Lanthanides in Catalysis

Question 1

What OS does catalysis with lanthanides occur in

Answer 1

1. +3
2. Only consider Ln(III) oxidation state, so oxidative addition and reductive elimination are not possible.

Question 2

What are the two types of reactivity seen by Cp*2LnR

Answer 2

1. Insertion of C=C unsaturation into polar Ln-R bonds
2. Sigma-bond metathesis

Question 3

What does Lanthanide catalysis rely on

Answer 3

1. Instead rely on steps that occur without change of oxidation state at the metal.
2. These steps occur via polarised 4-membered transition states where the orientation of the reactants is defined by the consistent Lnδ+-Xδ- polarisation.
3. This leads to high selectivity
4. Reaction can be reversed by beta-hydride elimination

Question 4

What is the product of sigma-bond metathesis defined by

Answer 4

1. Product of the σ-bond metathesis is defined by the polarisation of the element-hydrogen bond, inferred from the electronegatitivity of the element.

Question 5

What are the three fundamental catalytic transformations

Answer 5

1. Alkene polymerisation
2. Heterofunctionalisation

Question 6

Describe what alkene polymerisation involves

Answer 6

1. Successive C=C insertions
2. Where termination occurs via the reverse reaction, β-hydride elimination to reform Cp*2LnH and polymer with alkene
3. High pressure, rate of insertion> rate of elimination

Question 7

Describe what hydrofuncitonalisation involves

Answer 7

1. Combines Insertion and σ-Bond Metathesis to give a net addition of a hydrogen-element bond across an alkene.
2. The same reaction steps give two distinct products depending on whether the hydrogen-element bond is protic or hydridic.
3. Orientation of the alkene in the transition state and therefore the isomer produced depends entirely on the identity of the substituent, R, on the alkene

Question 8

What makes a protic sigma bond metathesis

Answer 8

1. X-H added where X=delta- and H= delta+
2. X= NR2, PR2, OR, C=-C-R (triple bond)
3. RH is side product

Question 9

What makes a protic sigma bond metathesis

Answer 9

1. H-E where H=delta- and E= delta+
2. E= BR2, SiR3
3. RE is side product

Question 10

How can you carry out protic heterofunctionalisation

Answer 10

1. First sigma bond metathesis Cp*2LnR + X-H, e.g. where X=NR2
2. Then insertion with Cp*LnX + =
3. Alkene has polarisation induced produces lanthanide alkyl which are very basic
4. In presence of proton source such as Amine Lanthanide alkyl is protonated and Cp*2LnX is produced

Question 11

What is heterofunctionalisation overall

Answer 11

1. Addition of H-X bond across an alkene

Question 12

How can you carry out hydridic heterofunctionalisation

Answer 12

1. First add E-H to Cp2LnR in hydridic sigma-bond metathesis to produce Cp2LnH
2. Then insert alkene which is polarised by hydride
3. Then reacts with ER to produce product and recycle Cp*LnH

Question 13

How to tell if reaction is heterofunctionalisation

Answer 13

1. Always alkene - unsaturated bond
2. Sigma bond with p-block group

Question 14

How can you tell if reaction is protic or hydridic

Answer 14

1. Anything on left side of periodic table split - B, Si, As, Sb are hydridic
2. Anything right is protic- C, P, Se, Te

Question 15

What are the steps when drawing catalytic cycle

Answer 15

1. Protic or hydridic
2. Draw reaction
3. Catalyst activation- may need or not- sigma bond metathesis
4. Analyse insertion- which geometry
5. Close the cycle via sigma bond metathesis
6. Write overall reaction

Question 16

Is n-bu electron donating or withdrawing

Answer 16

1. Donating
2. So preferes positive charge next to it as stabilises

Question 17

If it is a hydridic reaction and Cp*2LnH, does the catalyst need to be activated

Answer 17

1. No

Question 18

When talking about organolanthanide catalysis what elements does it also apply to

Answer 18

1. Scandium
2. Lanthanum
3. Yttrium
4. and group 2 e.g. Cu
5. Small and charge dense

Question 19

Is Ph electron withdrawing or donating

Answer 19

1. Withdrawing
2. Likes to have delta negative

Question 20

Apart from alkenes what other systems can be heterofunctionalised

Answer 20

1. Ketones
2. Imines
3. Easier to work out TS as have electronegative atom

Question 21

What is a cross-metathesis reaction

Answer 21

1. Combines both protic and hydridic sigma-bond metathesis
2. Notice by lack of alkene, ketone, aldehyde- no unsaturated species
3. Would see amine and silane for example - combination of protic and hydridic element-hydrogen bond
4. Create new maingroup- maingroup bond

Question 22

Describe catalytic cycle in cross metathesis

Answer 22

1. Cp2LnX + E-H produces X-E which is new P-P block bond and CP2LnH
2. Then add X-H (Protic)
3. Produces H2 and Cp*2LnX
4. Overall E-H + X-H –> X-E + H2

Question 23

What are ligands meant to impart

Answer 23

1. Solubility- covered in Me, Ph etc
2. Defined molecularity - monomeric centres
3. Enhance reactivity by reducing oligomerization etc
4. Introduce chirality

Question 24

What happens as the metal of a catalyst decreases in size

Answer 24

1. Higher charge density so higher reactivity
2. Due to lanthanide contraction
3. If Small and fast it do with charge density
4. If small but slow reaction its to do with steric access problems