Acyclic Ethers Research Articles

On the chemical characteristics and dynamics of n-alkane low-temperature multistage diffusion flames

We demonstrate experimentally, perhaps for the first time, the existence of low-temperature multistage diffusion flames of n-alkanes. Multistage diffusion flames of n-heptane, n-decane, and n-dodecane are established in an atmospheric counterflow burner. Planar laser-induced fluorescence, chemiluminescence, and thermometry are used to probe the structures of such flames. In the first flame zone, the majority of the fuel is partially oxidized via low-temperature peroxy chemistry. In the second flame zone, the intermediate species produced are further oxidized via intermediate-temperature chemistry. The two stages of the flame are coupled such that significant fuel and oxidizer leakage occur, respectively, from the first and second reaction zones. The fuel is then further consumed, in the second stage, after the radical pool is replenished by the oxidation of the intermediates. The structure of the n-alkane multistage flame is found to be consistent with that previously observed for acyclic ethers. Owing to the different classes of temperature-dependent chemistries dominating the first and second stages, the reaction zone structure of multistage diffusion flames is dramatically influenced by the reactant concentrations and flame temperatures. The first stage is relatively favored at lower temperatures whereas the second stage is favored at elevated temperatures. Moreover, near extinction where the flame temperature is low, the multistage flame dynamics are controlled by the first oxidation stage, governed by peroxy chemistry, whereas the second oxidation stage, governed by intermediate chemistry, is dominant near high-temperature ignition conditions. Finally, by doping the oxidizer with ozone, we demonstrate the role of ozone doping on the multistage flame structure and the existence of a separate low-temperature ozone-assisted burning mode.

Chelation‐promoted Efficient C−H/N−H Cross Dehydrogenative Coupling between Picolinamides and Simple Ethers under Copper Catalysis

AbstractA highly efficient copper‐catalyzed C−H/N−H cross dehydrogenative coupling between picolinamides and simple ethers was developed. The reaction was promoted by the chelation assistance of removable picolinyl group and exhibited excellent TON and TOF number. This method was applicable to both N‐aryl and alkyl picolinamides as well as various cyclic and acyclic ethers with good functional group compatibility. It also possessed the merit of air and moisture tolerance and easy operation.magnified image

Oxidative C—H Alkynylation of Unactivated Acyclic Ethers

Oxidative C—H Alkynylation of Unactivated Acyclic Ethers

Palladium(II) acetate catalyzed acylative cleavage of cyclic and acyclic ethers under neat conditions

During the development of a palladium catalyzed C–H activation cross-coupling reaction involving acyl halides, it was noted that palladium(II) acetate catalyzes the acylative cleavage of tetrahydrofuran (used as a solvent) at room temperature to afford the corresponding 4-chlorobutyl ester derivative. After optimization, the reaction was shown to work well with epoxides, oxetane and tetrahydrofuran, but only barely with oxanes at room temperature. Acyclic ethers systematically failed to react under similar conditions, but underwent complete conversion in a microwave reactor at 100 °C.

H-Atom Abstraction Reactions by Ground-State Ozone from Saturated Oxygenates.

Theoretical insights into H-abstraction by ozone from saturated species have been virtually nonexistent, in sharp contrast to the situation for reactions with unsaturated species. Our computed rate constants for reactions with tetrahydrofuran and its methyl derivatives, obtained at various levels of theory, show that abstraction occurs primarily at the carbons situated beside the heterocyclic oxygen, with least likely reaction from the methyl groups. All of the methods tested are in broad relative agreement with this conclusion and with recent experiments although they do differ widely in terms of their absolute values. Abstraction leads variously to the formation of hydrotrioxides, ROOOH, or more directly to R-OH + O2(1Δg). To understand some of the observed behaviors, dimethyl and diethyl acyclic ethers, methyl and ethyl hydroperoxides, and the alcohols methanol and ethanol were also included in this study. For the simplest system, CH3OH + O3 → H2C═O + H2O2, an all-electron scalar relativistic CCSD(T)/CBS approach yields a barrier height that differs by >5 kJ mol-1 when an additional multireference treatment up to CCSDT(Q) is considered.

