Handbook of Functionalized Organometallics Applications in S

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4 Polyfunctional Magnesium Organometallics for Organic Synthesis
Where possible we will try to point out the advantages of Grignard reagents and compare
them to other organometallics. A separate section will deal with recent developments
in transition-metal-catalyzed reactions, where Grignard reagents again occupy
a central position, especially in the interesting field of iron-catalyzed reactions.
4.2
Methods of Preparation of Grignard Reagents and their Uncatalyzed Reactions
4.2.1
Direct Oxidative Addition of Magnesium to Organic Halides
Grignard reagents are sensitive to air and moisture. An inert atmosphere is therefore
always advantageous for their preparation and further reactions. The usual
method used for the preparation of organomagnesium reagents is the reaction of
organic halides with magnesium metal in a polar, aprotic solvent like THF or
diethyl ether (Scheme 4.1, Eq. 1). For large-scale industrial process, [14] these volatile
and highly flammable ethers represent safety hazards and can be substituted
by ªbutyl diglymeº (C 4H 9OC 2H 4OC 4H 9) that possesses a high flashpoint (118 C)
and a low water solubility.
RX
2RMgX
Mg
THF or Et 2O
RMgX
R 2Mg
+
MgX 2
Scheme 4.1 Synthesis of Grignard reagents by oxidative
addition (Eq. 1) and Schlenk equilibrium (Eq. 2).
Magnesium turnings or powder are usually covered with a small amount of solvent
and to this suspension a solution of the organic halide is added. This reaction
is exothermic and cooling is often necessary after the induction period. The magnesium
metal is, as received or after exposure to air, covered with an ªoxideº layer
(mainly Mg(OH) 2), [11e] that passivates the metal. This ªoxideº layer is responsible
for the induction period that is normally observed in the synthesis of Grignard
reagents. An activation with the promoter 1,2-dibromoethane can help to reduce
this induction time. 1,2-Dibromoethane reacts with magnesium to ethene and
MgBr 2. It also accelerates the formation of the Grignard reagent, leading to an
activated magnesium metal surface [15]. The mechanism of this reaction is not
yet fully clarified, but a radical mechanism is generally accepted [11,16].
In solution, a Grignard reagent (RMgX) is in equilibrium (so-called Schlenk
equilibrium, Scheme 4.1, Eq. 2) with R 2Mg and MgX 2, depending on temperature,
solvent and the anion X. This equilibrium can be shifted to the right side, by pouring
a solution of RMgX into dioxane, which does not dissolve MgX 2, leaving R 2Mg
in solution. Most Grignard reagents (RMgX) or diorganomagnesium compounds
(1)
(2)