Tetrabutylammonium Decatungstate Research Articles

Optimization of a Decatungstate-Catalyzed C(sp3)-H Alkylation Using a Continuous Oscillatory Millistructured Photoreactor.

Tetrabutylammonium decatungstate (TBADT) has emerged as an efficient and versatile photocatalyst for hydrogen atom transfer (HAT) processes that enables the cleavage of both activated and unactivated aliphatic C–H bonds. Using a recently developed oscillatory millistructured continuous-flow photoreactor, investigations of a decatungstate-catalyzed C(sp3)–H alkylation protocol were carried out, and the results are presented here. The performance of the reactor was evaluated in correlation to several chemical and process parameters, including residence time, light intensity, catalyst loading, and substrate/reagent concentration. In comparison with previously reported batch and flow protocols, conditions were found that led to considerably higher productivity, achieving a throughput up to 36.7 mmol/h with a residence time of only 7.5 min.

Acylation of Aryl Halides and α-Bromo Acetates with Aldehydes Enabled by Nickel/TBADT Cocatalysis.

In this protocol aryl halides and α-bromo acetates are efficiently cross-coupled with an array of (hetero)aromatic and aliphatic aldehydes under the cooperative catalysis of nickel and tetrabutylammonium decatungstate as a hydrogen-atom-transfer photocatalyst. This method provides a concise approach to a variety of ketones with high compatibility of various functional groups.

Light-Mediated Asymmetric Aliphatic C–H Alkylation with Hydrogen Atom Transfer Catalyst and Chiral Phosphoric Acid

In the presence of tetrabutylammonium decatungstate and chiral spiro phosphoric acid, a light-mediated asymmetric C–H functionalization of unactivated hydrocarbons with exocyclic enones has been es...

Nickel/Photo-Cocatalyzed Asymmetric Acyl-Carbamoylation of Alkenes.

An unprecedented asymmetric acyl-carbamoylation of pendant alkenes tethered on aryl carbamic chlorides with both aliphatic and aromatic aldehydes has been developed via the cooperative catalysis of a chiral nickel-PHOX complex and tetrabutylammonium decatungstate. This reaction represents the first example of merging hydrogen-atom-transfer photochemistry and asymmetric transition metal catalysis in difunctionalization of alkenes. Using this protocol, a variety of oxindoles bearing a challenging quaternary stereogenic center are furnished under mild conditions in highly enantioselective manner.

A scalable continuous photochemical process for the generation of aminopropylsulfones.

An efficient continuous photochemical process is presented that delivers a series of novel γ-aminopropylsulfones via a tetrabutylammonium decatungstate (TBADT) catalysed HAT-process. Crucial to this success is the exploitation of a new high-power LED emitting at 365 nm that was found to be superior to an alternative medium-pressure Hg lamp. The resulting flow process enabled the scale-up of this transformation reaching throughputs of 20 mmol h-1 at substrate concentrations up to 500 mM. Additionally, the substrate scope of this transformation was evaluated demonstrating the straightforward incorporation of different amine substituents as well as alkyl appendages next to the sulfone moiety. It is anticipated that this methodology will allow for further exploitations of these underrepresented γ-aminopropylsulfone scaffolds in the future.

Photoinduced Generation of Superoxidants for the Oxidation of Substrates with High C−H Bond Dissociation Energies

