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Transcript of Lecture Notes Chem 51C S. King - UC Irvine, UCI...
79
Lecture Notes Chem 51C
S. King
Chapter 23 Substitution Reactions of Carbonyl Compounds at the α-Carbon
I. Acidity of α-Hydrogens The hydrogen atoms bound to the carbon adjacent to a carbonyl carbon of an aldehyde, ketone, or ester, are acidic enough to be removed by a strong base. Once deprotonated, the carbanion generated is stabilized by delocalization of the (−)-charge to the oxygen atom of the carbonyl.
compare acidities:
Q. Why is the α-hydrogen of a carbonyl compound so much more acidic than the typical H bound to carbon?
A. Three reasons: 1. The carbonyl is strongly electron withdrawing.
2. Loss of the α-hydrogen gives a resonance-stabilized anion.
R
O
C
H
C C
H
> >alcohols, H2O aldehydes & ketones HC CCH3esters !
R
O
C
H
CH2
80
3. The negative charge of the enolate is delocalized onto oxygen, an electronegative element.
Nitroalkanes, nitriles and N,N-disubstituted amides also have unusually acidic α-hydrogens. In each of these, electrons left behind when the proton is removed can be delocalized onto an atom that is more electronegative than carbon.
☞ Remember: Treatment of carboxylic acids and 1° & 2° amides with base does not result in removal of the α-hydrogen. Why? Their O–H, and N–H hydrogens are far more acidic (review acidity of carbonyl compounds in chapter 19).
H2C CH
H
CH2 H2C CH
H
OCompare:
H2C C
H
H2C N
H
Compare: N
O
O H2C C
H
O
N(CH3)2
H3C OH
O
H3C NH2
O
81
II. Enolization of Carbonyl Compounds Carbonyl compounds with α-hydrogens are in equilibrium with vinylic alcohol isomers called enols. The two isomers that interconvert are called tautomers.
Most ketones and aldehydes are considerably more stable than the corresponding enols:
There are exceptions, however:
O OH
H3O+ or HO!
H2O
CH2 CH
O OH
CH2 CCH3
O OH
O OH
O OH
Keto Form Enol form % Enol @ equilibrium
6.0 ! 10"5
6.0 ! 10"7
1.0 ! 10"6
4.0 ! 10"5
CH3CH
CH3CCH3
OHH
OH
H3O+ or HO!
H2O
82
The enols of β-dicarbonyls are also relatively stable:
Two factors that contribute to the stabilization of β-dicarbonyls:
• Conjugation of the carbonyl group with the enol double bond.
• Stable H-bonded 6-membered ring structure of the enol form.
Q: What determines the percentage of enol form present at equilibrium? A: The % of enol at equilibrium depends on the structure of the carbonyl compound, which determines the acidity of the α-hydrogens. The more
resonance stabilization, the greater the % of enol at equilibrium.
☛ The acidity or pKa can be correlated to the extent of keto-enol tautomerization at equilibrium:
O O OH O
H3O+ or HO!
H2O
83
• Ability of groups to stabilize an enolate anion: • A methylene group that is α to two carbonyl, nitro, or cyano groups is called an
active methylene group. The hydrogens of an active methylene group are easily removed by bases such as alkoxide because the resulting conjugate base is highly stabilized by resonance.
CH3 CH CH3
O O
CH3 CH OCH3
O O
H
N
O2N CH OCH2CH3
C
O
CH C N
H
H
H
CH3O CH OCH3
O O
H
CH2 CH3
O
H
CH2 OCH3
O
H
H
H
CH2NO2
CH2CN
84
Compare:
Q: How would you completely deprotonate acetone? A: The formation of an enolate is an acid-base equilibrium, so the stronger the base, the more enolate that forms. The most common base used to completely deprotonate a simple aldehyde or ketone is LDA:
Synthesis of LDA:
H3C
O
CCH3
+ OCH3
H3C
O
C + OCH3CCH3
O
N
LDA (lithium diisopropyl amide)
Li
NN H + CH3CH2CH2CH2Li-78°C
LiTHF
85
Typical reaction:
Why LDA?
A. Mechanism of Keto-Enol Tautomerization 1. Base Catalyzed: Step 1: loss of proton from α-carbon Step 2: protonation of carbonyl
♦ For most simple ketones, the keto form is more stable than the enol form. Nevertheless the enol is a crucial intermediate in a variety of reactions with ketones & aldehydes.
