Lecture 15 More Carbonyl Chemistry - University of Texas ...
Transcript of Lecture 15 More Carbonyl Chemistry - University of Texas ...
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Chemistry 328N
Lecture 15
March 3, 2016
More Carbonyl Chemistry
Shared Nobel Prize
with Sabatier in
1912
“student” of
Philippe Barbier
François Auguste Victor Grignard
1871-1935
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Chemistry 328N
Reaction Theme
The most common reaction of a carbonyl
group is addition of a nucleophile to form a
tetrahedral addition compound
Here it is an acid or electrophile Tetrahedral carbonyl addition compound
+ C
R
R
O CNu
O -
RR
Nu -
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Chemistry 328N
Carbon Nucleophiles
Addition of carbon nucleophiles is one of the most important types of nucleophilic additions to a C=O group; a new carbon-carbon bond is formed in the process!!!!
We will study the addition of these carbon nucleophiles
A Grignard reagent
An organolithium reagent
An anion of a terminal alkyne
Cyanide ion
RC C - NRMgX RLi - C
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Chemistry 328N
Victor Grignard
Shared Nobel
Prize with
Sabatier in 1912
“student” of
Philippe Barbier
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Chemistry 328N
Grignard Reagents
X = Cl, Br, I
Diethyl ether
Et2OTetrahydrofuran
THF
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Chemistry 328N
Crystal Structure of CH3CH2MgBr
L.J. Guggenberger and R.E. Randle , J. Amer. Chem. Soc. 90(20) 5357 (1968)
O
O
Mg
Br
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Chemistry 328N
Grignard Reagents
Given the difference in electronegativity
between carbon and magnesium, the C-Mg bond
is polar covalent, with C - and Mg +
– Grignard reagents behave like a carbanions
Carbanion: an anion in which carbon has an unshared
pair of electrons and bears a negative charge
– a carbanions are good nucleophiles and add efficiently to the
carbonyl group of aldehydes and ketones
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Chemistry 328N
Grignard Reagents
Addition of a Grignard reagent to formaldehyde followed by H3O
+ gives a 1°alcohol
This sequence (mechanism) is general and important!
Mg+2H3O
+
CH3CH2- MgBr
+
H
C
H
O CH3CH2 C
H
H
O- MgBr
+CH3CH2 C
H
H
OH +THF dil.
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Chemistry 328N
Grignard Reagents
Addition to any other RCHO gives a 2° alcohol
Mg+2H3O
+
CH3CH2- MgBr
+ C
H
O CH3CH2 C
H
O- MgBr
++
THF dil.
CH3CH2 C
H
OH
• You may change decorations at will…read pages 567-573
– but, be careful of acidic functions like -OH
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Chemistry 328N
Grignard Reagents
Addition to a ketone gives a 3° alcohol
Please try this with other Grignard reagents and other ketones
Mg+2H3O
+
CH3CH2- MgBr
+ CH3CH2 C O- MgBr
+
CH3
CH2
CH2
CH3
+THF dil.
CH3CH2 C OH
CH3
CH2
CH2
CH3
C O
CH3
CH2
CH2
CH3
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Chemistry 328N
Grignard Reactions
Mg+2H3O
+
CH3CH2- MgBr
+
H
C
H
O CH3CH2 C
H
H
O- MgBr
+CH3CH2 C
H
H
OH +THF dil.
Mg+2H3O
+
CH3CH2- MgBr
+ C
H
O CH3CH2 C
H
O- MgBr
++
THF dil.
CH3CH2 C
H
OH
Mg+2H3O
+
CH3CH2- MgBr
+ CH3CH2 C O- MgBr
+
CH3
CH2
CH2
CH3
+THF dil.
CH3CH2 C OH
CH3
CH2
CH2
CH3
C O
CH3
CH2
CH2
CH3
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Chemistry 328N
Grignard Reagents
Addition to CO2 gives a carboxylic acid
This is a great way to add a carbon
Mg+2H3O
+
CH3CH2- MgBr
+ CH3CH2 C O- MgBr
+
O
+THF dil.
CH3CH2 C OH
O
C OO
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Chemistry 328N
Grignard Reactions
MgBr
OTHF
O-MgBr
+
H3O+
Dilute
OH
These are very valuable and important reactions…
Please add to your card stock!
