Rearrangement or isomerization

II. Reaction with Preservation of the F Atoms a) Rearrangement or Isomerization... 

Abstract In this chapter, nanocluster catalyzed modifications of organic and silicon based polymers are described. The tailoring of the polymeric tanplates was carried out under mild conditions and led to hybrid polymers in quantitative yields. Detailed characterization studies indicated that the integrity of the polymeric templates was not compromised during the functionalization process. The nanoparticle catalysis was found to be quite effective and highly selective, hi most cases exclusive 6-hydrosilylation products were obtained without any rearrangement or isomerization reactions. Detailed characterization and property profiling of the new hybrid polymers is also presented. 

Intramolecular rearrangements or isomerizations of organic compounds primarily important in organic syntheses " produce new C—H bonds, e.g. ... 

Through AddUion + Other Reactions (without Substitution) 2.1. + Rearrangement or Isomerization (and Elimination) 2.1.1. Addition to the C = C Bond... 

In rearrangement or isomerization reactions, one has coupling of a set of discrete initial states i> to a quasi-continuous but still discrete set of final states f>, as shown in equation (39). 

Rearrangement or isomerization reactions proceed typically at carbocations or other electron-deficient positions of a molecule. In rearrangement reactions the substrate stabilizes itself by rearranging its structure without changing the number and type of its atoms. Thus, rearrangement reactions proceed without addition/release of molecules other than substrate and product. Rearrangement reactions of technical importance are the isomerization of linear alkanes to branched alkanes (important to increase the quality of fuels) and the rearrangement of cyclohexanone oxime to e-caprolactam (Scheme 2.2.4). 

Classify each of the following reactions as one of substitution, addition, elimination, condensation, rearrangement, or isomerization ... 

Alkyl- and arylthiazoles rearrange undernltraviolel irradiation in different solvents to yield the corresponding isothiazoles or isomeric thiazoles. With alkylthiazoles the overall yields are very low, and it is not possible to use this method preparatively. For arylthiazoles it is possible 2-arylthiazoles. for instance, can be used to prepare 3-arylisothiazoles that are otherwise very difficult to obtain. 

The parent thionine system 1 up to now has not been prepared probably because the C-S bond in valence isomeric forms is too weak giving rise to facile rearrangement or decomposition. The obvious synthetic route, photochemical transformation of cyclooctatetraenccpisulfide 2 (9-thiabicyclo[6.1.0]nona-2,4,6-triene), does not lead to 1, but intriguingly to another valence isomer, the sulfur-bridged homotropylidene system 3.20... 

Rearrangement of each isomer or isomeric mixture leads to only one of the three isomers. No side reaction such as "bromine dancing", trans-aimulare reaction etc, can be observed. 

Ammonium ylides can isomerize to (1,2) rearrangement products (Stevens rearrangement) or to (2,3) shift products (Sommelet-Hauser sigmatropic rearrangement) when allyl or benzyl are located on the nitrogen atom. A strong microwave effect is noticed (Eq. 66) [116]. 

A closer examination by ex situ analysis using NMR or gas chromatography illustrates that intrazeolite reaction mixtures can get complex. For example photooxygenation of 1-pentene leads to three major carbonyl products plus a mixture of saturated aldehydes (valeraldehyde, propionaldehyde, butyraldehyde, acetaldehyde)38 (Fig. 33). Ethyl vinyl ketone and 2-pentenal arise from addition of the hydroperoxy radical to the two different ends of the allylic radical (Fig. 33). The ketone, /i-3-penten-2-one, is formed by intrazeolite isomerization of 1-pentene followed by CT mediated photooxygenation of the 2-pentene isomer. Dioxetane cleavage, epoxide rearrangement, or presumably even Floch cleavage130,131 of the allylic hydroperoxides can lead to the mixture of saturated aldehydes. 

In general, sulfur-substituted allenes are accessible starting from alkyne precursors by a variety of transformations such as isomerization, rearrangement or addition reactions. The standard method for the synthesis of donor-substituted allenes is again the base-induced isomerization of alkynes. This very first method was applied for the preparation of achiral [11, 162, 163] and chiral [164] S-functionalized 1,2-dienes (Scheme 8.78). 

An increase of char yield is generally reflected as an improvement in oxygen index. In the styrylpyridine based polyesters and polycarbonate an intermolecular thermally induced Diels-Alder reaction has occurred through the double bond, this increased the char yield and decreased the flammability. The Fries rearrangement, as well as dimerization and isomerization, occurred simultaneously during the UV irradiation of p-VPPB, but no dimerization or isomerization occurred for p,p -BVPDPC, probably due to steric effects. 

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

Rule D. When a quarternary carbon atom is the alpha position to a secondary or tertiary double-bonded carbon atom, a pinacol type of rearrangement involving migration of a methyl group may occur. Examples of this type of rearrangement include isomerization of... 

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

Neither the anilide of cinnamic acid94 nor the diphenyl substituted acroyl anilide 138 95 yields any product of rearrangement or cyclodimerization. Upon irradiation of 138 in benzene solution in a Pyrex reactor, only the isomeric /9-lactams 139 (2.3%) and 140 (37%), in addition to dihydrocarbostyril 141 (5%), were isolated. The latter is the major product upon irradiation of alkyl substituted acroyl anilides.96 On the other hand, the closely related phenyl cinnamate rearranges regularly to the ortho- and para-positions97 and does not dimerize as the other alkyl esters of cinnamic acid.98... 

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