Dirhodium complexes of amino acid derivatives: separation and characterization by circular dichroism spectroscopy

This paper reports on the synthesis and chiroptical spectroscopic characterization of Rh-2(OAc)(4-n)(O-Phe-Z)(n) (n = 1-4, abbreviated as Rh2L1, Rh2L2, Rh2L3 and Rh2L4) complexes. Mixtures of these complexes were prepared in MeCN and CHCl3 solvents from a Rh-2(OAc)(4) core complex and the chiral N-protected amino acid N-benzyloxycarbonyl-L-phenylalanine (Z-Phe-OH). The components of the in situ mixture of complexes proved to be stable enough to be separated by flash chromatography followed by preparative HPLC. The in situ mixtures prepared at different Z-Phe-OH to Rh-2(OAc)(4) concentration ratios contained different amounts of the Rh2L1-Rh2L4 complexes. ECD spectra of the individual complexes were measured in MeCN, CHCl3 and EtOH. This is the first time that dinuclear rhodium complexes comprising 1-4 chiral ligands have been characterized by ECD spectroscopy. It was found that the structure of the complexes depends not only on the number of the chiral Z-Phe-O (-) ligands but also on the donor character of the solvent. (C) 2008 Elsevier Ltd. All rights reserved.