99334-85-7Relevant academic research and scientific papers
Radical ions in photochemistry. Carbon-carbon bond cleavage of radical cations in solution: Theory and application
Popielarz,Arnold
, p. 3068 - 3082 (2007/10/02)
The cleavage of radical cations of two series of alkanes, 1,1,2-triaryl- and 1,1,2,2-tetraarylalkanes, generated by photoinduced single electron transfer in acetonitrile-methanol, occurs with formation of radical and carbocation fragments. The radical cations of some unsymmetrically substituted alkanes cleave to give all four of the possible products, two hydrocarbons emanating from the radicals and two methyl ethers from the carbocations, in proportion to the oxidation potentials of the two possible radical fragments. There is an excellent linear correlation between the logarithm of the observed ratio of products and that calculated from the reported electrochemically determined oxidation potentials (r = 0.998, 5 points). The proportionality constant (1.27) for this relationship is close to unity which indicates that the product ratio is determined by the relative rates of cleavage in the two possible modes or by equilibration of the radicals and carbocations before separation of the geminate radical carbocation pair and not by equilibration upon reencounter of freely solvated radical and carbocation fragments. The effect of temperature on the relative oxidation potentials of the radicals studied is small and can be neglected when radicals of the same order (i.e., both secondary or both tertiary) and of similar size are compared. The ratio of products obtained upon cleavage of the radical cation at 25 °C can be used to determined standard oxidation potentials of radicals. The oxidation potential of the diphenylmethyl radical (0.350 V vs SCE) has been accepted as the primary standard and the (4-methyl-phenyl)phenylmethyl (0.265 V) and bis(4-methylphenyl)methyl (0.188 V) radicals are established as secondary standards against which the oxidation potentials of other radicals can be measured. Oxidation potentials of several 4-substituted cumyl radicals have been determined by this photochemical method. There is a good (r = 0.987, 5 points) linear correlation between the measured oxidation potentials and the σ+ substituent constants. The reaction constant is appropriately negative and large (p = -6).
Photochemically Generated Ion and Radical Pairs. Self-Destructive Charge-Transfer Complexes
Maslak, Przemyslaw,Chapman, William H.
, p. 6334 - 6347 (2007/10/02)
Irradiation of CT complexes between 4-methoxy-4'-X-bicumenes, 1a-e (X = H, a; OMe, b; Me, c; CF3, d; CN,) and tetranitromethane (2) results in efficient generation of radical and ion pairs (Φ ca. 0.20).The identity of the final products depends on the dynamics of these pairs, which in turn is dramatically influenced by substitution and solvent.In all cases, 2*- dissociates rapidly ( - (4-) and NO2*.In CH2Cl2, 1a*- reacts with 4- (aromatic trinitromethylation) and undergoes C-C bond fragmentation before diffusional separation of fragments can occur.Thus, a tetrad of reactive intermediates (cumyl radical, NO2*, p-methoxycumyl cation and 4-) are produced in a single solvent cage.The cumyl radical is oxidized by 2, and both cumyl cations undergo α-trinitromethylation.The cumyl cations undergo trinitromethylation in the original solvent cage which is kinetically equivalent to a contact ion pair (CIP).In CH3CN the fragmentation is accompanied by aromatic nitration (radical collapse).In this solvent, the p-methoxycumyl cations produced by cleavage reaction undergo trinitromethylation at the CIP stage, but cumyl cations produced by thermal oxidation of the cumyl radicals are trapped by 4- at the solvent-separated ion pair stage.In CH2Cl2, 1b-c*+ undergo exclusively fragmentation, completely within the solvent cage.The cleavage of 1d-e*+ is much slower, and the radical cations undergo instead aromatic trinitromethylation.The observed substituent effect on the rate of cleavage ( ρ+ = -2.2) indicates significant charge transfer across the scissile bond in the transition state for this process.The products of these reactions are predominantly derived from ion annihilation.The radical coupling processes are limited to radical cation/radical collapse that lead to nitrated products.
Radical ions in photochemistry. 16. The photosensitized (electron transfer) carbon-carbon bond-cleavage reaction of radical cations
Okamoto, Akio,Arnold, Donald R.
, p. 2340 - 2342 (2007/10/02)
The photosensitized (electron transfer) carbon-carbon bond cleavage of 1,1,2,2-tetraphenylethane (1b), 2-(4-methoxyphenyl)-1,1-diphenyl-2-methylpropane (1c), 1,1,2-triphenyl-2-methylpropane (1d), and 2-(4-trifluoromethylphenyl)-1,1-diphenyl-2-methylpropane (1e) has been studied with 1,4-dicyanobenzene (2) serving as the electron-accepting sensitizer.The oxidation potential of 1b-e have been measured by cyclic voltammetry.Estimation of the free-energy associated with the electron transfer between these donors (1b-e) and the first excited singlet state of 2, using the Weller equation, indicates the process is favorable in every case.There is, in every case, a one-to-one correspondence of products derived from reaction of the carbocation fragment and from the carbanion derived from the initially formed radical fragment.The efficiency of the reaction of 1b, which gives a good yield of diphenylmethane (3) and methyl diphenylmethyl ether, has been studied as a function of temperature.The observed activation energy (7.2 kcal mol-1) is equated to the bond dissociation energy of the central bond of the radical cation (1b).The ratio of products from the unsymmetric compounds is dependent upon the redox potentials of the fragment radicals and carbocations.