AbstractOxidation of substrates, such as methane, with large C−H bond dissociation energies usually requires harsh reaction conditions, such as high temperature and pressure. The use of photoexcited states of oxidants enables the oxidation of substrates which would otherwise be unable to react thermally. This Review focuses on photoinduced generation of strong inorganic and organic oxidants, which enables the oxidation of substrates with strong C−H bonds. For example, photoexcitation of a CeIV‐alkoxide complex results in the cleavage of Ce−O bond to produce an alkoxyl radical, which can abstract a hydrogen atom from methane to afford a methyl radical, leading to amination of methane with tert‐butyloxycarbonyl (Boc)‐protected monomethylhydrazine. Hydrogen atom abstraction from alkanes also occurs induced by the photoexcited state of tetrabutylammonium decatungstate, leading to alkylations and acylation of both aromatic and aliphatic N‐tosylimines. Photoexcitation of Bi‐ and V‐containing beta zeolites also results in hydrogen atom abstraction from methane to produce methanol at ambient temperature. The photoexcited state of a manganese(IV)‐oxo complex binding with two Sc3+ ions has a surprisingly long lifetime (6.4 μs), and is able to oxidize benzene to produce phenol. Chlorine radicals, that can be generated by photoinduced oxidation of chloride ion by the photoexcited states of oxidants or by photoexcitation of a chlorine dioxide radical, abstracts a hydrogen atom from alkanes including methane to produce alkyl radicals, which can be converted to the oxygenated products. Photoinduced oxidation of methane was also made possible by mixing photosystem II (PSII) and the membrane fraction of a particular form of methane monooxygenase. A PSII model reaction has been achieved by using p‐benzoquinone derivatives as plastoquinone analogues with a non‐heme FeII catalyst in the presence of H2O under photoirradiation, to yield dioxygen with the corresponding hydroquinone derivatives.

Photocatalyzed Generation of Nitrosocarbonyl Intermediates Under Solar Light Irradiation

Tetrabutylammonium decatungstate (TBADT) has emerged as a valuable photocatalyst for sunlight‐induced organic reactions. In this communication we wish to present its use for the photocatalytic oxidation of hydroxamic acids to generate fleeting nitrosocarbonyl intermediates. The reaction required a fine tuning of the conditions, especially in terms of solvent and additives, since an aqueous acetonitrile solution containing a mild base (NaHCO3) gave the best results. The obtained highly reactive nitrosocarbonyl species were then efficiently trapped with suitable dienes, such as cyclopentadiene and 1,3‐cyclohexadiene, to afford the corresponding Hetero Diels–Alder (HDA) cycloadducts in good yields. Dedicated photophysical experiments helped to elucidate the mechanistic scenario.

Radical Addition to N-Tosylimines via C-H Activation Induced by Decatungstate Photocatalyst.

A method for the radical addition toward N-tosylimines based on light promoted C-H activation by tetrabutylammonium decatungstate is described. The reaction proceeds under irradiation using 400 nm light emitting diodes in conventional glassware. The method can be applied for alkylations (cycloalkanes, ethers, dimethylformamide) and acylation (primary aliphatic aldehydes) of both aromatic and aliphatic N-tosylimines.

Efficiency and Selectivity Aspects in the C–H Functionalization of Aliphatic Oxygen Heterocycles by Photocatalytic Hydrogen Atom Transfer

The C–H to C–C conversion in aliphatic oxygen heterocycles (dioxolanes, 1,3-dioxane, or cyclic carbonates) by photocatalytic hydrogen atom transfer and subsequent trapping of the resulting radical with phenyl vinyl sulfone was investigated. The performance of three different photocatalysts, namely tetrabutylammonium decatungstate and the aromatic ketones thioxanthone and 9-fluorenone, was compared. The UV-light-absorbing decatungstate anion is more efficient and permits the use of a smaller excess of hydrogen donor than the aromatic ketones, although the ketones could be excited by visible light. Further intramolecular selectivity studies revealed that aromatic ketones afforded a higher proportion of functionalization at the acetalic versus the ethereal positions than did the decatungstate anion.

Photocatalytic hydroacylation of trifluoromethyl alkenes.

Hydrofunctionalization of trifluoromethyl alkenes is highly challenging, because the nucleophilic addition is oftentimes followed by β-F elimination in this case. By the use of tetrabutylammonium decatungstate (TBADT) as a hydrogen-atom-transfer (HAT) photocatalyst for acyl C-H activation, we successfully avoid the β-F elimination in the hydroacylation of trifluoromethyl alkenes with aldehydes. This light (390 nm) promoted reaction provides a facile and efficient access to various β-CF3 ketones in complete regiocontrol with high functionality tolerance and 100% atom economy.