Recent Advances in Functionalization of α‐C(sp3)–H Centres in Inactivated Ethers through Cross Dehydrogenative Coupling

Carbon–carbon and carbon–heteroatom bond construction through oxidative carbon–hydrogen bond activation has attracted much attention in synthetic organic chemistry. Recently, a variety of reactions of inactivated cyclic ethers (THF, THP and dioxane) and acyclic ethers with various coupling partners have been reported. Despite the absence of phenyl rings and high oxidation potentials, these ethers worked efficiently under oxidative conditions. This review focusses on recent reports of C–C and C–heteroatom bond formation in these cyclic and acyclic ethers through cross dehydrogenative coupling.

Direct alkylheteroarylation of alkenes via photoredox mediated C-H functionalization.

The direct alkylheteroarylation of alkenes with cyclic and acyclic ethers via distal heteroaryl ipso-migration has been accomplished through the design of a photoredox-mediated C-H functionalization pathway. The use of inexpensive ethers as radical precursors combined with a straightforward method makes this a practical approach.

Why do the Togni reagent and some of its derivatives exist in the high-energy hypervalent iodine form? New insight into the origins of their kinetic stability.

In a recent study published in ChemComm, H. F. Schaefer and coworkers showed that the Togni trifluoromethyltation reagent and some of its derivatives appear in a high-energy hypervalent form. The (kinetic) stability of these reagents is granted by the five-membered ring of their benziodoxole-based scaffold, which prevents isomerization to the (inactive) acyclic ether form. Whereas the thermodynamic stability of these reagents can be predicted on the basis of the trans influence of the electrophilic substituent, no such descriptor was found for their kinetic stability. In this study, we explore an array of Togni-type reagents, and show that the barrier to isomerization can be predicted based on the bond length between the iodine atom and the electrophilic substituent. For compounds, where this correlation does not hold, we have a reliable indication that the structure of the transition state is at variance with those in the series.

Direct C(sp3)-H Cross Coupling Enabled by Catalytic Generation of Chlorine Radicals.

Here we report the development of a C(sp3)-H cross-coupling platform enabled by the catalytic generation of chlorine radicals by nickel and photoredox catalysis. Aryl chlorides serve as both cross-coupling partners and the chlorine radical source for the α-oxy C(sp3)-H arylation of cyclic and acyclic ethers. Mechanistic studies suggest that photolysis of a Ni(III) aryl chloride intermediate, generated by photoredox-mediated single-electron oxidation, leads to elimination of a chlorine radical in what amounts to the sequential capture of two photons. Arylations of a benzylic C(sp3)-H bond of toluene and a completely unactivated C(sp3)-H bond of cyclohexane demonstrate the broad implications of this manifold for accomplishing numerous C(sp3)-H bond functionalizations under exceptionally mild conditions.

Pericyclic reactions in ether biofuels

Unimolecular decompositions of acyclic ethers through a pericyclic mechanism, i.e., alcohol elimination, have been shown to play a crucial role in the high-temperature combustion of these compounds. The production of new fuels derived from biomass has led to a renewed interest into the combustion chemistry of ethers. A large fraction of ethers produced as potential biofuels features a cyclic structure. The pericyclic reactions in these cyclic structures, with or without a lateral alkyl group, remains unknown. In this work, we performed a systematic theoretical study on the pericyclic reactions in acyclic and cyclic ethers. Envisaged concerted reactions includes the classical alcohol formation and a H2 eliminations that was recently shown to play a non negligible role in the thermal decomposition of tetrahydrofuran (Verdicchio et al., 2015). Theoretical calculations performed in this work demonstrated that H2 elimination in acyclic ethers is negligible. In the case of cyclic ethers (tetrahydrofuran and tetrahydropyran), the branching ratio of the unimolecular pericyclic reactions strongly depends on the presence of a lateral alkyl group bonded to the carbon atom in position 2. If an alkyl group is present, the alcohol formation is favored through an exo 4-center rearrangement, that we newly defined in this work. If no lateral alkyl group is available in position 2, endo alcohol formation and H2 eliminations are equivalently important. Reaction rate rules were established to include pericyclic decomposition reactions in detailed chemical kinetic models of ether combustion.

ChemInform Abstract: An Efficient Iron Catalyzed Regioselective Acylative Cleavage of Ethers: Scope and Mechanism.

AbstractRegioselective cleavage of cyclic and acyclic ethers in the presence of acid chlorides yields chloro‐esters and nonchlorinated esters, respectively.

ChemInform Abstract: Iron‐Catalyzed Direct α‐Arylation of Ethers with Azoles.

