Radical ions in photochemistry. 12. The photoaddition of olefins to cyano aromatic compounds in polar solvents

Arnold, Donald R.; Wong, P. C.; Maroulis, A. J.; Cameron, T. Stanley

doi:10.1351/pac198052122609

Cited by 66 publications

(26 citation statements)

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“…These include, in addition to tautomerization of the alkene (2): configurational isomerization (E-Z) of the alkene (7); cyclodimerization of the alkene, forming either cyclobutane or, in the case of conjugated alkenes, cyclohexene dimers (8,9); cycloaddition of the alkene to the aromatic ring at the 1,2-, 1,4-, or 2,4-position (lo), or at the cyano group (1 1); addition of nucleophile to the alkene radical cation followed either by addition of the resulting radical to the cyanoaromatic radical anion to give the substituted dihydroaromatic (12), or by substitution of the resulting radical for the cyano substituent on the ring (the photo-NOCAS reaction) (la, 13); substitution of the allylic radical for the cyano substituent on the aromatic ring (14); and, anti-Markovnikov addition of hydroxylic solvent to the alkene with the cyanoaromatic acting as a photosensitizer (15). The problem of understanding this complex behaviour is exacerbated by.…”

Section: Discussionmentioning

confidence: 99%

Radical ions in photochemistry. 21. The photosensitized (electron transfer) tautomerization of alkenes; the phenyl alkene system

Arnold¹,

Mines²

1989

Can. J. Chem.

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. MINES. Can. J. Chem. 67, 689 (1989). Alkenes, conjugated with a phenyl group, can be converted to nonconjugated tautomers by sensitized (electron transfer) irradiation. For example, irradiation of an acetonitrile solution of the conjugated alkene 1-phenylpropene, the electron accepting photosensitizer 1,4-dicyanobenzene, the cosensitizer biphenyl, and the base 2,4,6-trimethylpyridine gave the nonconjugated tautomer 3-phenylpropene in good yield. Similarly, 2-methyl-1-phenylpropene gave 2-methyl-3-phenylpropene, and l-phenyl-1-butene gave E-andZ-1-phenyl-2-butene. The reaction also works well withcyclic alkenes. For example, 1-phenylcyclohexene gave 3-phenylcyclohexene, and 1-(phenylmethylene)cyclohexane gave 1-(phenylmethyl)cyclohexene. The proposed mechanism involves the initial formation of the alkene radical cation and the sensitizer radical anion, induced by irradiation of the sensitizer and mediated by the cosensitizer. Deprotonation of the radical cation assisted by the base gives the ambident radical, which is then reduced to the anion by the sensitizer radical anion. Protonation of the ambident anion at the benzylic position completes the sequence. Reprotonation at the original position is an energy wasting step. Tautomerization is driven toward the isomer with the higher oxidation potential, which is, in the cases studied, the less thermodynamically stable isomer. The regioselectivity of the deprotonation step is dependent upon the conformation of the allylic carbon-hydrogen bond. The tautomerization of 2-methyl-1-phenylbutene gave both 2-phenylmethyl-1-butene and 2-methyl-1-phenyl-2-butene (E and Z isomers), while 2,3-dimethyl-1-phenylbutene gave only 3-methyl-2-phenylmethyl-1-butene. In the latter case, steric interaction of the methyls on the isopropyl group prevents effective overlap of the tertiary carbon-hydrogen bond with the singly occupied molecular orbital, thus inhibiting deprotonation from this site.Key words: photosensitized, electron transfer, alkene, tautomerization, radical cation. , on peut transformer des alcknes conjuguCs avec un groupement phCnyle en tautomkres qui ne le sont pas. Par exemple, l'irradiation du phCnyl-1 propkne, un alckne conjuguC, en solution dans I'acCtonitrile, en prCsence de dicyano-1,4 benzkne agissant comme photo-sensibilisateur acceptant des electrons, de biphCnyle agissant comme cosensibilisateur et de trimCthy1-2,4,6 pyridine agissant comme base, fournit de phCnyl-3 propkne, le tautomkre qui n'est pas conjuguCe, avec un bon rendement. De la m&me manikre, le methyl-2 phCnyl-1 propkne fournit du mCthyl-2 phCnyl-3 propkne alors que le phCnyl-1 butkne-1 conduit i un melange des phCnyl-1 butknes-2-(E) et -(a. La rCaction donne aussi de bons rtsultats avec les alcknes cycliques. Par exemple, le phinyl-1 cyclohexkne fournit du phCnyl-3 cyclohexkne alors que le (phCny1mCthylkne)-1 cyclohexane conduit au (phCnylmCthy1)-1 cyclohexene. Le micanisme proposC implique la formation initiale du cation radical de l'alckne et l'anion radical du sensibilisateur qui est induit...

