Oxoisoaporphines: Regioselective deuterium labelling involving the metastable hydrogenated photoproduct anions

During the ¹H NMR study of the photoreduction of aza-oxoisoaporphines, aza-OIAs, (7H-benzo[e]perimidin-7-ones) by amines we found an unexpected H/D isotopic exchange in the aromatic positions 4 and 6 of these molecules. This surprising exchange motivated us to probe it with previously studied oxoisoaporphines, OIAs, (1-aza-benzo[de]anthracen-7-ones) and 2,3-dihydro-oxoisoaporphines, 2,3-dh-OIAs, (2,3-dihydro-7H-dibenzo[de,h] quinolin-7-ones). All of these compounds photoreduce efficiently in the presence of aliphatic tertiary amines through a stepwise mechanism of electron-proton-electron transfer. This photoreaction generates an AH ^- anion hydrogenated either at the N-atom, for 2,3-dh-OIAs, or at the O-atom for aza-OIA and OIAs. These long-lived metastable photoproducts revert thermally to the initial oxoisoaporphines nearly quantitatively. In the presence of D₂O, regioselective exchange of the aromatic protons at positions 4 and 6 takes place to an extent greater than 90% under very mild conditions. This facile isotopic exchange reaction might be advantageously used to introduce deuterium, and likely tritium at these positions of aromatic oxoisoaporphines.