CONTROL OF THE DEPROTONATION ROUTE OF AN AMINIUM CATION BY THE SOLVENT POLARITY

We have studied the photoreaction of triethylamine DH with the triplet sensitizer 9,10-anthraquinone in a series of aprotic solvents with re lative permittivity epsilon(r) varying between 2 and 50. The results a re compared with those of an earlier investigation in which a series o f triplet sensitizers in acetonitrile was used. All these reactions ar e two-step processes: an aminium cation DH.+ is formed by photoinduced electron transfer, and then deprotonated at C(alpha) to give an alpha -aminoalkyl radical D.. The deprotonation of DH.+ can either occur wit hin the cage, by the sensitizer radical anion, or outside the cage, by surplus amine. The final reaction products (e.g. N,N-diethylvinylamin e) are independent of the deprotonation route. Nevertheless, a distinc tion between the two mechanistic alternatives is possible by using mea surements of chemically induced dynamic nuclear polarization (CIDNP). This technique is sensitive to radical pairs only, and the different a mine-based intermediates contained in the two possible kinds of radica l pairs (radical-ion pairs for both deprotonation routes, and pairs of neutral radicals for in-cage deprotonation only) give rise to charact eristically different signal patterns. The influence of solvent polari ty as well as sensitizer oxidation potential E(ox) on the deprotonatio n pathway (exclusive in-cage deprotonation at low values of epsilon(r) , or high E(ox), and exclusive out-of-cage deprotonation at high epsil on(r), or low E(ox) can be quantitatively explained by the dependence of the in-cage deprotonation rate on the driving force - DELTAG(dep)0 of this process, which shows a marked threshold behavior. If DELTAG(de p)0 is more negative than - 125 kJ/mol, proton transfer from DH.+ to t he sensitizer radical anion is faster than separation of the primarily formed radical-ion pair, so the aminium cations are deprotonated with in the cage. For DELTAG(dep)0 more positive than -100 kJ/mol, this rea ction is too slow to compete with escape from the cage. By the latter process, free aminium cations are formed, which are then deprotonated outside the cage by surplus amine.