reactions of enolates with carbonyl compounds
TRANSCRIPT
Reactions of Enolates with Carbonyl Compounds(The Aldol and Claisen reactions)
By Vijay Kumar Maurya
IntroductionFormation of enolates or enols by base or acid and then nucleophillic attack by enols and enolates on carbonyl group of aldehyde and ketone or acylating agents
A brief discussion on:• Explanation of reactions through mechanism• Specific names of reaction• Reaction conditions• Regio-selectivity• Control of the reaction• Uses of it
MechanismThe simplest available enolisable is acetaldehyde i.e. CH3CHO and when it is reacted with a small amount of NaOH,
As the final product contains aldehyde and alcohol group, so it is named as aldol
MechanismThe same reaction occurs with ketones(say )
But with more base, further reaction occurs because aldol product dehydrated rather easily under the reaction conditions to give stable conjugated unsaturated carbonyl compounds
Depending upon the reactions conditions base catalysed reaction gives sometime elimination product and sometimes aldol(i.e. high temperature, strong base long time will favor the elimination product
MechanismAcid catalyzed mechanism, enols are less nucleophilic than enolates, and the reactions occurs because the electrophilic carbonyl is protonated(i.e. the carbonyl carbon is activated)
MechanismIf there are two different carbonyl compounds, then it is called Cross Condensation reactions. This reaction to work well two conditions are there:• One partner only must be capable of enolisation• The other partner must be incapable of enolisation and be more
electrophilic than the other enolisable partner
If cross condensation reaction happens between acetaldehyde and formaldehyde, then
Reaction between acetaldehyde and formaldehyde
Why do we need to control the reaction?When a mixture of two carbonyl compounds(A and B) is taken both of them having α- hydrogen atom and treated with a base, then four aldol products can be formed, i.e.• If A acts as both electrophile and nucleophile• If B acts as both electrophile and nucleophile• If A acts as both electrophile and B as nucleophile• If B acts as both electrophile and A as nucleophile
To overcome this situation some specific enol equivalent can be used to control the reactions, like, lithium enolates, silyl enols ethers, etc
Control of the reactionUsing lithium enolates, prepared at low temperature(-78 ˚C ) in THF
Control of the reactionUsing silyl enols ethers, prepared in weak base such as tertiary amine and for trapping the enolate Me3SiCl.
UsesIn preparation of alcohol,
2 CH3CHO CH3CH(OH) CH2CHO CH3CH=
CHCHO
CH3CH2CH2CH2OHIn synthesis of vitamin A ,
In aldol condensation of glycol-aldehyde gives monosaccharides.
3 CH2(OH)CHO C6H12O6 ( Formose )
OH- H2O Δ
H2/Ni
NaOH
BF3
AcOH
Citral
References• A Guidebook to Mechanism in Organic Chemistry, by Peter
Sykes.
• Organic Chemistry: Jonathan Clayden, Nick Greeves and Stuart Warren.
A lot of interesting things are out there to be explored, so let us keep our eyes open.
A very thank to all …