Photodehydrocyclizations in stilbene-like compounds. Part V. Photochemistry of 2,2′-distyrylbiphenyl

The photochemistry of 2,2′-distyrylbiphenyl (I) has been investigated. Irradiation under nitrogen gives, as the kinetically controlled product, trans,trans,trans-1,2,2a,10b-tetrahydro-1,2-diphenylcyclobuta[I]phenanthrene (IIa), and as the thermodynamically-controlled product 4,5,9,10-tetrahydro-4,9-diphenylpyrene (IXa). Irradiation under nitrogen in the presence of iodine gives 7-phenylbenzo[c]chrysene (IV). The mechanisms are discussed. Heating (I) at 240° gives cis,cis,cis-1,2,2a,10b-tetrahydro-1,2-diphenylcyclobuta[I]phenanthrene (IIb) as the only primary product. The isomers (IIa) and (IIb) behave quite differently on heating and on irradiation. Under both conditions (IIa) is converted into (I) and (IIb) into stilbene and phenanthrene. The stereospecificity of the non-pericyclic conversions of (IIa and b) is remarked upon.