Acetylene radialenes

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

The compound 251 decarbonylates on photolysis to bis(4-hydroxyaryl) acetylene 253, which is easily oxidized to the quinonoid cumulene 254. This is also obtained by thermal decarbonylation of the product of oxidation of cyclopropenone 251, the diquinocyclopropanone 252. Likewise, the blue derivative of 3-radialene 256 (a phenylogue of triketo cyclopropane) is formed from tris-(4-hydroxyaryl) cyclopropenium cation 255 by oxidation34. ... 

Scheme 6.1 (A) Synthesis of radialenes 1 by oxidative acetylenic homocoupling [17], and (B) Synthesis ofTTF based [18]annu-lenes 2 and 3 by Pd-mediated homocoupling [25], Reagents and conditions (a) TBAF, THF (b) [Cu(OAc)2], 02, py/PhH (c) Cul, [PdCI2(PPh3)2], Et3N.

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. 

Other dimerizations initiated by acetylene-allene interconversions have also been reported, and dimerization has also been applied to other cumulenes. The structure of the solid-state photodimer of tetraphenylbutatriene has now been reassigned, the new bis-allene structure (251) proposed being substantiated by ozonolysis to the known diketone (252), and by X-ray analysis. The solid-state photochemical reaction thus contrasts with the thermal dimerization, which gives the radialene (253). ... 

Cycloaddition (Diels-Alder) reactions have been reported for [6]radia-lene (5) and its hexaalkyl derivatives 113 and 115, but not for the permethylated radialene 72, which was inert even to the reactive dienophiles TCNE and Af-phenyltriazolinedione [67]. The sterically least hindered radialene 5 reacted with acetylenic and olefinic dienophiles in a 1 3 ratio to give triphenylene derivatives such as 139 in low yield (Scheme 4.30) [5, 95]. On the other hand, radi-alenes 113 and 115 gave linear,/)-quinodimethane-type 1 2-adducts, when they were exposed to an excess of various common dienophiles inter alia maleic anhydride, tetracyanoethylene, />-benzoquinone, acrolein, ethyl acrylate, acetylenedi-carboxylic acid) [89, 96, 97]. The 1 1 adduct 140, which was isolated so far only from the reaction with an equimolar amount of TCNE (92% yield) [97], presumably prefers the second cycloaddition step in the linear (para) position (141) over that in the angular (meta) position (142) for steric reasons. 

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

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