Vibrational CD study on the solution phase structures of the MacMillan catalyst and its corresponding iminium ion

We demonstrate that VCD spectroscopy can reveal insights into the conformational preferences of the iminium ion obtained from MacMillan's imidazolidinone catalyst. For both the isolated and in situ generated iminium ion, the comparison of experimental and computed VCD spectra directly confirms that conformer 2b ("Houk-conformer") is the dominant structure in solution. This conclusion is reached without any in-depth interpretation of the spectroscopic data, just by visual comparison of the spectral signatures. For the parent catalyst 1 and its salts 1·HCl and 1·HClO₄, we report a comprehensive analysis of the conformational preferences in two solvents. VCD spectroscopy is subsequently shown to be able to reveal small conformational changes induced by solute-solvent and solute-anion interactions.

SuFEx-Click Approach for the Synthesis of Soluble Polymer-Bound MacMillan Catalysts for the Asymmetric Diels–Alder Reaction

Novel polymeric MacMillan catalysts were prepared from modified chiral imidazolidin-4-one monomers via sulfur(VI) fluoride exchange chemistry. The resulting polysulfates containing chiral imidazolidin-4-one units could be employed as polymeric organocatalysts for the asymmetric Diels–Alder reaction. With the use of these polysulfate catalysts, sufficient catalytic activity and enantioselectivity were obtained, which were similar to those obtained by monomeric catalysts in a homogeneous catalytic reaction. In addition, the polysulfate catalysts could be recovered and reused five times without a considerable loss of activity and selectivity.

Ionic, Core-Corona Polymer Microsphere-Immobilized MacMillan Catalyst for Asymmetric Diels-Alder Reaction

The improvement of the catalytic activity of a heterogeneous chiral catalyst is one of the most critical issues, as are its recovery and reuse. The design of a heterogeneous chiral catalyst, including the immobilization method and the support polymer, is of significance for the catalytic activity in asymmetric reactions. An ionic, core-corona polymer microsphere-immobilized MacMillan catalyst (ICCC) was successfully synthesized by the neutralization reaction of sulfonic acid functionalized core-corona polymer microsphere (CCM–SO3H) with a chiral imidazolidinone precursor. We selected the core-corona polymer microsphere as the polymer support for the improvement of catalytic activity and recovery. The MacMillan catalyst was immobilized onto the pendant position of the corona with ionic bonding. ICCC exhibited excellent enantioselectivity up to 92% enantiomeric excess (ee) (exo) and >99% ee (endo) in the asymmetric Diels-Alder (DA) reaction of (E)-cinnamaldehyde and 1,3-cyclopentadiene. ICCC was quantitatively recovered by centrifugation because of the microsphere structure. The recovered ICCC was reused without significant loss of the enantioselectivity.

Kinetics and Mechanism of Pyrrolidine Buffer-Catalyzed Fulvene Formation

Rapid synthesis of fulvenes is achieved using pyrrolidinium/pyrrolidine buffers in anhydrous acetonitrile. Time-dependent UV-vis absorption and NMR spectroscopy reveal that the rate and yield of fulvene formation depend strongly on both the presence of acid in the medium and the choice of solvent, and they are negatively affected by water. Kinetic data have been collected for various substrates, and the synthetic benefits of the adjusted reaction conditions are showcased. Enhancements of reaction rates are found in comparison to literature procedures. α-Unsaturated fulvenes that were previously difficult to access can now be obtained in good yields.

Reaction Mechanism of Organocatalytic Michael Addition of Nitromethane to Cinnamaldehyde: A Case Study on Catalyst Regeneration and Solvent Effects

The Michael addition of nitromethane to cinnamaldehyde has been computationally studied in the absence of a catalyst and the presence of a biotinylated secondary amine by a combined computational and experimental approach. The calculations were performed at the density functional theory (DFT) level with the M06-2X hybrid functional, and a polarizable continuum model has been employed to mimic the effect of two different solvents: dichloromethane (DCM) and water. Contrary to common assumption, the product-derived iminium intermediate was absent in both of the solvents tested. Instead, hydrating the C1–C2 double bond in the enamine intermediate directly yields the tetrahedral intermediate, which is key for forming the product and regenerating the catalyst. Enamine hydration is concerted and found to be rate-limiting in DCM but segregated into two non-rate-limiting steps when the solvent is replaced with water. However, further analysis revealed that the use of water as solvent also raises the energy barriers for other chemical steps, particularly the critical step of C–C bond formation between the iminium intermediate and nucleophile; this consequently lowers both the reaction yield and enantioselectivity of this LUMO-lowering reaction, as experimentally detected. These findings provide a logical explanation to why water often enhances organocatalysis when used as an additive but hampers the reaction progress when employed as a solvent.