Photocatalyzed Site-Selective C(sp3)-H Functionalization of Alkylpyridines at Non-Benzylic Positions.

Tetrabutylammonium decatungstate (TBADT)-photocatalyzed C-H functionalization of alkylpyridines was investigated. Unlike alkylbenzene counterparts, alkylation of α-C-H bonds did not proceed for the reaction of 2- and 4-alkylpyridines and reluctantly proceeded for 3-alkylpyridines, which allow site-selective C(sp3)-H functionalization at nonbenzylic positions. The observed nonbenzylic site selectivities are rationalized by the polar inductive effects of pyridyl groups in the SH2 transition states. Consecutive γ-functionalization and α-bromofunctionalization were successfully carried out in selected cases.

Sunlight decatungstate photoinduced trifluoromethylations of (hetero)aromatics and electron-poor olefins.

Tetrabutylammonium decatungstate has been exploited to photoinduce the trifluoromethylation of (hetero)aromatics and electron-poor olefins. The process made use of the cheap Langlois' reagent (CF3SO2Na) and occurred under sunlight irradiation in an acetonitrile/water mixture. The trifluoromethylation of (hetero)aromatics required the use of potassium persulfate as an additive and the mono- or bis-trifluoromethylated adducts were obtained, depending on the actual reaction conditions and the chosen substrate.

Smooth Photocatalyzed Benzylation of Electrophilic Olefins via Decarboxylation of Arylacetic Acids.

Arylacetic acids were used as sources of benzyl radicals under tetrabutylammonium decatungstate photocatalyzed conditions for the benzylation of electron-poor olefins. The reaction proceeds smoothly in a mixed aqueous medium (MeCN/H2O 2/1) in the presence of NaHCO3, NaClO4, and an electron transfer agent (biphenyl). The reaction tolerates a wide variety of functional groups on the aromatic ring (whether electron donating or electron withdrawing) and can be extended to heteroaromatic analogues. The olefins have the double role of radical trap and electron acceptor. The present approach can also be extended to arylpropionic acids (including the nonsteroidal anti-inflammatory drugs ibuprofen and flurbiprofen), as well as mandelic acid derivatives.

ChemInform Abstract: Decatungstate‐Photocatalyzed Si—H/C—H Activation in Silyl Hydrides: Hydrosilylation of Electron‐Poor Alkenes.

Tetrabutylammonium decatungstate has been used for the photocatalytic activation of the Si−H bond in trisubstituted silanes and applied for the hydrosilylation of electron-poor alkenes. The mechanism that occurs depends on the silyl hydride used, as supported by laser flash photolysis and EPR trapping experiments. Homolytic Si−H cleavage through a hydrogen atom transfer from the silane to the excited catalyst operates with dimethylphenylsilane and methyldiphenylsilane, for which the hydrosilylation yields were satisfactory. If we used tertiary silanes that have more labile Si−H bonds, such as triphenylsilane or tris(trimethylsilyl)silane, the reaction worked to some extent even under uncatalyzed conditions because of a radical chain reaction. Competition between C−H and Si−H cleavage was observed, however, if we used trialkylsilanes (e.g., triethylsilane). In favorable cases, the process was also efficient under flow conditions or if promoted by sunlight.

Decatungstate Photocatalyzed Acylations and Alkylations in Flow via Hydrogen Atom Transfer

