HOMOGENEOUS CATALYSIS - A RUTHENIUM-BASED LEWIS-ACID CATALYST FOR THE DIELS-ALDER REACTION

In order to circumvent some of the inherent drawbacks of using traditional Lewis acids to catalyze the Diels-Alder and other Lewis-acid-promoted reactions, a ruthenium-based catalyst has been devised. The readily prepared catalyst is trans-[Ru(salen)(NO)(H2O)]SbF6, where salen is the N,N'-bis(salicylidene)ethylenediamine dianion ligand. It has been fully cbaracterized; it is stable to oxygen and can be handled under normal laboratory conditions. At 1 mol % loadings, the catalyst accelerates Diels-Alder reactions by many orders of magnitude over the corresponding thermal reaction. In many cases, accelerations of > 10(5) are observed. The catalysis proceeds even in the presence of water. Slight product inhibition is observed, but the catalyst will continue to turn over substrates indefinitely. The binding of the Diels-Alder adduct of methacrolein and isoprene with the catalyst has been examined. Binding of the dienophile is stronger than that of the adduct. It is found that binding of the dienophile is endoergic, exchange of the aquo ligand is rapid and reversible, and a competitive equilibrium exists between the aquo species and the dienophile adduct. Overall, the catalyst is a mild Lewis acid which promotes Diels-Alder reactions of aldehyde and ketone dienophiles but not of alpha,beta-unsaturated esters. Its mild Lewis-acid characteristics, however, serve to suppress polymerization even with sensitive substrates.