Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors
J. Phys. Org. Chem., 2013, 26, 956-969
The generation of carbocations by laser flash photolysis of suitable precursors provides information about their reactivities toward nucleophiles on the nanosecond and microsecond time scale that cannot be obtained by conventional methods. We discuss the requirements that must be met by the precursors to achieve sufficient yields of the carbocations under the conditions of kinetic experiments. These include (i) efficient photogeneration of the carbocations; (ii) the stability of the precursor in the sample solution; (iii) the absorption of the precursor at the excitation wavelength; and (iv) sufficient lifetimes of the photogenerated carbocations for the observation of their reactivities toward the nucleophiles of interest. We provide an overview of the advantages and disadvantages of some precursors that are commonly employed for the generation of carbocations R+. This includes alkyl halides R–Cl, acetates R–OAc, aryl ethers R–OAr, ammonium salts R–NR′3+, and phosphonium salts R–PR′3+.