14.11 Alkane Synthesis Using Organocopper Reagents

1
14.11Alkane Synthesis Using Organocopper
Reagents
2
Lithium Dialkylcuprates

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

3
How?

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

Li
I
4
Lithium diorganocuprates are used toform CC
bonds



R'X
Ar
R'
ArCu
LiX
Ar2CuLi
5
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)

6
Example Lithium diphenylcuprate

(C6H5)2CuLi
CH3(CH2)6CH2I
diethyl ether
CH3(CH2)6CH2C6H5
(99)
7
Vinylic halides can be used

Br
(CH3CH2CH2CH2)2CuLi
diethyl ether
(80)
8
Aryl halides can be used

I
(CH3CH2CH2CH2)2CuLi
diethyl ether
CH2CH2CH2CH3
(75)
9
14.12An Organozinc Reagent forCyclopropane
Synthesis
10
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

11
Example
CH2CH3
CH2CH3
CH2I2, Zn/Cu
H2C
C
CH3
diethyl ether
CH3
(79)
12
Stereospecific syn-addition
CH2I2, Zn/Cu
diethyl ether
13
Stereospecific syn-addition
CH3CH2
H
H
CH2CH3
14
14.13Carbenes and Carbenoids
15
Carbene
name to give to species that contains adivalent
carbon (carbon with two bondsand six electrons)

• Carbenes are very reactive normally cannot be
  isolated and stored.
• Are intermediates in certain reactions.

16
Generation of Dibromocarbene
Br

Br
H
Br
17
Generation of Dibromocarbene

C


Br
Br
Br
18
Carbenes react with alkenesto give cyclopropanes
Br
KOC(CH3)3
CHBr3
(CH3)3COH
Br
(75)

• CBr2 is an intermediate
• stereospecific syn addition

19
14.14Transition-Metal Organic Compounds
20
Introduction

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

21
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.

22
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
23
(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 ? electrons to bind to Cr.

24
Ferrocene

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

25
14.15Homogeneous Catalytic Hydrogenation

• Wilkinsons Catalyst

26
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.
27
Mechanism of Homogeneous Hydrogenation
Steps 1 and 2 Catalyst is converted to the
active form.
This is the activeform of the catalyst.
28
Mechanism of Homogeneous Hydrogenation
Step 3 Alkene bonds to rhodium through ?
electrons.
29
Mechanism of Homogeneous Hydrogenation
Step 4 Rhodium-alkene complex rearranges.
30
Mechanism of Homogeneous Hydrogenation
Step 5 Hydride migrates from Rh to carbon.
31
Mechanism of Homogeneous Hydrogenation
Step 6 Active form of the catalyst is
regenerated.
32
14.16Olefin Metathesis
33
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.
34
Olefin Metathesis

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

35
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.

36
14.17Ziegler-Natta Catalysis of Alkene
Polymerization

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

37
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.

38
Karl Ziegler (1950)
Al(CH2CH3)3

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

39
Giulio Natta
Al(CH2CH3)3
polypropylene

• Natta found that polymerization of propene under
  Ziegler's conditions gave mainly isotactic
  polypropylene. This discovery made it possible
  to produce polypropylene having useful properties.

40
Karl Ziegler (1950)
Al(CH2CH3)3

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

41
Karl Ziegler (1950)
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.

42
Ziegler-Natta Catalysts

• Early Ziegler-Natta catalyst were a combination
  of TiCl4 and (CH3CH2)2AlCl, or TiCl3 and
  (CH3CH2)3Al.
• Currently used Ziegler-Natta catalyst
  combinations include a metallocene such as
  bis(cyclopentadienyl)zirconium dichloride.

43
Ziegler-Natta Catalysts

• Early Ziegler-Natta catalyst were a combination
  of TiCl4 and (CH3CH2)2AlCl, or TiCl3 and
  (CH3CH2)3Al.
• Currently used Ziegler-Natta catalyst
  combinations include a metallocene such as
  bis(cyclopentadienyl)zirconium dichloride.

44
Ziegler-Natta Catalysts
The metallocene is used in combination with a
promoter such as methyl alumoxane (MAO)
45
Mechanism of Coordination Polymerization
MAO
This is the activeform of the catalyst.
46
Mechanism of Coordination Polymerization
47
Mechanism of Coordination Polymerization
48
Mechanism of Coordination Polymerization
etc.