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Review

Lewis Acid-Initiated Ring-Opening Reactions of Five- and Six-Membered Cyclic Ethers Based on the Oxonium Ylide Intermediates

by
Dandan Jiang
,
Jun Xiao
,
Yingzhen Zhang
,
Kunming Liu
,
Juanhua Li
* and
Jinbiao Liu
*
Jiangxi Provincial Key Laboratory of Functional Molecular Materials Chemistry, Faculty of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology, 86 Hongqi Road, Ganzhou 341000, China
*
Authors to whom correspondence should be addressed.
Organics 2024, 5(3), 219-236; https://doi.org/10.3390/org5030011
Submission received: 3 June 2024 / Revised: 1 July 2024 / Accepted: 18 July 2024 / Published: 22 July 2024
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Abstract

:
In light of the small ring strain of five/six-membered cyclic ethers, constructing complex molecules via ring-opening reactions has consistently been a highly challenging topic in organic synthesis. Induced by Lewis acids, the charge redistribution in cyclic ethers forms oxonium ylide intermediates, thereby activating the C–O bond and subsequently facilitating nucleophilic attack for ring opening. In recent years, a variety of novel Lewis acids, encompassing those with new metal centers and frustrated Lewis pairs (FLPs), have been effectively utilized to induce the formation of oxonium ylides, offering a diverse array of methods for the ring opening of five/six-membered cyclic ethers. This review conveys the extensive application advancements of diverse Lewis acid types for cyclic ether ring-opening reactions over the past two decades, originating from the perspective of the classification of Lewis acids. Furthermore, the substrate applicability and chemical transformation efficiency of these Lewis acids in the ring-opening reactions of cyclic ethers have also been discussed herein.

1. Introduction

One of the primary motivations in organic synthesis is to explore novel avenues for accessing new chemical structures. Cyclic ethers, which are prevalent structural motifs in natural products, pharmaceuticals, and functional polymers, can introduce diverse molecular frameworks through C–O bond cleavage [1,2,3,4,5]. Due to the significant amount of ring strain, such as angle, torsional, and steric strain, small ring cyclic ethers like epoxides and oxetanes possess high reactivity, leading to ring-opening reactions when presented with Lewis acids or bases that have lower activation energy and higher reaction rates [6,7]. Up until now, extensive research on the ring-opening and subsequent conversion reactions of small ring cyclic ethers has progressed at a rapid pace, and several reviews have summarized recent advances in nucleophilic attack ring-opening reactions, ring expansion reactions, ring-opening polymerization, and ring-opening multi-component reactions [8,9,10].
In contrast, five- and six-membered cyclic ethers, including tetrahydrofuran (THF), tetrahydropyrane (THP), and 1,4-dioxane, are thermodynamically “inert ring-opening” substrates due to the extremely low ring strain. Additionally, five/six-membered cyclic ethers are widely recognized as the building blocks for constructing long-chain alkoxy skeletons that are extensively present in bioactive compounds, naturally occurring chemical feedstocks, and various synthetic intermediates [11,12,13]. Hence, developing ring-opening methodologies for five/six-membered cyclic ethers continues to be a challenging endeavor in synthetic organic chemistry. Among them, the Lewis acid-catalyzed cation polymerization of THF has been well known as the most widely used ring-opening reaction for a long time [14,15].
The ring opening of cyclic ethers by the anhydrides and halides of organic acids under a Lewis acid medium/catalyst has been very effective and is a common method for the cleavage of cyclic ethers. In this connection, the agents more commonly used are acid chlorides. The cleavage of ethers with acyl chloride has been reported using Lewis acids, such as BCl3 [16], PdCl2(PPh3)2 [17], Pd(OAc)2 [18], CoCl2 [19], NaI [20,21], lanthanide salts [22], graphite [23], aluminum complexes [24], and others [25,26,27,28,29,30]. A variety of other different reagents have been utilized for the cleavage of ethers but are not always satisfactory for complex molecules containing sensitive functionalities, and frequently long reaction times are required, or for the lack of regioselectivity in the cleavage of nonsymmetrical cyclic systems. In recent years, a variety of new Lewis acids have been reported to be used for the ring opening of five- and six-membered cyclic ethers.
In this review, a general overview of recent progress of the widely applicable various types of Lewis acids as catalysts for the ring-opening reactions of five- and six-membered cyclic ethers over the past two decades is presented. The substrate applicability of the ring-opening reactions of cyclic ethers using different Lewis acids, combined with the chemical transformation efficiency, is mainly focused on in this review.

