Catalytic reactions substituent effects

A theoretical study of the catalytic and substituent effects on the 2 - - 2-cycloaddition reactions between isocyanates and aldehydes was presented. Steady-state NOE coefficients have been used to develop a stereochemical model for the transition state for 2 -I- 2-cycloaddition of chlorosulfonyl isocyanate with chiral vinyl ethers. The asymmetric 2 -I- 2-cycloadditions of chlorosulfonyl isocyanate to chiral vinyl ethers derived from sugars and hydroxy/acids have been reviewed. 

The isolated Ru(0) nanoparticles were used as solids (heterogeneous catalyst) or re-dispersed in BMI PP6 (biphasic liquid-liquid system) for benzene hydrogenation studies at 75 °C and under 4 bar H2. As previously described for rhodium or iridium nanoparticles, these nanoparticles (heterogeneous catalysts) are efficient for the complete hydrogenation of benzene (TOP = 125 h ) under solventless conditions. Moreover, steric substituent effects of the arene influenced the reaction time and the decrease in the catalytic TOP 45, 39 and 18h for the toluene, iPr-benzene, tBu-benzene hydrogenation, respectively, finally. The hydrogenation was not total in BMI PPg, a poor TOE of 20 h at 73% of conversion is obtained in the benzene hydrogenation. 

The analysis of the influence of substituents in organic molecules upon rates and equilibria has led to the recognition that they operate in two different ways, either by changing the electronic density, in comparison with a reference substituent, at the reaction center of the molecule or by blocking the access to the reaction center. The same is true for heterogeneous catalytic reactions. However, the interaction of a molecule with a surface can disturb the normal effect of a substituent. 

The interpretation of slopes also requires meaningful rate data. When the reaction consists of a series of elementary steps (and this is always so with heterogeneous catalytic reactions), the rate coefficients obtained from a superficial treatment of a limited set of measurements may be composites of several rate and equilibrium constants for individual steps, in favorable cases constituting a product. As every step may be influenced by the substituents, the resulting effect can be easily attributed to a false elementary step. 

The idea of the decisive role of complexing reactions makes possible to unterstand such peculiarities of epoxy compound reactions with primary and secondary amines as a rather unusual, from the kinetic point of view, simultaneous occurrence of the autoacceleration and autoinhibition reactions, catalytic and inhibitive effects of various solvents, and obvious ortho-effect of a number of substituents in aromatic amines, the influence of tertiary amines as additives and some other 5,6,13.14.30,... 

A comparison of Figures 2 and 4 indicates that substituents have a qualitatively similar effect both on the oxidative addition reaction and on the catalytic cyanation. In both cases in fact there is a change in the slope on passing from electron-withdrawing to electron-releasing substituents. As to the effect of electron-withdrawing substituents, the sensitivity is lower in catalytic cyanation (p = 4.8) than in oxidative addition (p = 8.8) which is what is to be expected on the basis of the substituent effect in stoichiometric cyanation reaction of arylnickel complexes. 

On the other hand, Maruoka and coworkers were intrigued with the preparation of symmetrical N-spiro-type catalysts to avoid the independent synthesis of two different binaphthyl-modified subunits required for 1. Along this line, 4,4, 6,6 -tetra-arylbinaphthyl-substituted ammonium bromide (S, S)-13 was assembled through the reaction of aqueous ammonia with bis-bromide (S)-14 on the basis of previous studies on the substituent effect of this type of salt. The evaluation of (S,S)-13 as a chiral phase-transfer catalyst in the alkylation of 2 uncovered its high catalytic and chiral efficiency (Scheme 5.9) [9]. 

Only data for substituents of the requisite symmetry are included in Fig. 19. These results adhere to the correlation with excellent precision. This observation confirms the general utility of the procedure. One caution is necessary. The p-value determined for non-catalytic bromina-tion of the monosubstituted compounds is — 12.1. The reaction parameter is decreased to — 8.7 for the bromination process with the polymethylbenzenes. The large variation in substituent effects is presumably the consequence of the greater overall reactivity of the alkylated benzenes. The dependence of p on the nature of the substrate is an important problem worthy of further attention. 

Substituent effects on the rate of electrophilic amination of phenylmagnesium bromides, magnesium diphenylcuprates, and catalytic phenylzinc cyanocuprates with O-methylhydroxylamine in THF have been investigated in a competitive kinetic study.169 The mechanistic differences between these three reactions were discussed on the basis of the experimental results. 

Perform the regression analyses for the descriptors to assess the contribution of substituent effect(s) on the rate of a-chymotrypsin-catalyzed hydrolysis of p-nit-rophenyl esters. Referring to the catalytic triad of chymotrypsin, rationalize your results for the plausible reaction mechanism. 

Although the reaction is successful with many representatives of the purine family, it relies on the protonation of the N -nitrogen atom [81] meaning that substituents at N and N cannot be chosen at will, thus limiting the scope for steric, electronic and chiral diversification of a purine derived carbene ligand for application in homogenous catalysis. Furthermore, the NH functionality may have adverse effects in catalytic reactions and the yields of C-bound complexes are generally low. 

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