Chapter 14 Organometallic Compounds

1
Chapter 14Organometallic Compounds
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Organometallic Nomenclature
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Metal is the parent
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Carbon-Metal BondsinOrganometallic Compounds
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Electronegativities

• F 4.0 H 2.1
• O 3.5 Cu 1.9
• N 3.0 Zn 1.6
• C 2.5 Al 1.5
• H 2.1 Mg 1.2
• Li 1.0
• Na 0.9
• K 0.8

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Polarity of Bonds
d
d
d
d

• organometallics are a source of nucleophilic
  carbon

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Polarity of Bonds
CH3F
CH3Li
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Preparation of Organolithium Compounds
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Organolithium Compounds
normally prepared by reaction of alkyl halides
with lithium
same for ArX

• An oxidation-reduction reaction carbon is
  reduced

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Examples
diethylether
(CH3)3CCl 2Li
(CH3)3CLi LiCl
10C
(75)
diethylether
Br
2Li
Li
LiBr
35C
(95-99)
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Electron Bookkeeping
Li


Li

R
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Preparation of Organomagnesium Compounds
Grignard Reagents
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Grignard Reagents
prepared by reaction of alkyl halides with
magnesium
RMgX
same for ArX

• Diethyl ether is most often used solvent.
  Tetrahydrofuran is also used.

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Examples
diethylether
Mg
10C
(96)
diethylether
Br
Mg
MgBr
35C
(95)
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Electron Bookkeeping


Mg



R
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Order of Reactivity

• I gt Br gt Cl gtgt F
• RX gt ArX

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Forbidden Groups

• 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

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Forbidden Groups
Anything with an OH, SH, or NH group i.e. an
acidic hydrogen

• therefore cannot use H2O, CH3OH, CH3CH2OH, etc.
  as solvents
• cannot prepare Grignard reagent from substances
  such as HOCH2CH2Br, etc.

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Organolithium and Organomagnesium Compounds as
Brønsted Bases
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Brønsted basicity
d
d

• Grignard reagents (M MgX) and organolithium
  reagents (M Li) are strong bases.

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Example
H2O
CH3CH2CH2CH2Li

• water is a stronger acid than butane

CH3CH2CH2CH3

LiOH
(100)
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Example
MgBr
CH3OH

• methanol is a stronger acid than benzene

CH3OMgBr
(100)
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Table 14.2Approximate Acidities of Hydrocarbons
Hydrocarbon pKa (CH3)3CH 71 CH3CH3 62 CH4 60 Ethyl
ene 45 Benzene 43 Ammonia 36 Acetylene 26 Water 16

• Hydrocarbons are very weak acids.
• Their conjugate bases are very strong bases.
• Grignard reagents and organolithium reagents are
  strong bases.

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Acetylenic Grignard Reagents

• are prepared by an acid-base reaction

CH3CH2MgBr
stronger acid

CH3CH3
weaker acid
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Synthesis of Alcohols Using Grignard Reagents
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Grignard reagents act as nucleophilestoward the
carbonyl group
d
d
C

MgX
O



H3O

• two-step sequence gives an alcohol as the
  isolated product

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Grignard reagents react with

• formaldehyde to give primary alcohols
• aldehydes to give secondary alcohols
• ketones to give tertiary alcohols
• esters to give tertiary alcohols

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Grignard reagents react with

• formaldehyde to give primary alcohols

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Grignard reagents react with formaldehyde
H
H
H
d
d
H
C
O



H3O
H
H

• product is a primary alcohol

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Example
Mg
diethylether
H3O
(64-69)
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Grignard reagents react with

• formaldehyde to give primary alcohols
• aldehydes to give secondary alcohols

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Grignard reagents react with aldehydes
H
H
R'
d
d
R'
C
O



H3O
H
R'

• product is a secondary alcohol

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Example
Mg
CH3(CH2)4CH2Br
CH3(CH2)4CH2MgBr
diethylether
H3O
(84)
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Grignard reagents react with

• formaldehyde to give primary alcohols
• aldehydes to give secondary alcohols
• ketones to give tertiary alcohols

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Grignard reagents react with ketones
R"
R"
R'
d
d
R'
C
O



H3O
R"
R'

• product is a tertiary alcohol

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Example
Mg
CH3Cl
CH3MgCl
diethylether
O
H3O
(62)
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Synthesis of AlcoholsUsing Organolithium
Reagents

• Organolithium reagents react with aldehydes and
  ketones in the same way that Grignard reagents
  do.

