Bergman Cyclization in Polymer Chemistry and Material Science

Abstract

Bergman cyclization of enediynes, regarded as a promising strategy for anticancer drugs, now finds its own niche in the area of polymer chemistry and material science. The highly reactive aromatic diradicals generated from Bergman cyclization can undergo polymerization acting as either monomers or initiators of other vinyl monomers. The former, namely homopolymerization, leads to polyphenylenes and polynaphthalenes with excellent thermal stability, good solubility, and processability. The many remarkable properties of these aromatic polymers have further endowed them to be manufactured into carbon-rich materials, e.g., glassy carbons and carbon nanotubes. Whereas used as initiators, enediynes provide a novel resource for high molecular weight polymers with narrow polydispersities. The aromatic diradicals are also useful for introducing oligomers or polymers onto pristine carbonous nanomaterials, such as carbon nano-onions and carbon nanotubes, to improve their dispersibility in organic solvents and polymer solutions.

Recent advances of the Bergman cyclization in polymer science

“…Taking advantage of the designable structure of enediynes, it is very easy to modulate the reaction conditions and variegate the structure, performance, and application of the conjugated polymers. Although still in its infancy, the Bergman cyclization has already appeared as a valuable tool in polymer chemistry where it is applied in a twofold way: (1) the highly reactive p-benzynes generated by the Bergman cyclization can undergo homo-polymerization acting as symmetrical bifunctional monomers; (2) the highly reactive radicals at the chain ends of the conjugated polymers can also be used to covalently modify carbon materials to improve their solubility in organic solvents [3,28].…”

“…They found that cis-1,5-hexadiyn-3-ene underwent an exclusively thermal rearrangement to generate a 1,4-didehydrobenzene intermediate which could easily be trapped by external reagents to yield benzene derivatives (Scheme 1) [3]. However, Bergman's research did not receive much attention until the late 1980s when the enediyne-type core structures of many naturally occurring antibiotics, such as calicheamicin [4,5], dynemicin A [6], esperamicin A 1 [7,8] and kedarcidin chromophore [9] were reported.…”

“…The enediyne "warheads" of these antibiotics are readily triggered in vivo to generate 1,4-phenylene diradicals, which further cause DNA cleavage or crosslinking. The strong cytotoxicity arising from the Bergman cyclization has initiated a flurry of activities on the synthesis of biosimilar enediyne doi: 10.1007/s11426-015-5460-4 Scheme 1 Bergman cyclization of cis-1,5-hexadiyn-3-ene [3].…”

Application of named reactions in polymer synthesis

“…Due to the cytotoxicity of the biradical intermediate, it is widely used in pharmaceutical research and organic synthesis [71]. Since 1994, when Tour et al [72] described a radical polymerization route to obtain poly(p-phenylene)s and polynaphthalenes, diradicals from Bergman cyclization have been regarded as very promising and attractive monomers for preparing conjugated aromatic polymers and carbon-rich materials without transition-metal catalysts (Scheme 18) [73].…”

Synthesis and reactivity of anthracenyl-substituted arenediynes

“…12,[14][15][16] Moreover, the photophysical properties of polycyclic aromatic hydrocarbons containing the 5-membered ring are unusual compared to the analogous molecules with 6-membered rings. 3,[5][6][7][8][9][10][11][12][13]15 It is thus of interest to develop a better understanding of the structure-property relationships of the 5-membered ring containing polycyclic aromatic hydrocarbons.…”