Mechanism of Meerwein arylation of furan derivatives

Article abstract of DOI:10.1134/S1070428009090103

Catalytic arylation of furan-2-carbaldehyde with arenediazonium salts was studied. The yields of 5-arylfuran-2-carbaldehydes were found to depend on the solvent, catalyst, and anion in the arenediazonium salt. Redox catalysis and generation of aryl radicals are not sufficient conditions for the reaction to occur. The reaction is successful only under conditions ensuring formation of complex intermediates. A mechanism involving two Cu²⁺ ? Cu ⁺ catalytic series and generation of furan-2-carbaldehyde was proposed.

Full text of DOI:10.1134/S1070428009090103

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2009, Vol. 45, No. 9, pp. 1375–1381. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © N.D. Obushak, A.I. Lesyuk, Yu.I. Gorak, V.S. Matiichuk, 2009, published in Zhurnal Organicheskoi Khimii, 2009, Vol. 45, No. 9,
pp. 1388–1394.
Mechanism of Meerwein Arylation of Furan Derivatives
N. D. Obushak, A. I. Lesyuk, Yu. I. Gorak, and V. S. Matiichuk
Ivan Franko Lviv National University, ul. Kirilla i Mefodiya 6, Lviv, 79005 Ukraine
e-mail: obushak@in.lviv.ua
Received September 15, 2008
Abstract—Catalytic arylation of furan-2-carbaldehyde with arenediazonium salts was studied. The yields of
5
-arylfuran-2-carbaldehydes were found to depend on the solvent, catalyst, and anion in the arenediazonium
salt. Redox catalysis and generation of aryl radicals are not sufficient conditions for the reaction to occur. The
reaction is successful only under conditions ensuring formation of complex intermediates. A mechanism
2
+
+
involving two Cu ↔Cu catalytic series and generation of furan-2-carbaldehyde was proposed.
DOI: 10.1134/S1070428009090103
Interest in arylation of furan derivatives is largely
determined by prospects in searching for biologically
active substances among compounds of the furan
series. Many furan derivatives are used as medicines
furan derivatives follows a mechanism similar to one
of the above reactions. In the first case, the reaction is
redox catalytic, and it involves intermediate copper
complexes [14, 15], while in the second radical aryla-
tion occurs [7, 13]. The ESR and polarographic data
indicated generation of a radical adduct (arylfuryl radi-
cal) [16, 17], and radical mechanism of the process
was proposed.
[
1, 2]. Such medicines as Nitrafudan, Dantrolene,
Clodanolene, and Azimilide contain an arylfuran frag-
ment. Numerous arylfuran compounds exhibit versatile
biological activity [2–5]. A convenient method for the
synthesis of arylfurans is copper-catalyzed arylation of
furan derivatives with arenediazonium salts according
to Meerwein [4, 6–12]. These reactions were reported
mainly for 2-substitued furans which underwent aryla-
tion at the 5-position (Scheme 1).
We performed a series of experiments on the aryla-
tion of furfurol with a view to elucidate in detail
mechanism of this reaction. The following modes of
generation of aryl radicals were tried: decomposition
of N-nitrosoacetanilide, decomposition of diazonium
salts in alkaline medium, on heating, and in the pres-
ence of potassium iodide, thermal and catalytic decom-
position of diaryliodonium salts, reduction of diazo-
nium salts with ascorbic acid, and their electrochem-
ical reduction. The reagents and conditions were se-
lected in such a way that the formation of intermediate
complexes by the catalyst [Cu(I) or Cu(II)] with fur-
furol and diazonium salt be either favored or com-
pletely excluded.
Scheme 1.
N Cl
2
R1
+
R2
O
R2
CuCl2
O
R1
2
Using 4-nitrobenzenediazonium chloride (Ia) as
an example we examined decomposition of diazonium
salt in the presence of furfurol in alkaline medium
(Gomberg reaction conditions) and on heating
(Scheme 2). In the first case, 5-(4-nitrophenyl)furan-2-
carbaldehyde (IIa) was obtained in 9% yield, and in
the second case the yield of IIa was 11.5%. In both
cases, the reactions were accompanied by considerable
tarring. Standard Meerwein reaction conditions ensure
50–60% yield of aldehyde IIa. In the reaction of fur-
2
R = CHO, COOH, COOR, CH OH, COMe, CN,
COCHO, COCOOH.
