1,2-Rearrangement reaction

Chiral Br0nsted acid-catalyzed aza-Cope rearrangement reactions. 

Phosphoric acid-catalyzed asymmetric semipinacol rearrangement 

SCHEME 2.25 Phosphoric acid-catalyzed asymmetric pinacol rearrangement reaction. 

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At times, however, even the infomiation presented by "nis too detailed. If one wants to rigorously calculate the themial rate of rearrangement reactions, the initial vibrational state is not important. The relevant quantity is the sum of tire initial-state-selected probabilities... 

Rearrangement reactions. These reactions may be skeletal, in which case the symbol —<=i is... 

Electrochemical fluorination leads to fragmentation, coupling, and rearrangement reactions as well as giving the perfluorinated product. In addition, small amounts of hydrogen can be retained in the cmde product. The products are purified by treatment with base to remove the hydrogen-containing species and subsequently distilled. 

Squalene is also an intermediate in the synthesis of cholesterol. StmcturaHy, chemically, and biogeneticaHy, many of the triterpenes have much in common with steroids (203). It has been verified experimentally that squalene is the precursor in the biosynthesis of all triterpenes through a series of cyclization and rearrangement reactions (203,204). Squalene is not used much in cosmetics and perfumery formulations because of its light, heat, and oxidative instabiUty however, its hydrogenated derivative, squalane, has a wide use as a fixative, a skin lubricant, and a carrier of Hpid-soluble dmgs. 

Adenosylcobalamin (coenzyme B 2) is required in a number of rearrangement reactions that occurring in humans is the methylmalonyl-Co A mutase-mediated conversion of (R)-methylmalonyl-Co A (6) to succinjl-CoA (7) (eq. 1). The mechanism of this reaction is poorly understood, although probably free radical in nature (29). The reaction is involved in the cataboHsm of valine and isoleucine. In bacterial systems, adenosylcobalamin drives many 1,2-migrations of the type exemplified by equation 1 (30). 

C-Alkylations may be discussed under the headings of alkene reactions and A/-alkyl rearrangements. The isopropylation of benzene and naphthalene are two important examples of alkylation with alkenes (see Alkylation). Manufacture of j butylaniline, by heating /V-butylaniline with 2inc chloride, typifies the rearrangement reaction appropriate to and higher alkyl derivatives. 

Figure 4.6 The bifunctional enzyme PRA-isomerase (PRAI) IGP-synthase (IGPS) catalyzes two sequential reactions in the biosynthesis of tryptophan. In the first reaction (top half), which is catalyzed by the C-terminal PRAI domain of the enzyme, the substrate N-(5 -phosphoribosyl) anthranilate (PRA) is converted to l-(o-carboxyphenylamino)-l-deoxyribulose 5-phosphate (CdRP) by a rearrangement reaction. The succeeding step (bottom half), a ring closure reaction from CdRP to indole-3-glycerol phosphate (IGP), is catalyzed by the N-terminal IGPS domain.

In turn the oxazoline-containing polymer may then react very rapidly (e.g. at 240°C) with such groups as carboxyls, amines, phenols, anhydrides or epoxides, which may be present in other polymers. This reaction will link the two polymers by a rearrangement reaction similar to that involved in a rearrangement polymerisation without the evolution of water or any gaseous condensation products (Figure 7.14). 

A variety of bifunctional compounds react with the bismaleimides to form polymers by rearrangement reactions. These include amines, sulphides and aldoximes (Figure 18.41). 

Over the years many blends of polyurethanes with other polymers have been prepared. One recent example is the blending of polyurethane intermediates with methyl methacrylate monomer and some unsaturated polyester resin. With a suitable balance of catalysts and initiators, addition and rearrangement reactions occur simultaneously but independently to give interpenetrating polymer networks. The use of the acrylic monomer lowers cost and viscosity whilst blends with 20% (MMA + polyester) have a superior impact strength. 

Allyl esters undergo rearrangement reactions at 300°C and above. Two examples are shown, one of which is degenerate, since the product and reactant are identical ... 

