Stereoselectivity in Coordination Compounds

Alfred Werner conjectured as early as 1899 that octahedrally coordinated metal complexes should occur in nonidentical minor image isomers. For such objects. Lord Kelvin, in 1893, had coined the adjective chiral , a term never used by Werner. It can be proved by examination of the original sample of [Co(NC)2)2(en) Br, prepared by Edith Humphrey, a Ph.D. student of Wemer s, that crystals of optically pure samples were obtained in Werner s laboratory as early as 1899 or 1900. However, Werner did not publish die first successful resolution of an octahedral metal complex until 1911. Presently, interest in chirality in coordination compounds is booming, mainly because of the importance of coordination compounds in enantioselective homogeneous catalysis. Other interesting sq )plications are enantioselective interactions of chiral coordination species with biomolecules, and the stereoselective synthesis of multicenter systems. 

Many of the compounds used have additional functional groups, including ester, amide, ether, and acetal. These groups may be involved in coordination to samarium and thereby influence the stereoselectivity of the reaction. 

Contents K. Bemauer Diastereoisomerism and Dia-stereoselectivity in Metal Complexes M. S. Wrighton Mechanistic Aspects of the Photochemical Reactions of Coordination Compounds. 

After Werner s death in 1919, there was little activity in the field of stereochemistry of coordination compounds. An exception is found in the work of Yuji Shibata, who had been one of Werner s students, and who continued with excellent stereochemical work when he returned to Japan. His work on the enzyme-like activity of cobalt complexes furnishes especially interesting examples of stereoselectivity and of the catalytic action of such compounds (6). T. P. McCutcheon and V. L. King, Americans who had done their theses on stereochemical topics under Werner s guidance, did not continue in that field. King, who actually performed the first resolution of an asymmetric complex ( 7), went into industrial work. McCutcheon became a member of the faculty at the University of Pennsylvania and did research on complex compounds, but not on their stereochemistry. 

The first stereoselective syntheses of coordination compounds were performed by Jonassen and Huffman (35) who treated [Co(en)2CO3with jd-tartaric acid. This gave a mixture of D-and L-[Co(en)2d-tart]+, which, upon treatment with ethylene-diamine at 50°, gave a 70% yield of D-[Co(en)3]3+. Either the two tartrato isomers were formed in unequal amounts, or the less... 

A classical method for the preparation of enantiopure compounds is the resolution of racemate. However, it is much more effective to use the selective synthesis of the desired enantiopure substance via enantioselective approach. Stereoselective methods of synthesis have been widely developed in organic chemistry. The method of asymmetric synthesis has been known since the nineteenth century and asymmetric catalysis has witnessed an enormous amount of development in recent decades as shown in Chapter 3. In contrast, the asymmetric synthesis of coordination compounds has only recently become a subject of systematic investigation. This is no doubt related to the fact that the chirality of coordination compounds is a much more complex phenomenon than that of organic compounds, because of higher coordination and the multitude of possible central atoms. Furthermore, while in organic chemistry the chiral tetrahedral carbon centres can be prepared without racemization, in contrast T-4 metal centres are very often labile. In fact it is even difficult to prepare compounds with a metal centre coordinated to four different monodentate ligands, and thus the possibility of obtaining one enantiomer is excluded in most cases. 

R, S ) diastereomers with complete selectivity. Moreover, for the compounds containing three chiral stereocenters there was considerable stereoselectivity in favor of the (R, S )-anti diastereomer. For (56 X — AsPh), (R, S )-syn (R, S )-anti =5 4 for (56 X = PPh) and for (59), the corresponding ratios were 5 1 and 100 1 (no detection of minor diastereomer by NMR spectroscopy), respectively. The chelating (R, S ) diastereomers in each case were identified by preparing molyb-denum(O) carbonyl derivatives. The R, S )-syn diastereomers of the ligands gave fully coordinated /ac-tricarbonylmolybdenum complexes and the R, S )-anti forms the As2- or Pj-chelated cis-tetracarbonylmolybdenum complexes. 

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