Page 396 - Mechanism and Theory in Organic Chemistry

Electrophilic Aromatic Substitution 383

Acylations are most often carried out with BF, or AlCl, and an acyl halide,
anhydride, ester, or a carboxylic acid (Equation 7.67).

0
I I
Apparently the attacking species is most often an acyl cation, R-C=O -
Y = halide, OCR, OR, or OH
+
R-cEo.~~~
The action of nitrous acid on aromatic amines produces aromatic diazonium
ions (Equation 7.68), which are weak electrophiles. Correlation of the rate of
+
ArNH, + HNOz + ArN-N OH- + H,O (7.68)
diazonium coupling, as Reaction 7.69 is called, with pH shows that the reactive
species must be the free diazonium ion rather than ArN20H.15,

Some metals, such as mercury and thallium, that form covalent carbon-
metal bonds react in electrophilic aromatic substitutions. Both ionic [e.g.,
Hg(C10,) ,] and covalent [e.g., Hg(0Ac) ,] mercuric compounds react; the
attacking species, depending on the reagent and on the reaction conditions, may
be Hg2 + , HgX + , or HgX2.15, The only reagent that has been found to give high
0
II
yields of arylthallium compounds is T1(OCCF3)3.155 The nature of the attacking
0 0
Il + I1
species has not been studied, but presumably it is Tl(OCCF,), or Tl(OCCF,),.
0
I1
The products, ArTl(OCCF,) ,, 'are useful in organic synthesis because the thallium
group can be introduced into a substituted aromatic ring highly regiospecifically
and can then be replaced by another group such as I or CN. An example is
shown in Scheme 3. Regiospecific introduction of aromatic substituents by direct
means is often difficult to carry out (see p. 39 1).

lS2 F. R. Jensen and G. Goldman, in Friedel-Crafts and Related Reactions, G. A. Olah, Ed., Vol. 3,
p. 1003.
lS3 R. Wistar and P. D. Bartlett, J. Amer. Chem. Soc., 63, 413 (1941).
'54 (a) A. J. Kresge, M. Dubeck, and H. C. Brown, J. Org. Chem., 32, 745 (1967) ; (b) C. Perrin and
F. H. Westheimer, J. Amer. Chem. Soc., 85, 2773 (1963).
lS5 (a) E. C. Taylor and A. McKillop, Accts. Chem. Res., 3, 338 (1970) ; (b) A. McKillop, J. D. Hunt,
M. J. Zelesko, J. S. Fowler, E. C. Taylor, G. McGillivray, and F. Kienzle, J. Amer. Chem. SOG., 93,
4841 (1971); (c) E. C. Taylor, F. Kienzle, R. L. Robey, A. McKillop, and J. D. Hunt, J. Amer. Chem.
Soc., 93, 4845 (1971).