Palladium-catalyzed direct oxidation of alkenes with molecular oxygen: general and practical methods for the preparation of 1,2-diols, aldehydes, and ketones

Oxygenolytic cleavage of 1,2-diols with dioxygen by a mononuclear nonheme iron complex: Mimicking the reaction of myo-inositol oxygenase

A mononuclear iron(II) complex, [(TpPh2)FeII(OTf)(CH3CN)] (1) (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate, OTf = triflate) has been isolated and its efficiency toward the aliphatic CC bond cleavage reaction of 1,2-diols with dioxygen has been investigated. Separate reactions between 1 and different 1,2-diolates form the corresponding iron(II)-diolate complexes in solution. While the iron(II) complex of the tetradentate TPA (tris(2-pyridylmethyl)amine) ligand is not efficient in affecting the CC cleavage of 1,2-diol with dioxygen, complex 1 displays catalytic activity to afford carboxylic acid and aldehyde. Isotope labeling studies with 18O2 reveal that one oxygen atom from dioxygen is incorporated into the carboxylic acid product. The oxygenative CC cleavage reactions occur on the 1,2-diols containing at least one α-H atom. The kinetic isotope effect value of 5.7 supports the abstraction of an α-H by an iron(III)-superoxo species to propagate the CC cleavage reactions. The oxidative cleavage of 1,2-diolates by the iron(II) complex mimics the reaction catalyzed by the nonheme diiron enzyme, myo-inositol oxygenase.

K2S2O8-Mediated or Azobisisobutyronitrile-Catalyzed Regioselective Aerobic Oxidative Cleavage of 1-Arylbutadienes to Cinnamaldehydes

This work reveals the regioselective oxidative cleavage of 1-arylbutadienes to cinnamaldehydes mediated by K2S2O8 or catalyzed by azobisisobutyronitrile, a very common free radical initiator, in an easy to handle, simple procedure and free of transition metals. This approach demonstrates excellent regioselectivity, mild reaction conditions, and compatibility with a broad range of functional groups (45 examples).

Synergistic catalysis: cobalt-catalyzed cascade reaction for the oxidative cleavage of olefins to access esters

An efficient strategy for the oxidative cleavage of CC bonds in olefins to form esters with one or multiple carbon atoms less over heterogeneous cobalt/nitrogen-doped carbon catalyst with dioxygen as the oxidant was described. The protocol features a wide substrate range including the challenging inactive aliphatic and long-chain alkyl aryl olefins. The reactivity of the catalyst did not decrease after reused for seven times. Characterization and control experiments reveal that synergistic effects between the metallic Co nanoparticles and Co-Nx sites provide access to the excellent catalytic activity.

Effect of Solvent on the Rate of Ozonolysis: Development of a Homogeneous Flow Ozonolysis Protocol

Herein, we describe the effect of organic solvents on ozone solubility and the rate of ozonolysis reactions using rapid injection NMR spectroscopy. The tabulated solubility and kinetic data allowed for the design of a homogeneous ozonolysis flow reactor capable of delivering precise quantities of dissolved ozone to various olefin substrates.

Potential of Y. lipolytica epoxide hydrolase for efficient production of enantiopure (R)-1,2-octanediol

The recombinant Yleh from a tropical marine yeast Yarrowia lipolytica NCIM 3589 exhibited a high epoxide hydrolase activity of 9.34 ± 1.80 µmol min^-1 mg^-1 protein towards 1,2-epoxyoctane (EO), at pH 8.0 and 30 °C. The reaction product was identified as 1,2-Octanediol (OD) by GC-MS using EO and H₂O¹⁸ as substrate, affirming the functionality of Yleh as an epoxide hydrolase. For EO, the K_m, V_max, and k_cat/K_m values were 0.43 ± 0.017 mM, 0.042 ± 0.003 mM min^-1, and 467.17 ± 39.43 mM^-1 min^-1, respectively. To optimize the reaction conditions for conversion of racemic EO by Yleh catalyst to enantiopure (R)-1,2-octanediol, initially, Response Surface Methodology was employed. Under optimized reaction conditions of 15 mM EO, 150 µg purified Yleh at 30 °C a maximal diol production of 7.11 mM was attained in a short span of 65 min with a yield of 47.4%. Green technology using deep eutectic solvents for the hydrophobic substrate (EO) were tested as co-solvents in Yleh catalyzed EO hydrolysis. Choline chloride-Glycerol, produced 9.08 mM OD with an increased OD yield of 60.5%. Thus, results showed that deep eutectic solvents could be a promising solvent for Yleh-catalyzed reactions making Yleh a potential biocatalyst for the biosynthesis of enantiopure synthons.

