Organocatalyzed Synthesis of γ-Alkenyl Butenolides via Asymmetric Direct Vinylogous Conjugate Addition-Elimination of Substituted Furanone Derivatives to β-Phenylsulfonylenones

A diaminomethylenemalononitrile organocatalyst efficiently promoted the asymmetric direct vinylogous conjugate addition of α-angelica lactone derivatives to β-phenylsulfonylenones, affording the corresponding γ-alkenyl γ-butenolides in high yields with excellent enantioselectivities (up to 97% ee) after the elimination of the phenylsulfonyl group. This study reports the first successful example of a stereoselective reaction using β-phenylsulfonylenone as the direct alkenyl donor.

Electrostatically Enhanced 3- and 4-Pyridyl Borate Salt Nucleophiles and Bases

A variety of electrostatically enhanced 3- and 4-pyridylborate salt catalysts are reported and show significant improvement over an activated noncharged neutral control compound. Their nucleophilicity in a stoichiometric SN2 reaction and catalytic performance in a urethane synthesis are evaluated along with three methods for rapidly evaluating the basicity of these species. That is, qualitative titrations in CH2Cl2 and CHCl3 were carried out, two separate solution-state IR studies in CCl4 and CDCl3 are reported, and the proton affinities of the anionic components of the salts were computed. Charge differences between the anion and its protonated zwitterionic conjugate acid are evaluated along with the highest occupied molecular orbitals of the anions in relationship to some of the surprising reactivity findings that were observed in the two kinetic studies.

Spin-induced asymmetry reaction-The formation of asymmetric carbon by electropolymerization

We describe the spin polarization-induced chirogenic electropolymerization of achiral 2-vinylpyridine, which forms a layer of enantioenhanced isotactic polymer on the electrode. The product formed is enantioenriched in asymmetric carbon polymer. To confirm the chirality of the polymer film formed on the electrode, we also measured its electron spin polarization properties as a function of its thickness. Two methods were used: First, spin polarization was measured by applying magnetic contact atomic force microscopy, and second, magnetoresistance was assessed in a sandwich-like four-point contact structure. We observed high spin-selective electron transmission, even for a layer thickness of 120 nm. A correlation exists between the change in the circular dichroism signal and the change in the spin polarization, as a function of thickness. The spin-filtering efficiency increases with temperature.

New bifunctional 1,3-diamine organocatalysts derived from (+)-camphoric acid for asymmetric Michael addition of 1,3-dicarbonyl compounds to nitroolefins

Novel 1,3-diamine-derived bifunctional thiourea and squaramide organocatalysts were synthesized from (+)-camphoric acid. These catalysts were easily obtained in up to two to five synthetic steps, in a flexible approach that facilitates their structure variation. Their catalytic activity was examined in the asymmetric Michael addition of 1,3-dicarbonyl compounds to several trans-β-nitrostyrenes. Yields up to 98% and enantiomeric excesses up to 74% and high diastereoselectivities when applicable (dr up to 93:7) were obtained in these reactions showing that 1,3-diamine-derived bifunctional thioureas are efficient organocatalysts.

Horizons in Asymmetric Organocatalysis: En Route to the Sustainability and New Applications

Nowadays, the development of new enantioselective processes is highly relevant in chemistry due to the relevance of chiral compounds in biomedicine (mainly drugs) and in other fields, such as agrochemistry, animal feed, and flavorings. Among them, organocatalytic methods have become an efficient and sustainable alternative since List and MacMillan pioneering contributions were published in 2000. These works established the term asymmetric organocatalysis to label this area of research, which has grown exponentially over the last two decades. Since then, the scientific community has attended to the discovery of a plethora of organic reactions and transformations carried out with excellent results in terms of both reactivity and enantioselectivity. Looking back to earlier times, we can find in the literature a few examples where small organic molecules and some natural products could act as effective catalysts. However, with the birth of this type of catalysis, new chemical architectures based on amines, thioureas, squaramides, cinchona alkaloids, quaternary ammonium salts, carbenes, guanidines and phosphoric acids, among many others, have been developed. These organocatalysts have provided a broad range of activation modes that allow privileged interactions between catalysts and substrates for the preparation of compounds with high added value in an enantioselective way. Here, we briefly cover the history of this chemistry, from our point of view, including our beginnings, how the field has evolved during these years of research, and the road ahead.

