Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Homogeneous catalysis > Unit 1 > Flashcards

Unit 1 Flashcards

1
Q

What is the structure of Wilkinson’s catalyst and what reaction does it catalyse?

A

[Rh(PPh3)3(Cl)]
It catalyses the hydrogenation of alkenes to the alkane

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Calculate the NVE, oxidation state and dn configuration for Wilkinsons catalyst

A

16 e, Rh(I) and d8

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What are the steps in the catalytic cycle for the hydrogenation of an alkene using Wilkinson’s catalyst?

A
  1. OA of H2
  2. Alkene coord
  3. MI (RDS) of an H onto the alkene
  4. RE → alkane product
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the turn-over limiting step for the hydrogenation of an alkene using Wilkinson’s catalyst?

A

Migratory insertion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What factors effect the selectivity of Wilkinsons catalyst?

A

Unconjugated alkenes + alkynes hydrogenated
* Donor solvents (e.g. ethanol) speed up rxn since e density is increased → helps MI step (RDS)
* Less substituted alkenes preferred as highly substituted alkenes bind more weakly to M → MI has a higher activation barrier → decreases the rate
* Kinetic control: preference for the reduction of less substituted double bond
* high level of functional group tolerance → doesn’t reduce nitro groups

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the productivity of a catalyst?

A

Mass of product per moles of catalyst per unit time, g/mmol.h or g/mol.h

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is a catalysts turnover number (TON) and formula?

A

How many times a catalyst completes a complete cycle of the reaction in a given time before becoming deactivated

TON = moles of product / moles of catalyst

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the turnover frequency TOF of a catalyst?

A

Number of moles of product per moles of catalyst per unit time

TOF = TON / time (s⁻¹, h⁻¹, min⁻¹)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is methanol carbonylation?

A

Conversion of methanol to acetic acid by the addition of CO

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are the two processes used for methanol carbonylation

A
  • Monsanto process
  • BP’s Cativa process
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the catalyst for the Monsanto process of methanol carbonylation, its VE count, electronic configuration and geometry?

A

[Rh(CO)₂I₂]- , 16e, d8 and SP

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Draw the mechanism for the Monsanto Process of methanol carbonylation

A
  1. Organometallic cycle:
    * OA of methyl iodide (RDS)
    * Migratory insertion of CO → intermediate 18 e- acyl complex
    * Reductive elimination → acetic acid + regeneration of catalyst
  2. Organic cycle:
    * methanol reacts rapidly w/ HI - CH3I + H2O
    * Acyl iodide reacts w/ H2O (hydrolysed) → acetic acid that we want as a product which regenerates HI
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Why won’t methanol oxidatively add to Rh?

A

The hydroxyl group (-OH) is a weak leaving so it’s not good at OA which is an SN2 like reaction, need good LG + OH is not a good LG whereas I- = good LG

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Explain the OA step in the Monsanto process?

A

SN2-like
* [Rh(CO)2I2]– is highly nucleophilic (-ve charge)
* Neutral analogues of [Rh(CO)2I2]– e.g. [Rh(AsPh3)2(CO)I] = 105 times less reactive
* Can be accelerated by electron donor ligands (e.g. PEt3) but these are unstable in real reactor conditions: PEt3 + HI -> [HPEt3]I after ligand dissociation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What are the main issues with the Monsanto process?

A

*↑ [HI] and [H₂O] is needed to stabilise the catalyst to prevent formation of insoluble Rh(III) salts
*↑ [H₂O] → CO loss via WGS rxn:
H₂O + CO → CO₂ + H₂ → ∼ 30% of CO to be lost, + H2 → product purity issues
*HI is v corrosive + requires expensive reactor metallurgy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is the catalyst for BP’s Cativa process for methanol carbonylation?

A

[Ir(CO)₂I₂]-

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Why is the OA of iodide methane in BP’s Cativa process is much faster than the Monsanto Process?

