10.1002/anie.201004593
The study presents an innovative approach to C-C bond formation through NHC-catalyzed Michael addition to α,β-unsaturated aldehydes, utilizing redox activation. The researchers, Suman De Sarkar and Armido Studer, explore the use of N-heterocyclic carbenes (NHCs) to activate α,β-unsaturated aldehydes, which then react with various 1,3-dicarbonyl compounds to form dihydropyranones. They demonstrate that this method is effective with different nucleophiles and enals, achieving high yields and selectivity under mild conditions. The process involves a two-step umpolung reaction at the β-position of the α,β-unsaturated aldehyde, followed by a redox-type activation. The study also includes control experiments to rule out alternative mechanisms, such as kinetic O-acylation, and provides a proposed catalytic cycle for the process. This work contributes to the field of organocatalysis by offering a new strategy for conjugate addition reactions using soft C-nucleophiles and showcases the potential of NHCs in redox activation.
10.1021/ol802330h
The research focuses on the development of an asymmetric formal carbo [3 + 3] cycloaddition reaction using diphenylprolinol silyl ether as an organocatalyst. The reaction proceeds through a domino Michael/Knoevenagel condensation process, starting from R,?-unsaturated aldehydes and dimethyl 3-oxopentanedioate. The study investigates the influence of various reaction parameters, including the molar ratio of reagents, solvent, catalyst choice, and additives. The main reactants are cinnamaldehyde (R,?-unsaturated aldehyde) and dimethyl 3-oxopentanedioate. The reaction's success is highly dependent on the stoichiometry of the reactants, with optimal results achieved using equimolar amounts. The reaction's efficiency and enantioselectivity are influenced by the catalyst, with diphenylprolinol trimethylsilyl ether and its derivatives being tested. The addition of benzoic acid as an additive enhances the reaction yield and enantioselectivity. The analysis of the reaction products is performed using HPLC on a chiral phase to determine the enantiomeric excess (ee), and the structures of the synthesized compounds are confirmed by spectroscopic techniques such as 1H NMR, 13C NMR, and IR. The research demonstrates the synthesis of cyclohexenone derivatives with excellent enantioselectivity and further explores one-pot transformations to more complex cyclohexane derivatives, showcasing the synthetic utility of the developed method.
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