Page 1054 - Advanced Organic Chemistry Part B - Reactions & Synthesis

1030 + .
–
initiation ArN N + e Ar + N 2
CHAPTER 11 propagation Ar . + HCN(CH ) Ar H + . CN(CH )
3 2
3 2
Aromatic Substitution O O
Reactions
+ . +
.
ArN N + CN(CH ) Ar + N + C O + CH N H CH 2
2
3
3 2
O
This reaction can be catalyzed by FeSO . 97
4
11.2.1.2. Phenols from Diazonium Ion Intermediates. Aryl diazonium ions can be
converted to phenols by heating in water. Under these conditions, there is probably
formation of a phenyl cation.
+ H O
2
+
ArN N Ar + N 2 ArOH + H +
By-products from capture of nucleophilic anions may be observed. 53 Phenols can be
formed under milder conditions by an alternative redox mechanism. 98 The reaction
is initiated by cuprous oxide, which effects reduction and decomposition to an aryl
radical, and is run in the presence of Cu(II) salts. The radical is captured by Cu(II)
and converted to the phenol by reductive elimination. This procedure is very rapid and
gives good yields of phenols over a range of structural types.

+
ArN N + Cu(I) Ar. + N + Cu(II)
2
H O
. 2
III 2+
Ar + Cu(II) [Ar Cu ] ArOH + Cu(I) + H +
11.2.1.3. Aryl Halides from Diazonium Ion Intermediates. Replacement of diazonium
groups by halides is a valuable alternative to direct halogenation for the preparation of
aryl halides. Aryl bromides and chlorides are usually prepared by a reaction using the
appropriate Cu(I) salt, which is known as the Sandmeyer reaction. Under the classic
conditions, the diazonium salt is added to a hot acidic solution of the cuprous halide. 99
The Sandmeyer reaction occurs by an oxidative addition reaction of the diazonium ion
with Cu(I) and halide transfer from a Cu(III) intermediate.

+
I
III
ArN N + [Cu X ] – Ar Cu X + N 2
2
2
Ar Cu X 2 ArX + Cu X
III
I
Good yields of chlorides have also been obtained for reaction of isolated diazonium
tetrafluoroborates with FeCl -FeCl mixtures. 100 It is also possible to convert anilines
3
2
to aryl halides by generating the diazonium ion in situ. Reaction of anilines with
alkyl nitrites and Cu(II) halides in acetonitrile gives good yields of aryl chlorides and
bromides. 101
97 F. W. Wassmundt and W. F. Kiesman, J. Org. Chem., 60, 1713 (1995).
98
T. Cohen, A. G. Dietz, Jr., and J. R. Miser, J. Org. Chem., 42, 2053 (1977).
99
W. A. Cowdrey and D. S. Davies, Q. Rev. Chem. Soc., 6, 358 (1952); H. H. Hodgson, Chem. Rev., 40,
251 (1947).
100 K. Daasbjerg and H. Lund, Acta Chem. Scand., 46, 157 (1992).
101
M. P. Doyle, B. Sigfried, and J. F. Dellaria, Jr., J. Org. Chem., 42, 2426 (1977).