Rearrangement and Fragmentation Reactions of Free Radicals 12.7.1. Rearrangement Reactions... 

The elimination from sulfonates of secondary alcohols is frequently easier than more direct methods applied to the free alcohols. As with the latter, there are the possibilities of isomeric olefin formation and rearrangement reactions. In addition, displacement and hydrolysis may occur, but these side reactions can usually be suppressed. 

It was pointed out earlier that the low nucleophilicity of fluoride ion and its low concentration in HF solutions can create circumstances not commonly observed with the other halogen acids. Under such conditions rearrangement reactions either of a concerted nature or via a true carbonium ion may compete with nucleophilic attack by fluoride ion. To favor the latter the addition of oxygen bases, e.g., tetrahydrofuran, to the medium in the proper concentration can provide the required increase in fluoride ion concentration without harmful reduction in the acidity of the medium. 

The enamine-imine tautomerism of the indolenine system gives rise to rearrangement reactions of interest in indole alkaloid chemistry. Thus the synthesis of dihydroburnamicine (625) utilized the rearrangement of an acetoxyindolenine to an a-hydroxyalkyl indole, presumably through an intermediate enamine. Similarly 2,3-dialkyl indoles undergo oxidations to 2-acyl indoles (626-631). 

The di- r-methane rearrangement is a fairly recent reaction. One of the first examples has been reported in 1966 by Zimmerman and Grunewald with the isomerization of barrelene 8 to semibullvalene 9. This rearrangement reaction occurs in the presence of acetone as photosensitizer, and proceeds from the Ti-state. ... 

Yields of the di- r-methane rearrangement reaction strongly depend on substrate structure, and are ranging from poor to nearly quantitative. Acetone and acetophenone have been used as photosensitizers." ... 

Phenolic esters (1) of aliphatic and aromatic carboxylic acids, when treated with a Lewis acid as catalyst, do undergo a rearrangement reaction to yield ortho- and para-acylphenols 2 and 4 respectively. This Fries rearrangement reaction is an important method for the synthesis of hydroxyaryl ketones. 

As catalysts for the Fries rearrangement reaction are for example used aluminum halides, zinc chloride, titanium tetrachloride, boron trifluoride and trifluoromethanesulfonic acid7... 

For the Flofmann rearrangement reaction, a carboxylic amide 1 is treated with hypobromite in aqueous alkaline solution. Initially an iV-bromoamide 4 is formed. With two electron-withdrawing substituents at nitrogen the A -bromoamide shows NFI-acidity, and can be deprotonated by hydroxide to give the anionic species 5. 

The ketoxime derivatives, required as starting materials, can be prepared from the appropriate aromatic, aliphatic or heterocyclic ketone. Aldoximes (where R is H) do not undergo the rearrangement reaction, but rather an elimination of toluenesulfonic acid to yield a nitrile. With ketoxime tosylates a Beckmann rearrangement may be observed as a side-reaction. 

The pinacol rearrangement reaction is of limited synthetic importance although it can be a useful alternative to the standard methods for synthesis of aldehydes and ketones." Especially in the synthesis of ketones with special substitution pattern—e.g. a spiro ketone like 5—the pinacol rearrangement demonstrates its synthetic potential ... 

The required vicinal diols are in general accessible by standard methods. Pinacol itself can be obtained by dimerization of acetone. For the rearrangement reaction concentrated or dilute sulfuric acid is often used as catalyst. 

The zinc iodide formed in a Simmons-Smith reaction can act as Lewis acid, and thereby may catalyze rearrangement reactions however interfering side-reactions are generally rare. 

The synthetic applicability is rather limited, due to the various side-reactions observed, such as eliminations and rearrangement reactions. The attempted coupling of two different alkyl halides in order to obtain an unsymmetrical hydrocarbon, usually gives the desired product in only low yield. However the coupling reaction of an aryl halide with an alkyl halide upon treatment with a metal (the Wurtz-Fittig reaction) often proceeds with high yield. The coupling of two aryl halides usually does not occur under those conditions (see however below ) since the aryl halides are less reactive. 

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