Unsymmetrical Ethers Research Articles

Catalytic dehydrative cross-coupling of alcohols: An eco-friendly approach to unsymmetrical ethers

Developing green as well as sustainable protocols by employing cheap and readily available catalysts and/or reagents without using environmentally hazardous solvents is becoming more significant to the researcher community. Here, we introduce a green and metal-free catalytic method for the etherification of alcohols by using catalytic NH4I and oxone under mild conditions. The reagents we employed are inexpensive, abundant in nature, readily available, easy to handle and produce non-polluting by-products, which can be easily removable by simple aqueous work-up. This makes our approach more convenient, sustainable and economically viable. The present catalytic system does not require any organic solvent as reaction medium to facilitate the reaction. Using this green approach, we have synthesized a wide range of unsymmetrical ethers from two distinct types of alcohols (secondary benzylic and primary (benzylic or non-benzylic) alcohols) which demonstrated the efficiency of our protocol. Further, the gram scale reactions established the feasibility in scaling-up of our process.

Titanium tetrafluoride catalysis for the dehydrative conversion of diphenylmethanols to symmetric and unsymmetric ethers.

In contrast to the conversion of diphenylmethanol to the corresponding halides with an equivalent of titanium tetrachloride or -bromide, catalytic (50 mol%) titanium tetrafluoride converts benzhydrols in diethyl ether or dichloromethane to bis(benzhydryl) ethers within 0.5-1 h at room temperature. Cross ether formation with diphenylmethanols and primary aryl or aliphatic alcohols is achieved in the presence of 25 mol% TiF4 in refluxing toluene as solvent. A tentative mechanism involving a carbocation intermediate has been proposed.

The Bis(indolylmethyl) ethers: Design, Prototypical Synthesis, and Scope Studies.

The design, prototypical synthesis, isolation, and characterization of bis(indolylmethyl) ethers from corresponding indolylcarbinols is described. This approach involves very mild conditions and exhibits good scope for indolylcarbinols (both N-electron withdrawing group and N-electron donating group). Cross etherification between two electronically different indolylcarbinols is also demonstrated for the generation of unsymmetrical ethers. For the first time, the intermediacy of the bis(indolylmethyl) ethers for the formation of bis(indolyl)methanes from indolylcarbinols is proved experimentally and by 1H NMR analysis.

Polyether Synthesis through Reductive Etherification Reaction Strategy

AbstractThe reductive etherification reaction (RER) of carbonyl groups (aldehydes or ketones) through silane as a reducing agent together with Bronsted or Lewis acid affords the synthesis of symmetrical and unsymmetrical ethers. This strategy is applied at the macromolecular level for the first time in 1993, and isophthalaldehyde is self‐polymerized in the presence of triethylsilane (Et3SiH)/ tritylperchlorate (TrClO4) to yield polyethers with low to moderate molecular weights. Next, the polyethers with alternating structures are achieved by reacting isophthalaldehyde with bis(trimethylsilyl) ethers or diols as comonomers using reducing agent silane and Lewis acid. Moreover, in recent years, it is shown that polyether synthesis and post‐polymerization modification (PPM) of polymers proceeds smoothly and effectively with the RER strategy in the presence of chlorodimethylsilane (CDMS), which acts as both a reducing agent and a Lewis acid.

Silver(I)-Catalyzed Reductive Cross-Coupling of Aldehydes to Structurally Diverse Cyclic and Acyclic Ethers.

A range of medium-sized cyclic ethers (5 to 11 membered) have been effectively synthesized through intramolecular reductive coupling of dialdehydes initiated by 50 ppm to 0.5% of AgNTf2 with hydrosilane at 25 °C. The catalytic system is also suitable for the coupling of two different monoaldehydes to provide unsymmetrical ethers. This protocol features broad functional group compatibility, high product diversity, high efficiency, and utility in the late-stage modification of complex biorelevant molecules.

Selective Hydrodeoxygenation of Esters to Unsymmetrical Ethers over a Zirconium Oxide-Supported Pt-Mo Catalyst.