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Example

1. diethyl ether 2. H3O
(76)
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Synthesis of Acetylenic Alcohols
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Using Sodium Salts of Acetylenes
NaNH2
NH3
1. NH3

2. H3O
(65-75)
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Using Acetylenic Grignard Reagents

CH3CH2MgBr
diethyl ether

CH3CH3
2. H3O
(82)
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Retrosynthetic Analysis

• Retrosynthetic analysis is the process by which
  we plan a synthesis by reasoning backward from
  the desired product (the "target molecule").

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Retrosynthetic Analysis of Alcohols

• Step 1 Locate the carbon that bears the
  hydroxyl group.

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Retrosynthetic Analysis of Alcohols

• Step 2 Disconnect one of the groups attached to
  this carbon.

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Retrosynthetic Analysis of Alcohols
OH
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Retrosynthetic Analysis of Alcohols

• What remains is the combination of Grignard
  reagent and carbonyl compound that can be used to
  prepare the alcohol.

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Example

• There are two other possibilities. Can you see
  them?

CH3MgX
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Synthesis
Mg, diethyl ether
CH3Br
CH3MgBr
1.
2. H3O
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Preparation of Tertiary AlcoholsFrom Esters and
Grignard Reagents(also in Chapter 19)
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Grignard reagents react with esters
R'
R'
d
d
C

MgX
O

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

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Grignard reagents react with esters
R'
R'
d
d
C

MgX
O



CH3OMgX

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

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Example

2 CH3MgBr
1. diethyl ether 2. H3O

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

OH
(CH3)2CHCCH3
CH3
(73)
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Alkane Synthesis Using Organocopper Reagents
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Lithium Dialkylcuprates

• Lithium dialkylcuprates are useful synthetic
  reagents.
• They are prepared from alkyllithiums and a
  copper(I) halide.

2RLi CuX
R2CuLi LiX
customary solvents are diethyl ether and
tetrahydrofuran (THF)
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How?

• the alkyllithium first reacts with the copper(I)
  halide

Li
I
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How?

• the alkyllithium first reacts with the copper(I)
  halide

Li
I
then a second molecule of the alkyllithium
reacts with the alkylcopper species formed in
the first step
Li


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Lithium diorganocuprates are used toform CC
bonds



R'X
Ar
R'
ArCu
LiX
Ar2CuLi
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Example Lithium dimethylcuprate

(CH3)2CuLi
CH3(CH2)8CH2I
diethyl ether
CH3(CH2)8CH2CH3
(90)

• primary alkyl halides work best (secondary and
  tertiary alkyl halides undergo elimination)

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Example Lithium diphenylcuprate

(C6H5)2CuLi
CH3(CH2)6CH2I
diethyl ether
CH3(CH2)6CH2C6H5
(99)
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Vinylic halides can be used

Br
(CH3CH2CH2CH2)2CuLi
diethyl ether
(80)
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Aryl halides can be used

I
(CH3CH2CH2CH2)2CuLi
diethyl ether
CH2CH2CH2CH3
(75)
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An Organozinc Reagent forCyclopropane Synthesis
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Iodomethylzinc iodide
formed by reaction of diiodomethane withzinc
that has been coated with copper(called
zinc-copper couple)
Cu
CH2I2 Zn
ICH2ZnI

• reacts with alkenes to form cyclopropanes
• reaction with alkenes is called theSimmons-Smith
  reaction

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Example
CH2CH3
CH2CH3
CH2I2, Zn/Cu
H2C
C
CH3
diethyl ether
CH3
via
(79)
I CH2 ZnI
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Stereospecific syn-addition
CH2I2, Zn/Cu
diethyl ether
CH3CH2
CH2CH3
H
H
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Stereospecific syn-addition
CH3CH2
H
H
CH2CH3
CH2I2, Zn/Cu
diethyl ether
CH3CH2
H
H
CH2CH3
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Carbenes and Carbenoids
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Carbene
name to give to species that contains adivalent
carbon (carbon with two bondsand six electrons)

• (same chemistry with dichlorocarbenes)
• Carbenes are very reactive normally cannot be
  isolated and stored.
• Are intermediates in certain reactions.

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Generation of Dibromocarbene
Br

Br
H
Br


OC(CH3)3

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Generation of Dibromocarbene

C

Br
Br
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Carbenes react with alkenesto give cyclopropanes
Br
KOC(CH3)3
CHBr3
(CH3)3COH
Br
(75)

• CBr2 is an intermediate
• stereospecific syn addition

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End of Chapter 14