Chemistry Reference
In-Depth Information
+
(Br
3
)
-
(BrCl )
-
N
hexyl
,
N
n
(a)
CH
2
NMe
3
X
3
-
CH
2
NMe
3
X
-
(b)
Fig.
8
.42
Halogenations with
polyhalide-exchanged resins.
cyclohexene
2-pentene
styrene
stilbene
chalcone
CO
3
H
epoxides
or
SO
4
H
Fig.
8
.43
Epoxides from peracids.
polyvinylpyridine [130] (Fig. 8.42a); here the bromi-
nation occurs exclusively on the side chain of the
same alkylaromatics.
A similar poly(pyridinium hydrobromide-perbro-
mide) performs the selective bromination of methyl-
arylketones to bromomethyl derivatives; in this
polymer, the pyridine rings are grafted onto a poly-
methylmethacrylate polymer by the lithium salt of
a-picoline [131]; the hydrobromide is formed and
bromine is added to give the tribromide reagent.
Such trihalides can be obtained also from halide-
exchanged resins to which chlorine or bromine is
added (Fig. 8.42b) [132-134]; halogenations occur
regioselectively in the
para
-position of substituted
benzenes.
Similar polyhalides (dichloroiodide, tetrachloroio-
dide and dibromoiodide) [135] afford the iodination
of activated benzene and naphthalene derivatives
such as amines and phenols; the selectivity in
the monoiodination of the aromatic ring can be
monitored.
3.9 Carbodiimides
Dehydration of ureas and thioureas into carbodi-
imides [136] can be performed by the supported
triphenylphosphine-Br
2
reagent shown in Fig. 8.17.
3.10 Epoxides
Epoxidation of alkenes is performed with supported
percarboxylic and persulfonic acids; these are
obtained by treating polyvinylbenzoic acids or poly-
styrene sulfonic acids with hydrogen peroxide in the
presence of mineral acids [137-139] (Fig. 8.43).
The reduced acidic forms can be reoxidised but
the carboxylic reagent exhibits decreasing capacity
after repeated recyclings: some irreversible degrada-
tion occurs on the polystyrene backbone. Epoxides
can be obtained also from sulfonium ylides: a
polystyrene-supported sulfide is quaternised, trans-
formed into the ylide with a base and then reacted
with a carbonyl compound [140] (Fig. 8.44).
