Isoprene, photosensitized dimerization

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

The photosensitized dimerization of isoprene is considerably more complex than that of butadiene, yielding cyclobutanes (14)—(16) as well as four dimers of noncyclobutane types<9 11,18) ... 

The major products from the photosensitized dimerization of isoprene predicted. The ratio of diene dimers vary as the sensitizer is varied or not ... 

The dimerization of isoprene has been accompHshed by methods other than heating. Thus isoprene has been dimerized by uv radiation in the presence of photosensitizers to give a complex mixture of cyclobutane, cyclohexene, and cyclooctadiene derivatives (36,37). Sulfuric acid reportedly... 

Very early in the study of photosensitization it was discovered that salicylaldehyde, 2,4-dihydroxybenzophenone, and 2-hydroxy-4-methoxy-benzophenone do not sensitize piperylene isomerization, indicating that enolization is much faster than quenching by the diene.37 Later it was shown that 2-hydroxybenzophenone would sensitize the dimerization of isoprene in concentrated solution, but that the reaction was much less efficient than when benzophenone was used as a sensitizer (Table I).36... 

Conjugated dienes are dimerized by irradiation in the presence of various photosensitizers. Structural assignments have been made to the dimers of 1,3-butadiene, isoprene, - and cyclopentadiene. Formally, all of the observed products are consistent with the addition of triplet diene to another molecule to form diallylic biradicals [see reactions (13)]. 

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