Cyclic radialenes

In contrast, the synthesis of tetraethynylethene (TEE, C10H4) was described in 1991 and, since then, a rich variety of cyclic and acyclic molecular scaffolds incorporating this carbon-rich molecule as a construction module have been prepared. The majority of these compounds, such as the expanded radialenes or the oligomers and polymers of the poly(triacetylene) type, are highly stable and... 

Diels-Alder cycloadditions of sila- and germa[3]radialenes 541 with MTAD or PTAD provide the corresponding products 542 (Equation 73) <19930M1996, 1994AG723>. Similarly, phospha derivative 543 with MTAD gives 544 (Equation 74). On the other hand, the same reaction of cyclic 545 with MTAD is slower and the formed reaction mixture contains only small amount of a product probably analogous to 544 and compound 546 as the major product (Equation 75) <2000JA12507>. 

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... 

This comparison demonstrates that the exocyclic double bond length is little affected by the cyclic strain, which was also found for the radialenes see Section n.E. However, significant deviations were found for conjugated systems, and the same holds for the linear and branched dienes and polyenes. 

It is obvious that these compounds have in common an uninterrupted cyclic arrangement of cross-conjugated jr-systems. Compound 5 likewise contains the maximum number of exocyclic double bonds at a perimeter consisting only of sp2-hybridized carbon atoms. Thus, our definition allows one to call it a radialene, i.e. naphtharadialene on the other hand, it excludes hydrocarbons such as 6 [3,4,5,6-tetrakis(methylene)cyclohexene]. Although in the latter molecule all carbon atoms are indeed sp2-hybridized, the number of exocyclic double bonds has not reached its maximum. In 5, however, the number of double bonds cannot be increased further. 

Hexacyano[3]radialene (50) is a very powerful electron acceptor according to both experiment23,24 35 and MNDO calculations of LUMO energy and adiabatic electron affinity25. The easy reduction to the stable species 50" and 502- by KBr and Nal, respectively, has already been mentioned. Similarly, the hexaester 51 is reduced to 512-by Lil24. Most [3]radialenes with two or three quinoid substituents are reduced in two subsequent, well-separated, reversible one-electron steps. As an exception, an apparent two-electron reduction occurs for 4620. The reduction potentials of some [3]radialenes of this type, as determined by cyclic voltammetry, are collected in Table 1. Due to the occurrence of the first reduction step at relatively high potential, all these radialenes... 

TABLE 1. Reduction potentials of [3]radialenes ( 1/2, V) with quinoid substituents and of some related compounds, as determined by cyclic voltammetry (in CH2CI2 vs SCE)... 

Cyclononatriene, the smallest cyclic [3]cumulene isolated so far, polymerizes when its solutions are concentrated57. On the other hand, several radialenes have been isolated which represent cyclodimers of seven- and eight-membered 1,2,3-trienes (Scheme 9). 

Silirenes (140, equation 32) could also be involved in the transition-metal catalyzed decomposition of bis(diazoketones) 139 which provides the electron-rich [4]radialenes 14266,67. While the formation of 142 directly from silirene 140 cannot be excluded a priori, it is more reasonable to assume that 140 undergoes twofold ring-expansion to form the cyclic cumulene 141, which then provides 142 by a cyclodimerization reaction. The intermediacy of 141 is corroborated by the isolation of the Diels-Alder product 14366. 

Increase of cyclic cross-conjugation Fig. 8. Comparison of the end-absorptions of TEE dimers and expanded radialenes reveals that the degree of cyclic cross-conjugation in the radialenes increases with increasing donor strength of the peripheral substituents [29d],... 

Interestingly, the perethynylated core of an expanded [6] radialene contains 60 carbon atoms and can therefore be viewed as an isomer of Buckminsterfullerene C60. X-ray crystal structure analysis of 30b (Figure 10) revealed that the cyclic core adopts a non-planar, chairlike conformation, with an average torsional angle of 57.2° [29d]. Each buta-l,3-diynediyl moiety only deviates slightly from linearity. Thus, the corresponding bond angles vary from 174 to 180°. The six individual TEE units are almost planar, with deviations of no more than 0.03 A from the mean plane. 