The catalytic hydrodeoxygenation (HDO) of carbonyl oxygen in esters using H2 is an attractive method for synthesizing unsymmetrical ethers because water is theoretically the sole coproduct. Herein, we report a heterogeneous catalytic system for the selective HDO of esters to unsymmetrical ethers over a zirconium oxide-supported platinum–molybdenum catalyst (Pt–Mo/ZrO2). A wide range of esters were transformed into the corresponding unsymmetrical ethers under mild reaction conditions (0.5 MPa H2 at 100 °C). The Pt–Mo/ZrO2 catalyst was also successfully applied to the conversion of a biomass-derived triglyceride into the corresponding triether. Physicochemical characterization and control experiments revealed that cooperative catalysis between Pt nanoparticles and neighboring molybdenum oxide species on the ZrO2 surface plays a key role in the highly selective HDO of esters. This Pt–Mo/ZrO2 catalyst system offers a highly efficient strategy for synthesizing unsymmetrical ethers and broadens the scope of sustainable reaction processes.

Cobalt‐Catalysed Reductive Etherification Using Phosphine Oxide Promoters under Hydroformylation Conditions

A phosphine-oxide-promoted, cobalt-catalysed reductive etherification using syngas as a reductant is reported. This novel methodology was successfully used to prepare a broad range of unsymmetrical ethers from various aldehydes and alcohols containing diverse functional groups, and was scaled-up to multigram scale under comparably mild conditions. Mechanistic experiments support an acetalization-hydrogenation sequence.

Iodine catalysed synthesis of unsymmetrical benzylic ethers by direct cross-coupling of alcohols

Although symmetrical ethers can be synthesized easily from alcohols, synthesis of unsymmetrical ethers by dehydrative cross-coupling of alcohols is still a challenge. While dehydrative cross-coupling is environmentally appealing due to formation of water as the only byproduct, the chances for formation of symmetrical ethers always exist. The existing transition metal based methods give good selectivity, but the catalyst are costly and not readily available. Here, we present a simple, readily available, and cost-effective catalyst in the form of molecular iodine which catalyzes a highly selective cross-coupling of benzylic alcohols with benzyl, alkyl, and aryl alcohols to give their corresponding unsymmetrical ethers in good to excellent yield.

Molybdenum (VI)-catalyzed dehydrative construction of C[sbnd]O and C[sbnd]S bonds formation via etherification and thioetherification of alcohols and thiols

An inexpensive, easily available, environmentally benign, and efficient catalyst molybdenum(VI) dioxo (acetylacetonate)2 was used for the direct oxo- and thioetherification of alcohol. This method endures selective molybdenum catalyzed dehydrative synthesis of symmetrical ethers from benzylic secondary alcohols as well as unsymmetrical ethers from the reaction of benzylic secondary alcohols with primary alcohol. Furthermore, we have been also successful in the synthesis of Aryl thioether by using alcohol and thiols.

Phosphine Free Catalytic Selective Dehydrative Etherification of Secondary Alcohols by Using Ag(I)-Nhc Complexes

We herein report the phosphine-free catalytic selective dehydrative etherification of secondary alcohols into symmetrical/ unsymmetrical ethers under mild conditions by using well-defined, air-stable, Ag(I)-NHC complexes. We have synthesized several novel Ag(I) complexes bearing NHC- ligands, namely, [(S)-7-(R1)-6-(R2)2-2-(R3)-6,7-dihydro-4H-[1,2,3]triazolo[5,1-c][1,4]oxazin-3-ylidene}AgCl (1−2)b, (Scheme 3), were isolated for the first time and structurally characterized through various spectroscopic techniques including single crystal X-ray diffraction analyses. All of these novel Ag(I)-NHC complexes have shown an excellent catalytic activity to produce value added ethers from various secondary alcohols through dehydration process by using Ag(I)-NHC catalysts under mild conditions, respectively.

