Electron aromatic

Cyclooctatetraene provides a significant contrast to the preference of aromatic hydrocarbons for one-electron reduction. It is converted to a diamagnetic dianion by addition of two electrons. It is easy to understand the ease with which the cyclooctatetraene radical accepts a second electron because of the aromaticity of the 10-7t-electron aromatic system which results (Section 9.3). 

The latest syntheses of novel four- and five-membered cyclic anions of the heavier group 14 elements (Si, Ge, Sn) provided a fresh look at the question of aromaticity (or nonaromaticity) of the heavier analogs of 6jt-electron aromatic systems. Thus, the story of the heavy analogs of cyclopentadienyl anion (Section 2.3.2.5) was continued by the preparation of the stannole dianion by Saito and co-workers. ° The stannole dianion 68 Li2 was prepared by the metalation of the bis(Ll-stannole) with an excess of Li in THF at elevated temperatures however, it can be better prepared by the lithiation of hexaphenylstannole with Li in THF under reflux conditions " (Scheme 2.65). 

In contrast to 69 Li, which was formulated above as the bji-electron aromatic compound, other representatives of this class of cyclic compounds, the heavy analogs of cyclobutadiene dianion, were found to be nonaromatic. The two compounds of this type, disiladigermacyclobutadiene dianion 71 K2 and tetrasilacyclobutadiene dianion 72 K2, were synthesized by the reductive dehalogenation of the corresponding precursors 73 and 74 " with KCg (Scheme 2.61)... 

Since divalent sulfur is isoelectronic with the carbon-carbon double bond and can participate in Jt-electron delocalization, the anions derived from 1,2- (143), 1,3- (144), and 1,4-dithiepins (145) may be regarded as 1 Orc-electron aromatic systems. According to the prediction by Zahradnik and Parkanyi851 based on Hiickel s molecular... 

The triphenylcyclopropenium cation (2), first prepared by Breslow,14 is an unusually intriguing species a 2tt-electron aromatic species constituted by a three-membered carbon ring. In every respect the triphenylcyclopropenium cation displays traditional aromatic behavior, the same as benzene. 

Phthalocyanine (Pc) molecules consisting of four interconnected isoindole units are synthetic analogues of porphyrins. They represent planar aromatic macrocycles with an 18 Jt-electron aromatic system, which is delocalized over an arrangement of alternated carbon and nitrogen atoms. For years, phthalocyanines have... 

Aryl derivatives of 101 (R=Ar) are planar, and the bond lengths indicate a fully delocalized 10 re-electron aromatic system. By contrast, the (dimethyl-amino) derivative 102 adopts a folded structure. Dithiatetrazocines with exo-cyclic N(Me)Bu groups exhibit both cisltrans and ring inversion isomerism.257... 

Despite its 10-electron aromatic Jt-system, indolizine apparently participates as an eight-electron system in its reaction with acetylene dicarboxylate, although the process may be stepwise and not concerted. By carrying out the reaction in the presence of a noble metal as catalyst, the initial adduct is converted into an aromatic cyclazine. 

As outlined in CHEC-II <1996CHEC-II(8)79>, only a few ring systems 1-34 having a 1 On-electron aromatic structure isoelectronic with the pentalene dianion 58 have been described. Triazoles 59, having two adjacent... 

By following the same arguments as before, we conclude that the conformation will be lower in energy than the Cee conformation. The same analysis can be used to compare the relative energetics of the and Cx conformations. In conclusion, we can say that the C conformation will be favored over the Cee and Cx conformations due to a larger pi attractive nonbonded interaction which obtains in the 6 pi electron aromatic geometry of the conformation. 

The stabilization of 10B versus 10A of 38.1 kcal mol-1 is remarkably large for an uncharged homoaromatic. This demonstrates the power of 2e aromaticity 8B is 61.6 kcal mol-1 lower in energy than 8A. Less repulsion between the a skeleton electrons, the number of which is reduced by two in the 2n electron aromatics as compared with the classical isomers, certainly contributes to the huge energy differences. The additional stabilization by formation of BHB bridges is, however, of minor importance. Note that classical a skeletons are to be expected only for 8A2-and 10A2- [6, 20] (Scheme 3.2-6), the 2e reduction products of triborirane and triboretane. 

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

Benzene is the parent molecule of a class of molecules said to be aromatic. The key feature of aromatic molecules is the delocalization of some electrons over more than two carbon atoms ( free-range electrons ). Aromatic molecules are of great importance in the chemistry of life. 

One could expect diazadiboretidines to be converted into Huckel aromatic systems either by adding or by subtracting one pair of n-electrons. The addition of two electrons to diazadiboretidines of the type (RBNalkali metals. The dianions [(RBNiBu)2] are stable in solution and can be reconverted into the diazadiboretidines by oxidants. Because they contain six n-electrons, aromatic character may be attributed to the dianions (19). Cyclodimers of the type (R BNR)2 are also readily oxidized, but the adoption of an aromatic dication [(R BNR)2] as a product would be mere speculation at present. 

Conjugated ring systems offer an alternative mode for the stabilization of a carbanion center. The most common situation is where deprotonation completes a cyclic n system leading to a highly stabilized, aromatic anion. The best known example is cyclopentadiene, which leads to a six-electron, aromatic ring after... 

A very low reaction order, with respect to i-Cg, suggests a strong adsorption of i-Cg molecules on the Ni surface. Rostrup-Nielsen " also noticed a retarding effect of aromatics and higher hydrocarbons on the reaction rate. Due to the presence of % electrons, aromatics can strongly adsorb onto the catalyst surface and cause the reaction order to approach zero. 

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