Noah Vinter
The objective of alcohol-amine coupling reactions is to form C-N bonds, which are important in the production of bulk and fine chemicals, pharmaceuticals, and drug-like molecules. This can be achieved through direct alkylation or amination of alcohols with amines, using catalysts such as iron, copper, or manganese. These reactions are atom-efficient and environmentally benign, with the potential for large-scale applications. The choice of catalyst and reaction conditions depends on the specific alcohol and amine substrates involved, as well as the desired product.
| Characteristics | Values |
|---|---|
| Objective | To form C-N bonds through a "hydrogen-borrowing" reaction between amines and alcohols |
| Catalyst | Iron-tetraphenylcyclopentadienone tricarbonyl complex |
| Conversion rate | 99% in 48 hours |
| Conditions | Inert atmosphere, 10 mol % of 1 in toluene, 2-fold excess of benzyl alcohol, trimethylamine-N-oxide |
| By-products | Water, H2 |
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What You'll Learn
Oxidative amide coupling
The objective of alcohol-amine coupling reactions is to form amides, which are one of the most common structural features in bioactive molecules. Amide coupling reactions are the most frequently used reactions in drug discovery and medicinal chemistry.
The direct alkylation of amines with alcohols is an atom-efficient and environmentally benign process. Iron-based catalysts have been shown to carry out this process with high conversions and good substrate scope. The key challenge in direct amination of alcohols is matching alcohol dehydrogenation and imine hydrogenation steps. Conditions must be established under which the formed Fe-H species from the initial alcohol dehydrogenation step can reduce the imine sufficiently, negating the need for hydrogen gas.
Tertiary amides can be synthesized from tertiary amines and anhydrides in the presence of FeCl2 as a catalyst and tert-butyl hydroperoxide in water as an oxidant. Mechanistic studies indicate that the in situ-generated α-amino peroxide of the tertiary amine and iminium ion act as key intermediates.
Several catalytic methods for amide coupling have been reported, but none exhibit the synthetic utility of methods that use stoichiometric coupling reagents. Amide coupling reactions typically use stoichiometric coupling reagents to activate a carboxylic acid for nucleophilic attack by the amine coupling partner. Amide coupling with electron-deficient amines often produces sluggish results. However, the use of reagents such as EDC and DMAP, with a catalytic amount of HOBt, has provided excellent yields for amide derivatives with electron-rich amines and functionalized carboxylic acids.
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Direct alkylation of amines with alcohols
The objective of alcohol-amine coupling reactions is to form amines by converting carbon-oxygen bonds into carbon-nitrogen bonds. This is achieved by direct N-alkylation, which involves the coupling of primary alcohols and amines to form secondary amines.
The direct alkylation of amines with alcohols can be achieved through a "borrowing hydrogen" methodology. This method uses transition metal complexes to synthesize N-alkylated amines from alcohols. The use of Lewis acids, such as AlCl3, has been shown to be effective in producing N-alkylated amines without the need for ligands or additives. This process is advantageous due to its atom economy and environmental friendliness, as water is generated as a side product.
The choice of alcohol as a substrate for direct C-N bond formation is desirable for producing secondary and tertiary amines and N-heterocyclic compounds. Alcohols are readily available through various industrial processes, including fermentation or catalytic conversion of lignocellulosic biomass.
In terms of specific applications, Muldoon and coworkers reported the use of Cu/nitroxyl-catalyzed oxidation of hydroxylalkyl-substituted anilines, resulting in the formation of indoles and quinolines. Lu and coworkers developed a method for the preparation of benzoxazoles and benzothiazole products through the reaction of phenylene diamine with aromatic alcohols under solvent-free conditions. Additionally, Wu and coworkers prepared quinazolines through a three-component oxidative coupling reaction of 2-aminobenzyl alcohol, aryl aldehyde, and ammonium chloride reagents.
Overall, the direct alkylation of amines with alcohols is a versatile and environmentally friendly process that has found applications in various chemical synthesis fields, including pharmaceuticals, agrochemicals, and fine chemicals.
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C–N bond formation
The objective of alcohol-amine coupling reactions is to form C–N bonds, which are important in the production of bulk and fine chemicals, pharma intermediates, and amides.
Direct alkylation of amines with alcohols is an atom-efficient and environmentally benign process. Transition metal complexes based on precious metals have been used as catalysts for this transformation. However, the use of abundant, inexpensive, and environmentally friendly metals, such as iron, has been a goal for this process.