Reductive Etherification of Aldehydes and Ketones with Alcohols and Triethylsilane Catalysed by Yb(OTf)3: an Efficient One‐Pot Benzylation of Alcohols

AbstractThe one‐pot synthesis of symmetrical and unsymmetrical ethers from aldehydes and ketones can be conveniently performed using Yb(OTf)3 as catalyst and triethylsilane as reducing agent in presence of alcohols. This methodology leads to the synthesis of ether derivatives with good yields. Notably, this process resulted a useful tool to protect alcohols as benzyl ether derivatives using differently substituted benzaldehydes as protecting agents under mild conditions. A plausible mechanism was also proposed.magnified image

Iron-Catalyzed Selective Etherification and Transetherification Reactions Using Alcohols.

Simple and readily available iron(III) triflate turned out to be a cheap, environmentally benign, and efficient catalyst for the direct etherification of alcohols. The use of ammonium chloride as an additive (5 mol %, 1 equiv relative to catalyst) suppressed the side reactions completely and ensured the selective ether formation even on challenging substrates containing electron-donating substituents. This method allows the selective synthesis of symmetrical ethers from benzylic secondary alcohols and unsymmetrical ethers directly from secondary and primary alcohols. Moreover, transetherification of symmetrical ethers using primary alcohols is attained. The reaction progress of symmetrical ether and unsymmetrical ether formation followed zero-order and first-order kinetics, respectively. Electron paramagnetic resonance (EPR) measurements of the reaction mixture and simple iron(III) triflate clearly indicated that oxidation state of the metal center remains same throughout the catalysis. Mechanistic studies confirmed that the unsymmetrical ether formation occurs via the in situ formed symmetrical ethers.

Highly Efficient Heterogeneous Copper-Catalysed O-Arylation of Phenols by Nitroarenes Leading to Diaryl Ethers

The heterogeneous O-arylation of phenols by nitroarenes was achieved in DMF at 100 °C by using an MCM-41-immobilised bidentate nitrogen copper(II) complex [MCM-41-2N-Cu(OAc)2] as catalyst, yielding a variety of unsymmetrical diaryl ethers in good to excellent yields. This heterogeneous copper catalyst can be easily prepared by a simple procedure from commercially readily available and inexpensive reagents, recovered by filtration of the reaction solution and recycled at least seven times without significant loss of activity.

ChemInform Abstract: A Review on the Advancement of Ether Synthesis from Organic Solvent to Water

AbstractReview: 106 refs.

An efficient method for the preparation of dialkoxymethanes from dichloromethane with alcohols catalyzed by a Cu-NHC complex

A facile, rapid and efficient method for the preparation of dialkoxymethanes from dichloromethane with alcohols catalyzed by a Cu-NHC complex is reported. A variety of symmetrical dialkoxymethanes can be prepared under mild condition in excellent yields (up to 98%). The unsymmetrical ether is also obtained in 89% yield from the etherification of p-tolylmethanol and n-butyl chloride catalyzed by ICyCuCl complex at 80°C. The reaction provides a new method for the preparation of dialkoxymethanes under mild conditions in excellent yields.

Effect of the niobium state on the properties of NbSiBEA as bifunctional catalysts for gas- and liquid-phase tandem processes

