Catalysis 3

Turnover Frequency, TOF =

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).

Stereoselectivity is

the property of a chemical reaction in which a single reactant forms an unequal mixture of stereoisomers during the non-stereospecific creation of a new stereocenter or during the non-stereospecific transformation of a pre-existing stereocentre

An enantioselective reaction is where

one enantiomer is formed in preference to the other, an optically active product forms from an achiral starting material, using either a chiral catalyst, an enzyme or a chiral reagent.

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 the Wacker process

ethene to ethanal, catalysed by Pd2+ & Cu2+

what is Methanol carbonylation

the "new" route to ethanoic acid, catalyst is cis-[Rh(CO)2I2] -

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 Alkene hydrocyanation

use nickel cats to make organocyanides which are reduced and used in nylon manufacture

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

how can we quantify the screening effect of the ligands

by the space they block i.e. the apex angle of a cone which places the phosphorus centre 2.28A away from the metal and spreads to the van der Waals spheres of the outermost atoms

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 are Ancillary ligands

ligands that provide an appropriate electronic and steric environment around the central metal atom or to help stabilize the central atom during reactions, but they essentially do not participate in any transformation/reaction the coordination complex undergoes

wacker process

H2C=CH2 + PdCl4(2-) + H2O → CH3CHO + Pd + 4Cl- + 2H+

traditional synthesis of acetic acid

C6H12O6 → 2 C2H5OH + 2 CO2

C2H5OH + O2 → CH3COOH + H2O

industrial synthesis of acetic acid

CH3OH + CO → CH3COOH

conditions for industrial synthesis of acetic acid

• 180 deg

• 30-40 atm

• 99% selective

what is monsanto process

industrial synthesis of acetic acid

what is monsanto process catalysed by

• 16 electron square planar rhodium (I) complex

Mechanistic evidence for monsanto process

1. The reaction is zero-order in CH3OH.

2. The reaction is zero-order in CO

rate of monsanto process

rate ∝ [CH3OH]^0 [CO]^0 [Rh]^1 [HI]^1

what does the rhodium complex do in the monsanto process

catalyses the Water Gas Shift Reaction

whats the issue w the rhodium complex in the monsanto process catalysing the Water Gas Shift Reaction

we waste the feedstock CO and then the H2 liberated destroys the MeI thereby reducing our atom efficiency

how to reduce issue w the rhodium complex in the monsanto process catalysing the Water Gas Shift Reaction

Replacement of the rhodium complex by [Ir(CO)2(I)2]- overcame this problem as even though the reaction was slower, it operated with low levels of H2O

what is RDS of monsanto process when catalysed by [Ir(CO)2(I)2]-

migration of IrCH3

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

what is the oxo process used for

Alkene Hydroformylation - We take an alkene and CO and H2 and form an aldehyde

origional oxo process =

RCH=CH2 + CO + H2 —(organic solvent, Co compound0—> RCH2CH2CHO

cat for origional oxo process

Co2CO8 - is therefore 18 electron with a metal-metal bond. It reacts with H2 to form Co(CO)4(H) and this is called an unmodified catalyst

operating conditions for origional oxo process

• 1:1 mixture of CO and H2

• (total pressure 200-350 atm),

• temperature 110-180 oC

organic solvent for origional oxo process

alkene

what must the Co compound be for origional oxo process

soluble in organic solvents

In case of aliphatic olefins what are preferred products

linear aldehydes

rate for oxo process

rate ∝ ( [alkene]^1 [Co(CO)4H]^1 [pH2]^1 ) / ( [pCO]^1 )

what is the modified oxo catalyst

change CO for another 2 electron donor, like a phosphine. The result is a change in reaction selectivity.

implication of modified oxo cat

barrier to migration will be affected by congestion on the metal. We want a lower barrier to linear product and can use ligand bulk to introduce steric strain and favour this route.

Possible mechanisms for alkene isomerization under hydroformylation conditions

• Via an alkyl intermediate: this is a common route

• Via an η3-allyl intermediate

• proton migration

info on alkyl intermediate mechanism for alkene isomerization under hydroformylation conditions

1. This is the correct mechanism for HRh(CO)(PPh3)3

2. This seems logical as alkyl intermediates are intermediates in hydroformylation and the process Is rapid and reversible, but with DCo(CO)4 you should get DCH2CH=CHR

info on η3-allyl intermediate mechanism for alkene isomerization under hydroformylation conditions

1. This is the correct mechanism for [HCo(CN)5] 3-

2. 18-electron rule means that CO must be lost

3. With DCo(CO)4 you should get some DCH2CH=CHR

info on proton migration mechanism for alkene isomerization under hydroformylation conditions

Explains why no D incorporated into the product when DCo(CO)4 is used

Aldehydes react with ammonia to form

imines

Imines can be hydrogenated to form

an amine

what happens to relative Rates of Hydrogenation using Wilkinson's catalyst at 25oC when increase steric bulk of alkene

rate decreases for steric reasons - introducing selectivity

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

Water Gas Shift rxn

H2O + CO <-[M]→ CO2 + H2

wavenumber of free CO

2144cm-1

what is an unmodifie

allyl lig charge and e-s

-1

4e- donor

what does bite angle, P-M-P impact

catalyst form and selectivity

Mechanism of alkene hydrosilylation (Chalk-Harrod)

(i) Silylation product

(ii) Dehydrosilylation product, as named we add silicon and loose hydrogen

(iii) Can hydrogenate with released H2 so we are going to see other products, a problem

Strong trans effect =

strong σ-donor + strong π-acceptor

what is more trans labalising, ethene or cl-

ethene

comments on the traditional route to acetic acid

not clean or efficient

hydrolysis of an acid iodide by water is…

rapid

rxn of H2 w MeI

H2 + MeI ←[M]→ CH4 + HI

in the monsanto process, after changing the cat to [IrI3(CH3)(CO2)]-, what else was done to improve the rxn

• co cat Ru(CO)4I2 was added which:reduced effective conc of HI

• also Ru(CO)3I2 abstracts I- from [IrI3(CH3)(CO2)]- thus increasing rate of methyl migration

what does treating a soln of cis-[Rh(CO)I2]- w MeI at rt in absence of CO give

[Rh2(CO)2I6(CHCH3)2]2- which is isolable as its PhMe3N+ salt

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