Photochemistry of s-cis acyclic 1,3-dienes

Article abstract of DOI:10.1021/ja00170a019

We have examined the photochemistry of several s-cis acyclic 1,3-dienes and found that they close to cyclobutenes at 15 K. Extensive investigation of (E,E)-1,4-dideuterio-2,3-dimethyl-1,3-butadiene (DDMB) has shown that there are two pathways to thermal double bond isomerization: the first through an intermediate cyclobutene with a barrier of 48 kcal/mol and the second through an allylmethylene biradical with a barrier of 55 kcal/mol. Examination of the photochemistry of s-cis-DDMB shows for the first time that the s-cis conformer undergoes both electrocyclic closure and double bond isomerization. An excited-state surface is postulated that combines these two photochemical events and includes a concerted nonsynchronous disrotatory closure. This surface also allows prediction of the partitioning ratio between the two photochemical events.