Q1(g). Photochemical cleavage and alkene isomerization

Answer in brief

(i) Norrish-I cleavage gives an unsaturated aldehyde and, after CO loss, ring-contracted and acyclic hydrocarbons; (ii) alkene excitation gives an E/Z photostationary mixture.

Key points

• α-Cleavage of the excited cyclic ketone produces the ring-opened acyl/alkyl biradical •CO–CH(Me)–(CH₂)₃–CH•(Me).
• Intramolecular H transfer to the acyl radical gives 2-methylhept-5-enal, OHCCH(Me)CH₂CH₂CH=CHMe, as E/Z products.
• Decarbonylation gives a 1,5-biradical; ring closure yields cis/trans-1,2-dimethylcyclopentane and disproportionation yields hept-2-ene, with CO as coproduct.
• π–π* excitation of the Z alkene reduces its bond order; rotation through a twisted excited state and relaxation give E as well as recovered Z alkene.