_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Arene( tricarbonyl )chromium complexes
ali undergo Friedel-Crafts acylation. Attempts to acetylate (C 6 H 6 )Cr(CO), under
normal conditions (excess AICI,. CH 2 Cl 2 , OuC) lead to extensive decomposition.
The Perrier method, however, which employs preformed CH,co+ .AICI; and
hence avoids excess aluminium(IIl) chloride, is successful. Benzenetricarbonylchromium
is deactivated to electrophilic substitution compared with benzene
itself. Evidence from a variety of experiments. for example comparison of the pK"
values of benzoic acid (5.48 in 50% aqueous ethanol) and its Cr(CO), complex
(4.52) suggests that the Cr(CO), group is about as electron-withdrawing as a p­
nitro substituent. Some consequences ofthis property which have applications in
organic synthesis are discussed below.
10.2.1 Nucleophilic attack on arenetricarbonylchromium complexes
While arenetricarbonylchromium complexes are deactivated to electrophilic
substitution with respect to the free arene, their susceptibility to nucleophilic
substitution is enhanced. The chlorobenzene complex, for example, undergoes
ready substitution by sodium methoxide. In this way it resembles 1-chloro-4-
nitrobenzene.
VOM•
Cr
(C0) 3
-
Carbanions such as CH(C0 2 Et) 2 also react; in the early stages ofthe reaction ring
adducts ortho or meta to the chloro substituent are observed, but these are
gradually replaced by the substitution product. The reactivity of Cr(PhX)(CO),
towards nucleophiles decreases in the order X = F > Cl > Br > I.
These results led Semmelhack and his colleagues to study the addition of
carbanions to benzenetricarbonylchromium itself. Carbanions derived from
species with pK" ~ 2 5 in tetrahydrofuran form cyclohexadienyl anions which are
stable below 0°C in the absence of air. Oxidation with iodine affords substituted
benzenes. The sequence
ArH ------+ (ArH)Cr(CO),
COMPLEX
FORMATION
R
------> [ (ArHR)Cr(CO),r ------> ArR
ADDITION OF
NUCLEOPHILE
12
REMOVAL OF
METAL
provides a mild procedure for the nucleophilic substitution of an arene, a reaction
which is normally not possible on the free ligand.
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