♦ The enolate ion is a powerful nucleophile.
CH3
O
CCH3
N+-78°C
LiTHF
86
2. Acid Catalyzed: Step 1: protonation of carbonyl Step 2: loss of proton from α-carbon
♦ An enol has an electron-rich double bond. When electrons from this double bond attack an electrophile, the resulting cation is stabilized through resonance with the oxygen atom.
III. Reactions at the α-Carbon Enols and enolates are nucleophilic, and therefore, react with electrophiles such as halogens, alkyl halides, and other carbonyl groups. ☞ in base ☞ in acid
enolate ion is the nucleophile enol is the nucleophile
87
A. Racemization
Because the formation of enols, and the reverse reaction, conversion of enols into carbonyl compounds is catalyzed by both acids and bases, when a ketone or aldehyde with a chiral α-carbon is allowed to sit in solution with a trace of acid or base, racemization occurs.
Mechanism (base catalyzed):
B. Deuterium Exchange
α-Hydrogens can also be exchanged for deuteriums by a similar process in either acid or base.
CH3
O
trace base or acidH Et
O
D3O+ or DO!
D2O
88
Mechanism (acid catalyzed): C. α-Halogenation 1. Acid catalyzed Halogenation of Aldehydes and Ketones
Mechanism: In acid, an enol is the reactive intermediate
O
H3O+Br2
CH3CCH3
89
2. Base-Promoted Halogenation of Aldehydes and Ketones: In the base-promoted reaction, all enolizable H’s are substituted with halogens.
If a methyl ketone is used:
Mechanism: In base, an enolate ion is the reactive intermediate.
O
NaOHBr2
O
RCCH3NaOHBr2
90
When iodine & NaOH are used in is used in this reaction, it is called an iodoform reaction. In the presence of a methyl ketone, a yellow precipitate (HCI3)forms. This is a chemical test for the presence of a methyl ketone.
Q: Why is multiple halogenation seen with base-promoted halogenation,
but only monohalogenation with acid-catalyzed halogenation? A: Compare:
D. α- Halogenated Carbonyl Compounds in Synthesis
When the α-position is halogenated, it becomes electrophilic, and can therefore be attacked by nucleophiles.
Br
O
H
in base:
Br
O
in acid:
H
Nu:
OBr
91
compare with:
Now we can incorporate both electrophiles and nucleophiles into the α-position!
A-halo ketones can also be converted into α,β−unsaturated ketones:
IV. Direct Alkylation of Enolate Ions General Reaction:
OH
B:
OBr
C N
OBr Li2CO3
DMFLiBr
RO
O
CCH3
N+!78 °C
LiTHF
92
• This is an SN2 reaction and is subject to the same considerations as the SN2 reaction:
1° Alkyl halides or tosylates (AKA: alkylating agents) work great! 2° Alkyl halides or tosylates: Not great. 3° Alkyl halides: Doesn’t work at all. ✦ This reaction works well for esters and nitriles:
✦ The enolates of ketones can also react with alkyl halides, but the reaction is more complicated:
CH3CH2 C N1. LDA, THF, -78°C
2. CH3CH2Br
93
Problem: There are two sides where the enolate can form:
By adjusting the conditions, it is possible to establish either kinetic or thermodynamic control. To favor kinetic enolate: Ideal conditions for kinetic control of enolate formation
are those in which deprotonation is rapid, quantitative, and irreversible (strong base, aprotic solvent, no excess ketone, low temp.)
OCH3
H
H
H
OCH3
THF-78° C
OCH3
OCH3
+
PhCH2Br
OCH3PhCH2
LDA
(slight excess of LDA)
94
To favor thermodynamic Ideal conditions for thermodynamic control of enolate enolate: formation are those in which the enolates can be
interconverted rapidly so that equilibrium can be established (strong base, protic solvent or slight excess ketone, warmer temp.) The product composition will reflect the relative thermodynamic stability of the enolates (most stable enolate will predominate.)
How is this equilibrating?
Book uses NaOCH2CH3 in CH3CH2OH to form the thermodynamic enolate. This doesn’t work well! Why?