Asymmetric Henry reaction of trifluoromethyl enones with nitromethane using a N,N-dibenzyl diaminomethylenemalononitrile organocatalyst

A novel N,N -dibenzyl diaminomethylenemalononitrile organocatalyst efficiently promoted asymmetric Henry reactions of trifluoromethyl enones with nitromethane, affording corresponding highly functionalized products in high yields with excellent enantioselectivities (up to 90% ee). This study is the first to report the successful example of the asymmetric 1,2-additions of nitromethane to trifluoromethyl enones.

Diaminomethylenemalononitrile as a Chiral Single Hydrogen Bond Catalyst: Application to Enantioselective Conjugate Addition of α-Branched Aldehydes

An improved diaminomethylenemalononitrile organocatalyst, bearing a N,N-disubstituted structure, promoted enantioselective conjugate addition reaction of α-branched aldehydes with vinyl sulfone, affording adducts with excellent enantioselectivities (up to 96% ee). Mechanistic studies revealed that the diaminomethylenemalononitrile motif holds the vinyl sulfone substrate using a single hydrogen bond accompanied by multiple weak interactions, including electrostatic C-H⋅⋅⋅O interactions.

Catalytic stereoselective Mannich-type reactions for construction of fluorinated compounds

For its unique role in developing and designing new bioactive materials and healthcare products, fluoro-organic compounds have attracted remarkable interest. Along with ever-increasing demand for a wider availability of fluorine-containing structural units, a large diversity of methods has been introduced to incorporate fluorine atoms specially in a stereoselective fashion. Among them, catalytic Mannich reaction can proceed with a broad variety of reactants and open clear paths for the synthesis of versatile amine synthons in the synthesis of natural product and pharmaceutical molecules. This review provides an overview of the employment of catalytic asymmetric Mannich reactions in the synthesis of fluorine-containing amine compounds and highlights the conceivable distinct mechanisms.

Osmolyte-Induced Effects on the Hydration Behavior and the Osmotic Second Virial Coefficients of Alkyl-Substituted Urea Derivatives: Critical Assessment of Their Structure-Making/Breaking Behavior

We analyzed the hydration behavior of alkyl-substituted urea osmolytes in terms of their deviation from Lewis-Randall solution ideality and characterized their structure-making/breaking tendency according to a proposed solvation formalism that provides a rigorous cause-effect connection between the system microstructure and its solution thermodynamic nonidealities. After a brief introduction of the rationale behind the use of Lewis-Randall over alternative solution ideality references, we (i) assessed the effect of the nature and type of alkyl substitution on the osmolyte-induced perturbation of the solution microstructure as a function of composition, (ii) analyzed their microstructural responses to changes in temperature and pressure, and (iii) demonstrated the structure-breaking nature of urea and the magnifying behavior of its alkyl-substituted osmolytes, whose trend follows the increasingly positive deviation of the osmolyte solutions from Lewis-Randall ideality. Then, we discussed the falsifiability of a pair of frequently used conjectured structure-making/breaking criteria, supported by the derived exact relationships between the structure-making/breaking parameter, the solution thermodynamic nonideality, and the osmotic second virial coefficient of the aqueous osmolytes. Finally, we provided an outlook on how the proposed approach could guide the quest for a truly (microstructural to free energy) causative explanation for the denaturation mechanism of proteins.

Stereoselective organocascades: from fundamentals to recent developments

Reaction sequences where more bonds are sequentially formed (cascade reactions) may be started either by a stoichiometric or by a catalytic reagent, and proceed in an enantio- diastereo- or non-stereo- selective manner. A wide variety of such strategies has been developed, including both stoichiometric and catalytic ones.

Within the widely developed cascade reactions field, this chapter is not meant to be omni-comprehensive, but to offer an as much as possible complete overview on organocatalytic stereoselective methods. We embrace the more general definitions by Tietze and Denmark, considering as cascade reactions all those one-pot processes that involve two or more bond formations, where each subsequent step is enabled by a structural change caused by the previous one. We will include both two- and multi-component reactions where one or more organocatalysts may be responsible either for all or just some of the occurring transformations. Organocascades will be reported according to the number of involved catalytic cycles.

In the following paragraphs, only cascade reactions that are stereoselective by means of a chiral catalyst will be considered. It will be shown that multiple possibilities, relying on different catalysis modes, are available to achieve the same reaction sequence.