2. Lewis Acid-Initiated Ring-Opening Reactions of Five/Six-Membered Cyclic Ethers

2.1. Rare Earth Metal Center Lewis Acid-Initiated Ring-Opening Reactions of Five/Six-Membered Cyclic Ethers

China is the country with the largest reserves of Rare Earth resources in the world, boasting abundant Rare Earth mineral deposits. Moreover, due to their unique chemical properties, Rare Earth metals play an important role in the field of organic synthesis. Firstly, compared to other metals, Rare Earth metals have larger ionic radii [31,32], coupled with Lewis acidity and the presence of empty orbitals, which provide their compounds with a pronounced tendency to complexation and they therefore have high coordination numbers [33]. These features increase the diversity and possibility of constructing catalysts. Secondly, due to the similarity in redox and chemical properties of Rare Earth elements, combined with a substantial variation in the ionic radius in ion radii in these compound series [31], not only can the metal coordination sphere be designed, but an appropriate radius of the central atom can also be selected based on the specific characteristics of the catalyzed reaction, thereby optimizing the reactivity of metal complexes, making their catalytic activity stronger and reaction speed faster. Finally, the pronounced oxophilicity of Rare Earth metals lays the foundation for the potential high activity of their derivatives in the ring-opening reaction of cyclic ethers [14], as the first stage of this reaction is the coordination of the oxygen atom of the cyclic ether to the metal center.
In recent years, some progress was made in the involvement of Rare Earth metals as Lewis acid in the ring opening of five- or six-membered cyclic ethers, such as samarium [34,35,36,37], ytterbium [14], and, as will be mentioned later, lanthanum [38]. As early as 1997, Kang’s group reported the ring opening of THF, THP, and 1,3-dihydrobenzofuran with samarium(II) iodide in benzene-hexamethylphosphoramide (HMPA), but it has low yields due to unstable products [35]. However, in 2002, Yong’s group found that the cleavage reaction of tetrahydrofuran could be developed by using SmI2 on the basis of chelation control; they obtained functionalized acylated iodide compounds with both high regioselectivity and high chemical yields [36]. The high selectivity of branched tetrahydrofuran is influenced by the branching chain. Compared to other traditional metals, the samarium of acylsamarium C has stronger electrophilicity towards oxygen connected at the β-position, such as the hydroxyl oxygen atom of tetrahydrofurfuryl alcohol, forming the five-membered ring bidentate chelated oxonium ylide intermediate A, which generates corresponding iodine products with extremely high regioselectivity (>99:1) through an “a” attack (Scheme 1). The reason why oxonium ylide B does not provide high selectivity is due to the simple interaction between oxygen and samarium in cyclic ethers without substituents or alkyl substituents.
The Rare Earth metal triflate is a highly active Lewis acid, which is a useful water-tolerant and potentially recyclable catalyst in various reactions [39]. In 2004, Miles’ group published the acylative cleavage reactions of tetrahydrofuran catalyzed by ytterbium, scandium, or lanthanum triflate under conditions favoring low-molecular-weight products (Scheme 2) [14]. The following was most noteworthy: the acylative cleavage of THF with acetic anhydride catalyzed by ytterbium trifluoride to obtain dimeric compound 6 with moderate yield. The nucleophilic attack of carboxylate anions on intermediate A leads to monomeric product 7. However, within the coordination sphere of the Lewis acidic Rare Earth metals, the nucleophilic attack by another tetrahydrofuran forms oxonium ion B, which is then nucleophilically attacked by carboxylate anions to obtain dimer product 6. Alternatively, another tetrahydrofuran can be nucleophilically attacked to give oxonium ion C, ultimately resulting in trimeric or higher oligomers.
Lanthanum(III) nitrate hexahydrate is relatively non-toxic, inexpensive, insensitive to air, and used in various organic transformations. After reporting that Bi(NO3)3·5H2O can cleave cyclic ethers [40], Venkateswarlu’s group found that La(NO3)3·6H2O is also an effective and mild Lewis acid catalyst for the cleavage of cyclic ethers with acid chlorides in the presence of a catalytic amount of La(NO3)3·6H2O under solvent-free conditions in 2007 (Scheme 3) [38]. It is an efficient and facile method for the synthesis of chloroesters. The method does not require expensive reagents or special care to exclude moisture from the reaction medium.
In 2012, Ling’s group reported a series of Rare Earth trilates (RE(OTf)3) as the novel, highly efficient, hydrolytically stable and recyclable Lewis acid catalysts participating in the living/controlled bulk ring-opening polymerization (ROP) of THF with the presence of epoxides at room temperature [41]. This system has two differences from the traditional ring opening of the THF system in the past (Scheme 4). Firstly, this polymerization system does not add or form acids. Secondly, the stable five-membered secondary oxonium ion A′ formed by THF and RE(OTf)3 cannot undergo initiation, but rather complex A with higher reactivity and strain is susceptible to nucleophilic attack from THF, forming an alkyltetrahydrofuranium active center [42], thereby initiating the ROP of THF.
Studies in 2018 of the reduction of CpMe3Ln(CpMe = C5H4Me) in THF revealed another way of solvent decomposition in which there are in situ LnCpMe3/KC8 reactions. For Ln = La and Pr, this forms the decomposition product where a molecule of THF has been reduced by two electrons and ring-opened to obtain an alkoxyalkyl-bridged bimetallic Ln(III) complex (Scheme 5a) [43]. Under similar conditions in 2023, THF can also be ring-opened by the Y(NR2)3/KC8 reaction system (Scheme 5b) [44].