Trimethylenecyclopropane, [3]radialene (169), which is one of the benzene isomers and the smallest member of the radialene family, is characterized by a cyclic arrangement of cross-conjugated Ti-electron systems. The synthesis and properties of radialenes have been reviewed recently by Hopf and Maas The C—C and C=C bond lengths in 169,1.453 (3) and 1.343 (3) A, determined by are similar to those in its hexamethyl derivative... 

A polycyclic system with a l,3-dioxa-2-silacyclohepta-4,6-diene fragment was obtained in low yield from bis(diazocarbonyl)silane 671. In the first stage, a biscarbene or a biscarbenoid, generated in the presence of copper triflate or palladium acetate, forms 1,2-diacylcyclopropene, then rearranging into cyclic cumulene triene 672 by migration of a silicon atom to an oxygen atom. The latter dimerizes to [4]-radialen 673 or reacts with furan to afford 674 (Scheme 32) (89AG175()). 

Radialenes (49) are a homologous series of all-eA o-methylenecycloalkanes of molecular formula C H (Fig. 13-3) [60], Upon insertion of butadiynyl moieties into the cyclic framework between each pair of vicinal ejto-methylene units, the carbon-rich expanded radialenes 50 (C3 H ) are obtained. Starting from suitable protected tetraethynylethenes, the perethynylated expanded radialenes 51-53 were prepared via 54, as shown in Scheme 13-11 [41], They possess large carbon cores with silyl-protected peripheral valences and can be viewed as C40 + 8 (/Pr)3Si (51), Cjo + 10 (iPr)3Si (52), and C , + 12 (/Pr)3Si (53). The diameters of these large carbon-rich molecules are in the nanometer range with values of = 17 A (for 51), 19 A (for 52), and 22 A (for 53). 

Figure 4.2 [4]Radialenes obtained by cyclodimerization of acyclic and cyclic [3]cumulenes.

Estimation of resonance energy by spin-coupled valence bond calculations [129] and the application of newly developed aromaticity indices considering the cyclic x-electron delocalization [142, 143] have confirmed that [3]- and [6]radialene are nonaromatic. As far as the magnetic criterion for aromaticity of conjugated... 

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]. 

Polyalkenic conjugated hydrocarbons can be classified on the basis of the connectivity of their ethylene units. The linear polyenes 1 and annulenes 2 are comprised of vicinal linked units, whereas the dendralenes 3, and their cyclic counterparts the radialenes 4, contain geminal linkages (Figure 12.1). In addition to these four principal classes, hybrid molecules that exhibit both vicinal and geminal connectivity are possible, as illustrated by the fulvenes 5. 

Despite great advances in the DA chemistry of the dendralenes, their cyclic counterparts, the radialenes (see Chapter 4), remain comparatively neglected. The simplest family member, [3]radialene (173), can, in principle, undergo a DA reaction to generate the bicyclic triafulvene 174 (Scheme 12.36). Although this... 

Altogether there are three fundamental classes of oligo-olefins that exhibit cross-conjugation the dendralenes (acyclic and branched), the radialenes (cyclic and branched), and the fulvenes. The fulvenes contain both branched and linear conjugation within the same it-system, and are therefore hybrid systems. As they are widely regarded as cross-conjugated molecules in the chemical literature, we follow this convention here. 

Other species that are potentially aromatic such as the annulenes and radialenes have also been discussed, based on DFT computations and several experimentally prepared structures [112, 113]. The key conclusion of these studies is that aromaticity is an important consideration for these types of structures but the considerable strain often found in these molecules can override some of the aromatic stabilization. Owing to their cyclic delocalization virtually all carbomers are excellent electron acceptors, as evident from their high electron affinities [113]. 

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