Hydroxyl group catalysis

Bender and Glasson (1959), in studies of alcoholysis and hydrolysis of alkyl esters in aqueous alcohol, found that the rate of disappearance of ester is decreased by increasing alcohol concentration. However, product analysis led to the conclusion that both methanolysis and ethanolysis are faster than hydrolysis in alcohol-water mixtures. It was calculated that in pure water attack by hydroxide, methoxide and ethoxide ions would occur at about the same rates. 

Bruice and Lapinski (1958) reported that logarithms of second-order rate constants for reaction of p-substituted phenoxide ions with p-nitrophenyl acetate were a linear function of the p/ a-value of the phenol with a slope of 0-8. Phenolate ions cannot displace 

Since serine-195 is the site of acylation, a good deal of recent work has been directed towards determining the efficiency of neighboring alcohol and phenol groups as intramolecular nucleophiles in ester and amide hydrolysis. In comparison with acetamide and butyramide, the hydrolysis of 7-hydroxybutyramide [equation (17)] in the alkaline and neutral pH-range is accelerated 800-fold (Bruice and Marquardt, 1962). These reactions are attack of alkoxide ion on the neutral and protonated amide function, 

Intermolecular alcoholysis of carbamate esters will also take place, but the reactions are very slow. In comparison with intermolecular nucleophilic attack by a phenoxide ion of the same p.fifa on the unsubstituted ester [28], the effective molarity of the neighbouring phenoxide ion of [27] is 3 x 108 M. Thus, a phenoxide ion is an 

Part of the great efficiency of the intramolecular reactions of [26] and [27] is undoubtedly due to the correct alignment of the rigidly held nucleophile and carbonyl group. Molecular models show that in one of the conformations of [27] in which steric interactions are minimized, the phenoxide ion is immediately adjacent to the carbonyl group and in an excellent position for perpendicular attack (Bender, 1960) but other factors must also be important. Correct orientation would not explain why anionic nucleophiles are superior to neutral nucleophiles. Extensive studies have not been carried out with nitrogen nucleophiles in carbamate ester hydrolysis, but Hegarty and Frost (1972) found that carbamate [29] underwent elimination to an isocyanate. This can be contrasted with the 

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Imidazole Catalysis Hydroxyl Group Catalysis Mechanistic Conclusions... 

D. Piszkiewicz and T. C. Bruise, Glycoside hydrolysis. I. Acetamido and hydroxyl group catalysis in glycoside hydrolysis, J. Am. Chem. Soc., 89 (1967) 6237-6243. 

Bruice, T. C., T. H. Fife, J. J. Bruno, and N. E. Brandon, Hydroxyl group catalysis. II. The reactivity of the hydroxyl group to serine. The nucleophilicity of alcohols and the ease of hydrolysis of their acetyl esters nucleophilicity as related to their pK s , Biochemistry, 1,7-12 (1962). 

Fig. 33. The intramolecular neighboring hydroxyl group catalysis in hydrolysis of methyl cis-4-hydroxy-tetrahydroftiran-2.carboxylatc (122)...

Acetals are readily formed with alcohols and cycHc acetals with 1,2 and 1,3-diols (19). Furfural reacts with poly(vinyl alcohol) under acid catalysis to effect acetalization of the hydroxyl groups (20,21). Reaction with acetic anhydride under appropriate conditions gives the acylal, furfuryUdene diacetate... 

Structure Modification. Several types of stmctural defects or variants can occur which figure in adsorption and catalysis (/) surface defects due to termination of the crystal surface and hydrolysis of surface cations (2) stmctural defects due to imperfect stacking of the secondary units, which may result in blocked channels (J) ionic species, eg, OH , AIO 2, Na", SiO , may be left stranded in the stmcture during synthesis (4) the cation form, acting as the salt of a weak acid, hydrolyzes in aqueous suspension to produce free hydroxide and cations in solution and (5) hydroxyl groups in place of metal cations may be introduced by ammonium ion exchange, followed by thermal deammoniation. 

