Hydrogenation amino acid complexes

Kim, Chin, and co-workers have described a highly interesting oxyanion hole mimic that transforms L-amino acids to D-amino acids [97]. The mechanism involves stabilization of the enolate intermediate by an internal hydrogen bond array generated by urea group (Scheme 4.14). In the presence of an external base, such as triethylamine, the receptors readily promote the epimerization of a-amino acids, favoring the D-amino acids due to unfavorable steric interactions in the receptor-L-amino acid complex. These receptors can also be viewed as chiral mimics of pyridoxal phosphate [98]. 

The use of rhodium-amino acid complexes in catalytic hydrogenation has been reported by Rajca to be capable of reducing a wide variety of aromatic and heteroaromatic compounds under mild conditions (1 atm, 22 °C, DMF 1 atm = 101 kPa). Thus, the rhodium-anthranilic acid catalyzed hydrogenation of pyrrole under these mild conditions yields a 2 1 mixture of pyrrolidine and 2,5-dihydropyrrole (53% conversion after 8 h)." Recently, Lunn found that the hydrogenation of pyrrole can be carried out with a nickel-aluminum alloy, as generated with aqueous KOH, to give pyrrolidine in 58% yield, albeit relatively slowly (4 d, r.t.)."... 

Hemocyanin possesses a catalase-like action (10, 12), albeit a weak one, in that it decomposes hydrogen peroxide into water and oxygen just as the hemeprotein catalase. The catalase-like action of hemocyanin is due to the protein-bound copper since neither copper-free hemocyanin nor free copper ions exhibit activity under equivalent conditions. Copper-amino acid complexes have been tested for catalase-like action, and it was reported that only the copper-arginine complex possesses catalase-like activity (13). However, we have found (37) as have other investigators (29, 40), that other amino acid complexes of copper also decompose H202. 

The amino acid complexes [(q -arene)Ru(aa)Cl], where aa = N,0-chelated amino acid, undergo rapid epimerization even at temperatures below 0 °C [66,67]. Similar lability and epimerization has been found for sahcylaldiminato complexes [68]. In contrast, [(ri -p-cymene)Ru(imine)Cl] complexes are configurationally stable unless heated to high temperatures in polar solvents [69]. Configurationally-stable half sandwich Ru(If) arene complexes are ofmuch interest as potential asymmetric catalysts (e.g. for Diels-Alder and Mukaiyama reactions) [63]. The chloride ligand is usually removed in order to allow the complex to act as a Lewis add catalyst and exchange ofcoordinated water on [(r -l,3,5-trimethylbenzene)Ru(pyridyloxazoline-N,N )(H20)] is slow on the NMR timescale in acetone-water [70]. Chiral chloro arene Ru(II) catalysts for asymmetric transfer hydrogenation can be activated in situ by treatment with KOH in 2-propanol [71]. 

For most amino acids the R represents a complex organic group in glycine, the R represents H (hydrogen). Amino acids are usually known by their common rather than chemical names. For instance glycine, with the structural formula H2N-CH2-COOH, is known chemically as amino acetic acid. For a discussion of the structure of amino acids, consult Harbome (1984) and Bhushan and Martens (1996). In living aqueous systems, amino acids exist predominantly as dipolar ions referred to as zwitterions and represented by the formula... 

Racemases are enzymes that catalyze the inversion of the chiral center by deprotonation of the C , followed by reprotonation on the opposite face of the planar carban-ionic transition-state species [13,14], In order to overcome the high energetic barrier of racemization, for example, on a-amino acids, some racemases employ pyridoxal phosphate (PLP) as a cofactor to use the resonance-stabilized amino acid complex as an electron sink because the estimated pK values for the C of amino acids are high, in the range 21-32 [14,15]. The formation of an imine PLP-substrate covalent bond makes the pK value of a-hydrogen of amino acids low. The second class of enzymes includes proline, aspartate, and glutamate racemases and diaminopimelate epimer-ase, with a cofactor-independent two-base mechanism [14],... 

The principle of this method depends on the formation of a reversible diastereomeric complex between amino acid enantiomers and chiral addends, by coordination to metal, hydrogen bonding, or ion—ion mutual action, in the presence of metal ion if necessary. L-Proline (60), T.-phenylalanine (61),... 

The copper complex is very stable at neutral pH, but it fades very rapidly in the presence of hydrogen ions. Other complex formers such as tartaric acid or citric acid and thiourea interfere with the reaction and, therefore, should not be included in mobile phases used for the separation of amino acids [3]. 

An early success story in the field of catalytic asymmetric synthesis is the Monsanto Process for the commercial synthesis of l-DOPA (4) (see Scheme 1), a rare amino acid that is effective in the treatment of Parkinson s disease.57 The Monsanto Process, the first commercialized catalytic asymmetric synthesis employing a chiral transition metal complex, was introduced by W. S. Knowles and coworkers and has been in operation since 1974. This large-scale process for the synthesis of l-DOPA (4) is based on catalytic asymmetric hydrogenation, and its development can be... 

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