2.2. Other Metal Center Lewis Acid-Initiated Ring-Opening Reactions of Five/Six-Membered Cyclic Ethers

All the above reactions require acyl chloride or anhydrides as a nucleophilic reagent to participate in the ring opening of pentacyclic ether. However, under mild conditions, only Lewis acid and the THF reaction can open the ring of cyclic ether, which is very rare. During their study of MoCl5, Tamotsu Takahashi’s group unexpectedly found that the C–O bond cleavage could also occur in THF without benzoyl chloride in 2002 [45]. This C–O bond of tetrahydrofuran was cleaved by MoCl5, and in the presence of additional THF, the resulting C–O bond-cleaved species containing metal attacked the second THF to form dimerized compound 11 (Scheme 6).
In addition, the vast majority of Lewis acids examined to date fail to afford regioselective cleavage and suffer from the additional drawbacks of being air-sensitive (BCl3 [16]), environmentally toxic, or expensive (SmI2 [36]). The reason why Bi(III) complexes are favored as catalysts for organic synthesis is that they are multifunctional, low-cost, non-toxic, and easy to use in a laboratory [46,47]. James F. Costello’s research group described a mild, quantitative, regioselective methodology for the O-acylative cleavage of tetrahydrofurans using organic acid halides and catalytic amounts of Bi(III) halides in 2005 (Scheme 7) [48]. This system extends acyl bromide as an organic acid halide, resulting in yields as high as acyl chloride.
There has been considerable interest in the catalytic use of indium(III) halides [49,50] in organic synthesis. Due to their unique catalytic properties, indium(III) bromides have been widely used for a variety of organic transformations [51,52,53,54,55]. In 2007, J.S. Yadav’s group disclosed a mild, efficient, and practical methodology for the cleavage of cyclic ethers with acyl chloride under solvent-free conditions using indium(III) bromide as the novel catalyst (Scheme 8) [56]. This method offers several significant advantages such as high conversions, easy handling and high catalytic nature of indium compounds, solvent-free conditions, cleaner reaction profiles, and shorter reaction times, which makes it a useful and attractive process for the rapid cleavage of cyclic ethers in a single-step operation.
Zinc salts are advantageous as catalysts because they are abundant, inexpensive, and less toxic. The use of zinc for catalytic ring opening of cyclic ethers has been reported [57,58]; however, the scope and limitations of the method are desired, and no mechanistical investigations have been previously accounted for. In 2012, Enthaler and Weidauer investigated the ZnCl2-mediated cleavage of cyclic ethers using acid halides as nucleophiles to yield chloroesters with different chain length and subsequent functionalization into cyanoesters [59]. Interestingly, all applied zinc sources such as ZnBr2, Zn(OAc)2, Zn(acac)2, and Zn(OTf)2 lead to a quantitative conversion of chloride with excellent yields. In addition, both alkyl acid chlorides and a heteroaromatic acid chloride reacted with THF with high conversion and the sulfonic acid chloride also reacted in the presence of ZnCl2 (Scheme 9).
Jin-Quan Yu’s group developed a new one-pot synthesis of C2-hydroxypropyl-substituted imidazolinium salts by ring opening of five- and six-membered cyclic ethers, such as THF with N, N′-disubstituted diamines, in 2017 (Scheme 10) [60]. In essence, the ring opening of THF is the activation of the C–O bond of THF by carbon dioxide in air in the form of carbonic acid, which promotes the nucleophilic ring opening of THF, followed by the oxidation of the resultant diamine to give intermediate imidazolidine 23′, whose further oxidation by Hg(II)-mediated oxidation gives imidazolinium salt 23.
