Non-Kekul

Dimethylenefuran-, 3,4-dimethylenethiophene- and 3,4-dimethylenepyr-rolediyl radicals as non-Kekule molecules with tunable singlet-triplet energy spacings 97ACR238. 

Among the non-Kekule diradicals, tetramethyleneethane (TME, 7) has evoked lasting attention during the last two decades due to the controversy over its spin preference in the ground state between experiments and theoretical predictions [48-59], Now TME is known to be a slightly favored singlet diradical with a negligible... 

T gap (cf. references collected in Table 1). This correlates well with a disfavored cyclic six-orbital interaction by the phase discontinuity in the triplet state of 7 [29] (shown in Fig. 11). In addition, TME is an important topological unit which appears frequently in many non-Kekule diradicals (as exemplified by 15-17 in Fig. 13). 

The classification into Kekule and non-Kekule diradicals is mainly based on the difference in their resonance structures. From the proceeding discussions, however, such a classification does not closely relate to the relative stabilities and spin preference of TT-conjugated diradicals. For example, some non-Kekule diradicals, such as 1 and 8, prefer a triplet ground state, but some others (like 7) have a singlet ground... 

The singlet Kekule diradicals, i.e., the excited state of Kekule molecules, are destabilized by the orbital phase continuity while the triplet Kekule diradicals and the singlet and triplet non-Kekule diradicals are stabilized by the orbital phase continuity. 

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). 

Evidence for the trapping of a non-Kekule intermediate in m-nitro participation was obtained in a photo-retro-aldol type reaction94. Photolysis of 157 in aqueous acid solution... 

A general reaction mechanism for m-nitrobenzyl derivative is proposed (Scheme 8) which involves a non-Kekule intermediate100. The mechanism for the p-nitrobenzyl alcohol involves the highly polarized intermediate 163, which is consistent with the observed strong solvent effect and base catalysis of the reaction (equation 80). 

The sequential removal of H and H+ from isobutene-type structural units (so-called H2+ abstraction ) was also used to generate the radical anion of non-Kekule benzene , i.e. l,3-dimethylenecyclobutane-l,3-diyl (39) (Scheme 11). As shown by Hill and Squires161, this highly unusual, distonic C(,II(, isomer can be produced in pure form by reaction of O with 1,3-dimethylenecyclobutane (38). Working in a flowing afterglow mass spectrometer, subsequent reactions were again used to characterize this radical anion and differentiate it from other ( VdL, isomers. 

Bushby and Jarecki22 reported a preparation of precursors to conformationally constrained 8jt non-Kekule polyenes and their NMR data. 

A synthesis is described for the Z and E isomers of 2-(2/-butylallylidene)-6,7-diazabi-cyclo[3.2.2]nona-3,6-diene 11 and 12, which are potential precursors to conformationally constrained 8 r non-Kekule polyenes. 

The double degeneracy of NBMOs in m-[8] has nothing to do with the geometrical symmetry of the molecule, but, rather, with the connectivity of the two radical centres, or the phase relationship of the atomic orbitals in the conjugated system. Therefore, the term topological symmetry has been proposed to describe the connectivity of the carbon atoms carrying the n-electrons and the periodicity of the Jt-orbitals in this class of non-Kekule hydrocarbons. 

The stable anion-radical in Scheme 3.63 contains two perchlorotriphenylmethyl radical units linked by an all-trani-p-divinylbenzene bridge. At 200 K, the unpaired electron of the anion-radical is localized (within the ESR timescale) on one stilbenelike moiety only. At 300 K, thermal activation forces the nnpaired electron at one strong electrophilic center to move to another one. Such an electron transfer takes place between two eqnivalent redox sites (Bonvoisin et al. 1994). In contrast to this situation, no electron transfer was observed for the anion-radical that contains two perchlorotriphenylmethyl radical units linked by an all-trani -m-divinylbenzene bridge (Rovira et al. 2001). Such results can be ascribed to the localization of frontier orbitals in the meta-isomeric anion-radical because of the meta connectivity of this non-Kekule structure. 

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. 

The arrival of the third phase of non-Kekule chemistry now awaited two necessary developments. First, the Guoy balance technique was of limited sensitivity and yielded no stmctural information about the source of the paramagnetism thus, a most desirable development would be the appearance of a new and independent... 

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. 

A non-Kekule molecule conceptually formed by fusion of two TMM units and also predicted" " to have a triplet ground state is 2,4-dimethylenecyclo-butane diyl (20), which ultimately was prepared by two independent syntheses. Matrix ESR spectroscopy and gas-phase photodetachment photoelectron spectroscopy (PES) (see Section 4.1.4) eventually agreed that the ground state is triplet. 

To this point, it would be fair to say that the dominant question was the comparison of experiment with theory by attempts to designate the ground state and if possible to determine the energy separation between it and the next higher state. To a considerable extent, this line of development was driven by the advances in theory itself, whose predictions were made with ever-increasing confidence, sophistication, and refinement. However, one should not conclude that this is the extent and sole purpose of research in the non-Kekule domain. These molecules really... 

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