Construction of Alkylidene Fluorene Scaffolds Using Pd-Catalyzed Direct Arene/Alkene Coupling Strategy

A versatile synthetic method for the construction of alkylidene fluorenes and their heteroarene derivatives has been developed successfully by means of a Pd(II)-catalyzed direct C-H/C-H coupling of o-alkenyl biaryls. Use of the Pd(OAc)₂ catalyst under aerobic oxidation conditions gives rise to the corresponding alkylidene fluorenes having various functional groups and diversely fused polycyclic systems. The resulting products can serve as versatile synthetic building blocks for the construction of structurally diverse polycyclic arenes and heteroarenes.

Palladium-Catalyzed Chemo- and Regiocontrolled Tandem Cyclization/Cross-Coupling of 2-Benzyl-3-alkynyl Chromones with Aryl Iodides for the Synthesis of 4H-Furo[3,2-c]chromenes and Xanthones

A novel Pd-catalyzed chemo- and regiocontrolled tandem cyclization/cross-coupling reaction of 3-alkynyl chromone with aryl iodide was developed for the synthesis of 4H-furo[3,2-c]chromenes and xanthones. The difunctionalization of alkynes through O-attack/5-exo-dig and C-attack/6-endo-dig cyclization was reported by this rare approach, which was selectively controlled by the addition of KF or a bidentate phosphine ligand. A one-pot tandem process was demonstrated directly from γ-alkynyl-1,3-diketone for this method.

Differences in chemoselectivity in olefin oxidation by a series of non-porphyrin manganese(IV)-oxo complexes

High valent metal-oxo intermediates are versatile oxidants known to facilitate both oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions in nature. In addition to performing essential yet challenging biological reactions, these intermediates are known for their selectivity in favoring the formation of one oxidation product. To understand the basis for this selectivity, we explore the role of equatorial ligand field perturbations in Mn^IV-oxo complexes on chemoselectivity in cyclohexene oxidation. We also examine reactions of Mn^IV-oxo complexes with cyclohexene-d₁₀, cyclooctene, and styrene. Within this series, the product distribution in olefin oxidation is highly dependent on the coordination environment of the Mn^IV-oxo unit. While Mn^IV-oxo complexes with sterically encumbered, and slightly tilted, MnO units favor CC epoxidation products in cyclohexene oxidation, a less encumbered analogue prefers to cleave allylic C-H bonds, resulting in cyclohexenol and cyclohexenone formation. These conclusions are drawn from GC-MS product analysis of the reaction mixture, changes in the UV-vis absorption spectra, and kinetic analyses. DFT computations establish a trend in thermodynamic properties of the Mn^IV-oxo complexes and their reactivity towards olefin oxidation on the basis of the MnO bond dissociation free energy (BDFE). The most reactive Mn^IV-oxo adduct from this series oxidizes cyclohexene-d₁₀, cyclooctene, and styrene to give corresponding epoxides as the only detected products. Collectively, these results suggest that the chemoselectivity obtained in oxidation of olefins is controlled by both the coordination environment around the MnO unit, which modulates the MnO BDFE, and the BDFEs of the allylic C-H bond of the olefins.

Structural derivatization strategies of natural phenols by semi-synthesis and total-synthesis

Structural derivatization of natural products has been a continuing and irreplaceable source of novel drug leads. Natural phenols are a broad category of natural products with wide pharmacological activity and have offered plenty of clinical drugs. However, the structural complexity and wide variety of natural phenols leads to the difficulty of structural derivatization. Skeleton analysis indicated most types of natural phenols can be structured by the combination and extension of three common fragments containing phenol, phenylpropanoid and benzoyl. Based on these fragments, the derivatization strategies of natural phenols were unified and comprehensively analyzed in this review. In addition to classical methods, advanced strategies with high selectivity, efficiency and practicality were emphasized. Total synthesis strategies of typical fragments such as stilbenes, chalcones and flavonoids were also covered and analyzed as the supplementary for supporting the diversity-oriented derivatization of natural phenols.

A product-controllable aerobic oxidative cleavage of vicinal diols using vanadium-based photocatalyst

A photocatalytic controllable oxidative cleavage of C-C bond is developed with molecular oxygen as the oxidant. Herein, a series of vanadium oxide-based photocatalysts were synthesized and characterized by XPS, PL,...

