Enantioselective quenching, metal complexes

A particularly interesting paper deals with chiral discrimination in electronic energy transfer processes between dissymmetric metal complexes in solution . Time resolved luminescence measurements show that enantioselective excited state quenching occurs. 

Another example is given by the interaction of chiral metal complexes with fluorescent chiral analyte bearing metal binding sites. Enantioselective quenching of fluorescence can be observed in such cases as a result of coordination or ligand... 

Fig. 5 Examples of quenching enantioselective sensors involving metal ions (a) JV.W -dioxide of the 1,8-diacridylnaphtalene units which can form a scandium(m) complex (b) high enantioselective quenching obtained by the scandium complex of 11 for 2-aminopropanol (reprinted with permission from [64]. Copyright 2008 American Chemical Society) (c) triamino-diamido copper (II) complex (12) (d) Zn (II) complex with terpyridine based chiral ligand (13) (e) chiral oxazoline containing maleimido polymers (14)...

Metal-centered circularly polarized luminescence (CPL) can also be observed for rare earth complex. Since the luminescence from a chiral rare earth complex is circularly polarized, the chirality of the excited states can be determined. Most reported CPL studies have dealt with luminescent Eu " " and Tb + complexes. Racemic Tb " " complex 16 generates non-racemic excited states upon interaction with chiral species. Its CPL signals are enantioselectively quenched by biological vitamin B12 derivatives (Meskers and Dekkers, 2001), cytochrome c... 

The use of ethylene adduct lb is particularly important when the species added to activate catalyst la is incompatible with one of the reaction components. Iridium-catalyzed monoallylation of ammonia requires high concentrations of ammonia, but these conditions are not compatible with the additive [Ir(COD)Cl]2 because this complex reacts with ammonia [102]. Thus, a reaction between ammonia and ethyl ciimamyl carbonate catalyzed by ethylene adduct lb produces the monoallylation product in higher yield than the same reaction catalyzed by la and [Ir(COD)Cl]2 (Scheme 27). Ammonia reacts with a range of allylic carbonates in the presence of lb to form branched primary allylic amines in good yield and high enantioselectivity (Scheme 28). Quenching these reactions with acyl chlorides or anhydrides leads to a one-pot synthesis of branched allylic amides that are not yet directly accessible by metal-catalyzed allylation of amides. 

Marchelli used the copper(II) complex of histamine-functionalized P-cy-clodextrin for chiral recognition and separation of amino acids [27]. The best results were obtained for aromatic amino acids (Trp). Enantioselective sensing of amino acids by copper(II) complexes of phenylalanine-based fluorescent P-cyclodextrin has been recently published by the same author [28, 29]. The host containing a metal-binding site and a dansyl fluorophore was shown to form copper(II) complexes with fluorescence quenching. Addition of d- or L-amino acids induced a switch on of the fluorescence, which was enantioselective for Pro, Phe, and Trp. This effect was used for the determination of the optical purity of proline. 

The (-)-enantiomer of sparteine is obtained from natural sources as a commercially available and relatively inexpensive oil. The metallation of 1 with n-BuLi in hexane, in the presence of (-)"Sp, produced the a-carbanion 2 which was subsequently quenched affording the known phosphine oxide 3 in approximately 14% ee. Although it has been suggested that the low stereoselectivity is due to the disruption of the (-)-sp/BuLi complex by complexation of the oxygen atom to the lithium cation, the same reaction with unprotected dimethylphenylphosphine provides completely racemic products. Due to the low enantioselectivity, this method was initially ignored in phosphorus chemistry until a seminal paper of Evans and co-workers in which they described similar reactions with phosphine boranes and sulfides. In this case, the enan-tioselectivities were much higher and the method could be successfully applied... 

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