Extended radialenes

DT[5]radialene tetracation 33 (397 and 519 nm) except for the absorption at 682 nm. This implies that four positive charges are mainly localized at the radia-lene moieties. The observed band at 682 nm can be attributed to intramolecular CT from the central TTP core to the radialene units. 

---

Thermal or Ni(0)-catalyzed [2+2] cyclodimerization reactions of higher [ ]cumulenes give access to radialenes bearing cumulenic ir-systems ( extended radialenes, compare Scheme 4.13, Section 4.2.2, and Chapter 9). However, the success and the regioselectivity of the cycloaddition event depend strongly on the steric demand of the substituents at both ends of the cumulene [7, 8, 51]. Thus, a thermally induced cyclodimerization of a [4]cumulene furnished radialene 77... 

Ito and coworkers have designed an electrochemical route to the elusive hexaanionlc-extended radialene 146 (Scheme 4.32), which was presumably formed in a six-electron reduction of hexakis(6-azulenylethynyl)benzene (145). 

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. 

Radialene 52 has been envisaged as a precursor to hexaethynyl[3]radialene, a potential building block for carbon networks. However, desilylation under very mild conditions led to an unstable product of so far unknown identity. In this context, it should be mentioned that according to thermochemical calculations, the still unknown hexaethynyl[3]radialene has an increased conjugation energy with respect to vinylacetylene, probably because of partial relief of strain in the radialene corc26b. A remarkable aspect of 52 is its color the crystals are deep-red and a hexane solution has a purple color. In this respect, 52 differs from the yellow radialenes 50 and 51 and resembles [3]radialenes such as 27 and 38, which have much more extended --systems. 

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. 

Cyclodimerization of unsymmetrically substituted butatricncs such as 12 give both head-to-head and hcad-to-tail cycloaddition products. The structure of the head-to-head dimer was confirmed by its independent synthesis from the mixed cycloaddition of cumulenes 8 and 10,21 22 These dimerizations proceed by discrete nickel cyclopentanes which was established by the isolation of the 2-bispyridinenickel complex of the l.l,4,4-tetramelhylbuta-l,2,3-triene dimer.23 4-Radialenes with extended conjugation, potential organic conductors and semiconductors, have been prepared by similar methods as illustrated by the examples below.24,25... 

Dewar and coworkers39 have pointed out that an adequate description of polyenes has to be based on a set of reference molecules and reference bonds that comprises not only a C(sp3)—C(sp3) bond, but also C(sp2)—C(sp3), C(sp2)—C(sp2) reference bonds in order to separate normal cases of bond conjugation from exceptional cases which show bond delocalization. When this extended set of reference bonds is used, polyenes as well as radialenes and many cyclopolyenes are described as normal conjugated 7t-systems without any significant extra stabilization from bond delocalization39. 

Fig. 3. Extended carbon networks containing diyne fragments linked by (a) sp3 carbons, (b) sp2 carbons, and (c) [3]-radialenes.

Since tetraethynylethenes represent a repeat unit in more than one two-dimensional carbon network including 45 and 46 (Fig. 13-1) [1] the preparation of a specific network cannot be accomplished by simple oxidative polymerization of 20, but rather requires a more characteristic macrocydic precursor as starting material. Macrocyclic precursors to extended carbon sheets are perethynylated dehydroannulenes [56] and expanded radialenes, novel carbon-rich materials with interesting and unusual structures and functions. 

Charge-transfer complexes of variously functionalized, both electron-rich and electron-poor, [3]radialenes have attracted attention because of their magnetic properties [7] and as potential organic metals [17, 18]. Most of the [3]radialenes reported so far have been studied with respect to their oxidation and reduction chemistry (see the reviews [7, 8] and some examples mentioned in this chapter). With appropriate substitution patterns, the range of redox properties extends from the most electron-rich [3]radialene 31 to the very electron-poor radialene 24a, a strong oxidant [21]. The first one-electron reduction step of 18 (X = Y = CN) [16], 19, and 20 [17] occurs at potentials similar to TCNQ, and at potentials similar to chloranil for other derivatives of 18 (Scheme 4.3). 

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... 

One of the authors and his coworkers have reported a series of further n-extended DT[ ]radialenes (see Chapter 9) of 36, 37, and 38a,b (Figure 8.17) [47]. These radialene derivatives were designedto elucidate the electronic structure of [5]radi-alenes with different conjugation modes. 

Quite recently, fused TTF analogs having one or two [5]radialene unit(s) 40—42 have also been prepared (Figure 8.18), because the extended TTF moieties are of interest as multi-redox systems, components for molecular conductors, positive electrode materials for rechargeable batteries, and so on [48]. 

Figure 8.24 (a) Acetylene-extended DT[n]radialene series, (b) Synthesis of 77. 

---