The tritylone ether is used to protect primary hydroxyl groups in the presence of secondary hydroxyl groups. It is prepared by the reaction of an alcohol with 9-phenyl-9-hydroxyanthrone under acid catalysis (cat. TsOH, benzene, reflux, 55-95% yield).It can be cleaved under the harsh conditions of the WolfT-Kishner reduction (H2NNH2, NaOH, 200°, 88% yield), " and by electrolytic reduction (-1.4 V, LiBr, MeOH, 80-85% yield). It is stable to 10% HCl, 55 h. ... 

Most attempts to differentiate these hydroxyl groups with conventional derivatives resulted in the formation of a tetrahydrofuran. The dithiocarbonate can also be prepared by phase-transfer catalysis (Bu4N HS04T, 50% NaOH/H20, CS2, Mel, rt,. 5h) ... 

We have disclosed that the ligands 4c, 10, and 77, when complexed with a metal ion such as Zn2 +, Ni2+, or Co2+, become highly active toward the hydrolysis of p-nitrophenyl picolinate (7). The catalysis is most likely to occur through formation of a ternary complex in the transition state or in reactive intermediates. The metal ion in such a complex serves to activate the ligand hydroxyl group for nucleophilic attack and to orient the substrate into a favorable position to undergo the reaction. 

The decarbonylation of oxide-supported metal carbonyls yields gaseous products including not just CO, but also CO2, H2, and hydrocarbons [20]. The chemistry evidently involves the support surface and breaking of C - O bonds and has been thought to possibly leave C on the clusters [21]. The chemistry has been compared with that occurring in Fischer-Tropsch catalysis on metal surfaces [20] support hydroxyl groups are probably involved in the chemistry. 

It is evident that the supported clusters have a strong affinity for hydride ligands provided by the support. The process by which the support delivers these ligands is referred to in the catalysis literature as reverse hydrogen spillover. The opposite process (spillover), well known for supported metals [36], is shown by the theoretical results to be a redox process in reverse spillover, the support hydroxyl groups oxidize the cluster. 

The antibiotic activity of certain (3-lactams depends largely on their interaction with two different groups of bacterial enzymes. (3-Lactams, like the penicillins and cephalosporins, inhibit the DD-peptidases/transpeptidases that are responsible for the final step of bacterial cell wall biosynthesis.63 Unfortunately, they are themselves destroyed by the [3-lactamases,64 which thereby provide much of the resistance to these antibiotics. Class A, C, and D [3-lactamases and DD-peptidases all have a conserved serine residue in the active site whose hydroxyl group is the primary nucleophile that attacks the substrate carbonyl. Catalysis in both cases involves a double-displacement reaction with the transient formation of an acyl-enzyme intermediate. The major distinction between [3-lactamases and their evolutionary parents the DD-peptidase residues is the lifetime of the acyl-enzyme it is short in (3-lactamases and long in the DD-peptidases.65-67... 

The conclusions of the preceding discussion can be briefly summarized as follows. The formation of inclusion complexes in both the crystalline state as well as in solution has been convincingly demonstrated by spectral and kinetic techniques. Whereas the crystalline complexes are seldom stoichiometric, the solution complexes are usually formed in a 1 1 ratio. Although the geometries within the inclusion complexes cannot be accurately defined, it is reasonable to assume that an organic substrate is included in such a way to allow maximum contact of the hydrophobic portion of the substrate with the apolar cycloamylose cavity. The hydrophilic portion of the substrate, on the other hand, probably remains near the surface of the complex to allow maximum contact with the solvent and the cycloamylose hydroxyl groups. The implications of inclusion complex formation for specificity and catalysis will be elucidated in subsequent sections of this article. 

As noted in an earlier section of this article, the utility of the cycloamyloses as covalent catalysts is limited by the low reactivity of the catalytically active hydroxyl groups at neutral pH s and by the relatively slow rates of deacylation of the covalent intermediates. In an effort to achieve effective catalysis, several investigators have attempted to selectively modify the cycloamyloses by either (1) introducing an internal catalyst to facilitate deacylation or (2) introducing a more reactive nucleophile to speed acylation and/or deacylation. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

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