Reaction rate enhancement

The cycloadditions of cyclopentadiene 1 and its spiro-derivatives 109 and 110 with quinones 52, 111 and 112 (Scheme 4.20), carried out in water at 30 °C in the presence of 0.5% mol. of cetyltrimethylammonium bromide (CTAB), gave the endo adduct in about 3 h with good yield [72b]. With respect to the thermal Diels-Alder reaction, the great reaction rate enhancement in micellar medium (Scheme 4.20) can be ascribed to the increased concentration of the reactants in the micellar pseudophase where they are also more ordered. 

There are distinct advantages of these solvent-free procedures in instances where catalytic amounts of reagents or supported agents are used since they provide reduction or elimination of solvents, thus preventing pollution at source . Although not delineated completely, the reaction rate enhancements achieved in these methods may be ascribable to nonthermal effects. The rationalization of microwave effects and mechanistic considerations are discussed in detail elsewhere in this book [25, 193]. A dramatic increase in the number of publications [23c], patents [194—203], a growing interest from pharmaceutical industry, with special emphasis on combinatorial chemistry, and development of newer microwave systems bodes well for micro-wave-enhanced chemical syntheses. 

Despite the sometimes impressive reaction rate enhancements, second-order rate constants for most bimolecular reactions involving counterions actually decrease,with just a few remaining virtually constant or increas-ing. As discussed (vide supra), micellar rate constants for (pseudo) unimole-cular reactions are frequently lower than rate constants in water. Many of the... 

As discussed in Section 3.5, CDs have been used as enzyme mimics. Their catalytic activity usually results in only modest reaction rate enhancements, but in a very few specific cases these rates may reach values typical... 

Ultrasonically irradiated solution of p-NPA exhibits an observed rate constant that is enhanced by about two orders of magnitude in comparison to the same hydrolysis under ambient conditions at 25°C. Observations in unsonified solutions suggest that the reaction rate enhancement occurs at the cavitation-bubble interface. Both the enhanced hydrolysis rate and its pH independence may be explained by the existence of SCW around the... 

H. Kim and R. Kapral. Quantum equilibrium structure for strong nonadiabatic coupling Reaction rate enhancement. Chem. Phys, Lett., 440(4-6) 215-220, 2007. 

Another example of reaction-rate enhancement was reported for the microwave-assisted Paal-Knorr synthesis of a series of tetrasubstituted pyrroles [18]. Following the standard procedure, 1,4-dicarbonyl compounds were converted to pyrrole rings via acid-mediated dehydrative cyclization in presence of primary amines. The main limitation of the standard protocol is the harsh reaction conditions (reflux in acetic acid for extended times). The use of microwaves slashes the reaction times to few minutes, giving good isolated yields of the desired products (Scheme 15.5). 

Propinquity (proximity) effects are important in reaction rate enhancement. In the case of the following compounds, anhydrides (products formed on removal of water) form at different rates. Arrange the compounds in order of their rates of anhydride formation and explain the reasons for the ordering. 

SiMejCl Silver salts Carbonylation reaction Rate enhancement [llOd]... 

Enzymes derive both their selectivities and their reaction rate enhancements by the formation of enzyme-substrate complexes. This complex formation results in a transition state for the reaction, that lowers AG, the activation energy, but does not affect the net free energy for the conversion of substrate to product. This finding is represented in Figure 2.5, for the simple one-substrate enzyme reaction shown in Eq. 2.10 ... 

To make a direct comparison, when reaction shown in Figure 31 is carried out in [bmim][Tf2N] or in acetonitrile, the overall yields are 35 and 50%, respectively. These results confirm the stabilising role of the cyano groups and the reaction rate enhancement obtained in this reaction by using TSILs 29 or 30, compared to acetonitrile. Unfortunately, the consequence of a low catalyst stability makes recyclability impossible. Indeed, the recovered Co-containing ionic liquid phase does not promote a second cyclotrimerisation process, unless new NaBH4 is added in this case a halved yield in the second run is obtained. ... 

A second synthetic substrate, BTPNA, was investigated to clarify if this rate enhancement in CTAB reversed micelles is observed for all substrates (Figure 7). Two major differences are observed first, that there is no reaction rate enhancement in reversed micelles versus an aqueous based system, and second, that saturation kinetics are observed. 

S. Jensen, J.L. Olsen, H. Hansen, U.J. Quaade, Reaction rate enhancement of catalytic CO oxidation under forced thermal oscillations in microreactors with real time gas detection, in AIChE (Ed.), International Conference on Microreaction Technology (IMRET 8), Atlanta, 2005, p. 134h. 

Micelles concentrate reactants from the surrounding medium and provide microenvironments favorable to reaction. Rate enhancement arises from electrostatic and hydrophobic interactions between reactants and micelles. Rates may be strongly dependent on the struture of the reactant. For example, the hydrolysis of methyl ort/iobenzoate is catalyzed by micellar sodium dodecylsulfate, whereas the hydrolysis of methyl ortho-formate (which has less hydrophobic character and less affinity for the micelle core) is not ". The more pronounced the hydrophobic nature of the reactant, the more rapid is the micellar catalysis. ... 

Diels-Alder reaction. Rate enhancement of the Diels Alder reaction is noted in aqueous perfluorinated emulsions (from perfluorohexane and hthium perfluorooctane-sulfonate) ... 

The reaction rate as a function of agitation speed is shown in Fig. 9. The reaction rates increased with increasing agitation rate up to a speed of 140 rpm. In the range 140-170 rpm, the reaction rate enhancement... 

A qnasiclassical trajectory stndy of mode specific reaction rate enhancements in... 

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