Electrophilic and Nucleophilic Substitution in Aromatic Systems
Me,C -£o=o
C 11^
CO+Me,C PhCMe,
A useful synthetic application of Friedel-Crafts acylation is the use
of cyclic anhydrides in a two-stage process to build a second ring on -
to an aromatic nucleus:
O
II
ii
o
C.H.
A1C1,
o
IlOH
H.SO.
O
HCOC1 is very unstable but formylation may be accomplished by
e
protonating carbon monoxide to yield H0=0, i.e. by use of CO,
HC1 and A1C1 3 (the^rattermann-Koch reaction):
C.H„+HC=0
Aid.
PhCHO+H®
DIAZO COUPLING
Another classical electrophilic aromatic substitution is diazo coupling,
in which the effective electrophile has been shown to be the diazonium
-cation:
4 ® 9
Ph-N^N — PhN=N
4J " " "
This is, however, a weak electrophile compared with species such as
®N0 2 and will normally only attack highly reactive aromatic com
pounds such as phenols and amines; it is thus without effect on the
otherwise highly reactive Ph-OMe. Introduction of electron-
withdrawing groups into the o- or/j-positions of the diazonium cation
Rearrangement of R does not take place as with alkylation, but if
it is highly branched loss of CO can occur leading ultimately to
alkylation rather than the expected acylation: