On the mechanism of bifunctional squaramide-catalyzed organocatalytic Michael addition: a protonated catalyst as an oxyanion hole

Cinchona-Based Hydrogen-Bond Donor Organocatalyst Metal Complexes: Asymmetric Catalysis and Structure Determination

In this study, we describe the synthesis of cinchona (thio)squaramide and a novel cinchona thiourea organocatalyst. These catalysts were employed in pharmaceutically relevant catalytic asymmetric reactions, such as Michael, Friedel-Crafts, and A3 coupling reactions, in combination with Ag(I), Cu(II), and Ni(II) salts. We identified several organocatalyst-metal salt combinations that led to a significant increase in both yield and enantioselectivity. To gain insight into the active catalyst species, we prepared organocatalyst-metal complexes and characterized them using HRMS, NMR spectroscopy, and quantum chemical calculations (B3LYP-D4/def2-TZVP), which allowed us to establish a structure-activity relationship.

Regioselectivity Switch between Enantioselective 1,2- and 1,4-Addition of Allyl Aryl Ketones with 2,3-Dioxopyrrolidines

A vinylogous addition reaction of allyl aryl ketones with good yields and excellent regioselectivity catalyzed by squaramide catalysts has been developed. A series of chiral tertiary alcohols and bicyclic pyrrolidones could be synthesized in good to excellent yields, enantioselectivities, and diaseteroselectivities. Both experimental results and DFT calculations indicate that 1,2-addition reaction is favorable when the reaction is employed at a lower temperature, while the 1,4-addition/cyclization pathway is favorable when the reaction is employed at a higher temperature. Furthermore, the formation of compound 4 can potentially arise from either the 1,4-addition/cyclization pathway or retro-aldol reaction of compound 3, followed by subsequent 1,4-addition/cyclization.

Squaramide Organocatalyzed Addition of a Masked Acyl Cyanide to β-Nitrostyrenes

A method for the squaramide-organocatalyzed enantioselective addition of a silyl-protected masked acyl cyanide (MAC) reagent to various β-nitrostyrenes is described. Reactions are carried out in a freezer and provide products cleanly and in high enantioselectivities at very low catalyst loadings. Adducts are then unmasked, providing various oxidation state 3 functional groups, thereby highlighting the utility of these MAC reagents and a new strategy for the preparation of β-amino acids.

Unravelling the Development of Non-Covalent Organocatalysis in India

This review is devoted to underpinning the contributions of Indian researchers towards asymmetric organocatalysis. More specifically, a comprehensive compilation of reactions mediated by a wide range of non-covalent catalysis is illustrated. A detailed overview of vividly catalogued asymmetric organic transformations promoted by hydrogen bonding and Brønsted acid catalysis, alongside an assortment of catalysts is provided. Although asymmetric organocatalysis has etched itself in history, we aim to showcase the scientific metamorphosis of Indian research from baby steps to large strides within this field.

1 Introduction

2 Non-Covalent Catalysis and Its Various Activation Modes

3 Hydrogen-Bonding Catalysis

3.1 Urea- and Thiourea-Derived Organocatalysts

3.1.1 Thiourea-Derived Organocatalysts

3.1.2 Urea-Derived Organocatalysts

3.2 Squaramide-Derived Organocatalysts

3.2.1 Michael Reactions

3.2.2 C-Alkylation Reactions

3.2.3 Mannich Reactions

3.2.4 [3+2] Cycloaddition Reactions

3.3 Cinchona-Alkaloid-Derived Organocatalysts

3.3.1 Michael Reactions

3.3.2 Aldol Reactions

3.3.3 Friedel–Crafts Reactions

3.3.4 Vinylogous Alkylation of 4-Methylcoumarins

3.3.5 C-Sulfenylation Reactions

3.3.6 Peroxyhemiacetalisation of Isochromans

3.3.7 Diels–Alder Reactions

3.3.8 Cycloaddition Reactions

3.3.9 Morita–Baylis–Hilman Reactions

4 Brønsted Acid Derived Organocatalysts

4.1 Chiral Phosphoric Acid Catalysis

4.1.1 Diels–Alder Reactions

4.1.2 Addition of Ketimines

4.1.3 Annulation of Acyclic Enecarbamates

5 Conclusion

Hydrogen-Bond-Donor-Directed Switching of Enantioselectivity in the Vinylogous Aldol-Cyclization Cascade Reaction of Prostereogenic 3-Alkylidene Oxindoles with Isatins and o-Quinones

In this study, we reported a hydrogen-bond-donor-directed enantiodivergent vinylogous aldol-cyclization cascade reaction of 3-alkylidene oxindoles with isatins and o-quinones. Both enantiomers can be prepared by thiourea or squaramide cinchona alkaloid bifunctional organocatalysts with the same quinine scaffold. Kinetic study data provided the possible reaction mechanism for the vinylogous aldol-cyclization cascade reaction. The DFT calculation data showed the geometry of the generated dienolates from pronucleophiles dominated the observed switch of enantioselectivity.

