6-Nitrobenzisoxazole decarboxylation

These hydrophobic ammonium ions exert a medium effect on spontaneous, unimolecular reactions. Tri-n-octylmethylammonium chloride effectively speeds decarboxylation of 5-nitrobenzisoxazole carboxylate ion (24) (Kunitake et al., 1980), and tri-n-octyl ethylammonium mesylate or bromide... 

Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate [52] has been most widely investigated in aqueous systems, since this reaction is remarkably solvent dependent (Kemp and Paul, 1970 Kemp and Paul, 1975 Kemp et al.,... 

Fig. 17. Rate of release of 5-nitro-2-cyanophenolate during decarboxylation of 6-nitrobenzisoxazole-3-carboxylate at 25°C, pH 7.4, p. = 0.05. Initial concentration of substrate, So = 4.18xl0-6M (A) In presence of quaternized polyethylenimine...

Table X summarizes the kinetic parameters derived from the decarboxylation rates of nitrobenzisoxazole carboxylate catalyzed by each of four different lauryl polyethylenimines.

Comparing catalytic effects of different modified polyethylenimines on the decarboxylation of nitrobenzisoxazole carboxylate, we can discern several interesting features. Comparison of the 25% laurylated polymer in the quaternized and nonquaternized forms, A and D, respectively, in Table X, shows that the former is about three times more effective as a catalyst than the latter. For the quaternized polymer the first-order catalytic constant k2 and the second-order rate constant nk2IKM are greater and the binding of substrate (measured by XM ) is stronger. 

Fig. 18. Variation of initial rate constant VJPa for decarboxylation of 6-nitrobenzisoxazole-3-carboxylate as a function of substrate concentration under conditions of excess substrafe. Initial velocities were corrected for the spontaneous hydrolysis of substrate in absence of polymer. Polymer is (C2H4N)m (C 2H25)0 2Sm(C2H5), 75m. The curve was drawn according to (30). with nk2 = 0.458 sec-1, and KM = 8.59 x 10-5 M.

TABLE X. Kinetic Parameters for Catalyzed Decarboxylation of Nitrobenzisoxazole Carboxylate by Modified Polyethylenimines3... 

We have made no systematic studies of ionic strength effects. Ionic strength does not seem important in the catalytic hydrolysis of the acylnitrophenolates, which carry no charge. There are substantial salt effects in the decarboxylation reaction with nitrobenzisoxazole, which is an anionic substrate. These may reflect general ion cloud effects or anionic competition for binding to the polymer. 

Figure 8.10a is an example of some data in which the effect of added salt is more than a competitive ion-binding phenomenon. The reaction involved is the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, catalyzed by hexadecyl trimethyl ammonium bromide micelles ... 

Eschenmoser and collaborators (28) have reported the base induced fragmentation of a.B-epoxy-tosylhydrazones to produce acetylenic ketones (e.g. 69 71). Interestingly, in this fragmentation, both the triple-bond and molecular nitrogen are produced from the key intermediate 70 via an anti mode. The decarboxylation of nitrobenzisoxazole carboxylate (72) into 2-cyano-5-nitrophenol (73) can also be viewed as a trans-elimination (29, 30). 

The effects of the addition of sugars, long-tailed n-alkyl pyranosides, n-alkyl glycerol ethers and n-alcohols on the properties of di-n-hexadccyldi methyl ammonium bromide (DHAB) vesicles have been studied.54 Upon addition of most additives, an inhibiting effect on the decarboxylation reaction of 6-nitrobenzisoxazole-3-carboxylate anion has been observed relative to the reaction in vesicles without any additive. The largest inhibition was observed in the case of cholesterol. 

Finally, mention should be made of the polysoap-catalyzed decarboxylation of 6-nitrobenzisoxazole-3-carboxylate anion (Eg. I) studied by Kunitake and co-workers 21). This reaction is known to proceed faster in apolar solvents than polar ones. 

Our final reaction is an instructive example of specific anion solvation in a non-SN reaction. The decarboxylation rate of 6-nitrobenzisoxazole-3-carboxylate as the tetra-... 

Katritzky, A.R., Perumal, S. and Petrukhin, R. (2001a) A QSRR treatment of solvent effects on the decarboxylation of 6-nitrobenzisoxazole-3-carboxylates employing molecular descriptors. 

In an attempt to extract information on the mode of host-guest association from kinetics the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate 34 in the presence of quaternary ammonium hosts was investigated This reaction has been shovra to follow clean first order kinetics the absolute rate being extremely dependent on the solvent. Addition of an ammonium host 36, 24 or 25... 

The strongly solvent-dependent, unimolecular decarboxylation of sodium 6-nitrobenzisoxazole-3-carboxylate (6-NBIC, Figure 16.4) has been a popular kinetic probe for aqueous media containing surfactant aggregates. 

M. V. Scarpa, P. S. Araujo, S. Schreier, A. Sesso, A. G. Oliveira, H. Chaimovich, I. M. Cuccovia, Effect of vesicles of dimethyldioctadecylammonium chloride and phospholipids on the rate of decarboxylation of 6-nitrobenzisoxazole—3-carboxylate, Langmuir, 2000, 16, 993-999. 

