Chiral metal complexes configurational stability

An extensive review appeared on the configurational stability of enantiomeric organolithium reagents and the transfer of the steric information in their reactions. From the point of view of the present chapter an important factor that can be evaluated is the ease by which an inversion of configuration takes place at the metallation site. It happens that H, Li, C and P NMR spectra of diastereotopic species have been central to our understanding of the epimerization mechanism depicted in equation 26, where C and epi-C represent the solvated complex of one chiral species and its epimer, respectively. It has been postulated that inversion of configuration at the Li attachment site takes place when a solvent-separated ion pair is formed. This leads to planarization of the carbanion, its rotation and recombination to form the C—Li bond, as shown in equation 27, where Li+-L is the solvated lithium cation. An alternative route for epimerization is a series of... 

The synthesis of functionalized zinc organometallics can be accomplished with a variety of methods that have been developed in recent years. The intrinsic moderate reactivity of organozinc reagents can be dramatically increased by the use of the appropriate transition metal catalyst or Lewis acid. Furthermore, the low ionic character of the carbon-zinc bond allows the preparation of a variety of chiral zinc organometallics with synthetically useful configurational stability. These properties make organozinc compounds ideal inteimediates for the synthesis of complex and polyfunctionalized organic molecules. 

An important issue in these chiral complexes is the configurational stability at the metal centre. A complex may be configurationally stable over weeks while others epimerize or racemize rapidly even at low temperatures. Thus in one case complexes may be... 

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