Bifunctional Iminophosphorane-Catalyzed Enantioselective Sulfa-Michael Addition to Unactivated α,β-Unsaturated Amides

The first metal-free catalytic intermolecular enantioselective Michael addition to unactivated α,β-unsaturated amides is described. Consistently high enantiomeric excesses and yields were obtained over a wide range of alkyl thiol pronucleophiles and electrophiles under mild reaction conditions, enabled by a novel squaramide-based bifunctional iminophosphorane catalyst. Low catalyst loadings (2.0 mol %) were achieved on a decagram scale, demonstrating the scalability of the reaction. Computational analysis revealed the origin of the high enantiofacial selectivity via analysis of relevant transition structures and provided substantial support for specific noncovalent activation of the carbonyl group of the α,β-unsaturated amide by the catalyst.

Synthesis of Methyl 4,6-Di-O-ethyl-α-d-glucopyranoside-Based Azacrown Ethers and Their Effects in Asymmetric Reactions

Carbohydrate-based crown ethers have been reported to be able to generate asymmetric induction in certain reactions. Previously, it was proved that the monosaccharide unit, the anomeric substituent, and the sidearm could influence the catalytic activity of the monoaza-15-crown-5 macrocycles derived from sugars. In order to gain information about the effect of the flexibility, 4,6-di-O-ethyl-glucoside-based crown compounds were synthesized, and their efficiency was compared to the 4,6-O-benzylidene analogues. It was found that the absence of the two-ring annulation has a negative effect on the enantioselectivity in liquid-liquid two-phase reactions: in the Darzens condensation of 2-chloroacetophenone and in the epoxidation of chalcone. The same trend was observed in the solid-liquid phase Michael addition of diethyl acetamidomalonate. Surprisingly, in the solid-liquid phase cyclopropanation of benzylidenemalononitrile, one of the new catalysts was highly enantioselective (99% ee).

A bifunctional iminophosphorane squaramide catalyzed enantioselective synthesis of hydroquinazolines via intramolecular aza-Michael reaction to α,β-unsaturated esters

An efficient synthesis of enantioenriched hydroquinazoline cores via a novel bifunctional iminophosphorane squaramide catalyzed intramolecular aza-Michael reaction of urea-linked α,β-unsaturated esters is described. The methodology exhibits a high degree of functional group tolerance around the forming hydroquinazoline aryl core and wide structural variance on the nucleophilic N atom of the urea moiety. Excellent yields (up to 99%) and high enantioselectivities (up to 97 : 3 er) using both aromatic and less acidic aliphatic ureas were realized. The potential industrial applicability of the transformation was demonstrated in a 20 mmol scale-up experiment using an adjusted catalyst loading of 2 mol%. The origin of enantioselectivity and reactivity enhancement provided by the squaramide motif has been uncovered computationally using density functional theory (DFT) calculations, combined with the activation strain model (ASM) and energy decomposition analysis (EDA).

The activation of both aromatic and aliphatic ureas as N-centered nucleophiles in intramolecular Michael addition reactions to α,β-unsaturated esters was achieved under bifunctional iminophosphorane squaramide superbase catalysis.

Probing α-Amino Aldehydes as Weakly Acidic Pronucleophiles: Direct Access to Quaternary α-Amino Aldehydes by an Enantioselective Michael Addition Catalyzed by Brønsted Bases

The high tendency of α-amino aldehydes to undergo 1,2-additions and their relatively low stability under basic conditions have largely prevented their use as pronucleophiles in the realm of asymmetric catalysis, particularly for the production of quaternary α-amino aldehydes. Herein, it is demonstrated that the chemistry of α-amino aldehydes may be expanded beyond these limits by documenting the first direct α-alkylation of α-branched α-amino aldehydes with nitroolefins. The reaction produces densely functionalized products bearing up to two, quaternary and tertiary, vicinal stereocenters with high diastereo- and enantioselectivity. DFT modeling leads to the proposal that intramolecular hydrogen bonding between the NH group and the carbonyl oxygen atom in the starting α-amino aldehyde is key for reaction stereocontrol.

