Kinetic thermodynamic control

In an intramolecular aldol condensation of a diketone many products are conceivable, since four different ends can be made. Five- and six-membered rings, however, wUl be formed preferentially. Kinetic or thermodynamic control or different acid-base catalysts may also induce selectivity. In the Lewis acid-catalyzed aldol condensation given below, the more substituted enol is formed preferentially (E.J. Corey, 1963 B, 1965B). 

Chloro 1 3 butadiene (chloroprene) is the monomer from which the elastomer neoprene IS prepared 2 Chloro 1 3 butadiene is the thermodynamically controlled product formed by addi tion of hydrogen chloride to vinylacetylene (H2C=CHC=CH) The principal product under conditions of kinetic control is the allenic chlonde 4 chloro 1 2 butadiene Suggest a mechanism to account for the formation of each product... 

On reaction with acyl chlorides and acid anhydrides phenols may undergo either acylation of the hydroxyl group (O acylation) or acylation of the ring (C acylation) The product of C acylation is more stable and predominates under conditions of thermodynamic control when alu mmum chloride is present (see entry 6 m Table 24 4 Section 24 8) O acylation is faster than C acylation and aryl esters are formed under conditions of kinetic control... 

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Plot of signal versus time for an analytical system that is under (a) thermodynamic control and (b) under kinetic control. 

Conra.d-Limpa.ch-KnorrSynthesis. When a P-keto ester is the carbonyl component of these pathways, two products are possible, and the regiochemistry can be optimized. Aniline reacts with ethyl acetoacetate below 100°C to form 3-anilinocrotonate (14), which is converted to 4-hydroxy-2-methylquinoline [607-67-0] by placing it in a preheated environment at 250°C. If the initial reaction takes place at 160°C, acetoacetanilide (15) forms and can be cyclized with concentrated sulfuric acid to 2-hydroxy-4-methylquinoline [607-66-9] (49). This example of kinetic vs thermodynamic control has been employed in the synthesis of many quinoline derivatives. They are useful as intermediates for the synthesis of chemotherapeutic agents (see Chemotherapeuticsanticancer). 

The composition of the products of reactions involving intermediates formed by metaHation depends on whether the measured composition results from kinetic control or from thermodynamic control. Thus the addition of diborane to 2-butene initially yields tri-j iAbutylboraneTri-j -butylborane. If heated and allowed to react further, this product isomerizes about 93% to the tributylborane, the product initially obtained from 1-butene (15). Similar effects are observed during hydroformylation reactions however, interpretation is more compHcated because the relative rates of isomerization and of carbonylation of the reaction intermediate depend on temperature and on hydrogen and carbon monoxide pressures (16). 

Alkylation of pyrazinones and quinoxalinones may be carried out under a variety of conditions and it is usually observed that while O-alkylation may occur under conditions of kinetic control, to yield the corresponding alkoxypyrazines or alkoxyquinoxalines, under thermodynamic control the A-alkylated products are formed. Alkylation using trialkyl-oxonium fluoroborate results in exclusive O-alkylation, and silylation under a variety of conditions (75MI21400) yields specifically the O-silylated products. Alkylation with methyl iodide or dimethyl sulfate invariably leads to A-methylation. 

A parallel exists between the results of protonation and alkylation of pyrazolones since there is an alkyl derivative for each tautomer. The main difference is that the percentage of the different tautomers is thermodynamically controlled whereas that of alkyl derivatives is kinetically controlled. One has to remember that the alkyl derivatives thus obtained are the fixed compounds used in tautomeric studies. 

Basic Mechanistic Concepts Kinetic versus Thermodynamic Control, Hammond s Postulate, the Curtin-Hammett Principle... 

SECTION 4.4. BASIC MECHANISTIC CONCEPTS KINETIC VERSUS THERMODYNAMIC CONTROL, HAMMOND S POSTULATE. THE CURTIN-HAMMETT PRINCIPLE... 

Product composition may be governed by the equilibrium thermodynamics of the system. When this is true, the product composition is governed by thermodynamic control. Alternatively, product composition may be governed by competing rates of formation of products. This is called kinetic control. 

The idea of kinetic versus thermodynamic control can be illustrated by discussing briefly the case of formation of enolate anions from unsymmetrical ketones. This is a very important matter for synthesis and will be discussed more fully in Chapter 1 of Part B. Most ketones, highly symmetric ones being the exception, can give rise to more than one enolate. Many studies have shown tiiat the ratio among the possible enolates that are formed depends on the reaction conditions. This can be illustrated for the case of 3-methyl-2-butanone. If the base chosen is a strong, sterically hindered one and the solvent is aptotic, the major enolate formed is 3. If a protic solvent is used or if a weaker base (one comparable in basicity to the ketone enolate) is used, the dominant enolate is 2. Enolate 3 is the kinetic enolate whereas 2 is the thermodynamically favored enolate. 

Reaction of A with water under kinetic control conditions leads to the 5)5,19-cyclo-6 -ol (69a), whereas under conditions of thermodynamic control A rearranges to the isomeric cation B which reacts with water to give the B-homo-7)5-ol (70a). 

FIGURE 10.8 Energy diagram showing relationship of kinetic control to thermodynamic control in addition of hydrogen bromide to 1,3-butadiene. 

Reactant and product structures. Because the transition state stmcture is normally different from but intermediate to those of the initial and final states, it is evident that the stmctures of the reactants and products should be known. One should, however, be aware of a possible source of misinterpretation. Suppose the products generated in the reaction of kinetic interest undergo conversion, on a time scale fast relative to the experimental manipulations, to thermodynamically more stable substances then the observed products will not be the actual products of the reaction. In this case the products are said to be under thermodynamic control rather than kinetic control. A possible example has been given in the earlier description of the reaction of hydroxide ion with ester, when it seems likely that the products are the carboxylic acid and the alkoxide ion, which, however, are transformed in accordance with the relative acidities of carboxylic acids and alcohols into the isolated products of carboxylate salt and alcohol. 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

These results have led to the conclusion (11) that the formation of enammonium salts is kinetically controlled, while the protonation on the 3-carbon atom is subject to thermodynamic control, t Only tertiary enamines will be considered,... 

Dimethylborane+propene C2 and 2-propyldimethyl borane depict the regioisomeric transition state and addition product. Calculate the energies of these species relative to those of the alternative transition state and product. Given these energy differences, and the experimental observation that this addition is almost completely selective for the anti-Markovnikov product, does it appear that this reaction is under kinetic or thermodynamic control Explain. 

It was clearly shown by NMR spectroscopy that the addition of ammonia or primary or secondary alkylamines at position 5 of the 1,2,4-triazine 4-oxides to give the adducts 89 is a kinetically controlled process, while addition at position 3 to form the ring-opening products 85 is a thermodynamically controlled process. 

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