Metal absolute configuration

Figure B The absolute configuration of the optical isomers of a metal tris-chelates complex such as [Co(en)3]. (a) A configuration and (b) A configuration.

Metalated SAMP- or RAMP-hydrazones derived from alkyl- or arylethyl ketones 3 add to arylaldehydes both diastereo- and enantioselectively. Substituted / -hydroxy ketones with relative syn configuration of the major diastereomer are obtained with de 51-80% and 70-80% ee. However, recrystallization of the aldol adducts, followed by ozonolysis, furnishes diastereo- and enantiomerically pure (lS, S )-. yn-a-mcthyl-/3-hydroxy ketones 5 in 36-51% overall yield. The absolute configuration of the aldol adducts was established by X-ray crystallographic analysis. Starting from the SAMP- or RAMP-hydrazone either enantiomer, (S,S) or (R,R), is available using this methodology16. 

Double metalation of the protected (2/ .3S)-4-nitro-l, 2,3-butanetriol 4 and addition of benzaldehyde furnished, after removal of the protecting groups, a single crystalline diastereomer. The absolute configuration of the newly formed asymmetric carbons is assumed to be cither (4R,5S) or (4S,5R)ls. 

The absolute configuration of transition metal complexes. R. D. Gillard and P. R. Mitchell, Struct. Bonding (Berlin), 1970,7, 46-86 (165). 

Absolute configurations of metal complexes determined by X-ray analvsis. Y. Saito, Coord. Chem. Rev., 1974,13,305-337 (52). 

Utilization of the Pfeiffer effect and outer-sphere complexation for the prediction of absolute configurations of optically active metal complexes. S. Kirschner and I. Bakkar, Coord. Chem. Rev., 1982,43, 325-335 (27). 

Gillard RD, Mitchell PR (1970) The Absolute Configuration of Transition Metal Complexes. 7 46-86... 

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

C. J. Hawkins, Absolute Configuration of Metal Complexes, Wiley-Interscience, NY, 1971, Chap. 1. 

---