Zinc, hydrolysis catalyst

The effect of zinc catalysts on PET hydrolysis has been investigated by Campanelli et al.62 in the temperature range 250-280 °C. The catalytic effect of zinc salts was attributed to the electrolytic changes induced in the polymer-water interface during hydrolysis. Likewise, Kao et al.63 concluded the existence of an autocatalytic mechanism in PET hydrolysis because the depolymerization reaction is catalysed by the carboxyl groups produced during the reaction. 

It was first described in 1608 when it was sublimed out of gum benzoin. It also occurs in many other natural resins. Benzoic acid is manufactured by the air oxidation of toluene in the liquid phase at 150°C and 4-6 atm. in the presence of a cobalt catalyst by the partial decarboxylation of phthalic anhydride in either the liquid or vapour phase in the presence of water by the hydrolysis of benzotrichloride (from the chlorination of toluene) in the presence of zinc chloride at 100°C. 

The electrophile 4 adds to the aromatic ring to give a cationic intermediate 5. Loss of a proton from 5 and concomitant rearomatization completes the substitution step. Subsequent hydrolysis of the iminium species 2 yields the formylated aromatic product 3. Instead of the highly toxic hydrogen cyanide, zinc cyanide can be used. The hydrogen cyanide is then generated in situ upon reaction with the hydrogen chloride. The zinc chloride, which is thereby formed, then acts as Lewis acid catalyst. 

In 1965, Breslow and Chipman discovered that zinc or nickel ion complexes of (E)-2-pyridinecarbaldehyde oxime (5) are remarkably active catalyst for the hydrolysis of 8-acetoxyquinoline 5-sulfonate l2). Some years later, Sigman and Jorgensen showed that the zinc ion complex of N-(2-hydroxyethyl)ethylenediamine (3) is very active in the transesterification from p-nitrophenyl picolinate (7)13). In the latter case, noteworthy is a change of the reaction mode at the aminolysis in the absence of zinc ion to the alcoholysis in the presence of zinc ion. Thus, the zinc ion in the complex greatly enhances the nucleophilic activity of the hydroxy group of 3. In search for more powerful complexes for the release of p-nitrophenol from 7, we examined the activities of the metal ion complexes of ligand 2-72 14,15). 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Benzylacetophenone has been prepared by the reduction of benzalacetophenone with zinc and acetic acid1 and catalytic-ally with palladium and hydrogen 2 by the reduction of /3-duplo-benzylidene acetophenone monosulfide 3 by the oxidation of the corresponding car bind with chromic acid 4 by the hydrolysis of ethyl benzyl benzoylacetate 5 from acetophenone and benzyl chloride by the action of sodamide 6 and from benzoic and hydrocinnamic adds using as catalysts manganese oxide 7 and ferric oxide.8... 

Neutral PET hydrolysis usually takes place under high temperature and pressure in die presence of alkali metal acetate transesterification catalysts.28 It is diought diat the catalytic effect observed on the part of zinc salts is the result of electrolytic changes induced in die polymer-water interface during the hydrolysis process. The catalytic effect of zinc and sodium acetates is thought to be due to die destabilization of die polymer-water interface in the hydrolysis process. 

Lewis-Acid Catalyzed. Recently, various Lewis acids have been examined as catalyst for the aldol reaction. In the presence of complexes of zinc with aminoesters or aminoalcohols, the dehydration can be avoided and the aldol addition becomes essentially quantitative (Eq. 8.97).245 A microporous coordination polymer obtained by treating anthracene- is (resorcinol) with La(0/Pr)3 possesses catalytic activity for ketone enolization and aldol reactions in pure water at neutral pH.246 The La network is stable against hydrolysis and maintains microporosity and reversible substrate binding that mimicked an enzyme. Zn complexes of proline, lysine, and arginine were found to be efficient catalysts for the aldol addition of p-nitrobenzaldehyde and acetone in an aqueous medium to give quantitative yields and the enantiomeric excesses were up to 56% with 5 mol% of the catalysts at room temperature.247... 

Phosphate ester crystal structures have been determined of zinc 1,5,9-triazacyclononane including an interesting structure containing an oligophosphate bridged zinc unit.450 The zinc complex of 1,5,9-triazacyclododecane was studied as a hydrolysis catalyst for substituted phenyl acetates.451 Kinetic analysis suggested that hydrolysis occurs by a mechanism involving hydroxide attack of a metal-bound carbonyl. 

