Copper complexes ester hydrolysis

The use of a lipophilic zinc(II) macrocycle complex, 1-hexadecyl-1,4,7,10-tetraazacyclododecane, to catalyze hydrolysis of lipophilic esters, both phosphate and carboxy (425), links this Section to the previous Section. Here, and in studies of the catalysis of hydrolysis of 4-nitrophenyl acetate by the Zn2+ and Co2+ complexes of tris(4,5-di-n-propyl-2 -imidazolyl)phosphine (426) and of a phosphate triester, a phos-phonate diester, and O-isopropyl methylfluorophosphonate (Sarin) by [Cu(A(A(A/,-trimethyl-A/,-tetradecylethylenediamine)l (427), various micellar effects have been brought into play. Catalysis of carboxylic ester hydrolysis is more effectively catalyzed by A"-methylimidazole-functionalized gold nanoparticles than by micellar catalysis (428). Other reports on mechanisms of metal-assisted carboxy ester hydrolyses deal with copper(II) (429), zinc(II) (430,431), and palladium(II) (432). 

Micelle-forming copper complexes were found to effectively discriminate between enantiomers in the hydrolysis of a-amino esters (257). Hydrolysis of (.V)-phenylalanine p-nitrophenyl ester is 14-fold faster than its enantiomer, Eq. 223. Leucine affords 10-fold faster hydrolysis. The authors note that the micellar nature of these systems is extremely important for both rate of hydrolysis and selectivity (258). For example, the /V-mcthyl-dcrivcd ligand 419b leads to inhibition of the hydrolysis process, relative to catalysis by Cu(II) ion alone. 

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

It has been known for many years that the rate of hydrolysis of a-amino acid esters is enhanced by a variety of metal ions such as copper(II), nickel(II), magnesium(H), manganese(II), cobalt(II) and zinc(II).338 Early studies showed that glycine ester hydrolysis can be promoted by a tridentate copper(II) complex coupled by coordination of the amino group and hydrolysis by external hydroxide ion (Scheme 88).339 Also, bis(salicylaldehyde)copper(II) promotes terminal hydrolysis of the tripeptide glycylglycylglycine (equation 73).340 In this case the iV-terminal dipeptide fragment... 

Three new macrocyclic ligands (187) when complexed with zinc(II) could promote ester hydrolysis and a kinetic study of the hydrolysis of 4-nitrophenyl acetate in Tris buffer at pH 8.63 in 10% (v/v) MeCN was earned out with these.153 The hydrolysis of lipophilic esters is also catalysed by zinc(H) in a complex of a long alkyl-pendant macrocyclic tetraamine (188) in micellar solution.154 A study with a copper chloride-containing micelle has compared its effectiveness in the hydrolysis of esters and amides.155... 

A kinetic investigation of the hydrolysis of 8-acetoxyquinoline in solutions of pH between 1 and 9, and in the presence and absence of copper(II) ions, yielded some interesting results (205). In the absence of copper(II), it was found that the rate of hydrolysis was first-order with respect to 8-acetoxyquinoline, but the rate equation that fitted the kinetic data was quite complex since the ester and the ester cation reacted with both the hydrogen and the hydroxide ion. In the presence of copper(II), the hydrolysis of the ester occurred more rapidly and the rate equation was found to be first-order with respect to 8-acetoxyquinoline copper(II) and hydroxide ion. Therefore, the reaction intermediate (structure XXXVII) is presumably a 1 1 chelate of copper(II) which is attacked by hydroxide ion, just as in the case of the amino acid ester. 

The hydrolysis of an amino acid ester catalyzed by a copper complex is assumed to proceed through a coordination of the substrate to the complex followed by the hydrolysis of the coordinated substrate. The difference between the reactivities of... 

Two different binuclear copperdi) complexes have been prepared recently, one with a bridging phenoxy ligand having two bis-benzi-midazole arms (12, Fig. 14), and the second having a bis-cyclen-naphthalene ligand (13, Fig. 15) (352, 353). Both of them show bimetallic cooperativity for the hydrolysis of phosphate diesters, contrary to studies with the dinuclear cobalt complex (354). The pseudo-first-order rate constants for hydrolysis of the para-nitrophenylphosphate ester of propylene glycol by bis-benzimidazole-based copper complexes... 

Complexes of metal + ligand + protein or DNA can also catalyze the Diels Alder cycloaddition or oxidations with hydrogen peroxide. Copper complexes bound to DNA catalyzed the Diels-Alder cycloaddition with up to 99% ee [15, 16], Cu(phthalocyanine) complexed to serum albumin also catalyzed the enantioselective (98% ee) Diels-Alder reaction, but only with very high catalyst loading (10 mol%) and only with pyridine-bearing dienophiles (presumably to complex the copper) [17]. Achiral Cr(III) complexes or Mn(Schiff-base) complexes inserted into the active site of apomyoglobin variants catalyzed the sulfoxidation of thio-anisole with up to 13 and 51% ee, respectively [18, 19]. A copper phenanthroline complex attached to the adipocyte lipid-binding protein catalyzed the enantioselective hydrolysis of esters and amides [20]. 

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