Darzens reaction

Darzens reaction

The Darzens reaction (also known as the Darzens condensation or glycidic ester condensation) is the chemical reaction of a ketone or aldehyde with an α-haloester to form an α,β-epoxy ester, also called a "glycidic ester".[1][2] This reaction was discovered by the organic chemist Auguste George Darzens in 1904.[3]


Darzens reaction overview.png

Contents

Reaction mechanism

The reaction process begins when a strong base is used to form a carbanion at the halogenated position. Because of the ester, this carbanion is a resonance-stabilized enolate, which makes it relatively easy to form. This nucleophilic structure attacks another carbonyl component, forming a new carbon–carbon bond. These first two steps are similar to a base-catalyzed aldol reaction. The oxygen anion in this aldol-like product then does an intramolecular SN2 attack on the formerly-nucleophilic halide-bearing position, displacing the halide to form an epoxide.[4] This reaction sequence is thus a condensation reaction, since there is a net loss of HCl when the two reactant molecules join.

Darzensepoxide.png

The primary role of the ester is to enable the initial deprotonation to occur, and other carbonyl functional groups can be used instead. If the starting material is an α-halo amide, the product is an α,β-epoxy amide.[5] If an α-halo ketone is used, the product is an α,β-epoxy ketone.[4]

Any sufficiently strong base can be used for the initial deprotonation. However, if the starting material is an ester, the alkoxide corresponding to the ester side-chain is commonly in order to prevent complications due to potential acyl exchange side reactions.

Stereochemistry

Depending on the specific structures involved, the epoxide may exist in cis and trans forms. A specific reaction may give only cis, only trans, or a mixture of the two. The specific stereochemical outcome of the reaction is affected by several aspects of the intermediate steps in the sequence.

The initial stereochemistry of the reaction sequence is established in the step where the carbanion attacks the carbonyl. Two sp3 (tetrahedral) carbons are created at this stage, which allows two different diastereomeric possibilities of the halohydrin intermediate. The most likely result is due to chemical kinetics: whichever product is easier and faster to form will be the major product of this reaction. The subsequent SN2 reaction step proceeds with stereochemical inversion, so the cis or trans form of the epoxide is controlled by the kinetics of an intermediate step. Alternately, the halohydrin can epimerize due to the basic nature of the reaction conditions prior to the SN2 reaction. In this case, the initially formed diastereomer can convert to a different one. This is an equilibrium process, so the cis or trans form of the epoxide is controlled by chemical thermodynamics--the product resulting from the more stable diastereomer, regardless of which one was the kinetic result.[5]

Alternative reactions

Glycidic esters can also be obtained via nucleophilic epoxidation of an α,β-unsaturated ester, but that approach requires synthesis of the alkene substrate first whereas the Darzens condensation allows formation of the carbon–carbon connectivity and epoxide ring in a single reaction.

Subsequent reactions

The product of the Darzens reaction can be reacted further to form various types of compounds. Hydrolysis of the ester can lead to decarboxylation, which triggers a rearrangement of the epoxide into a carbonyl (4). Alternately, other epoxide rearrangements can be induced to form other structures.

The Darzens reaction

References

  1. ^ Darzens, G. (1905). Compt. Rend. 141: 766. 
  2. ^ Darzens, G. (1906). Compt. Rend. 142: 214. 
  3. ^ Darzens, G. (1904). Compt. Rend. 139: 1214. 
  4. ^ a b Jie Jack Li (2006). "Darzens glycidic ester condensation". Name Reactions (3rd. expanded ed.). Springer-Verlag. pp. 183–184. doi:10.1007/3-540-30031-7. http://www.springerlink.com/content/v764410x2k2117h7. 
  5. ^ a b Tung, C. C.; Speziale, A. J.; Frazier, H. W. (1963). "The Darzens Condensation. II. Reaction of Chloroacetamides with Aromatic Aldehydes". The Journal of Organic Chemistry 28 (6): 1514. doi:10.1021/jo01041a018.  edit

Review articles

  • Newman, M. S.; Magerlein, B. J. (1949). Org. React. 5: 413. 
  • Ballester, M. (1955). "Mechanisms of The Darzens and Related Condensations Manuel Ballester". Chem. Rev. 55: 283. doi:10.1021/cr50002a002. 
  • Rosen, T. (1991). Comp. Org. Syn. 2: 409–439. 

See also


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • Réaction de Darzens — La réaction de Darzens ou condensation de Darzens est une réaction chimique découverte par le chimiste Auguste Georges Darzens en 1904. La réaction a lieu entre une cétone ou un aldéhyde et un α halogénoester pour former un α,β epoxy ester. bilan …   Wikipédia en Français

  • Condensación de Darzens — La condensación de Darzens (Conocida también como condensación de Darzens or condensación del éster glicídico es una reacción de condensación de un compuesto carbonílico con un α haloéster para formar un α,β epoxi éster, (Denominado también éster …   Wikipedia Español

  • Johnson–Corey–Chaykovsky reaction — The Johnson–Corey–Chaykovsky reaction (sometimes referred to as the Corey–Chaykovsky reaction or CCR) is a chemical reaction used in organic chemistry for the synthesis of epoxides, aziridines, and cyclopropanes. It was discovered in 1961 by A.… …   Wikipedia

  • Friedel-Crafts reaction — The Friedel Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. [Friedel, C.; Crafts, J. M. Compt. Rend. 1877, 84 , [http://gallica.bnf.fr/ark:/12148/bpt6k30410/f1386.table 1392]… …   Wikipedia

  • Friedel–Crafts reaction — The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877.[1] There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. This reaction type is a form of… …   Wikipedia

  • Condensation reaction — The condensation of two amino acids to form a peptide bond (red) with expulsion of water (blue) A condensation reaction is a chemical reaction in which two molecules or moieties (functional groups) combine to form one single molecule, together… …   Wikipedia

  • Epoxide — A generic epoxide. An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are… …   Wikipedia

  • Ibuprofen — Systematic (IUPAC) name (RS) 2 (4 (2 methylpropyl)phenyl)propanoic acid Clinical data Trade names Advil, Motrin …   Wikipedia

  • Chan rearrangement — The Chan rearrangement is a chemical reaction that involves rearranging an acyloxy acetate (1) in the presence of a strong base to a 2 hydroxy 3 keto ester (2).[1] This procedure was rediscovered and employed in the Holton Taxol total synthesis …   Wikipedia

  • List of organic reactions — Well known reactions and reagents in organic chemistry include Contents: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z    See also   Ext …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”