Introduction to Palladium Catalysis II Flashcards
How does Pd(0) react?
It is electron rich (d¹⁰)
Nucleophilic
How dooes Pd(II) react?
Electron poor (d⁸)
Electrohilic
The Pd(0) compound can go oxidative addition with the following benzohalide
What is the transition state and the product
- Selective cis-addition of X and R to the meatal centre (concerted reaction)
- Metal is oxidised from Pd(0) to Pd(II), hence oxidative addition
How many ligands are there on a saturated Palladium
4
What is the opposing reaction for oxidative addition?
Reductive elimination
(it is essentially plucking palladium complex out and reforming a bond between x and y
The metal is reduced from Pd(II) to Pd(0) in the process
(x-y because doesn’t always have to occur between a carbon-halogen)
What is the key feature of the substitents to allow reductive elimination to occur?
The substituents undergoing reductive elimination (X and Y) need to be cis to each other
What is the driving force of a reductive elimination?
- Thermodynamics (formation of stronger bonds)
- Reductive elimination is easy and fast when strong bonds are formed
- Two halogens or C-X bond would be too weak and a lot slower
What occurs in transmetallation and why is it important?
- Transmetallation transfers a carbon ligand from one metal (e.g. Li, Mg, Cu, Zn, B, Si, Sn) to another metal (e.g. Pd) - note some of these aren’t metals like silicon or boron
- This step is important, because it allows the transfer of different groups onto palladium which can the be coupled together in the reductive elimination step
What is the role of the base in the Suzuki reaction?
The base acts as an activator, increasing the nucleophilicity by generating a negatively charged ‘ate’ complex, and speeds up transmetallation:
Why can Boronic acid alone not be used in a transmetallation within a Suzuki reaction?
Draw a mechanism for this
- Boronic acids are not very nucleophilic (C-B bond not very polarised) meaning the transmetallation is slow
- Other activators can be added in, e.g. CsF (F- has a very high affinity for B)
- The activators will drive electron density into B empty P orbital which will give a complex which is negatively charged
Name some standard reaction conditions for the Suzuki reaction
If the boronic acid/ester coupling partner for a Suzuki reaction is not available, a different transmetallating reagent can be used:
What can replace X
(the benefit of the bottom three are that they are very reactive meaning they do not require an activator)
How can we make bornic acids from transmetallation from organolithium
By reacting it with a trimethoxyboron group
OMe are leaving groups
How can we form a boronic ester from hydroboration of alkynes and alkenes
Alkyne displaces the hydrogen on boron
How can a Boronic ester be formed from Pd cross-coupling
