Fragmentation-recombination pathways

The first is the fragmentation-recombination pathway (see mechanism a. Scheme 4), with acrylate and an acrylate-derived radical as the intermediate state (Scheme 6). This possibility has been suggested only recently, and is based on the reported inhibition of 2-methyleneglutarate mutase by acrylate [47], The second suggested mechanism is the addition-elimination pathway (see reaction c. Scheme 4), with a substituted cyclopropylcarbinyl radical as the intermediate (Scheme 6) [6, 26],... 

The presence of a C=C double bond in the migrating group of the but-3-enyl radical introduces the possibility of the addition-elimination mechanism (path b, Scheme 7), where the appropriate intermediate is the previously discussed cyclopropylcarbinyl radical (5). We find a significant preference ca 100 kJ mof ) for the addition-elimination pathway compared with the fragmentation-recombination pathway. Thus it is more favorable, in the gas phase at least, for the migrating HC=CH2 group to stay bonded to the remaining framework rather than to become detached from it. The cyclopropylcarbinyl radical intermediate involved in the addition-elimination mechanism is predicted to lie in a well of depth 30 kJ mof. ... 

Fragmentation-Recombination Pathway for the Rearrangement of the Aminopropyl Radical... 

The pathway involving cyclization of a protonated migrating group provides a very appealing alternative to the fragmentation-recombination pathway. Given the lack of evidence for imine formation, it is interesting to note that the formation of a protonated imine (14-H ) in the model system can alternatively arise formally as the result of removal of a hydride ion from the parent (saturated) system (12), aminopropyl (path e. Scheme 9). 

Dimethyl ether was reacted over a radical initiator, pre-dried dibenzoyl peroxide supported a celite, at various temperatures (Table 2). Although dibenzoyl peroxide was short lived under these conditions sufficient was reactive in the timescale of the experiments, typically 5 min. Under all conditions only dimethoxyethane was observed as a dimerisation product showing that the methoxymethyl radical was generated under these conditions. The absence of other products, e.g. alkenes or ethanol, discounts the methoxymethyl radical fragmentation/recombination pathway (mechanism 1, Fig. 1). 

The product formation can be rationalized by a fragmentation-recombination pathway followed by oxy-Cope rearrangements, as depicted below. 

Although the Skraup/Doebner-von Miller reaction represents one of the most common reaction for the synthesis of quinoline core for more than a century, its mechanism is still dedebated. To date, both of the two more popular mechanistic explanations are involving fragmentation-recombination pathways. In the first one, initially the amine reacts with the aldehyde or ketone under acidic conditions to form an imine. Dimerization and Pictet-Spengler type cyclization forms a diazetine core. Protonation and subsequent cyclization-ring cleavage reaction assembles the isoquinoline nucleus. 

The second possible route for the rearrangement of iminopropyl radical involves the formation of a cyclic intermediate (16), and the subsequent elimination of the amino carbon to yield the product radical (path c. Scheme 9). The barrier for this addition-elimination pathway (52.4 kJ mol ) is significantly lower than the barrier for the fragmentation-recombination of the iminopropyl radical (118.0 kJ mol ) or the aminopropyl radical (97.2 kJ mol ). Additionally,... 

Figure 4. Comparison of the G3(MP2)-RAD(p) energy requirements for the fragmentation-recombination, addition-elimination and protonated pathways for the model systems of B 12-dependent carbon-skeleton mutases with migrating groups CH=CH2, CH=NH, CH=0 and CH2-NH2.

Catechol and related phenolics 13,16,19, 31, and 32 were also isolated after alkaline treatment of D-glucose and sucrose. Several other substituted acetophenones were isolated. The mechanism of formation of phenolic compounds from monosaccharides under alkaline conditions has yet to be thoroughly investigated. The similarity in the types of aromatic products from D-glucose and D-xylose indicates the formation of the same C2, C3, or C4 fragments, with subsequent recombination and cycliza-tion. Base-catalyzed aldol reactions are, no doubt, predominant pathways in the initial formation of these aromatic products. 

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