Molecular rearrangements aromatic reactions

Keywords Amines Aromatic compounds Ethers Heteroaromatic compounds Meisenheimer complexes Molecular rearrangements Olefination reaction Sulfides Sulfones... 

Studies of alkaloid synthesis provide another example of aminomethylation with concurrent molecular rearrangement (Fig. 69). In this case the methyleneimmonium ion is inserted into a tetrahydroisoquinolinc ring, and the cyclization occurs by reaction with a brominated aromatic ring activated by alkoxy groups.- - ... 

Cfi Aromatic Reactions Without Hydrogen. In the present study, tire aluminophosphate molecular sieves have been used alone and with added platinum and hydrogen to isomerize Cs aromatic feeds. In an initial screening study, a series of large to medium pore size molecular sieves were evaluated for catalytic activity for m-xylene rearrangements at 1000° F without added metal and hydrogen. 

Examples of the completely unsaturated 10 -electron hetero-aromatic ring system are known for all the above heterocycles. The chemical reactivity of these heterocyclic systems can be viewed as a microcosm of heterocyclic chemistry. Although little explored, the literature on these compounds is already replete with examples of imaginative synthetic entry, nucleophilic and electrophilic substitution reactions, mechanistically intriguing molecular rearrangements and compilations of spectral data, which now allow structure assignment to new reaction products with relative certainty. 

Recent data, published and unpublished, provide strong evidence that the common views on the reaction mechanism of the MTH reaction are not tenable. The data rather point to ethene and propene formation from an adsorbate hydrocarbon pool, probably of aromatic nature. There are strong indications that the catalytic cycle is based on arenes that are continually methylated by methanol/dimethyl ether, and dealkylations leading to ethene, propene and most likely also isobutene via molecular rearrangements. Penta- and hexamethylbenzene appear prone to undergo this reaction. However, there is also clear evidence that higher alkenes, if present in substantial amount, may take part in the classical homologation system. 

Studies of aromatic model hydrocarbons have demonstrated that the main reactions, which are responsible for mesophase formation, are molecular rearrangement, cleavage of aliphatic substituents (e.g., methyl groups) attached to aromatic rings, and dehydrogenative... 

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. 

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