ORGANOMETALLIC CHEMISTRY: MAGNESIUM & ZINC Flashcards
Compare C-Mg to C-Li
Higher electronegativity of Mg means more covalent bond, less polarisation, less reactive, less aggregation
What is the structural formula of a Grignard reagent
[RMgX.(L)k]n
What the effecting factors of Grignard formation
State of metal: Mg can be covered in oxide film, needs to be activated
Solvent: Cannot use protic solvents
Temperature: Higher temp than Li due to stability of C-Mg. Usually 0C
Wurtz coupling: If temp too high or C-I is used
What are the 3 activating mechanism of Mg
Riekes magnesium
Magnesium anthracene
Mg vapour
Explain Riekes magnesium activation
Lithium or potassium used to reduce MgCl2 to Mg
Explain Magnesium anthracene activation
Mg provides 2e- to anthracene in THF. Stable orange powder forms. More toluene added the THF adduct becomes unstable and decomposes. Mg2+ ion is reduced to activated Mg powder
Explain Mg vapour activation
Mg vapour co condenses with solvent and RX. Similar to MVS
Explain the radical mechanism of Grignard reagent formation
Single electron transfer (SET)
RX run over Mg metal surface.
SET from Mg to RX forming Mg+ and RX- (splits to R and X-)
Next SET from Mg to R forming Mg2+ and R-
Mg2+ and X - make MgX+
MgX+ and R- make RMgX
How can you tell if a reaction includes radicals
Add tetramethylpiperidine oxide (TMPO) which is a radical scavenger. If a stable product is formed then reaction includes radicals.
Why do Mg and Zn exist in Schlenk equilibrium and Li does not
Because of the 2+ state of Mg and Zn and because the reaction occurs in solution
How can we produce di-organomagnesium complexes
By Schlenk equilibrium and precipitating out the MgX2 salt.
By transmetallation with a less electropositive metal like R2Hg
How does aggregation of Mg complexes occur
It occurs through Mg-halide bonds instead of C-Mg due to the increased stability meaning there isn’t a need for further aggregation
Explain ligand scrambling in aggregates
An initial di-magnesium complex bridges by halogens that undergoes Schlenk equilibrium may rearrange to become R2Mg and MgX2 and the 2 L and add as another aggregate
What does the position of Schlenk equilibrium depend on
Temperature
Solvent (THF vs Et2O): THF strong donor so monomers dominate. Et2O weaker so makes dimers
Compare C-Zn to C-Mg and C-Li
Most covalent character of all
Bonding is more directional, controlled by orbitals and more energetically stable
Why can Zn complexes contain another functional group
Because C-Zn is not as reactive as C-Mg and C-Li which would attach to the functional group easily.
What are the 3 ways to synthesise organozinc complexes
- Oxidative zincation
- Halogen - Zn exchange
- Transmetallation
Explain oxidative zincation
Zn inserts into an RX bond. Firsty the Zn needs to be activated from Zn dust or ZnCl2 (riekes Zn)
Explain halogen - Zn exchange
Halogen substituent is replaced by Zn. Can be used to make dialkylzinc compounds
Explain transmetallation for Zn complex formation
Salt elimination and metathesis.
Eliminate salt by precipitation
Outline the structure of Zn complexe
4 coordinate tetrahedral complexes
What are the 2 main reactions of organic synthesis Zn complexes are involved in
Simmons-Smith
Reformatsky
Explain the Simmons-Smith reagent reaction
Produces an initial carbenoid which can be made a carbene by eliminating the metal with the halogen
The carbene can then be added to a C=C to add an extra carbon to the molecule by cyclopropanation
What is the reformatsky reaction
Produces beta-hydroxyester by condensing aldehydes with a-haloesters
Low reactivity of Zn complex prevents second nucleophilic addition to the ester group
Explain the key steps of the Reformatsky reaction
Oxidative addition of Zn into C-Br bond (forming an enolate)
1,2 Addition of the C-ZN bond across carbonyl of the aldehyde
Hydrolysis to quench the newly formed Zn alkoxide
Formation of the beta-hydroxyester through eleimination of ZnBr(OH) salt.
