Molecular rearrangement reactions, thermal

Reaction of 8-aminoquinoline 567 with 3,4-dichlorodithiazolium chloride gave the quinolyl iminodithiazole 568 whose thermal rearrangement gave 569 via a molecular rearrangement process (96MI2775) (Scheme 95). 

In summary, it should be pointed out, however, that the exact mechanism of the thermal rearrangement of sulfur rings and chains is still unknown and that further investigations are necessary in this connection. For any further discussion it may also be interesting to take into account the recent results on the molecular rearrangement reactions of cyclic selenium sulfides and of elemental selenium which take place at considerably lower temperatures compared with elemental sulfur and for which — as far as solid selenium is concerned — interesting ionic mechanisms have been proposed... 

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

Trithioisatoic anhydrides were also cyclized to thiazolo-[2,3-bjquinazolines (255) upon reaction with 2-aminoethanol or l-amino-2-dimethoxyethane (80PHA124 83ZC215). Double ring closures occured when jV-(2-methoxycarbonylphenyl)thiocarbamate reacted with 2-aminothioethanol (82JIC1117) to give 256. Thermal molecular rearrange-... 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

Not only must precursor fibers be self-supporting as extruded, they must also remain intact (e.g. not melt or creep) during pyrolytic transformation to ceramic fibers. Thus, precursor fibers (especially melt spun fibers) must retain some chemical reactivity so that the fibers can be rendered infusible before or during pyrolysis. Infusibility is commonly obtained through reactions that provide extensive crosslinking. These include free radical, condensation, oxidatively or thermally induced molecular rearrangements. 

Photoisomerization of c/.y-stilbene 191 Ionic fragmentation reaction 191 Cyclopropyl radical ring-opening 192 Ionic molecular rearrangement 193 Ene reaction 196 Thermal denitrogenation 198 Unimolecular dissociation 199 Sn2 reaction 200... 

Procyanidins are quite reactive and are therefore considered as some of the most unstable natural phenolic compounds [19-20]. They are subject to enzymatic oxidation by polyphenol oxidases as well as to spontaneous oxidation [21], Coupled oxidation reactions involving o-quinones of phenolic acids have been reported [22-24], Procyanidins are thermally labile [25] and can easily undergo molecular rearrangements in acidic or basic media [26]. In model solutions interflavanoid bonds of procyanidins were found to be unstable, but also new carbon-carbon bonds were formed... 

It not infrequently happens that a chain reaction and a molecular reaction take place concurrently and make contributions of comparable magnitude to the total observed chemical change. In the thermal decomposition of acetaldehyde vapour, for example, there are probably two major mechanisms, a direct molecular rearrangement CH3CHO = C0-[-CH4, and a chain process similar to (3) above. The activation energy, Ui, for the formation of radicals is very much higher than that for the rearrangement, Ii, and in consequence the number of molecules which initiate chains is smaller in about the ratio than the number which suffer simple... 

This chapter will discuss 1,3-dienes in a reaction with alkenes to give cyclohexene derivatives. This is a thermal reaction driven by interactions of molecular orbitals rather than ionic or polarized intermediates. In addition to the reaction of 1,3-dienes, 1,5-dienes undergo a rearrangement to a different 1,5-diene in what is known as a sigmatropic rearrangement. Similarly, allyl vinyl ethers rearrange to form alkenyl aldehydes or ketones. Both of these reactions tend to give difunctional molecules as products. 

It had become apparent, however, that many thermal reactions, including well known molecular rearrangements, decompositions and cycloadditions, were not amenable to study by these methods. The mechanistic information which they were able to provide about such thermo-reorganization reactions was so meager that half in jest, half in desperation [they were designated] No Mech-anism reactions [8]. 

Thermal degradation reactions of polymers can be divided into two broad groups molecular rearrangement reactions and reactions which involve ho-mol5d ic scission of bonds to produce free radicals. In both cases the chemistry... 

McNeill and Rincon studied thermal degradation of PC by means of TGA, TVA, DSC, FT-IR, mass spectrometry (MS) and gas chromatogra-phy-MS (GC-MS) method (see Fig. 2.5). PC is stable up to 300°C. Above that temperature, small quantities of phenol and p-cresol were detected, at 375-400°C CO2 appeared and then at T > 455°C CO and CH4 were formed the peak on the DTG curve was at F = 462°C. At 500°C the main products are q chc dimer and bisphenol A, with small quantities of CO2, p-cresol, p-ethyl phenol, phenol, p-vinyl phenol, p-isopropyl phenol, CO and CH4. In the absence of air and moisture, degradation of PC proceeds by hemolytic decomposition of the polymer chain, radical reactions, fragmentations and molecular rearrangements. 

The most intriguing hydrocarbon of this molecular formula is named buUvalene, which is found in the mixture of products of the reaction given above. G. SchrOder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicais of cyciooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of buUvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,6(X) different combinations of the carbon atoms are possible. 

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