Octaphenyl radialene

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. 

Radialenes bearing four to seven phenyl groups, but not octaphenyl[4]radia-lene, could be prepared from 3,4-bis(diphenylmethylene)cyclobutane-l,2-dione and 2,3-bis(diphenylmethylene)-4-benzylidenecyclobutan-l-one [49]. 

The formal cyclooligomerization of vinylidene units via copper or nickel car-benoids represents another route to [4]radialenes. To this end, a l,l-dibromo(or chloro)-alkene can be converted into a bis(l-halogenovinyl)cuprate at low temperatures, which, on warming, furnishes the [4]radialene, often accompanied by the corresponding dimerization product, the [3]cumulene. This method is so far the only route to octaphenyl[4]radialene (see Scheme 4.10, Section 4.2.1) and other octaaryl[4]radialenes [10, 37]. With l,l-dibromo-2-methylprop-l-ene (32) as the starting material, a mixture of [4]radialene 71 and [Sjradialene 84 as the major products is formed [76] (Scheme 4.17). While this method is not the preferred route to the former radialene, it represents the only known way for the preparation of 84. Treatment of dibromoalkene 32 with a Ni(0) complex [37] or activated nickel [10] does not improve the yield of radialene 71. 

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