Cross-conjugated dianions

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. 

Another C HgLij isomer is the dilithium salt of the cross-conjugated dianion of isobutylen the trimethylenemethane dianion 37 (Ds symmetry). This 67c... 

Interactions polarize bonds. Trimethylenemethane (TMM) and 2-buten-l,4-diyl (BD) dianions (Scheme 6a, b) are chosen as models for hnear and cross-conjngated dianions. The bond polarization (Scheme 7) is shown to contain cyclic orbital interaction (Scheme 6c) even in non-cyclic conjugation [15]. The orbital phase continnity-discon-tinnity properties (Scheme 6d, e) control the relative thermodynamic stabihties. 

Scheme 7 Polarization of the cross- (TMM) and linear (BD) conjugate dianions...

Experimental evidence for the six electron systems has been described in Sect. 2.1.4. Skancke reproduced the relative stabihty of the cross conjugated systems relative to the linear isomers by calculating the trimethylenemethane and buta-l,4-diyl dianions [27] and their dilithio salts [28]. For the four electron systems butadiene is more stable than trimethylenemethane. Experimental examination of the relative stabihties of two electron systems using the trimethylenemethane and buta-14-diyl dications needs to overcome the intrinsic instabihties of dications dissatisfying the octet rule. 

Trimethylenemethane (1) is a cross-conjugated it system with a diradical ground state. Its chemistry has been reviewed as early as 1972 by Dowd [8]. The Hiickel molecular orbital (MO) scheme resembles that of cyclobutadiene, and accordingly trimethylenemethane can be stabilized as a ligand in transition metal complexes. Trimethylenemethane complexes can generally be obtained by treatment of a trimethylenemethane dianion with a metal halide or by treatment of organic halides with low-valent metal complexes. In addition to these general methods, there are some more specialized procedures. 

The main use of this reaction is for the synthesis of bowl-shaped polycyclic aromatic hydrocarbons and fullerenes [325], Exploring the boundaries of these reactions resulted in the synthesis of compounds of different geometry, tt-electron structure, aromaticity, and with the presence of heteroatoms [324]. Various cross-conjugated enediynes exert anionic cycloaromatization to form fulvene and fulvalene anions and even heterocycles. In accordance with the concept of aromaticity, the anionic Bergman type cyclization is preferred to the classical Bergman cycloaromatization of linear enediynes. This anionic cyclization differs considerably from the classical Bergman cyclization and related cycliza-tions, as well as from other dianionic cyclizations [326]. 

C-C a bond is replaced by a C=C n bond. The relative stability can also be nnder-stood by the analogy of the cross and linear n conjugation in the TMM and BD dianions (Schemes 6 and 7). Lone pair orbitals for the anion centers correspond to the bonds. The branched butene is more stable than the hnear isomer. 

Epoxy resins, which are used as adhesives, are also thermoset polymers that form by cross-linking when the two components of the resin are mixed. One component is a low-molecular-mass linear polymer formed by the reaction of the conjugate base of bisphenol A with epichlorohydrin. The nucleophilic oxygens of the phenolate dianion can either displace the chlorine or open the epoxide ring of epichlorohydrin. A slight excess of epichlorohydrin is used to keep these polymer chains short and to ensure that the linear molecules have epoxide groups at their ends. 

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