Photo Bergman cycloaromatization

Jones, G. B., Russell, K. C. The photo-Bergman cycloaromatization of enediynes. CRC Handbook of Organic Photochemistry and... 

Irradiation of diethynyl sulfide 408 at 300 nm in hexane in the presence of 1,4-cyclohexadiene gives 3,4-diphenylthiophene 409 (photo-Bergman cyclization). The authors describe this as being the first five-membered ring cycloaromatization reaction. The low yield of the thiophene 409 is due to the production of many side products (Scheme 68) <20030L2195>. Naphthothiophene 411 and dinaphthothiophene 413 can be prepared from 410 and 412, respectively, by use of this method <2004T7191>. 

Photo-Bergman cyclization have been carried out on diethynyl sulfides 4 to produce 3, 4-disubstituted thiophenes 5. Matzger reported this as being the first five-membered ring cycloaromatization reaction <03OL2195>. The reaction is found to produce many side products thereby lowering the yield of the desired heterocycle. 

Transition metal catalysts can also be used in photochemical organic synthesis. For example, the photo-Bergman reaction (cycloaromatization see also Scheme 6.62) of (Z)-hex-3-en-l,5-diyne [(Z)-589], which is obtained by photostationary E Z isomerization (Section 6.1.1) from ( )-589, occurs via transition metal-catalysed cycloaromatization reaction followed by photochemical dissociation of the arene ligand from the complex 590 (Scheme 6.289).1530 The product, 1,2-di( -propyl)benzene (591), is obtained in 91% chemical yield. 

It was found that cyclization of benzannulated enediynes can be initiated by irradiation to give the same products as were obtained by thermal Bergman cyclization [238]. Photo-Bergman cyclization, which is also related to the chemistry of benzyne, has not been so extensively investigated. Terminal acyclic enediynes capable of cycloaromatization by thermal C -C route do not undergo photochemical initiation. However, introduction of one or two phenyl substituents in the acetylenic branches facilitates the C -C photocyclization of acyclic enediynes, but any increase in steric hindrance raises the activation barrier and the photochemical yield is significantly lower than in the case of their cyclic... 

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