An iron-tetraphenylcyclopentadienone tricarbonyl complex has been demonstrated as a catalyst for the formation of C–N bonds through a "hydrogen-borrowing" reaction between amines and alcohols. The reaction proceeds through the dehydrogenation of the alcohol to the corresponding aldehyde, with the temporary storage of one 'hydrogen equivalent' at the bifunctional iron complex. This is then converted to its reduced, hydride form. The key challenge is matching the alcohol dehydrogenation and imine hydrogenation steps, ensuring the formed Fe-H species can reduce the imine at a sufficient rate without requiring hydrogen gas.
The scope of this methodology includes the monoalkylation of anilines and benzyl amines with a wide range of alcohols and the use of diols in the formation of five-, six-, and seven-membered nitrogen heterocycles.
Other metals and catalysts have also been explored for alcohol-amine coupling reactions. For example, copper catalysts enable the oxidative coupling of alcohols and amines to form amides, with all four permutations of benzylic/aliphatic alcohols and primary/secondary amines viable. Ruthenium catalysts have also been used to mediate a direct coupling between an alcohol and an amine, liberating two molecules of dihydrogen.
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Acceptorless dehydrogenative coupling
The objective of alcohol-amine coupling reactions is to form amines by directly coupling an alcohol with an amine. This is achieved through alcohol dehydrogenation and imine hydrogenation.
ADC reactions are promoted by supported transition-metal catalysts in the absence of hydrogen acceptors. The design of these catalysts involves cooperation between the metal sites and the acid and/or base sites on the metal–oxide supports. Recent advancements in ADC reactions have focused on the use of ruthenium catalysts, group VI sulfide catalysts, and iron catalysts.
One example of an ADC reaction is the acceptorless dehydrogenative coupling of ethanol, which can yield ethyl acetate and hydrogen gas. Another example is the synthesis of unsymmetrical N-heterocyclic carbene–nitrogen–phosphine chelated ruthenium(II) complexes through the acceptorless dehydrogenative coupling of alcohols to esters.
Photoinduced hydrogen-atom transfer (HAT) catalysis has emerged as a promising strategy in acceptorless dehydrogenative coupling. This technique enables the cleavage of C–H bonds, facilitating the formation of C–C bonds and the synthesis of N-heterocycles.
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Synthesis of epoxyquinoid molecules
The objective of alcohol-amine coupling reactions is to form amines by converting carbon-oxygen bonds into carbon-nitrogen bonds. This process is known as direct N-alkylation and can be achieved through various methods, including the use of transition metal catalysts.
The synthesis of epoxyquinoid molecules, such as epoxyquinol A, B, and epoxytwinol A, involves the dimerization of 2H-pyran epoxyquinol monomers. Modifications of 2H-pyran precursors have been explored, including altering the epoxy alcohol and diene stereochemistry. Epoxyquinoid molecules can also be synthesized through the oxidation of hydroxylalkyl-substituted anilines, leading to the formation of indoles and quinolines.
One method for the synthesis of epoxyquinoid molecules involves the use of Cu/nitroxyl catalyst systems, which enable the efficient formation of imines, nitriles, and nitrogen heterocycles via the aerobic oxidative coupling of alcohols and amines. This approach takes advantage of the chemoselectivity of Cu/nitroxyl for alcohol over amine oxidation. Benzyl, cinnamyl, and phenylpropargyl alcohols are particularly effective coupling partners with various amine and aniline derivatives.
Another approach to synthesizing epoxyquinoid molecules is through iron-catalyzed direct alkylation of amines with alcohols. This method is atom-efficient and environmentally benign, utilizing iron as a readily available and inexpensive catalyst. The key challenge in direct amination of alcohols is matching alcohol dehydrogenation and imine hydrogenation steps to ensure the efficient formation of the desired products.
The synthesis of epoxyquinoid molecules has also been explored in the context of natural products, such as the potent antifungal agent (-)-jesterone, and its related molecules. These studies involve enantioselective syntheses and the exploration of chemical diversity within the epoxyquinoid family to identify potential therapeutic drugs and starting materials for new medicines.
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Frequently asked questions
The objective of the alcohol-amine coupling reaction is to form C-N bonds, which are important for the production of bulk and fine chemicals, pharmaceutical intermediates, and drug-like molecules.
Some methods include using iron-based catalysts, Cu/nitroxyl catalyst systems, and manganese-based catalysts.
Iron-based catalysts are inexpensive, readily available, and environmentally benign. They also have a wide substrate scope and can be used to form a variety of nitrogen heterocycles.
