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Preparation of Enantiomerically Pure Chelate Ligands L 2 = XCH 2 CH(OH)CH 2 Y from Epichlorohydrin – Conformation of Their L 2 Rh(COD) + Derivatives and Enantioselective Hydrogenation by L 2 Rh(COD) +
Author(s) -
Karas Jörg,
Huttner Gottfried,
Heinze Katja,
Rutsch Peter,
Zsolnai Laszlo
Publication year - 1999
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(199903)1999:3<405::aid-ejic405>3.0.co;2-t
Subject(s) - chemistry , aryl , enantioselective synthesis , steric effects , epichlorohydrin , chelation , enantiomer , medicinal chemistry , atropisomer , enantiomeric excess , stereochemistry , microcrystalline , organic chemistry , catalysis , crystallography , alkyl
Enantiomerically pure chelate ligands L 2 = XCH 2 CH(OH)CH 2 Y ( 1 ) are obtained from epichlorohydrin in a two‐step synthesis. × and Y may be different types of R 2 P donor groups, NR 2 or SR donors. The OH function of 1 may be transformed into an ether function under specialized conditions. Ligands 1 react with [Rh(COD)Cl] 2 in the presence of KPF 6 to give the coordination compounds 2 , [L 2 Rh(COD)] + PF 6 − , as orange, microcrystalline salts. The structures adopted by compounds 2 in the solid state have been analysed by X‐ray crystallography in selected cases, and it has been found that the six‐membered chelate cycles adopt twist as well as chair conformations depending on the nature of × and Y. In solution, compounds 2 generally show dynamic behaviour, which is in part due to the conformational flexibility of the six‐membered cycles. In cases where one of the PR 2 donor groups contains ortho ‐substituted phenyl substituents, rotational isomerism of these groups is an additional dynamic process. For some of these compounds, the nature of the dynamic behaviour has been analysed by variable‐temperature NMR experiments. Compounds 2 are found to be precatalysts in the hydrogenation of ( Z )‐2‐acetamidocinnamic acid. The rate of conversion is strongly influenced by the steric bulk of the substituents, with smaller substituents leading to higher rates. Enantiomeric discrimination is high only for those ligands that contain ortho ‐substituted aryl groups at their PR 2 donors. The maximum enantiomeric excess observed was 85% for × = PPh 2 , Y = P(2‐MeOPh) 2 .