ORGANOMETALLIC CHEMISTRY: LITHIUM Flashcards
When an electron donor/ Lewis base is added to a highly aggregated molecule what will happen
Deaggregation. This is due to the competition of other ligands wanting to bind to the metal centre
What is the benefit of using unsaturated lewis bases/donors
Allows for the donating atom to stick out from the bulky part of an organic fragment, less steric hinderance, better reaction
Explain the hierarchy of halogens in Li exchange
Fluorine does not participate in Li exchange as C-F bonds are very strong. Tend cause metalation. Weaker bonds like C-I tend to cause exchange
I > Br > H > Cl/F
What are the 4 methods of producing organolithium complexes
1: Directly from the metal
2: Lithiation (metallation), Li-H exchange
3: Lithium heteroatom exchange
4: Transmetallation
Explain organolithium formation directly from the metal
2Li(m) + RX -> RLi + LiX (salt)
EFFECTING FACTORS:
- Nature of halide: weaker C bond (faster), Too weak Wurtz coupling can occur
- Atmosphere: Reactive with air and moisture, must be inert
- Purity of metal: 1-2% of Na can be beneficial to ensure organolithium is formed
- Solvent: Hyrdocarbon or arene solvents are used to allow organolithium to form. Protic solvents avoided.
What is the reaction of Wurtz coupling
2Li(m) + 2RI -> R-R + 2LiI
What are the procedures used for synthesising organolithiums directly from the metal
Ultrasound: Equipment that activates metal by oxide layer breakdown and creates lattice dislocations for increased reactivity
Metal vapour synthesis: apparatus heats up metal destroying lattice (reactivity). Organic vapour funnelled in. Product condenses on N2 surface.
Explain organolithium formation by lithiation (metallation, Li-H exchange)
RLi + R’H ––> R’Li + RH (equilibrium)
Most acidic H is replaced
EFFECTING FACTORS
- Solvent: Donor solvents accelerate rate because deaggregation increases reactivity and bonding to Li makes carbon more anionic
- Special cases: activation by a-heteroatom like ortho directed lithiation
Explain Li formation by lithium heteroatom exchange
RLi + R’X ––> R’Li + RX
EFFECTING FACTORS
- Alkylation side reaction: Wurtz coupling (use C-Br)
- Metallation: H so acidic, easier to replace them instead
- 1,2 addition: RLi reacts over double bond instead of only Li attaching
What are the advantages and disadvantages of Lithium heteroatom exchange method of formation
Regiospecific: Position where Li will react is governed by halide.
Low temperature needed: So that LiX can be eliminated to give reactive intermediates.
Explain organolithium formation by transmetallation
RnM + Li ––> n RLi + M
Exchange of lithium will less electropositive metal.
Explain the general formula of an alkali metal
[RM.(L)k]n
R = organic group M = metal L = donor ligands k = no. of ligands n = degree of association
How are Li oligomers formed
Simple organic ligands like Me cannot stabilise a negative charge well. So aggregation occurs so that they can.
What can we learn from the bond lengths of solid state structures
If the bonds are unequal over a range then electrons are not evenly distributed and the bonding is more ionic/electrostatic.
What denticity cause what aggregation states
Monodentate donors: oligomers to tetramers
Bidentate: dimers
Tridentate: monomers
What factors determine the most reactive aggregate of a complex in solution
Charge density of metal: higher charge density, better Lewis acid, stronger M-L bonds. Less reactive
Bulk of organic moiety: Bulkier L = lower aggregation
Denticity of donors
size/radius: Bigger means more ligands can fit around M
What is the structural formula of lithium amides
[(R2N)M(L)k]n
What characteristics allow N-Li to be better than C-Li
being air and moisture sensitive
Strong base
Very weak nucleophile: Deprotonation favoured over addition
What is an open dimer
The most reactive structure of a lithium amide because 2 coordinate Li is unsaturated (only connected to 2 NR2) and therefore highly reactive. (3 coordinate Li connected to 1 NR2 and 2 L)
