Stereospecific Asymmetric N‑Heterocyclic Carbene (NHC)-Catalyzed Redox Synthesis of Trifluoromethyl Dihydropyranones and Mechanistic Insights

N-Heterocyclic carbene (NHC)-catalyzed redox asymmetric hetero-Diels–Alder reactions of α-aroyloxyaldehydes with β-trifluoromethyl enones generates synthetically useful dihydropyranones containing a stereogenic trifluoromethyl substituent in good yields (up to 81%) and excellent diastereoselectivity and enantioselectivity (up to >95:5 dr and >99% ee). The process is stereospecific, with use of either (<i>E</i>)- or (<i>Z</i>)-β-trifluoromethyl enones forming <i>syn</i>- or <i>anti</i>-dihydropyranone products, respectively. Mechanistic studies through in situ kinetic analysis of the reaction reveal key differences in reactivity between chiral NHC precursor <b>1</b> and an achiral NHC precursor