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Organometallic Reagents Carbon-Metal Bonds. R. X. R. M. Nucleophilic carbon. +. –. –. +. Organometallic Reagents. R. X + 2 Li. R. Li + Li X. Organolithium Reagents. normally prepared by reaction of alkyl halides with lithium. - PowerPoint PPT Presentation
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Organometallic ReagentsCarbon-Metal Bonds
R X+ –
R M– +
Nucleophilic carbon
Organometallic Reagents
same for Ar—X
is an oxidation-reduction reaction: carbon is reduced
Organolithium Reagents
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°CLi + LiBr
(95-99%)
Preparation of Organomagnesium 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.
Nobel Prize 1912
Born: 6 May 1871, Cherbourg, France
Died: 13 December 1935, Lyon, France
Affiliation at the time of the award: Nancy University, Nancy, France
Examplesdiethylether
–10°C(96%)
Br + Mg
diethylether
35°CMgBr
(95%)
Cl + Mg MgCl
Alkyl halides, vinyl halides, and aryl halides can all be used to form organolithium and organomagnesiumcompounds
However, these organometallic compounds cannot be prepared from compounds containing acidic groups (OH, NH2, NHR, SH, C=CH, CO2H)Cannot use H2O, CH3OH, CH3CH2OH, etc. as solventsCannot prepare from substances such as HOCH2CH2Br, etc.
Synthesis of Alcohols Grignard or Organolithium Reagents
Organolithium reagents react with carbonyls in the same
general way that Grignard reagents do.
• Direct formation of a Grignard reagent, involves an alkyl halide reacted with Mg metal.
Grignard Reagents
Indirect Grignard Reagents
Grignard Reagents can be prepared from other Grignard Reagents (trans-metallation)
Indirect Grignard Reagents
α-acetylenic Grignard Reagents are prepared by an acid-base Grignard reaction
CH3CH2MgBr
+CH3CH3
stronger acid
weaker acid
HC CH
HC CMgBr
+
Question• Select the best method to prepare
CH3CH2CºCMgI.• A) CH3CH2CºC-I + Mg (in diethyl
ether)• B) I-CH2CH2CºCH + Mg(OH)2 (in
diethyl ether)• C) CH3CH2CºCH + CH3MgI (in
diethyl ether)• D) CH3CH2CºCH + Mg (in diethyl
ether)
• A Grignard reagent, RMgX, is a very strong base and a good nucleophile; water/moisture MUST be avoided in its reaction.
Grignard Reactions
QuestionIf water can’t be used as a solvent, can other
protic solvents be used instead?
Yes (A) / No (B)
Example
MgBr
(100%)
+ CH3OH
+ CH3OMgBr
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
other aldehydes to give secondary alcohols
ketones to give tertiary alcohols
esters to give tertiary alcohols
Grignard reagents react with:
formaldehyde to give primary 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
ClMg
MgCl
C O
H
H
CH2OMgClH3O+
CH2OH
(64-69%)
formaldehyde to give primary alcohols
aldehydes to give secondary alcohols
Grignard reagents react with:
Question• The reaction between propanal and
ethylmagnesium chloride followed by H3O+ will produce
• A) B)
• C) D)
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
HH3O+
(84%)
CH3(CH2)4CH2Br CH3(CH2)4CH2MgBr
CH3(CH2)4CH2CHCH3
OMgBr
CH3(CH2)4CH2CHCH3
OH
Question
• What is the product of the reaction of ethylmagnesium bromide (CH3CH2MgBr) with butanal (CH3CH2CH2CH=O) followed by dilute acid?
• A) 2-hexanol• B) 1-butanol• C) 3-hexanol• D) 3-pentanol
Example
(76%)
H2C CHLi + CH
O
1. diethyl ether2. H3O+
CH2CHCH
OH
formaldehyde to give primary alcohols
aldehydes to give secondary alcohols
ketones to give tertiary alcohols
Grignard reagents react with:
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'
Question
• What is the product of the reaction shown at the right?
• A) B)
• C) D)
Example
diethylether
Mg
H3O+
(62%)
CH3Cl CH3MgCl
O
CH3ClMgOCH3HO
Question
• Two alcohols, each having the molecular formula C11H22O, are formed in the reaction of methyl lithium with 3-(R)-tert-butylcyclohexanone. These two alcohols are
• A) constitutional isomers.• B) enantiomers in equal amounts.• C) enantiomers in unequal amounts.• D) diastereomers.
Preparation of Tertiary AlcoholsFrom Esters and Grignard Reagents
Grignard reagents react with esters
R
MgX
C
O••
•• –MgX+
– +R C
O••
•• ••
diethylether
OCH3••
•• OCH3••
••
R'R'
but species formed is unstable and dissociates under the reaction conditions to form a ketone
Grignard reagents react with esters
R
MgX
C
O••
•• –MgX+
– +R C
O••
•• ••
diethylether
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
Example
2 CH3MgBr + (CH3)2CHCOCH3
O
1. diethyl ether2. H3O+
(CH3)2CHCCH3
OH
CH3
(73%)
Two of the groups attached to the tertiary carbon come from the Grignard reagent
Question• Which one of the compounds below will be
produced when 2 moles of methylmagnesium bromide is added to 1 mole of methyl propanoate (CH3CH2CO2CH3) followed by acid?
• A) CH3CH2COCH3
• B) CH3CH2CO2-MgBr• C) (CH3)2C(OH)CH2CH3
• D) CH3CH2CH2CH3
• E) 3-methyl-2-butanol
1) xs CH3CH2CH2MgBr, Et2O
2) H3O+
A.
B.
C.
D.
O
O
O
O
OH
OH
OH
What is the product of the following reaction?
Question
Grignard Reactions
Question
• Which one of the compounds below will be produced when methylmagnesium bromide is added to propanoic acid (CH3CH2CO2H)?
• A) CH3CH2COCH3
• B) CH3CH2CO2-MgBr• C) CH3CH2CH(OH)CH3
• D) CH3CH2CH2CH3