In 2022, Qiu’s group reported the development of selective multi-component reactions (MCRs) of carboxylic acids, ethers, and halogens by Cu(I)-catalyzed dual C–O bond cleavage of cyclic ethers (Scheme 11) [61]. Symmetrical cyclic ethers such as tetrahydropyran and 1,4-dioxane were well tolerated in this reaction. However, when using methyl-substituted tetrahydrofuran, the yield is good but the selectivity is low (1:1). In addition, the presence of TMSCF3 can greatly improve the reaction yield, as excessive NaI can induce its reaction with cyclic ethers to generate difluoromethylene oxonium A, which is then deprotonated with water to obtain intermediate B. Cu(I) reagents with Lewis acidic behavior can easily combine with carboxyhydroxyl groups, increasing the nucleophilicity of carboxylic acids while hindering the side reaction between difluorocarbene and carboxylic acid. Then, the nucleophilic attack is carried out on intermediate B to obtain ring-opening intermediate C. In the end, the oxygen atom of the carboxyhydroxyl group remained in product 24, while the oxygen atom of ether was lost by in situ exchange in the presence of NaI.
Metal oxides are rich in reserves, low cost, environmentally friendly, highly safe, and corrosion-resistant, so they are more widely available and easier to apply in the industry than other metal catalysts.
Re2O7 is primarily considered as a strong oxidant and was found to be a highly selective Lewis acid catalyst that breaks the ring of THF [62]. In 2007, Ehud Keinan et al. found that Re2O7 affects the heteroacylative dimerization of THF at room temperature (Scheme 12) [63]. This multi-component reaction involves THF, trifluoroacetic anhydride (TFAA), and a carboxylic acid to produce a nonsymmetrical diester in high yields. Two of the five oxygen atoms in the product originate from THF. One originates from rhenium oxide and the two carbonyl oxygens come from the carboxylic acid and TFAA.
As shown in Scheme 13, Re2O7 reacts with TFAA in THF to produce carboxylate complex A with high yields, which can be easily converted to B through a carboxylate exchange reaction. The complexation of the second carboxylate oxygen in B activates the trans oxo ligand to undergo nuclophilic attack on the adjacent THF ligand [64], resulting in dioxometallacycle C. The second THF coordinated to form D, followed by a nucleophilic attack of the alkoxide ligand on that THF, resulting in the formation of E in the second ring-opening step. The central oxygen in the diolate ligand in E can coordinate to Re, thereby shifting the equilibrium of carboxylate towards E′/E″. The carboxylate in E′/E″, a 14e rhenium dioxide, subsequently undergoes simple metal–oxygen bond metathesis with the concomitant coordination of a THF molecule, to produce 16e rhenium trioxide species F. Finally, the electrophilic cleavage of the metal oxygen bond in F with TFAA produces product 26 that regenerates the catalyst, A.
In 2021, for example, Chen’s group described an effective method for the synthesis of chloroesters by the reaction of ethers with acyl chlorides catalyzed by nano-ZnO at room temperature under a solvent-free condition (Scheme 14) [65]. This method applies to a range of ethers, including pentacyclic ethers and hexacyclic ethers such as 2-methyltetrahydrofuran and tetrahydropyran, and the products were obtained in moderate to good yields. All kinds of substituted aromatic chlorides can participate in the reaction smoothly, and 2-chloropyridine-chloride can quickly yield the ring-opening product with a high yield of 92%. However, thiophene carbonyl chloride, which can react smoothly in the zinc salt catalytic system, cannot participate in the reaction catalyzed by zinc oxide. ZnO can be reused up to three times, and the product yield after three cycles is 87%.