BF3·OEt2 catalyzed chemoselective CC bond cleavage of α,β-enones: an unexpected synthesis of 3-alkylated oxindoles and spiro-indolooxiranes

A BF₃·OEt₂ catalyzed highly chemoselective formal CC double bond cleavage reaction of α,β-enones with diazoamides for the synthesis of 3-alkylated oxindoles is developed. Boron trifluoride etherate is found to be an effective catalyst for the chemoselective C_α-C_β cleavage of enones to obtain 3-alkylated oxindoles. The product formation indicates a selective β-carbon elimination pathway of α,β-enones using the inexpensive BF₃·OEt₂ as a catalyst, transition metal-free conditions, an open-air environment, good functional tolerance and broad substrate scope. The synthetic utility of this protocol is highlighted by synthesizing spiro-indolooxiranes.

Visible-Light-Driven Oxidative Cleavage of Alkenes Using Water-Soluble CdSe Quantum Dots

The oxidative cleavage of C=C bonds is an important chemical reaction, which is a popular reaction in the photocatalytic field. However, high catalyst-loading and low turnover number (TON) are general shortcomings in reported visible-light-driven reactions. Herein, the direct oxidative cleavage of C=C bonds through water-soluble CdSe quantum dots (QDs) is described under visible-light irradiation at room temperature with high TON (up to 3.7×10⁴ ). Under the same conditions, water-soluble CdSe QDs could also oxidize sulfides to sulfoxides with 51-84 % yields and TONs up to 3.4×10⁴ . The key features of this photocatalytic protocol include high TONs, wide substrates scope, low catalyst loadings, simple and mild reaction conditions, and molecular O₂ as the oxidant.

A computational study of the reaction mechanism of 2,2-azobis(isobutyronitrile)-initiated oxidative cleavage of geminal alkenes

A computational study of 2,2-azobis(isobutyronitrile) (AIBN)-initiated aerobic oxidative cleavage of alkenes is carried out employing density functional theory (DFT) and high-level coupled-cluster methods, such as coupled-cluster singles and doubles with perturbative triples [CCSD(T)]. Our computations show that the barriers for the formation of dioxetane derivatives suggested by Xu and co-workers (J. Org. Chem., 2014, 79, 7220-7225) for the reaction mechanism of aerobic oxidative cleavage of alkenes are computed to be higher than 65 kcal mol^-1. This barrier is relatively high under the reaction conditions. Our results for the Xu mechanism indicate that the reaction does not proceed via the formation of a dioxetane ring under the reaction conditions. Our results demonstrate that the reaction of aerobic oxidative cleavage of geminal alkenes in the presence of AIBN is initiated by the peroxyl radical 9, contrary to the isobutyronitrile radical 2. Our results show that the 2-(2-hydroxyl-1,1-diarylethoxy)-2-methylpropanenitrile radical (15) does not appear throughout the reaction scheme and the reaction progresses over the 2-(2-hydroxyl-2,2-diarylethoxy)-2-methylpropanenitrile radical (13) rather than the 2-(2-hydroxyl-1,1-diarylethoxy)-2-methylpropanenitrile radical (15). Our results are in agreement with the experimental results for the aerobic oxidative cleavage of the geminal disubstituted alkenes. Our results also demonstrate that the epoxide derivatives can be formed as an intermediate under the reaction conditions. This reaction is not applicable for pyridine derivatives due to the conversion of vinylpyridine derivatives to N-oxide derivatives.

Iodine-Initiated Dioxygenation of Aryl Alkenes Using tert-Butylhydroperoxides and Water: A Route to Vicinal Diols and Bisperoxides

An environment-friendly and efficient dioxygenation of aryl alkenes for the construction of vicinal diols has been developed in water with iodine as the catalyst and tert-butylhydroperoxides (TBHPs) as the oxidant. The protocol was efficient, sustainable, and operationally simple. Detailed mechanistic studies indicated that one of the hydroxyl groups is derived from water and the other one is derived from TBHP. Additionally, the bisperoxides could be obtained in good yields with iodine as the catalyst, Na₂CO₃ as the additive, and propylene carbonate as the solvent, instead.

Photo-induced oxidative cleavage of C-C double bonds for the synthesis of biaryl methanone via CeCl3 catalysis

A Ce-catalyzed strategy is developed to produce biaryl methanones via photooxidative cleavage of C-C double bonds at room temperature. This reaction is performed under air and demonstrates high activity as well as functional group tolerance. A synergistic Ce/ROH catalytic mechanism is also proposed based on the experimental observations. This protocol should be the first successful Ce-catalyzed photooxidation reaction of olefins with air as the oxidant, which would provide inspiration for the development of novel Ce-catalyzed photochemical synthesis processes.