Distribution of Catalytic Species as an Indicator To Overcome Reproducibility Problems

Irreproducibility is a common issue in catalysis. The ordinary way to minimize it is by ensuring enhanced control over the factors that affect the reaction. When control is insufficient, some parameters can be used as indicators of the reaction performance. Herein we describe the use of the distribution of catalytic species as an indicator to map, track, and fine-tune the performance of catalytic reactions. This indicator is very sensitive and presents a quick response to variations in the reaction conditions. We have applied this new strategy to the conjugate addition of C-nucleophiles to enals via iminium intermediates, consistently achieving maximum turnover frequencies (TOF) regardless of the quality of the starting materials used. In addition, the present method has allowed us to efficiently reduce the catalyst loading to as little as 0.1 mol %, the lowest one described for this kind of reaction.

Enantioselective Silver and Amine Co-catalyzed Desymmetrizing Cycloisomerization of Alkyne-Linked Cyclohexanones

A silver(I) and amine co-catalyzed desymmetrization of 4-propargylamino cyclohexanones for the direct enantioselective synthesis of 2-azabicyclo[3.3.1]nonanes is described. Exploiting reactivity arising from dual activation of the pendant terminal alkyne by silver(I) and the ketone moiety through transient enamine formation, this synthetically relevant transformation is easy to perform, efficient and broad in scope. High enantioselectivity (up to 96 % ee) was achieved by exploiting a significant matching effect between the chirality of a cinchona alkaloid-derived aminophosphine ligand for the silver(I) salt and the 2-bis(aryl)methylpyrrolidine catalyst which was rationalized by DFT calculations. This allowed for the preparation of both enantiomers of the bicyclic product with near-identical stereocontrol.

Aromatic Interactions in Organocatalyst Design: Augmenting Selectivity Reversal in Iminium Ion Activation

Substituting N-methylpyrrole for N-methyindole in secondary-amine-catalysed Friedel-Crafts reactions leads to a curious erosion of enantioselectivity. In extreme cases, this substrate dependence can lead to an inversion in the sense of enantioinduction. Indeed, these closely similar transformations require two structurally distinct catalysts to obtain comparable selectivities. Herein a focussed molecular editing study is disclosed to illuminate the structural features responsible for this disparity, and thus identify lead catalyst structures to further exploit this selectivity reversal. Key to effective catalyst re-engineering was delineating the non-covalent interactions that manifest themselves in conformation. Herein we disclose preliminary validation that intermolecular aromatic (CH-π and cation-π) interactions between the incipient iminium cation and the indole ring system is key to rationalising selectivity reversal. This is absent in the N-methylpyrrole alkylation, thus forming the basis of two competing enantio-induction pathways. A simple L-valine catalyst has been developed that significantly augments this interaction.

Synthetic Strategies Toward the Decalin Motif of Maklamicin and Related Spirotetronates

Herein we describe a scalable approach to the decalin moiety of maklamicin. Key to the synthesis is an intramolecular Diels-Alder (IMDA) reaction that proceeds via an endo-axial transition state to generate the desired stereochemistry. We explored the diastereoselectivity of the IMDA reaction as a function of both chiral catalysis and acyclic precursor stereochemistry.

Improving catalyst activity in secondary amine catalysed transformations

The effect on catalyst performance of altering substituents at the 2-position of the Macmillan imidazolidinone has been examined. Condensation of L-phenylalanine N-methyl amide with acetophenone derivatives results in a series of imidazolidinones whose salts can be used to accelerate the Diels-Alder cycloaddition. Electron withdrawing groups significantly increase the overall rate of cycloaddition without compromise in selectivity. The most effective catalyst was shown to be efficient for a variety of substrates and the applicability of this catalyst to alternative secondary amine catalysed transformations is also discussed.