Expanded radialenes

Keywords Acetylene chemistry. Cross-coupling reactions, Cyclo[n]carbons, Expanded radialenes. Molecular scaffolding. Nanostructures, Perethynylated chromophores, Poly(triacetylene), Tetraethynylethene. 

Fig. 5. Insertion of buta-l,3-diynediyl fragments (a) into radialenes [108] produces carbon-rich expanded radialenes and (b) into dendralenes [109] produces expanded dendralenes...

The physical properties of the expanded radialenes were greatly enhanced upon donor functionalization, leading to the stable derivatives 76-78 with fully planar conjugated rr-chromophores [110]. These compounds exhibit large third-order nonlinear optical coefficients, can be reversibly reduced or oxidized, and... 

Fig. 6. Electronic absorption spectra of donor-functionalized expanded radialenes 76-78 in CHCI3 at 298 K...

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... 

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. 

A third route to higher or expanded radialenes (see Introduction) also rests on an insertion principle . It employs the rodlike butadiyne unit and inserts it between the neighboring double bonds of the basic radialene systems 181, thus converting these into the boxlike molecules 12 (equation 26)107,108. 

Molecular scaffoldings with tetraethynylethenes (TEEs, 3,4-diethynylhex-3-ene-l,5-diynes) and trans-1,2-diethynylethenes [DEEs, (E)-hex-3-en-l,5-diynes] are at a particularly advanced stage.114,37 38 441 A collection of dose to one hundred partially protected and functionalized derivatives have been prepared in the meantime, providing starting materials for the perethynylated dehydroannulenes and expanded radialenes shown in Figure 6.136 441 TEEs and DEEs, as well as dimeric derivatives substituted at the terminal alkynes with donor (D, p-(dimethyl-... 

Fig. 7. Expanded radialenes bearing peripheral substituents of different electron-donating strengths [29].

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],... 

Another interesting property of expanded radialenes is their strong ability to accommodate electrons upon reduction, as revealed by an electrochemical study by Gross, Gisselbrecht, and Boudon [29d]. Thus, in all three series, the first reduction occurred at anodically shifted potentials relative to the reference TEE dimers the following shifts were observed +170-440 mV for 29a-c relative to 19, +210-330 mV for 30a,b relative to 23, and +240-320 mV for 28a-c relative to 21 (Figure 9). Thus, the radical anions of expanded radialenes are very stable, particularly those of the expanded [3]- and [4]radialenes, whatever the nature of the substituents. 

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. 

Fig. 9. Comparison of first reduction potentials (ys. Fc/Fc+) for the three differently substituted series of TEE dimers/expanded radialenes [29d], Solvent THF for 19, 29a-c CH2CI2 for 21, 23, 28a-c, and 30a,b.

Fig. 10. X -ray crystal structure of expanded [6]radialene 30b H-atoms have been omitted for clarity. Atomic displacement parameters at 203 K are drawn at the 20 % probability level. The perethynylated C6o-core is highlighted in bold it adopts a chair-like conformation [29d].

This protocol is applied to the synthesis of the starting monomers for acetylenic nanoarchitectures. For example, geminally bis-deprotected teti aethynylethene 97 is prepared by Pd-catalyzed alkynylation followed by deprotection with K2CO3 in MeOH [Eq. (36)] [54c]. Perethynylated ethene 97 is a synthetic precursor of expanded radialenes, which are novel carbon-rich materials. 

Perethynylated Dehydroannulenes and Expanded Radialenes Large Carbon Cores on the Way to All-Carbon Sheets... 

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. 

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 13-3 Scheme to increase the C/H ratio by inserting butadiynyl moieties into radialenes (49), thereby producing expanded radialenes (50). 

Scheme 13-11 Synthesis of the persilylethynylated expanded radialenes 51 53. Shown for 51 in bold is the longest linearly -conjugated fragment in 51-53.

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