Kinetic control in contrast

Recently, Oppolzer s group reported on the synthesis and use of a new sulfinylating agent,107 the /V-sulfinyl sultam 82, as part of a broad program on the use of the versatile bornane-10,2-sultam 81 in asymmetric synthesis.108 The condensation of p-TolSOCI with 81 in THF, using dimethylaminopyridine (DMAP) as catalyst, gave the /V-(p-tolylsulfinyl)bornane- 10,2-sultam as a 6.2 1 diastereomeric mixture. Crystallization of the mixture from E O/hexane afforded pure 82 in 77% yield. X-ray analysis showed the absolute configuration at the sulfinyl sulfur to be (/ ). The reaction has been shown to be kinetically controlled, in contrast to the results obtained when n-BuLi was used instead of DMAR In the latter case, the reaction was under thermodynamic control, in accord with the result obtained by Evans with iV-sulfinyl oxazolidinone (Scheme 25). 

Shea [13] and Shea and Sasaki [41,51] used ketals (entry f) for imprinting synthetic polymers. Diketones with various distances between the keto groups were treated with polymerizable diols, to obtain template monomers. After polymerization and removal of the template, these polymers showed considerable selectivity for the original template when the imprinted polymers were reloaded with a mixture of diketones. Unlike the reloading experiments mentioned earlier, this bond formation takes place slowly, and therefore it seems to be kinetically controlled in contrast to the bonding reactions described so far, which are thermodynamically controlled. If the arrangement of functional groups is the same, the size and shape of the rest of the... 

This regioselectivity contrasts with the alkylation of allylsulfoxides under kinetic control, in which the a-product predominates. 

P-Keto esters and -keto amides, each substituted between the two carbonyl units with a 2-[2-(tri-methylsilyl)methyl] group, also undergo Lewis acid catalyzed, chelation-controlled cyclization. When titanium tetrachloride is used, only the product possessing a cis relationship between the hydroxy and ester (or amide) groups is product yields range from 65 to 88% (Table 8). While loss of stereochemistry in the product and equilibration of diastereomers could have occurred via a Lewis acid promoted retro aldol-aldol sequence, none was observed. Consequently, it is assumed that the reactions occur under kinetic, rather than thermodynamic, control. In contrast to the titanium tetrachloride promoted process, fluoride-induced cyclization produces a 2 1 mixture of diastereomeric products, and the nonchelating Lewis acid BF3-OEt2 leads to a 1 4.8 mixture of diastereomers. 

Since the SECM response depends on rates of both heterogeneous reactions (26) and (27), their kinetics can be measured using methodologies developed previously for ET processes. The mass-transfer rate for IT measurements by SECM is similar to that for heterogeneous ET measurements, and the standard rate constants of the order of 1 cm/s should be measurable. With relatively large pipet probes used in Ref. 49 (i.e., n > 10 /Ain), the mass-transfer coefficient was <0.01 cm/s, i.e., too low to measure the standard rate constant of potassium transfer [1.3 cm/s (51)]. The SECM response was diffusion controlled. In contrast, a kinetically controlled regeneration of the mediator was observed for a slower reaction of proton transfer assisted by 1,10-phenanthroline. 

Screening in 96- or 384-well plate formats allows precise fitness measurements. The accuracy of the detection system, kinetic assays (in contrast to end point assays), the possibility to normalize activity values (e. g., using measured cell concentrations), and better control over cell growth and protein expression contribute to the improved... 

The Diels-Alder reaction of maleic anhydride with 3,4-dimethoxyfuran affords endo- and exo-adducts at about the same rate, in contrast to furan, where the endo-isomer is the product of kinetic control. In both cases the exo-adduct is thermodynamically more stable. 3,4-Dimethoxyfuran and p-benzoquinone give the eiufo-compound (37), whereas in the reaction with 2,3-dimethyl-1,4-benzoquinone only the exo-adduct (38) was detected. The Diels-Alder adduct (40) of methyl acrylate to 2-amino-3-cyano-4,5-dimethylfuran (39) is readily... 

