Organometallic
Compounds: -
Compounds that contain
a metal-carbon bond, R-M, are known as Organometallic compounds. Organometallic
compounds of Li, Mg (Grignard reagents) are the most important organic
reagents. other metals have been utilized, for example Cu, B,
Na, and Zn.
Importance
of Organometallic compounds It provide a source of
nucleophilic carbon atoms which can react with electrophilic carbon to form a
new carbon-carbon bond. This is very
important for the synthesis of complex molecules from simple starting
materials. R-M = R-M+
The most important reactions is this chapter are the reactions of
Grignard reagents, RMgX, with the carbonyl groups in aldehydes, ketones and
esters to give alcohols. However, we will also look at some useful reactions
involving Cu, B.
Nomenclature:
Organometallic
compounds are normally named as substituted metals, e.g. alkyl metal or alkyl
metal halide. Organomagnesium compounds are generally referred to as Grignard
reagents. Examples: CH3Li = methyl lithium,
CH3MgBr = methyl magnesium bromide.
Physical
Properties: Organometallic are usually kept in
solution in organic solvents due to their very high reactivity (especially with
H2O, O2 etc.)
Structure:
Organosodium
and organopotassium compounds are essentially ionic compounds. Organolithiums and organomagnesiums have a
σ bond between a C atom and the metal:
C-M These are very polar,
covalent bonds due to the electropositive character of the metals.
Basicity:
Organolithium
and organomagnesium compounds are strong bases since the negative charge is on
carbon. Simple carbanions are strong bases, since the C is not very
electronegative (compared to N or O) In the presence of weak acids, RLi and
RMgX protonate giving the hydrocarbon.
Preparations
and Reaction of Organometallic Reagents
I.
Preparation of Organomagnesium Reagents
Organomagnesiums are formed by the reaction
of alkyl halides with magnesium metal.
Typical the reaction is carried in anhydrous diethyl ether or
tetrahydrofuran. The alkyl group can be primary, secondary or tertiary. Halide
reactivity: I > Br > Cl. R can be alkyl, vinyl or aryl. The reaction is conducted by adding the organic halide to a
suspension of magnesium in an etherial solvent, which provides ligands required to stabilize the organomagnesium compound.
Reactions
of Organomagnesium Reagents
Reaction
with Aldehydes and Ketones
Organomagnesium or
Grignard reagents react with the carbonyl group, C=O, in aldehydes or ketones
to give alcohols. It react with formaldehyde gives primary alcohol, with other
aldehydes gives secondary alcohols and ketones gives tertiary alcohols. Reaction
usually carried in ether Et2O or THF followed by acid hydrolysis. Reaction
type: Nucleophilic Addition.
