Title: 14.11 Alkane Synthesis Using Organocopper Reagents
114.11Alkane Synthesis Using Organocopper
Reagents
2Lithium Dialkylcuprates
- Lithium dialkylcuprates are useful synthetic
reagents. - They are prepared from alkyllithiums and a
copper(I) halide.
3How?
- the alkyllithium first reacts with the copper(I)
halide
Li
I
4Lithium diorganocuprates are used toform CC
bonds
R'X
Ar
R'
ArCu
LiX
Ar2CuLi
5Example 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)
6Example Lithium diphenylcuprate
(C6H5)2CuLi
CH3(CH2)6CH2I
diethyl ether
CH3(CH2)6CH2C6H5
(99)
7Vinylic halides can be used
Br
(CH3CH2CH2CH2)2CuLi
diethyl ether
(80)
8Aryl halides can be used
I
(CH3CH2CH2CH2)2CuLi
diethyl ether
CH2CH2CH2CH3
(75)
914.12An Organozinc Reagent forCyclopropane
Synthesis
10Iodomethylzinc 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
11Example
CH2CH3
CH2CH3
CH2I2, Zn/Cu
H2C
C
CH3
diethyl ether
CH3
(79)
12Stereospecific syn-addition
CH2I2, Zn/Cu
diethyl ether
13Stereospecific syn-addition
CH3CH2
H
H
CH2CH3
1414.13Carbenes and Carbenoids
15Carbene
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.
16Generation of Dibromocarbene
Br
Br
H
Br
17Generation of Dibromocarbene
C
Br
Br
Br
18Carbenes react with alkenesto give cyclopropanes
Br
KOC(CH3)3
CHBr3
(CH3)3COH
Br
(75)
- CBr2 is an intermediate
- stereospecific syn addition
1914.14Transition-Metal Organic Compounds
20Introduction
- Many organometallic compounds derivedfrom
transition metals have useful properties. - Typical transition metals are iron,
nickel,chromium, platinum, and rhodium.
2118-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.
22Example
- 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.
24Ferrocene
- 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.
2514.15Homogeneous Catalytic Hydrogenation
26Wilkinsons 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.
27Mechanism of Homogeneous Hydrogenation
Steps 1 and 2 Catalyst is converted to the
active form.
This is the activeform of the catalyst.
28Mechanism of Homogeneous Hydrogenation
Step 3 Alkene bonds to rhodium through ?
electrons.
29Mechanism of Homogeneous Hydrogenation
Step 4 Rhodium-alkene complex rearranges.
30Mechanism of Homogeneous Hydrogenation
Step 5 Hydride migrates from Rh to carbon.
31Mechanism of Homogeneous Hydrogenation
Step 6 Active form of the catalyst is
regenerated.
3214.16Olefin Metathesis
33Olefin 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.
34Olefin Metathesis
- The reaction is generally catalyzed a transition
metal complex. Typically Ru, W, or Mo are
used.Shown below is Grubbs catalyst.
35Ring-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.
3614.17Ziegler-Natta Catalysis of Alkene
Polymerization
- The catalysts used in coordination
polymerization are transition-metal organic
compounds.
37Ethylene oligomerization
Al(CH2CH3)3
- Triethylaluminum catalyzes the formation of
alkenes from ethylene. - These compounds are called ethylene oligomers
and the process is called oligomerization.
38Karl 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.
39Giulio 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.
40Karl Ziegler (1950)
Al(CH2CH3)3
- The ethylene oligomers formed under Ziegler's
conditions are called linear ?-olefins and have
become important industrial chemicals.
41Karl 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.
42Ziegler-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.
43Ziegler-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.
44Ziegler-Natta Catalysts
The metallocene is used in combination with a
promoter such as methyl alumoxane (MAO)
45Mechanism of Coordination Polymerization
MAO
This is the activeform of the catalyst.
46Mechanism of Coordination Polymerization
47Mechanism of Coordination Polymerization
48Mechanism of Coordination Polymerization
etc.