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CH10136 > Carbonyl chemistry- The Aldol Reaction > Flashcards

Carbonyl chemistry- The Aldol Reaction Flashcards

1
Q

What is an aldol reaction

A
  1. Key C-C bond forming reaction between 2 carbonyl species
  2. New C-C bond is formed from alpha-carbon to carbonyl carbon
  3. Requires enolate/enol and electrophilic carbonyl species
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2
Q

What are some generalisations about the aldol reaction

A
  1. Can be acid catalysed but enol under acid
  2. Can be base catalysed but enolate under base
  3. Any electrophilic carbonyl species is a suitable electrophile
  4. Any carbonyl species with an acid a-proton is a suitable nucleophile
  5. Can form multiple stereogenic centres
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3
Q

Describe the base catalysed aldol

A
  1. Only need a small amount of base- weak bases where pKa of conjugate acid is lower than ketone
  2. Only small amounts of enolate is formed but it is swamped by reactive carbonyl
  3. As catalysed by base this can go on to do further reactions and side reactions
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4
Q

What side reactions might take place after base catalysed aldol formation

A
  1. Acidic proton present which can be attacked by OH-

2. Forms a dehydration product with removal of OH group

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5
Q

Describe the acid catalysed aldol formation

A
  1. Only need small amount of acid
  2. Form the enol
  3. Again eliminate OH- but easier under acidic- E1
  4. Issues with selectivity
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6
Q

What is a self condensation- aldol

A
  1. a dimerisation of a single carbonyl compound to form dehydration product
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7
Q

What are issues with self condensation- aldol reactions

A
  1. Often the product can be further deprotonated as there are still acidic protons and base left
  2. Leads to by-product formation
  3. By-product formation can be restricted if enolisation can only occur at one carbon - e.g. benzene ring attached to one side or (CH3)3
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8
Q

What is a cross condensation

A
  1. Perform an aldol between 2 different carbonyl species
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9
Q

What are problems with cross condensation

A
  1. 4 possible aldol products- before thinking about condensation products or second enolisations
  2. Practically not most attractive- not much control
  3. some methods exist to control this
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10
Q

For crossed condensations to work well, what conditions must be met

A
  1. Only one carbonyl must be capable of enolisation - nucleophile
  2. The other partner must be incapable of enolisation and more electrophilic than the enolisable partner
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11
Q

What determines carbonyls electrophilicity

A
  1. How reactive they are
  2. electronegativity of C=O
  3. Acyl chlorides, acid anhydrides, aldehydes are all more electrophilic than ketones
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12
Q

What determines if something can be enolised

A
  1. Presence of acidic C-H
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13
Q

What are the issues of forming enol/enolates in presence of aldehyde and ketones particularly for cross condensation

A
  1. Multiple possible aldol products
  2. Multiple possible dehydration products
  3. By product formation
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14
Q

What is the solution to the issues of froming enol/enolates for cross condensation

A
  1. Using specific enol equivalents- only produce aldol
  2. Lithium enolate, enamine, silyl enol ether
  3. Have reactivity of enols/enolates, but can be prepared prior to any reaction with electrophilic carbonyl.
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15
Q

Describe use of lithium enolates in aldol reactions

A
  1. Lithium enolates can be prepared by reaction of LDA with ketone at low temperature
  2. Once formed they react with electrophilic partner
  3. No base present so only aldol product formed
  4. Doesn’t work for aldehyde as are too reactive so would quickly self condense
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16
Q

What are the two ways to produce a silyl enol ether

A
  1. Quench lithium enolate with Me3SiCl

2. Via weak base in presence of Me3SiCl- add together

17
Q

Which is more reactive lithium enolates or silyl enol ether and what does this mean for their aldol reaction

A
  1. Silyl enol ethers are less reactive
  2. Need to make electrophile more reactive
  3. Lewis acid is needed to get the ether to react
18
Q

How does the lewis acid make the electrophile more electrophilic

A
  1. Lowers the LUMO of carbonyl sufficiently for reaction to take place
19
Q

Describe the stereochemistry in aldol reactions

A
  1. Forming a new C-C bond
  2. Can form a single new stereogenic centre
  3. Other times can form multiple stereogenic centres

CH10136 (30 decks)

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