AbstractThe direct α‐arylation of cyclic and acyclic ethers with azoles (I), (IV), and (VI) is achieved, which features a novel Fe‐catalyzed cross‐dehydrogenative coupling process.

ChemInform Abstract: Cycloaddition Reactions of Propyne Iminium Salts and Enol Ethers.

Acetylenic iminium salts (propyne iminium salts) react with cyclic enol ethers in a thermal [2+2] cycloaddition reaction to afford condensed cyclobutene iminium salts. With acyclic enol ethers, a cycloaddition/electrocyclic ring-opening sequence occurs to give (1-alkoxybuta-1,3-dien-2-yl)methane iminium salts as mixtures of diastereoisomers. An analogous scenario is observed with 5-phenyl-2-vinylfuran as the nucleophilic olefinic component. With 1-methoxy-3-trimethylsilyloxy-1,3-butadiene, propyne iminium salts undergo a [4+2] cycloaddition. For all reactions, traces of acid must be quenched with solid K2CO3 or CaO to avoid immediate polymerization of the enol ether.

An efficient iron catalyzed regioselective acylative cleavage of ethers: scope and mechanism

A method involving iron catalyzed acylative cleavage of cyclic and acyclic ethers with acyl/aroyl chlorides has been studied to produce chloroesters and esters respectively. Examination of the scope revealed that less electron rich alkyl group in unsymmetric, acyclic ether was acylated while the chloride derived from the counterpart moiety was volatile and difficult to isolate. In contrast, α-branched cyclic ethers were converted to the corresponding primary ester and secondary chloride. Steric hindrance of ether also plays an important role in acylative C–O bond cleavage. The mechanism of ether cleavage is proposed to involve a single electron initiated SN1 dissociative pathway.

ChemInform Abstract: Highly Chemoselective and Regioselective Dehydrogenative Cross‐Coupling Reaction Between Pyridines and Ethers.

A highly chemoselective and regioselective dehydrogenative cross-coupling (DCC) reaction of unactivated pyridines with cyclic or acyclic ethers has been developed to give the corresponding 2- or 4-coupled pyridines in satisfactory yields. The DCC reaction affords an efficient and greener synthesis for a range of new pyridines. A possible mechanism involving radical substitution is also proposed.

Iridium Pincer-Catalyzed Dehydrogenation of Ethers Featuring Ethylene as the Hydrogen Acceptor

We describe efficient methods to dehydrogenate ethers by using iridium pincer complexes (iPr4Anthraphos)-Ir(H)(Cl), 4, iPr4PC(sp3)P–Ir(H)(Cl), 5, and (iPr4PCP)-Ir(H)(Cl), 6. At 120 °C, cyclic ethers were dehydrogenated with tert-butylethylene as the hydrogen acceptor with high turnover numbers (over 400 in many cases). Acyclic ethers such as diethyl ether can also be dehydrogenated catalytically with TONs up to 90. The efficient dehydrogenation of cyclic and acyclic ethers using ethylene as a more practical hydrogen acceptor has been demonstrated for the first time.

Highly Chemoselective and Regioselective Dehydrogenative Cross-Coupling Reaction between Pyridines and Ethers

A highly chemoselective and regioselective dehydrogenative cross-coupling (DCC) reaction of unactivated pyridines with cyclic or acyclic ethers has been developed to give the corresponding 2- or 4-coupled pyridines in satisfactory yields. The DCC reaction affords an efficient and greener synthesis for a range of new pyridines. A possible mechanism involving radical substitution is also proposed.

ChemInform Abstract: A General Metal‐Assisted Synthesis of α‐Halo Oxime Ethers from Nitronates and Nitro Compounds.

AbstractBis(oxy)enamines (I) and (XII) undergo an unprecedented reaction with cobalt(II) halides to produce a variety of cyclic and acyclic ethers of α‐halo oximes including previously unavailable trimethylsilyl ethers of α‐iodo oximes.

ChemInform Abstract: Practical and Highly Selective C—H Functionalization of Structurally Diverse Ethers.

AbstractAlkynylations, arylations and alkenylations of a broad variety of substrates (including cyclic and acyclic ethers and various heterocyclic substrates) are achieved.

ChemInform Abstract: Metal‐Free Amidation of Ether sp3 C—H Bonds with Sulfonamides Using PhI(OAc)2.

AbstractAliphatic cyclic and aliphatic linear acyclic ethers react to the desired α‐branched products.