HYDROGENATIONHYDROGENATION

HETEROGENEOUS CATALYSIS

H H

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)

REVIEW OF HYDROGENATIONREVIEW OF HYDROGENATION

Ch 14 Section 14.4 Hydrogenation of Alkynes

HETEROGENEOUS CATALYSTSHETEROGENEOUS CATALYSTS

Do not dissolve in the solvent.

These catalysts are usually powder-ed metals or metal oxides.

Traditional typeThe catalyst is suspended by con-stantly stirring the solution (this is a two phase, or heterogenous, system).Hydrogen is bubbled through thesolution

They adsorb hydrogen on the surface.

H HH

H H HH

H2

Pd, Pt, PtO2, Ni

... ..

Hydrogenation of AlkenesHydrogenation of Alkenes

+C C H H C C

H H

catalyst

The catalyst is Pt, PtO2, Pd, or Ni(in special cases Ru, Rh, Re)

a syn addition reaction

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

fine powder, veryporous, suspendedin solution

Hydrogenation of AlkynesHydrogenation of Alkynes

catalyst

catalyst = Pt, Pd, or Ni

+ 2 H2C CH R H C C R

H

H

H

H

With normal catalysts it isnot possible to stop at thealkene by the addition of one mole of H2 .

Alkenes are more reactive than the starting alkyne, andthe second addition proceedsat a faster rate than the first.

Alkynes react twice (two double bonds)

+ H2

Pd/BaSO4quinoline

heat oneequivalent

C CR R' C C

H

R

H

R'

stereospecific

Lindlar is a special catalyst that allowsthe hydrogenation of an alkyne to stopafter one mole of hydrogen is added.

N

quinoline

syn addition

Most amines, and compoundscontaining sulfur, reduce theactivity of catalysts or “poison”them.

Lindlar CatalystLindlar Catalyst

QUINOLINEQUINOLINE

QUINOLINE

QUINOLINE

POISONING MECHANISM IS UNKNOWN POISONING MECHANISM IS UNKNOWN

Quinoline binds to sites on the catalyst.Linear alkynes can approach and pick up hydrogens.Planar alkenes are blocked.

QUINOLINE

QUINOLINE

QUINOLINE C=CR R

R RQUINOLINE

QUINOLINE

R-C C-R

alkyne fitsalkene blocked

A POSSIBLE HYPOTHESIS IS GIVEN HERE

surface ofLindlar catalyst

bound quinoline

It is not uncommon that either specific or speciallydesigned catalysts are used to control the resultsin hydrogenation reactions.

For this reason a very large variety of catalysts areavailable to the organic chemist.

PdPd on CarbonPtPtO2 (Adam’s Catalyst)Ni (Raney Nickel)Pd on BaSO4 with quinoline (Lindlar Catalyst)

RhRu

etc., etc., etc. ….

DESIGNER CATALYSTSDESIGNER CATALYSTS

HOMOGENEOUS CATALYSISHOMOGENEOUS CATALYSIS

HYDROGENATION

HOMOGENEOUS CATALYSTSHOMOGENEOUS 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).

Fairly Modern

Hydrogen is transferred internally to the alkeneby a rearrangement (insertion).

Rh

Cl

L L

L

L = (Ph)3P:

WILKINSON’S CATALYSTWILKINSON’S CATALYSTHomogeneous catalysts are soluble in organic solvents.

Rh

Cl

L H

L

H

Cl

Rh

L H

L

H

C=C

C=C

H-C-C-H

Rh

L

L

H

C-C-H

Cl

reductiveelimination

alkene picomplex

I IIIH-H

insertion

H2

DISSOLVING METALDISSOLVING METAL REDUCTIONSREDUCTIONS

HR

H R

R C C R

Na + NH3 (liq) Na+ + (NH3)n. _ solvated

electrons

The solution is royal blue

SODIUM IN LIQUID AMMONIA

.

powerful reducing solution(electron donor)

Fe or Fe3+

NaNH2 + H2

catalyst

SODIUM AMIDE INLIQUID AMMONIA

strongbase

SODIUM IN LIQUID AMMONIASODIUM IN LIQUID AMMONIA

Dissolving Metal Dissolving Metal ReductionReduction

C CR R' C C

R

H R'

HNa

NH3 (liquid)

stereoselectiveanti

This reaction proceeds withanti addition (trans compound).

