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What Is A Metathesis Reaction
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Recent Advances In C—x Bond Metathesis Reactions
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Department of Organic Chemistry I and Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander University Erlangen-Nuremberg, Nikolaus-Fiebiger-Straße 10, 91058 Erlangen, Germany
Received: August 20, 2020 / Revised: September 3, 2020 / Accepted: September 4, 2020 / Published: September 21, 2020
Alkyne Cross Metathesis Reactions Of Extended Scope
The construction of carbon-carbon bonds is one of the most important tools for the synthesis of complex organic molecules. Among the many possibilities are carbonyl-alkyne and carbonyl-olefin metathesis reactions, which are used to form new carbon-carbon bonds between carbonyl derivatives and unsaturated organic compounds. As many different approaches are already established and offer reliable access to C=C bond formation via carbonyl-alkyne and carbonyl-olefin metathesis, the focus is now shifting towards cost-effective, sustainable and environmentally friendly metal catalysts. Iron, which is abundant on earth and is considered an environmentally friendly and inexpensive option compared to traditional metal catalysts, fulfills these requirements. Therefore, the focus of this review is on recent advances in iron-catalyzed C–O/C–O metathesis reactions of carbonyl–alkynes, carbonyl–olefins, and related compounds. There is still great research potential for environmentally and economically attractive and sustainable iron-based catalysts.
The formation of carbon-carbon bonds is one of the most important tasks in chemistry . For fast, efficient and selective coupling, catalysis via metal salts or metal complexes is an elegant option with diverse applications in academic research or industry [2, 3, 4]. Among the known efficient methods for C–C bond formation are metal-catalyzed cycloadditions, which provide attractive alternatives to photochemical , radical  or thermal [7, 8] cyclization reactions because they do not require expensive photosensitizers. and generally work in mild conditions. One of the most important methodologies is the metal-catalyzed olefin-olefin metathesis reaction (Scheme 1a), which is a powerful tool for the formation of carbon-carbon double bonds with a wide range of applications [9, 10, 11, 12]. This allows access to unsaturated compounds that are otherwise unobtainable, especially pharmaceuticals .
In addition to the olefin metathesis reaction, the carbonyl-alkyne and carbonyl-olefin metathesis reactions offer an alternative possibility for the formation of carbon-carbon bonds. Unlike olefin-olefin metathesis (as shown in Scheme 1a), carbonyl-alkyne (Scheme 1b) and carbonyl-olefin metathesis (Scheme 1c) do not follow the Chauvin mechanism, although the reaction proceeds through a formal [2 + 2] oxetene cycloaddition or oxetane as an intermediate [14, 15]. Since the reaction mechanism is different from olefin metathesis, the use of a different type of catalyst is required. The most common metal catalysts for these reactions are Lewis acids, which activate carbonyl and oxetene/oxetane during the reaction [16, 17].
A special case is the C–O/C–O metathesis reaction (as shown in Scheme 1d), which forms a new C–O bond, resulting in cyclic ether derivatives. Although the reaction does not form a new C–C bond, the reaction can be viewed as a metathesis of aliphatic ethers that closes the C–O/C–O ring .
File:metathesis Reaction Between R2pb And E(ch3)3 (e = Al, Ga).svg
While many catalytic systems mainly using organic Lewis acids  or metal-based Lewis acids  have already been reported, the development of new stable, cost-effective metal-containing, earth-rich catalysts is highly desirable in modern chemistry [21, 22] . Recently, FeCl
, which meets the above conditions, was established as a reliable catalyst for the metathesis of carbonyl-olefins and carbonyl-alkynes. Therefore, this review summarizes all recent advances in this field of research and the related Fe-catalyzed C–O/C–O metathesis reaction.
)–O ring closure reaction reported by Marandi et al . This single-bond “metathesis” reaction is a Lewis acid-catalyzed ring closure that converts linear diesters to five- or six-membered cyclic esters (Scheme 2).
