The N-heterocycliccarbene-catalyzedconjugateUmpolung of differently substituted α,β-unsaturatedaldehydes, e. g. cinnamaldehydes, a-methylcinnamaldehydes, and crotonaldehydes, is described. Coupling of these compounds with a variety of electrophilic aldehydes and ketones results in the selective formation of highly substituted β- and γ-butyrolactones.
<i>N</i>-Heterocyclic Carbene-Catalyzed Generation of Homoenolates: γ-Butyrolactones by Direct Annulations of Enals and Aldehydes
作者:Stephanie S. Sohn、Evelyn L. Rosen、Jeffrey W. Bode
DOI:10.1021/ja044714b
日期:2004.11.1
generation of reactive homoenolates from alpha,beta-unsaturated aldehydes. The catalyst-bound homoenolate reacts with electrophilic aldehydes leading, via the key intermediacy of an activated carboxylate, to gamma-butyrolactones in good yields and stereoselectivities. Importantly, this process demonstrates an unprecedented reaction mode for the generation of nucleophilic carbanions with a multifunctional
Imidazolylidenes with 2,6-diethylphenyl groups were effective catalysts, and several mechanistic studies, including a deuterium kinetic isotope effect study, revealed that both steric and kinetic effects were responsible for the enhanced catalytic activity.
A serial of imidazolium chiral ionic liquids starting from commercially available (â)-menthol as a chiral pool have been synthesized and successfully used for catalyzing conjugate umpolung of α,β-unsaturated aldehydes to stereoselectively synthesize γ-butyrolactones. The catalytic activities of these ionic liquids are examined and compared. The reaction of trans-cinnamaldehyde and p-methoxycarbonyl benzaldehyde as a model has been investigated in detail, and the reaction conditions have been optimized. Under the selected conditions, the ionic liquid catalyst can catalyze the annulations of trans-cinnamaldehyde with different aromatic aldehydes to afford differently substituted γ-butyrolactones. The recyclability of the ionic liquid catalyst is investigated, and the results have indicated that the catalyst can be recycled six times without obvious activity decreasing.
An efficient and flexible synthesis of a new class of chiral bifunctional NHC catalyst has been reported. These new imidazolylidene NHCs, bearing a (thio)urea function as a hydrogenbonddonor promoted efficiently highly diastereoselective trans‐cyclopentannulation of enals and enones in moderate to good yields (up to 69 % yield) along with excellent enantioselectivity (up to 96 % ee). This methodology