ELECTRON-DEFICIENT CARBOCATIONS - DIRECT OBSERVATION OF ALPHA-CARBONYLMETHYL CATIONS BY LASER FLASH-PHOTOLYSIS

The 9-carbomethoxyfluoren-9-yl cation was generated under stable ion conditions and characterized by spectral data (visible/NMR spectroscopy) and product identification of the benzene and toluene trapping reactions. The laser flash excitation of methyl alpha-bromodiphenylacetate and methyl 9-bromo-9-fluorenylcarboxylate generated both the corresponding cations and radicals. Rate constants for nucleophilic quenching by alcohols and bromide ion were measured for both the alpha-carbomethoxy cations and the unsubstituted diphenylmethyl and 9-fluorenyl cations. The results indicated an increase in kinetic stability upon replacement of alpha-hydrogen by alpha-carbomethoxy. Direct comparison of quenching rate constants in the same solvent showed an increase in reactivity of the 9-fluorenyl cations relative to the diphenylmethyl cations.