Photochemical Electrocyclic and Addition Reactions

Photochemical-concerted electrocyclic reactions of alkenes and dienes are symmetry allowed processes. For alkene system, the HOMO (SOMO) is the excited alkene 7i orbital and the LUMO is the n of ground-state alkene. The interactions of HOMO and LUMO produce two new o bonds in the product, cyclobutane. 

Similarly, photochemical reaction of 1,3-butadiene into cyclobutene and its reverse process are symmetry allowed processes in disrotatory mode of cyclization and opening of ring. 

Experimental results on the photochemical reaction of cyclobutene to butadiene ring opening indicated that the reaction is not straightforward concerted. The ring opening is accompanied by fragmentation products, acetylene and ethylene and rearrangement to methylene cyclopropane [12]. Similarly, trans- and cis- 

4- dimethylcyclobutenes 6 on direct irradiation give mixtures of three isomeric 

4- hexadienes in different ratios [13]. The photochemical cyclization of butadiene also gives a mixture of products [14]. 

---