Allylic oxidations catalyzed by dirhodium caprolactamate via aqueous tert-butyl hydroperoxide: the role of the tert-butylperoxy radical

Direct access to pyrrole anhydrides via oxidative self-coupling of pyrrole carboxaldehydes

An elegant synthesis of pyrrole-2-carboxylic acid anhydrides from pyrrole-2-carboxaldehydes using TBAI as a catalyst and tert-butyl hydroperoxide (TBHP) as an oxidant is described herein. Unlike the previous reports wherein pyrrole-2-carboxylic acids were invariably used, we report here for first time the oxidative self-coupling of N-benzyl pyrrole-2-carboxaldehydes for the synthesis of 1-benzyl-1H-pyrrole-2-carboxylic anhydrides. In addition, a one-pot synthesis of novel pyrrole-2-carboxamides from pyrrole-2-carboxaldehydes is also reported. The mechanistic investigation supports a previously unexplored oxidative self-coupling of a pyrrole acyl radical, leading to the synthesis of a carboxylic anhydride.

Synthesis of 6β-hydroxy androgens from a 3,5-diene steroid precursor to test for cytochrome P450 3A4-catalyzed hydroxylation of androstenedione

6β-Hydroxytestosterone is a biomarker for the activity of human cytochrome P450 3A4 (P450 3A4), the major drug metabolizing cytochrome P450 enzyme. Despite its significance, efficient routes for the chemical synthesis of 6β-hydroxytestosterone are rare. In this study, 6β-hydroxytestosterone was synthesized through the oxidation of a 3,5-diene precursor under the Uemura-Doyle reaction conditions using a dirhodium catalyst in the presence of tert-butylhydroperoxide. Mechanistic studies showed that some oxygen is incorporated from molecular oxygen and CH abstraction is partially rate-limiting. This reaction was used to synthesize 6β-hydroxyandrostenedione, which was used as a standard to test the hypothesis of whether P450 3A4 catalyzes the hydroxylation of androstenedione. Upon incubation of P450 3A4 with androstenedione, a hydroxylated product was formed, which matched the retention time of synthetic 6β-hydroxyandrostenedione. This reaction can be exploited to study other biochemical processes involving compounds with a 6 β -hydroxy-3-keto-Δ4 steroid backbone.

Synthesis of 2,5-dimethylhexene by isobutene dimerization with H2S co-feeding

This study investigates the effect of hydrogen sulfide (H2S) co-feeding on the synthesis of 2,5-dimethyl-1-hexene, 2,5-dimethyl-2-hexene, and 2,5-dimethylhexane (2,5-DMHs), useful compounds, using the dimerization of isobutene under mild pressure conditions. The dimerization of isobutene did not proceed in the absence of H2S, whereas the desired products of 2,5-DMHs were produced under H2S co-feeding conditions. The effect of reactor size on the dimerization reaction was then examined, and the optimal reactor was discussed. To enhance the yield of 2,5-DMHs, we changed the reaction conditions of the temperature, molar ratio of isobutene to H2S (iso-C4[double bond, length as m-dash]/H2S) in the feed gas, and the total feed pressure. The optimum reaction condition was at 375 °C and 2/1 of iso-C4[double bond, length as m-dash]/H2S. The product of 2,5-DMHs monotonously increased by an increment of total pressure from 1.0 to 3.0 atm with a fixed iso-C4[double bond, length as m-dash]/H2S ratio at 2/1.

Bioactive properties of the aromatic molecules of spearmint (Mentha spicata L.) essential oil: a review

Spearmint belongs to the genus Mentha in the family Labiateae (Lamiaceae), which is wildly cultivated worldwide for its remarkable aroma and commercial value. The aromatic molecules of spearmint essential oil,...

