Cycloaromatization reactions

Cycloaromatization reactions the testing ground for theory and experiment... 

Cycloaromatization reactions breaking 7t-bonds and breaking the rules 2... 

Scheme 1 Schematic representation of cycloaromatization reactions. Double lines correspond to the out-of-plane re-systems of a bis-alkyne reagent. Only orbitals of the in-plane re-system in the reactant and of new

Although data presented in the previous section illustrate the diversity of cycloaromatization reactions, most of the following discussion will concentrate on the Bergman cyclization - a reaction that has been studied intensively in recent decades due to its role in the mechanism of biological activity of natural anticancer antibiotics.8,9 We will take advantage of the large body of data produced by this... 

In order to activate remote substituent effects in cycloaromatization reactions, two orthogonal re-systems need to find a way to communicate. At first glance, this task... 

At this stage, both a- and re-effects are important. However, the influence of the 7r-effects dramatically increases. In fact, it is the formation of a 7t-aromatic system in the cycloaromatization reactions that makes these processes energetically feasible. 

Another example of a zwitterionic product of a cycloaromatization reaction was given by Kerwin and coworkers. Their skipped (aza)enediynes rearranged to yield (aza)eneyne-allenes that subsequently cyclized under addition of methanol (in a byproduct), which is consistent with a partitioning between a diradical and a zwitterionic reaction pathway (Scheme 15).66... 

The most obvious effect on cycloaromatization, as the name implies, is the formation of an aromatic system. By delocalizing electrons in an aromatic ring, the product gains a high degree of stability, which is reflected in the small endothermicity of the Bergman cyclization and the exothermicity of the Myers-Saito cyclization. Since the Schmittel and Schreiner cyclizations are not true cycloaromatization reactions per se, they do not have the beneficial effect of the formation of an aromatic system and are therefore much are more endothermic than their counterparts. 

These results, of course, contrast the significant effect of benzannelation in radical-anionic cycloaromatization reactions discussed above12 where not just the formation of a new aromatic cycle but also restoration of aromaticity in the previously existing cycle occurs at the same time in the cyclorearomatization process. 

We are omitting reactions such as the trimerization of acetylene on transition metal catalysts, even though they could be considered cycloaromatization reactions... 

Cross-interaction constants and transition-state structure in solution, 27, 57 Crown-ether complexes, stability and reactivity of, 17, 279 Crystalographic approaches to transition state structures, 29, 87 Cycloaromatization reactions the testing ground for theory and experiment, 42, 1 Cyclodextrins and other catalysts, the stabilisation of transition states by, 29, 1... 

Molecular orbital (MO) correlation diagrams, for cycloaromatization reactions, 7-9, 8/... 

Schmittel cyclization, 4f 30, see also Cycloaromatization reactions Schreiner cyclization, f 30, see also Cycloaromatization reactions Sensitizers, singlet oxygen, 233-234, 235-236... 

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