Chem 343 – Organic Reactions Chapter 8 Prepared by José Laboy, MS http://www.chem.wisc.edu/areas/clc (Resource page) Organometallics #1: Organometallic Compounds Organolithium Compounds
Br
Li
Li (2 equivalents)
+ LiBr
Organomagnesium Compounds (Grignard Reagents)
Br
Mg
MgBr
diethyl
ether
Lithium dialkyl cuprates (Gilman Reagents)
Li
CuI
Li
Cu
2
+ LiI
diethyl ether
Lithium dialkyl cyano cuprates
-2
CN
Li
CuCN
2
Cu
2 Li2+
diethyl ether
These organometallic reagents are used to make carbon-­‐carbon bonds, that is, lengthening hydrocarbon chains. Organolithium and Grignard reagents are strong bases and strong nucleophiles. Therefore the reacting reagent with organometallics should not have acidic groups, i.e., -­‐OH, -­‐NH2, etc. Example: MgBr
HO
Cl
diethyl ether
BrMg+
O
Cl
+ C3H8 (g)
For these two reagents chains lengthening by SN2 reaction mechanisms are the choice (see below). A second step, acid workup, which is a solution of a diluted acidic compound, is usually required to neutralize any un-­‐protonated chemical species generated in the synthesis. Cl
MgBr
1)
/ diethyl ether
2) Acid workup
Mechanism
Cl
MgBr
+ MgBrCl S N2 mechanism
This is especially important when the reagent being added is an epoxide*. Notice that in the example shown below an alkoxide was generated when a Grignard reagent was used. To generate the respective alcohol the alkoxide is “washed” with a solution of dilute acid. +MgBr
O
OH
O
CH3MgBr / Et2O
acid workup
Gilman reagents, on the other hand, are weaker bases. They can be used with 2° alkyl halides. Br
(CH3)2CuLi
* See Reaction of Epoxides with Organometallics.