Studies into the Stereochemical Control of Diels-Alder Reactions with ?6-Ruthenium Arene Complexes

Abstract

In this study, we examined the effect of ?6-ruthenium arene complexes on the stereocontrol of Diels-Alder (DA) reactions. In organic chemistry, ?6-ruthenium arene complexes can act as potent electron-withdrawing groups, thereby facilitating the nucleophilic aromatic substitution and deprotonation of benzylic and aromatic positions. Additionally, having the metal complex bound to a non-symmetrical arene provides a source of chirality, enabling stereoselective chemistry. It is well known that DA reactions with an electron-rich diene can be accelerated by electron-withdrawing substituents on the dienophile. Given that the ?6-ruthenium arene complexes are extremely electron-withdrawing, it follows that these moieties may be able to accelerate DA reactions when bound to the dienophile component. Indeed, previous studies by the Hitt research group has suggested that they can accelerate Diels-Alder reactions involving adjacent alkenes. However, the effect of using a chiral ruthenium arene substituent on the dienophile as a stereocontrol element has not been analyzed. In this vein, we have successfully synthesized a model dienophile substrate, [CpRu(?6-(ethyl 2-chlorocinnamate))]PF6, to be used in the DA reaction. The cis- and trans- isomers of this dienophile were separated by column chromatography. The trans- enriched dienophile substrate was then combined with 2,3-dimethyl-1,3-butadiene and the stereochemistry of the Diels-Alder product was examined via 1H NMR spectroscopy.