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 nucleophileeolefin 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-2-01 (17). and the aromatic, 1,4-dicyanobenzene (I), leads to cyclized 1:l (alk-4-eno1: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-(isopropy 14-cyanopheny I)-5-methyltetrahydrofuran (18,29%) and cis-2-(isopropyl 4-cyanopheny1)-5-methyltetrahydrofuran (19,24%) from 16 (reaction [5j), and r-2-(methyl4-cyanopheny1)-2,t-5-dimethyltetrahydrofuran (20, 13%), r-2-(methyl4-cyanophenyl)-2,~-5-dimethyltetrahydrofuran (21,7%), r-3-(4-cyanophenyl)-3,t-6-dimethyltetrahydropyran (22, 11 %), and r-3-(4-cyanopheny1)-3,c-6-dimethyltetrahydropyran (23,2%), from 17 (reaction [6j), rests mainly upon analysis of the 'H 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-en01 radical cation. The mechanistic implication of these results is discussed.Key words: photoinduced electron transfer, radical ions, cyclization of radical cations, intramolecular reactions of radical cations.

Direct Catalytic Anti-Markovnikov Hydroetherification of Alkenols

“…To date, however, this method remains significantly limited in scope and requires nearly stoichiometric quantities of the photooxidant that can often result in oxidant incorporation into the reaction products as well as undesired side reactivity. 16 Truly catalytic photosensitized anti-Markovnikov alcohol additions are limited to 1,1-diarylethylenes. 17–19 …”

“…To date, however, this method remains significantly limited in scope and requires nearly stoichiometric quantities of the photooxidant that can often result in oxidant incorporation into the reaction products as well as undesired side reactivity. 16 Truly catalytic photosensitized anti-Markovnikov alcohol additions are limited to 1,1-diarylethylenes. 17−19 After analysis of this body of literature, we proposed that the critical step preventing the development of catalytic protocols was the fate of putative radical intermediate 9.…”

“…Additionally, Gassman and Arnold have each demonstrated that single electron photooxidants can serve as effective single electron oxidants to access reactive olefin cation radicals ( 7 ). To date, however, this method remains significantly limited in scope and requires nearly stoichiometric quantities of the photooxidant that can often result in oxidant incorporation into the reaction products as well as undesired side reactivity . Truly catalytic photosensitized anti-Markovnikov alcohol additions are limited to 1,1-diarylethylenes. − …”

The Aromatic Carbon–Carbon ipso‐Substitution Reaction

“…[43] A further variation involved the presence of O-nucleophiles tethered to the alkene moiety as in w-alkenols; as a result, a cyclization ensued the initial ET step to yield aryl-substituted tetrahydropyrans or tetrahydrofurans by means of a tandem ArÀC, CÀO bond formation. [44] Despite their high electron-donor potentials, amines or ammonia can be used to some extent in place of alcohols. [45] To have a successful reaction, however, a cosensitizer (e.g., a triphenylbenzene) has to be used, and photoamination of alkadienes gave the corresponding 4-(4-aminobut-2-enyl)benzonitriles in a satisfying yield.…”