OrganometallicCompounds
Dr Seemal jelani
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Organometallic Compounds
An organic compound containing a carbon–metal bond
Organolithium compounds and Organomagnesium are two of the most common organometallic compounds
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A. Organometallic Mechanisms
• Oxidation State: The oxidation state of a metal is
defined as the charge left on the metal after all ligands
have been removed in their natural, closed-shell
configuration• This is a formalism and not a physical property!• Formalism is the study by analyzing and comparing
form and style—the way objects are made and their
purely visual aspects
• d-Electron Configuration: position in the periodic table
minus oxidation state.
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The d electron count is a Chemistry formalism used to describe the electron configuration of the valence electrons of a transitions metal center in a Coordination complex
The d electron count is an effective way to understand the geometry and reactivity of transition metal complexes
The formalism has been incorporated into the two major models used to describe coordination complexes;
Crystal field theory and ligand field theory
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18-Electron Rule A stable complex (with electron configuration of the
next highest noble gas) is obtained when the sum of the metal d-electrons, electron donated from the ligand, and the overall charge of the complex equals 18
In mononuclear, diamagnetic complexes, the total number of electrons never exceeds 18 (noble gas configuration)
The total number of electrons is equal to the sum of d-electrons plus those contributed by the ligands.
18 electrons = coordinatively saturated
< 18 electrons = coordinatively unsaturated.
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Diamagnetic
A diamagnetic compound has all electron spins paired, giving a net spin of zero. They are also weakly repelled by a magnet.
Descriptive term which indicates that a substance contains no unpaired Electrons and thus is not attracted to a magnetic field
NH3 is diamagnetic because all of the electrons in NH3 are paired.
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18-Electron rule
This was first proposed by N.V. Sigwick who extended the octet theory of G.N. Lewis and using it to explain coordination compounds
Ligands were considered Lewis bases and the metal ion in turn acted as a Lewis acid
The sum of all the electrons would assume a noble gas configuration of Kr, Xe or Rn.
As with most rules of the thumb, the 18 electron rule is not always obeyed
18 electrons are required to fill 5 d-orbitals, 1 s-orbital and 3 p-orbitals of a transition metal and hence reach the configuration of a noble gas.
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Organometallic Reagents
The Key Concepts:
Make a carbon negatively charged/polarlized so it is nucleophilic.
Reaction with electrophilic carbons can make carbon-carbon bonds.
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Victor Grignard
Grignard Reagents
• Discovered by Victor Grignard in 1900– Key factors are ethereal solvent
and water-free conditions
• Awarded Nobel Prize in 1912
Grignard, Victor , 1871–1935, French chemist. He shared the 1912 Nobel Prize in Chemistry for his work in organic synthesis based on his discovery (1900) of the Grignard Reagent. He taught at the Univ. of Nancy (1909–19) and at the Univ. of Lyons (from 1919 until the end of his career).
The First Organometallic Reagents…
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Grignard Reagents An Organomagnesium compound prepared by
addition of an alkyl, aryl, or alkenyl (vinylic) halide to Mg metal in diethyl ether or THF
Mg+
1-Bromobutane Butylmagnesium bromide(an alkyl Grignard reagent)
etherBr MgBr
+ether
MgBr MgBr
Bromobenzene Phenylmagnesiumbromide
(an aryl Grignard reagent)
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Br+ Mgo
n-butyl bromide
dry ether
tetrahydrofuran (THF)
or
Orecall: THF =
MgBr
Grignard Reagent
An Alternative to Grignard Reagents are Alkyl Lithiums
Both are prepared from alkyl, vinyl, and aryl halides under anhydrous conditions
Br+ 2 Lio
n-butyl bromide
dry ether
tetrahydrofuran (THF)
or Li
alkyl lithium + LiBr
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Grignard and Organolithium Reagents Given the difference in electronegativity between carbon and
magnesium (lithium), the C-Mg (C-Li) bond is polar covalent, with C- and Mg+ ( Li+ ) Grignard and Organolithium reagents behave like carbanions
Carbanions: an anion in which carbon has an unshared pair of electrons and bears a negative charge
MgBr
Grignard Reagent
-
+
Li
alkyl lithium
-
+
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Grignard and Organolithium ReagentsCarbanion: an anion in which carbon has an unshared pair
of electrons and bears a negative charge
Carbanions are strong bases--they are easily quenched by even very weak acids (water, alcohols, amines, amides, carboxylic acids, even terminal alkynes). A limitation to utility!
