Aqueous enantioselective organocatalytic Diels-Alder reactions employing hydrazide catalysts. A new scaffold for organic acceleration

One-step assembly of a MacMillan catalyst-based phenolic-type polymer

We report herein a "bottom-up" approach for the one-step assembly of a MacMillan catalyst-based phenolic-type polymer (Mac-CP). The resulting self-supported polymeric organocatalyst possesses homogeneously distributed and highly concentrated catalytic sites. Furthermore, Mac-CP is soluble in CH₃CN but insoluble in hexane. This unique property can be used to employ the polymer as an efficient catalyst in homogeneous organocatalysis and heterogeneous recycling. As a result, Mac-CP possesses comparable catalytic activity and enantioselectivity to its homogeneous counterpart in the asymmetric Diels-Alder reaction (95% yield, 93% enantiomeric excess (ee) for endo and 92% ee for exo).

Vibrational CD study on the solution phase structures of the MacMillan catalyst and its corresponding iminium ion

We demonstrate that VCD spectroscopy can reveal insights into the conformational preferences of the iminium ion obtained from MacMillan's imidazolidinone catalyst. For both the isolated and in situ generated iminium ion, the comparison of experimental and computed VCD spectra directly confirms that conformer 2b ("Houk-conformer") is the dominant structure in solution. This conclusion is reached without any in-depth interpretation of the spectroscopic data, just by visual comparison of the spectral signatures. For the parent catalyst 1 and its salts 1·HCl and 1·HClO₄, we report a comprehensive analysis of the conformational preferences in two solvents. VCD spectroscopy is subsequently shown to be able to reveal small conformational changes induced by solute-solvent and solute-anion interactions.

DABCO-based chiral ionic liquids as recoverable and reusable organocatalyst for asymmetric Diels-Alder reaction

New DABCO-based chiral ionic liquids were synthesized and evaluated in asymmetric Diels-Alder reaction of cyclopentadiene with α,β-unsaturated aldehydes or 4-phenyl-3-buten-2-one. Chiral ionic liquid of modified MacMillan catalyst having a DABCO cation and hexafluorophosphate anion acts as organocatalyst (5 mol%) for the Diels-Alder reaction of crotonaldehyde and cyclopentadiene producing 98% of the product and 87% ee (endo) in CH₃ CN/H₂ O (95/5) at 25°C in 2 h. The scope and limitations of the catalysis were also studied by using cyclopentadiene and α,β-unsaturated aldehydes, and the Diels-Alder products were obtained in 18%-92% yields with 68%-93% ee. The catalyst was recycled and reused up to 6 cycles with a slight drop in ee and conversion of the product.

Enamides and dienamides in phosphoric acid-catalysed enantioselective cycloadditions for the synthesis of chiral amines

This feature article describes how enamides and dienamides can participate in chiral phosphoric acid catalyzed enantioselective cycloadditions to prepare a wide range of cyclic amines.

Ruthenium-Catalyzed Enantioselective Hydrogenation of Hydrazones

Prochiral hydrazones undergo efficient and highly selective hydrogenation in the presence of a chiral diphosphine ruthenium catalyst, yielding enantioenriched hydrazine products (up to 99% ee). The mild reaction conditions and broad functional group tolerance of this method allow access to versatile chiral hydrazine building blocks containing aryl bromide, heteroaryl, alkyl, cycloalkyl, and ester substituents. This method was also demonstrated on >150 g scale, providing a valuable hydrazine intermediate en route to an active pharmaceutical ingredient.

Stereodivergent Synthesis of Camphor-Derived Diamines and Their Application as Thiourea Organocatalysts

A series of 18 regio- and stereo-chemically diverse chiral non-racemic 1,2-, 1,3-, and 1,4-diamines have been synthesized from commercial (1S)-(+)-ketopinic acid and (1S)-(+)-10-camphorsulfonic acid. The structures of the diamines are all based on the d-(+)-camphor scaffold and feature isomeric diversity in terms of regioisomeric attachment of the primary and the tertiary amine function and the exo/endo-isomerism. Diamines were transformed into the corresponding noncovalent bifunctional thiourea organocatalysts, which have been evaluated for catalytic activity in the conjugative addition of 1,3-dicarbonyl nucleophiles (dimethyl malonate, acetylacetone, and dibenzoylmethane) to trans-β-nitrostyrene. The highest enantioselectivity was achieved in the reaction with acetylacetone as nucleophile using endo-1,3-diamine derived catalyst 52 (91.5:8.5 er). All new organocatalysts 48-63 have been fully characterized. The structures and the absolute configurations of eight intermediates and thiourea derivative 52 were also determined by X-ray diffraction.

