In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl (RR'C=O) compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds. [1] [2] [3] [4]
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl (RR'C=O) compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds. [1] [2] [3] [4]
Enolate anions are electronically related to allyl anions. The anionic charge is delocalized over the oxygen and the two carbon sites. Thus they have the character of both an alkoxide and a carbanion. [5]
Although they are often drawn as being simple salts, in fact they adopt complicated structures often featuring aggregates. [6]
Deprotonation of enolizable ketones, aromatic alcohols, aldehydes, and esters gives enolates. [8] [9] With strong bases, the deprotonation is quantitative. Typically enolates are generated from using lithium diisopropylamide (LDA). [10]
Often, as in conventional Claisen condensations, Mannich reactions, and aldol condensations, enolates are generated in low concentrations with alkoxide bases. Under such conditions, they exist in low concentrations, but they still undergo reactions with electrophiles. Many factors affect the behavior of enolates, especially the solvent, additives (e.g. diamines), and the countercation (Li+ vs Na+, etc.). For unsymmetrical ketones, methods exist to control the regiochemistry of the deprotonation. [11]
The deprotonation of carbon acids can proceed with either kinetic or thermodynamic reaction control. For example, in the case of phenylacetone, deprotonation can produce two different enolates. LDA has been shown to deprotonate the methyl group, which is the kinetic course of the deprotonation. To ensure the production of the kinetic product, a slight excess (1.1 equiv) of lithium diisopropylamide is used, and the ketone is added to the base at −78 °C. Because the ketone is quickly and quantitatively converted to the enolate and base is present in excess at all times, the ketone is unable to act as a proton shuttle to catalyze the gradual formation of the thermodynamic product. A weaker base such as an alkoxide, which reversibly deprotonates the substrate, affords the more thermodynamically stable benzylic enolate.
Enolates can be trapped by acylation and silylation, which occur at oxygen. Silyl enol ethers are common reagents in organic synthesis as illustrated by the Mukaiyama aldol reaction: [13]
In addition to the use of strong bases, enolates can be generated using a Lewis acid and a weak base ("soft conditions"):
For deprotonation to occur, the stereoelectronic requirement is that the alpha-C-H sigma bond must be able to overlap with the pi* orbital of the carbonyl:
Extensive studies have been performed on the formation of enolates. It is possible to control the geometry of the enolate: [14]
For ketones, most enolization conditions give Z enolates. For esters, most enolization conditions give E enolates. The addition of HMPA is known to reverse the stereoselectivity of deprotonation.
The stereoselective formation of enolates has been rationalized with the Ireland model, [15] [16] [17] [18] although its validity is somewhat questionable. In most cases, it is not known which, if any, intermediates are monomeric or oligomeric in nature; nonetheless, the Ireland model remains a useful tool for understanding enolates.
In the Ireland model, the deprotonation is assumed to proceed by a six-membered or cyclic [19] monomeric transition state. The larger of the two substituents on the electrophile (in the case above, methyl is larger than proton) adopts an equatorial disposition in the favored transition state, leading to a preference for E enolates. The model clearly fails in many cases; for example, if the solvent mixture is changed from THF to 23% HMPA-THF (as seen above), the enolate geometry is reversed, which is inconsistent with this model and its cyclic transition state.
If an unsymmetrical ketone is subjected to base, it has the potential to form two regioisomeric enolates (ignoring enolate geometry). For example:
The trisubstituted enolate is considered the kinetic enolate, while the tetrasubstituted enolate is considered the thermodynamic enolate. The alpha hydrogen deprotonated to form the kinetic enolate is less hindered, and therefore deprotonated more quickly. In general, tetrasubstituted olefins are more stable than trisubstituted olefins due to hyperconjugative stabilization. The ratio of enolate regioisomers is heavily influenced by the choice of base. For the above example, kinetic control may be established with LDA at −78 °C, giving 99:1 selectivity of kinetic: thermodynamic enolate, while thermodynamic control may be established with triphenylmethyllithium at room temperature, giving 10:90 selectivity.