Arylation of furfurol was reported most commonly.
Reactions of furan compounds with diazonium salts
are usually carried out under conditions similar to
Meerwein reaction which is most typical of unsaturat-
ed compounds. Homolytic arylation of furan ring as
an aromatic system (Gomberg reaction) is also known
[
7, 13]. Presumably, copper-catalyzed arylation of
1
375

1
376
OBUSHAK et al.
to give cations which catalyzed the reaction. When
Scheme 2.
N Cl
2
analogous experiment was performed in the absence of
chloride ion (4-nitrobenzenediazonium tetrafluoro-
borate was used as starting compound), the yield of
aldehyde IIa decreased to 7%. Only traces of IIa were
formed from 4-O NC H N Cl and furfurol in the pres-
+
CHO
O
O N
2
Ia
2
6
4
2
ence of copper when no electric current was passed
through the system. Insignificant amounts of aldehyde
IIa (3–5%) were obtained by electrochemical reduc-
tion of 4-nitrobenzenediazonium tetrafluoroborate
CHO
NaOH, AcOH or Δ
O
O N
2
IIa
(
IVa) at a platinum electrode in the presence of furfu-
rol, regardless of the potential applied. Thus, in all
experiments described above the yields of 5-arylfuran-
furol with N-nitrosoacetanilide (III), which is known
as a good radical arylation reagent [18, 19], 5-phenyl-
furan-2-carbaldehyde (IIb) was formed in as poor as
2
-carbaldehydes ІІa and IIb were considerably lower
than in copper-catalyzed reactions of furfurol with di-
azonium salts or these compounds were not formed at
all. These data suggest that the reaction of furfurol
with arenediazonium salts follows a more complicated
mechanism than radical arylation. Redox catalysis is
not a sufficient condition, and it is not confined to re-
duction of diazonium ion. Insofar as electrochemical
version with arenediazonium chlorides is characterized
by a satisfactory yield, while arenediazonium tetra-
fluoroborates give rise to insignificant amounts of
5-arylfuran-2-carbaldehydes, chloride ion is likely to
be involved in complex formation as a ligand [14, 15].
1
0% yield (Scheme 3).
Scheme 3.
COMe
N
NO
+
CHO
O
III
CHO
O
Taking into account that arylation with diazonium
salts is usually carried out in aqueous acetone in the
IIb
presence of CuCl , it was presumed [10, 26] that such
2
Aryl radicals can also be generated by decom-
reactions are catalyzed by CuCl generated by reaction
position of diaryliodonium salts [20, 21]. However, we
failed to synthesize aldehydes IIa and IIb from furfu-
rol and diaryliodonium chloride according to Meer-
of CuCl with acetone. However, this statement cannot
2
be regarded as indisputable [14, 15]; therefore, we
examined in more detail the conditions for arylation of
furfurol with a view to optimize the yield and elucidate
effects of different factors on the reaction course. For
this purpose, the arylation of furfurol was performed
wein (water, acetone, CuCl ). Iodine-induced dediazo-
2
nation [21, 22] of 4-nitrobenzene- and benzenediazo-
nium tetrafluoroborates IVa and IVb (ArN BF + KI)
2
4
in the presence of furfurol gave only ~5% of arylfurans
IIa and IIb, though aliphatic unsaturated compounds
are known to readily undergo arylation under analo-
gous conditions [22, 23]. Ascorbic acid was reported to
effectively reduce diazonium salts to aryl radicals [24].
The reaction of 4-nitrobenzenediazonium chloride (Ia)
with furfurol in the presence of an equimolar amount
of ascorbic acid afforded only 15% of aldehyde IIa.
