Photosensitized (electron transfer) carbon-carbon bond cleavage of radical cations

Self Cite

. Can. J . Chem. 70, 272 ( 1992). Irradiation of an acetonitrile solution of cis 1-methyl-2-phenylcyclopentane ( l b cis); 1,4-dicyanobenzene (2), an electronaccepting photosensitizer; and 2,4,6-collidine (3), a nonnucleophilic base, leads to configurational isomerization of the cyclopentane; the photostationary state lies >99% in favour of the trans isomer. The mechanism proposed for this reaction involves formation of the radical cation of l b cis by photoinduced electron transfer to the singlet excited state of 2, deprotonation of the radical cation assisted by the base 3, reduction of the resulting benzylic radical by the radical anion 2;, and reprotonation of the benzylic anion to give both the cis and the tratls isomers of lb. The photostationary state is controlled by the relative rates of deprotonation of the radical cations of l b cis and trans; these rates are dependent upon the extent of overlap of the SOMO of the radical cation, which is largely associated with the phenyl ring, and the benzylic carbon-hydrogen bond. Molecular mechanics calculations (MM3 and MMP2) are used to calculate the preferred conformations of the isomers. The required orbital overlap is 3 1 % effective with the global minimum conformation of the cis isomer and essentially ineffective for the low-lying conformations of the trcrns isomer. This proposed mechanism is supported by Stern-Volmer quenching studies, which indicate that both isomers quench the singlet excited state of 2 at the diffusion-controlled rate, and by deuterium incorporation studies. When irradiation of the cis isomer is carried out in acetonitrile-methanol-0-d as solvent, isomerization is accompanied by deuterium exchange at the benzylic position; the trat~s isomer is stable under these conditions.Key words: photosensitized electron transfer, radical cation, deprotonation, configurational isomerization, conformation, molecular mechanics (MM3). conduit a une isomerisation configurationnelle du cyclopentane; 1'Ctat photostationnaire favorise I'isorqere trans i plus de 99%. Le mecanisme propose pour cette reaction implique la formation du cation radicalaire du produit l b cis par le biais d'un transfert d'electron photo-induit vers 1'Ctat excite singulet du produit 2, une deprotonation du cation radicalaire assistee par la base 3 , la reduction, par I'anion radicalaire 2;; du radical benzylique qui en resulte et un reprotonation de l'anion benzylique qui fournit les isomeres cis et trans du produit l b . L'etat photostationnaire est contr61C par les vitesses relatives de deprotonation des cations radicalaires des produits l b cis et trans; ces vitesses dependent du degre de recouvrement de la liaison carbone-hydrogene benzylique et des orbitales molCculaires SO du cation radicalaire qui sont trks associkes au noyau phknyle. On a fait appel a des calculs de mkcanique molCculaire (MM3 et MMP2) pour calculer les conformations privilCgiCs des isomkres. Le recouvrement orbitalaire requis est efficace a 3 1% dans la conformation globale minimale de I'isomkre cis et est ...

Section: Methodsmentioning

confidence: 99%

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

Perrott¹,

Arnold²

1992

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“…Oxidation potentials were measured by cyclic voltammetry using an apparatus similar to that previously described (23). The working electrode was a platinum sphere (1 mm diameter), while the counter electrode was a platinum wire.…”

Section: Cyclic Voltammetric Measurementsmentioning

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...

“…We also have recently pointed out that all of the thermochemical cycles describing the homolytic and heterolytic cleavage reactions of molecules and their radical ions can be conveniently represented in a thermochemical mnemonic (Scheme 1, eqs. 111- [7]) (17). In this mnemonic the vertical arrows represent the loss of R' as a radical (i.e., the homolytic bond dissociation energy).…”

Section: Introductionmentioning

confidence: 99%

“…[I]) with the difference between two potentials (eqs. [2]- [7]). It is important to keep in mind that an entropy correction must be applied to the bond dissociation enthalpies that are usually reported in the literature since the redox potentials are actually free energy differences.…”

Section: Introductionmentioning

confidence: 99%

Substituent and charge distribution effects on the redox potentials of radicals. Thermodynamics for homolytic versus heterolytic cleavage in the 1-naphthylmethyl system

Milne¹,

Wayner²,

DeCosta³

et al. 1992

Can. J. Chem. 70, 121 (1992). The electrochemical oxidation and reduction potentials of a number of substituted 1-methylnaphthalenes (la-1) and 1-naphthylmethyl radicals (2a-1') as well as 2-methylnaphthalene (3) and the 2-naphthylmethyl radical (4') have been measured by cyclic voltammetry and photomodulation voltammetry. The oxidation potentials correlate with a+ (p+ = -7.1 and -8.4 for 1 and 2' respectively) while the reduction potentials correlate with a (p-= 10.1 and 13.0 for 1 and 2' respectively). The relative magnitude of the p values can be rationalized when the charge density distribution in these systems is considered. This leads to the interesting conclusion that even though a full charge is placed in the IT-system of 1 when it is oxidized or reduced, the fraction of the charge that accumulates at C, is actually less than in 2+ or 2-where only 50-70% of the charge is delocalized into the ring. A correlation between p for the redox reactions of 1 , 2', benzyl, diphenylmethyl, and cumyl and the calculated (AMI) charge density at C, is established, implying that the sensitivity of the corresponding ions to substituent effects increases as the fraction of charge at that site increases. The redox data have been used in thermochemical cycles in order to estimate the substituent effect on the homolytic, mesolytic, and heterolytic cleavage reactions of 1 and its corresponding radical ions. The implication of these results on the C-C cleavage versus deprotonation of radical cations and on the photochemical homolysis versus heterolysis of naphthylmethyl halides and-acetates is discussed.