Paul Newman Flashcards
Discuss general features of heterogenous catalysts:
- Heterogeneous: catalyst in a different phase to substrates and/or products
- Good recycling, TTN, Quantity of catalyst
- Not good description, reaction conditions or selectivity
Discuss general features of homogeonous catalysts:
- all in single phase (usually in solution)
- Good selectivity, reaction conditions, description
- Not as good recycling, TTN
- Uses of transition metals have many applications, plastics, paints, nylons
- We will focus on carbonylation in L2
- Hydroformylation is the largest process by homocatalytic catalysis
What would the definition of a ‘perfect catalyst’ be?
Perfect catalyst: produces infinite product from a single reactant without poisoning
What is the ‘E factor’ what is the optimal value?
E factor = total waste/ total product (we want this low!)
What are 4 types of selectivity aimed for with modern catalysts:
Modern catalysis is more sophisticated than simple rate enhancement. The best systems are able to effect exceptional product control… four potential types of selectivity:
- Chemoselectivity: selective reaction of one functional group over another
- Regioselectivity: selective reaction of one or other ‘end’ of a functional group (terminal vs. terminal -1 position etc.)
- Diasterioselectivity: selective formation of one diastereomer from a substrate that contains a pre-defined stereocenter
- Enantioselectivity: selective formation of one enantiomer from an achiral substrate (need asymmetric catalyst)
Discuss the general features of homogenous catalytic cycles:
- Overall rate controlled by rate limiting step – slowest step in the cycle
- It does not necessarily correlate with the highest-energy TS but always the step for which the energy barrier is the highest
- Rate laws usually defined in terms of [precat] as this is the known parameter for [cat]
- The order in [precat] will be one if it all converts to active catalyst rapidly and there is no significant decomposition of catalytic species
- Deriving catalytic cycles is not trivial especially in cases where there are one or more reversible steps
- Concentration of active catalyst refers to the sum of concentrations of all catalytic species within the cycle – difficult to determine, kinetic studies challenging
- Catalyst resting state refers to (metal containing-species) of highest concentration: off-cycle (inhibits catalysis) or within the cycle
- We can plot each stage on catalytic cycles
What does
- TON
- TOF
Stand for?
TON (turn over number): moles of product/ mol (pre)catalyst
- stability/longevity; higher: more stable
TOF (turn over frequency): TON/unit time
- relates to reactivity
- want this to be large
How are catalytic cycles derived?
Deriving catalytic cycles:
- Identify rate limiting step; concentration will build up
- Often precedes resting state of catalyst
- Try to isolate intermediates: possible when such species are long-lived
- In situ spectroscopic analysis: NMR (insensitive, time frame, good time frame), IR (lower conc, faster, CO), EXAFS (quick, good data, expensive)
How is the rate limiting step of a reaction determined?
Determining rate limiting step:
Often involving resting state and, for reversible reactions, kinetics of catalysis are determined only by the events occurring between resting state and rate limiting step
- Need to derive full rate law (very difficult) dependence on one or more substrate can then aid identification of RDS.
Discuss the determination of the RLS of hydroformylation
Rate = k[Rh][3,3-dimethylbutene]0.1[H2]/[CO]
- Small dependence on alkene concentration as it is multiple steps before!
- First order for hydrogen
- Inverse dependence on [CO]
- Partial [alkene] because of earlier equilibria for all steps up to and including migratory insertion of CO – small reaction order signals this
By discussing isotopic labelling and kinetic isotope effects, Deuterium labelling in products. Consider late transition metal hydroamination for which there are two possible mechanisms:
What are these?
- Olefin activation
- Amine activation
Why does norbornene form two different isomers from hydroamination? How can this be studied?
With a rigid cyclic alkene such as norbornene, two different routes would lead to two different products. However, these would be indiscernible when RNH2 reagents were used but can be discriminated with RND2.
How can carbon-12 and carbon-13 labelling be used for determination of rate limiting step?
Need to have nucleophilic backside attack by amine if we have olefin activation. However, with amine we can get syn. Can also look at different reactivities using different isotopes. We can use selectively enriched substrates (expensive). Can then determine which positions have been enriched with carbon-13. If there is no change, then those carbon atoms have no effect on the rate. 1.030(4) is the key number. Activity of the C2 carbon is key. 12C reacts a little faster than 13C.