We also examined reactions of furfurol with electro-
chemically generated aryl radicals. The latter were
generated in two modes using an “inert” platinum
electrode and a copper anode [25]. Electrochemical
reduction of 4-nitrobenzenediazonium chloride (Ia) in
the presence of furfurol using a copper plate as anode
produced aldehyde IIa in 35% yield. Copper dissolved
Scheme 4.
N X
2
R
+
CHO
O
Ia, Ic–If, IVa, IVc–IVf
Va, Vc–Vf
CHO
Solvent, catalyst
–N , –HCl
O
R
2
IIa, IIc–IIf
R = 4-O
2
N (a), 3-O
2
N (c), 4-Cl (d), 4-Br (e), 3-Cl (f);
; V, X = HSO
I, X = Cl; ІV, X = BF
4
4
.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 9 2009

MECHANISM OF MEERWEIN ARYLATION OF FURAN DERIVATIVES
1377
using various solvents and catalysts and arenediazo-
nium salts with different anions (Scheme 4).
Arylation of furfurol with arenediazonium salts
Run X in
Yield,
%
Solvent
Catalyst
The arylation of furfurol with arenediazonium salts
Ia and Ic–If in aqueous chloroform, diethylene glycol
dimethyl ether, and acetonitrile afforded the corre-
sponding aldehydes II in poor yield, and the process
was accompanied by considerable tarring. The yield
of II increased in going to aqueous acetone, while the
most appropriate solvent systems (see table) were
DMF–water (1:1) and DMSO–water (1 :1). In the
latter system the yields of 5-arylfuran-2-carbaldehydes
increased on the average to 60%. Analogous yields
were obtained in water, but the reaction was very slow:
the conversion of initial diazonium salt was complete
in 3 days at room temperature.
no. ArN
2
X
0
0
1
2
BF
Cl
4
DMSO–water, 1:1 CuCl
DMSO–water, Cu(OAc)
:1.25
Water
DMSO–water, 1:1 CuCl2
Anhydrous DMSO CuCl
DMSO–water, CuSO
:1.25
Acetone–water,
:1
Acetone–water,
:1
Acetone–water,
:1
2
75–80
61–66
2
1
0
3
Cl
CuCl
2
55–60
50–60
44–48
40–46
04 Cl
0
0
5
6
BF
Cl
4
4
2
4
1
07
08
09
10
BF
Cl
Cl
Cl
CuCl
CuCl
CuCl
2
32–40
25–40
15–20
1
2
As catalyst we tried copper(II) acetate and cop-
per(II) sulfate in addition to commonly used CuCl2.
The catalyst strongly affected the arylation of furfurol
with arenediazonium chlorides Ia and Ic–If. The aryla-
tion efficiency was considerably lower when freshly
prepared copper(I) chloride was used as catalyst (run
nos. 9, 10). These findings contradict a scheme accord-
ing to which the initial step is electron transfer from
Cu(І) to diazonium ion to form aryl radical [26].
1
1
DMSO–water, 3:4 CuCl
DMSO–water, 1:1 CuCl
08–11
11 HSO
4
4
4
2
–
1
1
1
1
1
2
3
4
5
6
HSO
HSO
DMSO–water, 1:1 CuSO
Water CuCl
4
4
–
2
–
BF
BF
BF
4
4
4
DMSO–water, 1:1 CuSO
DMSO–water, 1:1 CuCl
–
In order to estimate the effect of counterion in
arenediazonium salt, the arylation of furfurol with
arenediazonium hydrogen sulfates Va and Vc–Vf and
tetrafluoroborates ІVa and IVc–IVf was examined.
Arenediazonium hydrogen sulfates Va and Vc–Vf
failed to react with furfurol (run nos. 11–13). The reac-
tions of furfurol with arenediazonium tetrafluoro-
borates ІVa and IVc–IVf were carried out in aqueous
acetone (1:1), anhydrous DMSO, and aqueous DMSO
–
–
DMSO–water, 1:1 Cu(OAc)
DMSO–water, 1:1 CuSO
NaCl, 1:4
2
17 HSO
4
4
–
55–58
equimolar amount) is crucial. In the reaction under
study chloride ion is not necessary for oxidation of
radical (radical adduct) with ligand transfer in the final
step, as in the classical Meerwein chloroarylation of
unsaturated compounds [15]. Presumably, chloride ion
behaves as an active ligand capable of being replaced
by diazonium ion or furfurol molecule in the copper
coordination sphere in the course of formation and
subsequent transformations of intermediate mixed-
ligand complexes. Probably, as in the arylation of un-
saturated compounds, such intermediate complexes are
arenediazonium tetrachlorocuprates(II) [14, 15, 27].
