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HETEROGENEOUS CATALYSIS. HYDROGENATION. REVIEW OF HYDROGENATION. CATALYTIC HYDROGENATION . IS COVERED IN SEVERAL PLACES IN THE TEXT,. PRINCIPALLY:. Ch3 Section 3.18 Hydrogenation of Alkenes. Ch4 Section 4.13 Relative Stabilities of Alkenes.

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slide2

REVIEW OF HYDROGENATION

CATALYTIC HYDROGENATION

IS COVERED IN SEVERAL PLACES IN THE TEXT,

PRINCIPALLY:

Ch3 Section 3.18 Hydrogenation of Alkenes

Ch4 Section 4.13 Relative Stabilities of Alkenes

Ch6 Section 6.12 Heat of Hydrogenation

Section 6.13 Resonance Energy of Benzene

Ch8 Section 8.11 Hydrogenation of the Double Bond

Pages 723-725 (Addition to Triple Bonds)

Ch 14 Section 14.4 Hydrogenation of Alkynes

slide3

HETEROGENEOUS CATALYSTS

Do not dissolve in the solvent.

Traditional type

The catalyst is suspended by con-

stantly stirring the solution (this is a

two phase, or heterogenous, system).

Hydrogen is bubbled through the

solution

H2

These catalysts are usually powder-

ed metals or metal oxides.

Pd, Pt, PtO2, Ni

.

.

.

.

.

They adsorb hydrogen on the surface.

H

H

H

H

H

H

H

slide4

Hydrogenation of Alkenes

( Sections 3.18, 4.13, 6.12, 6.13, 8.11 )

catalyst

+

C

C

H

H

C

C

H

H

fine powder, very

porous, suspended

in solution

a syn addition

reaction

The catalyst is Pt, PtO2, Pd, or Ni

(in special cases Ru, Rh, Re)

hydrogenation of alkynes
Hydrogenation of Alkynes

Alkynes react twice (two double bonds)

Alkenes are more reactive

than the starting alkyne, and

the second addition proceeds

at a faster rate than the first.

With normal catalysts it is

not possible to stop at the

alkene by the addition of

one mole of H2 .

slide6

Lindlar Catalyst

syn addition

Lindlar is a special catalyst that allows

the hydrogenation of an alkyne to stop

after one mole of hydrogen is added.

Most amines, and compounds

containing sulfur, reduce the

activity of catalysts or “poison”

them.

quinoline

slide8

DESIGNER CATALYSTS

It is not uncommon that either specific or specially

designed catalysts are used to control the results

in hydrogenation reactions.

For this reason a very large variety of catalysts are

available to the organic chemist.

Pd

Pd on Carbon

Pt

PtO2 (Adam’s Catalyst)

Ni (Raney Nickel)

Pd on BaSO4 with quinoline (Lindlar Catalyst)

Rh

Ru

etc., etc., etc. ….

slide10

HOMOGENEOUS CATALYSTS

Dissolve in the solvent to make a homogeneous

(one phase) solution.

These catalysts are usually a transition metal

complex that can bind both hydrogen and the

alkene reactant as ligands (many are based on

Rh or Ru complexes).

Hydrogen is transferred internally to the alkene

by a rearrangement (insertion).

Fairly Modern

slide11

L

L

Rh

L

Cl

insertion

WILKINSON’S CATALYST

Homogeneous catalysts are soluble in organic solvents.

H

L

H

III

H-H

I

Rh

H2

L

Cl

L = (Ph)3P:

C=C

H

H

L

C-C-H

L

reductive

elimination

H

Rh

Rh

L

L

Cl

Cl

H-C-C-H

C=C

alkene pi

complex

slide12

DISSOLVING METAL

REDUCTIONS

slide13

_

.

Na + NH3 (liq)

Na+ + (NH3)n

Fe or Fe3+

catalyst

SODIUM AMIDE IN

LIQUID AMMONIA

NaNH2 + H2

strong

base

SODIUM IN LIQUID AMMONIA

SODIUM IN LIQUID AMMONIA

solvated

electrons

.

powerful reducing solution

(electron donor)

The solution is

royal blue

dissolving metal reduction
Dissolving Metal Reduction

anti

This reaction proceeds with

anti addition (trans compound).

