ORGANOMETALLIC CHEMISTRY: MAGNESIUM & ZINC

Question 1

Compare C-Mg to C-Li

Answer 1

Higher electronegativity of Mg means more covalent bond, less polarisation, less reactive, less aggregation

Question 2

What is the structural formula of a Grignard reagent

Answer 2

[RMgX.(L)k]n

Question 3

What the effecting factors of Grignard formation

Answer 3

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

Question 4

What are the 3 activating mechanism of Mg

Answer 4

Riekes magnesium

Magnesium anthracene

Mg vapour

Question 5

Explain Riekes magnesium activation

Answer 5

Lithium or potassium used to reduce MgCl2 to Mg

Question 6

Explain Magnesium anthracene activation

Answer 6

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

Question 7

Explain Mg vapour activation

Answer 7

Mg vapour co condenses with solvent and RX. Similar to MVS

Question 8

Explain the radical mechanism of Grignard reagent formation

Answer 8

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

Question 9

How can you tell if a reaction includes radicals

Answer 9

Add tetramethylpiperidine oxide (TMPO) which is a radical scavenger. If a stable product is formed then reaction includes radicals.

Question 10

Why do Mg and Zn exist in Schlenk equilibrium and Li does not

Answer 10

Because of the 2+ state of Mg and Zn and because the reaction occurs in solution

Question 11

How can we produce di-organomagnesium complexes

Answer 11

By Schlenk equilibrium and precipitating out the MgX2 salt.

By transmetallation with a less electropositive metal like R2Hg

Question 12

How does aggregation of Mg complexes occur

Answer 12

It occurs through Mg-halide bonds instead of C-Mg due to the increased stability meaning there isn’t a need for further aggregation

Question 13

Explain ligand scrambling in aggregates

Answer 13

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

Question 14

What does the position of Schlenk equilibrium depend on

Answer 14

Temperature

Solvent (THF vs Et2O): THF strong donor so monomers dominate. Et2O weaker so makes dimers

Question 15

Compare C-Zn to C-Mg and C-Li

Answer 15

Most covalent character of all

Bonding is more directional, controlled by orbitals and more energetically stable

Question 16

Why can Zn complexes contain another functional group

Answer 16

Because C-Zn is not as reactive as C-Mg and C-Li which would attach to the functional group easily.

Question 17

What are the 3 ways to synthesise organozinc complexes

Answer 17

1. Oxidative zincation
2. Halogen - Zn exchange
3. Transmetallation

Question 18

Explain oxidative zincation

Answer 18

Zn inserts into an RX bond. Firsty the Zn needs to be activated from Zn dust or ZnCl2 (riekes Zn)

Question 19

Explain halogen - Zn exchange

Answer 19

Halogen substituent is replaced by Zn. Can be used to make dialkylzinc compounds

Question 20

Explain transmetallation for Zn complex formation

Answer 20

Salt elimination and metathesis.

Eliminate salt by precipitation

Question 21

Outline the structure of Zn complexe

Answer 21

4 coordinate tetrahedral complexes

Question 22

What are the 2 main reactions of organic synthesis Zn complexes are involved in

Answer 22

Simmons-Smith

Reformatsky

Question 23

Explain the Simmons-Smith reagent reaction

Answer 23

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

Question 24

What is the reformatsky reaction

Answer 24

Produces beta-hydroxyester by condensing aldehydes with a-haloesters

Low reactivity of Zn complex prevents second nucleophilic addition to the ester group

Question 25

Explain the key steps of the Reformatsky reaction

Answer 25

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.