Photochemical Dimerization of 2-Acyl-1,4-benzoquinones in the Presence of Rose Bengal

Article abstract of DOI:10.1021/jo01296a016

Irradiation of 2-acyl-1,4-benzoquinone 1 in the presence of rose bengal afford the quinone dimer 2-acyl-3-(3-acyl-4-hydroxyphenoxy)-1,4-benzoquinone 2 in good yields.This type of dimer was quite different from another dimer 3, which was obtained in the photoreaction of 2-acyl-1,4-benzoquinone 1 in the absence of rose bengal.In this photoreaction in the presence of rose bengal, the excited state of rose bengal played a very imprtant role.However, singlet oxygen generated by rose bengal sensitization is not involved in the photodimerization reaction of quinones.On the other hand, 2-acyl-1,4-benzoquinone 1 can quench the rose bengal sensitized photooxygenation of α-pinene and simultaneously dimerize to give the same dimer 2 mentioned above.In addition, photolysis of the acylquinoline 1 in a heterogeneous system using rose bengal coated silica gel as sensitizer also afford the same dimer obtained in the homogeneous system in comparable yields.