2.3. Free Element as Lewis Acid Precursor Participates in Ring-Opening Reactions of Five/Six-Membered Cyclic Ethers

Despite the above, many Lewis acids have been used for the cleavage of ethers with acyl chloride. However, there was no report on the use of elemental iodine for the cleavage of five/six-membered cyclic ethers with acyl chloride. In 2005, J. S. Yadav’s group disclosed a mild, efficient, and practical methodology for the cleavage of cyclic ethers with acyl chloride using iodine as the catalyst under solvent-free conditions (Scheme 15) [66]. The use of 10 mol% iodine under solvent-free conditions provided higher yields and reduced reaction times as compared to some other inexpensive reagents such as FeCl3 [67], ZnCl2 [68], and NaI [20,21]. Benzoyl chloride and its derivatives, as well as the acid chloride derived from cyhalothrin, can smoothly cleave tetrahydrofuran and obtain corresponding halogenated esters with good to excellent yields. However, two years later, Pasha and Manjula demonstrated that the cleavage of the tetrahydrofuran ring only required less iodine (0.5% equiv.) and a shorter reaction time (5 min) to obtain the same high yield (94%) [69].
In 2006, the group of Pasha developed a simple and economically viable method for the preparation of δ-chloroesters using catalytic amounts of inexpensive, readily available, and abundant zinc metal, THF, and acyl chlorides under sonic conditions (Scheme 16) [70]. Sonic conditions not only accelerate chemical reactions, but also reduce the number of steps required under normal conditions, allowing for the use of more cruder reagents and the initiation of reactions without any additives. However, in 2011, they discovered that magnesium, as a catalyst, could greatly reduce the reaction time (<10 min) without the need of sonic conditions [71].
When compared with expensive metal competitors, iron is not only a green catalyst, but also provides a wide range of organic transformations [72]. In 2015, Dong-Soo Shin’s group reported a mild and efficient method to provide esters and chloroesters via cleavage of cyclic ethers in the presence of iron powder as an environmentally friendly catalyst [73]. The α-branched cyclic ethers are converted to the relative primary ester and secondary chlorides. The steric hindrance of the ether also plays an essential role in the acylative C–O bond cleavage. In addition, it was found that good yields can be obtained from aroyl chlorides and aliphatic acyl chlorides.
The ether cleavage reaction follows the SN1 dissociative pathway (Scheme 17). First, in the presence of ether 36 and acyl/aroyl chloride 37, Fe(0) oxidizes to Fe(II)/Fe(III) by the loss of electrons through single electron transfer (SET) to generate free radical anion I. The formation of the acylium ion can be anticipated by the electronic loss of II or FeCl3/RCOCl system, which involves the formation of the concomitant bond with an incoming ether nucleophilic reagent. Therefore, the SN1 cleavage of resultant active oxonium ion III would be beneficial to generate more of substituted carbonium ion IV, from which the observed ester or chloroester can be obtained depending on the ether used.
Because none of the previous mechanisms involved single electron transfer (SET), the discovery of the method laid the foundation for developing new reaction pathways.