13C KIE = k12C/k13C
How does activation occur through bond breakage by catalysts?
All catalysis involves breaking and forming of bonds. Substrate molecules are activated towards bond-breaking by interaction with the catalyst. One of the simplest forms of activation is protonation (also applicable to Lewis Acid activation). In some ways this can be considered a passive activation as it does not result in bond cleavage. The lowering of energy of the LUMO and alterations in the orbital coefficients does, lead to increased rates of reactivity.
Sigma-bond cleavage: both the sigma-donation and pi-back donation despite H-H bond.
Metals are good pi-bases and cleave H-H bond to give cis dihydrides (oxidative addition, crucial for homogenous hydrogenation).
Certain transition metals in certain oxidations states are very good. Can also break pi-bond.
Activation with metals induces changes within pi-system.
Resonance forms show alkynes (and allenes and alkenes) can be activated towards external and internal (migratory insertion) nucleophilic attack.
Additionally, many coordination modes to metal which can affect the mechanism.
Sigma-bond cleavage: Both the σ-donation and the π-backdonation deplete the H-H bond so that metals that are good π-bases cleave the H-H bond to give cis-dihydrides (oxidative addition, crucial for homogeneous hydrogenation).
Certain transition metals in certain oxidation states are very good at this!
What is the general support structure of homogenous catalysts? How is their selectivity and activity determined?
All homogenous systems are metals supported by ligands. Activity and selectivity dependent on nature of supporting ligands. Often referred to as spectator ligands.
Describe the difference between
- Redox mechanism
- Non-redox mechanism
- Redox: induce change in metal oxidation state over the catalytic cycle. Often two of these processes per cycle. Ex. Oxidative addition, substrate association, migratory insertion, reductive elimination. Typical metals Pd(0), Rh(I).
- Non-redox: sigma-bond metathesis, substrate association, migratory insertion, sigma-bond metathesis. Typical metals can’t be oxidised: Ti(V), Ln(III), Ru(II). Ex. Di-hydrogen activation.
Discuss the difficulties in homogenous catalysis using Acetic Acid production (ethanoic acid) as an example:
- What are the names of the three mechansims?
- PTA (terephthalic acid) for polyethylene terephthalate (PET) – doesn’t let UV light through so products within stay preserved, plastic bottles
- Acetate esters particularly vinyl acetate monomer for polyvinyl acetate (PVA) for coatings
- Acetic anhydride: preservatives
- Others used in films, paints, sealants
- N.B acetic acid in vinegar usually comes from fermentation of ethanol not this process
Discuss Method 1 (Carbonylation of methanol) for the homogenous method of forming acetic acid:
High temperature and pCO to maintain catalyst stability and rate. 90% conversion from MeOH. By-products are a problem because of the high temperature! This is not an efficient process due to conditions and the amount this mechanism is being used is diminishing.
Discuss Method 2 (Monsanto process) for the homogenous method of forming acetic acid:
Monsanto process – and improvement 1970, using rhodium (Next Group 9 element)
Iodide now doesn’t take external route and instead is associated with the metal at every stage. [RhI2(CO)2]+ is the starting material. Nucleophilic attack of methyl iodide to give Rh(III) species. Eliminate cis orientated iodide.
Fundamental steps are the same as the previous mechanism. It is a faster mechanism than the Co species. It is a more nucleophilic species. I->Br>Cl-. Water is necessary but too much isn’t favoured. Build-up of HI unwanted: Corrosive and deactivates catalyst. We don’t want any ‘down time’ where no processing is happening. This can occur if insoluble compounds build up and deposit, requiring the reactor to be cleaned.
Insert cycle of other routes: We want to prevent production of the inactive compound therefore limit the amount of HI in the system. Catalyst deactivation:
Don’t worry about the second slide of the same process – just be aware than acetic anhydride can be produced.
Discuss Method 3 (Cativa process) for the homogenous method of forming acetic acid:
- modern processes making most acetic acid now
Same cycle but now with Iridium. It is more active, meaning we can use milder conditions and suppress unwanted side reactions. Much more efficient process. We are comparing relative rates of oxidative addition and reductive elimination.
