Rearrangement example mechanism

This reaction, for which the termprototmpic rearrangement is sometimes used, is an example of electrophilic substitution with accompanying allylic rearrangement. The mechanism involves abstraction by the base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more... 

Reimann et al.17 visualize this transformation as a formal example of a Wagner- Meerwein type of radical rearrangement. The mechanism suggested by these works is outlined in structures 65 to 68. The inter-... 

This is known as the van AIphen-Huttel rearrangement its mechanism, scope, and limitations were discussed in the preceding chapter.1 In cases where the 4//-pyrazole has been isolated, the migrating group R2 has most frequently been phenyl.40-46 In one example it has been methyl,47 and in an... 

The most important route is the conversion of pyrimidines into 1,3,5-triazines. The first one-step transformation was effected by Taylor and Jefford (62JA3744) by heating the pyrimidine (179) with benzenesulfonyl chloride in pyridine (equation 106). The reaction may be considered as an example of an abnormal Beckmann rearrangement. The mechanism of the reaction of the 4-aminopyrimidine (180) is probably dependent on the nature of the 2-substituent (180, R). If R is an electron-releasing moiety, pathway B seems more likely (Scheme 109). The 4-hydroxypyrimidine (179 R = OH) behaves similarly. Many 2-cyano-1,3,5-triazines may be synthesized by this method. 

Rearrangements.- Examples of, E-photoisomerization in imines have again been reported. The photoisomerizations of methanimine and of fluorinated methanimines have been examined and the study of the photoisomerization of protonated and unprotonated imines of 9-cis-, 11-cis-, 15-cis-, and all trans-retinals has attracted attention in view of the continuing interest in the mechanism of the visual process. Evidence from flash photolysis studies indicates that the photoisomerization of the E-hydrazone (1) to... 

Dimethylimidazole, however, gives the 5-hydroxymethyl derivative, and though it is usually difficult to induce substrates with electron-withdrawing groups to react, l-methyl-5-nitroimidazole forms the 2-hydroxymethyl product (see CHEC-I). Whilst the reaction mechanism is uncertain, it could involve A(-hydroxymethylation followed by rearrangement. Examples include 2-hydroxy-methylation of l,4-dimethyl-5-nitroimidazole in DMSO <87JMC150>, and 5-substitution of 4-bromo-... 

Although arynes are reluctant to rearrange, examples of ring contraction of an aryne have been recorded. An example of an aryne rearrangement to a carbene has been recorded it is probable that the aryne and carbene are in equilibrium under the vigorous conditions (flash vacuum pyrolysis at 960 C) of the reaction . A similar ring contraction in a heterocyclic system probably proceeds via a similar mechanism. ... 

A typical example of a proto-oncogene translocation is the membrane tyrosine kinase receptor RET [see review (Santoro et al. 2004)]. The outer membrane part consists of four cadherin-like domains the inner membrane domain has the tyrosine kinase activity. The gene was discovered in 1985 and was found to be activated by a DNA rearrangement, a mechanism giving the gene its name (Rearranged during Transfection). RET protein has several tyrosine residues that are auto-phosphory-lated. The phosphorylation of the tyrosine 905 is sug-... 

Rearrangement of Electron-Deficient Carbon Carbocation rearrangements are well known. One named example is the pinacol rearrangement, the mechanism and stereochemistry of which have been well studied. 

The lack of examples demonstrating the reactivity on C-2 may be the misleading impression that this atom is not electrophilic, contrary to what is indicated from charge diagrams. Such is not the case as the Cook s rearrangement demonstrates (209, 212). A logical mechanism for this reaction involves the tetrahedral intermediate (88) (Scheme 42). This... 

Many examples are known of rearrangement of azoles involving scrambling of the ring atoms to give a new isomeric azole molecule. Different mechanisms are involved. 

All the examples quoted in this section concerning fragmentations or rearrangements involve photochemistry. An interesting thermal reaction has been described (72TL2235) in which the pyrolysis of indazole between 700 and 800 °C leads to a mixture of (197) and (198 Scheme 15). A mechanism involving the 3// tautomer and the carbene seems reasonable. 

Scheme 6 depicts a typical penicillin sulfoxide rearrangement (69JA1401). The mechanism probably involves an initial thermal formation of a sulfenic acid which is trapped by the acetic anhydride as the mixed sulfenic-acetic anhydride. Nucleophilic attack by the double bond on the sulfur leads to an episulfonium ion which, depending on the site of acetate attack, can afford either the penam (19) or the cepham (20). Product ratios are dependent on reaction conditions. For example, in another related study acetic anhydride gave predominantly the penam product, while chloroacetic anhydride gave the cepham product (7lJCS(O3540). The rearrangement can also be effected by acid in this case the principal products are the cepham (21) and the cephem (22 Scheme 7). Since these early studies a wide variety of reagents have been found to catalyze the conversion of a penicillin sulfoxide to the cepham/cephem ring system (e.g. 77JOC2887).

For other mechanisms, the particle-scale equation must be integrated. Equation (16-140) is used to advantage. For example, for external mass transfer acting alone, the dimensionless rate equation in Table 16-13 would be transformed into the ( — Ti, Ti) coordinate system and derivatives with respect to Ti discarded. Equation (16-138) is then used to replace cfwith /ifin the transformed equation. Furthermore, for this case there are assumed to be no gradients within the particles, so we have nf=nf. After making this substitution, the transformed equation can be rearranged to... 

Another example of a reaction in which the stereochemistry of the process provides some valuable information about the mechanism is the thermal rearrangement of 1,5-dienes and substituted analogs ... 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

It should be noted that Scheme 5.1-44 shows idealized Friedel-Crafts allcylation reactions. In practice, there are a number of problems associated with the reaction. These include polyalkylation reactions, since the products of a Friedel-Crafts alkylation reaction are often more reactive than the starting material. Also, isomerization and rearrangement reactions can occur, and can result in a large number of products [74, 75]. The mechanism of Friedel-Crafts reactions is not straightforward, and it is possible to propose two or more different mechanisms for a given reaction. Examples of the typical processes occurring in a Friedel-Crafts alkylation reaction are given in Scheme 5.1-45 for the reaction between 1-chloropropane and benzene. 

Rearrangement polymerisation Here the mechanism resembles condensation polymerisation hut no stnall mbleculfr is split out. In the first example l 4-butane di6l reacts with hexaniethyiehe di-isocyanate to give 6,4-poly-... 

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the "expected" product, 2-chloro-3-methylbutane. 

There are two features of this example that are rather common. First, none of the steps in the reaction mechanism requires the collision of more than two particles. Most chemical reactions proceed by sequences of steps, each involving only two-particle collisions. Second, the overall or net reaction does not show the mechanism. In general, the mechanism of a reaction cannot be deduced from the net equation for the reaction , the various steps by which atoms are rearranged and recombined must be determined through experiment. 

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