Intramolecular rearrangement

The product (33) may undergo an oxydefluorination reaction to give the benzofuran derivative (34). Such intramolecular substitutions provide a useful synthetic methodology and often involve displacement of, or reaction with, a nitro group [112]. Some synthetic aspects are summarized in Section 6.5. 

Cyclization reactions of natural rubber and other polymers from conjugated dienes have been known for a long time. The reactions occur in the presence of Lewis and strong protonic acids. They result in loss of elastomeric properties and some unsaturation. Carbon cations form in the intermediate step and subsequent formation of polycyclic structures  

In a similar manner, polymers with pendant unsaturation undeigo cyclization reactions in benzene in the presence of BF3 or POCI3 yielding ladder structures. The exact nature of the initiation process is not clear. Water may be need for the initiation step  

The reactions result in formations of six-membered monocyclic and fused polycyclic units. These reactions of carbon cations should also lead to molecular rearrangements, like 1,2-shifts of protons, resulting in formations of five-membered rings and spiro cyclopentane repeat units.  

Cyclization reactions of polyisoprene can be catalyzed by TiCU and by sulfuric acid. 3-115,117 products appear similar in the infrared spectra with only a few minor differences. Also, there is only a small number of fused rings in the product. 

Polyacrylonitrile converts to a red solid when heated above 200 Only a small amount of 

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M.p. 127 5°C. Dibasic, readily diazotizes. Prepared by treating hydrazobenzene with hydrochloric acid, intramolecular rearrangement taking place. 

When hydrazobenzene, or 5ym-diphenylhydrazine, is warmed with acids, it undergoes an intramolecular rearrangement, with the formation of benzidine,... 

The mechanism of the Fries reaction is not known with certainty. One mechanism regards it as a true intramolecular rearrangement in which the acyl group migrates directly from the oxygen atom to the carbon atoms of the ring. Another scheme postulates that the ester is cleaved by the reagent... 

Ciesol and xylenol can be prepared by the methylation of phenol with methanol over both acid and base catalysts. It is postulated that phenol methylation on acid catalysts proceeds through the initial formation of anisole (methoxybenzene [100-66-3]) followed by intramolecular rearrangement of... 

The C alkylation has been rationalized 33) by initial N alkylation of 16 to 17 followed by an intramolecular rearrangement involving a six-membered... 

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). 

FIGURE 18.29 Vitamin B19 functions as a coenzyme in intramolecular rearrangements, reduction of ribonucleotides, and methyl group transfers. 

A number of intramolecular rearrangements of 1,2,3,4-tetrahydro-j8-carboline derivatives have been reported. Some of these lead into other naturally occurring heterocyclic ring systems and are therefore of particular interest. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Tricyclic 84 was prepared [91JCS(P1)1762] by the thermal intramolecular rearrangement of dichloro(pyrrolidinylcycloheptenyl)triazine 83. Its structure was confirmed by X-ray crystallography (Scheme 21). 

Intramolecular rearrangement of the initially formed radical may occur occasionally (e.g. backbiting - Section 4.4.3) or even be the dominant pathway (e.g. cyelopolymerization - Section 4.4.1, ring-opening polymerization - Section 4.4.2). These pathways can give rise to branches, rings, or internal unsaturation in the polymer chain. 

The authors conclude from these results that these (Z)/(E) rearrangements proceed through the ionization-rearrangement pathway. They assume that the ratios quoted arise solely from the difference in polarity of methanol and ethanol. For an intramolecular rearrangement (kY) one would not expect to find significantly different rates for these two solvents. 

Investigations of the constitution and the configuration at the azo groups in diazoanhydrides are difficult because they decompose very rapidly if dissolved in organic solvents, and their water solubility is not sufficient. The results obtained by Kauffmann et al. using HCN in water are in favor of the covalent structure 6.13 rather than of a diazonium-diazoate salt they obtained (in 93% yield) two equivalents of (Z)-4-chlorobenzenediazocyanide. In ethanol the products shown in Scheme 6-8 were obtained, i.e., the same products as obtained by Huisgen and Nakaten (1951, 1954) under similar conditions from (Zi)-diazoacetate, the intermediate in the intramolecular rearrangement of AT-nitrosoacetanilide. 

The situation is not as clearly solved in a positive or negative sense for arenediazo phenyl ethers. Here three alternatives have to be considered, namely an intramolecular rearrangement of the arenediazo phenyl ether (Scheme 12-11, A), and two types of intermolecular rearrangement, either by heterolytic dissociation into a diazonium ion and a phenoxide ion (B) or by homolytic dissociation into a radical pair or two free radicals (C). 

An intramolecular rearrangement of the conjugate acid of the triazene compound to form the oc-complex without an additional molecule of amine would correspond to a thermal [l,3]-sigmatropic rearrangement. However, such a mechanism can be ruled out on the grounds of the antarafacial pathway required from orbital symmetry considerations (Woodward-Hoffmann rules). 

As has been indicated, since there is a ring isotope effect there must be a degree of C-H bond breaking in the transition state of the rate-determining stage. Clearly further work is required in this system before a definitive mechanism can be established for the intramolecular rearrangement. 

N-substituted phenylhydroxylamine derivatives, e.g. N-acetyl and N-sulphonic acid, also form the para-aminophenol", but more bulky groups prevent reaction, it is thought by steric hindrance to the approach of the hydronium ion. -substituted phenylhydroxylamines, on the other hand form only the ortho product, it is thought via an intramolecular rearrangement, e.g. 

Cydizations and Other Intramolecular Rearrangements of Carbene Complexes... 

First, the rates of carbonylation of secondary and tertiary alkyl carbonium ions can now be compared quantitatively with the known rates of competing intramolecular rearrangements of these ions. The product distribution in the Koch synthesis of carboxylic acids depends, amongst other things, on these relative rates. 

Brick et al. have studied this bromination in more detail and showed that the extent of the bromination can be controlled by changing the ratio of the reagents. The first substitution was found to be in the para position but subsequent intramolecular rearrangements allowed the formation of 2-5-dibrominated species. Brick et al. also reported the functionalization of such species using Pd-catalyzed reactions such as Heck and Suzuki couplings to give fully substituted p-stilbenes, p-biphenyls, diarylamines, and methylcinnamates. Hydrogenation of... 

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