2.4. Ring Opening of Five/Six-Membered Cyclic Ethers by FLPs

As broadening implications of the chemistry of frustrated Lewis pairs (FLPs) continue to develop, it is interesting to note that this reactivity leads to an increasing number of this ring opening of THF in the context of the reactivity of FLPs.
A decade ago, Douglas W. Stephan’s group demonstrated that a combination of sterically encumbered donors with Lewis acids results in unique reactivity [74,75,76]. Such systems, referred to as “FLPs”, are comprised by Lewis acidic and basic centers but owe “frustrated” donor–acceptor dative bonds due to their steric bulk residues or the rigid linker system.
The unique structures of FLPs are potentially utilized for activating a broad variety of small molecules, including H2 [77,78,79,80,81,82], olefins [83], dienes [84], alkynes [85], boranes [86], disulfides [87], CO2 [88,89], and N2O [90,91]. In particular, the nucleophile attacks the Cα-carbon of a cyclic ether activated by FLPs, providing a novel pathway for low-ring-strain ring opening. In fact, in 1950, Wittig and Rȕckert [92] described the reaction of [Ph3C]− with THF (BPh3) to yield the ring-opened product, the borate anion [Ph3C(CH2)4OBPh3]−. Approximately 40 years later, Stephan’s group investigated and reported the related reaction between PCy3 and ZrCl4(THF)2 39, which led to the formation of the dimeric zwitterionic species (Cy3P(CH2)4OZrCl4)2 40 (Scheme 18) [93]. Following that, other transition-metal Lewis acids, including complexes derived from U [94,95], Sm [96], Ti [97], and Al [98] as well as Mn-carborane complexes [99], were also shown to induce similar ring opening in tetrahydrofuran.
Inspired by the reactivity of FLP [100,101], Stephan’s group subsequently added the addition of THF to the reaction mixture of 2,6-Lutidine/B(C6F5)3 in 2009. As a result, the ring opening of THF was shown to obtain 43 (Scheme 19) [102].
In addition, in 2012, Kiplinger et al. synthesized the thorium(IV) tetraiodide complex, using a green and mild method, but accidentally found ring-opening products of THF (complexes 45–47) during the reaction process, which are the first reported examples of THF ring opening mediated by thorium (Scheme 20) [103]. This case indicates that the FLPs formed by Th compounds have potential for ring-opening ability of cyclic ethers.
In 2013, Curran et al. did not produce expected 2,6-diisopropylphenyl(dipp)-Imd–BH2OPh complex 49 when conducting the reaction of 48 with lithium phenoxide (PhOLi) but surprisingly found that new tetrahydrofuran ring-opening adduct 50 (Ar = Ph) was produced with a yield of 51% (Scheme 21) [104].
Displacement of the triflate of 48 by a molecule of THF provides cationic complex A (formally a boronium ion) with a triflate counterion. Complex A has two potential sites for the nucleophilic attack: the boron atom, “a”, and the α-carbons “b” of the coordinated THF molecule. Attack by a nucleophile at boron gives the direct substitution products. Attack at carbon “b”, on the other hand, leads to the products of ring opening of tetrahydrofuran. Since both 49 and 50 were isolated and shown not to interconvert, the ratio of products is due to kinetic competition between paths “a” and “b” (Scheme 22).
Okuda et al. described that treatment of molecular aluminum hydride [(L)AlH2] (L = Me3TACD) with 2 equiv. of BPh3 in THF or THP at room temperature led to ring opening of the solvent molecule at the center of the aluminum (Scheme 23). In addition, in the presence of pinacolborane (HBpin) as the reducing agent, ring opening is catalytic, as the amounts of 51 and BPh3 can be reduced to 10 mol% and 20 mol%, respectively [105].
In 2021, Federmann et al. discovered that a frustrated Lewis pair composed of an acidic aluminum function (AlR2) and a basic phosphine entity, connected by a xanthene spacer, involves the ability to cleave THF (Scheme 24) [106]. Meanwhile, the ring-opening reaction rate is higher by a factor of 10 for R = C6F5, the fluorinated residues, than that for R = Mes. This is due to the more pronounced electron deficiency at the aluminum center bearing the pentafluorophenyl substituents. The latter causes a stronger electron donation of the THF oxygen atom to the Al center, which in turn leads to a more electrophilic α-carbon atom that is susceptible to the attack of the phosphine. In 2023, Su’s group theoretically demonstrated that this dimethylxanthene backbone Al/P-Rea (Rea = reactant) FLP-type molecule can be energetically feasible to cleave THF easily [107].
In 2021, two pairs of Lewis acid/base adducts, namely the bulky N-heterocyclic carbenes (NHCs)/phosphoranes PR5 (Scheme 25a) [108] and the bromo-borane/the bis-silylated [Ge9] cluster (Scheme 25b) [109], were reported. They do not correspond to typical FLPs, but nevertheless, a cleavage of the C–O bond of THF was observed.
The FLP system has demonstrated unique and superior reactivity in the field of ring opening of cyclic ethers, but there is still a lot of room for development. For instance, there are limitations in the stability and substrate applicability of FLPs. Specifically, FLPs are prone to deactivation under certain reaction conditions and exhibit low catalytic efficiency for some substrates. To overcome these limitations, the following possible strategies have been proposed: 1. Design new types of FLPs to enhance their stability and activity by altering the electronic properties of the ligands. 2. Optimize reaction conditions, such as temperature, solvent, and concentration, to improve the catalytic efficiency of FLPs. 3. Introduce auxiliary ligands to broaden the substrate scope of FLPs and enhance their selectivity.

