Traducción acerca de las principales reacciones de los compuestos organometálicos

1. 1
Organometallic Compounds
Summary
Compounds that contain a metal-carbon bond, R-M, are known as quot;organometallicquot;
compounds.
Organometallic compounds of Li, Mg (Grignard reagents) are amongst some of the most
important organic reagents.
Many other metals have been utilised, for example Na, Cu and Zn.
Organometallic compounds 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.
To rationalise the general reactivity of organometallics it is convenient to view them as
ionic, so R-M = R-M+
The most important reactions is this chapter are the reactions of organolithiums, RLi, and
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, Zn and Hg
(mercury).
Preparation of Organomagnesium Reagents
Reaction type: oxidation - reduction
Summary
 Organomagesiums are formed by the reaction of alkyl halides with magnesium
metal.
 Typical solvents are normally 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.

2. 2
Preparation of Organolithium Reagents
Reaction type: oxidation - reduction
Summary
 Organolithiums are formed by the reaction of alkyl halides with lithium metal.
 Typical solvents are normally anhydrous diethyl ether but pentane or hexane can
also be used.
 The alkyl group can be primary, secondary or tertiary.
 Halide reactivity : I > Br > Cl
 R can be alkyl, vinyl or aryl
 Other Group I metals (Na, K) can be used instead of Li.
Preparation of Organocopper Reagents
Summary
 The most useful organocopper reagents are lithium dialkylcuprates, R2CuLi.
 Lithium dialkylcuprates are formed by the reaction of 2 equivalents of an
organolithium with a copper (I) halide.
 Typical solvents are normally anhydrous diethyl ether or tetrahydrofuran.
 The alkyl group is usually primary. Secondary and tertiary are prone to
decomposition.
 Halide reactivity : I > Br > Cl
 R can be alkyl, vinyl or aryl.
Preparation of Organozinc Reagents
Reaction type: oxidation - reduction
Summary

3. 3
 Organozinc reagents, RZnX, are prepared in a fashion analogous to that of
organomagnesium reagents RMgX.
 They are much less reactive than either RLi or RMgX to aldehydes and ketones.
 The most common application of organozinc reagents is in the Simmons-Smith
reaction
Preparation of Acetylenic Reagents
Reaction type: acid-base
Summary
 In Ch9 we saw that terminal acetylenes can be deprotonated using sodium amide,
NaNH2
 Acetylenic Grignard reagents, RC≡CMgX, can also be prepared.
 Rather than starting from the acetylenic halides, they are prepared by an acid-base
reaction of the terminal acetylene with a second Grignard reagent.
 Acetylenic Grignards react in a similar fashion to other Grignard reagents.
Reactivity of Organometallics
As we have seen previously, the carbon attached to the metal is anionic in character, so it
reacts as a carbanion, a nucleophilic carbon.
In principle there are 3 important groups of reactions where nucleophiles attack
electrophilic C atoms.
For the organometallic reagents these types of reactions will result in the formation of
new C-C bonds.
Limitations will be discussed below.

4. 4
Organometallic
General Mechanism
Application
1. Nucleophilic Substitution
R2CuLi with alkyl
halides or
tosylates to give
alkanes
2. Nucleophilic Addition
RLi or RMgX with
aldehydes or
ketones to give 2o
or 3o alcohols
3. Nucleophilic Acyl Substitution
RLi or RMgX with
esters to give 3o
alcohols
Study Tip:
Note that the electrophilic C atoms can be recognised by realising that they are connected
to electronegative atoms (esp. halides or oxygen).
Limitations:
 Organolithium, RLi, and organomagnesium, RMgX, reagents are typically too basic
to be used in nucleophilic substitution reactions (1) with alkyl halides or tosylates
where they tend to cause elimination reactions or other side reactions.
 Organocuprates, R2CuLi, reagents are less reactive and do not react with
aldehydes, ketones or esters but can be reacted with alkyl halides or tosylates to
give alkanes without elimination.
 Nucleophilic acyl substitution (3) reactions of organolithium, RLi, and
organomagnesium, RMgX, reagents are most commonly seen with esters.