A New Dimeric Copper(II) Complex of Hexyl Bis(pyrazolyl)acetate Ligand as an Efficient Catalyst for Allylic Oxidations

A new dimeric copper(II) bromide complex, [Cu(L^OHex)Br(μ-Br)]₂ (1), was prepared by a reaction of CuBr₂ with the hexyl bis(pyrazol-1-yl)acetate ligand (L^OHex) in acetonitrile solution and fully characterized in the solid state and in solution. The crystal structure of 1 was also determined: the complex is interlinked by two bridging bromide ligands and possesses terminal bromide ligands on each copper atom. The two pyrazolyl ligands in 1 coordinate with the nitrogen atoms to complete the Cu coordination sphere, resulting in a five-coordinated geometry—away from idealized trigonal bipyramidal and square pyramidal geometries—which can better be described as distorted square pyramidal, as measured by the τ and χ structural parameters. The pendant hexyloxy chain is disordered over two arrangements, with final site occupancies refined to 0.705 and 0.295. The newly synthesized complex was evaluated as a catalyst in copper-catalyzed C–H oxidation for allylic functionalization through a Kharasch–Sosnovsky reaction without any external reducing agent. Using 0.5 mol% of this catalyst, and tert-butyl peroxybenzoate (Luperox) as an oxidant, allylic benzoates were obtained with up to 90% yield. The general reaction time was only slightly decreased to 24 h but a very significant decrease in the alkene:Luperox ratio to 3:1 was achieved. These factors show relevant improvements with respect to classical Kharasch–Sosnovsky reactions in terms of rate and amount of reagents. The present study highlights the potential of copper(II) complexes containing functionalized bis(pyrazol-1-yl)acetate ligands as efficient catalysts for allylic oxidations.

Ensemble effects on allylic oxidation within explicit solvation environments

Umbrella-sampling density functional theory molecular dynamics (DFT-MD) has been employed to study the full catalytic cycle of the allylic oxidation of cyclohexene using a Cu(ii) 7-amino-6-((2-hydroxybenzylidene)amino)quinoxalin-2-ol complex in acetonitrile to create cyclohexenone and H2O as products. After the initial H-atom abstraction step, two different reaction pathways have been identified that are distinguished by the participation of alkyl hydroperoxide (referred to as the "open" cycle) versus the methanol side-product (referred to as the "closed" cycle) within the catalyst recovery process. Importantly, both pathways involve dehydrogenation and re-hydrogenation of the -NH2 group bound to the Cu-site - a feature that is revealed from the ensemble sampling of configurations of the reactive species that are stabilized within the explicit solvent environment of the simulation. Estimation of the energy span from the experimental turnover frequency yields an approximate value of 22.7 kcal mol-1 at 350 K. Whereas the closed cycle value is predicted to be 26.2 kcal mol-1, the open cycle value at 16.5 kcal mol-1. Both pathways are further consistent with the equilibrium between Cu(ii) and Cu(iii) that has previously been observed. In comparison to prior static DFT calculations, the ensemble of both solute and solvent configurations has helped to reveal a breadth of processes that underpin the full catalytic cycle yielding a more comprehensive understanding of the importance of radical reactions and catalysis recovery.

Development of new and efficient copper(II) complexes of hexyl bis(pyrazolyl)acetate ligands as catalysts for allylic oxidation

In this study, two new hexyl bis(pyrazol-1-yl)acetate ligands and related copper(ii) complexes were prepared and fully characterized in the solid state and in solution. Their electronic and molecular structures were investigated by X-ray photoelectron spectroscopy and near edge X-ray absorption; their ligand molecular structural stability upon coordination to copper was also investigated. The Cu(ii) complexes were studied as new catalysts in copper-catalyzed C-H oxidation for allylic functionalization (the Kharasch-Sosnovsky reaction) avoiding the use of any external reducing agents. Using 5 mol% of these catalysts and tert-butylperoxybenzoate as the oxidant, allylic benzoates were obtained in up to 90% yield: the general reaction time was decreased to 6 h and a 5 to 1 ratio of the alkene and tert-butylperoxybenzoate was used to overcome the two most important limitations on their use in chemistry.

Catalytic Oxidation Processes for the Upgrading of Terpenes: State-of-the-Art and Future Trends

Terpenic olefins constitute a relevant platform of renewable molecules, which could be used as key intermediates for the perfumery, flavoring, and pharmaceutical industries. The upgrading of these cheap and available agro-resources through catalytic oxidation processes remains of great interest, leading to the formation of either epoxides via the oxidation of the olefinic bond or α,β-unsaturated ketones by the Csp3-H functionalization at the α-position of the double bond. This critical review summarizes some of the most relevant homogeneous or heterogeneous catalysts designed for the oxidation of some abundant terpenic olefins in the last decade (2008–2018).

