[2+2] Heterodimers of methyl phenanthrene-9-carboxylate and benzene

Both syn- and anti-[2+2] heterodimers (1 and 2) of methyl phenanthrene-9-ca rboxylate (9-MP) and benzene were synthesized from 9-MP and cyclohexa-1,4-d iene. Upon irradiation of 9-MP and excess methyl orthoformate of cis-cycloh exa-3,5-diene-1,2-diol followed by hydrolysis, the syn-[2+2] cycloadduct an d also four cyclodimers of 9-MP were produced. Triplet-sensitized photocycl oaddition followed by hydrolysis provided the anti-[2+2] cycloadduct. Dehyd roxylation of the cycloadducts under mild conditions gave 1 and 2. In contr ast to the previous reports on [2+2] dibenzenes and [2+2] heterodimers of a cenaphthylenes and benzene, kinetic analysis of the thermolyses of 1 and 2 indicated that 1 was kinetically less stable than 2. Photodissociation of 1 and 2 were found to be adiabatic with efficiencies of 0.33 and 0.39. An ex planation is provided for their thermoreversion and photoreversion.