Lecture 10 INTRO TO CATALYSIS

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A catalyst◦speeds up a reaction◦enables a reaction to proceed that otherwise does not without

it◦added in non-stoichiometric amounts ( ~ 10-6 to 10-1X)◦unchanged at the end of the reaction

INTRODUCTION TO CATALYSIS

Catalysis is a kinetic phenomenon:

INTRODUCTION TO CATALYSIS

A + B + [CAT] Ck1

k-1

K =k1

k-1

Reaction Coordinate

G

GReactants

Products

Ea

E acatalyzed

Catalyzed rxn proceeding through

an interm ediate

IMPORTANCE OF CATALYSIS

increasingly important in synthesisselectivity in production of fine chemicalsclean processes, high atom economy (bulk processes)production of high-tech products / materialsmild conditions (low energy consumption)environmentally friendly

IMPORTANCE OF CATALYSIS

Catalyst turnover frequencies, rates and numbers:

Turnover Frequency, TOF = number of turnovers per mol catalyst per unit time. For most relevant industrial applications TOF is in the range of 10-1 to 102 s-1

(= TON/time)

Turnover Number, TON = number of moles of substrate that a mole of catalyst can convert before becoming inactivated.

The lifetime of the catalyst before deactivation is measured in terms of total turnovers.

INTRODUCTION TO CATALYSIS

There are different types of catalysis:Acid-base catalysisElectrocatalysisPhotocatalysisRedox CatalysisHomogeneous CatalysisEnzymatic CatalysisHeterogeneous catalysisSupported CatalysisMetallodendritic CatalysisBiphasic Catalysis

INTRODUCTION TO CATALYSIS

The catalysts we look at are soluble complexes, or homogeneous catalysts, as opposed to catalysts such as palladium on carbon, or heterogeneous catalysts.

These terms are used because the catalyst and substrates for the reaction are in the same phase in the homogeneous, but not in the heterogeneous, type, where catalysis takes place at the surface of a solid catalyst.

Catalytic mechanisms are considerably easier to study in homogeneous systems, where such powerful methods as NMR can be used to both assign structures and follow reaction kinetics.

HOMOGENEOUS CATALYSIS

Homogeneous catalysts have the disadvantage that they can be difficult to separate from the product.

Homogeneous catalysts can also be chemically grafted on to solid supports for greater ease of separation of the catalyst from the reaction products.

Although the catalyst is now technically heterogeneous, it often retains the characteristic reactivity pattern that it showed as a homogeneous catalyst, and its properties are usually distinct from those of any of the classical heterogeneous catalysts—these are sometimes called “heterogenized” homogeneous catalysts.

HOMOGENEOUS CATALYSIS

HOMOGENEOUS CATALYSIS

The mechanistic ideas developed in homogeneous catalysis are also becoming more influential in the field of classical heterogeneous catalysis by suggesting structures for intermediates and mechanisms for reaction steps.

By bringing about a reaction at lower temperature, a catalyst can save energy in commercial applications. It often gives higher selectivity for the desired product, minimizing product separation problems and avoiding the need to discard the undesired product as waste.

HOMOGENEOUS CATALYSIS

Environmental concerns have promoted the idea of atom economy, which values a process most highly when all the atoms in the reagents are used to form the product, minimizing waste.

Catalysis is Green Chemistry !!!!

HOMOGENEOUS CATALYSIS

The selectivity can be changed by altering the ligands, allowing synthesis of products not formed in the un‐catalyzed process.

HOMOGENEOUS CATALYSIS

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

With growing regulatory pressure to synthesize drugs in enantiopure form, asymmetric catalysis has come to the fore, along with enzyme catalysis, as the only practical way to make such products on a large scale.

HOMOGENEOUS CATALYSIS

For most transition metal catalysts, the catalyzed pathway is completely changed from the pathway of the uncatalyzed reaction. Instead of passing by way of the high‐energy uncatalyzed transition state TS, the catalyzed reaction normally goes by a multistep mechanism in which the metal stabilizes intermediates that are stable only when bound to the metal.

Normally, the catalyst only increases the rate of a process but does not alter its position of equilibrium, which is decided by the relative thermodynamic stabilities of substrate and products.

For example, if the substrate S is slightly less stable than the product P, so the reaction will eventually reach an equilibrium favoring P.

The TS structure in the absence of the metal would be extremely unstable, but the energy of binding is so high that M.TS is now much more favorable than TS and the reaction all passes through the catalyzed route.

Different metal species may be able to stabilize other transition states TS—which may lead to entirely different products—hence different catalysts products can give different products from the same starting materials.

The slow step in a catalytic process is called the turnover limiting step. Any change that lowers the barrier for this step will increase the turnover frequency (TOF).

Changes in other barriers will not affect the TOF. For a high TOF, we require that none of the intermediates be bound too strongly (otherwise they may be too stable and not react further) and that none of the transition states be prohibitively high in energy

Indeed, the whole reaction profile must not stray from a rather narrow range of free energies, accessible at the reaction temperature. Even if all this is arranged, a catalyst may undergo a catalytic cycle only a few times and then “die.”

This happens if undesired deactivation reactions are faster than the productive reactions of the catalytic cycle itself.

HOMOGENEOUS CATALYSIS – GUIDING PRINCIPLES

start here

precatalyst

A

B

C

D

catalyst

substrate

substrate

products

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).

HOMOGENEOUS CATALYSIS – GUIDING PRINCIPLES

start here

precatalyst

A

B

C

D

catalyst

substrate

substrate

products

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

• Ancillary ligands insure stability and a good stereoelectronic balance.

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

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

Ligand:

(4-ClC6H4)3PPPh3

(4-CH3C6H4)3P(4-CH3OC6H4)3P

Relative rates for hydrogenation of cyclohexene:

1.74186

100

Simplistically, the relative ratessuggest that the rate-determining step 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

OXIDATIVE ADDITION OF H2 TO AN ALKENE