Solid State Packing and Photoreactivity of Alkali Metal Salts of trans,trans-Muconate

Three alkali-metal salts of trans,trans-muconate (muco) viz. Li2muco (1), Na2muco (2), and K2muco (3) have been prepared, and the influence of the crystal packing on the solid state photoreactivity has been investigated. Although the CC bonds of the muco ligands are oriented infinitely parallel in 1, it was found to be photoinert. In contrast, the muco ligands of 2 and 3 in the crystalline state undergo photodimerization yielding cycloocta-3,7-diene-1,2,5,6-tetracarboxylate which has been formed stepwise via the [2 + 2] cycloaddition reaction of a single pair of CC bonds and subsequent Cope rearrangement. This study demonstrates how the size of the metal ion can influence the crystal packing in metal organic salts.