Radialenes, preparation

Nierengarten, J.F., A. Herrmann, R.R. Tykwinski, M. Ruttimann, F. Diederich, C. Boudon, J.P. Gisselbrecht, and M. Gross, MethanofuUerene Molecular Scaffolding Towards CgQ- Substituted Poly(triacetylenes) and Expanded Radialenes, Preparation of a CgQ-CyQ Hybrid Derivative, and a Novel Macrocyclization Reaction, Helv. Chim. Acta, 80 293-316 (1997). 

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... 

The parent [5]radialene (5) has so far evaded preparation. The decamethyl derivative is, however, known, and this molecule is found to have a half-chair conformation, with approximately C2 symmetry179. There are, however, observations indicating that... 

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

The most recent strategy to prepare [3]radialenes is the treatment of 1,1-dihaloalkenes with activated nickel. Thus, the aryl-substituted [3]radialenes (Z,E,E)-30 and (E,E,E) 30, 27 and 32 were obtained together with the corresponding butatrienes (29, 28, 31) from the 1,1-dibromo- or 1,1-dichloroalkenes with the help of nickel activated by ultrasound (Scheme 4)11. It is worth mentioning that the mixed-substituted radialene 33 was produced, when the nickel carbenoid derived from 9-(dichloromethylene)xanthene was generated in the presence of butatriene 2811. 

A variety of functionalized [3]radialenes have been prepared starting from the appropriately substituted cyclopropanes or cyclopropenes. West and Zecher have pioneered the chemistry of [3]radialenes with quinoid substituents. The general strategy of this synthesis is outlined in Scheme 513. A tris(4-hydroxyphenyl)cyclopropenylium... 

In contrast to 51, hexacyano[3]radialene (50) proved difficult to obtain in pure form. Freshly prepared samples are bright-yellow, but mm brown on exposure to air and blue on contact with many solvents. Potassium bromide and sodium iodide reduce 50 to the radical anion and the dianion, respectively24. 

Radialenes represent the biggest and best known subset of the radialene family this is not surprising in view of the fact that more methods to prepare them exist than for any other class of radialenes. The major strategies are the transformation of appropriate cyclobutane derivatives, the thermal or Ni(0)-catalyzed cyclodimerization of butatrienes or higher cumulenes and the cyclotetramerization of (l-bromo-l-alkenyl)cuprates. 

Radialenes bearing four to seven phenyl substituents have been prepared from... 

The scope of this method is illustrated by the preparation of other peralkylated [4]radialenes, such as 103-10566 and the functionalized radialene 10670,71 (Scheme 12). Removal of the carboxylate groups from the latter provided the very electron-rich radialene 107. 

As we have mentioned, octaphenyl[4]radialene 109 can be obtained neither by photochemical nor by Ni(0)-catalyzed cyclodimerization of tetraphenylbutatriene. However, 109 has been prepared from l,l-dibromo-2,2-diphenylethene via organocuprate intermediates (Scheme 13)72. The correct choice of the added copper salt is crucial for the success of this transformation, but even then, formation of tetraphenylbutatriene (108) from the bis(l-bromovinyl)cuprate intermediate limits the yield of 109. 

Another [5]radialene ketone, the tetraallene 102a, has been prepared in 74% yield by treating the [5]cumulene 98a with stoichiometric amounts of Ni(CO)2(PPh3)267 the sterically very shielded ketone 102b is produced analogously in 32% yield from the corresponding terminally bridged [5]cumulene 98b (equation 11). In both cases the dimers of the [5]cumulenes, the peralkylated [4]radialenes are formed as well (see Section II.B, Scheme 11). 

Clearly, this is a rich playground for chemists interested in novel 7r-structures and topologies, and it is likely that many novel radialene-derived compounds will be prepared in the not too distant future. 

We thank Dr. Martina Glaser and Dr. Thomas Hopfner for useful discussions and help with the preparation of the manuscript. Our own work on radialenes has constantly been supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. 

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-... 

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... 

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. 

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