Enantioselective Protonation: Hydrophosphinylation of 1,1-Vinyl Azaheterocycle N-Oxides Catalyzed by Chiral Bis(guanidino)iminophosphorane Organosuperbase

Enantioselective protonation by hydrophosphinylation of diarylphosphine oxides with 2-vinyl azaheterocycle N-oxide derivatives was demonstrated using chiral bis(guanidino)iminophosphorane as the higher-order organosuperbase catalyst. It was confirmed by several control experiments that a chiral weak conjugate acid of the chiral bis(guanidino)iminophosphorane, instead of achiral diarylphosphine oxides, directly functioned as the proton source to afford the corresponding product in a highly enantioselective manner in most cases. Enantioselective protonation by a weak conjugate acid generated from the higher-order organosuperbase would broaden the scope of enantioselective reaction systems because of utilization of a range of less acidic pronucleophiles. This method is highlighted by the valuable synthesis of a series of chiral P,N-ligands for chiral metal complexes through the reduction of phosphine oxide and N-oxide units of the corresponding product without loss of enantiomeric purity.

Bench-stable imine surrogates for the one-pot and catalytic asymmetric synthesis of α-amino esters/ketones

N,O-Bis(tert-butoxycarbonyl)hydroxylamines are readily accessible as imine surrogates, which are bench stable and could quantitatively generate the corresponding imines for in situ applications. An unpresented catalytic asymmetric method for the synthesis of α-amino esters and ketones from novel imine surrogates, N,O-bis(tert-butoxycarbonyl)hydroxylamines, as well as its preliminary mechanistic studies are reported. A variety of optically enriched products were obtained in excellent yields and enantioselectivities (up to 99% yield and >99% ee).

Half-sandwich complexes of osmium containing guanidine-derived ligands

Pyridinyl- and phosphano-guanidino complexes of formula [(η6-p-cymene)OsCl(H2L)][SbF6] (cymene = MeC6H4iPr; H2L = N,N'-bis(p-Tolyl)-N''-(2-pyridinylmethyl)guanidine, H2L1 (1) and N,N'-bis(p-Tolyl)-N''-(2-diphenylphosphanoethyl)guanidine, H2L2 (2)) have been prepared from the dimer [{(η6-p-cymene)OsCl}2(μ-Cl)2] and H2L in the presence of NaSbF6. Treatment of complex 2 with HCl renders the phosphano-guanidinium complex [(η6-p-cymene)OsCl2(H3L2)][SbF6] (3). Compounds 1 and 2 react with AgSbF6 rendering the cationic aqua complexes [(η6-p-cymene)Os(H2L)(OH2)][SbF6]2 (H2L = H2L1 (4), H2L2 (5)). Addition of monodentate ligands L to compound 4 affords complexes of formula [(η6-p-cymene)Os(H2L1)L][SbF6]2 (L = py (6), 4-(NHMe)py (7), CO (8), P(OMe)3 (9)). Treatment of complexes 4 and 5 with NaHCO3 renders the monocationic complexes [(η6-p-cymene)Os(κ3N,N',N''-HL1)][SbF6] (10) and [(η6-p-cymene)Os(κ3N,N',P-HL2)][SbF6] (11), respectively, in which the HL ligand adopts a fac-κ3 coordination mode. The new complexes have been characterised by analytical and spectroscopic means, including the determination of the crystal structures of the compounds 1-4, 6, 8, and 11, by X-ray diffractometric methods. The phosphano-guanidino complexes 2 and 5 exhibit a temperature dependent fluxional process in solution. The new 18 electron complexes 1, 2, 6, and 8-10 are active catalysts for the Friedel-Crafts reaction between trans-β-nitrostyrene and N-methyl-2-methylindole. Conversions greater than 90% were obtained. Proton NMR studies support a mechanism involving the Brønsted-acid activation of trans-β-nitrostyrene through the NH functionalities of the coordinated guanidine ligands.