In general, kinetic enolates are favored by cold temperatures, conditions that give relatively ionic metal–oxygen bonding, and rapid deprotonation using a slight excess of a strong, sterically hindered base. The large base only deprotonates the more accessible hydrogen, and the low temperatures and excess base help avoid equilibration to the more stable alternate enolate after initial enolate formation. Thermodynamic enolates are favored by longer equilibration times at higher temperatures, conditions that give relatively covalent metal–oxygen bonding, and use of a slight sub-stoichiometric amount of strong base. By using insufficient base to deprotonate all of the carbonyl molecules, the enolates and carbonyls can exchange protons with each other and equilibrate to their more stable isomer. Using various metals and solvents can provide control over the amount of ionic character in the metal–oxygen bond.
As powerful nucleophiles, enolates react with a variety of electrophiles. The stereoselectivity and regioselectivity is influenced by additives, solvent, counterions, etc. When the electrophiles are alkyl halides, a classic problem arises: O-alkylation vs C-alkylation. Controlling this selectivity has drawn much attention. The negative charge in enolates is concentrated on the oxygen, but that center is also highly solvated, which leads to C-alkylation. [20]
Other important electrophiles are aldehydes/ketones and Michael acceptors. [21]
Regiospecific formation is the controlled enolate formation by the specific deprotonation at one of the α-carbons of the ketone starting molecule. This provides one of the best understood synthetic strategies to introduce chemical complexity in natural product and total syntheses. A prominent example of its use is in the total synthesis of progesterone illustrated in Figure "Regiospecific enolate formation in the total synthesis of progesterone".
When ketones are treated with base, enolates can be formed by deprotonation at either α-carbon. The selectivity is determined by both the steric and electronic effects on the α-carbons as well as the precise base used (see figure ""Masked functionality" for regiospecific enolate formation" for an example of this). Enolate formation will be thermodynamically favoured at the most acidic proton which depends on the electronic stabilization of the resulting anion. However, the selectivity can be reversed by sterically hindering the thermodynamic product and therefore kinetically favouring deprotonation at the other α-carbon centre. Traditional methods for regioselective enolate formation use either electronic activating groups (e.g. aldehydes) or steric blocking groups (e.g. 1,2-ethanedithiol protected ketone).
An enone can also serve as a precursor for regiospecific formation of an enolate, here the enone is a "masked functionality" for the enolate. This process is first described by Gilbert Stork [22] who is best known for his contributions to the study of selective enolate formation methods in organic synthesis. Reacting an enone with lithium metal generates the enolate at the α-carbon of the enone. The enolate product can either be trapped or alkylated. By using "masked functionality", it is possible to produce enolates that are not accessible by traditional methods.
The "masked functionality" approach to regiospecific enolate formation has been widely used in the total synthesis of natural products. For example, in the total synthesis of the steroid hormone progesterone, [23] Stork and co-workers used the "masked functionality" to stereospecifically construct one of the quaternary carbons in the molecule.
Aza enolates (also known as imine anions, enamides, metallated Schiff bases, and metalloenamines) are nitrogen analogous to enolates. [24] When imines get treated with strong bases such as LDA, highly nucleophilic aza enolates are generated.
The major benefit of using aza enolates is that they don't undergo self-condensation (i.e. aldol reaction for aldehydes) in a basic or neutral solution, but rather they favor alkylation on the alpha-carbon. [25] This is mainly because imines contain carbon-nitrogen double bonds unlike aldehydes, which contain oxygen-carbon double bonds. Since oxygen is more electronegative than nitrogen, it withdraws more electron density from the carbonyl carbon, inducing a greater partially positive charge on the carbon. Therefore, with more electrophilic carbon, aldehydes allow for better nucleophilic addition to the carbon on the carbon-oxygen double bond.
On the other hand, imine has less electronegative nitrogen which induces a weaker partially positive charge on the carbonyl-carbon. As a result, while imines can still react with organolithiums, they don't react with other nucleophiles (including aza enolates) to undergo nucleophilic additions. [26]
Instead, aza enolates react similarly to enolates, forming SN2 alkylated products. [25] Through nitrogen lone pair conjugation, β-carbon becomes a nucleophilic site, permitting aza enolates to undergo alkylation reactions. [27] Thus, aza enolates can react with numerous electrophiles like epoxides and alkyl halides to form a new carbon-carbon bond on β-carbon. [24]
Two potential reaction mechanisms are shown below:
Since epoxide is a three-membered ring molecule, it has a high degree of ring strain. Although the carbons in the ring system are tetrahedral, preferring 109.5 degrees between each atom, epoxide strains the ring angles into 60 degrees. To counter this effect, the nucleophilic aza enolates easily react with epoxides to reduce their ring strains.
