STOICHIOMETRICALLY SENSITIZED DECARBOXYLATION OCCURRING IN THE 2-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 acid s such as 3-indolepropionic acid (a) and 1-naphthylacetic acid (b) com bined with acridine (1) or phenanthridine (2) as an electron acceptor. The 1:1 hydrogen bonded crystals were prepared by recrystallization f rom the solutions. Irradiation of the crystals at -70 degrees C caused highly selective decarboxylation to give corresponding decarboxylated compounds alone in nearly quantitative yields, due to a smaller therm al motion of the radical species in the crystal lattice. Upon irradiat ing a crystal, a carboxylate radical and hydroacridine or hydrophenant hridine 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 pr iority over the shortest distance of 3.2-3.5 Angstrom resulting in the formation of a corresponding decarboxylated product and the regenerat ion of 1 or 2. Occurrence of radical coupling is low due to the longer coupling distance of 4.5-6.5 Angstrom estimated from the crystallogra phic data of the starting two-component molecular crystals. The hydrog en abstraction path in the crystal lattice could be confirmed by the r eaction of a deuterated crystal 1.bD in which the CO2H group was repla ced by CO2D, giving a deuterated l-methyl(CH2D)naphthalene as a produc t. Regeneration of 1 or 2 means that the acceptor plays the role of a stoichiometrical sensitizer, which can act in only one cycle, retainin g the initial crystal structure. Such a stoichiometrical sensitization is a novel photochemical process, which occurs specifically in the so lid state.