A

Ir has a higher e- density than Rh ∴ has greater nucleophilicity

OA is accelerated by stronger Ir-Me and Ir-I bond

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Why is the migratory insertion of CO in BP’s Cativa process slower than the Monsanto Process?

A

Ir has a higher e- density than Rh ∴ has greater nucleophilicity

Migratory insertion is slowed by stronger Ir-Me bond

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

How is the migratory insertion step sped up in BP’s Cativa process?

A

Rate = k[Ir][CO]/[Iodide] ∴ removing iodide ligand from will speed up reaction

Promoter = group 3 halide, e.g. InCl3 or Ru-CO complexes

Acts as iodide ‘shuttle’→ forms neutral [Ir(CH3)(CO)3I2] complex

  • Replace I w/ CO ∴ catalyst = neutral [Ir(CH3)(CO)3I2]
  • Migratory insertion is fast since CO = good π acceptor ligand (removes e- density from M → makes Ir-Me bond weaker → faster rxn
  • Add iodide back in + loose CO
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Draw the mechanism for BP’s Cativa Process?

A
  1. OA of MeI (fast)
  2. Rather than slow MI
    - Replace I w/ CO ∴ catalyst = neutral [Ir(CH3)(CO)3I2]
    * via a promoter (lewis acid I= v big,soft anion ∴ needs fairly soft lewis acid or TM Ru-CO complexes
    rather than slow MI
    loss of Iodide via promoter
    addition of CO
  3. Fast CO MI
    * Additon of iodide via promoter → back into main cycle
    * Forms acetyl complex
  4. Reductive elimination to regenerate catalyst
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What are the advantages of of BP’s Cativa Process?

A

Catalyst = more stable at ↓ [H2O] – WGSR not a problem

IR = cheaper than Rh

Product is purer – fewer distillation columns required

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What is hydroformylation?

A

Conversion of an alkene to a linear (desired) and branched aldehyde

Alkene + CO + H₂ → RCOH

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is the catalyst used in hydroformylation?

A

Pre-catalys = [Co(CO)₄(H)], Co (1), SP, d8

Active catalyst = [Co(CO)₃(H)]

Or Rh carbonyl complex

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Draw the mechanism for the unmodified Cobalt catalyst used in hydroformylation

A
  1. Alkene coordination
    Binds to M centre → TBP compound
  2. 1,2 migratory insertion
    * Co-H in cis orientation to Co-alkene
    * Hydride undergoes MI w/ alkene → alkyl
    * Side rxn: 2,1 migratory insertion of alkene → i-alkene (iso/ branched)
  3. CO coordination
    * 5 coordinate species
  4. Migratory insertion
    * Alkyl in cis orientation to carbonyl
    * Alkyl undergoes MI w/ carbonyl → acyl
    * Forms metallic ketone
  5. OA of H2
    * Concerted OA:
    * H2 = non-polar
    * Co (1) = low OS + 4 coordinate species
  6. Reductive elimination
    * Organic group in cis orientation to hydride
    * Forms n-aldehyde product (linear)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What is the RDS of unmodified cobalt hydroformylation?

A

OA of H2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What is the rate equation of unmodified cobalt hydroformylation?

A

Rate = k [Co][alkene][H2][CO]-1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Why is the unmodified cobalt hydroformylation reaction run at high CO pressure and temperature despite the inverse [CO] order?

A

Inverse order in CO due to need for CO dissociation from pre-catalyst to form 16e active species

Implies rxn should run under ↓ [CO] → faster catalyst decomp + ↓ l:b selectivity (bc of competitive Co-catalysed alkene isomerisation)

Rxn = run at high CO P + ↑ T to ↑ productivity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

What are the issues with the HCo(CO)4 catalysed used in hydroformylation ?

A

V high P (200+ bar)

Poor catalyst stability

High catalyst volatility

2 isomeric aldehydes in linear : branched ratios of 3 : 1.

29
Q

How could you improve the HCo(CO)₄ catalyst used for hydroformylation?