3. Conclusions

Tremendous progress has been achieved in the past decades in the research fields of ring opening of five- and six-membered cyclic ethers. Lewis acids play a pivotal role in the ring opening of cyclic ethers and are a class of highly significant substances. Outstanding examples of different types of Lewis acids, that is, Rare Earth element sources, metal halides, metal oxides, free elements, and FLPs, have been discussed herein. Under mild conditions, these substances can all participate in the ring-opening reactions of five- and six-membered cyclic ethers, yielding structurally complex target products. However, these catalytic systems are currently limited to a relatively narrow range of nucleophiles, and the ring-opening yields involving FLPs are generally low, making it difficult to obtain products with higher yields and more complex structures. In the future, our research will focus on discovering catalysts that can enhance chemical selectivity, broaden the substrate scope, and improve yields, thereby facilitating the synthesis of more complex molecular structures and constructing innovative catalytic systems. We will also expand the horizons of cyclic ether ring opening to encompass ring-opening polymerization, ring expansion, and multi-component reactions, thereby further expanding the perspective of applications in cyclic ether ring opening and synthesizing target molecules with complex structures in a more efficient and environmentally friendly approach.

Author Contributions

Conceptualization, J.L. (Jinbiao Liu) and K.L.; methodology, J.L. (Juanhua Li), D.J. and J.X.; validation, Y.Z.; writing—original draft preparation, D.J. and J.L. (Juanhua Li); writing—review and editing, K.L.; supervision, J.L. (Jinbiao Liu). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of China (2023YFC2908100), the Education Department of Jiangxi Province (No. GJJ2200820), the Jiangxi Provincial Key Laboratory of Functional Crystalline Materials Chemistry (2024SSY05161), and the National College Students’ Innovation and Entrepreneurship Training Program (202110407006).

