Reduction of Alkenes & Alkynes - Syracuse University

17 Pages·2015·0.37 MB·English

by  Nancy Totah

Reduction of Alkenes & Alkynes
Catalytic Hydrogenation: ! ! !
• ease of reduction
Reduction of Alkenes & Alkynes:
Catalytic Hydrogenation: ! ! !
H H H H
R R
H H
R R R R
• Heterogeneous Catalysts
- transition metal catalysts adsorbed on solid support

metal (reactivity): Pt > Pd, Rh ~ Ru > Ni
support: carbon, alumina, silica

reaction solvent: EtOAc > EtOH > HOAc

examples: 10% Pd/C, PtO , etc.
2

- can be poisoned (S, amines (Pt), etc.)

- CAUTION! H + O + Pd/C  can ignite!
2 2
Reduction of Alkenes & Alkynes:
Catalytic Hydrogenation: ! ! !
• Heterogeneous Catalysts
mechanism
H2 H H H H
catalyst
surface
H H H H
Reduction of Alkenes & Alkynes
Catalytic Hydrogenation: ! ! !
H H H H
R R
H H
R R R R
• Homogeneous Catalysts
- catalyst is soluble in reaction media

PF
examples: 6
.
PCy
PPh Ir 3
3
Ph3P Rh Cl N
Ph3P
Wilkinsen's catalyst Crabtree's catalyst
(Ph3P)3RhCl [Cy3PIr(COD)py]PF6

- NOT poisoned by sulfur
- very sensitive to steric effects
Reduction of Alkenes & Alkynes:
Catalytic Hydrogenation: ! ! !
• Homogeneous Catalysts
mechanism (Wilkinson’s catalyst)
I PPh
3
Ph P Rh Cl 16 e-
3
Ph P
3
- PPh
3
H
oxidative
H
R I addition
Ph P
3 Rh Cl
reductive Ph P
3
elimination
14 e-
H H
R
Ph P III Ph P III
3 Rh 3 Rh H
Ph P Ph P
16 e- 3 3 16 e-
Cl Cl
H
Ph P III H
3 Rh R
Ph P
olefin 3 Cl R
insertion
18 e-
Reduction of Alkenes & Alkynes
• steric effects
reactivity
R R R
> > > R > >> R
R R R R R R
R R R
H
2
OH OH
(PPh ) RhCl
3 3
H
H
stereochemistry
CH3 CH3 CH3 CH3
CO H
CO2H H2, Pd/C 2H
ethanol
H
CH CH CH
3 3 3
CH CH
H 3 3
2 +
PtO H H
2
70 : 30
Reduction of Alkenes & Alkynes
• directing effects
H
2
[Ir(cod)(PCy )(py)]PF
3 6
HO iPr HO iPr
H
2
[Rh(nbd)Diphos-4]PF
OH 6 OH
H
• chemoselectivity
O O
H , Pd/C
2
ethanol
Reduction of Alkenes & Alkynes
• reduction of alkynes (Lindlar catalyst)
H
2 ( ) OH
7
Pd/BaSO
( ) OH 4
7
quinoline
R SiMe H2 SiMe3
3
Pd/BaSO
4
R
quinoline
Reduction of Alkenes & Alkynes:
Diimide (diazene): ! ! !
• preparation
N N AcOH N N Cu(II)
H N-NH
KO2C CO2K - 2 CO2 H H O2 2 2
• mechanism
• general trends
- rate increases with increasing strain
- rate decreases with increasing substitution
- symmetrical C=C bonds reduced faster than polarized C=C bonds
HN=NH
Reduction of Alkenes & Alkynes:
LiAlH Reduction of Propargyl Alcohols: ! ! !
4
LiAlH
OH 4
R or Red-Al R OH
• mechanistic features
D H
1. LiAlH4 OH 1. LiAlD4
R OH 2. D2O R 2. H2O R OH
H D
• preparation of trisubstituted olefins
R R
OH 1. LiAlH , NaOMe (1:2) Me CuLi
4 2
R
2. I2 I OH Me OH
1. LiAlH , AlCl (60:1)
4 3
2. I
2
Me CuLi
2
R OH R OH
I Me
Corey J. Am. Chem. Soc. 1967, 89, 4245.