Photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction: Methanol, β-myrcene, and 1,4-dicyanobenzene. Intramolecular cyclization of an ene-diene radical cation

Article abstract of DOI:10.1139/v98-156

The photochemical nucleophile-olefin combination, aromatic substitution (photo-NOCAS) reaction of methanol, 7-methyl-3-methylene-1,6-octadiene (β- myrcene, 1), and 1,4-dicyanobenzene yields five 1:1:1-adducts: cis-2(4- cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (15), trans- 2-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylenecyclohexane (16), 1- (4-cyanophenylmethyl)-4-(1-methoxy-1-methylethyl)cyclohexene (17), 4[4- methoxy-3,3-dimethylcyclohex-(E)-1-ylidenyl]methylbenzonitrile (18), and 4- (1-vinyl-4-trans-methoxy,3,3-dimethylcyclohexyl)benzonitrile (19). All of these adducts are cyclic; variation in the product ratio as a function of methanol concentration indicates cyclization is occurring, 1,6-endo, with both the initially formed radical cation and with the intermediate β- alkoxyalkyl radicals. Evidence based upon comparison of the ionization and oxidation potential of β-myrcene with model alkenes and with conjugated dienes indicates the initial electron transfer involves the trisubstituted mono alkene moiety; the diene moiety, mono-substituted at a nodal position, has a higher oxidation potential. High-level ab initio molecular orbital calculations (MP2/6-31G(*)//HF/6-31G(}) provide useful information regarding the nature (relative energies and charge and spin distribution) of the intermediate radical cations, which supports the proposed reaction mechanism.