Chemistry Reference
In-Depth Information
CH
2
-NMe
3
+
F
-
n-octyl halides
chloroacetophenone
3-mesyloxysteroids
n-octyl fluoride
bromoacetophenone
3-fluorosteroids
Fig.
8
.52
Fluorination with fluoride-
exchanged resins.
N
HF
NH
+
HF]
-
[FH
F
cyclohexene
norbornene
1-methyl cyclohexene
3-hexyne
triphenylmethanol
cycloheptanol
cyclohexyl fluoride
2-norbornyl fluoride
1-methyl-2-fluorocyclohexane
3,3-difluorohexane
triphenylmethyl fluoride
cycloheptyl fluoride
Fig.
8
.53
Fluorination with
fluoroalkylamine.
3.14 Halides and dehalogenation reactions
poly(vinylpyridinium) bromotrichloride, obtained
by the addition of chlorine to a quaternised
polyvinylpyridine polymer [164] (Fig. 8.56a). These
ionic species are more selective than the neutral
ones, obtained by the direct addition of chlorine or
bromine to polyvinylpyridine [165] (Fig. 8.56b).
The selective a-monohalogenation of aliphatic
and aromatic ketones is performed in satisfactory
yield with iododichloride- and iodotetrachloride-
exchanged resins [166,167]; the tetrachloroiodate
has a higher reactivity than the dichloroiodate
(Fig. 8.57).
On the contrary, the removal of a halogen atom
by hydrogenolysis is a reaction that appears essen-
tially at the end of a synthetic sequence. Tin and
germanium hydrides are the most familiar reagents
and they react readily through a radical mechanism.
New polystyrene-supported organotin hydrides
are prepared by reduction with borohydrides or ditin
or tin halide compounds; hydrogenolysis is reported
for alkyl and benzylbromo derivatives [168-171]
(Fig. 8.58).
There are many methods of introducing halogens at
non-aromatic carbons and the choice between elec-
trophilic or nucleophilic reagents depends on the
nature of the molecule. From alkyl or benzyl halides
the halogen exchange is performed readily through
Finkelstein reactions with anion-exchange resins in
the required halide form [151,160]. The formation
of chlorides from primary alcohols is performed
in CCl
4
with polystyrene-supported triphenylphos-
phine [161,162] but secondary alcohols give a sub-
stantial amount of alkenes by elimination. With this
reagent it has been shown that the active species
is a triphenylphosphine dichloride, which can be
obtained directly with triphosgene (Fig. 8.55); high
yields in benzyl chlorides are obtained at room
temperature from the corresponding benzyl alcohols
[163]; the triphenylphosphine groups in the spent
reagent are regenerated with triphosgene.
The addition of halogens to activated alkenes such
as diarylethylene is performed with quaternary
