Enantioselective pi-complexation. Synthesis of enantiomerically pure planar chiral ruthenocenes

Reaction of half-sandwich complexes Cp*RuL(2)(ch), featuring a chiral. auxiliary ligand L(2)(ch) with prochiral cyclopentadienides Cp(pc-) gave enantiomerically pure (>95% eel or enriched ruthenocenes Cp*RuCp(pc) 4. For the chiral auxiliary L(2)(ch) were tested a pyridyloxazoline (ref 15), the diolefin nopadiene (ref 16), and the amino acids methionine and proline, which form relatively labile complexes with the Ru(II) fragment Cp*Ru+. For the prochiral cyclopentadienide Cp(pc)Li, Li derivatives of 1-tert-butyl-3-methylcyclopentadienide, 1-tert-butylindenyl, and 1-tert-butyl-3-benzylindenyl were used. Determination of enantiomeric excess was achieved through a derivatization procedure generating fulvene cation complexes [Cp(pc)RuC(5)-Me(4)CH(2)](+) (5), which, after the addition of (S)-alpha-phenylethylamine, yield mixtures of diastereoisomers CP(pc)RUC(5)H(4)CH(2)NHCH(Me)Ph (6). The absolute configuration of 4e (Cp(pc) 1-tert-butyl-3-benzylindenyl) was determined by X-ray structure analysis of the respective fulvene salt. Asymmetric induction was greatest with the amino acids and was reverted with the epimeric amino acid; thus, (S)-proline or -methionine gave (S)-ruthenocene 4e and vice versa.