Besides reacting with epoxides, aza enolates can also react with alkyl halides (or allyl halides as depicted above) to form a new carbon-carbon sigma bond. This reaction is one of the key steps in the synthesis of the male aggression pheromone, Oulema melanopus. [29] Aza enolate is generated by LDA reacting with pivaldehyde, which then reacts with an alkyl halide to form an Oulema melanopus intermediate.
Aza enolates can also be formed with Grignard reagents and react with other soft electrophiles, including Michael receptors. [24]
The aldol reaction is a reaction in organic chemistry that combines two carbonyl compounds to form a new β-hydroxy carbonyl compound. Its simplest form might involve the nucleophilic addition of an enolized ketone to another:
An enamine is an unsaturated compound derived by the condensation of an aldehyde or ketone with a secondary amine. Enamines are versatile intermediates.
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.
In organic chemistry, alkenols are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene (olefin) with a hydroxyl group attached to one end of the alkene double bond. The terms enol and alkenol are portmanteaus deriving from "-ene"/"alkene" and the "-ol" suffix indicating the hydroxyl group of alcohols, dropping the terminal "-e" of the first term. Generation of enols often involves deprotonation at the α position to the carbonyl group—i.e., removal of the hydrogen atom there as a proton H+. When this proton is not returned at the end of the stepwise process, the result is an anion termed an enolate. The enolate structures shown are schematic; a more modern representation considers the molecular orbitals that are formed and occupied by electrons in the enolate. Similarly, generation of the enol often is accompanied by "trapping" or masking of the hydroxy group as an ether, such as a silyl enol ether.
Lithium diisopropylamide is a chemical compound with the molecular formula LiN(CH 2)2. It is used as a strong base and has been widely utilized due to its good solubility in non-polar organic solvents and non-nucleophilic nature. It is a colorless solid, but is usually generated and observed only in solution. It was first prepared by Hamell and Levine in 1950 along with several other hindered lithium diorganylamides to effect the deprotonation of esters at the α position without attack of the carbonyl group.
The Robinson annulation is a chemical reaction used in organic chemistry for ring formation. It was discovered by Robert Robinson in 1935 as a method to create a six membered ring by forming three new carbon–carbon bonds. The method uses a ketone and a methyl vinyl ketone to form an α,β-unsaturated ketone in a cyclohexane ring by a Michael addition followed by an aldol condensation. This procedure is one of the key methods to form fused ring systems.
The Claisen condensation is a carbon–carbon bond forming reaction that occurs between two esters or one ester and another carbonyl compound in the presence of a strong base. The reaction produces a β-keto ester or a β-diketone. It is named after Rainer Ludwig Claisen, who first published his work on the reaction in 1887. The reaction has often been displaced by diketene-based chemistry, which affords acetoacetic esters.
The Henry reaction is a classic carbon–carbon bond formation reaction in organic chemistry. Discovered in 1895 by the Belgian chemist Louis Henry (1834–1913), it is the combination of a nitroalkane and an aldehyde or ketone in the presence of a base to form β-nitro alcohols. This type of reaction is also referred to as a nitroaldol reaction. It is nearly analogous to the aldol reaction that had been discovered 23 years prior that couples two carbonyl compounds to form β-hydroxy carbonyl compounds known as "aldols". The Henry reaction is a useful technique in the area of organic chemistry due to the synthetic utility of its corresponding products, as they can be easily converted to other useful synthetic intermediates. These conversions include subsequent dehydration to yield nitroalkenes, oxidation of the secondary alcohol to yield α-nitro ketones, or reduction of the nitro group to yield β-amino alcohols.