A

Employ a modified phosphine ligand:

Lower CO pressure

Linear : branched ratio improved

Or use a Rhodium modified catalyst:
* Good l:b regioselectivity, stability + cat activity (> Co by 1000x)

30
Q

How does the addition of phosphine ligands affect the catalyst stability and activity in hydroformylation reaction?

A
  1. Electronic effect
    * Phosphine ligand stabilises Co-CO bond via sigma donation to Co centre + back bonding to CO ligands
    *Makes Co-CO bond stronger against decomposition
    * ↓ CO P is required, 80 bar (stronger Co-CO bonds)
  2. Steric effect:
    * Linear selectivity is enhanced (l:b ratio ↑ 8 : 1)
    * bulkier system promotes linear product in 1,2 insertion step
31
Q

Give 2 examples of phosphine ligands used in phosphine modified catalyst?

A

PR3 = P (n-Bu)3, or phobane

32
Q

Draw the mechanism of the Phosphine modified Co catalyst?

A
  • Pre-cat = CoH(PR3)(CO)3]
  • Loss of CO → vacant site on active cat
  • Active cat: [CoH(PR3)(CO)2], 16e, SP
  1. Alkene coordination
    * Side rxns: hydrogenation of aldehyde → alcohols
  2. Migratory insertion
  3. CO coordination
  4. Migratory insertion
  5. OA of H2
  6. Reductive elimination
33
Q

What two hydrogenation reactions occur due to addition of phosphine ligands in Co hydroformylation reaction?

A
  1. Hydrogenation of aldehydes → Alcohols
  2. Hydrogenation of alkenes → alkanes - waste side product ~15 %
34
Q

Draw the mechanism for the hydrogenation of aldehydes to alcohols side reaction that occurs in the phosphine modified cobalt catalyst in hydroformylation

A
  1. Coordination of aldehyde formed from hydroformylation rxn
  2. Migratory insertion of aldehyde
    * Forms metal alkoxide species
  3. OA of H2
  4. Reductive elimination
    * Generates alcohol
35
Q

What is the rate in the Rhodium modified catalyst in hydroformylation reaction?

A

Rate = k [Rh][alkene][H2]0[CO]-1[PPh3]-1

36
Q
A
  1. Pre-cat = Rh (I), 18E, TBP, loss of PPh3 → active cat
  2. Active cat = Rh (1), 16 VE, SP
  3. Alkene coordination RDS
  4. MI
  5. CO coordination
  6. 1,1 MI
  7. OA
  8. RE
37
Q

How does using Rhodium increase the stability of the catalyst in hydroformylation

A

Rh is a 2nd row TM ∴ has a stronger M-L bond whilst Co is 1st row TM

38
Q

What is a disadvantage of Rh modified catalyst used in hydroformylation?

A

Rh is more expensive

39
Q

Name 2 processes that use Rh phosphine modified catalysts

A
  1. Ruhrchemie/Rhone–Poulenc process
  2. Eastman kodak process
40
Q

Draw structure of bisbi ligand used in Eastman Kodak process and comment on structure, activity

A
  • 9 membered chelates
  • V flexible w/ a large + variable bite angle (spans both 90-120 degrees in TBP complexes)
  • Catalyst activity 10x > phosphine mod cats
  • High l:b ratio 25:1
  • Dissociation of 1 P-donor does not (necessarily) -> ligand loss (chelate effect)
  • Despite being a 9 membered chelate, the unsaturated bonds lead to fewer degrees of rotational freedom than sat analogues -> higher chelate effect
    chelating diphosphine ligand with diaryl structure - when it coords to Rh makes 9 membered ring
41
Q

Draw structure of TPPTs ligand used in Ruhrchemie/Rhone–Poulenc process

A
  • V water soluble catalyst due to suffocated groups
  • Biphasic catalysis w/ reagents in organic phase
  • Rxn occurs at interface
  • Allows facile product separation + catalyst recycling
42
Q

Which alkenes use the Ruhrchemie/Rhone–Poulenc process

A

Propene + butene

43
Q

What is a phosphite

A

Instead of alkyl or arly have alkoxide group instead

44
Q

How effect do phosphite modified Rh based catalysts have on hydroformylation?

