Catalytic hydrolysis, intramolecular acetals

In aqueous Brij35 micellar solution, the copper(II) complex of macrocyclic Schifif base can only catalyze the hydrolysis of 1 by the mechanism that involves the nucleophilic attack of external hydroxide ion on the carbonyl carbon of substrate (ester), whereas the zinc(II) complex of same ligand can accelerate the hydrolysis of 1 more strongly than that of 4-nitrophenyl acetate by the intramolecular nucleophilic attack of zinc-bound hydroxide ion on carbonyl carbon of esters. The catalytic activity of Zn(II) complex is close to or higher than that of Cu(II) complex. The rate constants for the catalytic hydrolysis of bis(4-nitrophenyl) phosphate by complexes [(bpya)Cu]Cl2 and [(bpya)Zn]Cl2 (where bpya = 2,2 -dipyridylamine) in Brij35 micellar solution at 25°C and pH 7.02 are 1.2 X 10 times and 1.5 x 10 times higher than those for the spontaneous hydrolysis, respectively. ... 

In another estimate (Kirby and Percy, 1989), the carboxyl group in l-methoxymethoxy-8-naphthoic acid and the dimethylammonium group in the l-methoxymethoxy-8-A, A -dimethylnaphthylammonium ion are estimated to lead to rate increases by intramolecular catalysis of < ca. 900 and 1.9 X 10 compared to the value of ca. 1 x 10 calculated for the intramolecular catalytic effect of the carboxyl group in 2-methoxymethoxybenzoic acid. The salicylate ion remains the most efficient leaving group thus far discovered that can take part in hydrogen-bond catalysis of the hydrolysis of acetals. 

Mukaiyama aldol reactions using a catalytic amount of a Lewis acidic metal salt afford silylated aldols (silyl ethers) as major products, but not free aldols (alcohols). Three mechanistic pathways which account for the formation of the silylated aldols are illustrated in Scheme 10.14. In a metal-catalyzed process the Lewis acidic metal catalyst is regenerated on silylation of the metal aldolate by intramolecular or intermolecular silicon transfer (paths a and b, respectively). If aldolate silylation is slow, a silicon-catalyzed process (path c) might effectively compete with the metal-catalyzed process. Carreira and Bosnich have concluded that some metal triflates serve as precursors of silyl triflates, which promote the aldol reaction as the actual catalysts, as shown in path c [46, 47]. Three similar pathways are possible in the triarylcarbenium ion-catalyzed reaction. According to Denmark et al. triarylcarbenium ions are the actual catalysts (path b) [48], whereas Bosnich has insisted that hydrolysis of the salts by a trace amount of water generates the silicon-based Lewis acids working as the actual catalysts (path c) [47]. Otera et al. have reported that 10-methylacridinium perchlorate is an efficient catalyst of the aldol reaction of ketene triethylsilyl acetals [49]. In this reaction, the perchlorate reacts smoothly with the acetals to produce the actual catalyst, triethylsilyl perchlorate. 

Compound 3.13 (Scheme 2.22) is hydrolyzed with a half-life of 3.5 h in water at 39 °C, some 10 times faster than expected for the spontaneous hydrolysis of a methoxymethyl acetal of a tertiary alcohol, corresponding to a transition state stabilization of 14 kcal (58 kj) mol [47], and at the rate expected for the methoxymethyl derivative of an alcohol of pKa about 4. Perhaps significantly, this is almost equal to the pKa (4-18) of the catalytic COOH group of 3.13. (Which also confirms that there is no significant intramolecular hydrogen bonding in the reactant.)... 

Putting an acid catalytic group in the oxocarbonium ion portion of a hydrolysing acetal or ketal has resulted in the observation of intramolecular general acid catalysis only in the aromatic systems LVIII and LIX, but not in the aliphatic system (LX). Even so, the effective molarity of the carboxylic acid group in the hydrolysis of... 

It is therefore clear that any intramolecular catalysis of the hydrolysis of the ionized form of the mixed acetic salicylic anhydride can produce only a small rate enhancement. This is reasonable since the ratio in the rates of hydrolysis of the ionized to un-ionized forms of p-nitrophenyl 5-nitrosalicylate is about 30 [28], and a reduced catalytic effect would be expected with a less basic dinitrophenolate ion. It has recently been claimed, on the basis of the variation of fc(ionized) with atom fraction of deuterium for hydrolysis in HjO—D2O mixtures, that the acetic salicylic anhydride reacts with 39 per cent attack at salicyl carbon and 61 per cent attack at acetyl carbon [86]. This conclusion does, however, depend on several unproven assumptions. 

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