While studying the reaction of arenediazonium sulfate
(
1:1) in the presence of CuCl, CuSO , Cu(OAc) , and
4 2
CuCl . Copper(I) chloride, copper(II) sulfate, and cop-
2
per(II) acetate did not catalyze the process (run
nos. 14–16). The reaction was fast and efficient in
aqueous DMSO (1:1) in the presence of CuCl (run
2
no. 1): the yields of arylfurancarabaldehydes ІІ at-
tained 75–80%. The reaction in anhydrous DMSO (run
no. 5) occurred at a considerably lower rate (3–4 days),
and the yields were smaller.
It should be emphasized that the best results are
obtained with CuCl as catalyst, while CuCl does not
(IIc) with furfurol in the presence of CuSO , different
2
4
catalyze the reaction at all, other conditions being
equal (cf. run nos. 1 and 15). Obviously, the role of
DMSO is important: in going from DMSO to acetone
amounts of NaCl were added to the reaction mixture.
When the ArN HSO –CuSO –NaCl ratio attained
2 4 4
a value of 2:1:4, the yield of aldehyde IIc was the
(
run nos. 7, 9), the difference in the catalytic activity of
same as in the reaction with ArN Cl and CuCl . The
2
2
CuCl and CuCl becomes less significant. The pres-
ence of chloride ion (even in a catalytic rather than
above ratio conforms to the arenediazonium tetra-
2
+
2–
chlorocuprate(II) stoichiometry, (ArN ) CuCl .
2 2 4
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 9 2009

1
378
OBUSHAK et al.
Scheme 5.
pounds are capable of undergoing one-electron oxida-
tion with diazonium ions to give radical cations [29].
Cu2+
ArN2
Obviously, strong furfurol–CuCl complexation favors
CHO
CHO
2
O
electron transfer. The transformations shown in
Scheme 5 may be regarded as ligand interactions
inside mixed-ligand copper complexes. If the reaction
coordinate involves formation of radical adduct A
+
·
N2
Cu+
Ar
·
O
(such adducts were detected by ESR for both furfurol
17] and unsaturated compounds [32]), the reaction
[
scheme should comprise two catalytic cycles with
participation of two substrate molecules [14, 15]
H+
Ar
CHO
O
(
Scheme 6).
+
2+
We can conclude that the arylation of furfurol in-
The catalytic system Cu ↔Cu mediates electron
volves intermediate formation of copper coordination
transfer from furfurol to diazonium ion. Aryl radical
thus formed inside activated complex [arenediazonium
tetrachlorocuprate(ІІ)–furfurol or arenediazonium salt–
catalyst–furfurol] reacts with the second furfurol mole-
cule to give radical adduct A. The latter donates an
electron through copper(II) ion to radical cation B gen-
erated in the first cycle with formation of product C.
compounds. The complexation of CuCl with furfurol
2
was confirmed experimentally. The complex was iso-
lated in the reaction of CuCl with furfurol in acetone
2
or acetonitrile. The complex is stable; it decomposes at
1
40°C. Presumably, copper(II) ion coordinates furfurol
molecule at the carbonyl oxygen atom: the IR spec-
trum of the complex contained an absorption band at
–
1
–1
1
610 cm (C=O; 1680 cm in the IR spectrum of
EXPERIMENTAL
furfurol). In addition, the C=C stretching vibration
–
1
band was displaced from 1560 to 1525 cm upon
complex formation.
Arylation of furfurol according to Gomberg.
