Benzoin condensation

Last updated

In organic chemistry, the benzoin addition is an addition reaction involving two aldehydes (−CH=O). The reaction generally occurs between aromatic aldehydes or glyoxals (OCH=CHO), [1] [2] and results in formation of an acyloin (−C(O)CH(OH)−). In the classic example, benzaldehyde is converted to benzoin ( PhCH(OH)C(O)Ph). [3]

Contents

The benzoin condensation was first reported in 1832 by Justus von Liebig and Friedrich Wöhler during their research on bitter almond oil. [4] The catalytic version of the reaction involving cyanide was developed by Nikolay Zinin in the late 1830s. [5] [6]

Benzoin addition Benzoin condensation reaction.png
Benzoin addition

Reaction mechanism

The reaction is catalyzed by nucleophiles such as a cyanide or an N-heterocyclic carbene (usually thiazolium salts). The reaction mechanism was proposed in 1903 by A. J. Lapworth. [7] In the first step in this reaction, the cyanide anion (as sodium cyanide) reacts with the aldehyde in a nucleophilic addition. Rearrangement of the intermediate results in polarity reversal of the carbonyl group, which then adds to the second carbonyl group in a second nucleophilic addition. Proton transfer and elimination of the cyanide ion affords benzoin as the product. This is a reversible reaction, which means that the distribution of products is determined by the relative thermodynamic stability of the products and starting material.

Mechanism of the benzoin addition Benzoin mechanism.png
Mechanism of the benzoin addition

In this reaction, one aldehyde donates a proton and one aldehyde accepts a proton. Some aldehydes can only donate protons, such as 4-dimethylaminobenzaldehyde, whereas benzaldehyde is both a proton acceptor and donor. In this way it is possible to synthesise mixed benzoins, i.e. products with different groups on each half of the product. However, care should be taken to match a proton donating aldehyde with a proton accepting aldehyde to avoid undesired homo-dimerization.

Scope

The reaction can be extended to aliphatic aldehydes with base catalysis in the presence of thiazolium salts; the reaction mechanism is essentially the same. These compounds are important in the synthesis of heterocyclic compounds. The analogous 1,4-addition of an aldehyde to an enone is called the Stetter reaction.

In biochemistry, the coenzyme thiamine is responsible for biosynthesis of acyloin-like compounds utilizing the benzoin addition. This coenzyme also contains a thiazolium moiety, which on deprotonation becomes a nucleophilic carbene.

The asymmetric version of this reaction has been performed by utilizing chiral thiazolium and triazolium salts. Triazolium salts were found to give greater enantiomeric excess than thiazolium salts. [8] An example is shown below. [9]

Scheme 2. An intramolecular benzoin addition IntramolecularBenzoin.svg
Scheme 2. An intramolecular benzoin addition

Since the products of the reaction are thermodynamically controlled, the retro benzoin addition can be synthetically useful. If a benzoin or acyloin can be synthesized by another method, then they can be converted into the component ketones using cyanide or thiazolium catalysts. The reaction mechanism is the same as above, but it occurs in the reverse direction. This can allow the access of ketones otherwise difficult to produce.

See also

Related Research Articles

<span class="mw-page-title-main">Aldehyde</span> Organic compound containing the functional group R−CH=O

In organic chemistry, an aldehyde is an organic compound containing a functional group with the structure R−CH=O. The functional group itself can be referred to as an aldehyde but can also be classified as a formyl group. Aldehydes are a common motif in many chemicals important in technology and biology.

<span class="mw-page-title-main">Aldol condensation</span> Type of chemical reaction

An aldol condensation is a condensation reaction in organic chemistry in which two carbonyl moieties react to form a β-hydroxyaldehyde or β-hydroxyketone, and this is then followed by dehydration to give a conjugated enone.

In organic chemistry, a nitrile is any organic compound that has a −C≡N functional group. The name of the compound is composed of a base, which includes the carbon of the −C≡N, suffixed with "nitrile", so for example CH3CH2C≡N is called "propionitrile". The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, including methyl cyanoacrylate, used in super glue, and nitrile rubber, a nitrile-containing polymer used in latex-free laboratory and medical gloves. Nitrile rubber is also widely used as automotive and other seals since it is resistant to fuels and oils. Organic compounds containing multiple nitrile groups are known as cyanocarbons.

