Enantioselective Organocatalysis

Equilibrium acidities of BINOL type chiral phenolic hydrogen bonding donors in DMSO

The pK_a values of fifteen BINOL type chiral phenolic catalysts were determined by the overlapping indicator method in DMSO via UV spectrophotometric titrations. The pK_a values are in the range of 9.30–16.43.

Preparation of Poly(ionic liquid)s-Supported Recyclable Organocatalysts for the Asymmetric Nitroaldol (Henry) Reaction

A novel strategy for the embedding of quinine-based organocatalysts in polymerized ionic liquids-based hydrogels is presented. With this technique, the encapsulated organocatalyst was successfully recovered and reused for four cycles without any loss of enantioselectivity (up to 91% ee) for the asymmetric nitroaldol (Henry) reaction. In this study, high catalyst leaching was significantly reduced (<0.01%) by controlling the water content. After catalyst removal, evaporation of the solvent affords the product in 98% purity without any further purification.

Tetraalkylammonium Salts as Hydrogen-Bonding Catalysts

Although the hydrogen-bonding ability of the α hydrogen atoms on tetraalkylammonium salts is often discussed with respect to phase-transfer catalysts, catalysis that utilizes the hydrogen-bond-donor properties of tetraalkylammonium salts remains unknown. Herein, we demonstrate hydrogen-bonding catalysis with newly designed tetraalkylammonium salt catalysts in Mannich-type reactions. The structure and the hydrogen-bonding ability of the new ammonium salts were investigated by X-ray diffraction analysis and NMR titration studies.

Multivalent polyglycerol supported imidazolidin-4-one organocatalysts for enantioselective Friedel-Crafts alkylations

The first immobilization of a MacMillan’s first generation organocatalyst onto dendritic support is described. A modified tyrosine-based imidazolidin-4-one was grafted to a soluble high-loading hyperbranched polyglycerol via a copper-catalyzed alkyne–azide cycloaddition (CuAAC) reaction and readily purified by dialysis. The efficiency of differently functionalized multivalent organocatalysts 4a–c was tested in the asymmetric Friedel–Crafts alkylation of N-methylpyrrole with α,β-unsaturated aldehydes. A variety of substituted enals was investigated to explore the activity of the catalytic system which was also compared with monovalent analogues. The catalyst 4b showed excellent turnover rates and no loss of activity due to immobilization, albeit moderate enantioselectivities were observed. Moreover, easy recovery by selective precipitation allowed the reuse of the catalyst for three cycles.

Enantioselective synthesis of cis-decalins using organocatalysis and sulfonyl Nazarov reagents

The first organocatalytic synthesis of cis-decalins using sulfonyl Nazarov reagents is reported. The Jørgensen's catalyst directs this highly enantioselective synthesis using different cyclohexenal derivatives.

Chiral molecular tweezers: synthesis and reactivity in asymmetric hydrogenation

We report the synthesis and reactivity of a chiral aminoborane displaying both rapid and reversible H2 activation. The catalyst shows exceptional reactivity in asymmetric hydrogenation of enamines and unhindered imines with stereoselectivities of up to 99% ee. DFT analysis of the reaction mechanism pointed to the importance of both repulsive steric and stabilizing intermolecular non-covalent forces in the stereodetermining hydride transfer step of the catalytic cycle.

Computational organic chemistry: bridging theory and experiment in establishing the mechanisms of chemical reactions

Understanding the mechanisms of chemical reactions, especially catalysis, has been an important and active area of computational organic chemistry, and close collaborations between experimentalists and theorists represent a growing trend. This Perspective provides examples of such productive collaborations. The understanding of various reaction mechanisms and the insight gained from these studies are emphasized. The applications of various experimental techniques in elucidation of reaction details as well as the development of various computational techniques to meet the demand of emerging synthetic methods, e.g., C-H activation, organocatalysis, and single electron transfer, are presented along with some conventional developments of mechanistic aspects. Examples of applications are selected to demonstrate the advantages and limitations of these techniques. Some challenges in the mechanistic studies and predictions of reactions are also analyzed.

Direct access to pyrimidines through organocatalytic inverse-electron-demand Diels–Alder reaction of ketones with 1,3,5-triazine

An organocatalytic inverse-electron-demand Diels–Alder reaction of ketones with 1,3,5-triazine through enamine catalysis has been developed.

Organocatalysis by p-sulfonic acid calix[4]arene: a convenient and efficient route to 2,3-dihydroquinazolin-4(1H)-ones in water

An efficient and eco-friendly method is reported for the synthesis of 2,3-dihydroquinazolin-4(1H)-ones using p-sulfonic acid calix[4]arene as a recyclable organocatalyst in excellent yields in water at room temperature.

Linear dialdehydes as promising substrates for aminocatalyzed transformations

Linear dialdehydes: This article summarizes the recent utilization of linear dialdehydes as appropriate substrates for amine catalyzed cascade/domino transformations.

