Hydrogenation
All mechanisms involve metal hydrides. Oxidative addition Sigma-bond metathesis Heterolytic activation
R1
R2+ H2
R1
R2catalyst H
H
+ HM HM HM HM M
transition state
agostic bond
M = TM (usually 16 e) / Ln / An / group 13
Wilkinson’s catalyst: OA of H2 (hydride mechanism)
Simplistically, the relative ratessuggest that the rate-determiningstep is OA of H2.
H2 adds to the catalyst before the olefin.The last step of the catalytic cycle is irreversible. This is very useful because a kinetic product ratio can be obtained.
Ligand:
(4-ClC6H4)3PPPh3
(4-CH3C6H4)3P(4-CH3OC6H4)3P
Relative rates for hydrogenation of cyclohexene:
1.74186
100
RhAr3P
Cl PAr3
PAr3
RhCl
Ar3P S
PAr3
RhCl
Ar3P H
PAr3
H
S
RhCl
Ar3P H
PAr3
H
RhCl
Ar3P CH2CH2H
PAr3
H
S
PAr3
S ligand dissociation
H-H
oxidativeaddition
S
substratecoordination
insertion/migration
CH3CH3
reductiveelimination
S
rearrangement
irreversible
Cationic catalysts: OA of H2 (olefin mechanism)Cationic Rh complexes: Schrock and Osborn
Cationic Ir complexes: Crabtree
With cationic catalysts, because the metal center is a strong Lewis acid, the olefin coordinates first and then the oxidative addition of H2 occurs.
Rh+
Ph2P PPh2
2 H2
Solvent (S)
S S
Rh+
Ph2P PPh2
+
Ir+Py
PCy3
2 H2
Solvent (S)Ir+
S
S Py
PCy3
+
Sigma-bond metathesis
It is found for early TM, lanthanides, and actinides (d0fn configuration).
Cp2*Lu H
H-H
migratoryinsertion
substratecoordination
-bondmetathesis
CH3CH3
irreversible
Cp2*Lu
H
Cp2*Lu H
HH
substratecoordination
Cp2*Lu
H
HH
Heterolytic H2 activation
The protons of a dihydrogen ligand are more acidic than those in free H2: many H2 complexes can be deprotonated with NEt3.
Once H2 activation takes place, the rest of the catalytic cycle is similar to that for Wilkinson’s catalyst.
RuCl2(PPh3)2 hydrogenates selectively terminal double bonds over internal double bonds:
RuCl2(PPh3)3 + H2 RuCl(PPh3)3Cl
-
H
H
++ NEt3
RuPh3P
Ph3P ClH
PPh3
- Et3NHCl
H2
RuCl2(PPh3)2
25 oC, 1 atm
H2
RuCl2(PPh3)225 oC, 1 atm
+
H-transfer reactions
Reference: Morris, R. H. et al. Coord. Chem. Rev. 2004, 248, 2201
The ketone substrate does notcoordinate to the metal, but is
oriented in the second-coordinationsphere by forming hydrogen bonds.
Transfer hydrogenation avoidsthe use of high pressures of H2.
RuCl
H2N
NTs
Ph
Ph
NaO2CH or NaOiPr- NaCl
HCOOH or - iPrOH
Ru
HNN
Ts
PhPh
RuH
HN
NTs
Ph
Ph
C
O H
OC
H
H
RuH
N
NTs
Ph
PhH
HC O
RuH
HN
NTs
Ph
Ph
C
Ar
R
O H
C OAr
R
H
C
Ar
R
O H
Homolytic H2 activationA very early example of a homogeneous hydrogenation catalyst (Iguchi, 1942):
The resulting organic radical needs to be moderately stable: only “activated” alkenes willbe hydrogenated (formation of a conjugated radical).
HOOCCOOH H2
HOOCCOOH
70 oC, 1 atm
H2
25 oC, 1 atm
OH OH
Co(CN)53-
Co(CN)53-
(NC)5Co3- H H 2 HCo(CN)53-
HCo(CN)53- +
PhCOO-
Co(CN)53- +
PhCOO-
HCo(CN)53- +
PhCOO-
Co(CN)53- + Ph
COO-
Co(CN)53-
Asymmetric hydrogenation
The intermediate alkene adduct observed in the NMR spectra is not the one leading to the major product.
Enantioselectivity is determined in the first irreversible step after the enantiocenter is formed (not always the rate-determining step).
RhP
P S
S
Ph
Ph
o-An
o-An+
NHROOC
ArO Me
+fast
RhP
P
Ph
Ph
o-An
o-An+
NHROOC
ArO Me
minor intermediate
H2slow
RhP
P
H
Ph
Ph
o-An
oAn NH
ROOC
Ar
OMe
H
RhP
P
Ph
Ph
o-An
oAnNH
ROOC
Ar
OMe
H
ROOC NH
COMe
H Ph
(R)major product
Alkene isomerization: hydride mechanism
• All steps are reversible: TD ratio is formed if catalyst is active long enough.• Insertion to give the 1° alkyl is favored for many catalysts: nonproductive cycling.• The initial cis/trans ratio depends on the catalyst; the cis isomer is often favored. In the finalproduct mixture, the trans isomer is predominant.
It requires an M-H bond and a vacant site. It is often a side reaction.
M HR
M H
R
M
R
1o alkyl
M
R
2o alkyl
M H
R
R
Alkene isomerization: allyl mechanismIt is found for metal fragments that have 2 x 2e vacant sites but no hydrides: Fe(CO)3.
