Photochemical deuterium exchange in phenyl-substituted pyrroles and indoles in CD3CN–D2O

Abstract

A new mechanism of photochemical deuteration of some phenylpyrroles and indoles is reported. Irradiation of 2-phenylpyrrole (8), 2-phenylindole (9), and 7-phenylindole (12) in CH₃CN–D₂O gives rise to deuterium exchange at the C-atoms of the heterocycle and the adjacent phenyl ring. Photolysis of indole (7), 8, 9, 12 and N-methyl-2-phenylindole (10) in CD₃CN–D₂O also leads to deuteration at C-atoms with significantly higher yield. The mechanism most probably involves ejection of an electron on excitation and formation of radical cations that abstract D-atoms from CD₃CN or undergo bimolecular D-abstraction (photolysis in CH₃CN–D₂O). The other possible mechanism of deuterium exchange for 7, 8, 9 and 12 may take place via a homolytic N–D cleavage and recombination of the radical pair. Radical cations of 8, 9 and 10 were detected by laser flash photolysis. Steady state and time-resolved fluorescence of 8, 9, 10 and 12 showed that photoinduced intramolecular electron transfer probably does not occur. The fluorescence was quenched by acid and base and the rate constants determined by Stern–Volmer analyses. The estimated pK_a values for the protonation and deprotonation of the singlet excited states indicated that in neutral aqueous solutions ESPT probably does not take place, additionally corroborating the radical or radical cationic mechanism of deuteration.