Various substitution schemes are allowed in the reaction system, including benzyl groups with electron-donating and electron-accepting groups, as well as methyl groups and chain heteroatoms. In order to investigate whether the formation of dimethyl ether is the driving force of the reaction, experiments were performed with 1,5-pentanediol esterified with different alcohols (MeOH, PrOH and ButOH). Since the yield of cycloether was identical, it could be concluded that the formation of dimethyl ether is not the key driving force of the reaction. Additional studies have also shown that the formation of the cyclic ether is irreversible. In conclusion, the authors propose a bimolecular mechanism, which is supported by crossing experiments with methyl and propyl ether derivatives. The proposed mechanism is shown in diagram 3:
Palladium Catalyzed Ring Closing Reaction Via C–n Bond Metathesis For Rapid Construction Of Saturated N Heterocycles
, emphasizing the role of iron in the proposed mechanism. Although the mechanism is bimolecular and yields cross-products, the metal catalyst activates only oxygen and therefore strongly deviates from the Chauvin mechanism. In conclusion, the iron-catalyzed C–O/C–O ring closure reaction is the first of its kind to provide access to cyclic ethers, even if it cannot be considered a classical metathesis reaction.
Carbonyl-alkyne metathesis (the transfer of oxygen from the carbonyl to the carbon-carbon triple bond) is an atomically efficient reaction that peaked in the early 2000s as a gold-catalyzed reaction . Unlike stabilized Wittig reagents, the carbonyl-alkyne metathesis reaction offers a favorable alternative because it possesses full atomic efficiency for the synthesis of highly functionalized alkene derivatives. Discovery of the reaction catalyzed by AgSbF
Drew attention to cheaper Lewis acids . Although the reaction has been described with other Lewis acids, iron-based Lewis acids have been found to be among the most favorable, which can be explained by the HSAB theory:  Fe
, as it has a smaller ionic radius and therefore a more concentrated charge, which can be beneficial for the interaction, leading to better catalytic performance .
A) Synthesis Of A Model Poly(aryl Thioether) P1 By Reversible C−s…
It is important to note here that ferric chloride is used only to activate aldehydes and oxetenes in a concerted, albeit asynchronous, [2 + 2] cycloaddition, as shown in Scheme 4. The d-orbitals of iron do not participate in the cycloaddition Step. Therefore, the reaction does not follow the Chauvin mechanism, since the metal is not involved in the cycle formed in the transition structure. In general, the reaction itself is best described as an economical catalytic alternative to the Wittig reaction . However, carbonyl-alkyne metathesis consists of formal [2 + 2] and retro [2 + 2] cycloadditions and provides access to several heterocycles similar to olefin metathesis, which will be discussed in the next section.
The first example of iron-catalyzed carbonyl-alkyne metathesis was reported by Iana and co-workers in 2011 (Scheme 5) .
Similar reactions with the formation of 2H-chromene derivatives were previously described with different catalysts, for example, catalysts based on Ru  and Au . While previous methodologies suffer from the use of an expensive metal catalyst, the new method that uses iron as the Lewis acid dramatically reduces the cost of the reaction. A year later, the Jana research group published a new procedure for phenanthrene derivatives (Scheme 6).
In this case, the metathesis reaction is used to directly produce fully conjugated ring systems. In addition to the simple precursor synthesis (two-step palladium cross-coupling reaction), the method has good tolerance for functional groups including methoxy, methyl, fluorine, chloride and ether.
Solved For The Following Metathesis Reactions, Name The
An additional approach to carbonyl-alkyne metathesis was presented by Bera et al. in 2013 (Figure 7). Using 2,2′-functionalized diaryl esters, it was possible to synthesize the basic structures of dibenzo[b,f]oxepin and benzo[b]oxepin, which have many applications in medicinal chemistry. Furthermore, these ring systems represent the first examples of artificially synthesized seven-membered heterocycles reported in the literature to date.
In 2014, the Jana research group further extended the applicability of iron-catalyzed carbonyl-alkyne metathesis by synthesizing precursors inspired by their work published in 2011 (Scheme 5). By replacing oxygen with nitrogen and changing the chain length of the alkyl bridge to the alkyne fragment, a new library of six- or seven-membered nitrogen-containing heterocycles was created (Scheme 8). Again, these classes of compounds are already well established in pharmacological applications and may lead to the development of new biologically active compounds.
A recent work dealing with the metathesis of alkynes and olefins was carried out by Jalal et al. in 2017. This publication focused on the conversion of indole derivs.
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