The oxidative addition is the rate limiting step. Bonds to break in Ir are stronger. Operates faster at low concentrations of water assisting rate maintenance and processing. Must control water content.
The top-line shows an Ir-Ru mix. We know migratory insertion isn’t favourable for Ir so the Ru helps at this step. Cativa process heavily dependent on concentration of CO and Iodide.
Discuss the differences between netural and anionic cycles for the cativa process (homogenous catalysis of acetic acid):
Under industrial conditions the most likely process in anionic. For the Cativa process (anionic) the rate limiting step is migratory insertion. We can do this by adding iodide scavengers to improve this step of the reaction.
We can study the effect of promoters on the cativa process. CO are more nucleophilic in the neutral complex. We don’t need to know about the thermodynamics in detail.
Many iodide scavengers available however mixtures of certain activators and poisons can lead to increased rates compared to addition of activator alone. Much greater molar ratios of additives are needed. If rate decreases, they are called poisons. We can sometimes never fully understand the process.
Conclude the three methods of acetic acid production via homogenous catalysis:
Conclusion: BASF vs Monsanto vs Cativa
- Cativa best
- More reactive and lower loss due to formation of ‘inactive’ metal species
- More efficient: CO utilization > 90%
- Easier processing: only two stage distillation as opposed to three which reduces CO2 emissions and plant cost.
- More stable, less downtime to reclaim metal
Discuss the process and use of MMA: (Heterogenous)
- Outline the conventional mechansim
- What is the structure shown and why?
MMA production (Perspex®)
- Acrylic acid and the acrylates are an important class of industrial chemical intermediates. Acrylic acid itself is produced on the megatonne scale by a hetereogeneous catalytic process.
- Methyl methacrylate (MMA) is the most important member of the methacrylates (2 x 106 t/a) being used in the main for the production of perspex (plexiglass) which is a crystal-clear artificial glass with high hardness, fracture resistance and chemical stability.
- Good electrical properties and is slowly replacing many uses of glass
Mechanism:
The conventional route to MMA is the stoichiometric reaction between acetone and hydrocyanic acid followed by hydrolysis, dehydration and esterification:
Works well but:
- HCN (bad),
- 3 steps (eek!),
- loads of waste…. 2.4 t of ammonium bisulfate/sulfuric acid per tonne of MMA.
- A better option: PMMA (shown to the right). Low density, excellent light transmission, good electrical props, biocompatible
Discuss the process of carbonylation of propyne: (as a homogenous mechansim to make MMA)
- How do we chose a catalyst for this process?
- We want the first product, the close relative (right) is methyl crotonate. This is atom efficient is we use a catalyst for this process
- Choosing a catalyst for this process:
- As in most homogeneous catalysis we want stable, highly reactive catalysts that are selective for the desired MMA over methyl crotonate.
- PPh3 is slow but reasonably selective but diphenyl(2-pyridyl)phosphine gave an extraordinary rate and selectivity! Only works in the presence of Bronsted acid.
- Optimum conditions require a large excess of phosphine ligand (ratio of Pd:phosphine of around 1:20) and > 10 mol equivs of acid (containing a non-coordinating anion).
- Under these conditions the kinetics are zero order in acid and first order in palladium, propyne and methanol.
- Adding groups at the 6-position of the pyridine gives selectivity’s of >99.9% and no drop off in rate.
Why are these MMA catalysts so effective for the homogenous catalysis of propyne?
Increased steric pressure from the phosphine ligand should affect the two mechanisms in a mutually opposing way, i.e. MMA selectivity should decrease with increasing steric congestion for the hydride cycle (2,1 insertion disfavoured – non redox cycle) but increase for the methoxy cycle as the steric demands of the P,N ligand increase.
Hence the addition of propyne to the palladium-acyl complex and subsequent insertion determine the regioselectivity
Discuss the meachnism of the homogenous catalysis of propyne to give MMA, discuss the akyne coordination step:
- Which insertion step is favoured?
- Initial alkyne coordination has C≡C bond perpendicular to the Pd coordination plane
- Alkyne must ‘slip’ out of this coordination mode by translation / rotation towards the coordination plane to orientate itself to allow orbital overlap with the acyl carbon
- The 1,2 insertion is favoured on steric grounds
Discuss how the rate is affected by the alkyne insertion step for homogenous catalysis of propyne to Pd catalyst for acetic acid production to give MMA?