NbSiBEA zeolites contained isolated framework mononuclear Nb(V) (Nb0.7SiBEA) and a mixture of framework mononuclear and extra-framework polynuclear Nb(V) (Nb2.0SiBEA) were prepared by two-step postsynthesis method as evidenced by XRD, NMR, DR UV–vis and FTIR. DR UV–vis showed that two types of framework mononuclear Nb(V) are present in Nb0.7SiBEA, while Nb2.0SiBEA mainly contained isolated mononuclear Nb(V) in the framework of zeolite and polynuclear Nb(V) in the extra-framework position. FTIR with pyridine and 2,6-di-tert-butylpyridine as probe molecules showed that major amount Lewis and weak Brønsted acidic sites are formed by incorporation of niobium in the framework of zeolites as mononuclear Nb(V). The catalytic properties of Nb-containing zeolites were investigated in tandem processes of ethanol conversion into 1,3-butadiene (gas-phase) and synthesis of unsymmetrical ethers from aromatic aldehyde and aliphatic alcohol (liquid-phase). It has been found that Nb0.7SiBEA catalyst, containing only isolated framework mononuclear Nb(V) is more active than Nb2.0SiBEA in the conversion of ethanol and ethanol/acetaldehyde mixture into 1,3-butadiene, MPV reduction of crotonaldehyde with ethanol and etherification of 4-methoxybenzyl alcohol with 2-butanol. The higher specific activity (turnover number/frequency) of Nb0.7SiBEA than Nb2.0SiBEA catalyst has been revealed for gas- and liquid-phase tandem processes.

Radical additions of acyclic and cyclic ethers to alkenes via an allyl transfer reaction involving phthalimido-N-oxyl radical

Direct functionalization of rudimentary and cheap starting materials to yield complex value added products is of great interest to synthetic chemists. Particularly, direct functionalization of cheap commodity molecules that have been traditionally considered inert to known reactions are of widespread interest. We have previously demonstrated the functionalization of benzylic C–H bonds via an allyl transfer reaction using various allyl-phthalimido-N-oxyl substrates. In this work, we demonstrate the extension of our mild, metal-free, and neutral allyl transfer methodology to the direct functionalization of ethers. The C–H bond in α position to the ether oxygen in various acyclic and cyclic ethers was functionalized in high yields demonstrating wide substrate scope for this transformation. Furthermore, selective mono-functionalization of symmetrical cyclic ethers and regioselective functionalization of unsymmetrical ethers was achieved, thus demonstrating further utility of this reaction. Finally, kinetic chain length measurements were performed, which provided valuable insights into the initial rates and efficiency of the chain process involving the phthalimido-N-oxyl (PINO) radical.

Synthesis and Characterization of Sulfonated Carbon-Based Graphene Oxide Monolith by Solvothermal Carbonization for Esterification and Unsymmetrical Ether Formation

Herein, we report solvothermal carbonization of biomass, such as glucose, in the presence of a small amount of graphene oxide (GO) as a structure-directing agent that yields a carbon-based graphene oxide monolith (CGO monolith). The CGO monolith was functionalized with chlorosulfonic acid under mild reaction conditions to achieve sulfated carbon-based graphene oxide (SO3H@CGO monolith). It was used for the esterification of levulinic acid with benzyl alcohol and the etherification of benzyl alcohol with butanol as model reactions. The activity of the catalyst was compared with different solid acid catalysts, such as GO, CGO monolith, nanocrystalline sulfated titania (NCST), SO3H@HTC (sulfated hydrothermal carbon), amberlyst-15, and SO3H@GO for esterification of levulinic acid with benzyl alcohol. The SO3H@CGO monolith showed excellent conversion (97%) toward benzyl levulinate at 110 °C with low catalyst loading (0.008 g/cm3) and mole ratio. The catalyst was well characterized by SEM, EDXA, TEM, XRD, FTIR,...

ChemInform Abstract: Gold(I)‐Catalyzed Dehydrative Formation of Ethers from Benzylic Alcohols and Phenols.

AbstractA cross‐dehydrative reaction of two alcohols proceeds under mild reaction to give unsymmetrical ethers (III) using NHC—gold(I) complex IPR.

Hydrochloric acid as an efficient catalyst for intermolecular condensation of alcohols. A simple and highly efficient synthesis of unsymmetrical ethers from benzylic alcohols and alkanols

Benzylic alcohols and diarylmethanols with electron-donating substituents in the aromatic ring reacted with aliphatic alcohols in the presence of a catalytic amount of HCl to give the corresponding alkyl arylmethyl ethers. The reactivity of diarylmethanols in the intermolecular dehydration depended on the nature of substituents in the aromatic rings and structure of aliphatic alcohol.