Transcript Organic Chemistry Fifth Edition

14.11
Alkane Synthesis Using
Organocopper Reagents
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)]
How?
the alkyllithium first reacts with the copper(I)
halide
R
Cu
Li
R
Li+
I
Cu
I–
then a second molecule of the alkyllithium
reacts with the alkylcopper species formed in
the first step
R
Li+
R Li
R
Cu
R
Cu –
Lithium diorganocuprates are used to
form C—C bonds
R2CuLi +
R'X
R
R' + RCu + LiX
Ar2CuLi +
R'X
Ar
R' + ArCu + LiX
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)
Example: Lithium diphenylcuprate
(C6H5)2CuLi + CH3(CH2)6CH2I
diethyl ether
CH3(CH2)6CH2C6H5
(99%)
Vinylic halides can be used
(CH3CH2CH2CH2)2CuLi +
diethyl ether
CH2CH2CH2CH3
(80%)
Br
Aryl halides can be used
(CH3CH2CH2CH2)2CuLi +
diethyl ether
CH2CH2CH2CH3
(75%)
I
14.12
An Organozinc Reagent
for
Cyclopropane Synthesis
Iodomethylzinc iodide
formed by reaction of diiodomethane with
zinc 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 the
Simmons-Smith reaction
Example
CH2CH3
H2C
CH2I2, Zn/Cu
CH2CH3
C
CH3
diethyl ether
CH3
(79%)
Stereospecific syn-addition
CH3CH2
CH2CH3
C
H
CH2I2, Zn/Cu
CH3CH2
H
C
H
diethyl ether
CH2CH3
H
Stereospecific syn-addition
CH3CH2
H
C
H
CH2I2, Zn/Cu
CH3CH2
H
C
CH2CH3
diethyl ether
H
CH2CH3
14.13
Carbenes and Carbenoids
Carbene
name to give to species that contains a
divalent carbon (carbon with two bonds
and six electrons)
••
C
Br
Br
dibromocarbene
Carbenes are very reactive; normally cannot
be isolated and stored.
Are intermediates in certain reactions.
Generation of Dibromocarbene
Br
Br
C
– ••
H + •• OC(CH3)3
••
Br
Br
Br
–
•
C•
Br
+
H
••
OC(CH3)3
••
Generation of Dibromocarbene
••
C
Br
Br
Br
Br
C ••
Br
–
+
Br
–
Carbenes react with alkenes
to give cyclopropanes
KOC(CH3)3
Br
(CH3)3COH
Br
+ CHBr3
(75%)
CBr2 is an intermediate
stereospecific syn addition
14.14
Transition-Metal Organic Compounds
Introduction
Many organometallic compounds derived
from transition metals have useful properties.
Typical transition metals are iron, nickel,
chromium, platinum, and rhodium.
18-Electron Rule
The number of ligands attached to a metal
will be such that the sum of the electrons
brought by the ligands plus the valence
electrons of the metal equals 18.
When the electron-count is less than 18,
metal is said to be coordinatively unsaturated
and can take on additional ligands.
18-Electron rule is to transition metals as
the octet rule is to second-row elements.
Example
CO
OC
Ni
CO
CO
Nickel carbonyl
Ni has the electron
configuration [Ar]4s23d8
Ni has 10 valence
electrons
Each CO uses 2
electrons to bond to Ni
4 CO contribute 8
valence electrons
10 + 8 = 18
(Benzene)tricarbonylchromium
OC
Cr
CO
CO
Cr has the electron configuration [Ar]4s23d4
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.
Ferrocene
Fe
Fe2+ has the electron configuration [Ar]3d6
Each cyclopentadienide anion contributes 6 
electrons
Total 6 + 6 + 6 = 18
Organometallic compounds with cyclopentadienide
ligands are called metallocenes.
14.15
Homogeneous Catalytic Hydrogenation
Wilkinson’s Catalyst
Wilkinson’s 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.
P(C6H5)3
(C6H5)3P
Rh
Cl
P(C6H5)3
Wilkinson's Catalyst
Mechanism of Homogeneous Hydrogenation
Steps 1 and 2: Catalyst is converted to the active form.
