Pyrolysis rearrangement reactions

A unimolecular pyrolysis/rearrangement sequence leading to stable reaction products without the need for nucleophiles was reported by Katritzky et al. Thus, 6-(2-oxoalkyl)-l,3-dioxin-4-ones, when heated or irradiated, gave 6-substituted-4-hydroxy-2-pyrones in good yields <2005JOC4854>. a,7-Dioxoketenes are discussed as intermediates (Scheme 25). 

From the point of view of thermal decomposition behaviour, it is convenient to group the organic bromides and chlorides together because of kinetic features which are common to both these sets of compounds. The usual products of pyrolysis of brominated and chlorinated hydrocarbons are hydrogen halide and alkenes, but rearrangement reactions may also occur. Many of these decomposition reac-... 

Despite the high temperatures required for acetate and xanthogenate pyrolysis, the accessibility of these compounds from the corresponding alcohols provides for a useful route to alkenes. One advantage is that as no acidic or basic reagents are required, rearrangement reactions, which can interfere with alcohol dehydration, are avoided. For example, pyrolysis of lactol acetates has been used to prepare 4,5-dihy-drofurans, as illustrated in equation (58), - and pyrolysis of the corresponding thionocarbonate was used to dehydrate alcohol (128 Scheme 38). 

One of the most important reactions in the production of industrial aromatics is electrophilic aromatic substitution another prominent type of reaction is nucleophilic substitution, which is favored for aromatics with electron-withdrawing groups. Free radical reactions, which occur especially in thermal pyrolysis processes and in side-chain oxidation and chlorination reactions, are even more important, in quantitative terms, than electrophilic and nucleophilic substitution reactions. Typical examples are thermal cracking of naphtha and gas-oil fractions, the oxidation of naphthalene to phthalic anhydride, and the side-chain chlorination of toluene. Rearrangement reactions are less significant. 

Trost and Ornstein have developed a versatile reagent for the synthesis of 1,4-dienes with high E,E stereoselectivity. The method is based on the conversion of the lactol (237), prepared by the reaction of (236) with DMF, into the vinylcyclopropanes (238), which on flash vacuum pyrolysis rearrange to the... 

Heat s. Isomerization, thermal, Pyrolysis, Rearrangement, -Helferich-Schmitz-Hillebrecht reaction 22, 243 Helicenes 20, 645 suppl. 23 Helium 23, 719... 

Iminoboianes have been suggested as intermediates in the formation of compounds derived from the pyrolysis of azidoboranes (77). The intermediate is presumed to be a boryl-substituted nitrene, RR BN, which then rearranges to the amino iminoborane, neither of which has been isolated (78). Another approach to the synthesis of amino iminoboranes involves the dehydrohalogenation of mono- and bis(amino)halobotanes as shown in equation 21. Bulky alkah-metal amides, MNR, have been utilized successfully as the strong base,, in such a reaction scheme. Use of hthium-/i /f-butyl(ttimethylsilyl)amide yields an amine, DH, which is relatively volatile (76,79). 

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. 

The 1-azirines obtained from the vapor phase pyrolysis of 4,5-disubstituted 1-phthalimido-1,2,3-triazoles (157) have been found to undergo further thermal reactions (71CC1S18). Those azirines which contain a methyl group in the 2-position of the ring are cleaved to nitriles and phthalimidocarbenes, whereas those azirines which possess a phenyl substituent in the 2-position rearrange to indoles. 

Beckmann rearrangement, 4, 292 pyrolysis, 4, 202 synthesis, 4, 223 Wittig reaction, 4, 294 Wolff-Kishner reduction, 4, 291 Indole, 1-acyl-2,3-disubstituted photoisomerization, 4, 204 photo-Fries rearrangement, 4, 204 photoisomerization, 4, 42 synthesis, 4, 82 Indole, 2-acyl acidity, 4, 297 synthesis, 4, 337, 360 Indole, 3-acyl-acidity, 4, 297 cleavage, 4, 289 reduction, 4, 289 synthesis, 4, 360 Indole, 7-acyl-synthesis, 4, 246... 

Pyrazolo[3,4-c]pyrazole, tetrahydro-rearrangement, 5, 250 Pyrazolo[4,3-c]pyrazole, tetraaryl-electrophilic substitution, 6, 1035 oxidation, 6, 1034-1035 reduction, 6, 1035 vacuum pyrolysis, 6, 1035 Pyrazolo[ 1,2-n]pyrazole-1,5-diones synthesis, 6, 991 Pyrazolo[ 1,2-n]pyrazoles reactions, 6, 1038 ring opening, 6, 983... 

Hexafluoropropylene oxide (HFPO), which decomposes reversibly to di-fluorocarbene and trifluoroacetyl fluonde with a half-life of about 6 h at 165 °C [30], is a versatile reagent. Its pyrolysis with olefins is normally carried out at 180-2(X) °C, and yields are usually good with either electron-nch or electron-poor olefins [31, 32, 33, 34, 35, 36, 37] (Table 2). The high reaction temperatures allow the eyclopropanation of very electron poor double bonds [58] (equation 10) but can result in rearranged products [39, 40, 41] (equations 11-13)... 

This mechanism of a -elimination reaction is supported by experimental findings with " S- and C-labeled starting materials." The Chugaev reaction is analogous to the ester pyrolysis, but allows for milder reaction conditions—i.e. it occurs at lower temperatures. It is less prone to side reactions, e.g. the formation of rearranged products, and is therefore the preferred method. 

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