The Leuckart reaction is the

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

that converts aldehydes or

ketones In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bon ...

amines In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...

reductive amination Reductive amination (also known as reductive alkylation) is a form of amination that involves the conversion of a carbonyl group to an amine via an intermediate imine. The carbonyl group is most commonly a ketone or an aldehyde. It is considered ...

in the presence of heat. The reaction, named after

Rudolf Leuckart Karl Georg Friedrich Rudolf Leuckart (7 October 1822 – 22 February 1898) was a German zoologist born in Helmstedt. He was a nephew to naturalist Friedrich Sigismund Leuckart (1794–1843). Academic career He earned his degree from the Uni ...

, uses either

ammonium formate Ammonium formate, NH4HCO2, is the ammonium salt of formic acid. It is a colorless, hygroscopic, crystalline solid. Reductive amination Acetone can be transformed into isopropylamine as follows: :CH3C(O)CH3 + 2 HCO2− +NH4 → (CH3)2CHNHCHO + 2 ...

formamide Formamide is an amide derived from formic acid. It is a colorless liquid which is miscible with water and has an ammonia-like odor. It is chemical feedstock for the manufacture of sulfa drugs and other pharmaceuticals, herbicides and pesticide ...

as the nitrogen donor and

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...

. It requires high temperatures, usually between 120 and 130 °C; for the formamide variant, the temperature can be greater than 165 °C. Leuckart Reaction Scheme

History

The Leuckart reaction is named in honor of its developer, the German chemist

(1854–1899). He discovered that heating

benzaldehyde Benzaldehyde (C6H5CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. It is a colorless liquid with a characteristic almond-like odor. ...

with formamide does not produce benzylidenediformamide as anticipated, but

benzylamine Benzylamine is an organic chemical compound with the condensed structural formula C6H5CH2NH2 (sometimes abbreviated as PhCH2NH2 or BnNH2). It consists of a benzyl group, C6H5CH2, attached to an amine functional group, NH2. This colorless water ...

. In 1891, a colleague of Leuckart at the University of Göttingen, Otto Wallach, performed further reactions using alicyclic and terpenoid ketones as well as aldehydes, demonstrating the general application. Over the course of the past century, chemists have discovered several methods to improve the yield of the reaction and carry it out under less strenuous conditions. Pollard and Young summarized various ways in which amines can be formed: using either formamide or ammonium formate, or both, or adding formic acid to formamide. However, using just ammonium formate as the reagent produces the best yields. Using formamide produces low yields compared to ammonium formate but yields can be increased by using large amount of formamide, or using ammonium formate, ammonium sulfate, and magnesium chloride as

catalysts Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

Mechanism

Ammonium formate as reagent

Ammonium formate first dissociates into

formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Es ...

and

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

. Ammonia then performs a

nucleophilic attack In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

on the

carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a ...

carbon. The oxygen deprotonates hydrogen from nitrogen to form a hydroxyl. The hydroxyl is protonated using hydrogen from formic acid, which allows for water molecule to leave. This forms a carbocation, which is resonance stabilized. The compound attacks hydrogen from the deprotonated formic acid from previous step, forming a carbon dioxide and an amine.

Formamide as reagent

Formamide first nucleophilically attacks the carbonyl carbon. The oxygen is protonated by abstracting hydrogen from the nitrogen atom, subsequently forming a water molecule that leaves, forming N-formyl derivative, which is resonance stabilized. Water hydrolyzes formamide to give ammonium formate, which acts as a reducing agent and adds on to the N-formyl derivative. Hydride shift occurs, resulting in loss of carbon dioxide. Ammonium ion is added forming an imine and releasing ammonia. The imine goes through hydrolysis to form the amine, which is depicted in the scheme below. Imine hydrolysis

Applications

A notable example of the Leuckart reaction is its use in the synthesis of tetrahydro-1,4 benzodiazepin-5-one, a molecule that is part of the

benzodiazepine Benzodiazepines (BZD, BDZ, BZs), sometimes called "benzos", are a class of depressant drugs whose core chemical structure is the fusion of a benzene ring and a diazepine ring. They are prescribed to treat conditions such as anxiety disorders, ...

family. Many compounds in this family of molecules are central nervous system suppressants and are associated with therapeutic uses and a variety of medications, such as antibiotics, antiulcer, and anti-HIV agents. Researchers were able to synthesize tetrahydro-1,4-benzodiazepin-5-ones with excellent yields and purities by utilizing the Leuckart Reaction. Researchers performed the reaction via

solid-phase synthesis In chemistry, solid-phase synthesis is a method in which molecules are covalently bound on a solid support material and synthesised step-by-step in a single reaction vessel utilising selective protecting group chemistry. Benefits compared with no ...

and used formic acid as the reducing agent.

References