P. Wawrzyniak, M. Kindermann, Gabriele Thede
May 26, 2017
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Journal
European Journal of Inorganic Chemistry
Abstract
Diastereoisomer separation of the (1S)-camphanic acid 2-iso-propylphenylphosphanylphenyl 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 barrier for interconversion by rotation around the C-C axis (G# = 70 - 73 kJ/mol for 3a and 3c by 31P VT NMR). The P-asymmetric compounds 3d-f form 1:1 mixtures of stereoisomers and tend for 3dD and 3fD to co-crystallization, proved by crystal structure analyses, and in solution to 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 the (SP,Rax,1S)-isomer, which was separated by crystallization as enantiopure 3dE, characterized by 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] with yields up to 98%.