ENANTIOSELECTIVE PI-COMPLEXATION - SYNTHESIS OF ENANTIOMERICALLY PUREPLANAR CHIRAL RUTHENOCENES

Reaction of half-sandwich complexes CpRuL(2)(ch), featuring a chiral. auxiliary ligand L(2)(ch) with prochiral cyclopentadienides Cp(pc-) g ave enantiomerically pure (>95% eel or enriched ruthenocenes CpRuCp(p c) 4. For the chiral auxiliary L(2)(ch) were tested a pyridyloxazoline (ref 15), the diolefin nopadiene (ref 16), and the amino acids methio nine and proline, which form relatively labile complexes with the Ru(I I) fragment CpRu+. For the prochiral cyclopentadienide Cp(pc)Li, Li d erivatives of 1-tert-butyl-3-methylcyclopentadienide, 1-tert-butylinde nyl, and 1-tert-butyl-3-benzylindenyl were used. Determination of enan tiomeric excess was achieved through a derivatization procedure genera ting fulvene cation complexes [Cp(pc)RuC(5)-Me(4)CH(2)](+) (5), which, after the addition of (S)-alpha-phenylethylamine, yield mixtures of d iastereoisomers CP(pc)RUC(5)H(4)CH(2)NHCH(Me)Ph (6). The absolute conf iguration of 4e (Cp(pc) 1-tert-butyl-3-benzylindenyl) was determined b y 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)-rutheno cene 4e and vice versa.