Asymmetric catalysis by chiral hydrogen-bond donors

Bicyclic proline analogues as organocatalysts for stereoselective aldol reactions: an in silico DFT study

Density functional theory has been employed in investigating the efficiency of a series of bicyclic analogues of proline as stereoselective organocatalysts for the aldol reaction. Three classes of conformationally restricted proline analogues, as part of either a [2.2.1] or [2.1.1] bicyclic framework, have been studied. Transition states for the stereoselective C-C bond formation between enamines derived from [2.2.1] and [2.1.1] bicyclic amino acids and p-nitrobenzaldehyde, leading to enantiomeric products, have been identified. Analysis of the transition state geometries revealed that the structural rigidity of catalysts, improved transition state organization as well as other weak interactions influence the relative stabilities of diastereomeric transition states and help contribute to the overall stereoselectivity in the aldol reaction. These bicyclic catalysts are predicted to be substantially more effective in improving the enantiomeric excess than the widely used organocatalyst proline. Enantiomeric excesses in the range 82-95% are predicted for these bicyclic catalysts when a sterically unbiased substrate such as p-nitrobenzaldehyde is employed for the asymmetric aldol reaction. More interestingly, introduction of substituents, as simple as a methyl group, at the ortho position of the aryl aldehyde bring about an increase in the enantiomeric excess to values greater than 98%. The reasons behind the vital energy separation between diastereomeric transition states has been rationalized with the help of a number of weak interactions such as intramolecular hydrogen bonding and Coulombic interactions operating on the transition states. These predictions could have wider implications for the rational design of improved organocatalysts for stereoselective carbon-carbon bond-forming reactions.

Chiral amines as organocatalysts for asymmetric conjugate addition to nitroolefins and vinyl sulfones via enamine activation

Over the last decade the potential of organocatalysis has successfully been demonstrated. In particular, chiral amines such as pyrrolidine analogues have emerged as a broadly applicable class of organocatalyst for asymmetric conjugate addition via enamine activation. This Feature Article documents the development of these catalysts, emphasizing the design and mechanistic features that supply high selectivity in asymmetric Michael reactions.

One-pot approach to chiral chromenesvia enantioselective organocatalytic domino oxa-Michael–aldol reaction

A highly enantioselective (S)-diphenylpyrrolinol triethylsilyl ether promoted tandem oxa-Michael-aldol reaction of alpha,beta-unsaturated aldehydes with salicylaldehydes has been developed; the method affords one-pot access to chiral and synthetically useful chromenes in high yields and high enantioselectivities from readily available compounds.

Organocatalytic asymmetric Friedel–Crafts alkylation/cascade reactions of naphthols and nitroolefins

The asymmetric Michael-type Friedel-Crafts reaction of naphthols and nitroolefins promoted by bifunctional thiourea-tertiary amine organocatalysts (up to 95% ee) was investigated; on simply extending the reaction time further cascade reactions could occur to generate enantiopure dimeric tricyclic 1,2-dihydronaphtho[2,1-b]furanyl-2-hydroxylamine derivatives.

Organocatalytic Ring Opening Polymerization of Trimethylene Carbonate

A variety of organocatalysts has been surveyed in the ring opening polymerization of trimethylene carbonate. Excellent control was found for several of these catalysts yielding well-defined polycarbonates with molecular weights up to 50,000 g mol(-1), polydispersities below 1.08, and high end-group fidelity. Melt or bulk polymerization was accomplished without loss of control of molecular weight or polydispersity, and random ester-carbonate bulk polymerizations were also demonstrated. Furthermore, by combining disparate polymerization techniques using bifunctional initiators, the mild polymerization conditions allow for the preparation of new block copolymers. Hydrogen-bond activation of monomer and initiator/propagating species is proposed as the underlying mechanism, which can be tuned to mitigate adverse side reactions.

Evaluating β-amino acids as enantioselective organocatalysts of the Hajos–Parrish–Eder–Sauer–Wiechert reaction

A systematic study of the effect of substitution within the beta-amino acid framework indicates that both beta(2)- and beta(3)-amino acids catalyse the Hajos-Parrish-Eder-Sauer-Wiechert reaction with poor to reasonable levels of enantioselectivity. These results led to the evaluation of the conformationally constrained beta-amino acid (1R,2S)-cispentacin, which catalyses the Hajos-Parrish-Eder-Sauer-Wiechert reaction with comparable or higher levels of enantioselectivity to L-proline.

