Chapter 12 Alkene Reactions Part 2

I. Hydroboration—Oxidation A. 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:

3) Mechanism: BH3 electrophile, -bond nucleophile

H3B O

BH3HBH2 x 2

B(C C

H

)3

B

C C

B

C C

C C

H2B H HH2B

Filled p-orbitals

Empty p-orbital

Alkylborane

4) Boron binds to the less hindered Carbon (regioselective and syn addition) due primarily to steric factors

B. Oxidation of Alkylborane gives an Alcohol

1. Oxidation Mechanism

2. Result of Hydroboration—Oxidation is Anti-Markovnikov H2O addition

3. Example

B(RCHCH2R)3H2O2

NaOH, H2O3 RCH2CHOH

R

(CH3)2CHCH3CH=CH21. BH3, THF

2. H2O2, NaOH, H2O(CH3)2CHCH2CH2CH2OH

BR

-O OHB

R

O OH

-OH-

B ORNaOH

H2OROH + Na3BO3

CH3

H

HCH3

BBH3, THF H2O2, NaOH, H2O

OH

CH3

II. Oxidations of AlkenesA. Peroxycarboxylic Acids are useful oxidation reagents for alkenes

1) Generic:

2) MCPBA (metachloroperbezoic acid) is often used in small scale reactions

B. Epoxidation is the common name for the oxidation of an alkene to an oxacyclopropane (epoxide)

1) The reaction is syn stereospecific

2) Mechanism

3) More substituted alkenes react faster

RC O OH

O

C O OH

OCl

MCPBA

HH

O

CH3 CH3CH3H

HCH3 MCPBA

CH2Cl2

O

H

O

O

RO + RCOH

O

CH2Cl2

MCPBAO

C. Hydrolysis of Oxacyclopropanes leads to Vicinal Anti Dihydroxylation

D. Osmium Tetroxide oxidation gives Vicinal syn Dihydroxylation

1) Overall Reaction:

CH3H

HCH3

HH

CH3CH3

1. MCPBA, CH2Cl2

2. H2O

H

H

HO

CH3CH3

OHH

H

O

CH3

CH3

CH3H

O

CH3

H

CH3

H

HO

CH3H

OH2. H2O

1. MCPBA, CH2Cl2

racemic-2,3-butanediol

meso-2,3-butanediol

HH

HHOsO4

THFH

HH

H

O O

Os

O O

H2S

HH

H

H

HO OH

2) Mechanism

3) KMnO4 reacts identically and is used as a test for C=C

E. Ozonolysis

1. Ozone = O3 Reacts with Alkenes to give two carbonyl products

HH

H

H

HO OH

HH

HH

KMnO4H2O+

purple

colorless withBrown precipitateof MnO2

OO

O

O

Os

O

O

O

+8Os Reduction

HH

H

H

O OOs

O O

HH

HH

HO OHH2S+6

O3, CH2Cl2O O

O

an ozonide

Zn, CH3COOH

(Reduction)O O+

2) Mechanism

3) Reduction step is complex

4) Example

OO

O

OO

O

C C

Molozonide

C

O C

OO O O

O

C C

HCH3

CH3CH3CH2

+ O

H

CH3

O

CH3

CH3CH2

(Reduction)

Zn, CH3COOHO3, CH2Cl2

III. Radical Additions: Anti-Markovnikov ProductsA. 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

RO OR

2 RO

RO + HBr ROH + Br

Br + C C

H

H H

CH2CH3

C C

H

H

H

CH2CH3

Br

C C

H

H

H

CH2CH3

BrHBr

C C

H

H

H

CH2CH3

Br

H

Initiation

Propagation

4) The bromine radical attacks least substituted carbon, yielding the carbon radical that is most stabilized—the most substituted carbon

5) Only HBr works, HCl and HI reactions are endothermic by radicals

6) Popular Initiating peroxides:

B. Other Anti-Markovnikov Radical Additions

1) Thiols can add to alkenes by a radical mechanism initiated by peroxide

2) Halomethanes can add to alkenes by a radical mechanism initiated by peroxide

O O

Bis(1,1-dimethylethyl)peroxide

C

O

OO

C

O

Dibenzoyl Peroxide

CH3CH=CH2CH3CH2SH

ROORCH3CHCH2SCH2CH3

H

CH3(CH2)2CH=CH2HCCl3ROOR

CH3(CH2)2CHCH2CCl3

H

IV. Polymerization: producing plastics, and synthetic materialsA. Acid catalyzed polymerization

1. Alkenes can react with themselves if catalytic H+ is present

2. Cations (even carbocations) attack the -system

3. An acid is needed to initiate the polymerization: H2SO4, HF, BF3, etc…

4. Repeated attack leads to polymerization

C C C CC

monomer

C C C C C CC

polymer

H2C C

CH3

CH3

H+

H3C C

CH3

CH3

H2C C

CH3

CH3

+ H3C C

CH3

CH3

CH2 C

CH3

CH3

H3C C

CH3

CH3

CH2 C

CH3

CH3

+ H2C C

CH3

CH3

C

CH3

CH3

CH2 C

CH3

CH3

H3C C

CH3

CH3

CH2

5) Controlled reactions can be stopped at dimers or trimers or oligomers

B. Radical Polymerization is initiated by a radical instead of an acid

H2CH

C

CH3

CH3

CH2 C

CH3

H3C C

CH3

CH3

CH2-H+

C

CH3

CH3

CH2 C

CH3

H3C C

CH3

CH3

CH2

CH2

trimer

C

CH3

CH3

CH2 C

CH3

CH2

CH2

H

n

Oligomer or Polymer

Initiation Propagation

ROROCH2 C

CH3

CH3

CH2 C

CH3

CH3

+ H2C C

CH3

CH3

ROCH2 C

CH3

CH3

H2C C

CH3

CH3

1) Polyethylene has a branched structure and a MW of 1,000,000

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

C. Anionic Polymerization

1) Strong bases can also initiate polymerization of alkenes

RO + H2C CH2 ROCH2CH2CH2CH2

ROROCH2CHCH2CH2

CH2CH2CH2CH2

H2C CHCl+RO CH2CH

Cl n

H2C C

CH3

CH3

HOH2C C

CH3

CH3

H2C C

CH3

CH3

+ HOCH2 C

CH3

CH3

CH2 C

CH3

CH3

-OH