14.11 Alkane Synthesis Using Organocopper Reagents

1
14.11Alkane 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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14.12An 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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14.13Carbenes 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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14.14Transition-Metal Organic Compounds
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Introduction

• Many organometallic compounds derivedfrom
  transition metals have useful properties.
• Typical transition metals are iron,
  nickel,chromium, platinum, and rhodium.

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18-Electron Rule

• The number of ligands attached to a metalwill be
  such that the sum of the electronsbrought by the
  ligands plus the valenceelectrons of the metal
  equals 18.
• When the electron-count is less than 18, metal
  is said to be coordinatively unsaturatedand can
  take on additional ligands.
• 18-Electron rule is to transition metals asthe
  octet rule is to second-row elements.

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Example

• Ni has the electron configuration Ar4s23d8
• Ni has 10 valence electrons
• Each CO uses 2 electrons to bond to Ni
• 4 CO contribute 8 valence electrons
• 10 8 18

CO
OC
CO
Ni
CO
Nickel carbonyl
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Example
(Benzene)tricarbonylchromium

• Cr has the electron configuration Ar4s23d4
• Cr has 6 valence electrons
• Each CO uses 2 electrons to bond to Cr
• 3 CO contribute 6 valence electrons
• benzene uses its 6 p electrons to bind to Cr.

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Example
Ferrocene

• Fe2 has the electron configuration Ar3d6
• Each cyclopentadienide anion contributes 6 p
  electrons
• Total 6 6 6 18
• Organometallic compounds with cyclopentadienide
  ligands are called metallocenes.

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14.15Homogeneous Catalytic Hydrogenation

• Wilkinsons Catalyst

27
Wilkinsons Catalyst
Ni, Pt, Pd, and Rh can act as a heterogeneous
catalyst in the hydrogenation of
alkenes. However, tris(triphenylphosphine)rhodium
chloride was found to be soluble in organic
solvents. This catalyst was developed by Sir
Geoffrey Wilkinson, who received a Nobel Prize in
1973.
28
Mechanism of Homogeneous Hydrogenation
Steps 1 and 2 Catalyst is converted to the
active form.
This is the activeform of the catalyst.
29
Mechanism of Homogeneous Hydrogenation
Step 3 Alkene bonds to rhodium through ?
electrons.
30
Mechanism of Homogeneous Hydrogenation
Step 4 Rhodium-alkene complex rearranges.
31
Mechanism of Homogeneous Hydrogenation
Step 5 Hydride migrates from Rh to carbon.
32
Mechanism of Homogeneous Hydrogenation
Step 6 Active form of the catalyst is
regenerated.
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14.16Olefin Metathesis
34
Olefin Metathesis

• In crossed-olefin metathesis, one alkene is
  converted to a mixture of two new alkenes.

The reaction is reversible, and regardless of
whether we start with propene or a 11 mixture of
ethylene and 2-butene, the same mixture is
obtained.
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Olefin Metathesis

• The reaction is generally catalyzed a transition
  metal complex. Typically Ru, W, or Mo are
  used.Shown below is Grubbs catalyst.

36
Ring-Opening Metathesis

• Ring-opening metathesis is used as a method of
  polymerization.
• Usually, it is applied most often when ring
  opening creates a relief of strain, as in some
  bicyclic alkenes.

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14.17Ziegler-Natta Catalysis of Alkene
Polymerization

• The catalysts used in coordination
  polymerization are transition-metal organic
  compounds.

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Ethylene oligomerization
Al(CH2CH3)3

• Triethylaluminum catalyzes the formation of
  alkenes from ethylene.
• These compounds are called ethylene oligomers
  and the process is called oligomerization.

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Karl Ziegler
Al(CH2CH3)3

• Ziegler found that oligomerization was affected
  differently by different transition metals. Some
  gave oligomers with 6-18 carbons, others gave
  polyethylene.

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Karl Ziegler
Al(CH2CH3)3

• The ethylene oligomers formed under Ziegler's
  conditions are called linear a-olefins and have
  become important industrial chemicals.

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Karl Ziegler
Al(CH2CH3)3

• The polyethylene formed under Ziegler's
  conditions is called high-density polyethylene
  and has, in many ways, more desirable properties
  than the polyethylene formed by free-radical
  polymerization.

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Giulio Natta
Al(CH2CH3)3
polypropylene

• Natta found that polymerization of propene under
  Ziegler's conditions gave mainly isotactic
  polypropylene (methyl groups on same side). This
  discovery made it possible to produce
  polypropylene having useful properties.

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Ziegler-Natta Catalysts

• A typical Ziegler-Natta catalyst is a
  combination of TiCl4 and (CH3CH2)2AlCl, or TiCl3
  and (CH3CH2)3Al.
• Many Ziegler-Natta catalyst combinations include
  a metallocene.

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Mechanism of Coordination Polymerization
Al(CH2CH3)3

TiCl4
ClAl(CH2CH3)2

CH3CH2TiCl3
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Mechanism of Coordination Polymerization
Al(CH2CH3)3

TiCl4
ClAl(CH2CH3)2

CH3CH2TiCl3

CH3CH2TiCl3
CH3CH2TiCl3
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Mechanism of Coordination Polymerization
CH3CH2TiCl3
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Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2
CH3CH2TiCl3
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Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2
TiCl3
CH3CH2CH2CH2
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Mechanism of Coordination Polymerization
TiCl3
CH3CH2CH2CH2
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Mechanism of Coordination Polymerization
etc.
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End of Chapter 14