Although zinc, cadmium, and mercury are not members of the so-called main-group elements, their behavior is very similar because of their having complete d orbitals that are not normally used in bonding. By having the filled s orbital outside the closed d shell, they resemble the group IIA elements. Zinc is an essential trace element that plays a role in the function of carboxypeptidase A and carbonic anhydrase enzymes. The first of these enzymes is a catalyst for the hydrolysis of proteins, whereas the second is a catalyst for the equilibrium involving carbon dioxide and carbonate,... 

This reaction is essential in maintaining a constant pH in blood by the bicarbonate buffer system. Carbonic anhydrase, which contains a single zinc atom in its structure, has a molecular weight of about 30,000. In this structure, zinc is surrounded tetrahedrally by three histidine molecules and one water molecule. The exact role of the catalyst is not known, but it is believed to involve hydrolysis that can be represented as... 

The finding that the hydrolytic activity of the enzyme is retained after replacement of a tyrosine residue by phenylalanyl challenges the notion that a tyrosine acts as a general acid catalyst in peptide hydrolysis. It has been suggested that either the protonated Glu270 moiety or the zinc-water complex could perform the proton transfer [77]. 

Chiral amines, ArCH(R)NH2, can be prepared by addition of a dialkylzinc to A-(diphenylphosphinoyl)imines, ArCH=N—P(=0)Ph2, using a suitable auxiliary, followed by acid hydrolysis to cleave the phosphorus moiety. A series of 2-azanorbornylmethanols (65) give ee% up to 92%, and they also induce some enantioselectivity in additions to benzaldehyde. A highly organized transition state with two zincs is proposed one coordinates the nitrogens of substrate and catalyst and the other coordinates the oxygens. 

This type of alkoxylation chemistry cannot be performed with conventional alkali metal hydroxide catalysts because the hydroxide will saponify the triglyceride ester groups under typical alkoxylation reaction conditions. Similar competitive hydrolysis occurs with alternative catalysts such as triflic acid or other Brpnsted acid/base catalysis. Efficient alkoxylation in the absence of significant side reactions requires a coordination catalyst such as the DMC catalyst zinc hexacyano-cobaltate. DMC catalysts have been under development for years [147-150], but have recently begun to gain more commercial implementation. The use of the DMC catalyst in combination with castor oil as an initiator has led to at least two lines of commercial products for the flexible foam market. Lupranol Balance 50 (BASF) and Multranol R-3524 and R-3525 (Bayer) are used for flexible slabstock foams and are produced by the direct alkoxylation of castor oil. 

Hydrolysis of the enamine 14 furnishes citronellal (15) in high optical purity (ca. 99% ee) which gives 17 via ene cyclization with zinc bromide as catalyst. The diastereoselectivity of this step is the result of simple diastereoselection in a trans-decalin-like transition state 16. Catalytic hydrogenation converts the olefin 17 into (—)-menthol (18). Despite its elegance this novel route has not been able to replace the older resolution-based procedure described earlier in this section. 

The potentiometric pH titration disclosed monodeprotonation with a pK value of 7.6 at 25°C. On the basis of NMR spectroscopic and anionbinding studies of 13, the monodeprotonated species was assigned to the OH -bound ZnL complex 14, rather than the pendent alkoxide complex as seen with [12]aneN3 8b. Nevertheless, among all past zinc(II) complexes, 14 seems to be the most active catalyst for NA hydrolysis. From kinetic studies in 10% (v/v) CH3CN at 25°C and pH... 

Chapman and Breslow synthesized zinc(II) complexes of monomer and dimers derived from 1,4,7-triazacyclododecane with phenyl 48 and 4,4 -biphenyl linkers 49 (55). They were examined as catalysts for the hydrolysis of 4-nitrophenyl phosphate (NP2 ) and bis(4-nitrophenyl) phosphate (BNP ) in 20% (v/v) DMSO at 55°C. On the basis of the comparison of the pseudo-first-order rate constants, the dinuclear zinc(II) complexes 48 with 1,3-phenyl and 1,4-phenyl linkers are ca. 5 times more efficient than monomer or 49 in the hydrolysis of NP2, leading to the conclusion that the two zinc(II) ions are simultaneously involved in the hydrolysis, as in the enzyme alkaline phosphatase. For the hydrolysis of BNP, a longer dimer 49 is ca. six times more effective than 1,3-phenyl-linked dimer 48 and monomers. 

The zinc complex of (HOCH2)3CNH2(tris) is a most effective catalyst for the hydrolysis and aminolysis of benzylpenicillin, 1 and it is suggested that this observation may be of some relevance to the mode of action of zinc-dependent /3-lactamases. 

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