Bifunctional squaramides with benzyl-like fragments: analysis of CH⋯π interactions by a multivariate linear regression model and quantum chemical topology

A multivariate linear regression model and quantum chemical topology are used for the quantitative description of non-covalent interactions in the transition state of the Michael addition catalyzed by bifunctional squaramides.

Synthesis and Applications of Cinchona Squaramide-Modified Poly(Glycidyl Methacrylate) Microspheres as Recyclable Polymer-Grafted Enantioselective Organocatalysts

This work presents the immobilization of cinchona squaramide organocatalysts on poly(glycidyl methacrylate) solid supports. Preparation of the well-defined monodisperse polymer microspheres was facilitated by comprehensive parameter optimization. By exploiting the reactive epoxy groups of the polymer support, three amino-functionalized cinchona derivatives were immobilized on this carrier. To explore the effect of the amino linker, these structurally varied precatalysts were synthesized by modifying the cinchona skeleton at different positions. The catalytic activities of the immobilized organocatalysts were tested in the Michael addition of pentane-2,4-dione and trans-β-nitrostyrene with excellent yields (up to 98 %) and enantioselectivities (up to 96 % ee). Finally, the catalysts were easily recovered five times by centrifugation without loss of activity.

Organocatalytic 1,4-Addition of Azadienes with 3-Homoacyl Coumarins toward Highly Enantioenriched Benzofuran Coumarin Skeletons

A highly enantioselective 1,4-addition reaction of azadiene with 3-homoacyl coumarin has been accomplished by low amounts of bifunctional cinchona alkaloid catalysis under mild conditions. Varieties of benzofuran coumarin skeletons were obtained in moderate to high yields (up to 99%) with excellent enantioselectivities (up to 99% ee) and complete diastereoselectivity.

H-bond donor-directed switching of diastereoselectivity in the Michael addition of α-azido ketones to nitroolefins

The development of catalyst-controlled stereodivergent asymmetric catalysis is important for providing facile access to all stereoisomers of chiral products with multiple stereocenters from the same starting materials. Despite progress, new design strategies for diastereodivergent asymmetric catalysis are still highly desirable. Here we report the potency of H-bond donors as the governing factor to tune diastereoselectivity in a highly diastereoselective switchable enantioselective Michael addition of α-azido ketones to nitroolefins. While a newly developed bifunctional tertiary amine, phosphoramide, preferentially afforded syn-adducts, an analogous squaramide catalyst selectively gave anti-adducts. The resulting multifunctional tertiary azides can be converted to spiro-pyrrolidines with four continuous stereocenters in a one-pot operation. Mechanistic studies cast light on the control of diastereoselectivity by H-bond donors. While the squaramide-catalyzed reaction proceeded with a transition state with both squaramide N–H bonds binding to an enolate intermediate, an unprecedented model was proposed for the phosphoramide-mediated reaction wherein an amide N–H bond and an alkylammonium ion formed in situ interact with nitroolefins, with the enolate stabilized by nonclassical C–H⋯O hydrogen-bonding interactions.

We report the successful reversal of the diastereoselectivity in an unprecedented Michael addition of α-azido ketones to nitroolefins catalyzed by bifunctional tertiary amines, simply by varying the H-bond donor from phosphoramide to squaramide.

Catalytic enantioselective domino Michael/transannular aldol reaction under bifunctional catalysis

Chiral bifunctional tertiary amine/squaramides catalyze the enantioselective Michael/transannular aldol reaction on medium-sized cyclic ketoenones leading to bicycle[5.4.0]undecanes.

