Chemistry Reference
In-Depth Information
O
OH
O
O
OH
OEt
Carbonate
PTC, 180
°
C
Cl
10% PEG 400
microwave, 2 mi n
-
NaCl,
-
H
2
O
2
Y
Y
Y
+ EtOH + NaOH
X
O
O
O
O
yield 82%
with 10% TEBA, yield 70%
X= F, Cl, Br, I, NO
2
Y= O, NH
PTC= phase-transfer catalyst
Scheme 10.4
Scheme 10.2
activated aryl halides catalysed by cdyltrimethylam-
monium bromide (CTAB) [50] and the synthesis
of thioether-substituted benzaldehydes by reaction
of halobenzaldehydes with metal mercaptides catal-
ysed by TBAB [51]. Thioether-substituted aromatic
ketones also were prepared in a similar way [52].
Aromatic nucleophilic cyanation proceeds under
regular PTC conditions only with activated aryl flu-
orides. Thus, 4-fluoro-3-nitrobenzotrifluoride was
converted to 2-nitro-4-trifluoromethylbenzonitrile
by NaCN in the presence of Bu
4
NHSO
4
[53]. Less-
reactive substrates such as 2-nitrochlorobenzenes or
2-nitrobromobenzenes still reacted with NaCN under
phase-transfer conditions but required the presence
of copper salts as co-catalysts [54]. Cyanation of non-
activated benzenes normally called for the presence
of a noble metal catalyst. For example, aryl cyanides
were prepared from aryl iodides under inverse PTC
(see Chapters 4 and 7) conditions in the presence of
ZnCl
2
and PdCl
2
[PPh
2
(
m
-C
6
H
4
SO
3
Na)] [55].
NO
2
OCH
3
NO
2
NaOCH
3
TBAB
Br
2
4%
KOH (S)
oleum
Br
97% yield
Br
55ºC
81% yield
Scheme 10.3
Although PTC substitution of active aromatic chlo-
rine is a standard method for the synthesis of 4- or
2-nitroethers (such as nitroanisoles or phenetoles)
[43], the preparation of 3-nitroanisole is not straight-
forward. Methoxydenitration of 3-bromonitroben-
zene has been used in a unique procedure for the
synthesis of 3-bromoanisole (Scheme 10.3) [44].
Thiomethoxydenitration was used for the prepa-
ration of thioether-substituted aromatic ketones
[45].
Jiang
et al
. [46] reported a remarkable aromatic
substitution reaction. They applied microwave irra-
diation in the substitution of 2-chlorophenol with
ethoxide anion in the presence of 10 mol.% PEG-
400 or triethylbenzylammonium chloride (TEBA)
catalysts to obtain high yields of the corresponding
phenol ether (Scheme 10.4). This reaction was never
reported beforehand.
Although strong bases such as hydroxides usually
are required for aromatic substitution, some reac-
tions using carbonates also were recorded. A typical
example is the N-arylation of pyrrolidino fullerene
(Scheme 10.5) [47].
Similar aliphatic substitution of benzyl bromide
was achieved in 70% yield under microwave irradi-
ation [48].
Other recent citations include the sulfonation of
2,4-dinitrochlorobenzene with sodium sulfite catal-
ysed by PEG [49], the arylation of benzotriazole by
2.2 Phase-transfer catalysis in elimination
and isomerisation reactions
Phase-transfer catalysis dehydrohalogenation under
basic conditions remained the procedure of choice
for the syntheses of speciality olefins and acetylenes.
Some recent industrial examples are the prepara-
tion of
meta
-substituted styrenes via elimination of
a-bromoethylbenzenes [56], the conversion of a-
olefins to terminal acetylenes [57] and the produc-
tion of vinylidene chloride via the continuous PTC
elimination of 1,1,2-trichloroethane [58]. Other
examples include the production of fluorinated
unsaturated hydrocarbons by dehydrofluorination of
saturated fluorohydrocarcons containing —CHF—
CHF— in the presence of potassium carbonate [59]
and the surface modification of poly(vinyledene flu-
oride) by alkaline PTC treatment [60]. Synthesis of
ortho
-difluorobenzene derivatives was by the dehy-
