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Chapter 14
Organometallic Compounds
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Metal is the parent
Li
H2C
CHNa
Cyclopropyllithium
Vinylsodium
CH3CH2MgCH2CH3
CH3MgI
Diethylmagnesium
Methylmagnesium
iodide
Polarity of Bonds
+
R
–
X
–
R
+
M
organometallics are a
source of nucleophilic
carbon
Polarity of Bonds
CH3F
CH3Li
Organolithium Compounds
normally prepared by reaction of alkyl halides
with lithium
R
X + 2Li
R
Li + LiX
(same for Ar—X)
It is an oxidation-reduction reaction:
carbon is reduced and metal is oxidized.
Examples
diethyl
ether
(CH3)3CCl + 2Li
(CH3)3CLi + LiCl
–10°C
(75%)
diethyl
ether
Br + 2Li
Li + LiBr
35°C
(95-99%)
Electron Bookkeeping
R
X + Li •
[R X]
R •• Li
Li•
•–
R• +
+ Li+
•• X–
Preparation of
Organomagnesium Compounds:
Grignard Reagents
Grignard Reagents
prepared by reaction of alkyl halides
with magnesium
R
X + Mg
RMgX
(same for Ar—X)
Diethyl ether is most often used solvent.
Tetrahydrofuran is also used.
Examples
diethyl
ether
Cl + Mg
MgCl
–10°C
(96%)
diethyl
ether
Br + Mg
MgBr
35°C
(95%)
Electron Bookkeeping
R
•
X + Mg •
[R X]
•–
•
+ Mg+
•
Mg+
R •• Mg+ •• X–
R• +
•• X–
Order of Reactivity
I > Br > Cl >> F
RX > ArX
Forbidden Groups
certain groups cannot be present in
the solvent
the halide from which the Grignard reagent
is prepared
the substance with which the Grignard
reagent reacts
Forbidden Groups
Anything with an OH, SH, or NH group
therefore:
- cannot use H2O, CH3OH, CH3CH2OH,
etc. as solvents
- cannot prepare Grignard reagent from
substances such as HOCH2CH2Br, etc.
Organolithium and Organomagnesium
Compounds as Brønsted Bases
Brønsted basicity
– R
M
H
+
•• OR'
••
R
H
+ –• ••
M
• OR'
••
Grignard reagents (M = MgX) and
organolithium reagents (M = Li) are strong
bases.
Example
CH3CH2CH2CH2Li + H2O
CH3CH2CH2CH3
(100%)
+
LiOH
Example
MgBr + CH3OH
+
(100%)
CH3OMgBr
Acetylenic Grignard Reagents
are prepared by an acid-base reaction
CH3CH2MgBr +
HC
CH
stronger acid
CH3CH3
weaker acid
+
HC
CMgBr
Synthesis of Alcohols Using
Grignard Reagents
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Grignard reagents act as nucleophiles
toward the carbonyl group
– R
+
C
diethyl
ether
R
C
• O • + MgX
• •• •–
MgX O ••
••
H3O+
Two-step sequence
gives an alcohol as
the isolated product.
R
C
•• OH
••
Grignard reagents react with:
formaldehyde to give primary alcohols
aldehydes to give secondary alcohols
ketones to give tertiary alcohols
esters to give tertiary alcohols
Grignard reagents react with formaldehyde
– R
H
+ H
C
H
diethyl
ether
R
C
•• O ••
•• –
MgX O ••
••
H
+
MgX
H3O+
Product is a
primary
alcohol.
H
R
C
•• OH
••
H
Example
Mg
Cl
MgCl
diethyl
ether
H
C
O
H
CH2OH
(64-69%)
H3O+
CH2OMgCl
Grignard reagents react with aldehydes
– R
H
+ R'
C
MgX O ••
••
H
diethyl
ether
R
C
R'
•• O •• + MgX
•• –
H3O+
Product is a
secondary alcohol.
H
R
C
•• OH
••
R'
Example
Mg
CH3(CH2)4CH2Br
diethyl
ether
CH3(CH2)4CH2MgBr
H3C
C
O
H
CH3(CH2)4CH2CHCH3
OH
(84%)
H3O+
CH3(CH2)4CH2CHCH3
OMgBr
Grignard reagents react with ketones
– R
R"
+ R'
C
R"
diethyl
ether
R
C
•• O ••
•• –
MgX O ••
••
R'
+
H3O+
Product is a
tertiary alcohol.
MgX
R"
R
C
•• OH
••
R'
Example
Mg
CH3Cl
HO
(62%)
CH3
diethyl
ether
H3O+
CH3MgCl
O
ClMgO
CH3
Synthesis of Alcohols
Using Organolithium Reagents
Organolithium reagents react with
aldehydes and ketones in the same
way that Grignard reagents do.
Example
O
H2 C
CHLi +
CH
1. diethyl ether
2. H3O+
CHCH
OH
(76%)
CH2
Synthesis of Acetylenic Alcohols
Using Sodium Salts of Acetylenes
NaNH2
HC
CH
HC
NH3
CNa
O
HO
C
1. NH3
HC
CNa +
2. H3O+
(65-75%)
CH
Using Acetylenic Grignard Reagents
CH3(CH2)3C
CH + CH3CH2MgBr
diethyl ether
CH3(CH2)3C
CMgBr
1. H2C
2. H3O+
CH3(CH2)3C
CCH2OH
(82%)
+ CH3CH3
O
14.9
Retrosynthetic Analysis
Retrosynthetic analysis is the process by
which we plan a synthesis by reasoning
backward from the desired product (the
"target molecule").
Retrosynthetic Analysis of Alcohols
C
OH
Step 1 Locate the carbon that bears the
hydroxyl group.
Retrosynthetic Analysis of Alcohols
C
OH
Step 2 Disconnect one of the groups
attached to this carbon.
Retrosynthetic Analysis of Alcohols
C
OH
Retrosynthetic Analysis of Alcohols
MgX
C
O
What remains is the combination of Grignard
reagent and carbonyl compound that can be
used to prepare the alcohol.
Example
CH3
C
There are two
other
possibilities.
Can you see
them?
CH2CH3
OH
CH3MgX
O
C
CH2CH3
Synthesis
Mg, diethyl ether
CH3Br
CH3MgBr
O
1.
2. H3O+
CH3
C
OH
CH2CH3
C
CH2CH3