A Practical and Robust Zwitterionic Cooperative Lewis Acid/Acetate/Benzimidazolium Catalyst for Direct 1,4-Additions

A catalyst type is disclosed allowing for exceptional efficiency in direct 1,4-additions. The catalyst is a zwitterionic entity, in which acetate binds to Cu^II , which is formally negatively charged and serving as counterion for benzimidazolium. All 3 functionalities are involved in the catalytic activation. For maleimides productivity was increased by a factor >300 compared to literature (TONs up to 6700). High stereoselectivity and productivity was attained for a broad range of other Michael acceptors as well. The polyfunctional catalyst is accessible in only 4 steps from N-Ph-benzimidazole with an overall yield of 96 % and robust during catalysis. This allowed to reuse the same catalyst multiple times with nearly constant efficiency. Mechanistic studies, in particular by DFT, give a detailed picture how the catalyst operates. The benzimidazolium unit stabilizes the coordinated enolate nucleophile and prevents that acetate/acetic acid dissociate from the catalyst.

Rhodium(III)-Catalyzed [4 + 2] Annulation of N-Arylbenzamidines with Propargyl Alcohols: Highly Regioselective Synthesis of 1-Aminoisoquinolines Controlled by Noncovalent Interaction

A highly regioselective synthesis of 1-aminoisoquinolines has been explored via rhodium(III)-catalyzed C-H bond activation/annulation reactions of propargyl alcohols with N-arylbenzamidines. The imidamide was used as the directing group and the nitrogen source of the heterocycle and for regulating the regioselective migratory insertion of propargyl alcohol through a hydrogen bond. In this transformation, a specific isomer was obtained that would provide a new strategy for the synthesis of 1-aminoisoquinolines with biological activity.

Green Chemistry Meets Asymmetric Organocatalysis: A Critical Overview on Catalysts Synthesis

Can green chemistry be the right reading key to let organocatalyst design take a step forward towards sustainable catalysis? What if the intriguing chemistry promoted by more engineered organocatalysts was carried on by using renewable and naturally occurring molecular scaffolds, or at least synthetic catalysts more respectful towards the principles of green chemistry? Within the frame of these questions, this Review will tackle the most commonly occurring organic chiral catalysts from the perspective of their synthesis rather than their employment in chemical methodologies or processes. A classification of the catalyst scaffolds based on their E factor will be provided, and the global E factor (EG factor) will be proposed as a new green chemistry metric to consider, also, the synthetic route to the catalyst within a given organocatalytic process.

Catalytic Dynamic Kinetic Resolutions in Tandem to Construct Two-Axis Terphenyl Atropisomers

The defined structure of molecules bearing multiple stereogenic axes is of increasing relevance to materials science, pharmaceuticals, and catalysis. However, catalytic enantioselective approaches to control multiple stereogenic axes remain synthetically challenging. We report the catalytic synthesis of two-axis terphenyl atropisomers, with complementary strategies to both chlorinated and brominated variants, formed with high diastereo- and enantioselectivity. The chemistry proceeds through a sequence of two distinct dynamic kinetic resolutions: first, an atroposelective ring opening of Bringmann-type lactones produces a product with one established axis of chirality, and second, a stereoselective arene halogenation delivers the product with the second axis of chirality established. In order to achieve these results, a class of Brønsted basic guanidinylated peptides, which catalyze an efficient atroposelective chlorination, is reported for the first time. In addition, a complementary bromination is reported, which also establishes the second stereogenic axis. These bromo-terphenyls are accessible following the discovery that chiral anion phase transfer catalysis by C₂-symmetric phosphoric acids allows catalyst control in the second stereochemistry-determining event. Accordingly, we established the fully catalyst-controlled stereodivergent synthesis of all possible chlorinated stereoisomers while also demonstrating diastereodivergence in the brominated variants, with significant levels of enantioselectivity in all cases.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Catalytic 1,2-Rearrangements: Organocatalyzed Michael/Semi-Pinacol-like Rearrangement Cascade of 1,3-Diones and Nitroolefins

New types of organocatalytic 1,2-rearrangements, which resemble the Smiles-like or semi-pinacol-like rearrangement, of Michael adducts of 1,3-dicarbonyl-2-alkyl compounds and nitroalkenes have been realized. Unlike the well-known conjugate addition, the reaction affords the 1-phenyl-1-nitroalkanes via unprecedented rearrangement and cascade reactions. Structures of the appropriate products were unambiguously characterized by X-ray crystallography.

