Highly Diastereoselective Pinacol Coupling of Secondary Aliphatic Aldehydes Induced by a Catalytic System Consisting of Vanadium Complex, Chlorosilane, and Zinc Metal

Cr-Catalyzed Asymmetric Cross Aza-Pinacol Couplings for β-Amino Alcohol Synthesis

Chiral β-amino alcohols are crucial structural motifs found in pharmaceuticals, natural products, and chiral ligands in asymmetric catalysis. Despite previous advances, the development of catalytic approaches to access β-amino alcohols bearing vicinal stereocenters from readily available chemicals remains a prominent challenge. Herein, we describe the Cr-catalyzed asymmetric cross aza-pinacol coupling of aldehydes and N-sulfonyl imines. This protocol proceeds in a radical-polar crossover manner from the intermediacy of an α-amino radical instead of a ketyl radical. Key to the success is using a chiral chromium catalyst, which plays a triple role in the chemoselective single-electron reduction of the imine, fast radical interception to inhibit radical addition to imines, and chemo- and stereoselective addition to aldehydes instead of imines. This method provides a modular and efficient approach to accessing diverse β-amino alcohols bearing vicinal stereocenters.

Selective Pinacol-Coupling Reaction using a Continuous Flow System

The first continuous flow pinacol coupling reaction of carbonyl compounds was successfully achieved within only 2 min during a single pass through a cartridge filled with zinc(0). The optimized method allowed the efficient production of gram-scale value-added compounds with high productivity. The developed methodology is efficient for aromatic or α,β-unsaturated aldehydes but gives moderate results for more stable acetophenone derivatives. Moreover, the flow method displayed better results in terms of yield and selectivity in comparison to the corresponding batch methodology.

Synthesis of oxindoles via reductive CO2 fixation

The synthesis of 3-aryl-3-hydroxy-2-oxindoles, which are a structural motif found in various natural products and pharmaceutically active compounds, was conducted via reductive coupling of (2-aminophenyl)(aryl)methanone derivatives and CO₂ as a key step. The conditions employing Mg with chlorotrimethylsilane in DMA are the best for the reductive coupling. The reductive coupling and acid-catalyzed lactam formation can be performed in a one-pot reaction to give the oxindoles.

First pinacol coupling in emulsified water: key role of surfactant and impact of alternative activation technologies

For the first time, the influence of surfactants on the radical pinacol coupling reaction is investigated. The rate and selectivity of this reductive C-C coupling are compared under three different activation technologies: thermal activation, microwave irradiation, and sonication. The use of IgepalCO520, a neutral surfactant, led to the successful conversion of aromatic or α,β-unsaturated aliphatic carbonyl compounds in moderate to excellent yield (55-90 %). An insight on the potential mechanism involved in the reaction is also proposed, based on microscopic observations and particle size measurement.

Simple and expeditious pinacol coupling of non usual α,β-unsaturated carbonyl compounds in water

Using zinc (0) in a 5% v AcOH aqueous solution allowed the efficient pinacol coupling of aliphatic or aromatic unusual, α,β-unsaturated carbonyl compounds such as citral A in good to excellent yields (56–99%).

Tandem pinacol coupling–rearrangement of aromatic aldehydes with hydrogen catalyzed by a combination of a platinum complex and a polyoxometalate

Together with a strongly oxidizing polyoxometalate, H(5)PV(2)Mo(10)O(40), Pt(II)(N-(2,6-diisopropylphenyl)pyrazin-2-ylmethanimine)Cl2 forms a combined catalyst that was active in the tandem pinacol coupling-rearrangement of aryl aldehydes to give mostly the corresponding diarylacetaldehyde in high yields using molecular hydrogen as the reducing agent.

Vanadium-Catalyzed Pinacol Coupling Reaction in Water

A catalytic pinacol coupling using water as a solvent was performed by a catalytic amount of vanadium(III) chloride and metallic Al as a co-reductant. A combination forms a binary catalytic system, being in sharp contrast to the reaction in organic solvent, which requires a chlorosilane as an additive. Various aromatic aldehydes underwent the reductive coupling to give the corresponding 1,2-diols in moderate to good yields.

Aerobic oxidation of thiols to disulfides catalyzed by trichlorooxyvanadium

Thiols were converted into disulfide by the aerobic oxidation catalyzed by trichlorooxyvanadium in the presence of molecular sieves 3A.

Truly catalytic and enantioselective pinacol coupling of aryl aldehydes mediated by chiral Ti(III) complexes

A variety of chiral Ti(IV) complexes were reduced in situ with zinc in acetonitrile. The resulting chiral Ti(III) complexes were found to catalyze the pinacol coupling reaction stereoselectively. The best results were obtained from the Ti-SALEN complex, which was found to be an efficient catalyst at 10 mol % concentration. Various aromatic aldehydes were coupled to obtain chiral hydrobenzoin derivatives with high diastereoselectivity and enantioselectivity. A plausible mechanism is proposed that rationalizes the stereochemical outcome of the reaction.

A novel titanium-catalyzed cyclization of olefinic iodoethers to tetrahydrofurans

A catalytic reductive cyclization of olefinic iodoethers was achieved by use of cat. Cp(2)TiCl(2) in the presence of Mn and Me(3)SiCl. This protocol provides a versatile method for the selective formation of multisubstituted tetrahydrofurans.

Novel Zn/ZnI2-promoted cross-coupling of acrylic acid esters with arylaldehydes to alpha-aroyladipic acid esters

Cross-coupling of alkyl acrylates and arylaldehydes was achieved by treatment with metallic zinc and zinc(II) iodide in DMF, giving dialkyl alpha-aroyladipates in moderate yields.

Redox reactions via vanadium-induced electron transfer

One-electron reduction and oxidation induced by vanadium complexes are demonstrated to be useful in oxidative and reductive transformations of carbonyl compounds. The redox interaction between vanadium complexes and redox-active ligands is achieved with coenzyme PQQ and polyanilines that afford the corresponding redox systems.