Chapter 12 Alkene Reactions Part 2
I.
Hydroboration—Oxidation
A.
H3B O
Hydroboration
1) Borane, BH3, is stable in ether or THF because they stabilize the Lewis
acid by bonding as Lewis bases
2) BH3 will add to alkenes:
BH2
BH3
3)
Filled p-orbitals
H
x2
H
B(C C
)3
Alkylborane
Mechanism: BH3 electrophile, p-bond nucleophile
C C
C C
B
B
Empty p-orbital
C
C
H2B
H
H2B
H
4)
B.
B
Boron binds to the less hindered Carbon (regioselective and syn addition)
due primarily to steric factors
Oxidation of Alkylborane gives an Alcohol
R
H2O2
3 RCH2CHOH
B(RCHCH2R)3
NaOH, H2O
1. Oxidation Mechanism
R
B
-
O OH
2.
-OH
B
OR
NaOH
ROH + Na3BO3
H2O
O OH
Result of Hydroboration—Oxidation is Anti-Markovnikov H2O addition
(CH3)2CHCH3CH=CH2
3.
-
R
1. BH3, THF
2. H2O2, NaOH, H2O
(CH3)2CHCH2CH2CH2OH
Example
H
CH3
BH3, THF
CH3
B
H
CH3
H2O2, NaOH, H2O
OH
II.
Oxidations of Alkenes
A.
Peroxycarboxylic Acids are useful oxidation reagents for alkenes
Cl
O
B.
O
RC O OH
1)
Generic:
2)
MCPBA (metachloroperbezoic acid) is often used in small scale reactions
C O OH
MCPBA
Epoxidation is the common name for the oxidation of an alkene to an
oxacyclopropane (epoxide)
1) The reaction is syn stereospecific
CH3
H
2)
H
CH3
O
MCPBA
CH2Cl2
CH3
H
H
O + RCOH
O
H
3)
O
R
O
Mechanism
CH3
O
More substituted alkenes react faster
MCPBA
CH2Cl2
O
C.
CH3
Hydrolysis of Oxacyclopropanes leads to Vicinal Anti Dihydroxylation
H
H
O
1. MCPBA, CH2Cl2
CH3
CH3
1. MCPBA, CH2Cl2
CH3
H
D.
2. H2O
O
CH3
H
H
CH3
H
CH3
HO
CH3
H
H
H
O
H
Os
OsO4
THF
O
H
H
H2S
O
H
H
HO
H
H
CH3
H
OH
H
racemic-2,3-butanediol
Osmium Tetroxide oxidation gives Vicinal syn Dihydroxylation
1) Overall Reaction:
O
CH3
OH
H
meso-2,3-butanediol
HO
CH3
2. H2O
H
H
CH3
H
OH
H
H
2)
Mechanism
O
O
Os +8
O
3)
H
+6
Os Reduction
HO
H2S
O
H
HO
H2O
H
H
purple
OH
H
H
O
H
H
KMnO4 reacts identically and is used as a test for C=C
+ KMnO4
H
Os
O
H
H
O
O
H
H
H
OH
colorless with
Brown precipitate
H
of MnO2
H
O
E.
O
O
Ozonolysis
1. Ozone = O3 Reacts with Alkenes to give two carbonyl products
O3, CH2Cl2
O
O
O
an ozonide
Zn, CH3COOH
(Reduction)
O
+
O
2)
O
O
Mechanism
Molozonide
O
O
O
O
O
C
C
O
3)
Reduction step is complex
4)
Example
CH3
C C
CH3
H
O3, CH2Cl2
O
O
C C
CH3CH2
O
Zn, CH3COOH
(Reduction)
CH3CH2
CH3
O
CH3
O
+
O
H
III. Radical Additions: Anti-Markovnikov Products
A.
Radical Hydrobromination
1) HBr addition to alkenes is cleanly Markovnikov
2) If peroxides are present (RO—OR), the addition is Anti-Markovnikov
3) Peroxides provide entry into a Radical Addition Mechnism
Initiation
RO
Propagation

RO OR
2 RO
+ HBr
ROH + Br
H
Br +
C
H
Br
H C
H
Br
H
C
H
H C C
CH2CH3
H
C
CH2CH3
H
CH2CH3
Br H
HBr
H C C H
H CH2CH3
4)
5)
6)
The bromine radical attacks least substituted carbon, yielding the carbon
radical that is most stabilized—the most substituted carbon
Only HBr works, HCl and HI reactions are endothermic by radicals
Popular Initiating peroxides:
O
C
O O
O
O
C
O
Bis(1,1-dimethylethyl)peroxide
B.
Dibenzoyl Peroxide
Other Anti-Markovnikov Radical Additions
1) Thiols can add to alkenes by a radical mechanism initiated by peroxide
CH3CH=CH2
CH3CH2SH
ROOR
CH3CHCH2SCH2CH3
H
2)
Halomethanes can add to alkenes by a radical mechanism initiated by
peroxide
H
CH3(CH2)2CH=CH2
HCCl3
ROOR
CH3(CH2)2CHCH2CCl3
IV. Polymerization: producing plastics, and synthetic materials
A.
Acid catalyzed polymerization
1. Alkenes can react with themselves if catalytic H+ is present
C C
C
C C
C
C C C C C C
monomer
2.
3.
4.
CH3
H2C
C
polymer
Cations (even carbocations) attack the p-system
An acid is needed to initiate the polymerization: H2SO4, HF, BF3, etc…
Repeated attack leads to polymerization
CH3
+
H
H3C C
CH3
CH3
H3C
C
CH3
+
CH3
C
H3C
H2C
H3C
C
CH3
C
CH3
C
CH3
CH3
CH2 C
CH3
CH3
CH3
CH3
+
CH3
H2C
CH3
CH3
CH2 C
CH3
CH3
CH2
C
CH3
CH3
CH2 C
CH3
5)
Controlled reactions can be stopped at dimers or trimers or oligomers
CH3
CH3
CH2
H3C C
C
CH3
CH3
CH3
-H
CH2 C
H2C
CH2
H3C C
H
CH3
CH3
+
C
CH3
CH2 C
CH2
CH3
CH3
trimer
CH3
H
CH2
CH2 C
C
CH3
B.
CH3
Oligomer or Polymer
CH2
n
Radical Polymerization is initiated by a radical instead of an acid
CH3
H2C C
RO
CH3
Initiation
CH3
ROCH2
+
C
CH3
CH3
CH3
H2C C
ROCH2
CH3
C
CH3
Propagation
CH3
CH2 C
CH3
1)
RO +
Polyethylene has a branched structure and a MW of 1,000,000
RO
H2C CH2
ROCH2CH2CH2CH2
ROCH2CHCH2CH2
CH2CH2CH2CH2
2)
Polyvinylchloride (PVC) is made with regioselective polymerization
a) Radicals only add to unsubstituted end
b) Radicals next to Cl are very stable, so they are formed preferentially
c) A head-to-tail structure results
RO +
H2C CHCl
CH2CH
Cl
C.
n
Anionic Polymerization
1) Strong bases can also initiate polymerization of alkenes
CH3
H2C C
CH3
-
OH
+
HOH2C C
CH3
CH3
CH3
CH3
H2C C
HOCH2
CH3
C
CH3
CH3
CH2 C
CH3