Control of the Deprotonation Route of an Aminium Cation by the Solvent Polarity

Abstract

AbstractWe have studied the photoreaction of triethylamine DH with the triplet sensitizer 9,10‐anthraquinone in a series of aprotic solvents with relative permittivity· varying between 2 and 50. The results are compared with those of an earlier investigation in which a series of triplet sensitizers in acetonitrile was used. All these reactions are two‐step processes: an aminium cation DH is formed by photoinduced electron transfer, and then deprotonated at Cα to give an α‐aminoalkyl radical D′. The deprotonation of DH can either occur within the cage, by the sensitizer radical anion, or outside the cage, by surplus amine. The final reaction products (e.g. N,N‐diethylvinylamine) are independent of the deprotonation route. Nevertheless, a distinction between the two mechanistic alternatives is possible by using measurements of chemically induced dynamic nuclear polarization (CIDNP). This technique is sensitive to radical pairs only, and the different amine‐based intermediates contained in the two possible kinds of radical pairs (radical‐ion pairs for both deprotonation routes, and pairs of neutral radicals for in‐cage deprotonation only) give rise to characteristically different signal patterns. The influence of solvent polarity as well as sensitizer oxidation potential Eox on the deprotonation pathway (exclusive in‐cage deprotonation at low values of· or high Eox, and exclusive out‐of‐cage deprotonation at high· or low Eox) can be quantitatively explained by the dependence of the in‐cage deprotonation rate on the driving force ‐ΔGdep0 of this process, which shows a marked threshold behavior. If ΔGdep0 is more negative than ‐125 kJ/mol, proton transfer from DH to the sensitizer radical anion is faster than separation of the primarily formed radical‐ion pair, so the aminium cations are deprotonated within the cage. For ΔGdep0 more positive than ‐100 kJ/mol, this reaction 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.