MgBr
Grignard Reagent
-
+
OH- +
+alkane correspondingto original alkyl halide
O- MgBr+
+pKa = 16
pKa = ca. 50
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Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative chargeCarbanions are strong bases--they are easily quenched
by even very weak acids (water, alcohols, amines, carboxylic acids, amides, even terminal alkynes). A limitation to utility!
Li
alkyl lithium
-
+
NH2
- +
+alkane correspondingto original alkyl halide
NH- Li+
+
pKa = 40
pKa = ca. 50
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Limitations
Can’t make Grignards with acidic or electrophilic functional groups present in the molecule:
R2NH pKa 38-40
Terminal Alkynes pKa 25
ROH pKa 16-18
Carbonyls & Nitros pKa 11-27
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• Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge– Carbanions are also great nucleophiles– This is the reason for their great utility!
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• Key Point: Key Point:
• Great nucleophiles that add efficiently to electrophilic carbons, such as epoxides and carbonyl group of aldehydes, ketones and esters
• However, their basicity can be a limitation!
• Epoxides illustrate how many common organic functional groups contain electrophilic carbons
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Carbanions (nucleophiles) can react with electrophilic carbon centers in favorable cases
The net result is a carbon-carbon
Grignards and Organolithium reagents react with many oxygen-containing electrophiles, but not with alkyl halides.
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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 to give primary alcohols
Grignard reagents react productively with:
The one we are choosing for the sake of initial illustration
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Epoxides: The Example We Want to StressEpoxides: The Example We Want to Stress
MgBr
OTHF
O-MgBr
+H3O
+
Dilute
OHMgBr
OTHF
O-MgBr
+H3O
+
Dilute
OH
These is an extremely valuable reaction…
OH MgBr
+ OConsideredRetrosyn-thetically:
- +
New C-C bond
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Back to Work: A Related ExampleDetailed Mechanism Highlighting Retention of Stereochemistry
Key Points to Note:•Attack at least hindered carbon
Mechanism is SN2-like in initial step
•Single enantiomeric product
Chiral center not affected by reaction•Relief of ring strain helps drive
reaction
H3O+ breaks up initial salt
New C-C bond
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Two More Examples of Additions to EpoxidesTwo More Examples of Additions to Epoxides
MgBr
Grignard Reagent
-
+
O
+
1.
2. HCl, H2O
-
OH
Li
-
+
O
+
1.
2. HCl, H2O
-
HH
H
OH
H
Organolithium Reagent
New C-C bond
New C-C bond
Note: Stereochemistry at epoxide retained in product
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Gilman ReagentsLithium Diorganocopper reagents, known
more commonly as Gilman reagentsprepared by treating an alkyl, aryl, or alkenyl
lithium compound with Cu(I) iodide
2CH3CH2CH2CH2Li + CuIor THF
diethyl ether
(CH3CH2CH2CH2)2Cu- Li + + LiI
Butyllithium Copper(I) iodide
Lithium dibutylcopper (a Gilman reagent)
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Gilman Reagents Coupling with Organohalogen compounds
form new carbon-carbon bonds by coupling with alkyl and alkenyl chlorides, bromides, and iodides. (Note that this doesn’t work with Grignard or Organolithium reagents
They are too basic and do E2 eliminations.)
