Introduction to catalysis

Textbook H: Chapter 14

Textbook A: Part IV – Introduction

A + B + [CAT] Ck1

k-1

K =k1

k-1

Introduction to homogeneous catalysis 1850 Berzelius 1895 Ostwald: a catalyst is a substance that changes the rate of a chemical reaction

without itself appearing into the products. Definition: a catalyst is a substance that increases the rate at which a chemical

reaction approaches equilibrium without becoming itself permanently involved. Catalysis is a kinetic phenomenon.

is favorable

2

Reaction Coordinate

G

GReactants

Products

Ea

E acatalyzed

Catalyzed rxn proceeding through

an interm ediate

Homogeneous catalysis Homogeneous catalysts:

Are soluble metal complexes, usually mononuclear. TON (turnover number): moles of product/moles of catalyst TOF (turnover frequency): TON/time Resting state: the (pre)-catalytic species present in the highest concentration Turnover-limiting (rate-determining) step: smallest rate constant Can be characterized spectroscopically, but are more difficult to separate

than heterogeneous catalysts. Operate under moderate conditions.

Homogeneous processes without a heterogeneous counterpart: Pd-catalyzed oxidation of ethylene to acetaldehyde (Wacker process) Ni-catalyzed hydrocyanation of 1,3-butadiene to adiponitrile (DuPont) Rh- and Ru-catalyzed reductive coupling of CO to ethylene glycol An increasing number of enantioselective hydrogenation, isomerization, and

oxidation reactions.

Production of commodity chemicals

Operation Scale (million tonnes

per year)

Terephthalic acid and poly(ethylene terephthalate)

Acetic acid and acetyl chemicals

Aldehydes and alcohols via hydroformylation

Adiponitrile

Detergent-range alkenes via SHOP

Total fine chemicals manufacture

Olefin polymerization (60% uses Ziegler-Natta)

9

7

6

1

1

< 1

60

Selectivity

5

O

OH

O

OH

Chemoselectivity

O

OHydrogenation Hydrofomylation

Regioselectivity

OHR

OHR

OHR

Diastereoselectivity

Hydrogenation

COOR'

R NHCOR"

COOR'

R NHCOR"

COOR'

R NHCOR"Hydrogenation

Enantioselectivity

12 Principles of green chemistry1. Prevent waste

2. Increase atom economy

3. Use and generate no/less toxic chemicals

4. Minimize product toxicity during function

5. Use safe solvents and auxiliaries

6. Carry out processes with energy economy (ambient temperature and pressure)

7. Use renewable feedstocks

8. Reduce derivatives and steps

9. Use catalytic instead of stoichiometric processes

10. Keep in mind product life time (degradation vs. biodegradation processes)

11. Perform real-time analysis for pollution prevention

12. Use safe chemistry for accident prevention

Topics in Organometallic Chemistry 2005, vol. 16, Springer, Berlin

Guiding principles

The catalytically active species must have a vacant coordination site (total valence electrons = 16 or 14) to allow the substrate to coordinate.

Noble metals (2nd and 3rd period of groups 8-10) are privileged catalysts (form 16 e species easily).

In general, the total electron count alternates between 14/16 and 16/18.

Ancillary ligands insure stability and a good stereoelectronic balance.

One of the catalytic steps in the catalytic cycle is rate-determining.

start here

precatalyst

A

B

C

D

catalyst

substrate

substrate

products

General observations about catalysis Kinetic competence: catalysis is a kinetic

phenomenon, so the activity of a system may rely on a minor component of a catalyst (if an intermediate can be observed does not mean is a true intermediate in the catalytic cycle).

Homogeneous vs. heterogeneous: liquid Hg selectively poisons any heterogeneous platinum group (Ru, Os, Rh, Ir, Pd, Pt) metal catalyst.

Reversibility: if a catalytic cycle is formed only of reversible steps then a thermodynamic product ratio is obtained.

Chiral poisoning: an enatiomerically pure compound selectively binds to and poisons one enantiomer of a racemic catalyst.

Chiral amplification: the product of the catalytic reaction has a higher ee than expected because the major enantiomer of the catalyst acts as a poison for the minor enantiomer.

G*minor

G*major

PPh

P

Rh

PPh

P

Rh

2+

Asymmetric catalysis: introduction

In order to discriminate between enantiotopic atoms or faces of an achiral molecule, the energy difference of the two intermediates has to exceed 2.4 kcal/mol.

Chiral complexes as catalysts have the advantage of synthesizing, in principle, both enantiomers; enzymes produce only one enantiomer.

In order to pass the enantiomeric character on to the substrate, the ligand has to adopt C2 and not Cs symmetry.

PP

Cs symmetry

PP

C2 symmetry

Phosphine ligands in asymmetric hydrogenation

P P

DIPAMPo-MeO-Ph o-MeO-Ph

Ph Ph* *

Ligands containing a chiral P atom

Ligands with a chiral backbone

Ph2P PPh2

O*

O*

DIOP

Ligands with chiral substituents on P

P P

R

RR

R

* *

DuPHOS

PPh2

PPh2

S-BINAP

Ligands with axial (as)symmetry

Ligands with planar (as)symmetry

Fe

PPh2

PR2

CH3

JosiPhos

Monodentate ligands

O

OP N

Me

Me

Monophos