The importance of conformation in the reactivity of radical cations. Changing configuration at saturated carbon centres

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. 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...

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

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

Arnold¹,

Du²

1994

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“…All of the minima within 4 kJ mol-' of the global minimum are listed. We make the logical assumptions that the optimum conformation for benzylic bond cleavage will have a 90" dihedral angle between the plane of the aryl ring bearing the SOMO and the bond to break, and that the conformations of the neutral molecule and the radical cation are similar, at least with respect to the benzylic bonds (21). Furthermore, cleavage of the benzylic carbonxarbon bond, with formation of the a-oxy carbocation, requires overlap of a nonbonding pair of electrons on the oxygen with the vulnerable carboncarbon anti-bonding orbital (a*) (Fig.…”

Section: Deprotonation Of the Radical Cationmentioning

confidence: 99%

The effect of meta- or para-cyano substitution on the reactivity of the radical cations of arylalkenes and alkanes. Radical ions in photochemistry, Part 34

Arnold¹,

Du²,

Chen³

1995

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The effect of electron-withdrawing substituents, mera-orpnr~l-cyano, on the rcactivity of the radical cation of arylalkenes and alkanes has been determined. The radical cations were generated by single electron transfer (set) to an electron-accepting photosensitizer. Three reactions were studied: (i) the addition of nucleophile to the radical cation of arylalkenes, (ii) cleavage of the benzylic carbon-carbon bond of the radical cation of arylalkanes; and (iii) the deprotonation of the benzylic carbon-hydrogen bond of the radical cation of arylalkanes. The radical cations of 4-(1-pheny1ethenyl)benzonitrile (lb), 3-(1-phenylethenyl)benzonitrile (lc), 4-(2-methoxy-I-phenylethy1)benzonitrile (20). 3-(2-methoxy-I-phenylethy1)benzonitrile ( 2~) .cis-and tr.crrls-5-cyano-2-methoxy-1-phenylindane (60-cis and -rratls), and 6-cyano-3-phenylindene (7B) were generated, by single electron transfer to the lowest excited singlet state of 1,4-dicyanobenzene (3), in acetonitrile-methanol. The radical cations of lb, lc, and 70 react with methanol to yield the nrzri-Markovnikov adducts (20,2c, and 60-cis and 60-rmns). The radical cations of 2 0 . 2~. and 60-rrcrrzs cleave at the benzylic carbonsarbon bond to give products derived from the radical and carbocation fragments. The radical cation of 60-cis deprotonates from the benzylic position with subsequent formation of the diastereomer, 60-rrarls. This behaviour can be explained1 predicted on the basis of the proposed mechanisms for these reactions. Molecular orbital calculations (AM 1) support the conclusions.Key words: photosensitized, electron transfer, radical ions, radicals, molecular orbital calculations (AMI).RCsumC : On a dCterminC I'effet de substituants Clectroattracteur, les groupes rrle'ra-et pcrrcr-cyano, sur la rCactivitC de cations radicaux d'arylalcknes et alcanes. On a gCnCrC les cations radicaux par le transfert d'un seul Clectron i un photosensibilisateur accepteur d'klectron. On a CtudiC trois rCactions : (i) l'addition d'un nuclCophile sur le cation radical des arylalcknes; (ii) le clivage de la liaison carbone-carbone benzylique du cation radical des arylalcanes et (iii) la dCprotonation de la liaison carbone-hydrogkne benzylique du cation radical des arylalcanes. On a gCnCrC les cations radicaux des 4-(1-phCnylCthCny1)benzonitrile (10). 3-(1-phCnylCthCny1)-benzonitrile (lc), 4-(2-mCthoxy-1-phCny1Cthyl)benzonitrile (20). 3-(2-mCthoxy-I-phCnylCthy1)benzonitrile (2c), cis-et r,nr1~-5-cyan0-2-mCthoxy-I -phCnylindane (6b-cis et -fmrls) et 6-cyano-3-phCnylindkne (70) par un transfert d'un seul Clectron j. l'Ctat singulet excite le plus bas du 1,4-dicyanobenzkne (3), en solution dans l'acCtonitrile-mCthano1. Les cations radicaux des composCs lb, l c et 70 rkagissent avec le methanol pour donner des adduits arlri-Markovnikov (2b, 2c et 60-cis et 60-rrarls). Les radicaux cations 2 0 . 2~ et 60-rrans se clivent au niveau de la liaison carbone-carbone benzylique pour donner des produits dCrivCs des fragments radicalaires et carbocationiques. Le cation radica...

“…The most reasonable explanation for lack of efficient bond cleavage in 8 is that back electron transfer to regenerate the ground state is faster. Note that molecular models indicate that there is no particular stereo-electronic inhibition to bond cleavage, as has been observed for some radical cations (22). Finally, as mentioned above, the '~o t e that the authors have the electron "pushing" facts reversed in the rest of the paragraph and in Scheme I1 (19).…”

mentioning

confidence: 77%

Intramolecular electron transfer in the photochemistry of substituted 1-naphthylmethyl esters of benzoic acids

DeCosta¹,

Pincock²

1992

Direct excitation of the esters 5 in methanol solvent leads to rapid intramolecular exciplex formation (kc, = 1 0 '~s -' for X = CH30, Y = CN) with electron transfer from the naphthalene to the benzoate ring. This process dominates the usual fluorescence and reaction of the excited singlet state. The rate of this process can be varied over lo3 by suitable change in the substituents X and Y. The electron-transfer rates can be correlated with the two-parameter Hammett equation: log kc, = 8.48 -l .5u+ + 0.775. For cases where the rate of exciplex formation is slow, the usual homolytic carbon-oxygen bond cleavage occurs from the excited singlet state. The eventual products result from the ion pair since the rate of electron transfer in the radical pair to form the ion pair is considerably faster than the rate of decarboxylation of the benzoyloxy radical. IntroductionThe photochemistry of benzylic compounds with leaving groups (ArCH2-LG) is well known (1) to give products that result from both arylmethyl cations and radicals. The partitioning between these two intermediates is controlled by a variety of factors such as the leaving group, the structure of the aromatic ring including substituent effects, the nature of the excited state (singlet or triplet), and the solvent. In general, these factors are still so poorly understood that a reliable prediction about the products for any new system is impossible.