Enantioselective hydroxylation of nitroalkenes: an organocatalytic approach

An easy hydroxylation of aliphatic nitroalkenes in high yields and enantioselectivities is catalysed by bifunctional thiourea-cinchona alkaloids giving access to optically active nitroalcohols and aminoalcohols as final products.

Organocatalytic asymmetric hydrophosphination of nitroalkenes

The use of a bifunctional Cinchona alkaloid catalyst has provided a new organocatalytic strategy for the enantioselective addition of diphenylphosphine to a range of nitroalkenes, affording optically active beta-nitrophosphines (up to 99% ee after crystallization); this organocatalytic approach, providing a direct route to a new class of potentially useful enantiopure P,N-ligands, constitutes a bridge between the two complementary areas of asymmetric catalysis: organo- and metal-catalyzed transformations.

Bisoxazoline-Lewis Acid-Catalyzed Direct-Electron Demand oxo-Hetero-Diels−Alder Reactions of N-Oxy-pyridine Aldehyde and Ketone Derivatives

A general catalytic oxo-hetero-Diels-Alder reaction for pro-chiral aldehyde and ketone N-oxy-pyridines is presented. The catalytic and asymmetric oxo-hetero-Diels-Alder reaction of electron-rich dienes with N-oxy-pyridine-2-carbaldehyde and ketone derivatives, catalyzed by chiral copper(II)-bisoxazoline complexes, gives optically active six-membered oxygen heterocycles in moderate to good yields and with excellent enantioselectivities.

Enantioselective Direct Aza Hetero-Diels−Alder Reaction Catalyzed by Chiral Brønsted Acids

[Structure: see text] The first chiral Brønsted acid-catalyzed asymmetric direct aza hetero-Diels-Alder reaction has been described. The phosphoric acids, prepared from binol and H8-binol derivatives, have shown catalytic ability for the reaction of cyclohexenone with N-PMP-benzaldimine. A chiral phosphoric acid, derived from 3,3-di(4-chloropheneyl)-H8-binol, exhibited superior enantioselectivity, affording fairly good yields and enantioselectivities for the reaction of a range of aromatic aldimines with cyclohexenone.

Highly Enantioselective Organocatalytic Biginelli Reaction

A series of binol- and H8-binol-based phosphoric acids have for the first time been evaluated for their ability to catalyze Biginelli reactions of aldehydes, thiourea or urea, and beta-keto esters. A new chiral phosphoric acid, derived from 3,3'-diphenyl-H8-binol, exhibited superior catalytic activity and enantioselectivity compared to its structural analogues, affording high enantioselectivities ranging from 85 to 97% ee with a wide scope of substrates. A metal-free preparation of optically active monastrol was achieved on the basis of the current process.

Enantioselective Michael Addition of α-Substituted Cyanoacetates to Vinyl Ketones Catalyzed by Bifunctional Organocatalysts

A highly enantioselective Michael addition of alpha-substituted cyanoacetates to vinyl ketones was accomplished in the presence of simple bifunctional thiourea/tertiary amine organocatalysts. A number of alpha-aryl or alkyl cyanoacetates have been successfully applied to give multifunctional compounds with an all-carbon-substituted quaternary stereocenter in excellent enantioselectivities (82-97 % ee) and yields (61-99 %). The optical pure adducts could be smoothly converted to variously structured beta(2,2)-amino acid esters. Moreover, an interesting reaction model involving multiple hydrogen-bonding interactions amongst the thiourea/tertiary amine catalyst and the reactants has been proposed based on the absolute configuration of the adduct and computational studies.

