3-Nitro-4-acyloxybenzoic acid

Overberger and coworkers carried out an interesting study on the hydrolysis of various 3-nitro-4-acyloxybenzoic acid substrates (V) catalyzed by imidazole (VI) and poly[4(5)-vinylimidazole] (VII) in ethanol-water mixtures [Overberger et al., 1973]. 

In aqueous solution the catalytic esterolyses with polymeric catalysts is accelerated by hydrophobic bonding . The catalytic effect of synthetic polymers on hydrolysis has been reviewed by Kunitake and Okahata (57). Linear polyvinylimid-azole (Scheme 5) increases the rate of hydrolyses of 3-nitro-4-acyloxybenzoic acid by a factor of 103 with respect to monomeric imidazole 58-62). The primary reaction is the acylation of polyvinylimidazole. The effect is dependent on chain length of the acid and the solvent. The hydrophobic interaction causes an autocatalytic course of reaction rate. The acylated polyvinylimidazole is more hydrophobic than the starting polymer. At a 75% conversion the rate is five times higher than the initial rate. 

The preparation of pol)rmeric catalysts and substrates containing imidazole groups and nitro-phenyl esters, respectively, grafted onto crosslinked polystyrene beads has been described and the effects of the acyl chain length in the substrate in the aqueous alcohol solvent systems on the rate of hydrolysis of 3-nitro-4-acyloxybenzoic acid substrates catalyzed by insoluble catalysts were determined. 

In this study, insoluble pol)rmers containing imidazole groups were tested for esterolytic activity with several substrates of differing hydrophobic chain lengths. The substrates used were 3-nitro-4-acyloxybenzoic acid (Sn ). The concentration of substrate used does not allow the formation of substrate micelles, the concentration being below the critical micelle concentration for these esters (28,29). 

Non-ionic (hydrophobic) association of the polymeric catalyst-substrate has also been shown to increase the catalytic activity. Thus, the rate of hydrolysis of 3-nitro-4-acyloxybenzoic acid (6) by polyvinylimidazole, increases with increasing bulk of acyloxy group (Over-berger and Sannes, 1974). It was observed that the deacylation step of the intermediate polymeric acylimidazole is rate-determining. Since the acylated polymer becomes more hydrophobic, because of the extra group attached to the polymer, apolar associations increase with the chain length of the acyl group, and, hence increase the catalytic activity. 

It was shown (Overberger and Sannes, 1974) that the polymeric imidazole was about 10 times as reactive as the monomeric imidazole in the hydrolysis of p-nitrophenyl acetate and certain 3-nitro-4-acyloxybenzoic acids. The increased catalytic activity of the polymeric imidazoles has been attributed to the cooperative interactions of the imidazole moieties anchored on the polymer chain at regular intervals (Scheme 13-3). Although practical applications of polyvinylimidazoles as catalytic nucleophiles in ester hydrolysis have not been made, it appears that they can provide quite effective and reusable solid-phase catalysts for such reactions. 

The imidazole (14) and poly(vinylimidazole) (15) catalyzed hydrolysis of 3-nitro-4-acyloxybenzoic acids (16) is an interesting example of hydrophobic interactions,with studies having been carried out in ethanol-water mixtures. 

Copoly[1-methyl-4-vinylimidazole/4(5)-vinylimidazole], copoly-[l-methyl-5-vinylimidazole/4 (5)-vinylimidazole], copoly[l-ethyl-5-vinylimidazole/4(5)-vinylimidazole], were synthesized according to Scheme I and their catalytic activity towards 3-nitro-4-acyloxybenzoic acid substrates (Sn ) measured in 28.5% ethanol-water and in water (17,18). 

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