Expanded Chiral Surfaces for Asymmetric Anion–π Catalysis

Abstract

The insertion of a π-acidic surface of a naphthalenediimide (NDI) between a proline and a glutamate group affords trifunctional catalysts for the stereoselective addition of aldehydes to nitroolefins. In this report, phenyl sulfides are added to this central NDI surface. Oxidation of the sulfide donors into sulfoxide and sulfone acceptors increases both rate and stereoselectivity of the reaction. This dependence on π acidity provides corroborative support that anion–π interactions can contribute to asymmetric catalysis. Non-planar π surfaces around chiral sulfoxide connectors have a profound impact on stereoselectivity. Anti stereoisomers, with phenyl wings pointing in opposite directions from the central NDI surface, perform best in chloroform/methanol mixtures. With stronger anion–π interactions in more hydrophobic aromatic solvents, this trend inverts. Catalysis within π-box binding pockets between the two phenyl wings in syn architectures gives better selectivity under these conditions. The best results are obtained in toluene, whereas competitive π–π interactions with aromatic solvents of varied π acidity reduce the stereoselectivity. Diastereoselectivities up to 96% and enantiomeric excess values up to 91% with expanded surfaces exceed the performance of the original anion–π catalysts with identical chiral architecture (64% ee under identical conditions) and enters into the range of the best conventional catalysts.

Key words

anion–π interactions - enamine chemistry - proline catalysis - asymmetric catalysis - trifunctional catalysis - π surfaces

• References and Notes

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• 19 Stereoselective Additions of Aldehydes to Nitroolefins Using Catalysts 7–9 Solutions of substrates 1 (1.0 M) and 2 (0.5 M) and catalysts 5, 7–9 (50 mM) were prepared in CDCl₃/CD₃OD (1:1), benzene-d ₆, toluene-d ₈, nitrobenzene or 1,3-dimethoxybenzene, and stirred at 20 °C. With increasing reaction time, ¹H NMR spectra of aliquots diluted in CDCl₃ were recorded (see Figures S4, S5 in the SI). The concentration of product 3 was determined from the integration of pertinent resonances. The concentrations of product 3 were plotted against time, and the initial velocities were determined from the linear fitting (see Figure S6 in the SI). From apparent second-order rate constants, relative rate enhancements k _rel and transition-state stabilizations ΔE _a were determined. Product mixtures were analyzed by chiral HPLC (Chiralcel AD-H, n-hexane/i-PrOH 99.25:0.75, 0.8 mL/min, 1 mL/min, r.t.; detection: 254 nm, Figure 4; see also Figures S7–S11 in the SI).
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