Synthesis and characterization of two novel biological-based nano organo solid acids with urea moiety and their catalytic applications in the synthesis of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol), coumarin-3-carboxylic acid and cinnamic acid derivatives under mild and green conditions

Green and straight-forward syntheses of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives using nano structure organocatalysts, namely 2-carbamoylhydrazine-1-sulfonic acid and carbamoylsulfamic acid, have been developed.

Hyperbranched polyethylene-supported l-proline: a highly selective and recyclable organocatalyst for asymmetric aldol reactions

A recyclable l-proline-based organocatalyst system with unique hyperbranched polyethylene as a scaffold is prepared for highly stereoselective asymmetric aldol reactions.

Ordered short channel mesoporous silica modified with 1,3,5-triazine–piperazine as a versatile recyclable basic catalyst for cross-aldol, Knoevenagel and conjugate addition reactions with isatins

A recyclable triazine–piperazine immobilized silica supported material was explored as a heterogeneous catalyst for indole skeletal synthesized from isatins at RT.

Thiol-promoted selective addition of ketones to aldehydes

A simple and efficient thiol-mediated addition of ketones to aromatic and aliphatic aldehydes is reported. This thermodynamically controlled Pummerer/aldol reaction, which can tolerate both moisture and protic functional groups, provides a direct entry to syn-β-thioketones in high chemo- and regioselectivity. Mechanistic studies revealed that selective transformation of the aldehyde to an electrophilic thionium ion species concurrent with the generation of a nucleophilic vinyl sulfide coupling partner from the ketone is imposing cross-coupling over dimerization.

Highly enantioselective cross-aldol reactions of acetaldehyde mediated by a dual catalytic system operating under site isolation

Polystyrene-supported (PS) diarylprolinol catalysts 1 a (Ar = phenyl) and 1 b (Ar = 3,5-bis(trifluoromethyl)phenyl) have been developed. Operating under site-isolation conditions, PS-1 a/1 b worked compatibly with PS-bound sulfonic acid catalyst 2 to promote deoligomerization of paraldehyde and subsequent cross-aldol reactions of the resulting acetaldehyde in one pot, affording aldol products in high yields with excellent enantioselectivities. The effect of water on the performance of the catalytic system has been studied and its optimal amount (0.5 equiv) has been determined. The dual catalytic system (1/2) allows repeated recycling and reuse (10 cycles). The potential of this methodology is demonstrated by a two-step synthesis of a phenoperidine analogue (68% overall yield; 98% ee) and by the preparation of highly enantioenriched 1,3-diols 4 and 3-methylamino-1-arylpropanols 5, key intermediates in the synthesis of a variety of druglike structures.

Asymmetric organocatalysis combined with metal catalysis: concept, proof of concept, and beyond

Asymmetric catalysis has been considered to be the most intriguing means for building collections of functionalized optically active compounds. In particular, metal and organocatalysis have been well established to allow many fundamentally different reactions. Metal catalysis has enabled the participation of a much broader scope of chemical bonds in organic transformations than are allowed by organocatalysis, while organocatalysis permits a broader scope of functional groups to undergo a diverse range of enantioselective transformations, individually, simultaneously, or sequentially. Theoretically, the combination of organocatalysts and metal complexes could probably render new transformations through the simultaneous or sequential activation and reorganization of multiple chemical bonds if the superior features of both the catalysts are adopted. In 2001, both our research group and Takemoto's group separately described an asymmetric allylation of glycine imino esters with allyl acetate catalyzed by palladium complexes and chiral ammonium salts. In these cases, the oxidative addition of palladium complexes to allyl acetate formed the π-allylic fragments, while the chiral ammonium salts were actually responsible for controlling the stereoselectivity. These reactions in fact marked the beginning of asymmetric organo/metal combined catalysis. Since then, asymmetric organocatalysis combined with metal catalysis, including cooperative catalysis, relay catalysis, and sequential catalysis, has been a versatile concept for the creation of unknown organic transformations. Sequential catalysis describes a one-pot reaction involving two or more incompatible catalytic cycles. Alternatively, cooperative and relay catalyses require high compatibility of principally distinct catalysts and will be the focus of this Account. The catalysts in cooperative catalytic reactions must be able to simultaneously and individually activate both substrates to drive a bond-forming reaction, while relay catalysis is basically defined as a cascade process in which two or more sequential bond-forming transformations are independently catalyzed by distinct catalysts. In the past decade, we have discovered a variety of binary catalytic systems consisting of metals, including Rh(II), Pd(0), Au(I), and Mg(II), and chiral organocatalysts, including chiral phosphoric acids and quinine-based bifunctional molecules, for cooperative catalysis and relay catalysis, allowing the accomplishment of many unprecedented asymmetric transformations. In this Account, these achievements will be summarized, particularly focusing on the description of the concept and proof of the concept, to demonstrate the robustness of combined organo/metal catalysis in the creation of efficient enantioselective transformations. In addition, elegant studies from other laboratories using chiral phosphoric acid/Au(I) for the establishment of asymmetric cascade reactions involving the carbon-carbon triple bond functionality and typical combined organo/metal catalytic systems, very recently disclosed, will also be highlighted.