Catalytic reduction proceeds withsyn addition, hence we havea choice of methods.

MECHANISM OF Na-LIQUID-NHMECHANISM OF Na-LIQUID-NH33 REDUCTIONS REDUCTIONS

( from (NH3)n ). -

R

HR

H

anti addition

electron transfer 1

electron transfer 2

+2e+2H+

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

:NH2-

R C C R + e-

C CR

R.

.

..-H N

H

H

C CR

R H

. + e-. C CR

R H

- ..

H NH

H

All intermediates prefer the trans geometry.

radical-anion

radical

anion

OXYMERCURATIONOXYMERCURATION

R R

OH

H

Markovnikoff

CH2

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

OHH

CH3

CH3

CH3

OH

O H

H

ACID-CATALYZED HYDRATION OF AN ALKENEACID-CATALYZED HYDRATION OF AN ALKENE

+

:..

+

O H

H

H+

..

:

O

H

H:

..

: :

alcohol

Yields by this methodare not always good; not stereoselective.

Chapter 8

Works best if the productis a tertiary alcohol.

Rearrangementspossible.

Oxymercuration of an AlkeneOxymercuration of an Alkene

Hg(OCOCH3)2

H2O

NaBH4

C CR R C C Hg(OCOCH3)

OH

R C C H

OH

Hg(OCOCH3)2

Hg2+ + 2 CH3COO_

Mercuric Acetate

electrophile

BETTER METHOD

Follows Markovnikoff rule

Mechanism of Mechanism of OxymercurationOxymercuration

slow

Hg OCOCH3

OCOCH3

H HHg

OCOCH3

+ CH3COO_

+

Step 1

bridged ion

(2+)

Formation of Bridged Ion

Hg

H H

OCOCH3

H

Hg

O

H

H

OCOCH3

..

..HO

H

- H+

::

or

H

H

OHg

H

OCOCH3

..

..

+

Step 2

anti opening

Attack by Water

Reduction

H

Hg

O

H

H

OCOCH3

::

or

H

H

OHg

H

OCOCH3

..

..

NaBH4

H

H

O

H

H::

H

H

OH

H....

or + Hg

(ppt)

Step 3

reducingagent

Hg2+ + 2e- Hg0H:- H+ + 2e-

+

SODIUM BOROHYDRIDESODIUM BOROHYDRIDE

NaBHNaBH44

Na+ H B H

H

H

-

DONOR OF HYDRIDE

IONS ( H:- ) TO ASUITABLE ACCEPTOR

BH3 + H:- ( = H+ + 2e- )

Formal charge is onboron, but H is moreelectronegative thanboron (slightly):

B 2.0H 2.1

All 5 atoms share thecharge.

CH3

H CH3 slow

Hg(OCOCH3)2

H2O

O

25° CHgH

CH3

H

CH3

OCOCH3

HgH

CH3

H

CH3

OCOCH3

..:

+

- H+

H O

H

H

CH3

CH3

OH

Hg OCOCH3

H

+

EXAMPLE REACTION

continued …..

H

CH3

CH3

OH

Hg OCOCH3

H NaBH4

H

CH3

CH3

OHH

H

REMOVAL OF THE MERCURY ( REDUCTION )

H

CH3

CH3

OHH

H

H

CH3

CH3

overallresult

followsMarkovnikoffrule

1) Hg(OAc)2

2) NaBH4

HYDROBORATIONHYDROBORATION

H H

R R

H OH

Hydroboration-protonolysis

Hydroboration-oxidation

Anti-Markovnikoff

B

OR

RO OR

A PICTURE OF BORONA PICTURE OF BORON( a “schizophrenic” element )

trivalent

tetravalent

trivalentbackbonding

B RR

R

Incomplete octet; it is anelectrophile (wants electrons);nucleophiles will add; oftenstabilizes by dimerization orsolvation by an unreactiveelectron-donor solvent (ethers).

