CARBOCATION FORMATION VIA CARBENE PROTONATION STUDIED BY THE TECHNIQUE OF STOPPED-FLOW LASER-FLASH PHOTOLYSIS

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.