C. A. Bunton, M. J. Minch, Micellar catalyzed decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion. Tetrahedron Lett., 1970, 11, 3881-3884. 

T. Kunitake, Y. Okahata, R. Ando, S. Shinkai, S. Hirakawa, Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate catalyzed by ammonium bilayer-membranes - a comparison of the catalytic behaviour of micelles, bilayer-membranes, and other aqueous aggregates,... 

M. G. M. Jongejan, J. E. Klijn, J. B. F. N. Engberts, Vesicular catalysis of the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate. The effects of sugars, long-tailed sugars, cholesterol and alcohol additives, J. Phys. Org. Chem., 2006, 19, 249-256. 

Kinetic treatments are relatively simple for spontaneous reactions, both unimolecular and bimolecular water-catalyzed reactions, because only the transfer equilibrium of the substrate between solvent and the association colloid has to be considered, Eq. (3). For example, the rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion (3) is a useful indicator of medium polarity, and reaction is inhibited by solvents that hydrogen bond to the carboxylate moiety [96]. The reaction is accelerated by a variety of colloidal species that incorporate 3. including ionic and zwitterionic micelles [97,98] and O/W microemulsions [99]. Reactions are slightly slower in microemulsions derived from cetyltrimethylammonium bromide (CTABr) than in the corresponding aqueous micelles, but changes in the alcohol cosurfactant or the hydrocarbon, or in their relative concentrations, do not have major rate effects, and it appears that these microemulsion droplets are similar to aqueous micelles as submicroscopic reaction media. These observations are consistent with estimates of surface polarities [99-101] determined with bound fluorescent probes [102]. 

The water pool description of reverse micelles and O/W microemulsions is not appropriate if only small amounts of water are present. In that event the surfactants form ion pairs or small ion clusters with associated water [118]. These clusters are catalytically very effective in the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion (3), and for solutions of cationic surfactants and hydrophobic ammonium ions rate constants of reaction in CH2CI2 decrease significantly when there is sufficient water to generate water pool reverse micelles [119,120]. Similar results were obtained for the spontaneous hydrolysis... 

There is some kinetic evidence that the surface of o/w microemulsion droplets based on CTABr are similar to those of normal cationic micelles. Decarboxylation of 6-nitrobenzisoxazole carboxylate ion (9) is a sensitive probe of the polarity of a solvent or of a submicroscopic interface [64,65], and rates of decarboxylation of (9) in cationic o/w microemulsions are very similar to those in normal micelles of CTABr [155]. 

Horie et al.46 applied two fluorochromes (DNS and 6-p-toluidino-2-naphthalenesulfonate) to estimate the hydrophobicity of the microenvironment of cross-linked polystyrene containing quaternary ammonium groups. They found a correlation between the wavelength of the emission maximum of fluorophores and the rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate anion catalyzed by these gels. The hydrophobic microenvironment due to cross-linked matrices was suggested as a cause for this catalytic activity.47... 

There has recently been great interest in the synthesis of dendritic polymers, although applications of these have so far been few [59]. The first report of a reaction where a dendrimer is actually catalytic involved a biphasic system similar to phase transfer catalysis. The quaternary ammonium ion dendrimer (Figure 5.26) has 36 trimethylammonium functions, and catalyses the unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, and the hydrolysis of 4-nitrophenyldiphenyl phosphate [60]. 

Catalysis of 6-nitrobenzisoxazole-3-carboxylate (NBOC) decarboxylation by N,N dimethyl dialkylam-monium bromide vesicles is modulated by the bilayer fluidity [40], Catalytic efficiency increases with temperature, but at the phase transition there is a sharp increase in the catalysis. Several parameters, such as substrate binding constants, extent of ion dissociation and reactivity of NBOC, may change simultaneously at the Tc, com-... 

Despite the differences in morphologies of aqueous aggregates, the rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate is enhanced by micelles and bilayer membranes by an amount that is proportional to the hydrophobicity of the system. Activation-energy data suggest that membrane catalysis is governed mainly by the hydrophobicity and fluidity at temperatures above and below the phase-transition temperature. ... 

The rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion (eq 6) 9 highly sensitive to solvent. Polystyrene-supported catalysts having 2-15% divinylbenzene cross-linking and 22-92%... 

The unimolecular decarboxylation of 3-carboxybenzisoxazoles (Figure 10, often called Kemp decarboxylation) is enormously accelerated by aprotic, polar solvents. For example, reactions of 6-nitrobenzisoxazole-3-... 

Other multiparametric approaches have been appHed to the decarboxylation of 3-carboxy-6-nitrobenzisoxazole to further investigate the origin of the rate acceleration. In the latest of these approaches, Catalan and... 

Bunton CA, Minch MJ, Hidalgo J, Sepulveda L. Electrolyte effects on the cationic micelle catalyzed decarboxylation of 6-nitrobenzisoxazole-3-carboxylate anion. JAm Chem Soc. 1973 95 3262-3272. 

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