Introduction
Direct
reactions
Exchange
reactions
Insertion/addition
reactions
Reduction
reactions
Displacement
reactions
To present a systematic account of methods for the synthesis of organometallic compounds, giving some idea of their enormous range and diversity, is difficult. No system is entirely satisfactory, but the one adopted here is to divide the methods into five major categories.
- Direct reactions
- Exchange reactions
- Insertion/addition reactions
- Reduction reactions
- Displacement reactions
It will become clear that many methods are applicable to main groups as well as transition metals. Many organometallics are air and/or moisture sensitive. To prepare these organometallics, you need anhydrous conditions and inert atmospheres. Reactions are usually done in organic solvents. Many organometallics are toxic. top
General reaction
M is a reactive metal, eg: group I, group II and some of groups IIIand IV.
Examples
NB:
In this reaction group I gives alkylmetals RnM, group II to IV give alkylmetal halides RxMXy.
Effect of MX formation with group I metals provides large
Hf
0and drives reaction over to completain.The reactivity sequence is I > Br > Cl > F. It is nevertheless clear that the energy inputs needed to break down the metal lattice, ionise the metal and break the RX bond must be set against the energy released on formation of the RM and MX bonds. Since RM bonds are generally weak, we might expect that the formation of the MX bond would be a major influence in promoting the reaction. With this reasoning we would predict that chlorides would give best results since lattice energies are in the order MF > MCl > MBr > MI. As we have already seen, the actual reactivity order is RI > RBr > RCl > RF, indicating that kinetic rather than thermodynamic factors are dominant and that the rate of reaction is being controlled by energy required to break the C-X bond.
Nevertheless, influences which are likely to increase the negative free energy of the reaction must be considered favourable.
Thus, the use of alloys of sodium or magnesium with the primary metal the resulting formation of the relatively stable sodium or magnesium halides as well as the product often gives improved yields.
The synthesis of tetraethyllead is the basis of one of the commercial methods for preparation of the anti-knock tetraethyllead, Et4Pb. (Best results are obtained by using a 1:1 alloy and recycling the unreacted lead).
Use of metals in the vapour state not only removes the thermodynamically unfavourable metal lattice energy term, but also makes the metal kinetically more reactive. The technique consists of heating the metal, usually at temperatures of 1600oC in a high vacuum (10-7 -10-8 atm). The resulting metal vapour, and the other gaseous reactants, are condensed on a cooled surface where reaction occurs.
Activation of the metal can also be achieved by the use of ultrasound or microwave radiation.
eg: Using sonic radiation (20-50 kHz) organozinc reagents can be prepared by simply imadiating haloalkanes, lithium and zinc bromide for 10 minutes in mixture of toluene and tetrahydrofuran. Normally these reactions would take hours. top
There are three types of exchange reactions
Metallation
Metal-Metal (Transmetallation)
Metal-Halogen (Metathesis)
Metal-Hydrogen (Mercurations/Thallations)
Metallation
General reaction
For this reaction to proceed the acidity of R'H should exceed that of RH.This method is used mainly (but not exclusively) for the preperation of lithium compounds and, since the alkanes have very low acidities, lithium alkyls are normally used. The arenes with higher acidities are appropriate substrates and the method is particulary valuable for the preperation of aryllithium compounds.
Examples
NB: Y = MeO, Me2N, CONMe2, SO2Me, etc.
Metal-Metal (Transmetallation)
General reaction
Exchange is controlled by relative stabilities (
0) of RM' and RM. If RM is more stable than RM', the reaction will occur. So RM' is typically endothermic/weakly. Exothermic examples of weakly exothermic organometallics are the alkyls of Hg, Tl, Pb and Bi.Example
NB:
Whilst the direct reaction gave a alkylmagnesiumhalide product, metal-metal exchange gives a dialkylmagnesium compound.
Metal-Halogen (Metathesis)
This is the most widely used method of making metal-carbonbonds.
General reaction
Examples
NB: Li is more reactive than Mg. Li gives cleaner seperations.
NB: MXn can be a simple (WCl6) or a complex halide as here.
NB: One of several methods of making metalloceres eg. ferrocene
Metal-Hydrogen (Mercurations/Thallations)
General reaction
Examples
NB: Thiophene is a heteroaromatic species. It behaves as a 6 electron aromatic system.
More than MOAc group can be added.
General reaction
Insertion into M-H bond (Hydrometallation)
Examples
NB: Trialkylaluminium compounds form part of the Zeigler Natta catalyst system.
X=heteroatom, C etc.
Examples
NB: Note - stereospecific control of reaction. Very useful in pharmaceutical synthesis.
NB: Diazomethane is highly explosive. It is handled only at low temperatures and in special glasware. top
General reaction
Examples
General
Examples
- usually CO is displaced
- to aid the reaction use UV or warm gently (~80oC)
The numbers throughout the picture are pointing to the number of electrons according to the Eighteen Electron Rule (see introduction).
After you have studied the previous theory, you can practice some problems. top