Comparison of Cinchona Catalysts Containing Ethyl or Vinyl or Ethynyl Group at Their Quinuclidine Ring

Numerous cinchona organocatalysts with different substituents at their quinuclidine unit have been described and tested, but the effect of those saturation has not been examined before. This work presents the synthesis of four widely used cinchona-based organocatalyst classes (hydroxy, amino, squaramide, and thiourea) with different saturation on the quinuclidine unit (ethyl, vinyl, ethynyl) started from quinine, the most easily available cinchona derivative. Big differences were found in basicity of the quinuclidine unit by measuring the pKa values of twelve catalysts in six solvents. The effect of differences was examined by testing the catalysts in Michael addition reaction of pentane-2,4-dione to trans-β-nitrostyrene. The 1.6-1.7 pKa deviation in basicity of the quinuclidine unit did not result in significant differences in yields and enantiomeric excesses. Quantum chemical calculations confirmed that the ethyl, ethynyl, and vinyl substituents affect the acid-base properties of the cinchona-thiourea catalysts only slightly, and the most active neutral thione forms are the most stable tautomers in all cases. Due to the fact that cinchonas with differently saturated quinuclidine substituents have similar catalytic activity in asymmetric Michael addition application of quinine-based catalysts is recommended. Its vinyl group allows further modifications, for instance, recycling the catalyst by immobilization.

A nanoporous metal-organic framework as a renewable size-selective hydrogen-bonding catalyst in water

A novel squaramide-containing metal-organic framework (MOF) material has been designed and synthesized. A detailed X-ray crystal structure analysis showed that four squaramides of this MOF adopted two orientations in each dependent nanopore, confirming that two carbonyl and two N-H groups pointed simultaneously to the inside of the one-dimensional nanometer channel. The MOF was applied as an efficient bifunctional hydrogen-bonding catalyst for Michael additions of 1,3-dicarbonyl compounds to nitroalkenes in pure water, boosting the catalytic efficiency by up to approximately five times the value afforded by the homogeneous control and exhibiting a highly size-selective catalytic performance and good renewability. The catalytic mechanism was also discussed in detail. The present study provides a highly promising approach to achieving dual-activation catalytic centers in a single system, which function as microscopic chemical reactors that allow the interaction and fast transport of substrate molecules in their cavities.

Cinchona Alkaloids-Derivatives and Applications

Major Cinchona alkaloids quinine, quinidine, cinchonine, and cinchonidine are available chiral natural compounds (chiral pool). Unlike many other natural products, these alkaloids are available in multiple diastereomeric forms which are separated on an industrial scale. The introduction discusses in short conformational equilibria, traditional separation scheme, biosynthesis, and de novo chemical syntheses. The second section concerns useful chemical applications of the alkaloids as chiral recognition agents and effective chiral catalysts. Besides the Sharpless ethers and quaternary ammonium salts (chiral PTC), the most successful bifunctional organocatalysts are based on 9-amino derivatives: thioureas and squaramides. The third section reports the main transformations of Cinchona alkaloids. This covers reactions of the 9-hydroxyl group with the retention or inversion of configuration. Specific Cinchona rearrangements enlarging [2.2.2]bicycle of quinuclidine to [3.2.2] products are connected to the 9-OH substitution. The syntheses of numerous esterification and etherification products are described, including many examples of bi-Cinchona alkaloid ethers. Further derivatives comprise 9-N-substituted compounds. The amino group is introduced via an azido function with the inversion of configuration at the stereogenic center C9. The 9-epi-amino-alkaloids provide imines, amides, imides, thioureas, and squaramides. The syntheses of 9-carbon-, 9-sulfur-, and 9-selenium-substituted derivatives are discussed. Oxidation of the hydroxyl group of any alkaloid gives ketones, which can be selectively reduced, reacted with Grignard reagents, or subjected to the Corey-Chaykovsky reaction. The alkaloids were also partially degraded by splitting C4'-C9 or N1-C8 bonds. In order to immobilize Cinchona alkaloids the transformations of the 3-vinyl group were often exploited. Finally, miscellaneous functionalizations of quinuclidine, quinoline, and examples of various metal complexes of the alkaloids are considered.

Stereoselective synthesis of spirocyclohexadiene-pyrazolones via organic base and/or hydrogen bonding assisted [3 + 3] annulation reactions

A DABCO and a chiral hydrogen bonding-assisted catalyzed [3 + 3] cycloaddition of α-arylidene pyrazolinones and 2-benzylidenemalononitriles provided spirocyclohexadiene-pyrazolones with all-carbon quaternary centers in good yields with excellent ees and moderate to high drs.

