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Organometallic Compounds Dr Seemal jelani 1

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Organometallic Compounds. Organometallic Compounds. An organic compound containing a carbon–metal bond Organolithium compounds and Organomagnesium are two of the most common organometallic compounds. A. Organometallic Mechanisms. - PowerPoint PPT Presentation

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Page 1: Organometallic Compounds

OrganometallicCompounds

Dr Seemal jelani

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Page 2: Organometallic Compounds

Dr Seemal jelani

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Page 3: Organometallic Compounds

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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Page 4: Organometallic Compounds

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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Page 5: Organometallic Compounds

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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Page 6: Organometallic Compounds

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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Page 7: Organometallic Compounds

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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Page 8: Organometallic Compounds

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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Page 9: Organometallic Compounds

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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Page 10: Organometallic Compounds

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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Page 11: Organometallic Compounds

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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Page 12: Organometallic Compounds

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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Page 13: Organometallic Compounds

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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Page 14: Organometallic Compounds

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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Page 15: Organometallic Compounds

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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Page 16: Organometallic Compounds

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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Page 17: Organometallic Compounds

• 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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Page 18: Organometallic Compounds

• 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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Page 19: Organometallic Compounds

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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Page 20: Organometallic Compounds

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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Page 21: Organometallic Compounds

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

Dr Seemal jelani

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Page 22: Organometallic Compounds

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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Page 23: Organometallic Compounds

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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Page 24: Organometallic Compounds

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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Page 25: Organometallic Compounds

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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Page 26: Organometallic Compounds

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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Page 27: Organometallic Compounds

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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Page 28: Organometallic Compounds

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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Page 29: Organometallic Compounds

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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Page 30: Organometallic Compounds

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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Page 31: Organometallic Compounds

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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Page 32: Organometallic Compounds

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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Page 33: Organometallic Compounds

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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Page 34: Organometallic Compounds

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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Page 35: Organometallic Compounds

PracticePractice

OH

MgBr

H H

O+

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Page 36: Organometallic Compounds

PracticePractice

OHMgBr

+ O

MgBr

H H

O+

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Page 37: Organometallic Compounds

(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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Page 38: Organometallic Compounds

PracticePractice

OH

O

MgBr

+ C OO

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