B

R

RR

H-

Negatively-charged;has too many electrons;usually wants to expel either

H:- or R:- to an acceptor.

:Nu

:.. 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.

reactive

reactive

“GOOD”

3 NaBH4 + 4 BF3 ether

2 B2H6 + 3 NaBF4

B

H

H

H

HB

H

H

2

H

BH H

BHHH

Unusual “three-center” bonding,two electrons bond three atoms.

BH

HB

H

H

H

H

. ...

Preparation of DiboranePreparation of Diborane

B2H6 is a dimer of BH3

Mechanistically we can approximate B2H6 by using the BH3 structure.

sp2

empty 2p

122o

97o

a gas

BHH

H

BH

H

H

B

H

HH

-

+

DIBORANE ADDS TO ALKENES ANTI-MARKOVNIKOFFDIBORANE ADDS TO ALKENES ANTI-MARKOVNIKOFF

sp2

empty 2porbital

Since there are 3 B-Hbonds, addition canoccur three times(3 moles of alkene)

Boron adds to the carbon with the most hydrogens (!)which is the reverse of the Markovnikoff Rule.

concerted

synaddition

electrophile

R R

R

substituentstabilizes transientcarbocation center

hydride transfer

anti-Markovnikoff

B H

H

H

+

-B H

H

H

+

-

Concerted reaction,no intermediate;syn addition

Carbocation intermediatewould allow rotation; thereaction would not be stereospecific, giving bothsyn + anti addition.

Concerted versus Carbocation IntermediateConcerted versus Carbocation Intermediate

R R

R

Hydroboration of an Hydroboration of an AlkeneAlkene

R CH CH2R CH2 CH2

3

BBH3

B CH2CH2RRCH2CH2

RCH2CH2

a trialkylboron compound( or a trialkylborane )

3 moles

note abbreviation above

bubble B2H6 gas through the solution

“ “

CH3 H

H BH2

CH3 H

H BH2

anti-Markovnikoff

concerted

H added to the most-substitutedcarbon

syn addition

Stereochemistry of HydroborationStereochemistry of Hydroboration

B2H6

HYDROBORATION - PROTONOLYSIS

CH2CH3

CH3B CH2 CH

CH3

CH3

H CH2 CHCH3

CH3

CH3COOH

B2H6

Hydroboration - ProtonolysisHydroboration - Protonolysis

yields an alkane ( 3 moles )

trialkylborane

acetic acid

3

3

3

3

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.

Separate reactions:add diborane, whendone, add acetic acid.

BCH2

RR CH

CH3

CH3

O

R O H

BCH2

RR CH

CH3

CH3

O

R O H

B

CH2O

O HR

CH

CH3

CH3

R RB

O

OR

RR

CH2 CH

CH3

CH3H

..:

:

:: : :

..

..

..

..

....:

-

‘‘

‘ ‘

PROTONOLYSIS MECHANISM

+

+

-

6-ringconcerted

makes analkane

notice that thetrivalent boronhas resonance

the alkyl groupmoves with itselectrons

oxygenattached

After the first one, each of the other R groups reacts as well.

HYDROBORATION - OXIDATION

CH2CH3

CH3B CH2 CH

CH3

CH3

O CH2 CHCH3

CH3

B

B2H6

CH2 CHCH3

CH3

OH

H2O2 OH-

B(OH)3

+

trialkylborane

alcohol( 3 moles)

basichydrogenperoxide

trialkoxyborane

3

3

3

acid

H3O+

This work up step hydrolyzesthe trialkoxyboron compound.

Hydroboration - OxidationHydroboration - Oxidation

Each of these arrows is a separate reaction - first add B2H6, when complete add H2O2/NaOH, then finally hydrolyze with acid.

BCH2

RR CH

CH3

CH3B

CH2

RR CH

CH3

CH3

O

O H

B

CH2

OCH

CH3

CH3

R RB

OR

ORRO

O

O H

O H

OXIDATION MECHANISMOXIDATION MECHANISM

..: :

H-O-O-H + OH O-O-H + H-O-H::..