Thiazole, or 1,3-thiazole, is a 5-membered heterocyclic compound that contains both sulfur and nitrogen. The term 'thiazole' also refers to a large family of derivatives. Thiazole itself is a pale yellow liquid with a pyridine-like odor and the molecular formula C3H3NS. The thiazole ring is notable as a component of the vitamin thiamine (B1).

The Kolbe–Schmitt reaction or Kolbe process is a carboxylation chemical reaction that proceeds by treating phenol with sodium hydroxide to form sodium phenoxide, then heating sodium phenoxide with carbon dioxide under pressure, then treating the product with sulfuric acid. The final product is an aromatic hydroxy acid which is also known as salicylic acid.

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction which involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carboxylic acid.

<span class="mw-page-title-main">Benzoin (organic compound)</span> Chemical compound

Benzoin ( or ) is an organic compound with the formula PhCH(OH)C(O)Ph. It is a hydroxy ketone attached to two phenyl groups. It appears as off-white crystals, with a light camphor-like odor. Benzoin is synthesized from benzaldehyde in the benzoin condensation. It is chiral and it exists as a pair of enantiomers: (R)-benzoin and (S)-benzoin.

In organic chemistry, a cyanohydrin reaction is an organic reaction in which an aldehyde or ketone reacts with a cyanide anion or a nitrile to form a cyanohydrin. For example:

The Strecker amino acid synthesis, also known simply as the Strecker synthesis, is a method for the synthesis of amino acids by the reaction of an aldehyde with cyanide in the presence of ammonia. The condensation reaction yields an α-aminonitrile, which is subsequently hydrolyzed to give the desired amino acid. The method is used for the commercial production of racemic methionine from methional.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene is an organic molecule whose natural resonance structure has a carbon atom with incomplete octet, but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC), in which nitrogen atoms flank the formal carbene.

<span class="mw-page-title-main">Acyloin</span> Organic compounds of the form –C(O)CH(OH)–

In organic chemistry, acyloins or α-hydroxy ketones are a class of organic compounds of the general form R−C(O)CH(OH)−R', composed of a hydroxy group adjacent to a ketone group. The name acyloin is derived from the fact that they are formally derived from reductive coupling of carboxylic acyl groups. They are one of the two main classes of hydroxy ketones, distinguished by the position of the hydroxy group relative to the ketone; in this form, the hydroxy is on the alpha carbon, explaining the secondary name of α-hydroxy ketone.

In organic chemistry, umpolung or polarity inversion is the chemical modification of a functional group with the aim of the reversal of polarity of that group. This modification allows secondary reactions of this functional group that would otherwise not be possible. The concept was introduced by D. Seebach and E.J. Corey. Polarity analysis during retrosynthetic analysis tells a chemist when umpolung tactics are required to synthesize a target molecule.

<span class="mw-page-title-main">Pinacol rearrangement</span> Rearrangement of compound by charge rearrangement.

The pinacol–pinacolone rearrangement is a method for converting a 1,2-diol to a carbonyl compound in organic chemistry. The 1,2-rearrangement takes place under acidic conditions. The name of the rearrangement reaction comes from the rearrangement of pinacol to pinacolone.

The Stetter reaction is a reaction used in organic chemistry to form carbon-carbon bonds through a 1,4-addition reaction utilizing a nucleophilic catalyst. While the related 1,2-addition reaction, the benzoin condensation, was known since the 1830s, the Stetter reaction was not reported until 1973 by Dr. Hermann Stetter. The reaction provides synthetically useful 1,4-dicarbonyl compounds and related derivatives from aldehydes and Michael acceptors. Unlike 1,3-dicarbonyls, which are easily accessed through the Claisen condensation, or 1,5-dicarbonyls, which are commonly made using a Michael reaction, 1,4-dicarbonyls are challenging substrates to synthesize, yet are valuable starting materials for several organic transformations, including the Paal–Knorr synthesis of furans and pyrroles. Traditionally utilized catalysts for the Stetter reaction are thiazolium salts and cyanide anion, but more recent work toward the asymmetric Stetter reaction has found triazolium salts to be effective. The Stetter reaction is an example of umpolung chemistry, as the inherent polarity of the aldehyde is reversed by the addition of the catalyst to the aldehyde, rendering the carbon center nucleophilic rather than electrophilic.

<span class="mw-page-title-main">Nikolay Zinin</span>

Nikolay Nikolaevich Zinin was a Russian organic chemist.

