Titanium Salen Complexes with Appended Silver NHC Groups as Nucleophilic Carbene Reservoir for Cooperative Asymmetric Lewis Acid/NHC Catalysis

Abstract

Lewis acid catalysis and nucleophilic carbene catalysis are complementary fundamental concepts to accelerate and control chemical reactions of aldehyde substrates. Their efficient merger has recently been achieved using two separate catalyst species. The present report describes our efforts to develop a cooperative catalyst system which incorporates both features – Lewis acid and nucleophilic NHC – within the same catalyst entity. To generate free carbene moieties under very mild conditions, Ag‐NHC complexes were formed releasing the nucleophilic carbene upon treatment with PPh3. The result is the formation of an enol‐δ‐lactone as new enal dimerization product. Silver is essential for the reactivity mode thus suggesting that it plays a double role in the catalytic event.