7: Catalytic cycles

Question 1

Background of Lanthanide catalysts

Answer 1

• reactivity is dictated by polarizaion effects/ionic bonding and not redox steps
• highly reactive and FG tolerance of reactions is low

-Lanthanide OS remains as 3+ through cycle
-Catalysts are designed and normall include spectator ligands and a single reactive substituent eg R or H
Rates of ligand exchange are very fast- difficult to know true reactive species

Question 2

What are the fundamental steps of catalysiss

Answer 2

Insertion
Picture
Sigma Bond Metathesis
Picture

Question 3

What are the main catalyst cycles

Answer 3

1) The heterofunctionalisation of unsaturated subsrates

Question 4

Regiochemistry of the heterofunctionalisation of unsaturated substrates

Answer 4

Factors that stabilise the alkene insertion TS will determine the regiochemistry

Stabilisation of the partial negative charge is the dominant factor, as this lowers the energy of the TS

Ph/= –> Ph/_/X-Picture
2-1 insertion/anti markovnikov addition
Ph stabilises negative charge in benzyllic position

R/= –> R/ \NR2 - PICTURE
1,2-insertion markovnikov addition
although R destabilises - charge, it stabilises + charge
ALKYNE
syn addition of M-X bond- same face ‘cis’

PICTURE
initial products often undergo further isomerisation

Question 5

What is the pre catalyst and catalyst for C-H functionalisation

Answer 5

Picture

B(C6F5)3 - removes a hydride group

Question 6

Asymmetric Carbon Nitrogen Bond formation

Answer 6

-including chiral ligands on the metal ->
asymmetric processes aka. hydrosilylation/hydroamination
OVERALL PROCESS
Picture
Same cycle
Catalyst
Ln: Y or Lu -small
BINOL based asymmetric ligans w/big silyl groups - (R) stereochem
R has to be alkyl for a chiral product

ee relatively high: T =150C
challenging: ill defined coordination geometries and fast rates of ligand exchange on Ln complexes

Question 7

why are competitive rates so impportant

Answer 7

Sigma bond must be fast (sim rate to insertion)

If the rate of sequential insertion steps (k1) is much bigger sigma bond metathesis
Multiple insertion reactions will occur, giving a polymerisation reaction
Picture

Question 8

polyethylene

Answer 8

absense of X-H and sigma bond metathesis step
Ln Cp2 R and Ln Cp 2 H active catalysts
R/H end on the leading end
rxn proceeds by sequential alkene insertions
adding water to protonate off the polymer

add X-H and then bulk excess of ethene –> X at beginning

where sigma bond metathesis is slow - vary terminating (leading) group of the polymer by one pot addition of stoichiometry
HPR2 + n*ethene –cat-> H-polymer-PR2
slow rate due to non-favourable soft-hard interactions between the substrate and the catalyst
Ln are hard

Question 9

renewable monomer polymerisation

Answer 9

-Lantha- nide complexes have also een used for the polymerisation of renewable monomers eg. lactones and lactides which can be produced from sugars and used to make polyesters

Catalyst- La, Y, Lu
Rate of reaction La>Y>LU
selectivity for isotacticity Lu>Y>la
due to lanthanide contraction
bigger ions- faster catalysts and hence less selective

Question 10

Sigma bond metath only catalysis

Answer 10

-no insertion step
-protic and hydridic substrates substrates
these are coupled
Sc(Cp*)2-Me
cat +R3Si-H –> LnM-H
LnMH +Ch4 –> LnMCH3
LnMCH3 +R3Si-H –> R3SiCH3
PIctures of cycle

Question 11

Reverse bond polarity

Answer 11

Hydridic substances (silanes and boranes) reverse the polarity of the TS (H usually delta plue- but her delta-)
effects regiochemistry but not the actual product etc
picture of cycle
LnM-H bas instead of LnM-X in actual ring