Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction, Part 12. Factors controlling the regiochemistry of the reaction with alcohol as the nucleophile

Abstract

The photo-NOCAS reaction that combines methanol, serving as the nucleophile, and the radical cation of 4-methyl-1,3-pentadiene (14+•), substituting on the 1,4-dicyanobenzene radical anion (1−•), yields (E)-1-(4-cyanophenyl)-4-methoxy-4-methyl-2-pentene (15) as the major product. This regioisomer arises from bonding of methanol to C-4, the more heavily alkyl-substituted carbon of the diene, giving the less alkyl-substituted allylic radical. All previous examples of the photo-NOCAS reaction have yielded major adduct(s) having regiochemistry consistent with the anti-Markovnikov rule; the more heavily substituted (more stable?) β-alkoxyalkyl radical was the predominant intermediate. Empirically derived heats of formation and high-level ab initio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) provide convincing evidence that of the two alternative allylic radicals, generated upon addition of methanol to 14+•, that which has the more alkyl substituted allylic radical moiety is, in fact, not the more stable. Of course, the total structure of the intermediate must be considered; the stabilizing effect of alkyl substitution on the carbon next to the oxygen of the ether moiety cannot be ignored. Ab initio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) are reported for the radical cations of 2-methylpropene (2+•), 2-methyl-2-butene (6+•), 2-methyl-1,3-butadiene (9+•), 4-methyl-1,3-pentadiene (14+•), and 2,4-dimethyl-1,3-pentadiene (18+•) Calculations were also carried out on possible intermediates (bridged radical cations, distonic radical cations, and β-alkoxyalkyl radicals) involved upon reaction of these radical cations with methanol. Results of these calculations provide a basis for explaining/predicting the regiochemistry of the photo-NOCAS reaction involving methanol as the nucleophile: the major adduct(s) result(s) from attachment of methanol to that end of the alkene or diene which gives rise to the more stable intermediate radical. The more stable radical is not necessarily the more heavily alkyl substituted. Key words: photoinduced electron transfer, radicals, radical cations, ab initio molecular orbital calculations.