Enantioselective allylic substitution of Morita–Baylis–Hillman adducts catalyzed by planar chiral [2.2]paracyclophane monophosphines

Highly regio- and stereoselective (3 + 2) annulation reaction of allenoates with 3-methyleneindolin-2-ones catalyzed by a planar chiral [2.2]paracyclophane-based bifunctional phosphine-phenol catalyst

A planar chiral [2.2]paracyclophane-based phosphine-phenol catalyst catalyzed the (3 + 2) annulation reaction of ethyl 2,3-butadienoate with 3-methyleneindolin-2-ones to produce 2,5-disubstituted cyclopentene-fused C3-spirooxindoles in high yields with high regio-, diastereo-, and enantioselectivities. This catalyst was suitable for reactions of not only benzylideneindolinones but also alkylideneindolinones, the chiral phosphine-catalyzed reactions of which have not yet been reported. Density functional theory calculations suggested that the formation of hydrogen bonds between the phenolic OH group of the catalyst and the allenoate carbonyl group, rather than between the OH group and the carbonyl group of indolinone, contributed to the formation of an efficient reaction space at the enantiodetermining step.

DABCO-Catalyzed Mono-/Diallylation of N-Unsubstituted Isatin N,N′-Cyclic Azomethine Imine 1,3-Dipoles with Morita-Baylis-Hillman Carbonates

Allylation of N-unsubstituted isatin N,N′-cyclic azomethine imines with Morita-Baylis-Hillman carbonates in the presence of 1–10 mol% DABCO in DCM at room temperature, rapidly gave N-allylated and N, β-diallylated isatin N,N′-cyclic azomethine imine 1,3-dipoles in moderate to high yields. The reaction features mild reaction conditions, easily practical operation, and short reaction times in most cases. Furthermore, the alkylated products were transformed into novel bicyclic spiropyrrolidine oxoindole derivatives through the [3+2] or [3+3]-cycloaddition with maleimides or Knoevenagel adducts.

Organocatalytic atroposelective N-alkylation: divergent synthesis of axially chiral sulfonamides and biaryl amino phenols

Axial chirality exists ubiquitously in numerous natural products and has been extensively recognized for decades in pharmaceuticals and enantioselective transformations. The development of efficient methodologies to obtain enantiopure structures bearing...

Enantiopure planar chiral [2.2]paracyclophanes: Synthesis and applications in asymmetric organocatalysis

This short review focuses on enantiopure planar chiral [2.2]paracyclophanes (pCps), a fascinating class of molecules that possess an unusual three-dimensional core and intriguing physicochemical properties. In the first part of the review, different synthetic strategies for preparing optically active pCps are described. Although classical resolution methods based on the synthesis and separation of diastereoisomeric products still dominate the field, recent advances involving the kinetic resolution of racemic compounds and the desymmetrization of meso derivatives open up new possibilities to access enantiopure key intermediates on synthetically useful scales. Due to their advantageous properties including high configurational and chemical stability, [2.2]paracyclophanes are increasingly employed in various research fields, ranging from stereoselective synthesis to material sciences. The applications of [2.2]paracyclophanes in asymmetric organocatalysis are described in the second part of the review. While historically enantiopure pCps have been mainly employed by organic chemists as chiral ligands in transition-metal catalysis, these compounds can also be used as efficient catalysts in metal-free reactions and may inspire the development of new transformations in the near future.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Organocatalytic Asymmetric Allylic Alkylation of Morita-Baylis-Hillman Carbonates with Diethyl 2-Aminomalonate Assisted by In Situ Protection

With the aid of in situ protection by N-(2-formylphenyl)-4-methyl-benzenesulfonamide, enantioselective allylic alkylation of Morita-Baylis-Hillman carbonates with diethyl 2-aminomalonate was successfully realized. The corresponding adducts can be obtained in up to 99% yield with up to 98% ee as well as excellent regioselectivity. Besides, the adducts with opposite configurations were readily prepared by utilizing easily available and inexpensive quinine or quinidine as organocatalyst. Facile deprotection of the resulting adduct provides straightforward access to enantiopure α-methylene-γ-lactam.

DFT study of [2.2]-, [3.3]-, and [4.4]paracyclophanes: strain energy, conformations, and rotational barriers

The three smallest symmetrical paracyclophanes, having tethers with two, three, or four methylene groups, have been examined with four density functional methods (B3LYP, M06-2x, B97-D, ωB97X-D). The geometries predicted by functionals accounting for medium-range correlation or long-range exchange and/or dispersion are in close agreement with experiment. In addition, these methods provide similar estimates of the strain energy of the paracylcophanes, which decrease with increasing tether length. [4.4]Paracyclophane is nearly strain-free, reflecting the small out-of-plane distortion of its phenyl rings. Lastly, the barrier for interconversion of the conformers of [3.3]paracylcophane is computed in close agreement with experiment, and an estimate for phenyl rotation in [4.4]paracyclophane of about 19 kcal mol(-1) is predicted by the DFT methods employed.

Multifunctional chiral phosphine organocatalysts in catalytic asymmetric Morita-Baylis-Hillman and related reactions

Catalytic asymmetric synthesis has received considerable attention over the past few decades, becoming a highly dynamic area of chemical research with significant contributions to the field of organic synthesis. In the development of new catalysts, the concept of multifunctional catalysis described by Shibasaki and co-workers, namely, the combination of more than one functional group within a single molecule to activate the transformation, has proved a powerful strategy in the design of efficient transition metal-containing catalysts. A variety of reactions have since been addressed with multifunctional organocatalysts. One example is the Morita-Baylis-Hillman (MBH) reaction, in which a carbon-carbon bond is created between the alpha-position of an activated double-bond compound and a carbon electrophile. The seminal report on this reaction in 1972 described the prototypical couplings of (i) ethyl acrylate with acetaldehyde and (ii) acrylonitrile with acetaldehyde; the reaction is promoted by the conjugate addition of a nucleophilic catalyst to the alpha,beta-unsaturated aldehyde. Many variations of the MBH reaction have been reported, such as the aza-MBH reaction, in which an N-tosyl imine stands in for acetaldehyde. Recent innovations include the development of chiral molecules that catalyze the production of asymmetric products. In this Account, we describe the refinement of catalysts for the MBH and related reactions, highlighting a series of multifunctional chiral phosphines that we have developed and synthesized over the past decade. We also review similar catalysts developed by other groups. These multifunctional chiral phosphines, which contain Lewis basic and Brønsted acidic sites within one molecule, provide good-to-excellent reactivities and stereoselectivities in the asymmetric aza-MBH reaction, the MBH reaction, and other related reactions. We demonstrate that the reactivities and enantioselectivies of these multifunctional chiral phosphines can be adjusted by enhancing the reactive center's nucleophilicity, which can be finely tuned by varying nearby hydrogen-bonding donors. Artificial catalysts now provide highly economic access to many desirable compounds, but the general adaptability and reactivity of these platforms remain problematic, particularly in comparison to nature's catalysts, enzymes. The multifunctional organocatalysts described in this Account represent another positive step in the synthetic chemist's efforts to profitably mimic nature's catalytic platform, helping develop small-molecule catalysts with enzyme-like reactivities and selectivities.