Chemical reactions in the solid state. Part 1. Photochemical dimerization of 1-methyl-5,6-diphenylpyrazin-2-one

Abstract

1-Methyl-5,6-diphenylpyrazin-2-one (Ia) crystallizes from ethanol in two crystal modification; one is light stable and the other is light sensitive and undergoes photochemical [4 + 4] cyclodimerization in the solid state but not in a solution. The 1-ethyl derivative (Ib)is light stable. The crystal structures of the two modifications of (Ia) and (Ib), and that of the dimer, are described. The packing of the molecules in the unit cell plays a crucial role in the reactivity of the various compounds. The light-sensitive modification (Ia) is packed in parallel arrangement with short distances between the reacting centres. The light-stable modification has a different packing and the distances between possible reacting centres are too long to react (longer than 5.5 Å). The 1-ethyl derivative, although packed in a similar fashion to (Ia), is light stable. The distance between the possible reacting centres are longer than 4.2 Å, which is believed to be the distance limit for such a reaction to occur.