..

..

..

..

..

..

..

..

..

..

..

..

..:-

--

-

hydroperoxideion

trialkoxyborane

migration andcleavage of O-O(R group maintains configuration)

2 more times

internal SN2

-

adds intoempty orbital

cast-off

CH3

CH3

BH3CH3

B

H

CH3

H2O2/OH_

CH3

OH

H

CH3

Stereochemistry of Hydroboration-OxidationStereochemistry of Hydroboration-Oxidation

synaddition

retains cisstereochemistry

cis

syn

H3O+ (work up)

HYDROBORATION OF ALKYNESHYDROBORATION OF ALKYNES

Hydroboration-Protonolysis of Hydroboration-Protonolysis of AlkynesAlkynes

C CR R' C C

R

H

R'

B

C C

R

H

R'

H

3 + BH3

3

CH3COOHOnly one pi bond reacts

A second reaction sequence isrequired to react the second pi bond.

Hydroboration - Oxidation of AlkynesHydroboration - Oxidation of Alkynes

unfavoredtautomer !

EnolKetone

‘R C C R

OH

H

Only one pi bond reacts

immediatetautomerism

C CR R' C C

R

H

R'

B

‘

C C

R

H

R'

OH

3 + BH3

3

H2O2 / OH_

H3O+

C C

R

H OH

R

R C

H

H

C R

O

C C

H

H O

R

H

R

HO H

H

O

H

H

..

+

:....

+

:..

H

H

enolform

ketone oraldehyde

Most enols have high energy and rapidly convert to the lower energy keto form whichpredominates in the equilibrium.

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

enol

keto

ketoform

gained H

lost H

REVIEW / OVERVIEWREVIEW / OVERVIEW

Summary of Hydroboration ReactionsSummary of Hydroboration Reactions

BH3C C

R H

H H

C C

H B

R HH H

CH3COOH H2O2/OH_

C C

H H

R HH H

C C

H OH

R HH H

Anti-Markovnikoff

H

CH3 H

CH3

H

CH3 HCH3

OH

H

H

CH3 H

CH3

OHH

OXYMERCURATION VS. HYDROBORATION-OXIDATIONOXYMERCURATION VS. HYDROBORATION-OXIDATION

OXYMERCURATION HYDROBORATION-OXIDATION

1) Hg(OAc)2

2) NaBH4

1) B2H6

2) H2O2, NaOH

Markovnikoff Addition anti-Markovnikoff Addition

anti syn

result similar to 3M H2SO4/H2O

3) H3O+

TWO REACTIONS THAT ARE SIMILAR,TWO REACTIONS THAT ARE SIMILAR, BUT DIFFERENT !BUT DIFFERENT !

NH2-

+Na+

H.

H2

Na + NH3(liq). .(NH3)n

-+ Na +

MORE SODIUM-LIQUID AMMONIA CHEMISTRYMORE SODIUM-LIQUID AMMONIA CHEMISTRY

or Fe3+Fe

(gas)

Royal blueelectron solution

Clear sodium amidesolution

catalyst required for this step

AMIDE ION

N

H

H :.. -

Strong Base

ReducingSolution

x2

StrongBase

C C CH3CH3 H CH3

HCH3.(NH3)n+Na -

C C HCH3 C CCH3 :_

+Na

-..:N

H

H

SODIUM IN LIQUID AMMONIA

SODIUM AMIDE IN LIQUID AMMONIA

TWO DIFFERENT REAGENTS !TWO DIFFERENT REAGENTS !

Na +

Reducing Agent

Strong Base

=

=NaNH2 / NH3(l)

Na / NH3(l)

APPLICATION

CH3CH2CH2 OH CH CH3CH3

OH

SYNTHESIS PROBLEMSYNTHESIS PROBLEM

Show how to synthesize all four compounds from acetylene.

CH3 C

O

CH3 C C

CH3

H

H

CH3

Requires both Na / NH3(liq)and NaNH2 / NH3(liq).