Fulvenes radical anions

Whitlock et al.14 discovered a reductive cyclization of enediynes promoted by lithium naphthalenide that provides substituted fulvenes and suggested a dianionic mechanism (Scheme 6). However, even now it is still unclear whether the enediyne dianion is indeed the cyclizing species or whether the initially formed acyclic radical-anion cyclizes first to give a fulvene radical-anion which is further reduced by lithium to give the cyclic dianion. 

The diyne ene (84) undergoes photochemical cyclization on irradiation at 350 nm. The product obtained was identified as the phenanthrene derivative (85). " A variant of the photo-Bergman cyclization has been reported following the irradiation of the enediynes (86). The reactions are brought about using electron transfer from cyclohexa-1,4-diene in acetonitrile. This affords the products (87) in the yields reported under the structure. The products are formed by cyclization via the fulvene radical anion intermediate (88). " ... 

Various cross-conjugated enediynes undergo Bergman-type cycloaromatizations upon reduction with potassium metal, generating anions of fulvenes and fulvalene derivatives (Scheme l).6 Not all cross-conjugated enediynes yield cyclized dianions upon reduction some give uncyclized, Y-shaped, cross-conjugated dianions, whereas others apparently yield radical anions that either dimerize or persist as monomers. 

Kovalenko and Alabugin reported that Cj-C cyclization of benzannelated enediynes with tetra-fluoropyridinyl (TFP) substituents at the terminal alkyne carbons forms indenes rather than fulvenes (Scheme 30.22). The radical/anionic Cj-Cj cyclization of enediynes represents a new type of cycloaromatization reaction—the cyclorearomatization process driven by rearomatization in the vicinity of the TS. This process is triggered via photoinduced electron transfer (PET) [35-38]. Expaimental work unambiguously established PET as the triggering event for the Cj-C cascade and the intermediacy of the second PET step in the indene-forming cascade [39]. Unlike the stable benzene product... 

Hence, the anion-radical of diphenyl fulvene acquires a spin-charge distribution dictated by steric shielding at Ph2C node and six-n-electron delocalization in the C5H5 ring. Anion-radicals of sterically congested stilbenes represent examples that are quite different, but at the same time are similar in principle. Let us compare the E structures of stilbene and its congested derivative, namely, a,p-di(tert-butyl)stilbene in their neutral and anion-radical forms (Scheme 3.18). 

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