Bergman-type cyclization

Liu and colleagues developed a number of Bergman-type cyclization reactions leading to arene derivatives. PtCl2 (5 mol%) proved to be the catalyst of choice providing the cyclization products in 46-86% yield [246]. A valence tautomeric platinum biradical-carbenoid pair was considered, while for related cyclizations with nucleophiles present in the reaction mixture two-electron pathways were favored by the authors [247, 248]. 

Aral et al. investigated the photochemistry of the linear shaped aromatic enediynes (186) and (187). The quantum yield of both ( )- (Z) and (Z) ( ) isomerizations considerably decreased in (186) and (187) because of an increase in the number of photochemical processes that yielded nonfluorescent excited species. The compound (188) showed characteristic charge-transfer absorption due to the strong donor-acceptor chromo-phores. The enediynes (189) on Au nanoparticles underwent Bergman-type cyclization leading to a polymerized coating via biradical intermediates. 

Reduction of biphenyleneethene 3.686 by an alkali metal leads to the formation of a five-membered ring with six -ir-electrons that undergoes the Bergman type cyclization, affording the dianion 3.687 (Scheme 3.71B) [324-326]. The cyclization of this anion proceeds much... 

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]. 

Treitel, N., Eshdat, L., Sheradsky, T., et al. (2006) Reductive Bergman-type cyclizations of cross-conjugated enediynes to fulvene and fulvalene anions the role of the substituent. Journal of the American Chemical Society, 128(14), 4703-4709. 

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. 

The study of Bergman, Myers-Saito and related biradical cyclizations using an unrestricted broken spin symmetry approach refined by single-point energy coupled-cluster calculations has been reviewed, and a simple rule outlined for predicting biradical involvement in such Cope-type rearrangements radicals were found to be probable... 

Quantum chemical studies of cyclizations of enediynes and enyneallenes have been reviewed.180 The intermediates are computationally tractable as a result of the unrestricted broken spin symmetry (UBS) approach using GGA functionals for the description of open-shell biradicals. The intermediacy of biradicals in Cope-type rearrangements, to which the Bergman and Myers-Saito reactions belong, are shown to be predictable using a very simple rule biradicals are likely to be intermediates if they are stabilized either by allyl resonance or by aromaticity. 

A variety of methods are used to generate and study diyls including, to name a few, the thermal or photochemically promoted extrusion of nitrogen from a pyr-azoline, the Norrish type I photochemically initiated extrusion of carbon monoxide from a cyclic ketone, and the Bergman cyclization of enediynes [3-5]. In this chapter we focus upon the synthetic utility of diradicals, paying particular attention to the work of others as well as research originating from our laboratory. 

A third important development concerns the application of DFT to diradicals. As discussed in the section on this type of molecules, one of the main limitations of density functional theory is its ability to deal properly with singlet diradicals. Interestingly, Schreiner has found that pure density functionals such as BLYP perform much better than hybrid functionals (i.e., B3LYP) in calculations on dehydrobenzene singlet diradicals which are intermediates in the Bergman cyclization of enediynes. Apparently, the admixture of Hartree-Fock density appears to constitute a disadvantage in such cases. ... 

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