Arenes Bergman cyclization

The Bergman Cyclization (or Myers-Saito Cyclization) allows the construction of substituted arenes through the thermal or photochemical cycloaromatization of enediynes in the presence of a H donor such as 1,4-cyclohexadiene. 

The interest in the Bergman Cyclization was somewhat low, due to its limited substrate scope and the availability of alternative methods for the construction of substituted arenes. However, natural products that contain the enediyne moiety have been discovered recently, and these compounds have cytotoxic activity. 

More recently, O Connor and co-workers have reported the first catalytic Bergman cyclization using an iron(II) complex 32 (0.3 equiv) to catalyze cycloaromatization of acyclic enediynes. Mild catalytic activity (3 turnovers) was achievable when photolysis was combined with iron catalysis, enabling decomplexation of the arene product from the iron complex to... 

Under thermolysis conditions, cyclization of l,3-hexadiene-5-ynes occurs to give substituted arenes in a similar fashion to the Bergman cyclization. This variation on the Bergman cyclization is known as the Hopf... 

Until relatively recently, interest in the Bergman cyclization had been restricted due to its limited substrate scope and the availability of more practical methods for substituted arene construction. Since 1987 with the discovery of natural enediynes that possess cytotoxic activity, there has been a great surge in studies of the Bergman cyclization, specifically with respect to the mode of action of the natural enediynes. A number of these natural products were found to undergo Bergman cyclization under much milder conditions (37 °C or lower) than previously thought possible. ... 

Heating a benzene solution of 3,4-benzocyclodec-3-en-l,5-diyne 3.512 and 1,4-cyclohexadiene (CHD) at 84°C gives tetrahydroan-thracene with the half transformation period of 24 hours [258]. Complete cycloaromatization is achieved by heating the compound 3.512 at 100°C in nitromethane-d3 in the presence of 1,4-cyclohexadiene for 18 days. Scheme 3.27 [259] depicts an example of Bergman cyclization promoted by pentamethylcyclopentadienyl-ruthenium cation 3.518. Besides the formation of triflate 3.513 which has an arene fragment of enediyne complexed to the ruthenium cation 3.518, the rate of the Bergman cyclization is increased by the Ru-alkyne interaction. The more... 

The cyclization is induced thermally or photochemically. Most cyclizations have a high activation energy barrier and therefore temperatures around 200 °C are needed for the cycloaromatization. The Bergman Cyclizatic n forms a 1,4-benzenediyl diradical - a highly reactive species, that reacts with a H donor to give the corresponding arenes. 

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]. 

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