Kinetic acceleration

An estimate of the rate enhancement associated with the intramolecular phosphorylation can be made by using isopropyl p-nitrophenyl methyl-phosphonate as a model for the covalent intermediate formed in the initial step of the reaction of cycloheptaamylose with bis (p-nitrophenyl) me thy 1-phosphonate. The first-order rate constant for the alkaline hydrolysis of isopropyl p-nitrophenyl methylphosphonate at pH 9.86 can be obtained from the data of van Hooidonk and Groos (1970) kun = 1.4 X 10-5 sec-1. This value may be compared with the maximal rate constant for the reaction of cycloheptaamylose with bis(p-nitrophenyl) methylphosphonate— k2 = 1.59 X 10-1 sec-1 at pH 9.86—which must be a minimal value for the rate of the intramolecular phosphorylation. This comparison implies a kinetic acceleration of at least 104 which is similar to rate enhancements associated with the formation of cyclic phosphates from nucleoside phosphate diesters. 

In the calorimetric studies, the kinetic acceleration only became apparent when the calorimeter was stabilised to a constant temperature, rather than to a constant pre-cooling rate as had been the practice in the earlier work this improvement in technique had revealed the acceleration. However, the acceleration and the corresponding increase in conductivity were also observed in the isothermal dilatometric studies so that they cannot have been caused simply by the increase in temperature during the adiabatic reactions in the calorimeter. As is well-known [la] with this system, the degree of polymerisation of the polymer increases slightly as the concentration of the initiator is lowered (Table 1). 

MRR) to recover platinum. The extraction of palladium by the sulfide is slow with 2-3 hr to achieve equilibrium, which inhibits flow sheet design. However, it has been shown that tertiary amines can act as a kinetic accelerator, reducing equilibrium times to minutes, but no information has been published on the effect of these amines on palladium-platinum separation. High loading of the sulfides is possible, and stripping is easy with aqueous ammonia. The overall extraction and stripping equations are as follows ... 

The second selective extractant route (that of MRR) uses a hydroxy-oxime to remove palladium. The actual compound used has not been specified, but publications refer to both an aliphatic a-hydroxyoxime and an aromatic S-hydroxyoxime. The a-hydroxyoxime LIX63 has the faster extraction kinetics, but suffers from problems with stripping. For the (i-hydroxyoxime, a kinetic accelerator in the form of an amine (Primene JMT ) has been proposed. The precise mode of operation of this accelerator is unknown, but it may be a similar process to that proposed for the sulfide... 

The polyethylenimines are also effective in the cleavage of nitrophenyl-sulfate esters and nitrophenylphosphate esters. These have not yet been studied as extensively as the acyl esters, but interesting kinetic accelerations are already apparent. Nitrocatechol sulfate, for example, is very stable in aqueous solution at ambient temperature. In fact, even in the presence of 2 M imidazole no hydrolysis can be detected at room temperature. At 95°C in the presence of 2 M imidazole cleavage is barely perceptible. In contrast, a modified polyethylenimine with attached imidazole groups cleaves the sulfate ester at 20°C.34 Some kinetic parameters are compared in Table VI. It is obvious that accelerations of many orders of magnitude are effected by the polymer. 

Shiner and coworkers were the first to study quantitatively the /-silyl effect in solvolysis90-95. They found a distinct kinetic acceleration of the solvolysis of cis-3-(trimethylsilyl)cyclohexyl brosylate 230 compared with the alkyl reference 231 in TFA [fc(230)A(231) = 462] which is equivalent to a lowering of the ionization barrier by... 

Epilepsy affects 0.5% of the world s population and can have a multitude of underlying etiologies, including several mutations in CNS sodium channels. Sodium channel mutations linked to human epileptic syndromes typically shift activation to more hyperpolarized potentials, slow inactivation kinetics, accelerate recovery from inactivation, and/or increase the persistent current [48]. Seemingly paradoxically, some mutations appear to result in non-functional channels [48-50]. 

The allylation of 4-/ r/-butyl-2-hydroxycyclohexanone shows a high diastereoselectivity. When the hydroxyl substituent is oriented in the equatorial plane, kinetic acceleration accompanies exclusive entry of the allyl group from the equatorial direction (Equation (15)). The chelation in water has been revealed by competition experiments (Scheme 37).164 165... 

In the past few years our research laboratory has been working on the applications of a new technique, called microwave-accelerated metal-enhanced fluorescence (MAMEF), as applied to fluorescence-based bioassays. The MAMEF technique couples the benefits of low power microwave heating witii metal-enhanced fluorescence (MEF), to address the two major shortcomings of fluorescence-based bioassays currently in use today i.e., bioassay sensitivity and rapidity. In MAMEF, [1] the MEF phenomenon increases the sensitivity of the assays, while the use of low power microwave heating kinetically accelerates assays to completion within only a few seconds. 

The proof-of-principle of the MAMEF technique, which couples the benefits of MEF with low power microwave heating to kinetically accelerate the bioassays, was first demonstrated with a model protein-fluorophore system,[l] with biotinylated-BSA and fluorophore-labeled streptavidin, as shown in Figure 7.3A. The... 

In addition to their utility in increasing chemiluminescence intensity, silver nanoparticles, in combination with low power microwaves, have also been shown to kinetically accelerate the chemical reactions that produce chemiluminescence[14]. 

Spiro[2.5]octadienyl radical (146), the radical analog of the phenonium ion (105), is a very short-lived species but was detected by its visible absorption and fluorescence on generation by radical abstraction from 147 prior to facile ring-opening to 148 (equation 29). There appeared to be a modest (two-fold) acceleration compared to 1,3- or 1,4-cyclohexadiene in the rate of abstraction from 147, and this was attributed to stabilization by a favorable interaction between cyclopropyl and the adjacent semioccupied orbital in 146. Kinetic acceleration was also proposed to occur in hydrogen atom abstraction from spiro[2.n]alkanes (149). ... 

