Hydrogenation

Textbook H: Chapter 15.1 – 15.6

Textbook A: Chapter 14.1 – 14.2

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

Olefin additions

Textbook H: Chapter 16.1 – 16.5

Textbook A: Chapter 14.3 – 14.6

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.