Hydrogen-bonding Catalysts Research Articles

A Novel Hydrogen-Bonding Catalyst Class with Helical Chirality

A Novel Hydrogen-Bonding Catalyst Class with Helical Chirality

Design, synthesis, and application of chiral electron-poor guanidines as hydrogen-bonding catalysts for the Michael reaction

The first chiral electron-poor guanidine organocatalysts were synthesized. The presence of a tunable electron-withdrawing group on the guanidine moiety allows for the modulation of the catalysts’ activity. The application of this new class of organocatalysts to the Michael reaction is demonstrated.

Hydrogen-Bonding Catalysts are Hard to Predict

The authors synthesized various primary amine catalysts with fused hydrogenbonding donor moieties, among them 1-4, to study the influence of the pK a of the donor moiety, thus the hydrogen bonding ability in detail. The studied conjugate additions gave ambivalent results. However, it became clear that catalysts with either very acidic single hydrogen-bonding sites (1) or effective double hydrogen-bonding sites (3) are superior, since catalysts with less acidic (2) or without hydrogen-bonding abilities (4) were in most cases not comparably effective.

Helical Chiral 2-Aminopyridinium Ions: A New Class of Hydrogen Bond Donor Catalysts

Helical chiral 2-aminopyridinium ions were designed as a significantly more acidic (active) dual hydrogen-bonding catalyst than commonly used (thio)urea-based systems. The helicene framework was specifically utilized to position an inherently chiral barrier on the hydrogen-bonding side of the catalyst. The catalyst reactivity and enantioselectivity were successfully demonstrated in additions of 4,7-dihydroindoles to nitroalkenes (0.5-2 mol % catalyst loadings, up to 98:2 er).

Room temperature aldol reactions using magnetic Fe3O4@Fe(OH)3composite microspheres in hydrogen bond catalysis

Fe(3)O(4)@Fe(OH)(3) composite microspheres are highly active, environmentally friendly and easy to recycle catalysts for aldol reactions, which are catalyzed by a solid-liquid interfacial hydrogen bond catalyst at room temperature.

New architectures in hydrogen bond catalysis

New achiral sulfamide, phosphoric triamide, and thiophosphoric triamide compounds have been synthesized. Their activity as hydrogen bond catalysts for the Friedel–Crafts and Baylis–Hillman reactions compares favorably with that of a known and an active thiourea catalyst. The new compounds were also studied by X-ray crystallography and their solid state structures are described.

A New Class of Hydrogen-Bonding Catalysts: N-Sulfinyl Ureas

Chiral N-sulfinyl ureas have been developed as catalysts for an asymmetric aza-Henry reaction. Addition of nitroalkanes 3 to both aromatic and aliphatic N-Boc imines 2 in the presence of organocatalyst 1 (10 mol%) and i-Pr2NEt (50 mol%) gives protected nitroamines 4 in high yields, syn-diastereoselectivities, and enantio­selectivities. The authors present examples of enantioselective hydrogen-bonding-catalyzed additions to aliphatic N-Boc imines.

Design of Axially Chiral Dicarboxylic Acid for Asymmetric Mannich Reaction of Arylaldehyde N-Boc Imines and Diazo Compounds

An axially chiral dicarboxylic acid has been prepared as a new class of chiral hydrogen-bonding catalysts and applied to the highly enantioselective Mannich reaction of arylaldehyde N-Boc imines and diazo compounds.

Density functional study of noncovalent catalysis of the Diels–Alder reaction by the neutral hydrogen bond donors thiourea and urea

The Diels–Alder reaction between cyclopentadiene and methyl vinyl ketone has been investigated by density functional calculations at the B3LYP/6-31+G* level, both in the absence and presence of the hydrogen-bonding catalysts thiourea and urea. The computed transition structures are concerted, but highly asynchronous in all cases. The addition of thiourea or urea accelerates the reaction and slightly changes the endo/exo selectivity. The activation barriers are lowered by 2–3 kcal/mol, while the activation free energies show smaller reductions due to unfavorable entropic effects. The addition of BF3 causes a more pronounced decrease in the computed barriers by 8–11 kcal/mol. Despite their lower efficiency, hydrogen-bonding catalysts such as thiourea and urea may still offer an attractive alternative to Lewis acids because they do not suffer from product inhibition and can be used in aqueous solution.

Design of Hydrogen Bond Catalysts Based on a Modular Oxazoline Template: Application to an Enantioselective Hetero Diels—Alder Reaction.

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Design of Hydrogen Bond Catalysts Based on a Modular Oxazoline Template: Application to an Enantioselective Hetero Diels−Alder Reaction

[reaction: see text] A catalyst system that displays two hydrogen bond donating arms from a rigid oxazoline backbone and its utility in a hydrogen bond promoted enantioselective hetero Diels-Alder reaction are described.

Enantioselective Friedel–Crafts type addition of indoles to nitro-olefins using a chiral hydrogen-bonding catalyst – synthesis of optically active tetrahydro-β-carbolines

The enantioselective Friedel-Crafts addition of indoles to nitro-olefins using chiral hydrogen-bonding bis-sulfonamides as the catalysts has been developed. The reactions, in the presence of only 2 mol% catalyst, generally proceed in high yields and with enantioselectivities up to 64% ee, and the enantiomeric excess can be improved to >98% ee by recrystallization. Various synthetic transformations of the Friedel-Crafts adducts are demonstrated: the nitro group can easily be reduced to the corresponding amine and the product obtained can undergo a stereocontrolled Pictet-Spengler cyclization to give, for example, enantiopure tetrahydro-beta-carbolines. The X-ray structure of the chiral bis-sulfonamides has been determined and based on these structures the mechanism for the stereoselectivity in the reaction is discussed.

Aromatic nucleophilic substitutions with o- and p-fluoronitrobenzenes in aprotic solvents. Steric effects on the base-catalysed step

The kinetics of the reaction between o- and p-fluoronitrobenzene with n- and iso-propylamine were studied in toluene and DMSO at several amine concentrations and temperatures in the range 30–100 °C. The results show that, contrary to the previous assumption, primary steric effects due to branching in the amine do not produce a large decrease in the reaction rate when the first step is rate determining. However, reactions with bulky amines can be extremely slow because of the reduced power of the amine as a hydrogen-bond acceptor catalyst, when the second step is rate determining. The reaction rate may be increased by the addition of a non-nucleophilic base of another hydrogen-bond acceptor catalyst such as dimethyl sulphoxide.