Aromatic ring systems, comparison

In general, linear 7r-electron systems with Z1r = 2N electrons at the lowest energy levels have closed-shell singlet states while cyclic systems reach closed shell structures only when ZT = 4N + 2. Cyclic 7r-electron systems with Zn 4N + 2 will therefore exhibit multiplet ground states according to Hund s rules, and should be chemically reactive because of the unpaired electrons. Hiickel s rule that predicts pronounced stability for so-called aromatic ring systems with 4jV + 2 7r-electrons is based on this shell structure. The comparison with cyclic systems further predicts that ring closure of linear 7r-electron systems should be exothermic by an amount... 

As revealed by a comparison of the chromatographic behavior of 1 and 5 in Fig. 3-101, the introduction of a phenyl group into the molecule results in an increase in the retention at an identical number of phosphonic acid groups. This finding is explained by ji-ji interactions between the stationary phase and the aromatic ring system of the analyte. 

MO treatments are preferred in such cases. The manner in which these same cations, (CsHs) and (CeMce) can be treated in MO terms is worth illustrating here for the purpose of comparison with the localized bond schemes just discussed, and also to underline the relationship between these pyramidal systems and normal aromatic ring systems. 

Rhodium is the most active metal for the catalytic hydrogenation of aromatic ring systems. It is also very expensive (7 Dec 1992 price ca 44,000/Kg) because of its demand in autocatalysts. In comparison, at the same time the price of palladium was only 6% that of rhodium. Hence a bimetallic catalyst which can allow thrifting of the rhodium can offer a substantial precious metal inventory cost reduction. 

Comparison with penta-amminecobalt(m) complexes with the same carboxylato-ligands confirms that the bridging carboxy-group is not available for adjacent attack. The -phthalato-complex is believed to react via the precursor complex (19), but electron transfer via the aromatic ring system is not invoked. An outer-sphere type of electron transfer is suggested, on the basis of the value of and the ratio of rates of chromium(ii) and vanadium(n) reductions. ... 

Electrophilic attack on the nitrogen atom is strongly dependent on the availability of its electron pair. In comparison with the aliphatic amines this availability is decreased when the nitrogen atom belongs to an aromatic ring system since the electron pair is delocalized in the w system of the ring (45). This may explain the low reactivity of the pyrrole and indole derivatives. 

The ready availability of 1,2,5-thiadiazole 1,1-dioxides (177) (see above) has provided data for their comparison with the parent aromatic ring system, The NO-, NN-, and OO-dimethylated derivatives are obtained by suitable alkylation procedures. The action of an excess of phosphorus pentachloride gives the 3,4-dichloro-compound (181 X = Cl), which is rapidly hydrolysed back to the dione (177), and which undergoes ammonolysis or aminolysis to give a variety of 3,4-di(substituted amino)- ,2,5-thiadiazole 1,1-dioxides [e.g. (181 X = NR R )]. The amide-like character of the 3,4-diamino-derivative (181 X = NHa) has been noted, as has a pronounced tendency of the 3,4-di-alkoxy-derivatives (181 X = OMe) to transfer their 0-alkyl groups to the ring nitrogen on thermolysis. ... 

The double bond in vinyl arenes is activated as a result of its conjugation to the aromatic ring system. Hence, vinyl arenes generally react more smoothly in hydro-amination reactions in comparison to simple, unactivated alkenes, especially in intermolecular processes. 

In this initial section the reactivities of the major types of azole aromatic rings are briefly considered in comparison with those which would be expected on the basis of electronic theory, and the reactions of these heteroaromatic systems are compared among themselves and with similar reactions of aliphatic and benzenoid compounds. Later in this chapter all the reactions are reconsidered in more detail. It is postulated that the reactions of azoles can only be rationalized and understood with reference to the complex tautomeric and acid-base equilibria shown by these systems. Tautomeric equilibria are discussed in Chapter 4.01. Acid-base equilibria are considered in Section 4.02.1.3 of the present chapter. 

It would be expected that the stabilization of the adsorbed species by an extended conjugated system should increase with the number of aromatic rings in the adsorbed azahydrocarbon. However, data suitable for comparison are available only for phenanthridine, benzo-[/]quinoline, and benzo[h] quinoline. The large difference in the yields of biaryl obtained from the last two bases could be caused by steric interaction of the 7,8-benz-ring with the catalyst, which would lower the concentration of the adsorbed species relative to that with benzo[/]quinoline. The failure of phenanthridine to yield any biaryl is also noteworthy since some 5,6-dihydrophenanthridine was formed. This suggests that adsorption on the catalyst via the nitrogen atom is possible, but that steric inhibition to the combination of the activated species is involved. The same effect could be responsible for the exclusive formation of 5,5 -disubstituted 2,2 -dipyridines from 3-substi-tuted pyridines, as well as for the low yields of 3,3, 5,5 -tetramethyl-2,2 -bipyridines obtained from 3,5-lutidine and of 3,3 -dimethyl-2,2 -... 

The H-NMR spectrum features confirmed the proposed C ring structure as it showed a 4H aromatic AB system. An isolated aromatic proton signal at 8 5.93 indicated a pentasubstituted A ring. The 3H singlet at 8 3.65 showed that one of these was a methoxy substituent. The other two were thought to consist of OH and the pyrrolidine ring. The actual positions of substitution were determined by comparison with various derivatives and analogs. 

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