Cu-Catalyzed Aromatic Metamorphosis of 3-Aminoindazoles

We present a novel Cu-catalyzed aromatic metamorphosis of 3-aminoindazoles via oxidative cleavage of two C-N bonds of 3-aminoindazoles. This unprecedented reactivity of 3-aminoindazoles allows one to forge diverse nitrile-containing triphenylenes in decent yields via generation of the cyano group in situ. The current study reveals that 3-aminoindazoles could be harnessed as radical precursors via oxidative denitrogenation, the reaction mechanism of which was supported by density functional theory calculations.

Isolation and Synthesis of Novel Meroterpenoids from Rhodomyrtus tomentosa: Investigation of a Reactive Enetrione Intermediate

Rhodomyrtusials A-C, the first examples of triketone-sesquiterpene meroterpenoids featuring a unique 6/5/5/9/4 fused pentacyclic ring system were isolated from Rhodomyrtus tomentosa, along with several biogenetically-related dihydropyran isomers. Two bis-furans and one dihydropyran isomer showed acetylcholinesterase (AChE) inhibitory activity. Structures of the isolates were unambiguously established by a combination of spectroscopic data, ECD analysis, and total synthesis. Bioinspired total syntheses of six isolates were achieved in six steps utilizing a reactive enetrione intermediate generated in situ from a readily available hydroxy-endoperoxide precursor.

Electrochemical benzylic oxidation of C–H bonds

An electrochemical benzylic C–H oxidation reaction that is mediated by tert-butyl peroxyl radical is reported.

Copper-catalyzed desymmetrization of prochiral 4,4-disubstituted cyclopentenes via a site-selective allylic oxidation: a concise total synthesis of untenone A

The desymmetrization of prochiral 4,4-disubstituted cyclopentenes via a site-selective copper-catalyzed allylic oxidation is described.

Chemoenzymatic Synthesis of the Intermediates in the Peppermint Monoterpenoid Biosynthetic Pathway

A chemoenzymatic approach providing access to all four intermediates in the peppermint biosynthetic pathway between limonene and menthone/isomenthone, including noncommercially available intermediates (-)- trans-isopiperitenol (2), (-)-isopiperitenone (3), and (+)- cis-isopulegone (4), is described. Oxidation of (+)-isopulegol (13) followed by enolate selenation and oxidative elimination steps provides (-)-isopiperitenone (3). A chemical reduction and separation route from (3) provides both native (-)- trans-isopiperitenol (2) and isomer (-)- cis-isopiperitenol (18), while enzymatic conjugate reduction of (-)-isopiperitenone (3) with IPR [(-)-isopiperitenone reductase)] provides (+)- cis-isopulegone (4). This undergoes facile base-mediated chemical epimerization to (+)-pulegone (5), which is subsequently shown to be a substrate for NtDBR ( Nicotiana tabacum double-bond reductase) to afford (-)-menthone (7) and (+)-isomenthone (8).

Cobalt(II) Complexes with N,N,N-Scorpionates and Bidentate Ligands: Comparison of Hydrotris(3,5-dimethylpyrazol-1-yl)borate Tp* vs. Phenyltris(4,4-dimethyloxazolin-2-yl)borate ToM to Control the Structural Properties and Reactivities of Cobalt Centers

Scorpionate ligands Tp* (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and ToM (tris(4,4-dimethyloxazolin-2-yl)phenylborate) complexes of cobalt(II) with bidentate ligands were synthesized. Both Tp* and ToM coordinate to cobalt(II) in a tridentate fashion when the bidentate ligand is the less hindered acetylacetonate. In crystal structures, the geometry of cobalt(II) supported by the N₃O₂ donor set in the Tp* complex is a square-pyramid, whereas that in the ToM complex is close to a trigonal-bipyramid. Both Tp*- and ToM-acac complexes exhibit solvatochromic behavior, although the changing structural equilibria of these complexes in MeCN are quite different. In the bis(1-methylimidazol-2-yl)methylphenylborate (LPh) complexes, Tp* retains the tridentate (к³) mode, whereas ToM functions as the bidentate (к²) ligand, giving the tetrahedral cobalt(II) complex. The bowl-shaped cavity derived from the six methyl groups on ToM lead to susceptibility to the bulkiness of the opposite bidentate ligand. The entitled scorpionate compounds mediate hydrocarbon oxidation with organic peroxides. Allylic oxidation of cyclohexene occurs mainly on the reaction with tert-butyl hydroperoxide (TBHP), although the catalytic efficiency of the scorpionate ligand complexes is lower than that of Co(OAc)₂ and Co(acac)₂. On cyclohexane oxidation with meta-chloroperbenzoic acid (mCPBA), both ToM and Tp* complexes function as catalysts for hydroxylation. The higher electron-donating ToM complexes show faster initial reaction rates compared to the corresponding Tp* complexes.