A solution of 1 g of sodium hydroxide and 16 g of
sodium acetate trihydrate in 50 ml of water was added
to a mixture of 12.4 ml (0.15 mol) of furfurol and
60 ml of acetone, and an aqueous solution of 4-nitro-
benzenediazonium chloride (Ia) (prepared by diazo-
tization of 0.1 mol of 4-nitroaniline) was added drop-
wise under stirring, maintaining the pH value at 5–6
with the aid of acetate buffer. When nitrogen no longer
evolved (5 h at 25°C), the mixture was diluted with
water, and the oily material was separated and washed
with water, alcohol, and diethyl ether. The product
Our results indicate that the reaction is not initiated
+
2
+
·
according to the scheme ArN + Cu → Ar + N +
2
2
+
Cu , for the best results were obtained with copper(II)
salts under conditions that do not favor their reduction.
Then the initial step is copper(II)-mediated electron
transfer from the substrate molecule (furfurol) to
diazonium ion, as in conventional Meerwein arylation
[13, 14, 28]. As a result, furfurol radical cation is
formed (Scheme 5), which is fairly typical for hetero-
cyclic compounds [29–31]. Some heterocyclic com-
Scheme 6.
CHO
O
·
Ar
CHO
O
A
·
Cu+
Cu2+
Ar
CHO
CHO
O
O
H+
N2
+
·
Cu2+
Cu+
ArN2
Ar
CHO
O
B
C
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 9 2009

MECHANISM OF MEERWEIN ARYLATION OF FURAN DERIVATIVES
1379
solidified and was additionally purified by column
chromatography on Al O using benzene as eluent.
Yield of aldehyde IIa 2 g (9%), mp 209–210°C.
Reaction of 4-nitrobenzenediazonium chloride
with furfurol in the presence of ascorbic acid.
A cold solution of diazonium salt Ia prepared by
diazotization of 0.05 mol of 4-nitroaniline was added
dropwise to a mixture of 7.5 ml (0.09 mol) of furfurol,
2
3
The reaction with thermal decomposition of diazo-
nium salt was carried out in a similar way at 35–40°C
without addition of NaOH and AcONa. The product
was isolated as described above. Yield of compound
IIa 2.5 g (12%).
8
.8 g (0.05 mol) of ascorbic acid, 70 ml of acetone,
and 35 ml of water. The mixture was stirred at 20–
5°C until nitrogen no longer evolved. The oily mate-
2
rial was separated and washed with several portions of
water and then with alcohol and diethyl ether. The
solid residue was purified by column chromatography
on Al O using benzene as eluent to isolate 1.6 g
Reaction of N-nitrosoacetanilide with furfurol.
A mixture of 16.4 g (0.1 mol) of N-nitrosoacetanilide
(
III), 12.4 ml (0.15 mol) of furfurol, and 120 ml of
2
3
acetone was stirred at 18–20°C (~4 h). When nitrogen
no longer evolved, the mixture was diluted with water
and extracted with diethyl ether. The extract was dried
(
15%) of 5-(4-nitrophenyl)furan-2-carbaldehyde (IIa).
Arylation of furfurol with 4-nitrobenzenediazo-
nium salts under conditions of electrochemical re-
duction. The reaction was carried out in a four-necked
reactor equipped with a thermometer, bubble counter,
and two electrodes connected to a P5827M potentio-
stat. A copper plate was used as a working electrode,
and a platinum electrode served as an auxiliary elec-
trode. The reaction mixture was stirred with a mag-
netic stirrer.
over MgSO , the solvent was distilled off, and the
residue was distilled under reduced pressure. Yield of
4
5
1
-phenylfuran-2-carbaldehyde 1.7 g (10%), bp 132–
35°C (2 mm).
