Page 79 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

2.5 ORGANOLITHIUM AND ORGANOMAGNESIUM COMPOUNDS 59
There is good evidence for the existence of radical intermediates in this process.
Treatment of the Mg/RX reaction mixture with the stable organic radical 2,2,6,6-
tetramethylpiperidinyl-1-oxy (TEMPO, see structure below) results in high yields of a
TEMPO-R adduct, strongly indicating the intermediacy of alkyl radicals:

R O N O N
(2.30)
R

TEMPO

A reagent such as TEMPO that specifically reacts with radicals is called a radical trap;
such compounds are clearly very useful as mechanistic probes.
The organomagnesium compounds described above are known as Grignard reagents,
after the French chemist François Auguste Victor Grignard (1871–1935; Nobel Prize win-
ner in chemistry in 1912), who correctly identified them as organometallic compounds and
recognized their importance to organic chemistry. Although the Grignard reagents are com-
monly written as RMgX, there is strong evidence for the following equilibrium in ether, the
solvent in which Grignard reagents are typically generated:

X
R Mg X R Mg Mg R MgR + MgX 2 (2.31)
2
X
A variety of oligomeric structures are also known for organolithiums.
Organolithium and organomagnesium reagents represent a reasonable approximation to
carbanions; indeed, they are frequently referred to as such. As highly basic compounds,
they are moisture sensitive (as well as air-sensitive) and need to be handled accordingly.
The hydrolysis of n-butyllithium may be depicted as follows:

O
H H
+ − (2.32)
+ Li OH
Li

n-Butyllithium and other organolithiums are most commonly employed as strong bases,
for example, to deprotonate carbonyl compounds and thereby generate enolate anions, as
mentioned in Section 1.15. Grignard reagents, on the other hand, are most often employed
as nucleophiles toward carbonyl compounds, resulting in, after aqueous work-up (not
shown explicitly below), alcohols with carbon frameworks larger than those of the starting
materials:
R 2 R 1
Ether
R 1 Mg Br + C O C OH (2.33)
− 78 °C 2
R 3 R R 3