Efficient and Highly Selective Solvent-Free Oxidation of Primary Alcohols to Aldehydes Using Bucky Nanodiamond

Selective oxidation of alcohols to aldehydes is widely applicable to the synthesis of various green chemicals. The poor chemoselectivity for complicated primary aldehydes over state-of-the-art metal-free or metal-based catalysts represents a major obstacle for industrial application. Bucky nanodiamond is a potential green catalyst that exhibits excellent chemoselectivity and cycling stability for the selective oxidation of primary alcohols in diverse structures (22 examples, including aromatic, substituted aromatic, unsaturated, heterocyclic, and linear chain alcohols) to their corresponding aldehydes. The results are comparable to reported transition-metal catalysts including conventional Pt/C and Ru/C catalysts for certain substrates under solvent-free conditions. The possible activation process of the oxidant and substrates by the surface oxygen groups and defect species are revealed with model catalysts, ex situ electrochemical measurements, and ex situ attenuated total reflectance. The zigzag edges of sp² carbon planes are shown to play a key role in these reactions.

Catalytic Allylic Oxidation of Cyclic Enamides and 3,4-Dihydro-2H-Pyrans by TBHP

Allylic oxidation of heteroatom substituted cyclic alkenes by tert-butyl hydroperoxide (70% TBHP in water) using catalytic dirhodium caprolactamate [Rh₂(cap)₄] forms enone products with a variety of 2-substituted cyclic enamides and 3,4-dihyro-2H-pyrans. These reactions occur under mild reaction conditions, are operationally convenient to execute, and are effective for product formation with as low as 0.25 mol% catalyst loading. With heteroatom stabilization of the intermediate allylic free radical two sites for oxidative product formation are possible, and the selectivity of the oxidative process varies with the heteroatom when R = H. Cyclic enamides produce 4-piperidones in good yields when R = alkyl or aryl, but oxidation of 2H-pyrans also gives alkyl cleavage products. Alternative catalysts for TBHP oxidations show comparable selectivities but give lower product yields.

nBu4NI-Mediated oxidation of methyl ketones to α-ketoamides: using ammonium, primary and secondary amine-salt as an amine moiety

Presented is the first example of synthesizing an array of primary-, secondary-, and tertiary-α-ketoamides in moderate to excellent yields with a catalyst nBu₄NI from methyl ketones and amine/ammonium salts under mild conditions using oxidant TBHP.

Oxidative cyclization of alkenoic acids promoted by AgOAc

Alkenoic acids derived from salicylic acid and analogues undergo an unexpected oxidative cyclization process triggered by AgOAc leading to 4H-benzo[d][1,3]dioxin-4-ones. The process is affected by the substitution on the aryl and the allyl units.

Dirhodium(II)-Catalyzed Sulfide Oxygenations: Catalyst Removal by Coprecipitation with Sulfoxides

The dirhodium(II) carboxylate complex Rh2(esp)2 (esp = α,α,α',α'-tetramethyl-1,3-benzenedipropanoate) was shown to catalyze the sulfoxidation of organic sulfides using tert-butyl hydroperoxide as the oxidant. Due to the unique structure of Rh2(esp)2 and its stable Rh2(II,II) catalyst resting state, the rhodium catalyst is able to precipitate as a Rh2(esp)2-sulfoxide complex following the reaction which makes separation of the catalyst from the products very convenient. The precipitated Rh2(esp)2-sulfoxide complexes could be reused to catalyze sulfide oxygenation reactions without considerable loss of activity. Mechanistic studies suggest that the axial ligands fine-tune the redox potential of the dirhodium(II,II) compounds and determine the predominant catalyst species in the oxidation reaction.

Mixed-ligand complexes of paddlewheel dinuclear molybdenum as hydrodehalogenation catalysts for polyhaloalkanes

A mixed-ligated dimolybdenum complex Mo₂(OAc)₂[CH(NAr)₂]₂ in combination with 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene and ⁿBu₄NCl exhibited high catalytic activity for hydrodehalogenation reactions.

We developed a hydrodehalogenation reaction of polyhaloalkanes catalyzed by paddlewheel dimolybdenum complexes in combination with 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD) as a non-toxic H-atom source as well as a salt-free reductant. A mixed-ligated dimolybdenum complex Mo₂(OAc)₂[CH(NAr)₂]₂ (3a, Ar = 4-MeOC₆H₄) having two acetates and two amidinates exhibited high catalytic activity in the presence of ⁿBu₄NCl, in which [ⁿBu₄N]₂[Mo₂{CH(NAr)₂}₂Cl₄] (9a), derived by treating 3a with ClSiMe₃ and ⁿBu₄NCl, was generated as a catalytically-active species in the hydrodehalogenation. All reaction processes, oxidation and reduction of the dimolybdenum complex, were clarified by control experiments, and the oxidized product, [ⁿBu₄N][Mo₂{CH(NAr)₂}₂Cl₄] (10a), was characterized by EPR and X-ray diffraction studies. Kinetic analysis of the hydrodehalogenation reaction as well as a deuterium-labelling experiment using MBTCD-d₈ suggested that the H-abstraction was the rate-determining step for the catalytic reaction.

Rh2(esp)2-catalyzed allylic and benzylic oxidations

The dirhodium(ii) catalyst Rh₂(esp)₂ allows direct solvent-free allylic and benzylic oxidation by T-HYDRO with a remarkably low catalyst loading.

nBu4NI-catalyzed intermolecular C–O cross-coupling reactions: synthesis of alkyloxyamines

nBu₄NI-catalyzed cross-coupling of benzyl and allylic compounds with N-hydroxyphthalimide for the synthesis of alkyloxyamines were realized for the first time.

Synthesis of α-peroxyesters via organocatalyzed O-H insertion of hydroperoxides and aryl diazoesters

The synthesis of α-aryl peroxyesters, an unprecedented class of organic peroxide, via hydrogen-bond donor catalyzed O-H insertions of hydroperoxides and α-aryl diazoesters is reported. The method is applicable to a diverse set of substrates and the corresponding α-peroxyesters are typically isolated in high yield. Both thermogravimetric analysis and reactions with traditional peroxide reducing agents demonstrate the stability of α-peroxyesters.

Fabrication of nitrogen-modified annealed nanodiamond with improved catalytic activity

Annealed ultradispersed nanodiamond (ADD) with sp(2) curved concentric graphitic shells is an interesting hybrid material consisting of the remarkable surface properties of graphene-based nanomaterials and the intrinsic properties of a diamond core. In this case, based on its specific properties and surface oxygen functional groups, nitrogen-modified ADD powders have been tunably synthesized via three different preparation methods in a calcination treatment process. The detailed formation and dynamic behaviors of the nitrogen species on the modified ADD during the preparation process are revealed by elemental analysis, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption. Moreover, we study the catalytic performance on the metal-free nitrogen-modified ADD catalysts by means of selective oxidation of benzylic alcohols as a probe reaction. The results indicate that the modified ADD catalysts exhibit a higher catalytic activity than pristine ADD. By correlating XPS data with catalytic measurements, we conclude that the pyridinic nitrogen species plays a pivotal role in the catalytic reaction. Our work provides valuable information on the design of modified carbon materials with more excellent properties.

Recent advances in cholesterol chemistry

This review article presents advances in cholesterol chemistry since 2000. Various transformations (chemical, enzymatic, electrochemical, etc.) of cholesterol are presented. A special emphasis is given to cholesterol oxidation reactions, but also substitution of the 3β-hydroxyl group, addition to the C5-C6 double bond, C-H functionalization, and C-C bond forming reactions are discussed.

Allylic oxidation of alkenes catalyzed by a copper-aluminum mixed oxide

A strategy for the allylic oxidation of cyclic alkenes with a copper-aluminum mixed oxide as catalyst is presented. The reaction involves the treatment of an alkene with a carboxylic acid employing tert-butyl hydroperoxide as the oxidant. In all cases, the corresponding allylic esters are obtained. When L-proline is employed, the allylic alcohol or ketone is obtained. The oxidation of cyclohexene and valencene has been optimized by design of experiments (DoE) statistical methodology.