Organometallic compounds
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Organometallic Compounds. Chapter 15. Guest Lecturer: Prof. Jonathan L. Sessler. AWOL!. Organometallic Compounds. Organometallic compound: a compound that contains a carbon-metal bond In this chapter, we focus on organometallic compounds of Mg, Li, and Cu

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OrganometallicCompounds

Chapter 15

Guest Lecturer: Prof. Jonathan L. Sessler


AWOL!


Organometallic Compounds

  • Organometallic compound: a compound that contains a carbon-metal bond

  • In this chapter, we focus on organometallic compounds of Mg, Li, and Cu

    • these classes illustrate the usefulness of organometallics in modern synthetic organic chemistry

    • they illustrate how the use of organometallics can bring about transformations that cannot be accomplished in any other way

    • several more recent reactions of organometallic compounds are discussed in Chapter 24


Organometallic Reagents

The Key Concepts:

Make a carbon negatively charged/polarlized so it is nucleophilic.

Reaction with electrophilic carbons can make carbon-carbon bonds.

This is a Big Deal!


The First Organometallic Reagents…

Grignard Reagents

  • Discovered by Victor Grignard in 1900

    • Key factors are ethereal solvent and water-free conditions

  • Awarded Nobel Prize in 1912

Victor Grignard

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).


M

g

B

r

B

r

M

g

Butylmagnesium bromide

(an alkyl Grignard reagent)

B

r

M

g

M

g

B

r

Phenylmagnesium

bromide

(an aryl Grignard reagent)

Grignard Reagents

  • Grignard reagent: an organomagnesium compound

    • prepared by addition of an alkyl, aryl, or alkenyl (vinylic) halide to Mg metal in diethyl ether or THF

ether

+

1-Bromobutane

ether

+

Bromobenzene


An Alternative to Grignard Reagents are Alkyl Lithiums

Both are prepared from alkyl, vinyl, and aryl halides under anhydrous conditions


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

  • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge


Grignard and Organolithium Reagents

  • Carbanion: 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!


Grignard and Organolithium Reagents

  • Carbanion: 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, carboxylic acids, amides, even terminal alkynes). A limitation to utility!


Limitations

  • Can’t make Grignards with acidic or electro-philic functional groups present in the molecule:

    R2NH pKa 38-40

    Terminal AlkynespKa 25

    ROHpKa 16-18

    Carbonyls& NitrospKa 11-27


Grignard and Organolithium Reagents

  • 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!


  • Key Point: Grignard and Organolithium Reagents

    • 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


Grignard and Organolithium Reagents

  • Carbanions (nucleophiles) can react with electrophilic carbon centers in favorable cases. The net result is a carbon-carbon bond--a big deal!

  • Grignards and organolithium reagents react with many oxygen-containing electrophiles, but not with alkyl halides.

  • We’ll illustrate this with epoxides.

  • Recall, acidic protons will “kill” our reagents and/or won’t allow them to be generated in the first place


Grignard reagents react productively with:

formaldehyde to give primary alcohols

aldehydesto give secondary alcohols

ketonesto give tertiary alcohols

esters to give tertiary alcohols

CO2to give acids

epoxides to give primary alcohols

The one we are choosing for the sake of initial illustration


Epoxides: The Example We Want to Stress

New C-C bond

d+

d-

Considered

Retrosyn-

thetically:

These is an extremely valuable reaction…

Worth celebrating!


Prof. Iverson Would Normally Play His Trumpet Here!

But, he is gone…


Another Kind of Celebration!

  • Cost of Attending USC (per year): $50,000

  • Gymnastics, Tumbling and Cheerleading Lessons: $20,000

  • Keeping Hair Just The Right Shade of Blond: $10,000

  • Cheering for the Wrong Team:Priceless!


Back to Work: A Related Example

Detailed Mechanism Highlighting Retention of Stereochemistry

New C-C bond

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


Two More Examples of Additions to Epoxides

New C-C bond

New C-C bond

Note: Stereochemistry at epoxide retained in product


Key point: Orientation Matters!


Gilman Reagents

  • Lithium diorganocopper reagents, known more commonly as Gilman reagents

    • prepared by treating an alkyl, aryl, or alkenyl lithium compound with Cu(I) iodide


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.)

  • Example

New C-C bond


Gilman Reagents

  • coupling with a vinylic halide is stereospecific; the configuration of the carbon-carbon double bond is retained

New C-C bond


Gilman Reagents

  • A variation on the preparation of a Gilman reagent is to use a Grignard reagent with a catalytic amount of a copper(I) salt


Gilman Reagents

  • Reaction with epoxides

    • regioselective ring opening (attack at least hindered carbon)

New C-C bond


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.


Feeling Lost?

Fortunately, Dr. Iverson will be back on Monday!


Meanwhile….

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!


Grignard Reactions

These are valuable and important reactions…

Please add to your card stock!


••

diethylether

OCH3

••

••

OCH3

R

C

R

••

O

MgX

••

••

••

Grignard reagents react with esters

R'

R'

but species formed is unstable and dissociates under the reaction conditions to form a ketone

d+

d–

C

+

MgX

O

••

••


••

diethylether

OCH3

••

••

OCH3

R

C

R

••

O

MgX

••

••

••

R

R'

C

O

Grignard reagents react with esters

R'

R'

this ketone then goes on to react with a second mole of the Grignard reagent to give a tertiary alcohol

d+

d–

C

+

MgX

O

••

••

–CH3OMgX

••

••


Example

O

+

(CH3)2CHCOCH3

2 CH3MgBr

Two of the groups attached to the tertiary carbon come from the Grignard reagent

1. diethyl ether

2. H3O+

OH

(CH3)2CHCCH3

CH3

(73%)


Practice


Practice


The Same Chemistry is seen With Organolithium Reagents

O

+

CH

H2C

CHLi

1. diethyl ether

2. H3O+

CHCH

CH2

(76%)

OH


Practice


Organometallics are one of the classic ways of activating CO2


End of Lecture. Any Questions?


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