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Aldol condensation

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Aldol condensation

An aldol condensation is an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by a dehydration to give a conjugated enone.

Aldol condensation overview

Aldol condensations are important in reaction mechanisms.[1][2][3] In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a β-hydroxy ketone, or "aldol" (aldehyde + alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals.[4][5][6]

The Aldol reaction

The name aldol condensation is also commonly used, especially in biochemistry, to refer to just the first (addition) stage of the process—the aldol reaction itself—as catalyzed by aldolases. However, the aldol reaction is not formally a condensation reaction because it does not involve the loss of a small molecule.

The reaction between an aldehyde/ketone and a carbonyl compound lacking an alpha-hydrogen (cross aldol condensation) is called the Claisen-Schmidt condensation. This reaction is named after two of its pioneering investigators Rainer Ludwig Claisen and J. G. Schmidt, who independently published on this topic in 1880 and 1881.[7][8][9] An example is the synthesis of dibenzylideneacetone.

Contents

  • Mechanism 1
  • Condensation types 2
  • Aldox process 3
  • Scope 4
  • See also 5
  • References 6

Mechanism

The first part of this reaction is an aldol reaction, the second part a dehydration—an elimination reaction(Involves removal of a water molecule or an alcohol molecule). Dehydration may be accompanied by decarboxylation when an activated carboxyl group is present. The aldol addition product can be dehydrated via two mechanisms; a strong base like potassium t-butoxide, potassium hydroxide or sodium hydride in an enolate mechanism,[10] or in an acid-catalyzed enol mechanism.

Enolate mechanism :Enol Mechanism
Animation zum basenkat. Reaktionsmechanismus der Aldolkondensation Animation zum säurekat. Reaktionsmechanismus der Aldolkondensation
animation, base catalyzed animation, acid catalyzed

Condensation types

It is important to distinguish the aldol condensation from other addition reactions to carbonyl compounds.

Aldox process

In industry the Aldox process developed by Royal Dutch Shell and Exxon, converts propylene and syngas directly to 2-Ethylhexanol via hydroformylation to butyraldehyde, aldol condensation to 2-ethylhexenal and finally hydrogenation.[11]

Aldox process

In one study crotonaldehyde is directly converted to 2-ethylhexanal in a palladium / Amberlyst / supercritical carbon dioxide system:[12]

Crotonaldehyde Aldol process

Scope

Ethyl 2-methylacetoacetate and campholenic aldehyde react in an Aldol condensation.[13] The synthetic procedure [14] is typical for this type of reactions. In the process, in addition to water, an equivalent of ethanol and carbon dioxide are lost in decarboxylation.

Aldol condensation of Ethyl 2-methylacetoacetate and campholenic aldehyde

Ethyl glyoxylate 2 and diethyl 2-methylglutaconate 1 react to isoprenetricarboxylic acid 3 (isoprene skeleton) with sodium ethoxide. This reaction product is very unstable with initial loss of carbon dioxide and followed by many secondary reactions. This is believed to be due to steric strain resulting from the methyl group and the carboxylic group in the cis-dienoid structure.[15]

Isoprenetricarboxylic acid

Occasionally an aldol condensation is buried in a multistep reaction or in catalytic cycle such as the one sketched below:[16]

Ru Catalyzed Cyclization of Terminal Alkynals to Cycloalkenes

In this reaction an alkynal 1 is converted into a cycloalkene 7 with a ruthenium catalyst and the actual condensation takes place with intermediate 3 through 5. Support for the reaction mechanism is based on isotope labeling.[17]

The reaction between menthone and anisaldehyde is complicated due to steric shielding of the ketone group. The solution is use of a strong base such as potassium hydroxide and a very polar solvent such as DMSO in the reaction below:[18]

A Claisen-Schmidt reaction

Due to epimerization through a common enolate ion (intermediate A) the reaction product has (R,R)-cis-configuration and not (R,S)-trans-configuration as in the starting material. Because it is only the cis isomer that precipitates from solution, this product is formed exclusively.

See also

References

  1. ^ Wade, L. G. (2005). Organic Chemistry (6th ed.). Upper Saddle River, NJ: Prentice Hall. pp. 1056–1066.  
  2. ^ Smith, M. B.; March, J. (2001). Advanced Organic Chemistry (5th ed.). New York: Wiley Interscience. pp. 1218–1223.  
  3. ^ Mahrwald, R. (2004). Modern Aldol Reactions 1, 2. Weinheim, Germany: Wiley-VCH. pp. 1218–1223.  
  4. ^  
  5. ^ Mukaiyama T. (1982). "The Directed Aldol Reaction". Organic Reactions 28: 203–331.  
  6. ^ Paterson, I. (1988). "New Asymmetric Aldol Methodology Using Boron Enolates". Chemistry and Industry (London: Paterson Group) 12: 390–394. 
  7. ^ Claisen, L.; Claparède, A. (1881). "Condensationen von Ketonen mit Aldehyden".  
  8. ^ Schmidt, J. G. (1881). "Ueber die Einwirkung von Aceton auf Furfurol und auf Bittermandelöl in Gegenwart von Alkalilauge".  
  9. ^ March, J. (1985). Advanced Organic Chemistry: Reactions, Mechanisms and Structure (3rd ed.). Wiley Interscience.  
  10. ^ Nielsen, A. T.; Houlihan., W. J. (1968). "The Aldol Condensation". Organic Reactions 16: 1–438.  
  11. ^ For example BG 881979 
  12. ^ Seki, T.; Grunwaldt, J.-D.; Baiker, A. (2007). "Continuous catalytic "one-pot" multi-step synthesis of 2-ethylhexanal from crotonaldehyde".  
  13. ^ Badía, C.; Castro, J. M.; Linares-Palomino, P. J.; Salido, S.; Altarejos, J.; Nogueras, M.; Sánchez, A. (2004). "(E)-6-(2,2,3-Trimethyl-cyclopent-3-enyl)-hex-4-en-3-one" (pdf).  
  14. ^ Ethyl 2-methylacetoacetate (2) is added to a stirred solution of sodium sulfate and the solvent evaporated under reduced pressure to yield a residue that was purified by vacuum distillation to give 3 (58%).
  15. ^ Goren, M. B.; Sokoloski, E. A.; Fales, H. M. (2005). "2-Methyl-(1Z,3E)-butadiene-1,3,4-tricarboxylic Acid, "Isoprenetricarboxylic Acid"".  
  16. ^ Varela, J. A.; Gonzalez-Rodriguez, C.; Rubin, S. G.; Castedo, L.; Saa, C. (2006). "Ru-Catalyzed Cyclization of Terminal Alkynals to Cycloalkenes".  
  17. ^ The ruthenium catalyst, [CpRu(CH3CN)3]PF6, has a cyclopentadienyl ligand, three acetonitrile ligands and a phosphorus hexafluoride counterion; the acidic proton in the solvent (acetic acid) is replaced by deuterium for isotopic labeling. Reaction conditions: 90°C, 24 hrs. 80% chemical yield. The first step is formation of the Transition metal carbene complex 2. Acetic acid adds to this intermediate in a nucleophilic addition to form enolate 3 followed by aldol condensation to 5 at which stage a molecule of carbon monoxide is lost to 6. The final step is reductive elimination to form the cycloalkene.
  18. ^ Vashchenko, V.; Kutulya, L.; Krivoshey, A. (2007). "Simple and Effective Protocol for Claisen-Schmidt Condensation of Hindered Cyclic Ketones with Aromatic Aldehydes". Synthesis 2007 (14): 2125–2134.  
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