An asymmetric hydrocyanation/Michael reaction of α-diazoacetates via Cu(i)/chiral guanidine catalysis

An asymmetric one-pot hydrocyanation/Michael reaction of α-aryl diazoacetates with trimethylsilyl cyanide, tert-butanol, and N-phenylmaleimides has been realized using a chiral guanidinium salt/CuBr catalyst.

Proline derived guanidine catalysts forge extensive H-bonded architectures: a solution and solid state study

The preparation of a range of amino acid derived guanidine organocatalysts is reported together with their application to the Michael addition of 2-hydroxy-1,4-napthoquinone to β-nitrostyrene, achieving a maximum ee of 56%. Some insight into the mechanism was sought by using X-ray crystallography and a detailed study of the intra- and intermolecular hydrogen bonding is reported.

Catalysts assemble! We present the design and synthesis of a large family of amino acid derived guanidine organic moieties as catalysts in the solution state and building blocks towards extended H-bonded architectures upon crystallisation.

Enantioselective carbene insertion into the N-H bond of benzophenone imine

Efficient enantioselective insertion of α-diazoesters into the N-H bond of N-sp2-hybridized benzophenone imine was realized by using Rh2(esp)2 and chiral guanidine cooperative catalysis. Both aliphatic and aromatic substituted α-amino esters were obtained in high yields (up to 99%) and good enantioselectivities (up to 95.5 : 4.5 er) under mild reaction conditions.

Cooperative Catalysis of Chiral Guanidine and Rh2(OAc)4 in Asymmetric O-H Insertion of Carboxylic Acid: A Theoretical Investigation

We reported a mechanistic study on asymmetric O-H insertion reaction of α-diazoester with carboxylic acid using Rh₂(OAc)₄/chiral guanidine-amide as the cocatalyst by density functional theory [B3LYP-D3(BJ)/def2-TZVP//B3LYP-D3(BJ)/[6-31G**, SDD] (SMD, Et₂O)]. The catalytic reaction included two stages: (i) formation of Rh-carbene species, subsequently by the construction of C-O bond forming enol and (ii) chiral guanidinium salt-assisted H-transfer to the enol. In cooperative catalysis, Rh₂(OAc)₄ helped to form an enol intermediate via high-reactivity Rh-carbene species, while the in situ-formed guanidium carboxylate acted as a chiral proton shuttle to construct a hydrogen bonding net for the stereo-determinant protonation. The repulsions between the phenyl group of the enol intermediate and the cyclohexyl as well as the ortho-substituted isopropyl group of chiral guanidine played important roles in controlling stereoselectivity. A disadvantageous steric arrangement in si-face attack weakened the stabilizing electrostatic and orbital interaction of reacting species in the H-transfer step, enhancing the pathway to form a predominant product with R-configuration in the two competing pathways. A model was proposed to explain the asymmetric induction of chiral guanidine-amide in protonation.

Synergistic promotion by intramolecular hydrogen bonding: a bi-functionally catalyzed cascade reaction for the synthesis of enantiopure chromenopyrrolidines

An effective organocatalytic asymmetric cascade reaction of o-hydroxy aromatic aldimines and β,γ-unsaturated-α-ketoesters was developed.

Metal-catalysed reactions enabled by guanidine-type ligands

A review of metal–guanidine complexes, which are selective and powerful catalysts for organic transformations, asymmetric synthesis, and polymerisation.

Asymmetric Catalytic Double Michael Additions for the Synthesis of Spirooxindoles

Asymmetric cascade double Michael additions to construct 2'-substituted 3,3'-spirooxindoles by using a chiral guanidine organocatalyst has been developed. A series of spirooxindole derivatives containing dihydrofuran or pyrrolidine subunits were obtained with good to excellent diastereo- and enantioselectivities. The method showed great tolerance of a number of aromatic and aliphatic alkynones. The strategy gave access to the asymmetric synthesis of (-)-salacin for the first time.

Asymmetric synthesis of 3-aminodihydrocoumarins via the chiral guanidine catalyzed cascade reaction of azlactones

A new asymmetric preparative method of 3-aminodihydrocoumarins via reactions of azlactones and 2-nitrovinylphenols using chiral bifunctional guanidine catalysts.

A homodinuclear cobalt complex for the catalytic asymmetric Michael reaction of β-ketoesters to nitroolefins

A homodinuclear Co₂/aminophenol sulfonamide complex has been developed for the asymmetric Michael reaction.