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Section: Discussionmentioning

confidence: 99%

Radical ions in photochemistry. 21. The photosensitized (electron transfer) tautomerization of alkenes; the phenyl alkene system

Arnold¹,

Mines²

1989

Can. J. Chem.

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“…2Kz = 0 and K , > 0 represent a borderline case for which the following relations hold: For I , < I , the steady state concentrations for R 1 = tert-butyl and R, = phenoxyl are + co (8) 2R,-P,…”

Section: Tert-b Utyl/246-tri-tert-butylphenoxylmentioning

confidence: 99%

Unusual selectivities of radical reactions. Experimental studies on alkyl versus phenoxyl reaction systems

Rüegge

Fischer

1989

Int J of Chemical Kinetics

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Radical reactions involving two or more intermediates and many mutual reaction channels may lead to the specific formation of one cross-reaction product if one species is rather persistent and if transient and persistent species are produced with equal rates. A previous kinetic analysis of the phenomenon revealed that the concentrations of the intermediates and the selectivities of product formation depend critically on the relative initiation rates. The present experimental ESR studies on systems involving simultaneously generated phenoxyl radicals as persistent and alkyl radicals as transient species confirm the predicted behavior. They also lead to absolute rate constants for reactions of alkyl with phenoxyl radicals and for hydrogen atom transfer from phenols to alkyl radicals.

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“…The photo-NOCAS reaction could conceivably yield eight isomeric adducts with 2-methylnorbornene; six have been detected (gctms) and the five major products, those formed in greater than 1 % yield, were isolated and characterized (Table 1, reactions [20]- [24]). Neither of the sterically hindered isomers, those having both the methoxy and the 4-cyanophenyl groups in the endo position, were among the isolated products.…”

Section: Resultsmentioning

confidence: 99%

“…The results support the hypothesis that coupling occurs at the position of greatest spin density. This is also the position that minimizes the localization energy in the resulting anion (2,20).…”

Section: Direct and Photosensitized Irradiationsmentioning

confidence: 99%

The photochemical nucleophile–olefin combination, aromatic substitution reaction (Part 2): methanol – cyclic olefins, 1,4-dicyanobenzene

Arnold¹,

Snow²

1988

Can. J. Chem.

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Direct irradiation of acetonitrile-methanol (3:l) solutions of 1,4-dicyanobenzene and the cyclic olefins, cyclohexene, 1-methylcyclohexene, norbornene, and 2-methylnorbornene, leads to formation of regio-and stereoisomers of the 1:l:l (alcohol:olefin:aromatic) adducts. This reaction can be photosensitized by electron transfer; addition of electron donors, biphenyl or phenanthrene, to the irradiation mixture generally increases the efficiency and yield of adduct formation. The efficiency of the reaction and the ratio of isomeric adducts are also affected by the addition of salts, particularly magnesium perchlorate. All of the possible regio-and stereoisomers from cyclohexene and I -methylcyclohexene have been identified, two from cyclohexene and four from 1-methylcyclohexene. Three of the four possible isomers from norbornene were characterized; the endo,endo isomer was not detected. There are eight possible isomers from 2-methylnorbornene; six were detected and five have been isolated and identified. The two sterically hindered isomers, those having both the 4-cyanophenyl and the methoxy groups in the endo position, and exo-3-(4-cyanophenyl)-endo-2-methoxy-exo-2-methylnorbornane, were not characterized. The structures of the products were established largely on the basis of the 'H and I3C nuclear magnetic resonance spectra. The mechanism of the reaction is discussed, with emphasis on those factors that may affect the product ratio. The most striking observation is that the reaction is regioselective when magnesium perchlorate is added to the irradiation mixture.

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Radical ions in photochemistry. 12. The photoaddition of olefins to cyano aromatic compounds in polar solvents

Cited by 66 publications

References 0 publications

Radical ions in photochemistry. 21. The photosensitized (electron transfer) tautomerization of alkenes; the phenyl alkene system

Radical ions in photochemistry. 21. The photosensitized (electron transfer) tautomerization of alkenes; the phenyl alkene system

Unusual selectivities of radical reactions. Experimental studies on alkyl versus phenoxyl reaction systems

The photochemical nucleophile–olefin combination, aromatic substitution reaction (Part 2): methanol – cyclic olefins, 1,4-dicyanobenzene

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