Vanadium-Catalyzed Asymmetric Oxidation of Sulfides Using Schiff Base Ligands Derived from β-Amino Alcohols with Two Stereogenic Centers

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Formation of α-chiral centers by asymmetric β-C(sp3)-H arylation, alkenylation, and alkynylation

The enzymatic β-C-H hydroxylation of the feedstock chemical isobutyric acid has enabled the asymmetric synthesis of a wide variety of polyketides. The analogous transition metal-catalyzed enantioselective β-C-H functionalization of isobutyric acid-derived substrates should provide a versatile method for constructing useful building blocks with enantioenriched α-chiral centers from this abundant C-4 skeleton. However, the desymmetrization of ubiquitous isopropyl moieties by organometallic catalysts has remained an unanswered challenge. Herein, we report the design of chiral mono-protected aminomethyl oxazoline ligands that enable desymmetrization of isopropyl groups via palladium insertion into the C(sp³)-H bonds of one of the prochiral methyl groups. We detail the enantioselective β-arylation, -alkenylation, and -alkynylation of isobutyric acid/2-aminoisobutyric acid derivatives, which may serve as a platform for the construction of α-chiral centers.

Radical reactions of diamine bis(phenolate) vanadium(iii) complexes. Solid state binding of O2 to form a vanadium(v) peroxo complex

[VCl₃(THF)₃] reacted with the sodium salt of a tripodal diamine bisphenolate ligand precursor Na₂L2 to give a paramagnetic d² complex [V(L2)Cl] (2). The reaction of 2 with oxygen is strongly dependent on the experimental conditions, affording [VO(L2)Cl] (6) or [V(η²-O₂)(L2)Cl] (7). The formation of 7 involves the direct addition of O₂ to V(iii) in the solid state with oxidation to V(v) without significantly disturbing the structure of 2. DFT calculations showed that compound 7 is an intermediate in the formation of 6 from 2. The reaction involves the cleavage of the η¹-O-O bond in a proposed dimeric species. The overall reaction of 2 moles of vanadium(iii), complex 2, and one mole of O₂ to yield two moles of product 6 is a favourable process with ΔG₂₉₈ = -38.3 kcal mol^-1. 7 is the first non-oxido peroxidovanadium(v) complex obtained directly from the reaction of a crystal and the second example of a structurally characterized complex of that type. Reactions of V(L1)Cl (1) and V(L2)Cl (2) with one equivalent of the nitroxyl radical TEMPO in toluene also result in the formation of oxido V(v) complexes, VO(L1)Cl (5) and VO(L2)Cl (6). The reaction of VO(OⁱPr)₃ with Na₂L2 afforded [VO(L2)(OⁱPr)] (8) in high yield. A major isomer having the V[double bond, length as m-dash]O moiety in the equatorial plane was characterised by X-ray diffraction although solution NMR data showed the presence of a minor species with the oxido ligand trans to the tripodal nitrogen, as in 6. Complexes 6 and 8 are very active and selective sulfoxidation catalysts of thioanisole, but no enantiomeric excess was obtained.

Ti(Phen)(OC2H5)2Cl2: a highly efficient pre-catalyst for selective oxidation of organic sulfides to sulfoxides by hydrogen peroxide

A structurally characterized titanium complex [Ti(Phen)(OC₂H₅)₂Cl₂] (Phen = 1,10-phenanthroline) is reported as a homogeneous pre-catalyst for the selective oxidation of organic sulfides to sulfoxides with a stoichiometric amount H₂O₂ in CH₃OH at room temperature.

Symmetric diarylsulfoxides as asymmetric sulfinylating reagents for dialkylmagnesium compounds

At −78 °C, primary dialkylmagnesium compounds reacted with diarylsulfoxides when 1.5 equiv of the dilithium salt of (S)-BINOL was added as a promotor. Alkyl aryl sulfoxides resulted in up to quantitative yield and with up to 97% ee. This demonstrates the feasibility of asymmetric sulfinylations by achiral sulfinylating agents (from the perspective of Alkyl2Mg) as well as the feasibility of asymmetric sulfoxide–magnesium exchanges (from the perspective of Ar2SO).

Bimetallic titanium complex catalyzed enantioselective oxidation of thioethers using aqueous H2O2 as a terminal oxidant

This study represents a highly efficient sulfide oxidation catalytic system using a non-toxic Ti-catalyst and environmentally benign aqueous hydrogen peroxide.

Selective Oxidation of Sulfides to Sulfoxides Catalysed by Deep Eutectic Solvent with H2O2

A Brønsted acidic deep eutectic solvent based on choline chloride and p-toluenesulfonic acid (ChCl/ p-TsOH, 1:1) was prepared and utilised for the selective oxidation of sulfides with H2O2 as the oxidant. Broad substrate compatibility, good yields and selectivities, the reusability of the catalyst as well as the gram-scale synthesis are the major advantages of this protocol. Moreover, the use of ChCl/ p-TsOH, instead of neat p-TsOH, can greatly reduce the acid sewage especially in large-scale synthetic processes.

In situ-generated chiral iron complex as efficient catalyst for enantioselective sulfoxidation using aqueous H2O2 as oxidant

This study represents the rare combination of non-toxic Fe based catalyst/H₂O₂ as an efficient catalytic protocol for asymmetric sulfoxidation reaction.