Stetter Reaction, Benzoin Condensation and Pinacol Coupling

Stetter Reaction, Benzoin Condensation and Pinacol Coupling [Pg.34]

NHC catalysis has been efficiently employed in an intramolecular crossed-benzoin reaction of symmetrical reactants. This desymmetrization strategy can be applied to (g) asymmetric synthesis with chiral NHCs. As an example, bis(acyloin) (91) containing three contiguous quaternary bridgehead chiral centres was synthesized and structurally characterized by X-ray crystallography. [Pg.34]

NHC-catalysed umpolung reactions of both simple and a, -unsaturated aldehydes have been studied by NMR spectroscopy and X-ray diffraction key intermediates characterized include diamino enols, diamino dienols, azolium enolates, and the first report of an azolimn enol (92). Interconversion of these species has been followed by NMR kinetics, with mechanistic characterization further supported by DPT calculations. [Pg.34]

The intermolecular Stetter reaction of benzaldehyde and cyclopropene, catalysed by NHCs, has been modelled using DFT. The roles of water and of bases such as DBU in affecting the free energy and controlling the diastereomeric ratio are examined. [Pg.35]

NHC catalysis of Stetter and benzoin reactions by triazolidenes has been investigated via in situ observation of intermediate 3-(hydroxybenzyl)azolium salts of the benzaldehyde substrates. Equilibrium constants for their formation are reported, together with rate constants for hydrogen-deuterium exchange at the a-carbon. [Pg.35]

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