Catalytic reduction proceeds with

syn addition, hence we have

a choice of methods.

slide15

MECHANISM OF Na-LIQUID-NH3 REDUCTIONS

electron transfer 1

.

-

All intermediates prefer

the trans geometry.

( from (NH3)n )

+2H+

+2e

radical-

anion

anion

anti addition

radical

:NH2-

electron transfer 2

Electrons come from the complex and the hydrogens from the ammonia.

slide16

OXYMERCURATION

Markovnikoff

slide17

ACID-CATALYZED HYDRATION OF AN ALKENE

Rearrangements

possible.

+

..

:

..

+

Chapter 8

:

:

:

+

..

alcohol

:

Yields by this method

are not always good;

not stereoselective.

Works best if the product

is a tertiary alcohol.

oxymercuration of an alkene

BETTER METHOD

Oxymercuration of an Alkene

Mercuric Acetate

Hg(OCOCH3)2

_

Hg2+

+ 2 CH3COO

electrophile

Follows Markovnikoff rule

mechanism of oxymercuration
Mechanism of Oxymercuration

bridged ion

(2+)

Formation of Bridged Ion

Step 1

slide20

anti opening

Attack by Water

Step 2

slide21

H:-

H+ + 2e-

Hg2+ + 2e-

Hg0

+

reducing

agent

Reduction

Step 3

slide22

SODIUM BOROHYDRIDE

NaBH4

Formal charge is on

boron, but H is more

electronegative than

boron (slightly):

H

-

B 2.0

H 2.1

Na+

H

B

H

All 5 atoms share the

charge.

H

DONOR OF HYDRIDE

IONS ( H:- ) TO A

SUITABLE ACCEPTOR

BH3 + H:-

( = H+ + 2e- )

slide23

EXAMPLE REACTION

continued …..

slide24

REMOVAL OF THE MERCURY ( REDUCTION )

1) Hg(OAc)2

2) NaBH4

overall

result

follows

Markovnikoff

rule

slide25

Hydroboration-

protonolysis

HYDROBORATION

Hydroboration-

oxidation

Anti-Markovnikoff

slide26

A PICTURE OF BORON

( a “schizophrenic” element )

Incomplete octet; it is an

electrophile (wants electrons);

nucleophiles will add; often

stabilizes by dimerization or

solvation by an unreactive

electron-donor solvent (ethers).

trivalent

reactive

:Nu

Negatively-charged;

has too many electrons;

usually wants to expel either

H:- or R:- to an acceptor.

tetravalent

-

reactive

Boron is most stable when it is

trivalent and has attached elements

with unshared pairs ( esp. oxygen )

that can stabilize it through back-

bonding resonance. Borates and

boric acid are especially stable.

..

trivalent

backbonding

:

“GOOD”

slide27

.

.

H

H

H

B

B

.

.

H

H

H

Preparation of Diborane

a gas

B2H6 is a dimer of BH3

97o

122o

Unusual “three-center” bonding,

two electrons bond three atoms.

sp2

H

H

B

H

empty 2p

Mechanistically we can approximate B2H6 by using the BH3 structure.

slide28

concerted

DIBORANE ADDS TO ALKENES ANTI-MARKOVNIKOFF

substituent

stabilizes transient

carbocation center

R

R

hydride transfer

electrophile

sp2

empty 2p

orbital

R

Since there are 3 B-H

bonds, addition can

occur three times

(3 moles of alkene)

syn

addition

anti-Markovnikoff

Boron adds to the carbon with the most hydrogens (!)

which is the reverse of the Markovnikoff Rule.

slide29

d+

R

-

d

Concerted versus Carbocation Intermediate

R

R

+

-

Carbocation intermediate

would allow rotation; the

reaction would not be

stereospecific, giving both

syn + anti addition.