Data Availability Statement

The data presented in this study are available on reasonable request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Scheme 1. The mechanism of SmI2-catalyzed ring opening of cyclic ethers.
Scheme 1. The mechanism of SmI2-catalyzed ring opening of cyclic ethers.
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Scheme 2. Yb(OTf)3-catalyzed ring opening of THF.
Scheme 2. Yb(OTf)3-catalyzed ring opening of THF.
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Scheme 3. La(NO3)3·6H2O-catalyzed ring opening of cyclic ethers.
Scheme 3. La(NO3)3·6H2O-catalyzed ring opening of cyclic ethers.
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Scheme 4. RE(OTf)3 initiated the ROP of THF.
Scheme 4. RE(OTf)3 initiated the ROP of THF.
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Scheme 5. (a) The ring opening of THF by the LnCpMe3/KC8 reaction system (Ln = La, Pr). (b) The ring opening of THF by the Y(NR2)3/KC8 reaction system.
Scheme 5. (a) The ring opening of THF by the LnCpMe3/KC8 reaction system (Ln = La, Pr). (b) The ring opening of THF by the Y(NR2)3/KC8 reaction system.
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Scheme 6. The mechanism of MoCl5-catalyzed ring opening of THF.
Scheme 6. The mechanism of MoCl5-catalyzed ring opening of THF.
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Scheme 7. Bi(III) halide-catalyzed ring opening of cyclic ethers.
Scheme 7. Bi(III) halide-catalyzed ring opening of cyclic ethers.
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Scheme 8. In(III) halide-catalyzed ring opening of cyclic ethers.
Scheme 8. In(III) halide-catalyzed ring opening of cyclic ethers.
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Scheme 9. ZnCl2-catalyzed ring opening of cyclic ethers.
Scheme 9. ZnCl2-catalyzed ring opening of cyclic ethers.
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Scheme 10. Plausible mechanism for carbon dioxide-catalyzed cleavage.
Scheme 10. Plausible mechanism for carbon dioxide-catalyzed cleavage.
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Scheme 11. The MCR of carboxylic acids, ethers, and halogens by Cu(I) catalyzation.
Scheme 11. The MCR of carboxylic acids, ethers, and halogens by Cu(I) catalyzation.
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Scheme 12. Re2O7-catalyzed heteroacylative ring-opening dimerization of THF.
Scheme 12. Re2O7-catalyzed heteroacylative ring-opening dimerization of THF.
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Scheme 13. Proposed catalytic cycle Re2O7-catalyzed heteroacylative ring-opening dimerization of THF.
Scheme 13. Proposed catalytic cycle Re2O7-catalyzed heteroacylative ring-opening dimerization of THF.
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Scheme 14. ZnO-catalyzed ring opening of cyclic ethers.
Scheme 14. ZnO-catalyzed ring opening of cyclic ethers.
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Scheme 15. Iodine-catalyzed ring opening of cyclic ethers.
Scheme 15. Iodine-catalyzed ring opening of cyclic ethers.
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Scheme 16. Zinc-catalyzed ring opening of cyclic ethers.
Scheme 16. Zinc-catalyzed ring opening of cyclic ethers.
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Scheme 17. Plausible mechanism for iron-catalyzed cleavage.
Scheme 17. Plausible mechanism for iron-catalyzed cleavage.
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Scheme 18. Zr complexes participate in the ring opening of tetrahydrofuran.
Scheme 18. Zr complexes participate in the ring opening of tetrahydrofuran.
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Scheme 19. B(C6F5)3 participates in the ring opening of tetrahydrofuran.
Scheme 19. B(C6F5)3 participates in the ring opening of tetrahydrofuran.
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Scheme 20. Th compounds participate in the ring opening of tetrahydrofuran.
Scheme 20. Th compounds participate in the ring opening of tetrahydrofuran.
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Scheme 21. Dipp-Imd–BH2OTf participates in the ring opening of THF.
Scheme 21. Dipp-Imd–BH2OTf participates in the ring opening of THF.
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Scheme 22. Coordination of 48 with THF and two sites of nucleophilic attack on A.
Scheme 22. Coordination of 48 with THF and two sites of nucleophilic attack on A.
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Scheme 23. Ring opening of THF and THP by the (HBPh3)− anion.
Scheme 23. Ring opening of THF and THP by the (HBPh3)− anion.
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Scheme 24. THF ring opening on dimethylxanthene scaffold with Lewis acidic AlR2 site to produce 56 via intermediate 55.
Scheme 24. THF ring opening on dimethylxanthene scaffold with Lewis acidic AlR2 site to produce 56 via intermediate 55.
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Scheme 25. (a) Lewis acid/base adducts of NHCs/PR5 open ring of THF. (b) Lewis acid/base adducts of DAB/[Ge9] cluster open ring of THF.
Scheme 25. (a) Lewis acid/base adducts of NHCs/PR5 open ring of THF. (b) Lewis acid/base adducts of DAB/[Ge9] cluster open ring of THF.
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MDPI and ACS Style

Jiang, D.; Xiao, J.; Zhang, Y.; Liu, K.; Li, J.; Liu, J. Lewis Acid-Initiated Ring-Opening Reactions of Five- and Six-Membered Cyclic Ethers Based on the Oxonium Ylide Intermediates. Organics 2024, 5, 219-236. https://doi.org/10.3390/org5030011

AMA Style

Jiang D, Xiao J, Zhang Y, Liu K, Li J, Liu J. Lewis Acid-Initiated Ring-Opening Reactions of Five- and Six-Membered Cyclic Ethers Based on the Oxonium Ylide Intermediates. Organics. 2024; 5(3):219-236. https://doi.org/10.3390/org5030011

Chicago/Turabian Style

Jiang, Dandan, Jun Xiao, Yingzhen Zhang, Kunming Liu, Juanhua Li, and Jinbiao Liu. 2024. "Lewis Acid-Initiated Ring-Opening Reactions of Five- and Six-Membered Cyclic Ethers Based on the Oxonium Ylide Intermediates" Organics 5, no. 3: 219-236. https://doi.org/10.3390/org5030011

APA Style

Jiang, D., Xiao, J., Zhang, Y., Liu, K., Li, J., & Liu, J. (2024). Lewis Acid-Initiated Ring-Opening Reactions of Five- and Six-Membered Cyclic Ethers Based on the Oxonium Ylide Intermediates. Organics, 5(3), 219-236. https://doi.org/10.3390/org5030011

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