H-bonding additives act like Lewis acid catalysts

Friedel–Crafts alkylation of indoles with nitroolefins in the presence of β-cyclodextrin in water under neutral conditions

Various indolyl nitroalkanes were prepared by the reaction of indoles with nitroolefins in the presence of β-cyclodextrin in water under neutral conditions. β-Cyclodextrin can be recovered and re-used several times without loss of activity.Key words: β-cyclodextrin, nitroolefins, indoles, neutral conditions, water.

Organocatalytic enantioselective Michael addition of 2,4-pentandione to nitroalkenes promoted by bifunctional thioureas with central and axial chiral elements

Two novel bifunctional amine-thiourea organocatalysts 1 and 2, which both bear central and axial chiral elements, have been developed to promote enantioselective Michael reaction between 1,3-dicarbonyl compounds and nitro olefins. The catalyst 2 afforded the desired products with good levels of enantioselectivity (up to 96% ee), showing clearly that two chiral elements of 2 are matched, and enhance the stereochemical control.

Hypervalent iodine(III)-mediated regioselective N-acylation of 1,3-disubstituted thioureas

Reaction of asymmetrical 1,3-disubstituted thioureas with diacetoxyiodobenzene (DIB) produces regioselectively N-acetylurea in shorter time. Regioselectivity is dependent on the pKa's of the amine attached to the thiourea moiety with acylation taking place toward the amine having a lower pKa. This is the first example of DIB being employed as an N-acetylating agent. A mechanism for this novel transformation is also proposed. Mild reaction conditions, shorter reaction times, high efficiencies, environmentally benign methods, and facile isolation of the desired product make the present methodology a most suitable alternative.

The catalytic acylcyanation of imines

The catalytic acylcyanation of aldimines with acylcyanides and a direct three-component variant involving the generation of an imine in situ have been developed. Furthermore, a highly enantioselective version has been established, culminating in the first organocatalytic asymmetric three-component Strecker reaction. Jacobsen thiourea catalysts were found to catalyze the reaction with excellent enantioselectivities, whereas binol phosphates (binol = 1,1'-bi-2,2'-naphthol) proved to be catalytically active but only modestly enantioselective. A large number of different substrates could be used in the processes described, thus illustrating the potential of our reaction for the generation of diversity within the attractive alpha-amino carbonyl framework. Furthermore, a novel cyclic amidine was obtained from the reaction of acetyl cyanide with ketimines.

Asymmetric catalysis with bifunctional cinchona alkaloid-based urea and thiourea organocatalysts

Cinchona alkaloid derivatives modified to include a (thio)urea component have emerged in the last three years as readily accessible, robust and tunable bifunctional organocatalysts for a range of synthetically useful transformations. The origins of these catalysts and the major developments in their application in enantioselective synthesis are reviewed.

Bifunctional-thiourea-catalyzed diastereo- and enantioselective aza-Henry reaction

Bifunctional thiourea 1a catalyzes aza-Henry reaction of nitroalkanes with N-Boc-imines to give syn-beta-nitroamines with good to high diastereo- and enantioselectivity. Apart from the catalyst, the reaction requires no additional reagents such as a Lewis acid or a Lewis base. The N-protecting groups of the imines have a determining effect on the chirality of the products, that is, the reaction of N-Boc-imines gives R adducts as major products, whereas the same reaction of N-phosphonoylimines furnishes the corresponding S adducts. Various types of nitroalkanes bearing aryl, alcohol, ether, and ester groups can be used as nucleophiles, providing access to a wide range of useful chiral building blocks in good yield and high enantiomeric excess. Synthetic versatility of the addition products is demonstrated by the transformation to chiral piperidine derivatives such as CP-99,994.

Direct asymmetric intermolecular aldol reactions catalyzed by amino acids and small peptides

In nature there are at least nineteen different acyclic amino acids that act as the building blocks of polypeptides and proteins with different functions. Here we report that alpha-amino acids, beta-amino acids, and chiral amines containing primary amine functions catalyze direct asymmetric intermolecular aldol reactions with high enantioselectivities. Moreover, the amino acids can be combined into highly modular natural and unusual small peptides that also catalyze direct asymmetric intermolecular aldol reactions with high stereoselectivities, to furnish the corresponding aldol products with up to >99 % ee. Simple amino acids and small peptides can thus catalyze asymmetric aldol reactions with stereoselectivities matching those of natural enzymes that have evolved over billions of years. A small amount of water accelerates the asymmetric aldol reactions catalyzed by amino acids and small peptides, and also increases their stereoselectivities. Notably, small peptides and amino acid tetrazoles were able to catalyze direct asymmetric aldol reactions with high enantioselectivities in water, while the parent amino acids, in stark contrast, furnished nearly racemic products. These results suggest that the prebiotic oligomerization of amino acids to peptides may plausibly have been a link in the evolution of the homochirality of sugars. The mechanism and stereochemistry of the reactions are also discussed.

