si), furnished the 2-(1S)-camphanoyloxy-biphenylphosphanes 3a-c as 1:1 mixtures of diastereomers with low barriers for interconversion by rotation around the C-C axis (Delta G# = 70-73 kJ mol(-1) for 3a and 3c by P-31 VT NMR spectroscopy). The P-asymmetric compounds 3d-f form 1:1 mixtures of stereoisomers. There is a tendency to cocrystallization of two preferred diastereoisomers, as shown by the crystal structure analyses of 3d(D) and 3f(D), and in solution, there is a tendency toward partial isomerization to the sterically less-favored atropisomers. The [RhCl(cod)(3d(D))] complex 4d(D), however, seems stable in solution. Excess 2d(Li) reacted with (1S)-camphanoyl chloride preferentially to form the (S-P,R-ax,1S) isomer, which was separated by crystallization as enantiopure 3d(E), characterized by single-crystal XRD. Preliminary screening tests of this ligand in Rh-catalyzed asymmetric hydrogenations of N-(1-phenylvinyl) acetamide allowed high conversion and up to 59 % ee. Hydrosilylation of acetophenone proceeded with 78 % conversion and 48 % ee; Suzuki-Miyaura couplings of 1-bromo-2-naphthol with PhB-(OH)(2), in the presence of 3b/[Pd(OAc)(2)], gave yields up to 98 %.