Concerted reaction,

no intermediate;

syn addition

hydroboration of an alkene
Hydroboration of an Alkene

bubble B2H6 gas through the solution

3 moles

note abbreviation above

a trialkylboron compound

( or a trialkylborane )

slide31

concerted

Stereochemistry of Hydroboration

B2H6

syn addition

H added to the

most-substituted

carbon

anti-Markovnikoff

slide33

Hydroboration - Protonolysis

trialkylborane

3

3

3

acetic acid

3

Separate reactions:

add diborane, when

done, add acetic acid.

yields an alkane

( 3 moles )

This gives the same result as catalytic reduction - why do this?

Typically yields are high 90-100% - a big advantage.

This reaction will work in some cases where catalytic methods won’t.

slide34

..

..

..

..

PROTONOLYSIS MECHANISM

-

+

..

:

:

the alkyl group

moves with its

electrons

-

:

:

:

:

:

makes an

alkane

..

+

..

6-ring

concerted

notice that the

trivalent boron

has resonance

oxygen

attached

After the first one, each of the

other R groups reacts as well.

slide36

Hydroboration - Oxidation

trialkylborane

3

3

basic

hydrogen

peroxide

OH-

H2O2

acid

H3O+

3

alcohol

( 3 moles)

+

trialkoxyborane

B(OH)3

This work up step hydrolyzes

the trialkoxyboron compound.

Each of these arrows is a separate reaction - first add B2H6, when

complete add H2O2/NaOH, then finally hydrolyze with acid.

slide37

..

..

..

..

..

..

..

..

..

..

..

..

OXIDATION MECHANISM

-

-

..

:

:

H-O-O-H + OH

O-O-H + H-O-H

..

cast-off

-

-

..

:

:

internal SN2

adds into

empty orbital

hydroperoxide

ion

migration and

cleavage of O-O

(R group maintains

configuration)

2 more times

-

trialkoxyborane

:

slide38

Stereochemistry of Hydroboration-Oxidation

syn

addition

cis

(work up)

H3O+

retains cis

stereochemistry

syn

slide40

Only one pi bond reacts

Hydroboration-Protonolysis of Alkynes

A second reaction sequence is

required to react the second pi bond.

slide41

R

\'

R

BH

R

\'

R

C

C

C

C

3

H

B

Only one pi bond reacts

/ OH

H

O

2

2

R

\'

R

C

C

H

O

H

Hydroboration - Oxidation of Alkynes

3

+

_

H3O+

3

immediate

tautomerism

Ketone

Enol

unfavored

tautomer !

slide42

..

:

H

..

+

..

..

:

+

H

TAUTOMERS

Tautomers are in equilibrium and differ in the position a proton.

enol

form

keto

form

ketone or

aldehyde

Most enols have high

energy and rapidly

convert to the lower

energy keto form which

predominates in the

equilibrium.

enol

lost H

keto

gained H

slide45

OXYMERCURATION VS. HYDROBORATION-OXIDATION

result similar to

3M H2SO4/H2O

OXYMERCURATION

HYDROBORATION-OXIDATION

1) Hg(OAc)2

1) B2H6

2) NaBH4

2) H2O2, NaOH

3) H3O+

anti

syn

Markovnikoff Addition

anti-Markovnikoff Addition

slide47

Reducing

Solution

-

.

+

N

a

N

H

H

+

2

MORE SODIUM-LIQUID AMMONIA CHEMISTRY

Royal blue

electron solution

Fe

or Fe3+

catalyst required

for this step

AMIDE ION

Strong

Base

x2

Clear

sodium amide

solution

Strong Base

(gas)

H

2

slide48

TWO DIFFERENT REAGENTS !

SODIUM IN LIQUID AMMONIA

=

Reducing Agent

Na / NH3(l)

Strong Base

=

SODIUM AMIDE IN LIQUID AMMONIA

NaNH2 / NH3(l)

slide50

SYNTHESIS PROBLEM

Show how to synthesize all four compounds

from acetylene.

Requires both Na / NH3(liq)

and NaNH2 / NH3(liq).