Non-Kekule compounds

A number of authors in the the field of non-Kekule compounds have given summaries of one or another aspect of the historical record.However, it may be helpful to relate briefly some of the main occurrences, events that in retrospect can be seen to mark the emerging (and sometimes temporally overlapping) eras in the field. 

Earliest among these was the synthesis by Schlenk and Brauns of the bis(tri-arylmethyls) 1 and 2, the hrst non-Kekule compounds. 

A crucial methodological step forward was the discovery " that one could observe weU-defined electron spin resonance (ESR) spectra of frozen solutions of triplet species in random orientation. By the early 1960s, spectra of the triplet states of a number of carbenes had been recorded. Thus, when Dowd showed that photolysis of frozen matrices of the diazene (11) or the ketone (12) (Scheme 5.1) gave TMM (4), the spectroscopic tools for the characterization of this key non-Kekule compound lay to hand. Trimethylenemethane was the first non-Kekule molecule to be identified by ESR spectroscopy. 

Another series of non-Kekule compounds whose parent is 1,8-dimethylene-naphthalene (19) " dates from the same era. Extensive reviews " of that series are given elsewhere and will not be repeated here. 

The very nature of non-Kekule species as reactive intermediates suggests that studies of them under conditions far from those used in conventional investigations of the synthesis and reactions of stable molecules are indispensable. These requirements frequently are met by immobilizing the species in crystalline hosts or randomly oriented matrices, as is described in Chapter 17 by Bally in this book. Although some information available from crystal studies usually must be sacrificed in the random matrix technique, the latter is usually far more convenient, and most smdies of non-Kekule compounds in solids have used it. 

Thus, a disjoint non-Kekule compound like TME might well have a singlet ground state, since the singlet is expected to be stabilized by a favorable electron correlation effect. This conclusion seems to conflict with the ESR experiments on TME. 

We can summarize this section on TME and its hydrocarbon derivatives with the observation that it is not easy to test the major predictions of the theoretical model of disjoint non-Kekule compounds. Whether or not the singlet is actually the ground state in any given case depends on subtle particularities of structure and conditions of measurement. Much of the contention of the last decade or more in this area focused on these difficulties, but many of those difficulties are suppressed in the case of tetramethylenebenzene (TMB). 

The non-Kekule compounds we have considered so far are all stmcturally related to TMM and TME, and the disjoint and parity methods for predicting qualitatively the ground-state spin give similar results. However, the two methods do not agree in the case of another type of non-Kekule structure, of which the parent compound is the biradical 2,3,4-trimethylenepentane-l,5-diyl (56), commonly called penta-methylenepropane (PMP) (Scheme 5.11). 

It may be fairly said that the field of non-Kekule compounds has expanded from a few curious and esoteric molecules to an active domain of inquiry and a potential source of materials for practical application. Theory and experiment have interacted fruitfully in these developments, but some serious limitations remain. For example, for many years, experimentalists have operated under the imperative of structural simplification. The ideal is to construct molecules that embody the features of theoretical interest but are as free as possible of complicating impedimenta such as extra substituents or other structural elements left over from the synthesis itself and not readily removed. 

Cyclobutadiene (l)15 is the first member of the series of cyclic conjugated hydrocarbons (Kekule compounds) cyclopropenylidene (2), of cyclic conjugated carbenes trimethylenemethane (3), of the non-Kekule hydrocarbons. 

One of the ways to generate the tetramethylenethane-type diradical 350, an important reference compound in connection with non-Kekule hydrocarbons [147], consists in the thermal isomerization of hydrocarbon 34 at -100 °C [43, 148], Under the reaction conditions, the six-membered ring of 350 ruptures to yield [4]dendralene (3,4-bismethylene-1,5 -hexadiene) (351). 

The critical compilation in the first part of this paper was developed mainly for the purpose of showing the importance of the troublesome label that organic people have attached to the word aromaticity. But there is another field in which this label was. so much impressed on people s minds that it was out for many years of the question to study the aromaticity of non-planar compounds, because of the fundamentally necessary assumption , due to Kekule, that a cyclic molecule cannot be aromatic if it is not planar This belief was founded on the fact that, in the classical benzene series, the It delocalization is due to the overlap of atomic orbitals which quick-... 

The next example of two rings going-on one deals with the interrelated chemistry of bicydo[3.1.0]hexane and cyclohexane derivatives. One important interrelationship involves general non-Kekule species an extensive but still largely unexplored class of neutral organic compounds. These species are characterized by an inadequate number of double bonds compared to their more conventional isomers. (The reader may thus recognize the non-Kekule label as applicable for the compounds in the earlier 2,4-dimethylenebicyclobutane-l,3-dimethylenecyclobutadiene discussion because, however drawn, the latter has but two double bonds while the isomeric species, 3,4-... 

Although the term aromaticity was synonymous with benzene and benzenoid compounds from the Kekule era well into this century, several workers realized that a number of compounds that do not contain benzene rings possess electron configurations which appear to confer the special kind of stability exhibited by the benzene nucleus, and thus appear to have aromatic character. These compounds are termed non-benzenoid aromatics. ... 

D. P. Craig, in Non-benzenoid Aromatic Compounds (D. Ginsburg, ed.), p. 1. Interscience, New York, 1959 Theoretical Organic Chemistry, Kekule Symp., p. 20. Butterworth, London, 1959. 

Of polycyclic non-benzenoid hydrocarbons by far the most numerous and the most investigated series consists of the azulenes. The parent compound is azulene itself, bicyclo[5,3,0]decapentaene (VI). Like benzene it can be drawn as a polyene in two equivalent Kekule forms. It has a peripheral conjugated system involving 10 TT-electrons. Its structure is discussed on the following pages. 

To understand this particular research, we need to pay special attention to the Kekule structure for pyrene (Fig. 14.17). The total number of tt electrons in pyrene is 16 (8 double bonds = 16 tt electrons). Sixteen is a non-Hiickel number, but Hiickel s rule is intended to be applied only to monocyclic compounds and pyrene is clearly tetracyclic. If we disregard the internal double bond of pyrene, however, and look only at the periphery, we see that the periphery is a planar ring with 14 T7 electrons. The periphery is, in fact, very much like that of [I4]annulene. Fourteen is a Hiickel number An A- 2, where n = 3), and one might then predict that the periphery of pyrene would be aromatic by itself, in the absence of the internal double bond. 

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