Mechanism aldol condensation

Early patents indicated that because water inhibits the aldol condensation mechanism, it was necessary to dry recycle acetone to less than 1% water (139—142). More recent reports demonstrate DAA production from waste acetone containing 10—50% water (143), and enhanced DAA production over anion-exchange resins using acetone feeds that contain 3—10% water (144,145). 

The alkadienals could be formed from the autoxidation of PUFA and may contribute desirable arenas to freshly prepared foods (19). Further degradation of alkadienals often increased undesirable flavors. Josephson and Lindsay demonstrated that 2,4-decadienal could produce 2-octenal and ethanal (20) and 2,6-nonadienal could produce 4-heptenal and ethanal (21) via retro-aldol condensation mechanisms. Hsieh et al. (22) reported that iscmers of various alkadienals and alkatrienals gave green, greasy and oxidized fish oil odors in crude menhaden fish oil. 

Nucleophiles 20.5 Aldol Condensation Mechanisms in Motion Mechanism of the Aldol Condensation (page 873) Coached Tutorial Problem Aldol Condensations (page 879)... 

Only the aldol condensation mechanism is shown, not the dehydration. 

As illustrated in Scheme 4, the gas-phase base-catalyzed aldol condensation mechanism proceeds through a bifunctional pathway because of the necessary presence of both the surface oxygen anions and Lewis acid sites. However, the role of the latter is limited to the initial activation of the reactant molecules and to stabilization of the carbanionic intermediates. A discussion on the bifunctional nature of aldol condensation reactions can be formd in Refs. (8) and (14). 

The mechanism of the reaction between aromatic aldehydes and esters probably involves the intermediate formation of an aldol (hence the name— Claisen aldol condensation) ... 

It is also possible to carry out the aldol condensation under acidic conditions. The reactive nucleophile is then the enol. The mechanism, as established in detail for acetaldehyde, involves nucleophilic attack of the enol on the protonated aldehyde. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

Tire mechanism of the Claisen condensation is similar to that of the aldol condensation and involves the nucleophilic addition of an ester enolate ion to the carbonyl group of a second ester molecule. The only difference between the aldol condensation of an aldeiwde or ketone and the Claisen condensation of an ester involves the fate of the initially formed tetrahedral intermediate. The tetrahedral intermediate in the aldol reaction is protonated to give an alcohol product—exactly the behavior previously seen for aldehydes and ketones (Section 19.4). The tetrahedral intermediate in the Claisen reaction, however, expels an alkoxide leaving group to yield an acyl substitution product—exactly the behavior previously seen for esters (Section 21.6). The mechanism of the Claisen condensation reaction is shown in Figure 23.5. 

Base-induced eliminative ring fission, in which both the double bond and the sulfone function take part, has been observed in thiete dioxides253. The reaction can be rationalized in terms of initial Michael-type addition to the double bond of the ring vinyl sulfone, followed by a reverse aldol condensation with ring opening. The isolation of the ether 270c in the treatment of 6c with potassium ethoxide (since the transformation 267 -> 268 is not possible in this case) is in agreement with the reaction mechanism outlined in equation 101253. 

Ketones can react with themselves in a process known as aldol condensation. The mechanism for this reaction in acidic solution is shown here. Write the overall reaction, identify any intermediates, and determine the role of the hydrogen ion. 

Classical examples of this type of reaction are the various dimethylaminobenz-aldehyde reagents (q.v.) and vanillin-acid reagents, of which one, the vanillin-phosphoric acid reagent, is already included in Volume 1 a. The aldol condensation of estrogens is an example for the reaction mechanism (cf. Chapter 2, Table 6). According to Maiowan indole derivatives react in a similar manner [1]. Longo has postulated that catechins yield intensely colored triphenylmethane dyes [2]. 

Lipase from C.antarctica also catalyzes carbon-carbon bond formation through aldol condensation of hexanal. The reaction is believed to proceed according to the same mechanism as the Michael additions [113]. Lipase from Pseudomonas sp. 

The solid base catalysed aldol condensation of acetone was performed over a CsOH/Si02 catalyst using a H2 carrier gas. The products observed were diacetone alcohol, mesityl oxide, phorone, iso-phorone and the hydrogenated product, methyl isobutyl ketone. Deuterium tracer experiments were performed to gain an insight into the reaction mechanism. A mechanism is proposed. 

This possible mechanism should be evaluated in relation to the catalysts. If the catalytic action is to be ascribed to the acid character of the catalysts, the condensation under consideration may differ from the ordinary aldol condensation, which is catalyzed preferentially by basic agents. Nevertheless, many condensations of the aldol type are effected with the aid of acidic reagents. Moreover, the condensation of sugars with dicarbonyl compounds has been carried out in aqueous alcoholic media which are non-acidic hence, there also exists the possibility of a mechanism catalyzed simultaneously by acid and by base, somewhat like that suggested by Lowry46 in another connection. A transition state with an amphiprotic structure has been postulated. Its formation can be catalyzed by either acids or bases. 

