The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 10: intramolecular reactions involving alk-4-enols and 1,4-dicyanobenzene

Abstract

Our study of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include alk-4-enols. Irradiation of acetonitrile solutions of the alk-4-enols, 6-methyl-5-hepten-2-ol (16) and 5-methyl-5-hexen-2-ol (17), and the aromatic, 1,4-dicyanobenzene (1), leads to cyclized 1:1 (alk-4-enol:aromatic) adducts. The addition of biphenyl (5) to the irradiation mixture, serving as a codonor, increases the yields and the efficiency of formation of the adducts. The structures assigned to the products trans-2-(isopropyl 4-cyanophenyl)-5-methyltetrahydrofuran (18, 29%) and cis-2-(isopropyl 4-cyanophenyl)-5-methyltetrahydrofuran (19, 24%) from 16 (reaction [5]), and r-2-(methyl 4-cyanophenyl)-2,t-5-dimethyltetrahydrofuran (20, 13%), r-2-(methyl 4-cyanophenyl)-2,c-5-dimethyltetrahydrofuran (21, 7%), r-3-(4-cyanophenyl)-3,t-6-dimethyltetrahydropyran (22, 11%), and r-3-(4-cyanophenyl)-3,c-6-dimethyltetrahydropyran (23, 2%), from 17 (reaction [6]), rests mainly upon analysis of the 1H and 13C nuclear magnetic resonance spectra. The structures of 19, 22, and 23 were firmly established by X-ray crystallography. The observed ratio of regioisomers indicates a strong preference for 1,5-exo-trig, relative to 1,6-endo-trig, cyclization of the intermediate alk-4-enol radical cation. The mechanistic implication of these results is discussed. Keywords: photoinduced electron transfer, radical ions, cyclization of radical cations, intramolecular reactions of radical cations.