Rate:
- High rates only with 2-pyridyl.
- Optimum - large excess of ligand and acid.
- Rate is 1st order in [propyne] suggesting that irreversible propyne insertion is rate limiting and the final step to release MMA is turnover limiting.
- This final step is expected to be rel slow (electrostatic barrier) unless (2-py)Ph2P facilitates proton transfer to the palladium alkenyl complex (hydride cycle)
- The ligand acts as a proton messenger
- Terminating step in the methoxy cycle is also turnover limiting and requires nucleophilic attack of MeO(H) at the metal-acyl group.
- This may be assisted by the 2-pyridyl nitrogen through pre-association of the MeOH through H-bonding to increase the local concentration of MeOH
What properties are necessary for effective proton transfer for MMA production via the homogenous catalysis of propyne?
1) The phosphine needs to bind to the metal to enable approach of the proton to the alkenyl function;
2) The ligand should be sufficiently basic to act as a proton carrier but not so basic as to resist giving the proton up;
3) The proton bearing nitrogen should be close to the phosphorus anchor;
4) The monodentate phosphine should not be so strongly bound that it inhibits coordination of the reactant molecules.
Discuss the formation of MMA through the alkoxycarbonylation of ethene:
The alternative route to that of Shell. Initiated by ICI Acrylics, now operated by Mitsubishi-Rayon and SAMIC after buying the process from Lucite (Lucite alpha process). MMA – precursor to PMMA. 2 stage process, but amazing efficiency of the first step. Has to be the ligand shown.
What are the two main mechansims for the alkoxycarbonylation of ethene for MMA production?
Discuss the difference in rate between the two mechanisms for MMA production through alkoxycarbonylation of ethene:
First step is the same as the process before but instead we have changed the nature of the unsaturated substrate. We have gone from an alkyne to an alkene. If we don’t have MeOH, we get a copolymer. These are useful in their own ways. Relative kinetics are important. When we have competing reactions the kinetics are important. Both cycles pretty much the same as before.
The hydride mechanism is believed to operate in the selective methoxycarbonylation of ethene to give methyl propanoate, but the ethene / CO copolymerisation is believed to operate through either cycle: the difference lies in the relative rates of methanolysis which is fast (but still rate limiting) for the production of methyl propanoate but very slow for copolymerisation. Both are operating but one will be dominant.
Discuss the selectivity of the alkoxycarbonylation mechansim to form MMA:
- discuss the general mechanistic steps
Very useful ligand for other transformations. Irrespective of what alkene is selected, we only gain one product. Terminal alkenes are less stable. Even with terminal alkenes we are achieving our product. This is telling us that the catalyst is converting internal alkenes to terminal alkenes too. It is also acting as an alkene isomerise. We are converting both alkenes to the same terminal alkene.
- Phosphine is bound to palladium
- Terminal alkene comes in, migratory insertion etc
- If internal alkene, we get coordination then migratory insertion, gets a secondary alkyl, but then isomerisation occurs (this process is called beta hydride elimination)
How can carbon monoxide be combined with alkenes?
- Show the mechanism:
Mechanism:
Alkene insertion appears to be the rate limiting step but the kinetics are not always clear cut and rates of CO insertion can be rate determining. Bidentate phosphines are strongly preferred as monodentates promote alkyl propanoate formation. Rates tend to be greater for diphosphines with butyl and propyl backbones rather than ethyl backbones. We don’t need to worry about formation of the catalyst.
How can carbon monoxide be combined with alkenes?
-Discuss the ligand insertion step
Ethylene and CO come in and insert. Whenever we have a metal acyl bond, we only insert ethene. Whenever we have a metal alkyl, we only insert CO. This is what is controlling the alternation! CO would never insert into an acyl because the resulting product is very unstable. Even if it was to insert, this would be reversible and not favoured. If CO doesn’t come in and insert on the alkyl, the alkene can come in and insert. We could get polyethylene. The reason why this doesn’t happen is because the kinetics are slow. The M-C bond is weak. Ethene is a poorer strength ligand than CO so it finds it more difficult to displace.
