Photocycloaddition of Alkenes (Dienes) to Dienes ([4þ 2]/[4þ 4])

Among the earliest photochemical reactions was photodimerization of anthracene 1 to give dimer 2 (Sch. 1), reported by Fritzsche in 1867 [1-3]. While not technically a reaction of 1,3-dienes, it was the first [4þ4] cycloaddition reaction, and predates the first [2þ2] photocycloaddition by a decade [3]. More than fifty years would pass before investigations of 1,3diene photochemistry would be undertaken, and cycloaddition reactions were not reported until the early 1960s. The initial investigations, mercurysensitized reactions of dienes, reported fragmentations [4]. Seminal investigations by Srinivasan at the University of Rochester in 1960 [5] and they by Hammond at the California Institute of Technology [6] led to the first examples of photo-[2þ2], [4þ2] and [4þ4] cycloadditions by 1,3-dienes. Continuing studies of diene photochemistry have refined our understandings of the interrelated aspects of the chemistry that include conformation, exiplex and intermediate lifetimes. Several reviews have concentrated on 1,3-diene photochemistry [7-9].