1,2-Addition reaction kinetic control

As the OH addition reactions are controlled by kinetics rather than by thermodynamics, the primary OH adduct radicals are not necessarily the thermodynamically most stable radicals. Thus, a number of rearrangement and water elimination reactions take place. For example, Thy60H is more stable than ThySOH by 42 kj mol but Thy60H is formed preferentially. As has been shown in the case of 1,3-dimethyluracil in a detailed product study, acid catalysis converts the kinetically favored C6 OH adduct into the... 

In some reactions the 1,2-addition is kinetically controlled and reversible, so that longer reaction times or higher temperatures may result in the formation of an increased amount of 1,4-adduct. In some cases a similar effect can be attained by performing the addition in the presence of a certain amount of hexamethylphosphoric triamide (HMPT). This solvent effect may be the consequence of a reversal of the 1,2-addition, but there are also cases in which HMPT promotes a kinetically controlled 1,4-addition [10]. 

When the major product of a reaction is the one that is formed at the fastest rate we say that the reaction is governed by kinetic control Most organic reactions fall into this category and the electrophilic addition of hydrogen bromide to 1 3 butadiene at low temperature is a kmetically controlled reaction... 

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). 

With pteridine (1) the covalent hydration is a complex matter since the general acid-base catalyzed reaction provides a good example of a kinetically controlled addition to the... 

Addition of hydrogen chloride to 2-methyl-1,3-butadiene is a kinetically controlled reaction and gives one product in much greater amounts than any isomers. What is this product ... 

Another way to assess thiophene s reactivity is to compare the intermediate ions formed by addition of N02. Examine the structures, charge distributions and electrostatic potential maps of thiophene+nitronium at C2 and thiophene+nitronium at C3. Draw all of the resonance contributors needed to describe these structures. Which, if either, better delocalizes the positive charge Compare the energies of the two intermediates. Which product should form preferentially if the reaction is under kinetic control Are these results consistent with FMO theory ... 

When both a-positions of the oxime possess active hydrogen, the regiochemistry of the Hoch-Campbell reaction prefers the side with more available hydrogens— indicating the process is kinetically controlled. In case of oxime 36, azirine 37 was not formed. Instead, azirine 38 was obtained exclusively. Addition of the third equivalent of the Grignard reagent delivered aziridine 39 as a mixture of two diastereomers. 

The electrophilic addition of HBr to 1,3-butadiene is a good example of how a change in experimental conditions can change the product of a reaction. The concept of thermodynamic control versus kinetic control is a useful one that we can sometimes take advantage of in the laboratory. 

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

Both primary and secondary amines add to a /S-unsaturated aldehydes and ketones to yield /3-amino aldehydes and ketones rather than the alternative imines. Under typical reaction conditions, both modes of addition occur rapidly. But because the reactions are reversible, they generally proceed with thermodynamic control rather than kinetic control (Section 14.3), so the more stable conjugate addition product is often obtained to the complete exclusion of the less stable direct addition product. 

Lithiated areneacetonitriles react with a,/i-unsaturated ketones at low temperatures using short reaction times to give both 1,2- and 1,4-adducts. The 1,2-addition is reversible and under thermodynamic control (higher temperatures and longer reaction times) only the 1,4-adducts, i.e., <5-oxonitriles, arc obtained. When lithiated arylacetonitrile is added to 2-substituted 2-cy-cloalkenones in THF or in THF/HMPA mixtures at — 70-0°C, followed by protonation or alkylation under kinetically controlled conditions, predominantly cis- or fnms-2,3-disubstitut-ed cycloalkanones respectively, are obtained. 

Doping of solid reactant involves the introduction of a controlled amount of an impurity into solid solution in the host lattice. Such impurities can be selected to cause the generation or destruction of those electronic or structural defects which participate in the rate process of interest. Thus, the influence of the additive on kinetic behaviour can provide evidence concerning the mechanism of reaction [46,47]. Even if the... 

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