Radialenes structure

Considerably more structure data are available for [4]radialenes than for their smaller homologs168-177. The structure of the parent molecule 4 has not been determined yet, but its vibrational spectrum is in agreement with a planar molecule of symmetry178. Most [4]radialene structures are, however, found to be puckered 9, 22.1°168 10, 26.5°172 ... 

Secondly, the carbon framework holding the exocyclic double bonds could be extended. This is demonstrated by naphtharadialene 5, a highly reactive intermediate which has been generated by thermal dehydrochlorination from either the tetrachloride 178 or its isomer 179106. Radialene 5 has not been detected as such in these eliminations rather, its temporary formation was inferred from the isolation of the thermolysis product 180 which was isolated in 15% yield (equation 25). Formally, 5 may also be regarded as an [8]radialene into whose center an ethylene unit has been inserted. In principle, other center units—cyclobutadiene, suitable aromatic systems—may be introduced in this manner, thus generating a plethora of novel radialene structures. 

The spherical Jt-electron conjugated system of [60]fullerene contains the [lOjcyclo-phenacene system, which may be unraveled by removal of the top and bottom two [5]radialene structures (20jt electrons) at both poles of [60]fullerene. The approach shown in Fig. 2.4 constitutes double application of the method that removes one radialene moiety out of conjugation to leave a 50jt-electron bowl-shaped conju-... 

The radialene structure of this cyclodimer was elucidated in 1974. The photo dimer derived from Ph2C=C=C=C(CF3)2 has a different structure 3, as evidenced by C -NMR... 

Nickel(0)-mediated [2-1-2] cyclodimerization reactions of [5]cumulenes 79 have been investigated in detail by lyoda and coworkers [50a, 51, 75]. Here again, the regioselectivity depends on the steric demand of the substituents and on the reactivity of the double bonds as well. Thus, [4]radialene structures 80-82 have been obtained, and in the presence of Ni(CO)2(PPh3)2 the [5]radialenone 83 was isolated as a significant by-product in some cases (Scheme 4.16). For example, tetraarylhexapentaenes form extended octaaryl[4]radialenes 80 (see 57, Scheme 4.13) treatment of tetra-ferf-butylhexapentaene with Ni(CO)2(PPhj)2... 

Misaki and coworkers have pe pared several new electron-rich [5]radialenes structurally related to 106, but containing one or two vinylogous 1,3-dithiole groups, and have studied their redox behavior by cyclic voltammetry [148-150]. 

RS)2C —CH=C(SR)2 CIO ."" While the latter product is a useful source of tetrathio-allenes, " the synthetic uses of the cyclopropenium salt, and its electronic structure,are particularly intriguing. Although a substantial contribution of the hetero-[3]radialene structure (31) to the ground-state... 

B-N 140pm. Structurally, this cyclohexaborane derivative resembles the radialenes, particularly the isoelectronic [Ce(=CHMe)6] in which the Ce ring likewise adopts the chair conformation. 

Exocyclic double bonds at cyclic systems, which contain cross-conjugated double bonds, cannot be considered as a subgroup of radialenes and shall therefore be treated separately, although many of the structural features are comparable. However, in these systems the exocyclic and endocyclic double bonds are competing with each other as sites for Diels-Alder reactions, cycloadditions and electrophilic attacks. The double bond character of both, as measured by its distance, can provide some evidence for the selec-tivities. If no strain and conjugation are expected, the double bonds should be comparable... 

Two questions related to the structure of radialenes are of special interest ... 

FIGURE 11. Structures of substituted puckered [4]radialenes (distances in A)... 

The latter two molecules are, however, special cases, where forces other than those inherent in the [6]radialene system are determining the preferred conformation. Hexakis(ethylidene)cyclohexane is the only radialene molecule where structure results obtained in the solid state181, as well as in the gas phase182, are available for comparison (Table 21). 

Planar conformations of radialenes with five or more ring atoms will always be more or less destabilized due to nonbonded repulsions, unless special structural effects that stabilize a planar conformation are present. The available experimental data indicate, however, that radialene systems generally prefer planar conformations, if steric effects are not taken into account. 

We are aware that these simple rules must and will be extended as the structural complexity of the known radialenes evolves. Unfortunately, however, the catchiness of the term radialene has led to its inflationary use in recent years. Although we think it is justified to call molecules such as 11 and 12 expanded [4]- (n = 1) and [6]radi-alenes (n = 3) — by widening our above definition, since sp-hybridized carbon atoms are allowed —we hesitate to call compounds of the type 13 heteroradialenes. 

Since our earlier review1 appeared not so long ago, it makes no sense to repeat here all facets of the radialene family. Therefore, we focus here on the synthesis and chemical transformation of the radialenes, and we suggest the reader consult our earlier review for information on structural and spectroscopic data as well as the use of radialenes as building blocks for organic conductors and organic ferromagnets, as these topics will not... 

Radialenes which are structurally related to 44, i.e. cyclopropanes bearing two quinoid and another acceptor-substituted methylene substituent, were obtained by condensation of bis(4-hydroxyphenyl)cyclopropenones with active methylene compounds, followed by oxidation (Scheme 6)19. Radialenes 45a-f are brilliantly colored solids that are blue or blue-violet in solution but appear metallic gold or red in reflected light. Instead... 

Hexamethyl[3]radialene (25) does not undergo Diels-Alder-reactions with the typical electron-poor dienophiles, probably because of the full substitution at the diene termini. With TCNE, however, a violet-blue charge-transfer complex is formed which disappears within 30 min at room temperature to form a 1 1 adduct (82% yield) to which structure 55 was assigned9. Similar observations were made with tris(2-adamantylidene)cyclopropane (34), but in this case cycloaddition product 56 (81% yield) was identified its allenic moiety is clearly indicated by IR and 13C NMR data12. 

Dichlorocarbene adds to 150 to give the monoadduct 158 as well as the two bis-adducts 159 and 160 (equation 18)101. An ortho -bis-adduct could not be detected among the cyclopropanation products—possibly because of steric hindrance in the immediate vicinity of 158. The anti-arrangement of the two cylopropane rings in 159 and 160 was established by X-ray structural analysis101. Formally, the latter adducts represent hybrids of [6]radialenes and [6]rotanes ( rotaradialenes ). 

Clearly, this is a rich playground for chemists interested in novel 7r-structures and topologies, and it is likely that many novel radialene-derived compounds will be prepared in the not too distant future. 

We have already pointed out in the Introduction (see above) that the first review article on radialenes is only a few years old1. In this first summary we have enclosed a comprehensive survey and discussion of the structural and spectroscopic properties of the radialenes. Since progress in this latter area has not been very rapid in the last few years, we do not address here again these aspects of the radialenes. Furthermore, nothing new can be added to the statement that all radialenes are nonaromatic and that they have localized endocyclic single bonds and exocyclic double bonds1 (for recent discussions of ji-ji interaction in [5]- and [3]radialene, see elsewhere903 109). 

The present volume deals with the properties of dienes, described in chapters on theory, structural chemistry, conformations, thermochemistry and acidity and in chapters dealing with UV and Raman spectra, with electronic effects and the chemistry of radical cations and cations derived from them. The synthesis of dienes and polyenes, and various reactions that they undergo with radicals, with oxidants, under electrochemical conditions, and their use in synthetic photochemistry are among the topics discussed. Systems such as radialenes, or the reactions of dienes under pressure, comprise special topics of these functional groups. 

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