Organocatalytic synthesis of 5-hydroxyisoxazolidine catalyzed by camphor sulfonyl hydrazines through aza-Michael addition/cyclization

A series of chiral 5-hydroxy isoxazolidines has been successfully synthesized through camphor sulfonyl hydrazine-catalyzed asymmetric aza-Michael addition reaction between N,O-protected hydroxyamines and enals. Moderate yields with moderate to good enantioselectivities (up to 96% enantiomeric excess [ee]) were achieved. It provides an alternative asymmetric approach to preparing isoxazolidine derivatives.

Chiral Imidazolidin-4-one with catalytic amount of Dicationic ionic liquid act as a recoverable and reusable Organocatalyst for asymmetric Diels-Alder reaction

Imidazolidin-4-one is used as a recoverable organocatalyst for the asymmetric Diels-Alder reaction in the presence of catalytic amount of dicationic ionic liquid and trifluoroacetic acid as a co-catalyst. The Diels-Alder reaction between model substrate cyclopentadiene and crotonaldehyde gave the product in 95% conversion and 87% ee of the endo-product. The catalyst was shown better reusability when the 20 mol% of dicationic ionic liquid was used and catalyst was reused upto 5 cycles, conversion remains upto 3 recycles but ee of endo-9 was slightly droped.

Organocatalytic asymmetric Michael addition between 3-subsituted oxindoles and enals catalyzed by camphor sulfonyl hydrazine

3,3-Disubstituted oxindoles containing vicinal stereogenic carbon centers have been synthesized through organocatalytic asymmetric Michael addition between 3-substituted oxindoles and enals catalyzed by chiral camphor sulfonyl hydrazines (CaSHs).

Novel binaphthyl and biphenyl α- and β-amino acids and esters: organocatalysis of asymmetric Diels-Alder reactions. A combined synthetic and computational study

Asymmetric catalysis of the Diels-Alder reaction between cyclopentadiene and cinnamaldehydes has been studied using as catalysts a range of novel α- and β-aminoacids and aminoesters with binaphthyl and biphenyl backbones, providing enantioselectivities of up to 62% ee. B3LYP/6-31G* calculations, including free energy corrections, have been carried out on a binaphthyl catalyst example to identify transition state structures and to aid in the identification of major enantiomers. The calculated product ratios agree well with the experimental data; the transition states identified involve preferential approach of cyclopentene along a trajectory adjacent to the acid/ester group. The four lowest energy transition states display a stabilizing dipolar interaction between the carbonyl group oxygen atom and a terminal proton of the diene unit.

Asymmetric Diels-Alder Reaction of α-Substituted and β,β-Disubstituted α,β-Enals via Diarylprolinol Silyl Ether for the Construction of All-Carbon Quaternary Stereocenters

The asymmetric Diels-Alder reaction of α-substituted acrolein proceeds in the presence of the trifluoroacetic acid salt of trifluoromethyl-substituted diarylprolinol silyl ether to afford the exo-isomer with both excellent diastereoselectivity and high enantioselectivity. In the Diels-Alder reaction of a β,β-disubstituted α,β-unsaturated aldehyde, good exo-selectivity and excellent enantioselectivity was obtained when the perchloric acid salt of the bulky triisopropyl silyl ether of trifluoromethyl substituted diarylprolinol was employed as an organocatalyst in the presence of water. In both cases, all-carbon quaternary stereocenters are constructed enantioselectively.

An Organocatalytic Cope Rearrangement

The first example of an organocatalytic Cope rearrangement is reported. Acyclic and cyclic acyl hydrazides catalyze the rearrangement of 1,5-hexadiene-2-carboxaldehydes via iminium ion formation. A correlation between ring size and catalyst activity was observed for the cyclic hydrazides, with seven- and eight-membered-ring catalysts being the most active. Diazepane carboxylate 5 c (10 mol %) catalyzed the rearrangement of a range of dienes at room temperature in acetonitrile using triflic acid as a co-catalyst. Preliminary proof of principle for asymmetric catalysis was provided by rearrangement of 3,3-dimethyl-7-phenyl-1,5-heptadiene-2-carboxaldehyde in the presence of a novel 7-substituted diazepane carboxylate.

Caveat in the stereochemical outcome of the organocatalytic Diels–Alder reaction in PEG-400

The organocatalytic Diels–Alder reaction in non-conventional solvent (PEG-400) has yielded cycloaddition products with diastereoselectivities hitherto not reported in detail using classical reaction conditions.

Improving catalyst activity in secondary amine catalysed transformations

The effect on catalyst performance of altering substituents at the 2-position of the Macmillan imidazolidinone has been examined. Condensation of L-phenylalanine N-methyl amide with acetophenone derivatives results in a series of imidazolidinones whose salts can be used to accelerate the Diels-Alder cycloaddition. Electron withdrawing groups significantly increase the overall rate of cycloaddition without compromise in selectivity. The most effective catalyst was shown to be efficient for a variety of substrates and the applicability of this catalyst to alternative secondary amine catalysed transformations is also discussed.