AbstractA set of atom‐economical radical conjugate addition reactions of alkyl and acyl radicals onto electron‐poor olefins or diisopropyl azodicarboxylate has been carried out on a multi‐gram scale under continuous flow conditions. The process is based on the use of the inorganic photocatalyst tetrabutylammonium decatungstate (TBADT), which is capable to activate selectively a variety of CH bonds in different substrates, including aldehydes, amides, ethers and alkanes. A wide range of synthetic targets has been synthesized according to this procedure, including 1,4‐difunctionalized derivatives (e.g., 1,4‐diketones or keto esters). All the reactions have been carried out in a meso‐scale flow photoreactor consisting in coils of UV‐transparent FEP tubing (fluorinated ethylene propylene) wrapped around a water‐cooled 500 W medium pressure mercury vapor lamp. The use of such a photochemical apparatus caused a reduction of the residence time, a notable increase of the STY (space time yield) and of the productivity values with respect to those measured for the corresponding batch processes, often leading to an improvement of the overall sustainability of the process (process mass intensity – PMI – values down to 10 kg kg−1).magnified image

ChemInform Abstract: Sunlight Photocatalyzed Regioselective β‐Alkylation and Acylation of Cyclopentanones.

Sunlight-induced direct regioselective β-alkylation of cyclopentanones with electron-deficient alkenes was accomplished by using tetrabutylammonium decatungstate (TBADT) as the catalyst. The regiochemistry can be rationalized by a polar transition state for an SH2 reaction. In the presence of CO, the reaction gave the three-component β-acylation product in good yield.

(nBu4N)4W10O32-catalyzed selective oxygenation of cyclohexane by molecular oxygen under visible light irradiation

The development of mild and efficient process for the selective oxygenation of organic compounds by molecular oxygen (O2) can be one of the key technologies for synthesizing oxygenates. Here, the photo-oxygenation of cyclohexane to cyclohexanol and cyclohexanone over tetrabutylammonium decatungstate (W10O324−) was carried out under visible light irradiation and pure O2 atmosphere. The W10O324− was found to be active to this photo-oxygenation in a pure acetonitrile (MeCN), which can achieve ca. 8.1% cyclohexane conversion with ca. 64.3% selectivity for cyclohexanone under sustained visible light irradiation of 12h. Notably, the above-described photo-catalysis oxygenation was improved to some extent in the presence of some acidic additives such as10M HCl, H2SO4, or H3PO4 aqueous solution and benzenesulfonic acid, providing ca. 12.4–14.3% cyclohexane conversion with ca. 59.7–69.4% cyclohexanone selectivity. Moreover, this acid-promoted effect was further magnified in the case of adding a suitable amount of water, which can lead to enhancing conversion (16.8–20.0%) and improving cyclohexanone selectivity (68.2–78.3%). This acidic aqueous solution-promoted effect was also observed upon the W10O324−-photocatalyzed oxygenation of other substrates like toluene, ethylbenzene and butanone. This is likely due to the acidic aqueous solutions playing important roles in improving photo-redox cycling of W10O324− and preserving its stability, as supported by the UV–vis spectra and cyclic voltammetry measurements.

Sunlight photocatalyzed regioselective β-alkylation and acylation of cyclopentanones

Visible light induced direct β-C–H/C–C conversion of cyclopentanones was accomplished by using tetrabutylammonium decatungstate, TBADT, as the photocatalyst.

Efficient C–H/C–N and C–H/C–CO–N Conversion via Decatungstate-Photoinduced Alkylation of Diisopropyl Azodicarboxylate

Tetrabutylammonium decatungstate (TBADT) accelerated the addition of C-H bonds to the N═N double bond of diisopropyl azodicarboxylate (DIAD) under irradiation conditions. The photoinduced three-component coupling between cyclic alkanes, CO, and DIAD was also achieved to give the corresponding acyl hydrazides.

Microwave-assisted oxidation of alcohols by hydrogen peroxide catalysed by tetrabutylammonium decatungstate

AbstractThis work deals with catalytic activity of tetrabutylammonium decatungstate(VI) in the oxidation of selected alcohols with hydrogen peroxide as an oxidant using 1,2-dichloroethane/water or acetonitrile/water as a solvent system. Different forms of heating were compared. The highest conversions of substrates were achieved in the two phase system acetonitrile/water using microwave irradiation combined with elevated pressure. Finally, optimum parameters for these reactions in a microwave pressurised reactor were established and discussed.