Aldol reaction protic solvents

For the other broad category of reaction conditions, the reaction proceeds under conditions of thermodynamic control. This can result from several factors. Aldol condensations can be effected for many compounds using less than a stoichiometric amount of base. Under these conditions, the aldol reaction is reversible, and the product ratio will be determined by the relative stability of the various possible products. Conditions of thermodynamic control also permit equilibration among all the enolates of the nucleophile. The conditions that permit equilibration include higher reaction temperatures, protic solvents, and the use of less tightly coordinating cations. 

Aldehydes and ketones with an a hydrogen atom undergo a base-catalyzed carbonyl condensation reaction called the aldol reaction. For example, treatment of acetaldehyde with a base such as sodium ethoxide or sodium hydroxide in a protic solvent leads to rapid and reversible formation of 3-hydroxybutanal, known commonly as aldol (aidehyde + alcohol), hence the general name of the reaction. 

It has also been shown that dimethylsilyl enolates can be activated by diisopropylamine and water and exhibit a high reactivity toward iV-tosyl imines to give Mannich-type reaction products in the absence of a Fewis acid or a Bronsted acid.51 For example, the reaction of [(1-cyclohexen-l-yl)oxy]dimethylsilane with 4-methyl-A -(phenylmethylene)benzene sulfonamide gave re/-4-methyl-N- (f )-[(15)-(2-oxocyclohexyl)phenyl-methyl] benzenesulfonamide (anti-isomer) in 91% yield stereoselectively (99 1 anti syn) (Eq. 11.30). On the other hand, Fi and co-workers reported a ruthenium-catalyzed tandem olefin migration/aldol and Mannich-type reactions by reacting allyl alcohol and imine in protic solvents.52... 

Reaction between Two Molecules of the Same Aldehyde. Hydroxide or alkoxide bases are used in protic solvents, and the reaction is quite feasible. Many aldehydes have been converted to aldols and/or their dehydration products in this manner. The most effective catalysts are basic ion-exchange resins. Of course, the aldehyde must possess an a hydrogen. 

The intramolecular aldol condensation is well known, and aldol reactions are often used to close five- and six-membered rings. Because of the favorable entropy (p. 303), such ring closures generally take place with ease when using hydroxide or alkoxide bases in protic solvents. In aprotic solvents with amide bases,... 

The classical aldol reaction usually employs a basic catalyst in a protic solvent. However, this process is often associated with undesirable side products. The... 

Condensation Reactions. Traditionally, intermolecular aldol condensation reactions have been performed under equilibrating conditions using weaker bases than r-BuOK in protic solvents. Since the mid-1970s, new methodology has focused on directed aldol condensations which involve the use of preformed Lithium and (jroup 2 enolates, (Troup 13 enolates, and transition metal enolates. Although examples of the use of f-BuOK in intramolecular aldol condensations are limited, complex diketones... 

An aldol addition is an equilibrium reaction when it is conducted in a protic solvent with a base such as hydroxide or an alkoxide. The mechanism for an aldol addition involving an aldehyde is shown on the next page. 

We shall therefore consider crossed aldol condensations by two general approaches that allow control over the distribution of products. The first approach hinges on structural factors of the carbonyl reactants and the role that favorable or unfavorable aldol addition equilibria play in determining the product distribution. In this approach relatively weak bases such as hydroxide or an alkoxide are used in a protic solvent such as water or an alcohol. The second approach, called a directed aldol reaction, involves use of a strong base such as LDA in an aptotic solvent. With a strong base, one reactant can be converted essentially completely to its enolate, which can then be allowed to react with the other carbonyl reactant. 

One possible reaction for 60 is an intramolecular condensation with the other carbonyl (see Chapter 22, Section 22.6, for reactions of this type), but that would lead to a four-membered ring product, 61. The activation barrier to form this strained ring is high, so this reaction is slow (see Chapter 8, Section 8.5.3). The reaction conditions favor thermodynamic control (protic solvent, hydroxide, heat see Chapter 22, Section 22.4.2), which means that enolate anion 60 is in equilibrium with the neutral diketone. Further reaction with hydroxide generates the kinetic enolate anion 62 as part of the equilibrium mixture. If 62 attacks the carbonyl in an intramolecular aldol reaction (Chapter 22, Section 22.6), a six-membered ring is formed (63) in a rapid and highly favorable process. 

When the aldol-type addition is carried out under classical conditions (protic solvents and basic or acid catalysis), by-products such as dimers, polymers and a,P-unsaturated carbonyl compounds are frequently formed. Variants such as the Mukaiyama reaction [2] and the use of lanthanide triflates as catalysts [3] generally reduce these side-reactions. A significant contribution has also come from investigations on the use of water as the reaction medium. 

Several reaction conditions feature in this traditional transformation. First, the reaction is run in protic solvents and can be mediated either by acid or by base. Second, the reaction is reversible, particularly under these conditions. Finally, the enol or the enolate, vhich acts as a nucleophile, is inevitably generated in the presence of the aldehyde or ketone that functions as an electrophile. The aldol reaction performed under these conditions (Scheme 1.1) vas the state of the art until the early nineteen-seventies, vhen the chemistry of preformed enolates emerged. 

It is reported that enol ethers react with acetals or ketals, promoted by Lewis acids, to give aldol-type adducts these reactions of alkyl enol ethers are, however, often accompanied by undesired side reactions [21]. Further, is difficult to perform crossed-aldol reactions selectively because conventional aldol reactions are conducted under equilibrium conditions using a basic or acidic catalyst in protic solvents [22]. Detailed studies of this new aldol reaction of silicon enolates, however, reveal a number of advantages over conventional methods. 

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