I1. Li, pentane2. CuI CuLi
Br
2-Methyl-1-dodecene
1-Iododecane
or THFdiethyl ether
2
R'Br R2CuLiBr R'-R RCu LiBror THF
diethyl ether+ + +
•Example
New C-C bond
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Gilman ReagentsCoupling with a vinylic halide is stereospecific; the
configuration of the carbon-carbon double bond is retained
I CuLi2
diethyl ether+or THF
Lithiumdibutylcopper
trans-1-Iodo-1-nonene
trans-5-Tridecene
New C-C bond
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Gilman ReagentsA variation on the preparation of a Gilman reagent is to
use a Grignard reagent with a catalytic amount of a copper(I) salt
CH3(CH2)7
C C
(CH2)7CH2Br
H HCH3(CH2)4MgBr
Cu+
THF
CH3(CH2)7
C C
(CH2)12CH3
H H
+
(Z)-1-Bromo-9-octadecene
(Z)-9-Tricosene(Muscalure)
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Gilman Reagents
Reaction with epoxides regioselective ring opening (attack at least hindered
carbon)
Styrene oxide(racemic)
1-Phenyl-3-buten-1-ol(racemic)
1. (CH2=CH)2CuLi
2. H2O, HCl
O OH
New C-C bond
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Interim Summary of Introduction to Organometallic Reagents…
• Organolithium reagents and Grignard reagents are very basic but also great nucleophiles
• They react with epoxides at the less hindered site to give a two-carbon chain extended alcohol
• They do not couple with alkyl-, aryl-, or vinyl halides.
• Gilman reagents react with epoxides as do Organolithium reagents and Grignard reagents.
• However, they also add to alkyl-, aryl-, and vinyl halides to make new C-C bonds.
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Back to 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.
Mg+2H3O
+
CH3CH2- MgBr
+
H
C
H
O CH3CH2 C
H
H
O- MgBr
+CH3CH2 C
H
H
OH +THF dil.
Meanwhile….
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Mg+2H3O
+
CH3CH2- MgBr
+ CH3CH2 C O- MgBr
+
O
+THF dil.
CH3CH2 C OH
O
C OO Mg+2H3O
+
CH3CH2- MgBr
+ CH3CH2 C O- MgBr
+
O
+THF dil.
CH3CH2 C OH
O
C OO
Grignard Reactions
These are valuable and important reactions…
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Grignard reagents react with esters
but species formed is but species formed is unstable and dissociates unstable and dissociates under the reaction conditions under the reaction conditions to form a ketoneto form a ketone
R
MgX
CC
OO
•••••••• ––
MgXMgX++
–– ++RR CC
OO••••
•••• ••••
diethyldiethyletherether
OCHOCH33••••
•••• OCHOCH33••••
••••
R'R'R'R'
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Grignard reagents react with esters
this ketone then goes on this ketone then goes on to react with a second to react with a second mole of the Grignard mole of the Grignard reagent to give a tertiary reagent to give a tertiary alcoholalcohol
RR
MgXMgX
CC
OO
•••••••• ––
MgXMgX++
–– ++RR CC
OO••••
•••• ••••
diethyldiethyletherether
OCHOCH33••••
•••• OCHOCH33••••
••••
R'R'R'R'
––CHCH33OMgXOMgX
CC
OO
RR R'R'
••••••••
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Example
Two of the groups Two of the groups attached to the attached to the tertiary carbon tertiary carbon come from the come from the Grignard reagentGrignard reagent
2 CH2 CH33MgBrMgBr ++ (CH(CH33))22CHCCHCOCHOCH33
OO
1. diethyl ether1. diethyl ether
2. H2. H33OO++
(CH(CH33))22CHCCHCCHCH33
OHOH
CHCH33
(73%)(73%)
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PracticePractice
OH
MgBr
H H
O+
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PracticePractice
OHMgBr
+ O
MgBr
H H
O+
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(76%)(76%)
HH22CC CHLiCHLi ++
CHCH
OO
1. diethyl ether1. diethyl ether
2. H2. H33OO++
CHCH22CHCHCHCH
OHOH
The Same Chemistry is seen With Organolithium ReagentsThe Same Chemistry is seen With Organolithium ReagentsDr Seemal jelani
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PracticePractice
OH
O
MgBr
+ C OO
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