+ H2
Cl
then
- (C6H5)3P
P(C6H5)3
Cl
Rh
(C6H5)3P
H
P(C6H5)3
Rh
(C6H5)3P
P(C6H5)3
then
- (C6H5)3P
H
+ H2
This is the active
form of the catalyst.
Mechanism of Homogeneous Hydrogenation
Step 3: Alkene bonds to rhodium through  electrons.
Cl
Cl
H
P(C6H5)3
Rh
P(C6H5)3
Rh
(C6H5)3P
H
H
(C6H5)3P
H
+ CH2=CHCH3
CH2=CHCH3
Rhodium-alkene complex
Mechanism of Homogeneous Hydrogenation
Step 4: Rhodium-alkene complex rearranges.
Cl
H
P(C6H5)3
Cl
Rh
(C6H5)3P
H
Rhodium-alkene complex
P(C6H5)3
Rh
(C6H5)3P
CH2=CHCH3
H
CH2CH2CH3
Mechanism of Homogeneous Hydrogenation
Step 5: Hydride migrates from Rh to carbon.
Cl
H
P(C6H5)3
Rh
(C6H5)3P
CH2CH2CH3
[(C6H5)3P]2RhCl
+ CH3CH2CH3
Mechanism of Homogeneous Hydrogenation
Step 6: Active form of the catalyst is regenerated.
Cl
[(C6H5)3P]2RhCl
+ H2
H
P(C6H5)3
Rh
(C6H5)3P
H
14.16
Olefin Metathesis
Olefin Metathesis
In crossed-olefin metathesis, one alkene is converted
to a mixture of two new alkenes.
catalyst
2 CH3CH=CH2
CH2=CH2 +
CH3CH=CHCH3
The reaction is reversible, and regardless of
whether we start with propene or a 1:1 mixture of
ethylene and 2-butene, the same mixture is
obtained.
Olefin Metathesis
The reaction is generally catalyzed a transition metal
complex. Typically Ru, W, or Mo are used.Shown
below is Grubb’s catalyst.
Cl
PCy
Ru
Cl
PCy
CH
Cy =
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.
=HC
CH=
catalyst
-80oC
n
Norbornene
Polynorbornene
14.17
Ziegler-Natta Catalysis of
Alkene Polymerization
The catalysts used in coordination
polymerization are transition-metal organic
compounds.
Ethylene oligomerization
n H2C
CH2
Al(CH2CH3)3
CH3CH2(CH2CH2)n-2CH
CH2
Triethylaluminum catalyzes the formation of
alkenes from ethylene.
These compounds are called ethylene
oligomers and the process is called
oligomerization.
Karl Ziegler (1950)
n H2C
CH2
Al(CH2CH3)3
CH3CH2(CH2CH2)n-2CH
CH2
Ziegler found that oligomerization was
affected differently by different transition
metals. Some gave oligomers with 6-18
carbons, others gave polyethylene.
Giulio Natta
n H2C
CHCH3
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.
Karl Ziegler (1950)
n H2C
CH2
Al(CH2CH3)3
CH3CH2(CH2CH2)n-2CH
CH2
The ethylene oligomers formed under
Ziegler's conditions are called linear -olefins
and have become important industrial
chemicals.
Karl Ziegler (1950)
n H2C
CH2
Al(CH2CH3)3
CH3CH2(CH2CH2)n-2CH
CH2
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.
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.
Cl
Zr
Cl
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.
Ziegler-Natta Catalysts
The metallocene is used in combination
with a promoter such as methyl alumoxane
(MAO)
O—Al—O—Al
CH3
CH3
n
Mechanism of Coordination Polymerization
Cl
Zr
MAO
Cl
Zr
Cl
CH3
– Cl–
This is the active
form of the catalyst.
+
Zr
CH3
Mechanism of Coordination Polymerization
CH3
+
Zr
CH2
H2C
+
Zr
CH2
CH2
H2C
CH3
+
Zr
CH2
CH3
Mechanism of Coordination Polymerization
+
Zr
H2C
CH2CH2CH3
CH2
CH2
H2C
+
Zr
CH2
CH2
+
Zr
CH3
CH2CH2CH2CH2CH3
Mechanism of Coordination Polymerization
etc.
H2C
CH2
+
Zr
CH2CH2CH2CH2CH3