New insights into the asymmetric Diels–Alder reaction: the endo- and S-selective retro-Diels–Alder reaction

The endo- and S-selective retro-Diels–Alder reactions have been proposed and verified in an imidazolethione-catalyzed asymmetric Diels–Alder reaction.

Synthesis of MacMillan catalyst modified with ionic liquid as a recoverable catalyst for asymmetric Diels–Alder reaction

MacMillan catalyst was modified with imidazolium ionic liquid by ester linkage and acts as recoverable and reusable catalyst for asymmetric Diels–Alder reactions.

Stereoselectivity through a network of non-classical CH weak interactions: a prospective study of a bicyclic organocatalytic scaffold

The preferred stereoisomeric product of this catalytic Diels–Alder reaction is in part determined by noncovalent CH⋯π interactions.

Recent progress in asymmetric Biginelli reaction

The Biginelli reaction, which involves the interaction of ethyl acetoacetate, urea, and an appropriate aryl aldehyde, was first discovered by Pietro Biginelli about 120 years ago. The Biginelli products (3,4-dihydropyrimidin-2(1H)-ones) are interesting materials due to their significant pharmacological and structural profiles. In the last decades, the asymmetric synthesis as a powerful tool has an effective impact on the Biginelli products and has increased their potencies and applications as drugs. Having the importance of this subject in mind, in this review we wish to present the recent rapid progress of asymmetric Biginelli reaction.

A quantitative approach to nucleophilic organocatalysis

The key steps in most organocatalytic cyclizations are the reactions of electrophiles with nucleophiles. Their rates can be calculated by the linear free-energy relationship log k(20 °C) = s(N)(E + N), where electrophiles are characterized by one parameter (E) and nucleophiles are characterized by the solvent-dependent nucleophilicity (N) and sensitivity (s(N)) parameters.Electrophilicity parameters in the range -10 < E < -5 were determined for iminium ions derived from cinnamaldehyde and common organocatalysts, such as pyrrolidines and imidazolidinones, by studying the rates of their reactions with reference nucleophiles. Iminium activated reactions of α,β-unsaturated aldehydes can, therefore, be expected to proceed with nucleophiles of 2 < N < 14, because such nucleophiles are strong enough to react with iminium ions but weak enough not to react with their precursor aldehydes. With the N parameters of enamines derived from phenylacetaldehyde and MacMillan's imidazolidinones one can rationalize why only strong electrophiles, such as stabilized carbenium ions (-8 < E < -2) or hexachlorocyclohexadienone (E = -6.75), are suitable electrophiles for enamine activated reactions with imidazolidinones. Several mechanistic controversies concerning iminium and enamine activated reactions could thus be settled by studying the reactivities of independently synthesized intermediates.Kinetic investigations of the reactions of N-heterocyclic carbenes (NHCs) with benzhydrylium ions showed that they have similar nucleophilicities to common organocatalysts (e.g., PPh(3), DMAP, DABCO) but are much stronger (100-200 kJ mol(-1)) Lewis bases. While structurally analogous imidazolylidenes and imidazolidinylidenes have comparable nucleophilicities and Lewis basicities, the corresponding deoxy Breslow intermediates differ dramatically in reactivity. The thousand-fold higher nucleophilicity of 2-benzylidene-imidazoline relative to 2-benzylidene-imidazolidine is explained by the gain of aromaticity during electrophilic additions to the imidazoline derivatives. O-Methylated Breslow intermediates are a hundred-fold less nucleophilic than deoxy Breslow intermediates.

Effect of counterion structure on rates and diastereoselectivities in α,β-unsaturated iminium-ion Diels-Alder reactions

The use of cyclic α,β-unsaturated iminium-ion dienophiles is documented in two highly diastereoselective Diels-Alder (DA) reactions. The dienophilic counterion was found to have a significant effect on reactivity.

A novel hydrazide type organocatalyst for enantioselective Diels-Alder reactions

The development of a new class of hydrazide type organocatalyst, (4R,5R)-1,3-bis(isopropylamino)-4,5-dihenylimidazolidin-2-one 2a, for enantioselective Diels-Alder reactions between cyclopentadiene and α,β-unsaturated aldehydes are presented. The new organocatalyst 2a promoted the reaction, affording Diels-Alder adducts in good yields with good levels of enantioselectivity.

Mechanistic investigation of the enantioselective intramolecular stetter reaction: proton transfer is the first irreversible step

A study on the mechanism of the asymmetric intramolecular Stetter reaction is reported. This investigation includes the determination of the rate law, kinetic isotope effects, and competition experiments. The reaction was found to be first order in aldehyde and azolium catalyst or free carbene. A primary kinetic isotope effect was found for the proton of the aldehyde. Taken together with a series of competition experiments, these results suggest that proton transfer from the tetrahedral intermediate formed upon nucleophilic attack of the carbene onto the aldehyde is the first irreversible step.