The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 5: methanol–monoterpenes (α- and β-pinene, tricyclene, and nopol), 1,4-dicyanobenzene

Abstract

. Can. J. Chem. 72,403 (1994).The reactivity of the radical cations of a-and P-pinene (8 and 9), tricyclene (la), and nopol (23) has been studied. The radical ions were generated, in acetonitrile-methanol (3:1), by single electron transfer (set) to the singlet excited state of 1,4-dicyanobenzene (1). Biphenyl (3) was used as a codonor. The cyclobutane rings of the initially formed radical cations of a-and P-pinene (8' ' and 9") cleave to distonic radical cations that react as tertiary alkyl cations and allylic radicals. The results of a b initio molecular orbital calculations (STO-3G) are consistent with the observation that the positive charge is largely associated with the tertiary alkyl moiety while the spin density is largely distributed over the allylic radical. There was no evidence, experimental or theoretical, indicative of a bonding interaction between the cationic and allylic moieties of these distonic radical cations. The cyclobutane ring cleavage is irreversible. The radical cation of tricyclene (18) also gives 1 : 1: 1 (nucleophile:cyclopropy1:aromatic) adducts. The regio-and stereochemistry of the adducts, and the nucleophile selectivity, are all indicative of nucleophile-assisted cleavage of the radical cation (18"). The radical cation of nopol (23") also cleaves to the distonic radical cation. The cyclobutane ring cleavage must be rapid; intramolecular 1,5-endo cyclization of the hydroxyl group cannot compete. DONALD R. ARNOLD et XINYAO Du. Can. J. Chem. 72,403 (1994).On a CtudiC la rCactivitt des cations radicalaires des a-et P-pinbnes (8 et 9), du tricyclbne (18) et du nopol (23). On a gtntrts les ions radicalaires dans un mtlange acttonitrile-mtthanol(3 : 1) par un transfert d'un seul tlectron (set) 2i l'ttat excitt singulet du 1,4-dicyanobenzbne (1). On a utilist le biphtnyle (3) comme codonneur. Les noyaux cyclobutane des cations radicalaires qui se foment initialement B partir des a-et P-pinbnes (8'' et 9") se clivent en cations radicalaires distoniques qui rtagissent comme cations alkyles tertiaires et radicaux allyliques. Les rtsultats de calculs d'orbitales molCculaires a b initio (STO-3G) sont en accord avec l'observation que la charge positive est associte largement avec la portion alkyle tertiaire alors que la densitt de spin est largement distribuke sur le radical allylique. 11 n'y a pas d'indications, soit exptrimentale ou thtorique, pour suggtrer l'existence d'une interaction de liaison entre les portions cationiques et allyliques de ces cations radicalaires distoniques. Le clivage du noyau cyclobutane est irrtversible. Le cation radicalaire du tricyclbne (18) donne aussi des adduits 1 : 1 : 1 (nuclCophile : cyclopropyle : aromatique). La rtgio-et la sttrtochimie des adduits et la stlectivitt du nucltophile indiquent toutes que le clivage du cation radicalaire (18") est assist6 par le nucltophile. Le cation radicalaire du nopol (23") se clive aussi en cation radicalaire distonique. Le clivage du noyau cyclobutane doit Etre rapide car la cyclisation intramoltculaire 1,5-endo du grou...

Organic Photoredox Catalysis

“…In general, it was found that the presence of Ph–Ph in the reaction increased the yields of the deconjugated adducts. This is intriguing as the oxidation potential of Ph–Ph [ E ox (Ph-Ph • + /Ph-Ph) = +1.85 V vs SCE] is below that of nearly all of the substrates investigated. The authors speculate that Ph–Ph plays a crucial role in limiting back electron transfer processes that may hamper reactivity.…”

Triphenylpyrylium Tetrafluoroborate-Sensitized Photochemistry of the Terpenes Sabinene, α-Phellandrene, and α- and γ-Terpinene

“…[23][24][25][26][27] As a result, appropriate donor molecules may undergo a net conversion amounting to ''substitution'' of an activated H atom by a p-cyanophenyl group. With DCB as sensitizer, the geminate radical anion is a poor base as well as a poor hydrogen-atom acceptor; however, it readily undergoes aromatic substitution with loss of cyanide ion.…”

“…The radical cations of strainedring terpenes were generated with 1,4-dicyanobenzene/ phenanthrene as sensitizer/co-sensitizer in the presence of 5  methanol as nucleophile. [24,25] The terpenes studied under these conditions include cyclopropane derivatives (tricyclane), [26] dicyanobenzene (DCB). [23][24][25] Details of the reaction sequence leading to the threecomponent products are particularly well-established for olefins; this reaction is known as the photo-NOCAS reaction (for ''photo-induced nucleophile-olefin-combination-aromatic-substitution'').…”

Photochemistry of Cyclobutanes: Synthesis and Reactivity

“…Me containing molecules such as α-and β-pinene and nopol (505) 329 . The use of DCB(1,4-dicyanobenzene) as the electron-accepting sensitizer converts the two pinenes into the cation radicals shown in Scheme 14.…”