Enantioselective synthesis of spiro[4H-pyran-3,3'-oxindole] derivatives catalyzed by cinchona alkaloid thioureas: Significant water effects on the enantioselectivity

An efficient stereoselective three-component reaction for the synthesis of functionalized spiro[4H-pyran-3,3'-oxindole] derivatives was realized through an organocatalyzed domino Knoevenagel/Michael/cyclization reaction using a cinchonidine-derived thiourea as the catalyst. Using water as the additive was found to improve the product ee values significantly. Under the optimized conditions, the reactions between isatins, malononitrile, and 1,3-dicarbonyl compounds yield the desired spirooxindole products in good yields (71-92%) and moderate to high ee values (up to 87% ee).

Thiourea-Catalyzed Enantioselective Malonate Addition onto 3-Sulfonyl-3'-indolyl-2-oxindoles: Formal Total Syntheses of (-)-Chimonanthine, (-)-Folicanthine, and (+)-Calycanthine

A general approach to bispyrroloindolines via a key thiourea-catalyzed addition of malonates to 3-sulfonyl-3'-indolyl-2-oxindoles is reported. The enantioselelective process is found to be highly effective (up to 94% ee), where a C-C bond formation leads to the synthesis of a number of 2-oxindoles with an all-carbon quaternary stereocenter.

Enantioselective Synthesis of 5,5-Disubstituted Hydantoins by Brønsted Base/H-Bond Catalyst Assisted Michael Reactions of a Design Template

A new method for the enantioselective synthesis of 5,5-disubstituted (quaternary) hydantoins was developed on the basis of an organocatalytic Michael reaction approach involving the use of 2-benzylthio-3,5-dihydroimidazol-4-ones as key hydantoin surrogates. The method is general with respect to the substitution pattern at the hydantoin N¹ (alkyl, aryl, acyl), N³ (aryl), and C⁵ (linear/branched alkyl, aryl) positions and affords essentially single diastereomeric products with enantioselectivities higher than 95 % ee in most cases. Among the bifunctional Brønsted base/H-bond catalysts examined, a known squaramide-tertiary amine catalyst and a newly prepared squaramide-tertiary amine catalyst provide the highest selectivity so far with either nitroolefins or vinyl ketones as the acceptor components. Kinetic measurements support a first-order rate dependence on both reaction partners, the donor template and the Michael acceptor, whereas competitive ¹ H NMR spectroscopy experiments reveal the high ability of the template for catalyst binding.

Conformational control of nonplanar free base porphyrins: towards bifunctional catalysts of tunable basicity

No metal needed: nonplanar free base porphyrins act as bifunctional organocatalysts revealing a new mode of action for porphyrins.

Enantioselectivity in CPA-catalyzed Friedel–Crafts reaction of indole and N-tosylimines: a challenge for guiding models

Reaction models derived from theoretical investigations for predicting the enantioselectivity of organocatalytic reactions are very useful, but difficult to formulate for the Friedel–Crafts reaction of indole and N-tosylimines.

Catalytic Asymmetric Synthesis of Quaternary Barbituric Acids

The catalytic asymmetric α-functionalization of prochiral barbituric acids, a subtype of pseudosymmetric 1,3-diamides, to yield the corresponding 5,5-disubstituted (quaternary) derivatives remains essentially unsolved. In this study 2-alkylthio-4,6-dioxopirimidines are designed as key 1,3-diamide surrogates that perform exceedingly in amine-squaramide catalyzed C-C bond forming reactions with vinyl ketones or Morita-Baylis-Hillmann-type allyl bromides as electrophiles. Mild acid hydrolysis of adducts affords barbituric acid derivatives with an in-ring quaternary carbon in unprecedented enantioselectivity, offering valuable materials for biological evaluations.

Mechanism and Origins of Stereoselectivity in the Cinchona Thiourea- and Squaramide-Catalyzed Asymmetric Michael Addition of Nitroalkanes to Enones

We report density functional theory calculations that examine the mechanism and origins of stereoselectivity of Soós' landmark discovery from 2005 that cinchona thioureas catalyze the asymmetric Michael addition of nitroalkanes to enones. We show that the electrophile is activated by the catalyst's protonated amine and that the nucleophile binds to the thiourea moiety by hydrogen bonding. These results lead to the correction of published mechanistic work which did not consider this activation mode. We have also investigated the corresponding cinchona squaramide-catalyzed reaction and found that it proceeds by the same mechanism despite the differences in the geometry of the two catalysts' hydrogen-bond-donating groups, which demonstrates the generality of this mechanistic model.