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

Abstract

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...

Matrix Isolation and Spectroscopic Characterization of Perfluorinatedortho- andmeta-Benzyne

“…All the reaction products were isolated by radial chromatography from the reaction mixture, and characterised by 1 H and 13 C NMR and mass spectrometry. All the dithiane34 precursors of anions 3a – d and 4‐benzylbenzonitrile ( 24 )35 are well known and exhibited physical properties identical to those reported in the literature. 1 H and 13 C NMR spectra were recorded at 400.16 and 100.62 MHz respectively on a 400 spectrometer, and all spectra were reported in δ (ppm) relative to Me 4 Si, with CDCl 3 as solvent at 30 °C.…”

Charge‐Transfer Interactions in 4‐Donor 4′‐Acceptor Substituted 1,1‐Diphenylethenes

“…Investigation of the (steady-state) photophysics of the compounds under study is also relevant for another reason: although the photophysics of 1,1-diphenylethene is well documented, since the compound is frequently employed in photochemical studies, [24,25] knowledge of the photophysics and photochemistry of substituted 1,1-diphenylethenes is less well founded. [26] It is shown here that 1D2 exhibits intramolecular CT emission, and the nature of the emitting state is discussed.…”

“…This shows that the 1,1-diphenylethene moiety does not behave as a simple spacer. In addition to its already known electrondonating behaviour [26,43] it also exhibits electron-accepting behaviour.…”

“…Interestingly, 1,1-diphenylethene was previously found to behave as an electron donor in intermolecular photoinduced electron transfer reactions. [26,43] The redox activity of the 1,1-diphenylethene-based unit thus comprises both electron-donating and electron-accepting functions.…”

Radical Cation Fragmentation Reactions in Organic Synthesis

“…7 Substitution of the aromatic ring was also shown to be an important factor in determining the facility of bond cleavage (Scheme 3.5). 8 Cyano-substituted ether 7 cleaved under oxidizing conditions, while methoxy-substituted ether 9 did not. These results are contrary to the ease of radical cation formation, but are consistent with fragmentation proceeding more readily from nonstabilized radical cations.…”