Chapter 14Chapter 14Organometallic CompoundsOrganometallic Compounds
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Metal is the parent Li
Cyclopropyllithium Vinylsodium
H2C CHNa
CH3CH2MgCH2CH3
Diethylmagnesium
CH3MgI
Methylmagnesiumiodide
Polarity of Bonds
R X+ –
R M– +
organometallics are a source of nucleophilic
carbon
Polarity of Bonds
CH3F CH3Li
(same for Ar—X)
It is an oxidation-reduction reaction:
carbon is reduced and metal is oxidized.
Organolithium Compounds
R X + 2Li R Li + LiX
normally prepared by reaction of alkyl halides with lithium
Examples
(CH3)3CCl + 2Li
diethylether
–10°C
(CH3)3CLi + LiCl
(75%) Br + 2Li
diethylether
35°C
Li + LiBr
(95-99%)
Electron Bookkeeping
R X + Li [R X] + Li+•–
R• + •• X–
•
Li•LiR••
Preparation of Organomagnesium Compounds:
Grignard Reagents
(same for Ar—X)
Grignard Reagents
R X + Mg RMgX
prepared by reaction of alkyl halides with magnesium
Diethyl ether is most often used solvent. Tetrahydrofuran is also used.
Examplesdiethylether
–10°C(96%)
Br + Mg
diethylether
35°C
MgBr
(95%)
Cl + Mg
MgCl
R X + Mg
Electron Bookkeeping
[R X] + Mg+•–
R• + •• X–
•• •
R••Mg+
Mg+•
•• X–
I > Br > Cl >> F
RX > ArX
Order of Reactivity
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
Forbidden Groups
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
OR'••
••••–
M+
– +
Grignard reagents (M = MgX) and organolithium reagents (M = Li) are strong bases.
Example
(100%)
+ H2O
+ LiOH
CH3CH2CH2CH2Li
CH3CH2CH2CH3
Example MgBr
(100%)
+ CH3OH + CH3OMgBr
Acetylenic Grignard Reagents
are prepared by an acid-base reaction
CH3CH2MgBr
+CH3CH3
stronger acid
weaker acid
HC CH
HC CMgBr
+
Synthesis of Alcohols Using Synthesis of Alcohols Using
Grignard ReagentsGrignard Reagents
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Grignard reagents act as nucleophilestoward the carbonyl group
R
MgX
C
O••
••
– +
–
MgX+
R C
O••
•• ••
diethylether
••
R C
OH••
H3O+
Two-step sequence gives an alcohol as the isolated product.
formaldehyde to give primary alcohols
aldehydes to give secondary alcohols
ketones to give tertiary alcohols
esters to give tertiary alcohols
Grignard reagents react with:
Grignard reagents react with formaldehyde
R
MgX
C
O
••
•• –MgX
+
– +R C
O••
•• ••
••
R C
OH••
H3O+
diethylether
Product is a primary alcohol.
H HH
H
H
H
Example
diethylether
Cl
Mg
MgCl
C O
H
H CH2OMgCl
H3O+
CH2OH
(64-69%)
Grignard reagents react with aldehydes
R
MgX
C
O
••
•• –
– +R C
O••
•• ••
••
R C
OH••
H3O+
diethylether
Product is a secondary alcohol.
H R'H
R'
H
R'
MgX+
Example
diethylether
Mg
C O
H3C
H
H3O+
(84%)
CH3(CH2)4CH2Br CH3(CH2)4CH2MgBr
CH3(CH2)4CH2CHCH3
OMgBr
CH3(CH2)4CH2CHCH3
OH
Grignard reagents react with ketones
R
MgX
C
O
••
•• –MgX+
– +R C
O••
•• ••
••
R C
OH••
H3O+
diethylether
Product is a tertiary alcohol.
R" R'R"
R'
R"
R'
Example
diethylether
Mg
H3O+
(62%)
CH3Cl CH3MgCl O CH3ClMgO CH3HO
Organolithium reagents react with Organolithium reagents react with aldehydes and ketones in the same aldehydes and ketones in the same
way that Grignard reagents do.way that Grignard reagents do.
Synthesis of AlcoholsUsing Organolithium Reagents
Example
(76%)
H2C CHLi +
CH
O
1. diethyl ether
2. H3O+ CH2CHCH
OH
Synthesis of Acetylenic Alcohols
Using Sodium Salts of Acetylenes
HC CH
NaNH2
NH3
HC CNa
HC CNa +1. NH3
2. H3O+
O HO C CH (65-75%)
CHCH3(CH2)3C + CH3CH2MgBr
CMgBrCH3(CH2)3C + CH3CH3
diethyl ether
1. H2C O2. H3O+
CCH2OHCH3(CH2)3C
(82%)
Using Acetylenic Grignard Reagents
14.9Retrosynthetic Analysis
Retrosynthetic analysis is the process by Retrosynthetic analysis is the process by which we plan a synthesis by reasoning which we plan a synthesis by reasoning backward from the desired product (the backward from the desired product (the "target molecule")."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
C
O
What remains is the combination of Grignard reagent and carbonyl compound that can be used to prepare the alcohol.
MgX
Example C
OH
CH3
CH2CH3
C CH2CH3
O
CH3MgX
There are two other possibilities.
Can you see them?
Synthesis
C
OH
CH3
CH2CH3
C CH2CH3
O
1.
2. H3O+
CH3BrMg, diethyl ether
CH3MgBr
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