Dinuclear Zinc-AzePhenol Catalyzed Asymmetric Aza-Henry Reaction of N-Boc Imines and Nitroalkanes under Ambient Conditions

Dinuclear Zn-Catalytic System as Brønsted Base and Lewis Acid for Enantioselectivity in Same Chiral Environment

Zinc (Zn) is a crucial element with remarkable significance in organic transformations. The profusion of harmless zinc salts in the Earth's outer layer qualifies zinc as a noteworthy contender for inexpensive and eco-friendly reagents and catalysts. Recently, widely recognized uses of organo-Zn compounds in the field of organic synthesis have undergone extensive expansion toward asymmetric transformations. The ProPhenol ligand, a member of the chiral nitrogenous-crown family, exhibits the spontaneous formation of a dual-metal complex when reacted with alkyl metal (R-M) reagents, e.g., ZnEt2. The afforded Zn complex possesses two active sites, one Lewis acid and the other Brønsted base, thereby facilitating the activation of nucleophiles and electrophiles simultaneously within the same chiral pocket. In this comprehensive analysis, we provide a thorough account of the advancement and synthetic potential of these diverse catalysts in organic synthesis, while emphasizing the reactivity and selectivities, i.e., dr and ee due to the design/structure of the ligands employed.

An efficient catalytic method for hydrophosphination of heterocumulenes with diethylzinc as precatalyst without a solvent

Commercially available compound ZnEt₂ acts as an efficient precatalyst for the solvent-free hydrophosphinations of heterocumulenes using Ph₂PH as reagent. As far as we knew, this has been not reported in group 12 metal catalyzing reactions. A suggested mechanism of this reaction is explored, and the intermediate [{Ph₂PC(NⁱPr)₂}ZnEt]₂ is obtained and characterized by a single-crystal X-ray structural analysis.

Dinuclear zinc-catalyzed enantioselective formal [3 + 2] cycloaddition of N-2,2,2-trifluoroethylisatin ketimines with low reactivity aurone derivatives

Highly enantioselective formal [3 + 2] cycloaddition of N-2,2,2-trifluoroethylisatin ketimines with aurone derivatives of low reactivity using chiral dinuclear zinc catalysts has been developed via a Brønsted base and Lewis acid cooperative activation model. These transformations involving a domino Michael/Mannich reaction sequence led to efficient construction of a range of chiral spiro[benzofuran-pyrrolidine] scaffolds bearing three biologically relevant heterocyclic moieties and two adjacent spiro quaternary stereocenters in high yields (up to 95%) and with good enantioselectivities (up to 99% ee).

A new asymmetric activation strategy for hydrazones as acyl anion equivalents in the bimetallic catalyzed carbonyl-ene reaction

A new asymmetric activation strategy for hydrazones as acyl anion equivalents is developed in the bimetallic catalyzed carbonyl-ene reaction of isatins and hydrazones. Under mild conditions, optically active functionalized 3-hydroxy-2-oxindoles were furnished in up to 98% yield with up to 97% enantioselectivity. In this process, formaldehyde tert-butylhydrazone which is seldom employed in asymmetric carbonyl-ene reactions accelerated by a metallic catalyst can be activated well by a Brønsted base. A possible catalytic cycle is proposed.

Novel Chiral Thiourea Derived from Hydroquinine and l-Phenylglycinol: An Effective Catalyst for Enantio- and Diastereoselective Aza-Henry Reaction

A series of chiral thiourea bearing multiple H-bond donors derived from hydroquinine has been reported. The aza-Henry reaction of isatin-derived ketimines and long-chain nitroalkanes catalyzed by these chiral thioureas can achieve high enantioselectivity (78-99% ee) and excellent diastereoselectivity (up to 99:1). This work is the first report on long-chain nitroalkanes as substrates with excellent diastereoselectivity in metal-free catalytic systems.