Enantioselective silyl protection of alcohols catalysed by an amino-acid-based small molecule

Reliable, selective and environmentally friendly chemical transformations are crucial to the development of new therapeutics and the design of novel materials. Chiral catalysts that can be easily prepared and used to obtain organic molecules of high enantiomeric purity are critical to modern chemical synthesis. The development of protecting groups that shield reactive functionalities has also proved indispensable in the preparation of complex biologically active molecules. Here we present a chiral catalyst that promotes the enantioselective protection of a secondary alcohol as one of the most commonly used protected forms of an alcohol: a silyl ether. The catalyst is a small, simple molecule that can be prepared in three steps from commercial materials without the need for rigorously controlled conditions. Enantioselective silylations are performed with commercial silyl chlorides and produce yields of up to 96 per cent at an enantiomeric ratio of up to 98:2. Chiral catalysts for selective formation of commonly used protecting groups such as silyl ethers should significantly enhance the ability of chemical synthesis to deliver, in a more practical and efficient manner, important organic molecules.

A mechanistic rationalization of unusual kinetic behavior in proline-mediated C-O and C-N bond-forming reactions

Kinetic evidence supports the role of the reaction product in the catalytic cycle of proline-mediated alpha-aminoxylation and alpha-amination reactions, providing both design principles as well as a model for the evolution of efficiency in catalysis.

Enantioselective Friedel−Crafts Reaction of Indoles with Carbonyl Compounds Catalyzed by Bifunctional Cinchona Alkaloids

In this communication, we report an efficient asymmetric Friedel-Crafts reaction that, unprecedently, is applicable to a wide range of both indoles and carbonyls. The use of a readily accessible catalyst in combination with a high enantioselectivity that is insensitive to reaction concentration, temperature, air, and moisture should allow this reaction to provide useful enantioselective access to new chiral indole derivatives. [reaction: see text]

Design of Brønsted Acid-Assisted Chiral Brønsted Acid Catalyst Bearing a Bis(triflyl)methyl Group for a Mannich-Type Reaction

[Structure: see text] A new Brønsted acid-assisted chiral Brønsted (chiral BBA) acid catalyst (1) was developed by substituting a hydroxy group of optically active 1,1'-bi(2-naphthol) with a stronger Brønsted acidic group such as a bis(trifluoromethanesulfonyl)methyl group. The enantioselective Mannich-type reaction of ketene silyl acetals with aldimines catalyzed by (R)-1 in the presence of stoichiometric achiral proton sources gave (S)-beta-amino esters in high yield with moderate to good enantiomeric excesses.

Enantioselective construction of quaternary carbon centre catalysed by bifunctional organocatalyst

The bifunctional thiourea-tertiary amine derivatives of simple chiral diamines serve as highly enantioselective catalysts for the Michael addition of alpha-substituted cyanoacetates to vinyl sulfones, giving an efficient protocol for the construction of an all-carbon substituted quaternary stereocentre.

Electrostatic repulsion as an additional selectivity factor in asymmetric proline catalysis

The metal free, single amino acid-catalyzed asymmetric desymmetrization (ADS) of meso-compounds with nitrosobenzene has been investigated using DFT. In this communication, we describe the role of electrostatic and dipole-dipole interactions in amino acid-catalyzed reactions, which has not previously been invoked in discussions of these important reactions.

Hydrophobic amplification of noncovalent organocatalysis

The effects of hydrogen-bonding organocatalysts and water for the acceleration of epoxide openings with a variety of nucleophiles are additive and lead to excellent yields of the catalyzed reactions in water.

Parallel sheet structure in cyclopropane γ-peptides stabilized by C–H⋯O hydrogen bonds

A three-residue trans-cyclopropane gamma-peptide adopts an infinite parallel sheet structure in the solid state stabilized by intermolecular C-H...O hydrogen bonds, as demonstrated by single crystal X-ray diffraction.

Structural optimization of thiourea-based bifunctional organocatalysts for the highly enantioselective dynamic kinetic resolution of azlactones

This article describes the synthesis of a library of structurally diverse bifunctional organocatalysts bearing both a quasi-Lewis acidic (thio)urea moiety and a Brønsted basic tertiary amine group. Sequential modification of the modular catalyst structure and subsequent screening of the compounds in the alcoholytic dynamic kinetic resolution (DKR) of azlactones revealed valuable structure-activity relationships. In particular, a "hit-structure" was identified which provides e.g.N-benzoyl-tert-leucine allyl ester in an excellent enantiomeric excess of 95%.