Asymmetric Organocatalyzed Aza-Henry Reaction of Hydrazones: Experimental and Computational Studies

The first asymmetric catalyzed aza-Henry reaction of hydrazones is presented. In this process, quinine was used as the catalyst to synthesize different alkyl substituted β-nitrohydrazides with ee up to 77 %. This ee was improved up to 94 % by a further recrystallization and the opposite enantiomer can be obtained by using quinidine as the catalyst, opening exciting possibilities in fields in which the control of chirality is vital, such as the pharmaceutical industry. Additionally, experimental and ab initio studies were performed to understand the reaction mechanism. The experimental results revealed an unexpected secondary kinetic isotope effect (KIE) that is explained by the calculated reaction pathway, which shows that the protonation of the initial hydrazone and the C-C bond forming reaction occur during a concerted process. This concerted mechanism makes the catalysis conceptually different to traditional base-promoted Henry and aza-Henry reactions.

Chiral Thioureas-Preparation and Significance in Asymmetric Synthesis and Medicinal Chemistry

For almost 20 years, thioureas have been experiencing a renaissance of interest with the emerged development of asymmetric organocatalysts. Due to their relatively high acidity and strong hydrogen bond donor capability, they differ significantly from ureas and offer, appropriately modified, great potential as organocatalysts, chelators, drug candidates, etc. The review focuses on the family of chiral thioureas, presenting an overview of the current state of knowledge on their synthesis and selected applications in stereoselective synthesis and drug development.

Stereoselective synthesis of hydrazinodihydrofurans via cascade Michael addition-substitution involving the reaction of curcumin and other β-dicarbonyls with α-hydrazinonitroalkenes

Highly diastereoselective synthesis of 2-hydrazinated 2,3-dihydrofurans in good to excellent yields involving an interrupted Feist-Bénary type reaction by treating a wide variety of 1,3-dicarbonyl compounds, including curcumins, with α-hydrazinated nitroalkenes is reported here. The first ever enantioselective reaction of α-hydrazinonitroalkenes has also been carried out with two selected 1,3-dicarbonyls, dimedone and cyclohexanone by employing an l-t-leucine derived squaramide as the chiral organocatalyst to afford the enantio-enriched 2-hydrazinodihydrofurans as single diastereomers in good yields and with good enantioselectivities.

Proline bulky substituents consecutively act as steric hindrances and directing groups in a Michael/Conia-ene cascade reaction under synergistic catalysis

In this study, we report a highly stereoselective and versatile synthesis of spiro pyrazolones, promising motifs that are being employed as pharmacophores. The new synthetic strategy merges organocatalysis and metal catalysis to create a synergistic catalysis using proline derivatives and Pd catalysts. This protocol is suitable for late-stage functionalization, which is very important in drug discovery. Additionally, a thorough computational study proved to be very useful to elucidate the function of the different catalysts along the reaction, showing a peculiar feature: the -CPh2OSiMe3 group of the proline catalyst switches its role during the reaction. In the initial Michael reaction, this group plays its commonly-assumed role of bulky blocking group, but the same group generates π-Pd interactions and acts as a directing group in the subsequent Pd-catalyzed Conia-ene reaction. This finding might be very relevant especially for processes with many steps, such as cascade reactions, in which functional groups are assumed to play the same role during all reaction steps.

First aromatic amine organocatalysed activation of α,β-unsaturated ketones

The first example of a chiral aromatic amine used to activate α,β-unsaturated ketones in aminocatalysis.

First Organocatalytic Asymmetric Synthesis of 1-Benzamido-1,4-Dihydropyridine Derivatives

Preliminary results concerning the first asymmetric synthesis of highly functionalized 1-benzamido-1,4-dihydropyridine derivatives via the reaction of hydrazones with alkylidenemalononitriles in the presence of β-isocupreidine catalyst are reported. The moderate, but promising, enantioselectivity observed (40⁻54% ee), opens the door to a new area of research for the asymmetric construction of new chiral 1,4-dihydropyridine derivatives, whose enantioselective catalytic preparation are still very limited. Moreover, the use of hydrazones for the enantioselective construction of chiral 1,4-dihydropyridines has been overlooked in the literature so far. Therefore, our research represents a pivotal example in this field which remains still unexplored.

Cinchona-Diaminomethylenemalononitrile Organocatalyst for the Highly Enantioselective Hydrophosphonylation of Ketones and Enones

The use of diaminomethylenemalononitrile (DMM) organocatalyst to promote the challenging 1,2-hydrophosphonylation of simple ketones and enones, which are also called α,β-unsaturated ketones, is proposed and validated. This reaction provided the corresponding chiral α-hydroxy phosphonates in high to excellent yields and with enantioselectivity up to 96% ee.