Trapping Double Alcohols In Carbonyl Ethers: A Guide

Ethers are pivotal compounds in the chemical industry, with a wide range of applications in bulk and fine chemistry. Ether derivatives are used in several areas of the chemical industry and academia. Traditionally, non-catalytic routes employing (over) stoichiometric amounts of metal hydrides and/or an excess of Lewis/Brønsted acids have been used for etherification. However, these methods generate large amounts of waste. In recent years, catalytic reductive alcohol etherifications with carbonyl-based moieties (aldehydes/ketones and carboxylic acid derivatives) have emerged as a potential tool for ether synthesis. This method employs carbonyl-based compounds and CO2 as alkyl sources in alcohol reductive etherifications using external reducing agents (Si−H reagents or H2). This process allows for the selective production of both symmetrical and asymmetrical ethers with increased molecular complexity.

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Carbonyl-based compounds and CO2 as alkyl sources

Carbonyl-based compounds and CO2 are used as alkyl sources in alcohol reductive etherifications. This process involves the use of external reducing agents such as Si-H reagents or H2. The use of carbonyl-based compounds and CO2 as alkyl sources has emerged as a potential tool for the selective production of both symmetrical and asymmetrical ethers with increased molecular complexity.

Carbonyl-based compounds, including aldehydes, ketones, and carboxylic acid derivatives, can undergo catalytic reductive alcohol etherifications to form ether derivatives. These derivatives have a wide range of applications in the chemical industry, making the development of more effective and sustainable production protocols highly desirable. The carbonyl group is one of the most common functional groups in compounds isolated from biological sources, such as retinal (an aldehyde required for vision) and α-ionone (a fragrant ketone found in irises).

CO2, as the most abundant C1 carbon source, is a renewable and environmentally friendly compound that has gained prominence in the chemical industry. Heterogeneously catalyzed reactions with CO2 often involve the carboxylation of simple alcohols to form new C-O bonds and produce alkyl carbonates. Additionally, C-N bond formation has been explored through carbamate synthesis, and C-C bond formation has been demonstrated in the preparation of various carboxylic acids.

The use of carbonyl-based compounds and CO2 as alkyl sources in reductive etherifications offers several advantages. These processes allow for the efficient production of ether derivatives with high structural complexity, including both symmetrical and asymmetrical ethers. Furthermore, related transformations such as ester-to-ether reductions and acetal/ketal-to-ether reductions have also been explored, contributing to the versatility of this approach.

In summary, carbonyl-based compounds and CO2 play a significant role as alkyl sources in alcohol reductive etherifications. The utilization of these sources enables the production of a diverse range of ether derivatives with increased molecular complexity. The ongoing advancements in catalytic protocols and the exploration of related transformations further enhance the potential of this field, making it an important area of focus for both academic and industrial applications.

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Ester-to-ether catalytic reductions

Ethers are pivotal compounds for the chemical industry, with a wide range of applications in bulk and fine chemistry. In recent years, catalytic reductive alcohol etherifications with carbonyl-based moieties (aldehydes/ketones and carboxylic acid derivatives) have emerged as a potential tool for ether synthesis.

Ethers can be obtained by the direct reduction of esters. This is an attractive method due to the availability of esters from renewable sources. The ester-to-ether transformation was first achieved using non-catalytic procedures in the presence of (over)stoichiometric amounts of metal hydrides or thiocarbonylating reagents. However, catalytic strategies for ester-to-ether reduction are still relatively undeveloped, as esters tend to yield alcohols under reductive conditions instead of the desired ethers.

Despite this challenge, recent progress has been made in catalytic ester-to-ether reductions. Carbonyl-based compounds and CO2 are used as alkyl sources in alcohol reductive etherifications, employing external reducing agents such as Si−H reagents or H2. These protocols enable the efficient production of a wide range of symmetrical and asymmetrical ether derivatives with high structural complexity.

In 2016, the group of Tulchinsky reported the first example of alkyl 4‐alkoxypentanoates synthesis via Pd/C-catalyzed simultaneous esterification/reductive etherification of LA. The reaction was performed at high temperatures (200–220 °C) with an excess of alcohol, resulting in four different etherified esters with moderate to good yields (54–77 %). Subsequently, Guan and colleagues achieved a higher yield of 93% by using a silica-modified carbon-supported palladium nanocatalyst (Pd/SiO2‐C) and an H‐BETA zeolite as an acidic co-catalyst.