The electrical modulus and other dielectric properties by the impedance spectroscopy of LaCrO3 and LaCr0.90Ir0.10O3 perovskites

We have prepared LaCrO₃ (LCO) and 10% Ir doped LCO samples by the solid state reaction method and studied the electrical modulus and the other dielectric properties of the samples by means of the impedance spectroscopy in the −100 °C to 100 °C range, with steps of 20 °C. It has been clearly observed that the dielectric properties change due to Ir doping. The absolute dielectric constant value of Ir doped LCO has decreased and this reduction was attributed to decreasing Cr⁶⁺ ions which may play a vital role in space charge polarization and charge hopping. A plateau region appeared in the temperature-dependent real electrical modulus M′ versus f curves of the pure LCO sample while almost no plateau region is visible in the Ir doped LCO sample. The temperature-dependent imaginary modulus M′′ versus f curves has two peaks at each temperature; one of the peaks is at low frequency and the other at the high frequency region, which shifts through higher frequency region with increasing temperature. This originates from free charge accumulation at the interface with the increase of the temperature. Furthermore, it has been seen that the Ir doped LCO sample has higher impedance and resistance values than the undoped LCO sample at the same frequency and temperature. This phenomenon was attributed to doped Ir ions behaving like a donor in LCO because LCO is a p-type compound. Moreover, the activation energy values of 0.224 eV and 0.208 eV for LCO and of 0.161 eV and 0.265 eV for the Ir doped LCO have been obtained from the slopes of the ρ_dcvs. (kT)⁻¹ curves, respectively. Also the activation energies were calculated from the slopes of the f_maxvs. (kT)⁻¹ curves and the obtained results from low frequency region were in good agreement with ρ_dcvs. (kT)⁻¹ ones.

In this work, LaCrO₃ and Ir doped LaCrO₃ compounds were synthesized and their frequency dependent electrical properties were studied.

Highly enantioselective Michael addition of cyclohexanone to nitroolefins catalyzed by pyrrolidine-based bifunctional benzoylthiourea in water

Organocatalysis and aqueous reactions are identified as the focus of the greening of chemistry. Combining these two strategies effectively remains an interesting challenge in organic synthesis. Herein, we used pyrrolidine-based benzoylthiourea 1c to catalyze the asymmetric Michael addition of cyclohexanone to various nitroolefins in water to afford the corresponding compounds in moderate to good yields, and with excellent diastereoselectivities (up to >99:1 dr) and enantioselectivities (up to 99% ee).

First example of an organocatalytic asymmetric Mannich reaction between aldimines of glycinates and sulphonyl imines

The catalytic enantioselective Mannich-type reaction between glycinate Schiff base and imines has been one of the most efficient routes for accessing α,β-diamino acids. However, the glycinate Schiff bases used in the references were almost ketimines. Only several examples of aldimines were used in the presence of metal catalyst. We developed the first example of an asymmetric direct Mannich reaction using aldimines of glycinates instead of ketimines of glycinates. The reaction was well catalyzed by chiral guanidine with high yield (up to 92%) and moderate stereoselectivity (up to 65%).

Enantioselective Organocatalyzed Transformations of β-Ketoesters

The β-ketoester structural motif continues to intrigue chemists with its electrophilic and nucleophilic sites. Proven to be a valuable tool within organic synthesis, natural product, and medicinal chemistry, reports on chiral β-ketoester molecular skeletons display a steady increase. With the reignition of organocatalysis in the past decade, asymmetric methods available for the synthesis of this structural unit has significantly expanded, making it one of the most exploited substrates for organocatalytic transformations. This review provides comprehensive information on the plethora of organocatalysts used in stereoselective organocatalyzed construction of β-ketoester-containing compounds.

C 2-Symmetric pyrrolidine-derived squaramides as recyclable organocatalysts for asymmetric Michael reactions

Novel sustainable organocatalysts applicable to asymmetric synthesis of active pharmaceutical ingredients are developed.

Enantioselective synthesis of 2,3-disubstituted trans-2,3-dihydrobenzofurans using a Brønsted base/thiourea bifunctional catalyst

The enantioselective synthesis of 2,3-disubstituted trans-2,3-dihydrobenzofuran derivatives via intramolecular Michael addition has been developed using a bifunctional tertiary amine–thiourea catalyst.

Organocatalytic 1,4-Addition Reaction of 2-Formyl(thio)esters to Vinylketones: An Efficient Access to Acyclic Chiral Building Blocks with a Quaternary Carbon Stereocenter

2-Formyl(thio)esters were utilized as pronucleophiles to obtain less-accessible acyclic chiral building blocks bearing versatile functional groups on a quaternary carbon atom for enantioselective 1,4-addition to vinylketones. To achieve high enantioselectivity in the present 1,4-addition reaction, thiourea-tertiary amines containing a bulky chiral backbone were developed as catalysts, and several derivatizations of the products were performed to demonstrate the synthetic utility of the products.