Photoisomerization of 5-Membered Heterocycles

Abstract

Photolysis of substituted furans, thiophenes, and pyrroles resulted in positional isomerization as a consequence of the scrambling of the ring carbon atoms, Two intermediates have been proposed for this photoisomerization, namely, the Dewar form (1), and the ring contracted form (2). Recently, we confirmed the formation of both intermediates in the case of the parent furan, thiophene, and pyrrole by a combination of solution phase photolysis-trapping and low-temperature matrix photolysis-FTIR experiments. As the next step, we have now carried out similar photolysis of selected deuterium-labeled furan and thiophene in an effort to determine the symmetry of the reaction path leading to (1) and (2). Deuterium was chosen in order to minimize the electronic and steric effects of the substituent. The observed scrambling of the deuterium label in (1) and (2) indicates that photoisomerization takes place prior to their formation.