The Shapiro reaction or tosylhydrazone decomposition is an organic reaction in which a ketone or aldehyde is converted to an alkene through an intermediate hydrazone in the presence of 2 equivalents of organolithium reagent. The reaction was discovered by Robert H. Shapiro in 1967. The Shapiro reaction was used in the Nicolaou Taxol total synthesis. This reaction is very similar to the Bamford–Stevens reaction, which also involves the basic decomposition of tosyl hydrazones.
In stereochemistry, a chiral auxiliary is a stereogenic group or unit that is temporarily incorporated into an organic compound in order to control the stereochemical outcome of the synthesis. The chirality present in the auxiliary can bias the stereoselectivity of one or more subsequent reactions. The auxiliary can then be typically recovered for future use.
In organic chemistry, umpolung or polarity inversion is the chemical modification of a functional group with the aim of the reversal of polarity of that group. This modification allows secondary reactions of this functional group that would otherwise not be possible. The concept was introduced by D. Seebach and E.J. Corey. Polarity analysis during retrosynthetic analysis tells a chemist when umpolung tactics are required to synthesize a target molecule.
The Darzens reaction is the chemical reaction of a ketone or aldehyde with an α-haloester in the presence of a base to form an α,β-epoxy ester, also called a "glycidic ester". This reaction was discovered by the organic chemist Auguste Georges Darzens in 1904.
In organosilicon chemistry, silyl enol ethers are a class of organic compounds that share the common functional group R3Si−O−CR=CR2, composed of an enolate bonded to a silane through its oxygen end and an ethene group as its carbon end. They are important intermediates in organic synthesis.
In organic chemistry, aldol reactions are acid- or base-catalyzed reactions of aldehydes or ketones.
Acetoacetic ester synthesis is a chemical reaction where ethyl acetoacetate is alkylated at the α-carbon to both carbonyl groups and then converted into a ketone, or more specifically an α-substituted acetone. This is very similar to malonic ester synthesis.
Nitrile anions is jargon from the organic product resulting from the deprotonation of alkylnitriles. The proton(s) α to the nitrile group are sufficiently acidic that they undergo deprotonation by strong bases, usually lithium-derived. The products are not anions but covalent organolithium complexes. Regardless, these organolithium compounds are reactive toward various electrophiles.
In organic chemistry, the Baylis–Hillman, Morita–Baylis–Hillman, or MBH reaction is a carbon-carbon bond-forming reaction between an activated alkene and a carbon electrophile in the presence of a nucleophilic catalyst, such as a tertiary amine or phosphine. The product is densely functionalized, joining the alkene at the α-position to a reduced form of the electrophile.
The Enders SAMP/RAMP hydrazone alkylation reaction is an asymmetric carbon-carbon bond formation reaction facilitated by pyrrolidine chiral auxiliaries. It was pioneered by E. J. Corey and Dieter Enders in 1976, and was further developed by Enders and his group. This method is usually a three-step sequence. The first step is to form the hydrazone between (S)-1-amino-2-methoxymethylpyrrolidine (SAMP) or (R)-1-amino-2-methoxymethylpyrrolidine (RAMP) and a ketone or aldehyde. Afterwards, the hydrazone is deprotonated by lithium diisopropylamide (LDA) to form an azaenolate, which reacts with alkyl halides or other suitable electrophiles to give alkylated hydrazone species with the simultaneous generation of a new chiral center. Finally, the alkylated ketone or aldehyde can be regenerated by ozonolysis or hydrolysis.
Alpha-substitution reactions occur at the position next to the carbonyl group, the α-position, and involve the substitution of an α hydrogen atom by an electrophile, E, through either an enol or enolate ion intermediate.
Rearrangements, especially those that can participate in cascade reactions, such as the aza-Cope rearrangements, are of high practical as well as conceptual importance in organic chemistry, due to their ability to quickly build structural complexity out of simple starting materials. The aza-Cope rearrangements are examples of heteroatom versions of the Cope rearrangement, which is a [3,3]-sigmatropic rearrangement that shifts single and double bonds between two allylic components. In accordance with the Woodward-Hoffman rules, thermal aza-Cope rearrangements proceed suprafacially. Aza-Cope rearrangements are generally classified by the position of the nitrogen in the molecule :
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