Asymmetric catalysis by chiral hydrogen-bond donors

Hydrogen bonding is responsible for the structure of much of the world around us. The unusual and complex properties of bulk water, the ability of proteins to fold into stable three-dimensional structures, the fidelity of DNA base pairing, and the binding of ligands to receptors are among the manifestations of this ubiquitous noncovalent interaction. In addition to its primacy as a structural determinant, hydrogen bonding plays a crucial functional role in catalysis. Hydrogen bonding to an electrophile serves to decrease the electron density of this species, activating it toward nucleophilic attack. This principle is employed frequently by Nature's catalysts, enzymes, for the acceleration of a wide range of chemical processes. Recently, organic chemists have begun to appreciate the tremendous potential offered by hydrogen bonding as a mechanism for electrophile activation in small-molecule, synthetic catalyst systems. In particular, chiral hydrogen-bond donors have emerged as a broadly applicable class of catalysts for enantioselective synthesis. This review documents these advances, emphasizing the structural and mechanistic features that contribute to high enantioselectivity in hydrogen-bond-mediated catalytic processes.

Organocatalytic claisen rearrangement: theory and experiment

A combined computational and experimental study on the Claisen rearrangement of a 2-alkoxycarbonyl-substituted allyl vinyl ether in the presence of thioureas as potential noncovalent organocatalysts has been performed. DFT calculations employing different basis sets were utilized to predict a catalytic cycle for the thiourea-catalyzed Claisen rearrangement. The nature of the transition state in the presence and absence of thioureas was studied in detail. Critical geometrical data of the transition state that are indicators for the relative barrier height of the Claisen rearrangement are discussed. Although we did observe a significant transition state stabilization, due to endergonic conformational changes and endergonic complexation the overall effect on the barrier is small, in accordance with experimental results.

Organocatalysis Mediated by (Thio)urea Derivatives

Over the last decade the potential for N,N-dialkyl(thio)urea derivatives to serve as active metal-free organocatalysts for a wide range of synthetically useful reactions susceptible to the influence of general acid catalysis has begun to be realised. This article charts the development of these catalysts (with emphasis on the design principles involved), from early "proof-of-concept" materials to contemporary active chiral (bifunctional) promoters of highly selective asymmetric transformations.

Thiourea-Catalyzed Asymmetric Michael Addition of Activated Methylene Compounds to α,β-Unsaturated Imides: Dual Activation of Imide by Intra- and Intermolecular Hydrogen Bonding

A thiourea-catalyzed asymmetric Michael addition of activated methylene compounds to alpha,beta-unsaturated imides derived from 2-pyrrolidinone and 2-methoxybenzamide has been developed. In the case of 2-pyrrolidinone derivatives, the reaction with malononitrile proceeded in toluene with high enantioselectivity, providing the Michael adducts in good yields. However, the nucleophiles that could be used for this reaction were limited to malononitrile due to poor reactivity of the substrate. Further examination revealed that N-alkenoyl-2-methoxybenzamide was the best substrate among the corresponding benzamide derivatives bearing different substituents on the aromatic ring. Indeed, several activated methylene compounds such as malononitrile, methyl alpha-cyanoacetate, and nitromethane could be employed as a nucleophile to give the Michael adducts in good to excellent yields with up to 93% ee. The results of spectroscopic experiments clarified that this enhanced reactivity can be attributed to the intramolecular hydrogen-bonding interaction between the N-H of the imide and the methoxy group of the benzamide moiety. Thus, the key to the success of the catalytic enantioselective Michael addition is dual activation of the substrate by both intramolecular hydrogen bonding in the imide and intermolecular hydrogen bonding with thiourea 1a, as well as the activation of a nucleophile by the tertiary amine of the bifunctional thiourea.

Pyrrolidine−Thiourea as a Bifunctional Organocatalyst: Highly Enantioselective Michael Addition of Cyclohexanone to Nitroolefins

[reaction: see text] Asymmetric Michael additions of cyclohexanone to both aryl and alkyl nitroolefins in the presence of 20 mol % of organocatalyst 2b and 10 mol % of n-butyric acid afford adducts 5 with high diastereoselectivities and enantioselectivities.

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.

Highly enantioselective addition of ketones to nitroolefins catalyzed by new thiourea–amine bifunctional organocatalysts

A new and effective organocatalytic system: primary amine derived chiral thiourea catalyst and AcOH-H2O additive, which converts different ketones to gamma-nitroketones in high yields (82-99%) and enantioselectivities (90-99%) has been described.

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%.

Thiourea-catalyzed enantioselective cyanosilylation of ketones

The new chiral amino thiourea catalyst 3d promotes the highly enantioselective cyanosilylation of a wide variety of ketones. The hindered tertiary amine substituent plays a crucial role with regard to both stereoinduction and reactivity, suggesting a cooperative mechanism involving electrophile activation by thiourea and nucleophile activation by the amine.

Camphor Ionic Liquid: Correlation between Stereoselectivity and Cation−Anion Interaction

[structure: see text] As a halogen-free anion for an imidazolium room temperature ionic liquid, the use of a camphorsulfonate causes an increase in the number of free (naked) imidazolium cations, which produces an effective endo/exo stereoselective Diels-Alder reaction.