Cyclic conjugated systems

Monocyclic conjugated polyenes are referred to as annulenes, and there exists ample experimental evidence to support the conclusions based on application of HMO theory to neutral and charged annulenes. The relationship between stability and structure in cyclic conjugated systems will be explored more fully in Chapter 9. 

The pattern of experimental results on charged species with cyclic conjugated systems is summarized in Table 9.1. It is consistent with the applicability of HiickeTs rule to charged, as well as neutral, conjugated planar cyclic structures. 

Homoaromaticity is a term used to describe systems in which a stabilized cyclic conjugated system is formed by bypassing one saturated atom. The resulting stabilization would, in general, be expected to be reduced because of poorer overlap of the orbitals. The properties of several such cationic species, however, suggest that substantial stabilization does exist. The cyclooctatrienyl cation is an example ... 

Derive the tc-MO patterns for these three molecules by treating them as derivatives of the three-, five-, and seven-membered cyclic conjugated systems. Explain the relationship between the derived MO pattern and the observed properties and stabilities of the molecules. 

Chemists in the early 1900s believed that the only requirement for aromaticity was the presence of a cyclic conjugated system. It was therefore expected that cyclooctatetraene,. as a close analog of benzene, would also prove to be unusually stable. The facts, however, proved otherwise. When cyclooctatetraene was first prepared in 1911 by the German chemist Richard Willstatter, it was found not to be particularly stable but to resemble an open-chain polyene in its reactivity. 

In the preceding chapter, we looked at aromaticity—the stability associated with benzene and related compounds that contain a cyclic conjugated system of An + 2 77 electrons. In this chapter, we ll look at some of the unique reactions that aromatic molecules undergo. 

The approach makes use of a bipyrroledicarbaldehyde 54 and a tripyrranedicarboxylic acid 55 which are condensed with subsequent decarboxylation in the presence of acid and oxygen. The presence of oxygen, as in many other syntheses of porphinoid macrocycles, is necessary to adjust the oxidation level of the chromophore which is in sapphyrins a 2271-aromatic cyclically conjugated system. Many sapphyrins 56 with different substitution patterns have been synthesized according to this general scheme. As in all McDonald-type condensations at least one of the components has to be symmetric because otherwise mixtures of constitutional isomers would be formed. 

The anisotropy of the magnetic susceptibility of a cyclic conjugated system, attributable to induced ring currents in its rc-electron network, is one of the important quantities indicative of 7t-electron delocalization. The method used for the calculation of the magnetic susceptibilities of nonalternant hydrocarbons is the London-Hoarau method taken together with the Wheland-Mann SCF technique . The resonance integral is assumed again to be of exponential form but... 

The results of our calculations based on both the static and dynamic theories show that most of the nonbenzenoid cyclic conjugated systems examined exhibit in a greater or lesser degree a marked double-bond fixation. The static theory indicates that even in benzene there exists a hidden tendency to distort into a skewed structure and that such a tendency is actually realized in [4n-f-2] annulenes larger than a certain critical size. In nonalternant hydrocarbons bond distortion is a rather common phenomenon. Fulvenes, fulvalenes and certain peri-condensed nonalternant hydrocarbons undergo a first-order bond distortion, and... 

Fig. 1 summarizes cyclizations of the two prototype molecules, (Z) hex-3-ene-l,5-diyne and (Z) hept-3,5,6-triene-l-yne. In all of these processes, bonds are formed from in-plane 7t-orbitals in the presence of an orthogonal 7i-system. However, it is clear that the properties of the newly formed cyclic conjugated systems can be quite... 

In a review, Gorelik51 has shown that magnetic, structural, and energetic properties are determined by the electronic structure of cyclic conjugated systems, which are stabilized by a cyclic delocalization of electrons. Chemical reactivity cannot serve satisfactorily as a general criterion of aromaticity. 

In addition to the major classes of homoaromaticity considered so far, in which the cyclic conjugated array is interrupted by a saturated linkage(s), there is another class of homoaromaticity. In this class a regular cyclically conjugated system is perturbed by homoconjugation(s) [transannular interaction )] (Scott, 1986). This latter class is exemplified by the bridged annulenes. 

With push-pull ethylenes in which the donor part is a cyclic conjugated system with An + 2 ir electrons and/or the acceptor part is one with An tt electrons, the possibility exists for aromatic stabilization of the transition state to C=C rotation. Several such systems with both carbocylic and heterocyclic ring components have been studied. 

Reductive Generation of Anionic Species 1101 Tab. 2 Structure formula of annulenes and cyclic conjugated systems (1-17)... 

Kekule structure org chem A molecular structure of a cyclic conjugated system that is depicted with alternating single and double bonds. ka-k3,la. strak-char j ketal oRG CHEM 1. Former term for the =CO group, as in dimethyl ketal (acetone). 2. Any of the ketone acetates from condensation of alkyl orthoformates with ketones in the presence of alcohols. ke,tal ... 

More precisely, Eq. (33) represents the difference between the magnetic susceptibility of a cyclic conjugated system and that of a hypothetical cyclic system with localized double bonds in which the ring current vanishes. A molecule is aromatic when A>0 and antiaromatic when A<0, and at A 0 it is nonaromatic [61JCP1996 68JA811 75MI2 83JMS( 102)377]. 

Examples treated in this book are summarized in Table 9. One frequently found type consists of ring expansions of cyclic conjugated systems with an exocyclic ylide function, such as pyridine /V-oxides or /V-imides incorporation of the exocyclic half of the ylide function expands the ring by one member. 

SAMPLE SOLUTION (a) The crucial point is the number of it electrons in a cyclic conjugated system. If there are (4n + 2) it electrons, the ion is aromatic. Electron counting is easiest if we write the ion as a single Lewis structure and remember... 

A number of larger cyclic conjugated systems have been prepared, many of them by Sondheimer and co-workers.16 [10]Annulene and [12]annulene are subject to considerable steric difficulties and probably are not planar, but the larger molecules are big enough to accommodate hydrogen atoms inside the rings and so can have trans double bonds and still be planar or nearly so. Likely conformations for some of these compounds are shown in Structures 39, 40, and 41. 

This widely used criterion is manifested in (a) bond length equalization due to cyclic delocalization and (b) planarity of the cyclic systems13-16. The planarity of cyclic conjugated systems is regarded as a manifestation of aromaticity, while the non-planarity of cyclic conjugated systems is often taken as an indication of antiaromaticity. 

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