• This is a great way to add two carbons
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Chemistry 328N
Grignard reagents react with esters
R
MgX
C
O
•••• –
MgX+
– +R C
O••••
••
diethyl
ether
OCH3
••
•• OCH3
••
••
R'R'
but species formed is
unstable and dissociates
under the reaction
conditions to form a ketone
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Chemistry 328N
Grignard reagents react with esters
R
MgX
C
O
•••• –
MgX+
– +R C
O••••
••
diethyl
ether
OCH3
••
•• OCH3
••
••
R'R'
–CH3OMgX
C
O
R R'
••
••
this ketone then goes
on to react with a
second mole of the
Grignard reagent to
give a tertiary alcohol
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Chemistry 328N
Example
2 CH3MgBr + (CH3)2CHCOCH3
O
1. diethyl ether
2. H3O+
(CH3)2CHCCH3
OH
CH3
(73%)
Two of the groups attached to the tertiary carbon come from the Grignard reagent
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Chemistry 328N
formaldehyde to give primary alcohols
aldehydes to give secondary alcohols
ketones to give tertiary alcohols
esters to give tertiary alcohols
CO2 to give acids
epoxides give primary alcohols
Grignard reagents react with:
O
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Chemistry 328N
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Chemistry 328N
Grignard Reagents
Problem: 2-phenyl-2-butanol can be
synthesized by three differentcombinations of a Grignard reagent anda ketone. Show each combination
C-CH 2 CH3
OH
CH3
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Chemistry 328N
Practice Problem
1. Starting from benzene, write a synthetic path to the structures below. You are free to use any reagents or reactants you choose, but you must start with benzene.
C O
H
H
Cl
0
Cl
Br
COOH
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Chemistry 328N
The Wittig Reaction
(C6H5)3P C+
A
B
••
–+
+C C
R
R'
A
B
(C6H5)3P O+
••
–••
••
C O
R
R'
••
••
“ylide”
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Chemistry 328N
The Nobel Prize in Chemistry 1979
Georg Wittig 1897-1987
University of Heidelberg
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Chemistry 328N
Preparation of Ylides
The ylid is made in a two step process. The first
step is a nucleophilic substitution reaction that
Forms a phosphonium salt
(C6H5)3P •• + CH
A
B
X (C6H5)3P CH
A
B
+
+ X–
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Chemistry 328N
Preparation of Ylides
(C6H5)3P C
A
B
+••
–
H+ base
In the second step, the phosphonium salt istreated with a strong base in order to removea proton from the carbon bonded to phosphorus.
(C6H5)3P C
A
B
+H
base ••–
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Chemistry 328N
Phosphonium ylidesResonance stabilized
R is usually C6H5 (phenyl)
Carbon is negatively polarized and nucleophilic
(C6H5)3P C+
A
B
••
–(C6H5)3P C
A
B
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Chemistry 328N
The Wittig Reaction
(C6H5)3P C+
A
B
••
–+
+C C
R
R'
A
B
(C6H5)3P O+
••
–••
••
C O
R
R'
••
••
“ylide”
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Chemistry 328N
Mechanism
C O
R
R'
••
••
P(C6H5)3
+C
A
B–
••
OC
C P(C6H5)3
R
R'
B
A
••••
Step 1
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Chemistry 328N
Mechanism
OC
C P(C6H5)3
R
R'
B
A
••••
Step 2
P(C6H5)3+
–O••••
••
R'R
A B
C
C
+
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Chemistry 328N
Example
+
+ (C6H5)3P O+
••
–••
••
(C6H5)3P CH2
+ –
••O••
••
CH2
THF
(86%)
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Chemistry 328N
Triphenylphosphine oxide
O
P(C6H5)3
••••
P(C6H5)3+
–O••••
••
This substance is very stable
Nearly insoluble in many solvents, i.e. pentane
Oral, mouse: LD50 = 1380 mg/kg;
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Chemistry 328N
Wittig Reaction
More examples
Don’t plan to control the E/Z ratio…you get generally get a mixture
CH2 C
O
H+ Ph3P CHCH3
H H
CH3
+ Ph3P O
THF
87% Z isomer
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Chemistry 328N
The Wittig Reaction
See... sometimes you can control it
we won’t try to do that in this class
The reaction has broad “scope”
CH2 C
O
H+ Ph3P CHCOCH2CH3
O
H
H
O
O
+ Ph3P O
THF
100% E isomer
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Chemistry 328N
An Example: Write the structure
of the products
O + P
3