The triazol-5-ylidenes are a group of persistent carbenes which includes the 1,2,4-triazol-5-ylidene system and the 1,2,3-triazol-5-ylidene system. As opposed to the now ubiquitous NHC systems based on imidazole rings, these carbenes are structured from triazole rings. 1,2,4-triazol-5-ylidene can be thought of as an analog member of the NHC family, with an extra nitrogen in the ring, while 1,2,3-triazol-5-ylidene is better thought of as a mesoionic carbene (MIC). Both isomers of this group of carbenes benefit from enhanced stability, with certain examples exhibiting greater thermal stability, and others extended shelf life.

<span class="mw-page-title-main">Zincke reaction</span> Chemical reaction

The Zincke reaction is an organic reaction, named after Theodor Zincke, in which a pyridine is transformed into a pyridinium salt by reaction with 2,4-dinitro-chlorobenzene and a primary amine.

<span class="mw-page-title-main">Phenanthridine</span> Chemical compound

Phenanthridine is a nitrogen heterocyclic compound that is the basis of DNA-binding fluorescent dyes through intercalation. Examples of such dyes are ethidium bromide and propidium iodide. It is an isomer of acridine.

Benzaldehyde (C6H5CHO) is an organic compound consisting of a benzene ring with a formyl substituent. It is among the simplest aromatic aldehydes and one of the most industrially useful.

Furoin or 1,2-di(furan-2-yl)-2-hydroxyethanone is an organic compound with formula C10H8O4. It can be produced from furfural by a benzoin condensation reaction catalyzed by cyanide ions.

References

  1. Menon, Rajeev S.; Biju, Akkattu T.; Nair, Vijay (2016). "Recent advances in N-heterocyclic carbene (NHC)-catalysed benzoin reactions". Beilstein Journal of Organic Chemistry. 12: 444–461. doi:10.3762/bjoc.12.47. PMC   4901930 . PMID   27340440.
  2. Enders, Dieter; Niemeier, Oliver; Henseler, Alexander (2007). "Organocatalysis by N-Heterocyclic Carbenes". Chemical Reviews. 107 (12): 5606–5655. doi:10.1021/cr068372z. PMID   17956132.
  3. Roger Adams; C. S. Marvel (1921). "Benzoin". Organic Syntheses. 1: 33. doi:10.15227/orgsyn.001.0033.
  4. F. Wöhler, J. Liebig (1832). "Untersuchungen über das Radikal der Benzoesäure" [Studies on the radicals of benzoic acid]. Annalen der Pharmacie (in German). 3 (3): 249–282. doi:10.1002/jlac.18320030302. hdl: 2027/hvd.hxdg3f .
  5. N. Zinin (1839). "Beiträge zur Kenntniss einiger Verbindungen aus der Benzoylreihe" [Contributions to the knowledge of some compounds from the benzoyl series]. Annalen der Pharmacie (in German). 31 (3): 329–332. doi:10.1002/jlac.18390310312. Archived from the original on 2022-07-09. Retrieved 2020-09-11.
  6. N. Zinin (1840). "Ueber einige Zersetzungsprodukte des Bittermandelöls" [Study of some decomposition products of bitter almond oil]. Annalen der Pharmacie (in German). 34 (2): 186–192. doi:10.1002/jlac.18400340205. Archived from the original on 2022-07-09. Retrieved 2019-06-28.
  7. Lapworth, A. (1904). "CXXII.—Reactions involving the addition of hydrogen cyanide to carbon compounds. Part II. Cyanohydrins regarded as complex acids". Journal of the Chemical Society, Transactions. 85: 1206–1214. doi:10.1039/CT9048501206. Archived from the original on 2022-07-09. Retrieved 2019-06-28.
  8. Knight, Roland; Leeper, F. (1998). "Comparison of chiral thiazolium and triazolium salts as asymmetric catalysts for the benzoin addition". J. Chem. Soc., Perkin Trans. 1 (12): 1891–1894. doi:10.1039/A803635G.
  9. D. Enders, O. Niemeier & T. Balensiefer (2006). "Asymmetric Intramolecular Crossed-Benzoin Reactions by N-Heterocyclic Carbene Catalysis". Angewandte Chemie International Edition . 45 (9): 1463–1467. doi:10.1002/anie.200503885. PMID   16389609.

Commons-logo.svg Media related to animation of the Benzoin condensation reaction mechanism at Wikimedia Commons