hydrogenation heterogeneous catalysis. catalytic hydrogenation is covered in several places in the...
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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).