One of the several mechanisms for decarboxylation is the reverse of the familiar carboxylation reaction of organometallic compounds or carbanions. Many of the acids RCOOH that are readily decarboxylated in basic media are compounds for which the corresponding R( ) is a comparatively stable carbanion.405 The postulated intermediate has actually been trapped or diverted in a few cases as the product of an aldol condensation.406... 

The mechanism of the reaction is undoubtedly as follows when the sulfuric acid and acetone are in contact for long periods of time, several molecules of the acetone condense to form aldol condensation products. These do not break down into mesitylene until the temperature is raised in the second part of the experiment. 

The formation mechanisms and the nature of chromophores in PET are still a matter of discussion. Postulated chromophores are polyenaldehydes from the aldol condensation of acetaldehyde [73] and polyenes from polyvinyl esters [69], as well as quinones [74, 75], Goodings [73] has proposed aldol condensation as forming poly conjugated species by subsequent reactions of acetaldehyde molecules (Figure 2.16). 

However, cyanide ion is not suitable for inducing a benzoin-type condensation between two aliphatic aldehydes, since the basic character of this ion induces an aldol condensation between them. In Nature, nevertheless, condensations of this type take place easily. As Breslow proposed in 1958 [8], such condensations are catalysed by thiamine pyrophosphate 6 (or cocarboxylase), the active part of which is the conjugate base of the "thiazolium cation present in it. According to Breslow [8a], the mechanism is, in fact, identical to that described for the cyanide ion (see Scheme 5.7) that is to say, the conjugate base of thiamine (TPP ) reacts with an "aldehyde equivalent -such as an a-ketoacid 2- to generate the corresponding "active aldehyde" 8 with umpoled reactivity, which then reacts with the electrophile to give finally, after elimination of "thiamine anion", a 1,2-D system (9). 

A possible mechanism for the P-alkylation of secondary alcohols with primary alcohols catalyzed by a 1/base system is illustrated in Scheme 5.28. The first step of the reaction involves oxidation of the primary and secondary alcohols to aldehydes and ketones, accompanied by the transitory generation of a hydrido iridium species. A base-mediated cross-aldol condensation then occurs to give an a,P-unsaturated ketone. Finally, successive transfer hydrogenation of the C=C and C=0 double bonds of the a,P-unsaturated ketone by the hydrido iridium species occurs to give the product. 

In the Clemmensen reduction of 1,4-cyclohexanedione, all the products isolated from the reduction of 2,5-hexanedione were found in addition to 2,5-hexanedione (20%) and 2-methylcyclopentanone (6%). The presence of the two latter compounds reveals the mechanism of the reduction. In the first stage the carbon-carbon bond between carbons 2 and 3 ruptured, and the product of the cleavage, 2,5-hexanedione, partly underwent aldol condensation, partly its own further reduction [927], The cleavage of the carbon-carbon bond in 1,4-diketones was noticed during the treatment of 1,2-diben-zoylcyclobutane which afforded, on short refluxing with zinc dust and zinc chloride in ethanol, an 80% yield of 1,6-diphenyl-1,6-hexanedione [75<5]. 

The aldol formed by the aldol reaction, especially if heated, can react further. The heating causes dehydration (loss of H2O), and the overall reaction involving an aldol reaction followed by dehydration is the aldol condensation. The product of an aldol condensation, favored by the presence of extended conjugation, is an a,(3-unsaturated aldehyde (an enal) or ketone. The mechanism for dehydration (Figure 11-13) begins where the mechanism of the aldol reaction (Figure 11-12) ends. This process works better if extended conjugation results. The aldol reaction and condensation are reversible. 

The Claisen condensation bears some resemblance to the Aldol condensation seen in Chapter 11. The initial step in the mechanisms are very similar in that in both cases a resonance-stabilized ion is formed. 

The condensation of aldehydes and ketones with active hydrogen atoms is called Knoevenagel condensation. It is related to an aldol condensation and commonly is used to produce enones (a compound with a carbon-carbon double bond adjacent to a carbonyl). The process requires a weak base (an amine). A typical excimple and mechanism eire presented in Figure 15-22. 

Diacetone alcohol is a solvent used in hydraulic fluids and printing inks. Recall that the aldol condensation is an example of a variety of carbanion reactions used to make large molecules from smaller ones. An aldehyde or a ketone with at least one hydrogen on the carbon next to the carbonyl will react to give the aldol condensation. The mechanism is given as follows. 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

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