A

200x activity of PPh3 analogue

Easier CO dissociation (phosphite more pi acidic than phosphine)

BUT dissociated phosphite readily undergoes hydrolysis → cat deactivation

45
Q

Name a process that utilises phosphite modified Rh catalyst

A

Union carbide process

46
Q

Draw structure of phosphite ligand used Union carbide process

A

Rh cat = 50x more active than PPh3 analogue

Ring provides kinetic stability against hydrolysis

tBu group makes ligand environment more hydrophobic

Reduces hydrolysis

Increases steric bulk → maximising l:b ratio (=80)

47
Q

What is cross coupling?

A

Coupling of a boronic ester / acid with an aryl halide / vinyl halide

48
Q

Draw the mechanism for the Suzuki cross coupling

A

Loss of 2 PPh3 ligands to form active e- rich catalyst w/ vacant bonding sites

  1. OA of aryl halide (Pd inserts between halide and C) Pd = oxidised + carbon is reduced
  2. Transmetallation (RDS)
    * base reacts w/ boronic acid –
    * exchange R group on B for X group on Pd (2 metals exchange R groups)
    NaOH → R-B(OH)₃⁻ → X-B(OH)₃⁻ → NaX + B(OH)₃
  3. Ligand swap with the metal
  4. Trans-cis-isomerization
    * now have 2 alkyl or aryl group on cis orientation
  5. Reductive elimination –
    * Concerted step
    * Doesn’t occur if the 2 regions are trans to each other
  • 2 organic regions form single bond + give Pd 2 e-‘s back → pd reduced to Pd 0
  • generates product + addition of phosphine regenerates catalyst
49
Q

Why would the rate of a Pd(PR₃)₄ catalysed Suzuki reaction increase when R follows:

CMe₃ > CHMe₂ > (CH₂)Me > Me

A

Oxidative addition favoured with bulky phosphine ligands since the Pd will more readily lose one of the bulky phosphines during OA in order to remove steric strain

50
Q

Why is a strong base such as NaOH used in the Suzuki reaction?

A

Anionic, 4 CN boron compounds are significantly more amenable to transmetallation.

This can be achieved by adding a hard base, such as hydroxide. Therefore R-B(OH)₂ doesn’t work but R-⁻B(OH)₃ does

51
Q

Why can’t you use sp3 carbon centres in the Suzuki reaction?

A

β-hydride elimination can occur which will just reduce the alkyl chain to an alkene and release HX as a byproduct

52
Q

Why does the OA rate of Suzuki cross-coupling increase with more bulky sterically hindered ligands?

A

Usually steric crowding ≠ fast addn

However, Low CN = important

↑ steric crowding by PPh3 LGs facilitates LG de-coordination + enables rxn to proceed faster

53
Q

Why are aryl-bromides in the Suzuki cross coupling?

A
  • Aryl iodides = easily cross-coupled, while aryl chlorides = v slow + aryl fluorides dont undergo cross-coupling.
  • C-X bond strength affects OA Ea
  • aryl-iodides → side rxns so aryl-bromide used
54
Q

What is the role of coordination in Suzuki cross-coupling reactions?

A

Initial coordination between TM + halide (X) brings the boronic acid or Grignard reagent closer to M center.

Stronger coordination is observed for the Grignard reagent compared to the boronic acid.

The presence of a base enhances the reactivity of the boronic acid and aids in the initial coordination between the substrate and the transition metal catalyst.

Replacing the halide (X) with OH- creates a stronger interaction, promoting efficient transmetallation.

55
Q

What is the heck reaction?