The cationic cyclopropyl to allyl rearrangement is always disrotatory. If a cyclopropyl cation 9 or 14 is involved, both disrotatory modes are electronically equivalent, and their ratio is determined by steric interactions in the transition state. If, however, cyclopropane 16 rearranges to an allyl cation 17 in a synchronous reaction, bypassing the cyclopropyl cation 4, only the disrotatory mode, that moves the electron density of the breaking bond to the rear side of the leaving group X, occurs. This anchimeric assistance causes a kinetic acceleration of the rearrangement by a factor of at least 10 to 10 . °... 

The amorphization reaction slows down as the thickness of the amorphous interlayer increases, and the intermetallic NiZr phase forms when the amorphous interlayer exceeds a critical thickness d rit. Due to the much slower diffusion in the intermetallic phase, the reaction rate decreases. The second increase above 350°C is attributed to the reaction kinetics accelerated again at higher temperatures. Besides the growing NiZr phase, the intermetallic NiZr2 phase forms. The consumption of Ni leads to the second decrease of the reaction... 

Furthermore, the postulate of a strongly preferred orientation for the nucleophilic attack corroborated the concepts invoked at the time to understand the high kinetic accelerations of enzymic or intramolecular reactions, such as Koshland s concept of orbital steering in enzyme catalysis [46]. Finally, the hypothesis offered grounds for attractive interpretations of a puzzling pattern of regioselectivity in the reactions of imides and cyclic anhydrides [47,48]. 

A numerical solution, based on the model presented for a formation-dissolution mechanism, was derived by Miller (1981). The following two Figs 4.13 and 4.14 demonstrate the effect of micelles on adsorption kinetics. The effect of the rate of formation and dissolution of micelles, represented by the dimensionless coefficient nkfC Tj /D, becomes remarkable for a value larger than 0.1. Under the given conditions (D /D, =1, c /c , =10, n=20) the fast micelle kinetics accelerates the adsorption kinetics by one order of magnitude. 

Alkoxyl radical cleavages have log A terms of about 13 as expected for fragmentation reactions. Increasing alkyl substitution, such that the product alkyl radicals are increasingly more stable, results in considerable kinetic accelerations [59]. One should note that the kinetics of alkoxyl radical fragmentations arc sensitive to... 

In general, any system that has a nucleophilic substituent situated properly for back-side displacement of a leaving group at another carbon atom of the molecule can be expected to display neighboring-group participation. The extent of the rate enhancement depends on how effectively the group acts as an internal nucleophile. The existence of participation may be immediately obvious from the structure of the product if a derivative of the cyclic intermediate is stable. In other cases, demonstration of kinetic acceleration or stereochemical consequences may provide the basis for identifying nucleophilic participation. 

The rate of hydrolysis of propane sultone in aqueous solution is essentially independent of pH over the pH range 4-9, consistent with a BAL-E1 mechanism (equation 96). At higher pH values, however, in aqueous aprotic solvents the rate of hydrolysis increases and is attributable to an increasing contribution to the overall rate from concurrent bimolecular attack at sulphur (equation 97). Oxygen-18 tracer experiments confirmed that at pH >12, the hydrolysis of propane sultone proceeds with 14% sulphur-oxygen bond fission. The relative rates of hydrolysis at pH > 7 in 65% aqueous acetone of five-membered six-membered open-chain sulphonates (propane sultone, butane-1,4-sultone and of ethyl ethanesulphonate) were found to be 37 1 7126. The enthalpies of activation of all three compounds were very similar and the difference in rates were attributed to differences in entropies of activation ( — 17.1, —24.0 and —17.9 e.u., respectively). These data, however, are composite values. It is not possible to compare the kinetic acceleration for attack at sulphur in aliphatic sultones because both the six-membered sultone and the open-chain sulphonate hydrolyse exclusively with carbon-oxygen bond fission, within the limits of experimental detection. 

In recent years the importance of entropy contributions to the increased rates of reaction of five-membered cyclic esters has been recognised. Thus the high reactivity of the oxaphospholan ring has been shown to arise from a combination of both enthalpy and entropy strain153. On the other hand, entropy strain is the main cause of kinetic acceleration in the alkaline hydrolysis of cyclic sulphite and sulphinate esters154,155. 

These data also agree with Brown s results on the solvolysis of 2-aryl-2-bicyclo-[2,l,l]hexyl-p-nitrobenzoates for which the value of was found to be —4.31, i.e. still more negative than that of 2-aryl-2-endo-norbomyl-p-nitrobenzoates (q = —3.72). The acetolysis rate of the secondary tosylate 142 is 3 times as low as that of 2-endo-norbomyl tosylate and 1000 times as low as that of 2-exo-epimer i.e. there are no data in favour of kinetic acceleration due to a-participation. 

The effects of alkyl substitution in substituted ethenes decrease in the order 1,2-dialkyl < 2-alkyl < 2,2-dialkyl - trialkyl < tetraalkyl. This indicates that both electronic and steric factors are important in determining the nucleophilicity of the alkene toward radical cations. For example, alkyl substitution increases the nucleophilicity of the alkene, thus rendering 2,2-dialkyl and tetraalkylalkenes more reactive toward styrene radical cations than less-substituted alkenes such as 1-hexene. On the other hand, the kinetic acceleration that results fiom electronic effects of additional alkyl groups is offset by steric hindrance, as demonstrated by the low reactivity of 1,2-dialkyl-... 

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