Oxidative Cleavage of Alkenes by O2 with a Non-Heme Manganese Catalyst

The oxidative cleavage of C=C double bonds with molecular oxygen to produce carbonyl compounds is an important transformation in chemical and pharmaceutical synthesis. In nature, enzymes containing the first-row transition metals, particularly heme and non-heme iron-dependent enzymes, readily activate O₂ and oxidatively cleave C=C bonds with exquisite precision under ambient conditions. The reaction remains challenging for synthetic chemists, however. There are only a small number of known synthetic metal catalysts that allow for the oxidative cleavage of alkenes at an atmospheric pressure of O₂, with very few known to catalyze the cleavage of nonactivated alkenes. In this work, we describe a light-driven, Mn-catalyzed protocol for the selective oxidation of alkenes to carbonyls under 1 atm of O₂. For the first time, aromatic as well as various nonactivated aliphatic alkenes could be oxidized to afford ketones and aldehydes under clean, mild conditions with a first row, biorelevant metal catalyst. Moreover, the protocol shows a very good functional group tolerance. Mechanistic investigation suggests that Mn–oxo species, including an asymmetric, mixed-valent bis(μ-oxo)-Mn(III,IV) complex, are involved in the oxidation, and the solvent methanol participates in O₂ activation that leads to the formation of the oxo species.

Poly(ethylene glycol) dimethyl ether mediated oxidative scission of aromatic olefins to carbonyl compounds by molecular oxygen

A simple, and practical oxidative scission of aromatic olefins to carbonyl compounds using O₂ as the sole oxidant with poly(ethylene glycol) dimethyl ether as a benign solvent has been developed. A wide range of monosubstituted, gem-disubstituted, 1,2-disubstituted, trisubstituted and tetrasubstituted aromatic olefins was successfully converted into the corresponding aldehydes and ketones in excellent yields even with gram–scale reaction. Some control experiments were also conducted to support a possible reaction pathway.

A simple and practical O₂ oxidized scission of monosubstituted, gem- and 1,2-disubstituted, trisubstituted and tetrasubstituted aromatic olefins to aldehydes and ketones in PEGDME has been developed.

Endorsing Organic Porous Polymers in Regioselective and Unusual Oxidative C═C Bond Cleavage of Styrenes into Aldehydes and Anaerobic Benzyl Alcohol Oxidation via Hydride Elimination

Oxidative cleavage of styrene C═C double bond is accomplished by employing a nitrogen-rich triazine-based microporous organic polymer as an organocatalyst. We report this regioselective reaction as first of its kind with no metal add-ons to afford benzaldehydes up to 92% selectivity via an unusual Wacker-type C═C bond cleavage. Such a reaction pathway is generally observed in the presence of a metal catalyst. This polymer further shows high catalytic efficiency in an anaerobic oxidation reaction of benzyl alcohols into benzaldehydes. The reaction is mediated by a base via the in situ generation of hydride ions. This study is supported by experiments and computational analyses for a free-radical transformation reaction of oxidative C═C bond cleavage of styrenes and a hydride elimination mechanism for the anaerobic oxidation reaction. Essentially, the study unveils protruding applications of metal-free nitrogen-rich porous polymers in organic transformation reactions.

Radical-mediated aerobic oxidation of substituted styrenes and stilbenes

A 2,2-azobis(isobutyronitrile)-catalyzed oxidative cleavage of alkenes with molecular oxygen as the oxidant was described.

Thiol-initiated photocatalytic oxidative cleavage of the CC bond in olefins and its extension to direct production of acetals from olefins

Oxidative cleavage of a broad scope of olefins is realized over ZnIn₂S₄ under visible light, using air as oxidant and thiol as initiator. Coupled with the condensation between aldehydes/ketones and alcohols, this strategy can be used to yield acetals directly from olefins.

Visible light-induced aerobic dioxygenation of α,β-unsaturated amides/alkenes toward selective synthesis of β-oxy alcohols using rose bengal as a photosensitizer

The first visible light-induced aerobic dioxygenation of alkenes for the selective synthesis of β-oxy alcohols was developed using non-toxic rose bengal as a photosensitizer.