Hydrogenative alcohol etherifications with CO2

Ethers are pivotal compounds in the chemical industry, with a wide range of applications in bulk and fine chemistry. Ether derivatives have a variety of applications in several areas of chemical industry and academia. Hence, the development of more effective and sustainable protocols for their production is highly desired.

Catalytic reductive alcohol etherifications with carbonyl-based compounds or CO2 and other related transformations have emerged as a potential tool for the selective production of both symmetrical and asymmetrical ethers. These processes constitute appealing routes for the selective production of ethers with increased molecular complexity.

Reductive etherification employs carbonyl-based compounds and CO2 as alkyl sources in alcohol reductive etherifications using external reducing agents such as Si-H reagents or H2. By applying such protocols, an extensive range of symmetrical/asymmetrical ether derivatives with high structural complexity can be efficiently produced.

In recent years, several contributions have been made in homogeneous or heterogeneous catalysis areas employing different reducing agents (mainly hydrosilane-based compounds or molecular hydrogen). These processes constitute convenient strategies to perform alcohol etherifications using readily available carbonyl-based compounds as alkyl sources.

Additionally, efficient reductive etherification methods with carbonyl compounds and alcohols using hydrogen as a reducing agent have been widely developed employing heterogeneous catalysts. These systems present the large advantage of being reusable catalysts. However, a limitation of many of them is encountered when aromatic compounds are reacted, due to the tendency of nanostructured metal-based systems to reduce aromatic fragments.

Alcohol etherifications using carbonyl-based compounds

Ether derivatives are pivotal compounds for the chemical industry, with a wide range of applications in bulk and fine chemistry. They have applications in several areas of chemical industry and academia. Hence, the development of more effective and sustainable protocols for their production is highly desired.

Catalytic reductive alcohol etherifications with carbonyl-based compounds (aldehydes/ketones and carboxylic acid derivatives) have emerged as a potential tool for ether synthesis. These processes constitute appealing routes for the selective production of both symmetrical and asymmetrical ethers (including O-heterocycles) with increased molecular complexity.

Reductive etherification employs carbonyl-based compounds and CO2 as alkyl sources in alcohol reductive etherifications using external reducing agents (Si−H reagents or H2). By applying such protocols, a wide range of symmetrical/asymmetrical ether derivatives with high structural complexity can be efficiently produced.

One example of ether synthesis involves the use of an oxocarbenium ion interception strategy, which allows for the direct synthesis of ethers from alcohols and aldehydes. This strategy involves the use of phosphines as "sacrificial" nucleophiles, which results in the formation of phosphonium salts that can be decomposed to release ether products by hydrolysis. This method does not require the use of hydrogen or hydride donors as reductants, making it a novel approach to ether synthesis.

Ether derivatives and their applications

Ethers are a class of organic compounds that contain an ether group, which is a single oxygen atom bonded to two separate carbon atoms, each part of an organyl group. They have the general formula R−O−R′, where R and R′ represent the organyl groups. Ethers can be symmetrical, with the formula ROR, or unsymmetrical, with the formula ROR'.

Symmetrical ethers can be made from the acid-catalyzed dehydration of primary alcohols. For example, ethanol can be heated at 130-140 °C to produce diethyl ether. This process involves three key steps. Firstly, one equivalent of alcohol is protonated to its conjugate acid, which has a good leaving group, OH2 (water). Next, another equivalent of alcohol performs a nucleophilic attack at carbon (SN2), leading to the displacement of OH2 and the formation of a new C-O bond. Finally, the product is deprotonated by another equivalent of solvent, resulting in the ether product.

Ethers have a wide range of applications in the chemical industry, including as solvents, emulsifiers, cleaning agents, and lacquers. For instance, glycol ethers, which are ether derivatives of dihydroxy alcohols formed from ethylene or propylene, are used in many chemical products and manufacturing operations. However, they have also been shown to have toxic effects, such as severe skin irritation.

In recent years, catalytic reductive alcohol etherifications with carbonyl-based moieties (aldehydes, ketones, and carboxylic acid derivatives) have emerged as a potential tool for the selective production of both symmetrical and asymmetrical ethers. These processes allow for an increased molecular complexity of the resulting ethers. Additionally, ester-to-ether catalytic reductions and hydrogenative alcohol etherifications with CO2 have also undergone important advances.

The Williamson ether synthesis is another method for the formation of ethers, although it is not commonly used due to the significant waste cogenerated. This reaction involves treating a parent alcohol with a strong base to form an alkoxide, followed by the addition of an appropriate aliphatic compound bearing a suitable leaving group (R–X).

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