A

Coupling of alkene with a aryl halide / vinyl halide

56
Q

What is one key difference between the Suzuki reaction and Heck and Buchwald-Hartwig amination

A

Heck reaction and Buchwald-Hartwig amination have no transmetallation

57
Q

Draw the mechanism for the Heck reaction

A

Low CN Pd species
Ligands: 2 bulky phosphine ligands

  1. OA of Aryl halide
  2. Ligand substitution
    * Alkene comes in + displaces 1 of the remaining L ligands in a L subs rxn
  3. Migratory insertion (2,1-insertion = mimics sterics)
    * alkene in cis orientation to Pd Aryl e.g. Pd sigma cabron bond
  4. B-hydride elimination
    * Have beta hydrogens and low CN Pd species
  5. Ligand substitution
  6. Reductive elimination -> trans isomer
58
Q

What is Buchwald-Hartwig amination?

A

Coupling of an aryl halide with an amine.

Ar-X + NHR₂ → Ar-NR₂

59
Q

Why is a strong base e.g. (NaOt-Bu) needed for Buchwald-Hartwig amination?

A
  • Forms new C-N bond Ar-Nr2 = aryl amine
  • Deprotonation of NH occurs after coord to Pd (↑ acidity at this stage)
  • Wide range of N nucleophiles tolerated
60
Q

Draw the Buchwald-Hartwig amination mechanism?

A

Low CN Pd (0)
1. OA of aryl halide
2. Nucleophilic substitution type rxn:
* Instead of transmetallation
* Amine acts as a nucleophile → nu sub rxn happens on Pd
* by coordinating amine to metal increases acidity of proton compared to free amine - sufficiently acidic - strong base come in deprotonates it -lose base salt make key intermediate
3. Reductive elimination

61
Q

Intramolecular amination

A

Aryl halide + amine in same molecule

Xantpos - chelating phosphine LG

62
Q

Intermolecular amination

A

Using trifalates → acts v similarly to aryl bromide

BINAP ligand (more complex phosphine ligand)

63
Q

What are the two cycles involved in alkene hydrogenation?

A

The neutral cycle using Wilkinson’s catalyst and the cationic cycle with Rh(I) catalyst

64
Q

In the neutral cycle of alkene hydrogenation, what is the order of steps?

A
  • OA 1st
  • Alkene coord 2nd
  • Solvent used is neutral 2e- donor
65
Q

What are the key components of the cationic cycle in alkene hydrogenation?

A
  • Rh (I)
  • Alkene coord 1st
  • OA 2nd
  • Chelating phosphine
  • Weakly coordinating anion e.g. BF4 → don’t act as LG bc -ve charge more diffuse
66
Q

How can chiral diphosphine ligands contribute to stereoselective catalytic hydrogenation?

A

By coordinating with prochiral alkenes. The attack of the metal on different faces of the prochiral alkene leads to two diastereomers:
1. Sterically favoured - less steric bulk
2. Sterically unfavoured – steric clash between 2 bulky R groups
Examples:
A + B = different (P-chiral) e.g. DIPAMP LG
A + B = same but chiral backbone sets conformation of chelate ring

67
Q

What is a key complication in achieving stereoselectivity in alkene hydrogenation?

A

The most stable diastereomer does not necessarily lead to the major product. Stereoselectivity is influenced by the specific reaction pathways and energy barriers (Ea) for each diastereomer

68
Q

Provide a real example of stereoselective hydrogenation?

A

L-DOPA, used in the treatment of Parkinson’s disease, undergoes asymmetric hydrogenation. The rate of OA for the minor diastereomer is significantly faster: 600x > major diastereomer
* Steps after OA (MI, etc) = fast + irreversible
* Faster OA of minor diasteromer ‘locks in’ that stereochemical outcome

69
Q

Draw the mechanism for the stereoselective alkene hydrogenation of L-DOPA formation?

A
  • pre catalyst + substrate in centre
  • 2 possible routes:
    1.Coordination to form intermediate I = favoured
  • OA
  • MI
  • RE → product or can go to intermediate I’ = least favoured
    2. Coordination to form intermediate I’ = disfavoured

Homogeneous catalysis (2 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions