Stoichiometrically Sensitized Decarboxylation Occurring in the Two-Component Molecular Crystals of Aza Aromatic Compounds and Aralkyl Carboxylic Acids

Highly selective photodecarboxylation could be achieved by utilizing a series of two-component molecular crystals of aralkyl carboxylic acids such as 3-indolepropionic acid (a) and 1-naphthylacetic acid (b) combined with acridine (1) or phenanthridine (2) as an electron acceptor. The 1:1 hydrogen bonded crystals were prepared by recrystallization from the solutions. Irradiation of the crystals at −70 °C caused highly selective decarboxylation to give corresponding decarboxylated compounds alone in nearly quantitative yields, due to a smaller thermal motion of the radical species in the crystal lattice. Upon irradiating a crystal, a carboxylate radical and hydroacridine or hydrophenanthridine radical are produced via electron transfer from the acid to 1 or 2 and subsequent proton transfer followed by decarboxylation. Next hydrogen abstraction by an active aralkyl radical occurs in highest priority over the shortest distance of 3.2−3.5 A resulting in the formation of a corresponding decarboxylated product a...