2‐(1S)‐Camphanoyloxy‐2′‐phosphanylbiphenyl Ligands – Synthesis, Structure, and Preliminary Tests in Transition‐Metal Catalysis

Abstract

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 2aOH and 2b–fSi, 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 (ΔG# = 70–73 kJ mol–1 for 3a and 3c by 31P 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 3dD and 3fD, and in solution, there is a tendency toward partial isomerization to the sterically less‐favored atropisomers. The [RhCl(cod)(3dD)] complex 4dD, however, seems stable in solution. Excess 2dLi reacted with (1S)‐camphanoyl chloride preferentially to form the (SP,Rax,1S) isomer, which was separated by crystallization as enantiopure 3dE, 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 %.