Highly Efficient Nanosized Mesoporous CuMgAl Ternary Oxide Catalyst for Nitro-Alcohol Synthesis: Ultrasound-Assisted Sustainable Green Perspective for the Henry Reaction

Nanosized mesoporous CuMgAl ternary oxide catalysts were prepared by thermal decomposition of CuMgAl-layered double hydroxides at 500 °C with nominal Cu/Mg/Al ratios of 1:1:1 (Cu-LDH-I), 1.5:0.5:1 (Cu-LDH-II), and 2:0:1 (Cu-LDH-III). The synthesized catalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, CO2-TPD, and N2 physisorption analysis techniques. The catalytic activity of the synthesized materials was investigated for the Henry reaction between nitromethane and numerous aldehyde derivatives under ultrasonic irradiation. The three CuMgAl ternary oxide catalysts exhibited a high catalytic activity, forming nitro alcohol products with 100% atom economy. The CuMgAl-I catalyst derived from Cu-LDH-I offered high turnover frequencies (TOFs in the synthesis of all of the nitro alcohols in shorter reaction times). The superior catalytic activity of the CuMgAl-I sample is attributed to the synergistic effect between the physicochemical properties of the catalysts and ultrasonic irradiation. The HRTEM analysis of the used CuMgAl-I catalyst revealed the evidence for the cavitation collapse, which causes localized deformation and surface erosion. Moreover, the synthesized catalysts also exhibited robust sustainable activity that resisted deactivation over repeated usage. The present example of ultrasonic-assisted catalyzed organic synthesis represents a novel strategy for the solvent-free green synthesis of nitro-alcohols by the Henry reaction with 100% atom economy.

Regioselective Synthesis of Functionalized Dihydropyrones via the Petasis Reaction

A novel synthesis methodology for the construction of functionalized dihydropyrones has been developed with amines, glyoxylic acid, and 4-substituted-1,2-oxaborol-2(5H)-ols from the Petasis reaction. Mechanistic investigation indicated the intermolecular S_N2 cyclization to provide 3,6-dihydro-2H-pyran-2-ones (3,6-DHP) and 5,6-dihydro-2H-pyran-2-ones (5,6-DHP) in one step with moderate to excellent yields.

Azetidine-Derived Dinuclear Zinc-Catalyzed Asymmetric Conjugate Addition of Bioactive Heterocycles to β,γ-Unsaturated α-Keto esters

A general AzePhenol dinuclear zinc catalytic system has been successfully developed and applied into the asymmetric Michael addition of 4-hydroxyl pyrones and 4-hydroxycoumarins to β,γ-unsaturated α-keto esters. Excellent yields (up to 99%) and high enantioselectivities (up to 94% ee) are obtained for a wide range of substrates under mild conditions in the absence of additives. This bimetallic catalytic approach represents a new and effective asymmetric synthetic protocol to access a variety of bioactive compounds with pharmacological interest. The possible mechanism is proposed to explain the origin of the asymmetric induction.

Comprehension of the α-Arylation of Nitroalkanes

Background:

α-Aryl substituted nitroalkanes are important synthetic intermediates for the preparation of pharmaceutical molecules, natural products, and functional materials. Due to their scare existence in Nature, synthesis of these compounds has attracted the attention of synthetic and medicinal chemists, rendering α-arylation of nitroalkanes of an important research topic. This article summarizes the important advances of α- arylation of nitroalkanes since 1963.

Results:

After a brief introduction of the synthetic application and the reactions of nitroalkanes, this article reviewed the synthetic methods for the α-arylated aliphatic nitro compound. The amount of research on α-arylation of nitroalkanes using various arylation reagents and the discovery of elegant synthetic approaches towards such skeleton have been discussed. This review described these advances in two sections. One is the arylation of non-activated nitroalkanes, with an emphasis on the application of diverse arylation reagents; the other focuses on the arylation of activated nitroalkanes, including dinitroalkanes, trinitroalkanes, α-nitrosulfones, α-nitroesters, α-nitrotoluenes, and α-nitroketones. The synthetic application of these methods has also been presented in some cases.

Conclusion:

In this review, we described the progress of α-arylation of nitroalkanes. Although the immense amount of research on α-arylation of aliphatic nitro compounds has been achieved, many potential issues still need to be addressed, especially the asymmetric transformation and its wide application in organic synthesis.

Crystallization Does It All: An Alternative Strategy for Stereoselective Aza-Henry Reaction

An efficient and experimentally straightforward method for the stereoselective synthesis of a variety of β-nitro-α-amino carboxylic acids via aza-Henry (nitro-Mannich) reaction of aldimines is disclosed, yielding either anti- or a rarely reported syn-configuration. The reaction operates directly on free glyoxylic acid and generates imine species in situ. Crystallization-controlled diastereoselectivity enables isolation of the target compounds in high enantio- and diastereomeric purities by a simple filtration.

Dinuclear zinc cooperative catalytic three-component reactions for highly enantioselective synthesis of 3,3′-dihydrofuran spirooxindoles

A new dinuclear zinc cooperative catalytic enantioselective three-component reaction via a domino Knoevenagel/Michael/cyclization sequence has been developed.