Tetraethynylethene derivatives

The state of research on the two classes of acetylenic compounds described in this article, the cyclo[ ]carbons and tetraethynylethene derivatives, differs drastically. The synthesis of bulk quantities of a cyclocarbon remains a fascinating challenge in view of the expected instability of these compounds. These compounds would represent a fourth allotropic form of carbon, in addition to diamond, graphite, and the fullerenes. The full spectral characterization of macroscopic quantities of cyclo-C should provide a unique experimental calibration for the power of theoretical predictions dealing with the electronic and structural properties of conjugated n-chromophores of substantial size and number of heavy atoms. We believe that access to bulk cyclocarbon quantities will eventually be accomplished by controlled thermal or photochemical cycloreversion reactions of structurally defined, stable precursor molecules similar to those described in this review. 

Tetraethynylethene (20) and its differentially protected derivatives are versatile building blocks for two-dimensional all-carbon networks and carbon-rich nanomaterials [1]. In addition, they attract interest for their fully cross-conjugated 7c-electron system [33], The first tetraethynylethene derivative, 21a, was reported in 1969 by Hori and co-workers [34], and the persilylated and peralkylated derivatives 21b-d were prepared in the mid-1970 s by Hauptmann [35]. In 1991, Hopf et al. [36] summarized this early synthetic work (Scheme 13-5) and reported the X-ray crystal structure of 21a the authors also suggested in their paper the potential of substituted tetraethynylethenes as monomers for new polymers. Also In 1991, Rubin et al. [37] reported the first synthesis of the parent compound 20 by a synthetic route, which, after suitable modifications, provided access to tetraethynylethenes with any desired substitution and protection pattern. These transformations are the subject of this Section the application of these compounds as precursors to two-dimensional all-carbon networks tmd carbon-rich nanomaterials will be discussed in the following sections. 

Scheme 6.43 A twofold Stille coupling involving a bisstannylated tetraethynylethene derivative [183].

Scheme 8. Oligomers and polymers with a poly(triacetylene) (PTA) backbone that are derived from tetraethynylethenes (TEEs)...

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-... 

In a general synthetic route (Scheme 13-6) [37], dialkynylketone 22 was converted [38] into the dibromomethylene derivative 23 and subsequent Pd(0)-catalyzed alkynylatlon [39] afforded the protected tetraethynylethenes 21 d and 24a/b. The X-ray crystal structure of 21 d showed... 

The synthesis of monodeprotected tetraethynylethenes starts from the unsymmetrically protected dibromomethylene derivatives 26a/b that are prepared as shown in Scheme 13-6 for 23 [40-42]. Table 13-2 shows the reaction conditions for the palladium-catalyzed ethynylation to 27a-d and the subsequent monodeprotection to 28a-d. Remarkable is the high-yielding kinetically controlled removal of a trimethylsilyl in the presence of triethylsilyl and triisopropylsilyl protecting groups in the synthesis of 28 b. 

Table 13-2 (A) Preparation of tetrasubstituted tetraethynylethenes from unsymmetrically protected dibromomethylene derivatives...

Tetraethynylmethane (39), a potential monomer for a three-dimensional superdiamonoid carbon network [1], was elusive for many years [51, 52], until its synthesis was accomplished in 1993 by Feldman and co-workers [53]. The key step in the synthesis was the acid-mediated Johnson orthoester variant of the Claisen rearrangement, which provided the central quaternary methane C-atom with suitable functional groups for the ultimate transformation into 39 [Scheme 13-9(b)]. Solid 39, like tetraethynylethene (20), decomposes rapidly at room temperature in either the presence or absence of oxygen. The earlier efforts to prepare tetraethynylmethane had yielded the peralkynylated derivatives 40-42 [Scheme 13-9(c, d)] [51, 52]. Tetraethy-nylallene represents another potential precursor for a three-dimensional carbon network [1], but remains elusive of the perethynylated [K]cumulenes, so far only the silyl-protected [3]cumulenes 43a and 43b [Scheme 13-9 (e)] have been prepared [54]. With 44 [Scheme 13-9 (f)], the first transition metal complex of a perethynylated ligand is now available [55]. 

Figure 13-1 Planar all-carbon networks 45 and 46 derived from tetraethynylethene.

Diederich et al. [183] synthesized a novel light-driven fully reversible molecular switch by a cross[Pg.457]

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