Kinetics rearrangements

Toth et al. have thoroughly studied the rearrangement their kinetic determinations suggest a general acid-catalyzed mechanism (Scheme 115) (1578). Some points remain unclear, however why is the intermediate (181) written as a transition state when it is known that a tetrahedral intermediate (181b or 181b ) could as well be postulated How does this... 

Nitraminothiazole. treated for 12 hr with 96% sulfuric acid, gives 2-amino-5-nitrothiazole (194). The mechanism of this rearrangement is not yet quite resolved even for nitraminobenzene derivatives (617). The series of kinetic determinations and appropriate labeling performed by Toth et al. provide, however, precious hints for this difficult problem (1578. 1579). 

Alternatively, a number of investigators (69—73) have proposed, on the basis of plausible kinetic arguments, that the conjugate olefin is produced by a rearrangement of alkylperoxy radicals (eq. 24). 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

Table 27 Kinetic Data for Autotrope Rearrangements in Pyrazoles...

The course of the reaction is dependent on the configuration of the oxime. The (Z)-oxime gave 1,2-benzisoxazoles as the primary product while the (E)-oxime generally produced a Beckmann rearrangement product with or without subsequent cyclization to a benzisoxazole (Scheme 167) (67AHC(8)277). Bunnett conducted a kinetic study on the reaction shown in Scheme 167 and determined that cyclization to intermediate (551) was the rate determining step (61JA3805). 

The time required for atmospheric chemical processes to occur is dependent on chemical kinetics. Many of the air quality problems of major metropolitan areas can develop in just a few days. Most gas-phase chemical reactions in the atmosphere involve the collision of two or three molecules, with subsequent rearrangement of their chemical bonds to form molecules by combination of their atoms. Consider the simple case of a bimolecular reaction of the following type-. 

The third approaeh to synthetic polymers is of somewhat less commereial importance. There is in fact no universally accepted deseription for the route but the terms rearrangement polymerisation and polyaddition are commonly used. In many respects this process is intermediate between addition and condensation polymerisations. As with the former teehnique there is no moleeule split out but the kinetics are akin to the latter. A typical example is the preparation of polyurethanes by interaction of diols (di-alcohols, glycols) with di-isocyanates Figure 2.7). 

Even though the rearrangements suggest that discrete carbocation intermediates are involved, these reactions frequently show kinetics consistent with the presence of at least two hydrogen chloride molecules in the rate-determining transition state. A termolecular mechanism in which the second Itydrogen chloride molecule assists in the ionization of the electrophile has been suggested. ... 

All these kinetic results can be accommodated by a general mechanism that incorporates the following fundamental components (1) complexation of the alkylating agent and the Lewis acid (2) electrophilic attack on the aromatic substrate to form the a-complex and (3) deprotonation. In many systems, there m be an ionization of the complex to yield a discrete carbocation. This step accounts for the fact that rearrangement of the alkyl group is frequently observed during Friedel-Crafts alkylation. 

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). 

The C-nor-D-homosteroid rearrangement was discovered by Hirschmann and co-workers at the time that Wintersteiner and his collaborators established that the steroid alkaloids jervine and veratramine incorporate a 14 (1312) abeo-nng system. This was the predecessor of the family of simultaneous ring contraction-expansion reactions. Solvolysis of the 12j5-methanesulfonate (144a) gives mainly the kinetic reaction product, the C-nor-D-homo exocyclic olefin (145) along with some 13(17)-ene (146a). 

Rearrangement studies give an interesting insight into the specific effect of fluonne on the thermodynamic stability and rearrangement kinetics of fluonnated cyclopropanes Fluorine decreases the thermodynamic stability of the cyclopropyl nng, in contrast with the generally observed effect of fluonne increasing the stability of molecules to which it is introduced [124]... 

The strength of the carbon-carbon bond adjacent to a difluoromethylene group IS hardly affected by the presence of fluorine on the cyclopropyl ring When heated, 2,2 difluoromethylenecyclopropane undergoes methylene-cyclopropane rearrangement [/2 ] Under kinetic control, 2,2 difluoro 1 methylenecyclopropane and (difluoromethylene)cyclopropane are formed in a 2 1 ratio, although the latter IS slightly more stable [129] (equation 27)... 

The preference for O-acylation of phenols fflises because these reactions ffle kinetically controlled. O-acylation is faster than C-acylation. The C-acyl isomers are more stable, however, and it is known that aluminum chloride is a very effective catalyst for the conversion of fflyl esters to fflyl ketones. This isomerization is called the Fries rearrangement. 

Espenson and Livesey have solved the second-order kinetics problem (in the special case) when A is unknown. Combine Eqs. (2-16) and (2-51) and rearrange to give... 

From this expression, it is obvious that the rate is proportional to the concentration of A, and k is the proportionality constant, or rate constant, k has the units of (time) usually sec is a function of [A] to the first power, or, in the terminology of kinetics, v is first-order with respect to A. For an elementary reaction, the order for any reactant is given by its exponent in the rate equation. The number of molecules that must simultaneously interact is defined as the molecularity of the reaction. Thus, the simple elementary reaction of A P is a first-order reaction. Figure 14.4 portrays the course of a first-order reaction as a function of time. The rate of decay of a radioactive isotope, like or is a first-order reaction, as is an intramolecular rearrangement, such as A P. Both are unimolecular reactions (the molecularity equals 1). 

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