In contrast to CL hydrolytic polymerization, in which a thermodynamic equilibrium is established in most cases among linear maciomolectilar chains, monomer, and higher (cydic and linear) oligomers, the anionic route is very often under stringent kinetic control. In this respect, the residual monomer content, which is 7-8 wt.% in hydrolytic polymerization, can be far bdow the value predicted on the basis of thermodynamic considerations ( 3 wt.%) and, depending on the reaction conditions chosen, found to be as low as 1 wt.% or even less, ° as shown in Figure 22. 

In contrast, a preference for the 1,2-syn diastereomers is observed under conditions favoring kinetic control. In the presence of NaHC03, 204 and 207 were thus obtained as the major products (dr=l 3 and 1 4, respectively) [158]. The modest observed diastereoselectivity is the result of the... 

In contrast, physical adsorption is a very rapid process, so the rate is always controlled by mass transfer resistance rather than by the intrinsic adsorption kinetics. However, under certain conditions the combination of a diffiision-controUed process with an adsorption equiUbrium constant that varies according to equation 1 can give the appearance of activated adsorption. 

Figure 10 shows that Tj is a unique function of the Thiele modulus. When the modulus ( ) is small (- SdSl), the effectiveness factor is unity, which means that there is no effect of mass transport on the rate of the catalytic reaction. When ( ) is greater than about 1, the effectiveness factor is less than unity and the reaction rate is influenced by mass transport in the pores. When the modulus is large (- 10), the effectiveness factor is inversely proportional to the modulus, and the reaction rate (eq. 19) is proportional to k ( ), which, from the definition of ( ), implies that the rate and the observed reaction rate constant are proportional to (1 /R)(f9This result shows that both the rate constant, ie, a measure of the intrinsic activity of the catalyst, and the effective diffusion coefficient, ie, a measure of the resistance to transport of the reactant offered by the pore stmcture, influence the rate. It is not appropriate to say that the reaction is diffusion controlled it depends on both the diffusion and the chemical kinetics. In contrast, as shown by equation 3, a reaction in solution can be diffusion controlled, depending on D but not on k. 

Kinetic studies have shown that the enolate and phosphorus nucleophiles all react at about the same rate. This suggests that the only step directly involving the nucleophile (step 2 of the propagation sequence) occurs at essentially the diffusion-controlled rate so that there is little selectivity among the individual nucleophiles. The synthetic potential of the reaction lies in the fact that other substituents which activate the halide to substitution are not required in this reaction, in contrast to aromatic nucleophilic substitution which proceeds by an addition-elimination mechanism (see Seetion 10.5). 

As the individual components of a mixture are moved apart on the basis of their differing retention, then the separation can be partly controlled by the choice of the phase system. In contrast, the peak dispersion that takes place in a column results from kinetic effects and thus is largely determined by the physical properties of the column and its contents. 

Interestingly, the cycloaddition of 2-azadiene 44 with N-methylmaleimide in 2.5m LT-DE gave predominantly exo-adduct in contrast to the thermal cycloaddition that is mainly enJo-selective (Scheme 6.27). A similar but not so dramatic increase in cxo-selectivity was also observed [47] for the cycloaddition of 44 with N-phenylmaleimide. The reaction is kinetically controlled, but the origin of the high cxo-selectivity observed in LT-DE is unclear the polar medium probably favors the more polar exo transition state. 

In contrast to the results of the reaction of tertiary and secondary alkyl cations with carbon monoxide (Figs. 1-5), which were obtained under thermodynamically controlled conditions, the results of the carbonylation with the vinyl cations were obtained under kinetically controlled conditions. This presents a difficulty in explaining the occurrence of the 1,2-CH3 shift in the reaction 16->-17, because it involves a strong increase in energy. The exclusive formation of the Z-stereoisomer 18 on carbonylation of the 1,2-dimethylvinyl cation 16 is remarkable, but does not allow an unambiguous conclusion about the detailed structure— linear 19 or bent 20—of the vinyl cation. A non-classical structure 21 can be disregarded, however, because the attack... 

In electrocatalysis, in contrast to electrochemical kinetics, the rate of an electrochemical reaction is examined at constant external control parameters so as to reveal the influence of the catalytic electrode (its nature, its surface state) on the rate constants in the kinetic equations. 

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