NaOEt is not strong enough to quantitatively remove the alpha hydrogen!
OCH3 LDA
THF20° C
OCH3
OCH3
+
PhCH2BrO
CH3
CH2Ph
(slight excess of ketone)
or:NaH
room temp.(slight excess of ketone)
95
So, the alkyl halide has to be present when the enolate forms:
Competing reactions:
1. Alkyl halide reacts with CH3CH2O−
2. Aldol reaction (Chapter 24) 3. Reaction with kinetic enolate
V. Alternatives to Direct Alkylation of Enolate Ions Fortunately, there are other ways to alkylate ketones that work better!
A. Alkylation and Acylation of the α-Carbon via an Enamine Intermediate
Enamines react with electrophiles in the same way that enolates do:
OCH3
+ CH3CH2O!
OCH3
+ CH3CH2OH
OCH3 NaOCH2CH3
HOCH2CH3PhCH2Br
O HN
H3O+ (cat)
recall:
96
Typical reaction:
Advantages: • Better yields • Unsymmetrical ketones can be selectively alkylated at the least substituted side. • Enamines can also be acylated.
Q: Why does the enamine form on the least substituted side? A: The less substituted enamine is more sterically favored. Conjugation between
the N atom and the π-bond means that the bolded bonds must all be in the same plane.
Another example:
O HN
H3O+ (cat)CH3CH2 Br HCl
H2O
HN
H3O+ (cat)
O
CH3 CH3CH2 Br1.2. HCl, H2O
N
HH
CH
HH
N
CH
HH
H
H
H
H
H
H
H
H
H
HN
H3O+ (cat)
O
CH3 C Cl
O
H3C
97
B. Alkylation of an Acetoacetic Ester Unlike esters and ketones, the enolates of β-dicarbonyl compounds form completely with alkoxide bases such as sodium ethoxide.
The enolates of active methylene compounds are nucleophilic and react with alkyl halides and tosylates in typical SN2 reactions to introduce alkyl groups to the α-position.
The process can be repeated by adding additional base and methyl bromide.
This is an SN2 reaction:
• Best with methyl, primary, and primary allylic and primary benzylic substrates.
• 2˚ substrates: • 3˚ substrates:
H3C OEt
O O
+ EtO
H CCH3
CO2Et+ CH3 Br
O
EtOH
H CCH3
CO2Et
O
CH3
EtO!
EtOH
98
1. Decarboxylation of β-dicarbonyls
The loss of carbon dioxide from a carboxylic acid is called Decarboxylation.
Certain carboxylic acids readily decarboxylate: ✦ β-Keto acids
Mechanism: ✦ Malonic acids
2. Acetoacetic Ester Synthesis
Acetoacetic esters are useful reagents for the preparation of methyl ketones of the type shown here:
R C OH
O
H3C OH
O O
r.t.H3O+
HO OH
O OH3O+
!
H3C OEt
O O
H3C CH2
O
R
or H3C CH
O
R
R'
99
Strategy: 1. Alkylate acetoacetic ester 2. Hydrolyze ester 3. Decarboxylate
Example:
☛ Retrosynthetic Analysis:
H CCH3
CO2Et
O
H
1. NaOEt
2. CH2BrCH3
CH
R'
CH3R
O
100
V. Alkylation of Acetic Acid: The Malonic Ester Synthesis
Diethylmalonate is a useful reagent for the preparation of substituted acetic acids:
NOTE: Carboxylic Acids cannot be alkylated in the alpha position directly! Why not?
We can alkylate the corresponding ester and saponify:
Drawbacks:
• more expensive • requires inert atmosphere techniques • better for small scale reactions • enolate is a stronger base than a stabilized enolate, ∴ more E2
EtO OEt
O O
HO CH2
O
R
or HO CH
O
R
R'
H3C OH
OB
CH3 OEt
O LDA
-78 °CTHF
101
Conceptually malonic ester synthesis is similar to the acetoacetic ester synthesis:
1. Alkylate diethyl malonate 2. Hydrolyze both esters 3. Decarboxylate
Example:
☛ Retrosynthetic Analysis:
H CO2Et
H CO2Et 2. CH3I 1. NaOEt
CH
R'
OHR
O
102
Example: Outline a malonic ester synthesis if the following carboxylic acid:
OH
O