Topics in Current Chemistry (2019) 377:35
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instead of enals. Singha et al. also recently demonstrated enantioselective [5 + 2]
annulation of enals with vinylethylene carbonates by this NHC/Pd cooperative
catalysis (Fig. 18) [30].
Namitharan et  al. developed a cooperative relay catalysis strategy using chiral
triazolium NHC and a copper catalyst (Fig. 19) [31]. The chiral spirocyclic lactam
products were obtained with good enantioselectivity. The process is initiated by Cucatalyzed activation of terminal alkyne followed by reaction with TsN 3 to generate
ketenimine as a key intermediate. This ketenimine intermediate is subsequently activated by the chiral NHC catalyst to form an azolium enamide intermediate that can
react with electrophilic substrates such as reactive ketones and imines to form the
final product. Based on mechanistic studies, triazolium NHC ligation did not quench
the function of Cu to catalyze the [3 + 2] cycloaddition of azide and terminal alkyne.
Fig. 18 NHC/Pd-catalyzed [5 + 2] annulation reactions
Fig. 19 NHC/Cu cooperative relay catalysis
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