Aldol condensation And rearrangment reactions

An aldol condensation is an organic reaction in which 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 condensations are important in organic synthesis, providing a good way to form carbon–carbon bonds. The Robinson annulation reaction sequence features an aldol condensation; the Wieland-Miescher ketone product is an important starting material for many organic syntheses. Aldol condensations are also commonly discussed in university level organic chemistry classes as a good bond-forming reaction that demonstrates important reaction mechanisms. 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.

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 reactions between an aldehyde/ketone and a carbonyl compound lacking an alpha-Hydrogen(Cross Aldol condensation) is called Claisen-Schmidt condensation. These reactions are named after two of its pioneering investigators Rainer Ludwig Claisen and J. G. Schmidt, who independently published on this topic in 1880 and 1881 An example is the synthesis of dibenzylideneacetone.

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, or in an acid-catalyzed enol mechanism.

Condensation types

 

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

• When the base is an amine and the active hydrogen compound is sufficiently activated the reaction is called a Knoevenagel condensation.
• In a Perkin reaction the aldehyde is aromatic and the enolate generated from an anhydride.
• A Claisen condensation involves two ester compounds.
• A Dieckmann condensation involves two ester groups in the same molecule and yields a cyclic molecule
• A Henry reaction involves an aldehyde and an aliphatic nitro compound.
• A Robinson annulation involves a α,β-unsaturated ketone and a carbonyl group, which first engage in a Michael reaction prior to the aldol condensation.
• In the Guerbet reaction, an aldehyde, formed in situ from an alcohol, self-condenses to the dimerized alcohol.
• In the Japp–Maitland condensation water is removed not by an elimination reaction but by a nucleophilic displacement.
  

  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.
  

  

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

  

  Scope

  Ethyl 2-methylacetoacetate and campholenic aldehyde react in an Aldol condensation. The synthetic procedure 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.
  
  
  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.
  
  
  Occasionally an aldol condensation is buried in a multistep reaction or in catalytic cycle such as the one sketched below:
  
  
  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.
  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:
  
  
  Due to epimerization through a common enolate ion (intermediate A) the reaction product has (R,R) cis configuration and not (R,S) trans as in the starting material. Because it is only the cis isomer that precipitates from solution this product is formed exclusively.
  

   

  Rearrangement reaction

   

  A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule. In the example below the substituent R moves from carbon atom 1 to carbon atom 2:
  
  
  Intermolecular rearrangements also take place.
  A rearrangement is not well represented by simple and discrete electron transfers (represented by curly arrows in organic chemistry texts). The actual mechanism of alkyl groups moving, as in Wagner-Meerwein rearrangement, probably involves transfer of the moving alkyl group fluidly along a bond, not ionic bond-breaking and forming. In pericyclic reactions, explanation by orbital interactions give a better picture than simple discrete electron transfers. It is, nevertheless, possible to draw the curved arrows for a sequence of discrete electron transfers that give the same result as a rearrangement reaction, although these are not necessarily realistic. In allylic rearrangement, the reaction is indeed ionic.
  Three key rearrangement reactions are 1,2-rearrangements, pericyclic reactions and olefin metathesis.
  
  
  

  1,2-rearrangements

  A 1,2-rearrangement is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. Examples are the Wagner-Meerwein rearrangement:
  
  and the Beckmann rearrangement:
  
  

  Pericyclic reactions

  A pericyclic reaction is a type of reaction with multiple carbon-carbon bond making and breaking wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Examples are hydride shifts
  
  and the Claisen rearrangement:
  
  

  Olefin metathesis

  Olefin metathesis is a formal exchange of the alkylidene fragments in two alkenes. It is a catalytic reaction with carbene, or more accurately, transition metal carbene complex intermediates.
  
  In this example, a vinyl compound is dimerized with the expulsion of ethene
  .
  

  Beckmann rearrangement

   

  The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853–1923), is an acid-catalyzed rearrangement of an oxime to an amide. Cyclic oximes yield lactams.
  
  
  This example reaction starting with cyclohexanone, forming the reaction intermediate cyclohexanone oxime and resulting in caprolactam is one of the most important applications of the Beckmann rearrangement, as caprolactam is the feedstock in the production of Nylon 6.
  The Beckmann solution consists of acetic acid, hydrochloric acid and acetic anhydride, and was widely used to catalyze the rearrangement. Other acids, such as sulfuric acid or polyphosphoric acid, can also be used. sulfuric acid is the most commonly used acid for commercial lactam production due to its formation of an ammonium sulfate by-product when neutralized with ammonia. Ammonium sulfate is a common agricultural fertilizer providing nitrogen and sulfur.
  
  

  Curtius rearrangement

   

  The Curtius rearrangement (or Curtius reaction or Curtius degradation), as first defined by Theodor Curtius, is a chemical reaction that involves the rearrangement of an acyl azide to an isocyanate. Several reviews have been published
  

  

  The isocyanate can be trapped by a variety of nucleophiles. Water is often added in order to hydrolyze the isocyanate to an amine. When done in the presence of tert-butanol, the reaction generates Boc-protected amines, useful intermediates in organic synthesis.
  Carboxylic acids 1 can be easily converted to acyl azides 3 using diphenylphosphoryl azide 2.
  

  

  Likewise, when the Curtius reaction is performed in the presence of benzyl alcohol, Cbz-protected amines are formed.
  
  

  Hofmann rearrangement

   

  The Hofmann rearrangement is the organic reaction of a primary amide to a primary amine with one fewer carbon atom.
  

  

  The reaction is named after its discoverer - August Wilhelm von Hofmann. This reaction is also sometimes called the Hofmann degradation or the Harmon Process, and should not be confused with the Hofmann elimination
  .
  

  Wolff rearrangement

   

  The Wolff rearrangement is a reaction in organic chemistry in which an α-diazocarbonyl compound is converted into a ketene by loss of dinitrogen with accompanying 1,2-rearrangement. The Wolff rearrangement yields a ketene as an intermediate product, which can undergo nucleophilic attack with weakly acidic nucleophiles such as water, alcohols, and amines, to generate carboxylic acid derivatives or undergo [2+2] cycloaddition reactions to form four-membered rings. The mechanism of the Wolff rearrangement has been the subject of debate since its first use. No single mechanism sufficiently describes the reaction, and there are often competing concerted and carbene-mediated pathways; for simplicity, only the textbook, concerted mechanism is shown below. The reaction was discovered by Ludwig Wolff in 1902. The Wolff rearrangement has great synthetic utility due to the accessibility of α-diazocarbonyl compounds, variety of reactions from the ketene intermediate, and stereochemical retention of the migrating group. However, the Wolff rearrangement has limitations due to the highly reactive nature of α-diazocarbonyl compounds, which can undergo a variety of competing reactions.
  
  
  The Wolff rearrangement can be induced via thermolysis, photolysis, or transition metal catalysis. In this last case, the reaction is sensitive to the transition metal; silver (I) oxide or other Ag(I) catalysts work well and are generally used. The Wolff rearrangement has been used in many total syntheses; the most common use is trapping the ketene intermediate with nucleophiles to form carboxylic acid derivatives. The Arndt-Eistert homologation is a specific example of this use, wherein a carboxylic acid may be elongated by a methylene unit. Another common use is in ring-contraction methods; if the α-diazoketone is cyclic, the Wolff rearrangement results in a ring-contracted product. The Wolff rearrangement works well in generating ring-strained systems, where other reactions may fail.