H. Koshima et al., STOICHIOMETRICALLY SENSITIZED DECARBOXYLATION OCCURRING IN THE 2-COMPONENT MOLECULAR-CRYSTALS OF AZA AROMATIC-COMPOUNDS AND ARALKYL CARBOXYLIC-ACIDS, Journal of the American Chemical Society, 119(43), 1997, pp. 10317-10324
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.