15876-05-8Relevant articles and documents
Carbocation formation via carbene protonation studied by the technique of stopped-flow laser-flash photolysis
Belt, Simon T.,Bohne, Cornelia,Charette,Sugamori,Scaiano
, p. 2200 - 2205 (1993)
Photolysis of diaryl diazo compounds in the presence of hydroxylic protonating substrates in acetonitrile leads to the formation of the corresponding carbocations. Quantum yields for these processes have been determined for the first time; for example, photolysis of diphenyldiazomethane in 1:1 acetonitrile/trifluoroethanol produces Ph2CH+ with a quantum yield of 0.007. The products, formed either by direct insertion or through the carbocation route, are the corresponding ethers. Kinetics analysis leads to the conclusion that carbocations are formed by protonation of the singlet carbene. Electron-rich substituents enhance carbocation yields; for example, (4-MeOC6H4)2CH+ is produced with a quantum yield of 0:12 in 1:1 acetonitrile/methanol or acetonitrile/trifluoroethanol. Most of the diazo precursors employed decompose rapidly in the presence of acids or strongly protonating solvents. In order to overcome this problem, we have developed the technique of stopped-flow laser-flash photolysis, which permits the study of photoreactions in thermally unstable mixtures. Strong acids (e.g., perchloric) in concentrations up to 10 mM fail to yield any carbocations. This is probably due to the short lifetime of the carbene singlet that cannot be trapped at low acid concentrations, as well as to a low-energy singlet carbene in equilibrium with the triplet following intersystem crossing; higher concentrations of acid induce diazo decomposition within the stopped-flow mixing time.
Photohomolysis and photoionization of substituted tetraphenylethanes and C - C fragmentation of 1,1,2,2-tetra(p-R-phenyl)ethane radical cations (R = H, CH3, OCH3, Cl)
Faria, Joaquim L.,McClelland, Robert A.,Steenken, Steen
, p. 1275 - 1280 (2007/10/03)
On photolysis of a series of tetraphenylethanes in 2,2,2-trifluoroethanol (TFE) solution with 248 nm light, homolysis of the central C-C bond occurs to yield the corresponding substituted diphenylmethyl radicals, in a process requiring one quantum of light. A second process takes place under conditions of high photon fluxes, namely biphotonic photoionization to produce a radical cation, which subsequently undergoes efficient C-C scission of the aliphatic central bond to yield the radical and carbocation fragments. Photoionization and photohomolysis are the preferred processes of excited state deactivation in the solvents acetonitrile, TFE, and 1,1,1,3,3,3-hexafluoroisopropanol. The lifetime of the radical cation could be directly determined by following the formation rates of the fragments in solution. The cations were characterized by their UV absorption spectra and electrophilic reactivities.On photolysis of a series of tetraphenylethanes in 2,2,2-trifluoroethanol (TFE) solution with 248 nm light, homolysis of the central C - C bond occurs to yield the corresponding substituted diphenylmethyl radicals, in a process requiring one quantum of light. A second process takes place under conditions of high photon fluxes, namely biphotonic photoionization to produce a radical cation, which subsequently undergoes efficient C - C scission of the aliphatic central bond to yield the radical and carbocation fragments. Photoionization and photohomolysis are the preferred processes of excited state deactivation in the solvents acetonitrile, TFE, and 1,1,1,3,3,3-hexafluoroisopropanol. The lifetime of the radical cation could be directly determined by following the formation rates of the fragments in solution. The cations were characterized by their UV absorption spectra and electrophilic reactivities.
Stabilities of Carbocations in Solution. 14. An Extended Thermochemical Scale of Carbocation Stabilities in a Common Superacid
Arnett, Edward M.,Hofelich, Thomas C.
, p. 2889 - 2895 (2007/10/02)
Until now, thermodynamic stabilities of carbocations have been limited to (1) relatively stable resonance delocalized ions which are ranked on the pKR scale and (2) relatively unstable aliphatic and alicyclic ions which have been compared in the gas phase by ion cyclotron resonance or by calorimetry in SbF5/SO2ClF superacid at -50 to -120 deg C in our laboratory.The present paper will present an extensive series of new measurements which is designed to close the gap between the stable triarylmethyl cations and the unstable ions so as to put them all on a common energy scale.Carbinols were used as precursors in SbF5/HSO3F/SO2ClF at -40 deg C.As we reported recently (J.Am.Chem.Soc., 104, 3522(1982)), these conditions are necessary to avoid complications which appear to be introduced by ion-pairing when the alcohols are treated with the SbF5/SO2ClF system which was used previously to ionize alkyl chlorides.The results from the present work place 39 typical carbocations representing saturated, secondary, and tertiary, and aliphatic, bicyclic, and substituted cumyl, benzhydryl, and trityl systems on a common scale.Correlations and interpolation equations for relating other measurements in the gas phase and solution will be presented.The results provide useful comparisons of the ?+ and ?C+ scales for correlating carbocation stabilities and provide new data for several classic questions in the field such as the ranking of methyl, phenyl, and cyclopropyl groups for stabilizing ions and also the reactions of carbocations with water.
