Catalysis 3

Turnover Frequency, TOF =

number of turnovers per mol catalyst per unit time (often per second).

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, often per hour

what is The lifetime of the catalyst before deactivation is measured in

terms of total turnovers

The selectivity of a reaction is

the ratio of the desired product formed (in moles) to the undesired product formed (in moles).

An enantioselective reaction is where

one enantiomer is formed in preference to the other

what is the degree of enantioselctivity defined by

ee

ee =

(R-S)/(R+S) x100

what is Regioselectivity

forming one isomer over another by adding for example to a double bond in one direction

what is Chemoselectivity

the preferential outcome of a chemical reaction over a set of possible alternative reactions. Hence we might target one functional group in the presence of others.

what does catalysis do

It lowers the activation energy of a process and hence increases the rate of reaction. It cannot make a thermodynamically unfavourable process occur

Often pathways are …

multistep and the slow step is turnover or rate limiting.

Form of hom cat

Soluble metal complex

Form of het cat

Metal, might be supported on a surface, metal oxide etc

phase of hom cat

Liquid, same phase

phase of het cat

Gas/liquid

Liquid/solid

Temp of hom cat

Less than 250oC: ligand stability an issue

Temp of het cat

High 250-500oC

Activity of hom cat

Very high rates but Expensive, lifetime important

Activity of het cat

• Sensitive to poisons

• Problems due to diffusion

• Moderate

Selectivity of hom cat

Most important benefit of homogeneous catalysis

Selectivity of het cat

Lower selectivity but fine if products are cheap

Heat Transfer of hom cat

Facile (easy) can stir or cool

Heat Transfer of het cat

High energy process so can be a problem as specific heat capacity of gases low

Product Separation of hom cat

Difficult to separate can trick the system through biphasic catalysis, grafted catalysts on supports

Product Separation of het cat

Easy to separate catalyst as in a different phase. For example simply filter

Catalyst Re-use of hom cat

Difficult to recover

Short service life

This can be expensive

Catalyst Re-use of het cat

Readily recycled / regenerated Long-lived

Reaction Mechanism of hom cat

Realistic models available We can explore reactions and high levels of understanding possible

Reaction Mechanism of het cat

Poor mechanistic understanding in comparison to homogeneous catalysis

Advantages of hom cat

• Uniform catalyst

• one reaction site

• reproducible

• can be modified by changing the ligands

Advantages of het cat

• Cheap

• scale

• robustness

• ease of separation

what is Hydroformylation (Oxo) process

• alkenes to straight chain aldehydes, catalysed by Co or Rh complexes

• catalyst is HCo(CO)4

what catalyses Alkene isomerisation

HCo(CO)4 (or Rh complexes)

what is Alkene hydrogenation

• alkene + hydrogen

• forms Rh-H complex

• gives alkane e.g. propene → propane

what is Alkene hydrosilyation

• utilisation of Karsted’s catalyst and hexachoroplatinic acid.

• Formation of organosilanes by addition of silicon hydride bond across an unsaturated centre.

what is the ultimate goal with a catalyst

to combine the fast rates and high selectivities of homogeneous catalysts with the ease of recovery /recycle of heterogeneous catalysts

how do cats react

• substitution

• ligand loss

• migration

• insertion

• reductive elimination

Ligands which bind through a lone pair on phosphorus are important. There are two classes of such ligands:

• Phosphines (P-C-bond)

• Phosphites (P-O-bond)

diff btwn Phosphines (P-C-bond)and Phosphites (P-O-bond) ligands

• both are neutral 2e- donors

• phosphites hv electroneg O which sucks e- density twrds it so less able to donate the 2e-s

what is rate for catalyst activation by dissociation of a ligand

rate is first order

how can we Weaken bonds to ligands

we can add steric bulk to the ligands to increase bond lengthens and thereby weaken bonds which will aid ligand loss

what can we do Once we create a vacant site

we can bind the reactants and they can be functionalised

what do Ancillary ligands ensure

stability and a good stereo-electronic balance.

what does IR Stretch around 2000 cm-1 imply

terminal carbonyl ligands

what does IR Stretch around 1700 cm-1 imply

acyl (ketone) ligands.

what does migration play a role in

bond forming steps

In case of aliphatic olefins what are preferred products

linear aldehydes

Aldehydes react with ammonia to form

imines

Imines can be hydrogenated to form

an amine

If the metal centre becomes bigger…

steric effects will be less important

ox st and stability

higher ox st = less stable

diff isomeric prods…

are likely to react diff

wavenumber of free CO

2144cm-1

allyl lig charge and e-s

-1

4e- donor

what does bite angle, P-M-P impact

catalyst form and selectivity

Strong trans effect =

strong σ-donor + strong π-acceptor

what is more trans labalising, ethene or cl-

ethene

hydrolysis of an acid iodide by water is…

rapid

rxn of H2 w MeI

H2 + MeI ←[M]→ CH4 + HI

using reductive aminases for production of chiral amines, what are you going from and to

ketone to chiral amine via iminium ion intermediate

using TA’s for production of chiral amines, what are you going from and to

ketone to chiral amine