Carbonic acid chelate coordination

The enantioselectivity is the result of chelation of the chiral ligand to the zinc. The TS of the addition is believed to involve two zinc atoms. One zinc functions as a Lewis acid by coordination at the carbonyl oxygen and the other is the source of the nucleophilic carbon. The proposed TS for aminoalcohol A, for example, is shown below.149... 

We note that, with the lone participation of a carboxyl group in the coordination, the coordination modes examined earlier for the complexes of carbonic acids (Sec. 2.2.5.4, formulae 276 283) are possible. Additionally, a mixed binding amongst ligands is observed in many complexes of aminoacids, for example chelates 277, 414, and 0,0 -bridge 279 [714],... 

The imidazole ring of histidine can act as an acid-base catalyst, a nucleophile, or a chelator (coordinator) of metal ions. The first and third functions were illustrated by chy-motrypsin and carbonic anhydrase, respectively. 

The condensation reaction of coordinated amines with formaldehyde and nitro-alkanes is believed [166] to proceed through an imine (or aminol) intermediate sensitive to attack of the carbon acid conjugate base on the imine carbon atom (Scheme 2-11). The intramolecular path shown in this diagram is entropically favourable, so construction of the six-membered chelate cycle becomes possible and its formation proceeds in high yield. It should also be pointed out that the first condensation step involves nucleophilic attack on an HCHO molecule by a coordinated amido ligand arising from deprotonation of the metal-bound primary amine. 

Aldolases are a class of lyases that catalyze the reversible, stereoselective addition of an aldol donor component (nucleophile) to an acceptor component (electrophile) [9,75-81]. Typical products are 3-hydroxy ketones, a structural element that is frequendy incorporated in the framework of complex natural products. Mechanistically, aldolases promote the abstraction of the aldol donor a-proton, thereby generating a carbon nucleophile bound at the active site. In type I aldolases, an enamine is produced by covalent binding to a conserved lysine residue while in the type 11 aldolases, an enediol is formed by chelating coordination to an essential transition metal cation (mostly Zn +), which is acting as a Lewis acid promotor. Aldolases are often highly selective for the donor substrate, tolerating... 

This class of fluxes typically undergoes what is described as simple acid-base reactions with metal oxides, although chelation, coordinate covalent bonding, (i.e., formation of complexes), and electrochemical interactions (i.e., oxidation and reduction) may also be involved [102,104]. These types of reactions will be explained later. However, there can be subtle yet significant differences in performance related to molecular structure, carbon chain length, melting point, and boiling point. 

Reactions of the Hydroxyl Group. The hydroxyl proton of hydroxybenzaldehydes is acidic and reacts with alkahes to form salts. The lithium, sodium, potassium, and copper salts of sahcylaldehyde exist as chelates. The cobalt salt is the most simple oxygen-carrying synthetic chelate compound (33). The stabiUty constants of numerous sahcylaldehyde—metal ion coordination compounds have been measured (34). Both sahcylaldehyde and 4-hydroxybenzaldehyde are readily converted to the corresponding anisaldehyde by reaction with a methyl hahde, methyl sulfate (35—37), or methyl carbonate (38). The reaction shown produces -anisaldehyde [123-11-5] in 93.3% yield. Other ethers can also be made by the use of the appropriate reagent. 

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