Crossover experiment to distinguish between allyl and hydride isomerization mechanisms:
MR
M
R
M
R
M HM
R
R
R
D+
Fe3(CO)12+
D
Alkene additions
With transition metal catalyststhe control of selectivity is not
simply explained by the stabilityof the carbocation intermediate:anti-Markovnikov products are
possible.
Reference: Beller, M. et al. Angew. Chem. Int. Ed. 2004, 43, 3368
R + E-Nu R
E
Nu
R
Nu
E
Anti-MarkovnikovMarkovnikov
E = H, BR2, Si, Hg, SnNu = halogen, CN, CHO, OH, CO, COOR, NR2
Hydrocyanation of ethylene
The reductive elimination(rate-determining step)
is promoted by the additionof electron-withdrawing
phosphite ligands.
If styrene is used instead ofethylene, a benzyl intermediate
forms.
NiLL
Ni LL
H
CN
Ni LH
CN
Ni LH3C-H2C
CN
Ni LL
H3CH2C
CN
HCN
- L
L
CH3CH2CN
L = P(OR)3
not observed,but consistent with kinetics
NiP
P CN
Hydrocyanation of butadiene
Ni HCN
Ni
CN
NC
CN CN CN
NC
X
CN
1
2
3
4 5
Initial addition of HCN leads to 1:2 in a 1:2 ratio, but they equilibrate to 1:9 via the retro-reaction (involves C-C breaking, but the allyl cyanide species is stable).
Although 3 is more stable than 4, formation of 4 is kineticallycontrolled.
Lewis acids are added as cocatalysts. They accelerate the reductive elimination (formation of 5), probably bycomplexation to CN, thus decreasing the electron density at the metal.
Adiponitrile (5) is a precursor to 1,6-hexanediamine, one of the components of 6,6-nylon and polyurethanes.
Hydroboration
Reference: Crudden, C. M.; Edwards, D. Eur. J. Org. Chem. 2003, 24, 4695
The alkene could insert eitherin the Rh-H or the Rh-B bond(both are cis to the coordinatedolefin). Data supporting both pathways have been reported.
Hydrosilylation
Platinum catalysts for hydrosilylation (used in industry):
Hydrosilylation can be applied to alkenes, alkynes, and carbonyl compounds.
There are only few examples of selective dehydrogenative silylations.
Reference: Marciniec, B. Silicon Chemistry 2002, 1, 155
Although molecular catalysts are proposed for Rh, Ru, Ir, Pd, large clusters or small colloids are active for Pt.
R+ 2R'3Si-M-H
R
H
M
R
H
RR'3Si R+ Hydrosilylation
R R+
R'3Si Dehydrogenative silylation
M = Fe, Ru, Os, Co, Rh, Ir
H2PtCl6
Si
O
SiSi
OSi
Si
O
Si
Pt Pt
Speier
Lukevics Karstedt
(Bu4N)2PtCl6
Chalk-Harrod mechanism for hydrosilylation
The two side products, vinylsilane and alkane are always formed in a 1:1 ratio.
PtL
L L
L
PtL
L H
SiR3
PtL
H
SiR3
PtL
L CH2CH3
SiR3
HSiR3
- 2 L- L
H2C=CH2
L
cis-transisomerization
+ 2 L
CH3CH2SiR3
PtL
L CH2-CH2
H
SiR3
PtL
H
H
R3Si
PtL
H
HPt
L
L CH2CH3
H
cis-transisomerization
L
-hydrideelimination
- L
H2C=CH2
R3Si
L
majorproduct
CH3CH3
start here
Asymmetric palladium catalystsCatalytic asymmetric hydrosilylation of alkenes is a useful tool for the synthesis
of optically active alcohols.
Unlike all other metal catalysts, Pd catalysts give products with the SiR3 group at the secondary carbon. This selectivity is determined by the steric constraints of the ligand. The X (OMe) group has little influence on the course of the reaction and it is not coordinated during the catalytic cycle.
[(C3H5)PdCl]2S-MeO-MOP
SiCl3
HSiCl3
10% yield
90% yield
OH
1. EtOH/NEt32. H2O2, KF
93% ee
OMe
PPh2
S-MeO-MOP
A M Tondreau, P. J. Chirik et al. Science 2012, 335, 567-570Published by AAAS
Iron catalysts for alkene hydrosilylation
Early transition metals
and lanthanidesHydroamination was first developed with Ln catalysts.
Ln complexes work best in intramolecular versions.
Alkynes are more reactive than alkenes.
Late TMs: oxidative amination
a The overall mechanism for the Pd-catalyzed oxidation of olefins.
b C–heteroatom bond-forming by nucleophilic attack on a coordinated olefin
c Migratory insertion into an M–heteroatom bond
John F. Hartwig - Nature 455, 314-322(18 September 2008)
Relative rates of olefin insertion into alkyl, amide and alkoxo complexes
Rationalization for why the rates of olefin insertion into late TM alkoxides and amides are faster than into late TM alkyls, based on the destabilization of the alkoxo and amide reactants and the stabilization of the products of insertion into the alkoxo and amido complexes by an M–Y dative bond
John F. Hartwig - Nature 455, 314-322(18 September 2008)
Recently discovered reactions of transition-M–heteroatom bonds
John F. Hartwig - Nature 455, 314-322(18 September 2008)
a RE to form C–N, C–O and C–S
bonds in amines, ethers,
and thioethers
b OA of amine N–H bonds
c Migratory insertions of olefins
into metal amides
and metal alkoxides
d [2 + 2] Cycloadditions
between olefins and M–imido
or M–oxo complexes.
These reactions are analogues of classic reactions occurring at M–C bonds and have only recently been discovered.
Top Related