Merging CF3SO2Na photocatalysis with palladium catalysis to enable decarboxylative cross-coupling for the synthesis of aromatic ketones at room temperature

By merging CF₃SO₂Na-mediated photocatalysis with palladium catalysis, an efficient decarboxylative coupling strategy of α-keto acids and aryl boronic acids has been developed for the synthesis of aromatic ketones.

Highly efficient oxidative cleavage of olefins with O2 under catalyst-, initiator- and additive-free conditions

Without employing any external catalyst, initiator and additives, an efficient and eco-friendly protocol has been developed for the synthesis of carbonyl compound via 1,4-dioxane- promoted oxidation of olefins with atmospheric O₂ as the sole oxidant.

Flavor-Toxicant Correlation in E-cigarettes: A Meta-Analysis

Flavors in electronic cigarette (ECIG) liquids may increase ECIG aerosol toxicity via intact distillation or chemical transformation. For this report, we performed a meta-analysis of the literature to categorize the compounds found in flavored ECIG liquids into a few chemical classes and to predict their possible chemical transformations upon ECIG liquid aerosolization. This analysis allowed us to propose specific correlations between flavoring chemicals and aerosol toxicants. A literature search was conducted in November 2019 using PubMed. Keywords included terms related to ECIGs and flavors. Studies were included if they reported chemical ingredients of flavored liquids and clearly stated the commercial names of these liquids. The obtained data were visualized on a network diagram to show the common chemical compounds identified in flavored ECIG liquids and categorize them into different chemical classes. The systematic literature review included a total of 11 articles. Analysis of the data reported gave a total of 189 flavored liquids and 173 distinct chemical compounds that were categorized into 22 chemical classes according to their functional groups. The subsequent prediction of chemical transformations of these functional groups highlighted the possible correlation of flavor compounds to aerosol toxicants.

Oxidation of Long-Chain α-Olefins Using Environmentally-Friendly Oxidants

Studies on the oxidation of α-olefins via the two-stage method are presented. The new method consisted of oxidizing C30+ α-olefins with hydrogen peroxide (2 equiv.) and subsequent oxidation with oxygen. Products with high acid numbers (29–82 mgKOH/g) and saponification numbers (64–140 mgKOH/g) were obtained and compared with products obtained using only hydrogen peroxide or oxygen. It was demonstrated that H₂O₂ can be partially replaced by oxygen in the oxidative cleavage reaction of α-olefins. N-hydroxyphthalimide in combination with Co(acac)₂ demonstrated high activity in the oxidation stage using oxygen.

Computational Study for the Reaction Mechanism of N-Hydroxyphthalimide-Catalyzed Oxidative Cleavage of Alkenes

A computational study of N-hydroxyphthalimide-catalyzed aerobic oxidative cleavage of alkenes is carried out employing density functional theory and high-level coupled-cluster methods, such as coupled-cluster singles and doubles with perturbative triples [CCSD(T)]. Our results demonstrate that the reaction proceeds through the alkoxyl radicals, as opposed to the mechanism suggested by Jiao and co-workers (Org. Lett. 2012, 14, 4158-4161), in which the reaction proceeds via formation of the dioxetane ring. The barriers for the formation of dioxetane derivatives are computed to be higher than 50 kcal mol^-1, while the barriers for the formation of alkoxyl radicals are as low as 13 kcal mol^-1. Our results also demonstrate that epoxide derivatives can be formed as intermediates or byproducts under the reaction conditions.

Tandem Reaction Approaches to Isoquinolones from 2-Vinylbenzaldehydes and Anilines via Imine Formation-6π-Electrocyclization-Aerobic Oxidation Sequence

Two distinctive transition-metal-promoted aerobic oxidation protocols have been developed for the synthesis of isoquinolones from 2-vinylbenzaldehydes and aniline derivatives. Thus, the one-pot tandem reaction sequence of imine formation, thermal 6π-electrocyclization, followed by either Cu(OAc)₂-mediated or Pd(OAc)₂-catalyzed aerobic oxidation protocol allowed the ready access to isoquinolone derivatives. The control experiments revealed that the 1,2-dihydroisoquinoline intermediates from the 6π-electrocyclization of 1-azatrienes were aerobically oxidized to isoquinolones in the presence of either Cu(OAc)₂ or Pd(OAc)₂ catalyst.

Adipic acid formation from cyclohexanediol using platinum and vanadium catalysts: elucidating the role of homogeneous vanadium species

The applicability of platinum and vanadium catalysts for the production of adipic acid from cyclohexanediol is severely limited by the instability of the latter species in liquid phase reactions.