Understanding selectivities in ligand-free oxidative cyclizations of 1,5- and 1,6-dienes with RuO4 from density functional theory

Abstract

Quantum-chemical calculations using density functional theory were carried out to investigate the mechanism of the oxidative cyclization of 1,5- and 1,6-dienes with ruthenium tetroxide. Current experimental results show different selectivities for the formation of tetrahydrofuran and tetrahy-dropyran derivatives. Our theoretical data correctly reproduce the experimental selectivities. Transition structures for the first [3+2]-cycloaddition of RuO4 with ethene and for the second [3+2]-cycloaddition with two ethene molecules, 1,5-hexadiene, and 1,6-heptadiene were calculated. For the formation of tetrahydrofuran and tetrahydropyran derivatives we observed two reaction pathways. The transition structure for the formation of cis-tetrahydrofuran derivatives was found to be more stable than the trans-tetrahydrofuran-forming transition structure by about 40 kJ mol -1. By comparison to the reaction with two ethene molecules it was shown that the linking alkyl chain causes the energy gap between stereoisomers by a directing influence. In the tetrahydropyran reaction the trans-tetrahydropyran-forming transition structure was less than 4 kJ mol -1 more stable than the transition structure leading to the cis-tetrahydropyran. The obtained geometries showed that for tetrahydropy-rans the energy gap between stereoisomers is not caused by the linking alkyl chain. © 2010 Verlag der Zeitschrift fur Naturforschung, Tübingen.