Addition Reactions of Carbon-Carbon Pi Bonds – Part 3

Addition Reactions of Carbon-CarbonPi Bonds – Part 3

Ni

H2

O3

CH2O C

O

(CH2)14CH3

COH C

O

(CH2)7CH=CH(CH2)7CH3

CH2O P

O

O

OCH2CH2N(CH3)3

Miscellaneous Pi Bond Addition Reactions

Do all carbon-carbon pi bond addition reactions...

...involve electrophiles and carbocations?

...follow Markovnikov's rule?

•Anti-Markovnikov addition of H2O

•No electrophile

•Markovnikov's rule irrelevant

•Catalytic Hydrogenation

Examples•Hydroboration-Oxidation

CH31. BH3

2. NaOH, HOOH

H

CH3

H

OHH

}

CH3

CH3

H2

Pt

CH3

H

H

CH3

Chem 14D Lecture Supplement: Addition Reactions of Carbon-Carbon Pi Bonds Part 3 -- Page 1

Catalytic Hydrogenation

Catalytic hydrogenation: Addition of H2 in the presence of a catalyst.

Example:

Catalyst usually = Pt, Pd, Ni

Why catalyst needed?

•Bonds lost: C-C pi and H-H sigma Bonds gained: 2 x C-H sigma

•Bonds strengths: Sigma > pi

•Therefore ΔG < 0 (Keq > 1) ... but ΔG‡ (one or more mechanism steps) too high

•Catalyst alters mechanism but not Keq

CH3

CH3Pt

CH3

H

H

CH3

H H

Catalytic Hydrogenation

CH3

CH3

H2

Pt

CH3

H

H

CH3Syn addition

Why syn addition? You are not responsible for this mechanism.

•H2 is not an electrophile

•H2 symmetrical so Markovnikov's rule irrelevant

New bonds on same alkene face

Pt surface

H

R

RR

R

H

Pt surface

H H

RR R

R

Pt surfacePt surface

H2

Pt surface

H2

Pt surface

H H

Pt surface

H2

Pt surface

H HC C

R

R R

R

Pt surface

H2

Pt surface

H HC C

R

R R

R

Pt surface

H H

R

R R

R

Pt surface

H

R

RR

R

H


Catalytic Hydrogenation - ApplicationPartially Hydrogenated Vegetable Oil and Margarine

Polyunsaturated fat•Lower melting point (oil)•Susceptible to air oxidation

Monounsaturated fat•Higher melting point (margarine)•Less susceptible to air oxidation

Trans fatHealth concerns

Saturated fatHealth concerns

H2, Ni

and/or

H2, Ni

Addition of Br2 and Cl2

Example: Anti addition

New bonds on opposite alkene faceCH3

CH3Br Br

Br

Br

CH3

CH3

•Br2 has no H so Markovnikov's Rule does not apply

•Mechanism: You are not responsible for this mechanism

Br2 polarized bypi electrons

•Similar reaction for Cl2

Backside attack

CH3

CH3

δ+ δ−Br Br Br

CH3

CH3BrBr

Br

CH3

CH3


Hydroboration-Oxidation

Example:

Hydroboration Oxidation

Same as:

•Syn, anti-Markovnikov addition of water

•Compare with H3O+, which gives Markovnikov addition

•Electrophile = BH3 Open octet on boron

CH3BH3

BH2

HCH3

HOOH

HO-

OH

HCH3

CH3

OH

HCH3

1. BH3

2. HOOH, HO-

CH3BH3

BH2

HCH3

Hydroboration-Oxidation

•Hydroboration-oxidation used broadly in laboratory, industrial synthesis

Example:

Artemisinin (anti-malarial)

CH3

CH3

HO

1. BH3

2. HOOH, HO-

H

CH3

CH3

HO

1. BH3

2. HOOH, HO-

CH3

CH3

HO

HOHH H

CH3

CH3

HO

1. BH3

2. HOOH, HO-

CH3

CH3

HO

HO

Additional steps

O

O

O

CH3

CH3H

O

O

H3C

HH H

H H


H

CH3

H3C

FAQ: Addition Reaction StereochemistrySyn or anti or mixture?

•Catalytic hydrogenation – syn anti

•Br2 or Cl2 – syn anti

•Hydroboration/oxidation – syn anti

•HBr, H3O+, etc. – syn anti ???

Syn addition

Anti addition

Addition reaction stereochemistry:

Conclusion: H-Br + alkene gives... Anti addition Syn addition A mixture is formed

H

CH3

Br

CH3CH3

CH3

H Br

H

CH3

H3C

Br

CH3

CH3

H Br

Br

H

CH3

CH3

Br

Explore via mechanism...

Ozonolysisozone destruction

Cleavage of C=C or by reaction with ozone (O3).

Example:

Addition stereochemistry: Syn Anti But none at the end

Orientation: Markovnikov addition Anti-Markovnikov addition Both Neither

Electrophile: Alkene O3 Both Neither

Mechanism: You are not responsible for this mechanism

C C

Ph

H

CC

H

CH3

OO O

Ph

H

CC

H

CH3

OO O

C C

OO

O

H

Ph H

CH3Ph

H

CC

H

CH3

OO O

C C

OO

O

H

Ph H

CH3

OO

CO

CH

Ph H

CH3Ph

H

CC

H

CH3

OO O

C C

OO

O

H

Ph H

CH3

(CH3)2SOO

CO

CH

Ph H

CH3Ph

H

CC

H

CH3

OO O

C C

OO

O

H

Ph H

CH3

(CH3)2S

CPh O

H O

H

CCH3

+

OO

CO

CH

Ph H

CH3

Ph

H

CC

H

CH3C

Ph O

H O

H

CCH3

+1. O3

2. (CH3)2S


FAQ: What Must I Know About These Reactions?

You should know...

•Reactants and products

•Stereochemistry: Syn addition, anti addition, mixture, etc.

•Orientation: Markovnikov versus anti-Markovnikov addition

You are not responsible for mechanisms of...

•Catalytic hydrogenation

•Addition of Br2/Cl2

•Hydroboration-oxidation

•Ozonolysis

Familiarity with mechanisms = easier to remember stereochemistry, etc.

Biological Examples: Ozonolysis ofPulmonary Surfactant

Pulmonary surfactant assists diffusion of O2 from air into bloodstream

•Surfactant = surface active agent

•Requires both polar and nonpolar regions

•↓ nonpolar portion = ↓ surfactant effect = less efficient O2 diffusion

CCH2O

CO

O

(CH2)14CH3

H

CH2O P

O

O

OCH2CH2N(CH3)3

C

O

(CH2)7CH=CH(CH2)7CH3

O3

CCH2O

CO

O

(CH2)14CH3

H

CH2O P

O

O

OCH2CH2N(CH3)3

C

O

(CH2)7COOH + HOOC(CH2)7CH3


Biological Examples: Steroid Biosynthesis

Steroid skeleton:

Biosynthetic pathway: Start by gathering the necessary number of carbons...

Acetyl CoA SCoA = Good leaving group

Many enzymatic steps

O2, squalene epoxidaseSqualene

O

Squalene oxide

CH3C SCoA

O

15 x


Form the rings...

O

HO

Enzymatic protonation

HO



HO

HO

HOHO

HO

CH3

R

HO

RH

Biological Examples: Steroid BiosynthesisNow for some carbocation fates...

HO

R

H

HO

R

Enz H

HO

R


HO

HO


Almost done...

Lanosterol

19 moreenzymatic steps

Cholesterol

•Precursor to all other steroids


Documents

Addition Reactions of Carbon-Carbon Pi Bonds – Part 3