How can carbon monoxide be combined with alkenes?
-Discuss the ligand effects on chain length and rate
How can carbon monoxide be combined with alkenes?
-Discuss the sterics influence the rate
- If we change the support ligand, we will change the relative rate
- Transition metals are extremely sensitive to their surroundings
- If we go from 4- to 5-membered chelate this improves and 6 is better
- We don’t need to recall this but just understand the significance of changing the ligand
- More difficult to rationalise R groups
How are aldehydes produced?
- What are the names of the differences of the two mechanisms?
Direct oxidation of ethene: Hoechst-Wacker process. Alcohols derived from them are more important. This is not a CH3 this is an H! No change in the oxidation state of Pd! Need reinsertion as we need ‘beta deuteride elimination’ step as mentioned below
Two regimes of high and low chloride concentration. Positive dependence on concentration of alkene. Negative dependence on chloride. Low chloride concentration is also pH depdendent. External or intramolecular attack of water.
What is olefin metathesis?
- What are the different types?
In simple terms alkene (olefin) metathesis (or more properly π-bond metathesis) is a means of obtaining an equilibrium mixture of alkenes from any given combination of alkenes. If ethene is produced, we can remove this and use it as a driving force.
Discuss the nature of the homogenous metathesis catalysts of olefin metathesis:
Discuss the eludication of the mechanism of the metathesis catalysis:
What is the clincher in terms of olefin metathesis?
An alkene that cannot undergo further reactions. Anthracene cannot react. We can remove alkene as soon as it’s formed. We trap isotopically labelled ethene.
What are the accepted general schemes for catalytic oldefin cross metathesis?
How do NHCs aid olefin metathesis?
NHC promotes the loss of phosphine due to trans effect.
What is the use of metathesis in commerical industry?
Agrochemicals,
Comerical processes
pharamaceuticals
Why are ploymers of carbon dioxide useful?
Discuss alternating copolymerization catalysis (tend to be Lewis acidic metals)
Discuss ring opening copolymerisations - polymerisation of carbon dixoide
- Discuss mechansism
We are taking two monomers and making a copolymer. Change is whether we use carbon dioxide or an anhydride.
- Controlled Polymerizations (predictable Mn, narrow Ð)
- 500 < Mn < 20,000 g/mol
- Controllable end-groups - Polyols
- Block copolymer synthesis
Very simple mechanism. On the left, green compound reacts at anhydride first, then attacks epoxide as it is activated by the Lewis acidic metal. Internal nucleophilic attack then occurs.
- Scaleable synthesis (hundreds g)
- 3 steps (vs. 15 steps for ‘best’ [(salen)CoX)]
Discuss properties of low pressure, high activity catalysis for copolymerisation of carbon dioxide:
- Maximum TON > 40,000 g/g and TOF > 9000 h-1
- No co-catalyst needed
- Catalysts air/water tolerant (inert)
- Activity matches literature (salenCoX) but operates at a fraction of the CO2 pressure
- Bimetallic species are fantastic compounds
Discuss the chain shuttling mechanism for carbon dioxide copolymerisation:
Discuss catalyst syntheses for carbon dioxide polymerisation using chain shittling mechansism:
- DOSY: monomer in d8-THF
- MALDI: [M-Br]+
- COSY and HMQC: peak assignment
- Heterodinuclear is thermodynamically most stable
What is the point of caralysts for copolymerisation of carbon dioxide using chain shuttling mechanism?
Reaction with anhydride instead of carbon dioxide shows even more pronounced:
We can rationalise this because of the chain shutting mechanism. The ring opening of the epoxide (arrow is the wrong way around) was the rate determining step. This changes when bimetallic because the increased Lewis acidity of manganese. The mixed system with manganese or zinc means RDS faster.
How does carbon dioxide capture work?
How do different catalysts change the monomer mixtures of carbon dioxide capture?
Selective catalysis from monomer mixtures
- Using bimetallic catalyst
- All we are doing in Ring opening
Show the production of block-co-polymers from carbon dioxide:
- Block copolymers
- High Selectivity
- Ester enchained first
- Carbonate enchained second
- Good Control
- 8400<mn>
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Exam question 2013-14:
Exam question 2013-14:
Exam question: 2013-14
