Benzoin condensation


Benzoin condensation

The benzoin condensation is a condensation reaction between two aromatic aldehydes, particularly benzaldehyde. The reaction is catalyzed by a nucleophile such as the cyanide anion or an N-heterocyclic carbene. The reaction product is an aromatic acyloin with benzoin as the parent compound. ["Benzoin" Roger Adams and C. S. Marvel Organic Syntheses, Coll. Vol. 1, p.94 (1941); Vol. 1, p.33 (1921) [http://www.orgsyn.org/orgsyn/prep.asp?prep=cv1p0094 Article] ] An early version of the reaction was developed in 1832 by Justus von Liebig and Friederich Woehler during their research on bitter almond oil. [cite journal
title = Untersuchungen über das Radikal der Benzoesäure
author = Wöhler, Liebig
journal = Annalen der Pharmacie
volume = 3
issue = 3
pages = 249–282
year = 1832
url =
doi = 10.1002/jlac.18320030302
] The catalytic version of the reaction was developed by Nikolay Zinin in the late 1830s, [cite journal
title = Beiträge zur Kenntniss einiger Verbindungen aus der Benzoylreihe
author = N. Zinin
journal = Annalen der Pharmacie
volume = 31
issue = 3
pages = 329–332
year = 1839
url =
doi = 10.1002/jlac.18390310312
] [cite journal
title = Ueber einige Zersetzungsprodukte des Bittermandelöls
author = N. Zinin
journal = Annalen der Pharmacie
volume = 34
issue = 2
pages = 186–192
year = 1840
url =
doi = 10.1002/jlac.18400340205
] and the reaction mechanism for this organic reaction was proposed in 1903 by A. J. Lapworth. ["CXXII. — Reactions involving the addition of hydrogen cyanide to carbon compounds. Part II. Cyanohydrins regarded as complex acids" Arthur Lapworth, Journal of the Chemical Society, Transactions, 1904, 85, 1206 - 1214. doi|10.1039/CT9048501206]

Reaction mechanism

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.

The cyanide ion serves three different purposes in the course of this reaction. It acts as a nucleophile, facilitates proton abstraction, and is also the leaving group in the final step. The benzoin condensation is in effect a dimerization and not a condensation because a small molecule like water is not released in this reaction. For this reason the reaction is also called a benzoin addition. In this reaction, the two aldehydes serve different purposes; one aldehyde donates a proton and one aldehyde accepts a proton. 4-Dimethylaminobenzaldehyde is an efficient proton donor while 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.

cope

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 addition is also possible with enones; for instance methyl vinyl ketone is a reagent in the Stetter reaction.

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

In one study, a custom-designed N-heterocyclic carbene (NHC, the framework is related to thiazolium salts) was found to facilitate an enantioselective intramolecular benzoin condensation ("Scheme 2"). [cite journal | author = D. Enders, O. Niemeier and T. Balensiefer | title = Asymmetric Intramolecular Crossed-Benzoin Reactions by N-Heterocyclic Carbene Catalysis | year = 2006 | journal = Angewandte Chemie International Edition | volume = 45 | issue = 9 | pages = 1463–1467 | doi = 10.1002/anie.200503885 ]

This finding was confirmed in another study with a slightly modified NHC using DBU as the base instead of potassium tert-butoxide ("Scheme 3"). [cite journal | author = H. Takikawa, Y. Hachisu, J. W. Bode and K. Suzuki | title = Catalytic Enantioselective Crossed Aldehyde-Ketone Benzoin Cyclization | year = 2006 | journal = Angewandte Chemie International Edition | volume = 45 | issue = 21 | pages = 3492–3494 | doi = 10.1002/anie.200600268 ]

References


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