Bidentate compounds

Mclsaac, L. D. Baker, J. D. Krupa, J. F. LaPointe, R. E. Meikrantz, D. H. Schroeder, N. C. "Study of Bidentate Compounds for the Separation of Actinides from Commercial LWR Reprocessing Waste," Allied Chemical Idaho Chemical Programs Report ICP-1180, Idaho Falls, Idaho, February 1979. 

A review8 with more than 186 references discusses the synthesis of Rh and Pd complexes with optically active P,N-bidentate ligands and their applications in homogeneous asymmetric catalysis. The effect of the nature of the P,N-bidentate compounds on the structure of the metal complexes and on enantioselectivity in catalysis was examined. Allylic substitution, cross-coup-ling, hydroboration and hydrosilylation catalyzed by Rh or Pd complexes with optically active P,N-bidentate ligands are considered. 

Proline is a stable, nontoxic, cyclic, secondary pyrrolidine-based amino acid with an increased pK value. Thus, proline is a chiral bidentate compound that can form catalytically active metal complexes (Melchiorre et al. 2008). Bidentate means that proline has not only one tooth but also a second one to bite and react. The greatest difference to other amino acids is a Lewis-base type catalysis that facilitates iminium and enamine-based reactions. It is especially noteworthy that cross-aldol condensations of unprotected glycoladehyde and racemic glyceralde-hyde in the presence of catalytic amounts of the Zn-(proline)2 gave a mixture of pentoses and hexoses (Kofoed et al. 2004). Again, proline seems to play the decisive role. The conditions are prebiotic the reaction proceeded in water for seven days at room temperature. It is remarkable that the pentose products contained ribose (34%), lyxose (32%), arabinose (21%), and xylose (12%) and that all are stable under the conditions. Thus, the diastereomeric and enantiomeric selection observed support the idea that amino acids have been the source of chirality for prebiotic sugar synthesis. 

The aim of this chapter is to discuss the recent examples of chiral recognition in organometallic and coordination compounds. Most of the examples presented are related to octahedral tris(bidentate) compounds with helical chirality. The homo-chiral... 

A typical series of bidentate compounds are (8.138). While chain length does not appear greatly to affect bond angles at the P atom, it may decide between cis or trans isomer formation. Thus when molybdenum hexacarbonyl reacts with a two-carbon phosphine the trans compound (8.139a) is formed, but with a single carbon phosphine the cis derivative (8.139b) is obtained. 

The D-enantiomer of such bidentate compounds was generally retained more than L-enantiomer. Mixtures of methanol/acetonitrile/water or dichloromethane/methanol were often used as eluents. Chiral recognition based on CLEC was also involved in the enantiomer separation of amino acids and (3-adrenergic blocking agents on silica gel plates coated with the copper(II) complex of enantiomeric amino acids (L-proline, L-arginine, and lR,3R,5R-2-azobicyclo [3.3.0]octan-3-carboxylic acid). 

Since cyclometalation reactions proceed extremely easily, many reactions have been tried with many substrates, and the number of reaction substrates employed has increased rapidly as a result. Many representative substrates are shown in Fig. 5.11 [58]. These substrates are derived from bidentate compounds or tridentate compounds. The tridentate substrates are usually called pincer compounds. Tridentate pincer compounds are used as catalysts, with an phasis on their stability as catalysts. 

Finally, a few references have appeared in which bidentate compounds have been cyclized using well understood chemistry to produce some novel heterocycles. There are numerous examples of 5- and 6-membered diazaphos-phorus heterocycles but there are far fewer with smaller or larger rings. Dichlorophosphinyl carbamates, prepared from isocyanates, have been cyclized with diamines to afford the novel benzodiazaphosphepin system (395). Oxadiazepins (396) have been prepared by reaction of the amino-oximes with formaldehyde and 1,3,4,6-thiatriazepins (397) were obtained by Moss and Taylor using cyclic thioureas. 

Applications of CLEC in planar chromatography are concerned with the enantiomeric separation of several polar organic compounds, particularly a-amino acids and their derivatives, a-hydroxy acids, and peptides. These applications were almost the same tested on the commercially available Chiralplates, whose preparation was identical to that adopted for homemade plates. The o-enantiomer was retained more than L-enantiomer for all bidentate compounds, such as neutral... 

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