Decomposition of diphenyliodonium chloride in
the presence of furfurol. A mixture of 12 g
(
(
0.038 mol) of diphenyliodonium chloride, 6.6 ml
0.08 mol) of furfurol, 1.5 g of copper(II) chloride
The reactor was charged with 50 ml of acetone and
dihydrate, 50 ml of acetone, and 50 ml of water was
heated for 8 h under reflux with stirring. The mixture
was extracted with diethyl ether, the extract was dried
1
2.5 ml (0.15 mol) furfurol, approximately a half
(
0.1 mol) of a solution of 4-nitrobenzenediazonium
chloride (Ia) was added, and a required voltage was
over MgSO , the solvent was distilled off, and the
4
supplied to the electrodes. In all cases the current den-
residue was distilled under reduced pressure. We thus
isolated iodobenzene and unreacted furfurol. No
2
sity was 7 mA/cm . Nitrogen evolved at 20–30°C. The
addition of diazonium solution lasted 1 h. The mixture
was stirred until nitrogen no longer evolved (~6 h).
The organic layer was separated, washed with water,
and evaporated, and the residue was subjected to
column chromatography on Al O using benzene as
5
-phenylfuran-2-carbaldehyde (ІІb) was formed when
the reaction was carried out in aqueous dioxane with
no catalyst or in the presence of CuCl.
Reaction of benzenediazonium tetrafluoroborate
with furfurol in the presence of potassium iodide.
Benzenediazonium tetrafluoroborate (IVb), 13.7 g
2
3
eluent to isolate 3.8 g (35%) of 5-(4-nitrophenyl)furan-
2
-carbaldehyde (IIa).
(
1
0.07 mol), was added in portions to a mixture of
1.6 ml (0.14 mol) of furfurol, 11.6 g of potassium
The reaction of 11.8 g (0.05 mol) of 4-nitroben-
zenediazonium tetrafluoroborate (IVa) with 6.6 ml
0.08 mol) of furfurol in a mixture of 50 ml of acetone
iodide, 45 ml of acetone and 15 ml of water. Vigorous
evolution of nitrogen was observed at 0–10°C. The
mixture was treated with sodium thiosulfate until the
aqueous phase turned colorless and was then extracted
with diethyl ether. The extract was dried over MgSO4
and evaporated, and the residue was distilled under
reduced pressure to isolate iodobenzene and 0.6 g (5%)
of 5-phenylfuran-2-carbaldehyde (ІІb), bp 120–123°C
(
and 50 ml of water was carried out for 5 h at 30–35°C,
and the process was accompanied by strong tarring.
The organic layer was separated, washed with water,
and evaporated, and the residue was subjected to col-
umn chromatography on Al O using benzene as eluent
2
3
to isolate 0.8 g (7%) of aldehyde IIa and 0.35 g (5%)
of 4-nitrophenol.
(
2 mm). The reaction of 4-nitrobenzenediazonium
tetrafluoroborate with furfurol was carried out in
a similar way. After treatment with sodium thiosulfate,
the precipitate was filtered off, washed with water, and
purified by column chromatography on Al O using
In experiments with inert electrodes, 11.8 g
(0.05 mol) of 4-nitrobenzenediazonium tetrafluoro-
borate (IVa) reacted with 6.6 ml (0.08 mol) of furfurol,
the working electrode (cathode) being a platinum plate.
The reaction was carried out for 8 h at 30–36°C. By
2
3
benzene as eluent. Yield of aldehyde IIa 0.6 g (4%).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 9 2009

1
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OBUSHAK et al.
column chromatography on Al O using benzene as
Reaction of 3-nitrobenzenediazonium hydrogen
2
3
eluent we isolated 0.6 g (5%) of aldehyde IIa.
-(3-Nitrophenyl)furan-2-carbaldehyde (IIc).
Run no. 1. A mixture of 0.35 g of CuCl ·2H O, 2.5 ml
sulfate with furfurol in the presence of CuSO and
NaCl (run no. 17). A solution of 3-nitrobenzene-
diazonium hydrogen sulfate, prepared from 6.9 g
4
5
2
2
(
0.05 mol) of 3-nitroaniline, 6 ml of concentrated sul-
(
0.03 mol) of furfurol, 20 ml DMSO, and 20 ml of
furic acid, 25 ml of water, and 3.6 g (0.052 mol) of
sodium nitrite in 15 ml of water, was added dropwise
under stirring at 23–26°C to a mixture of 2 g
water was stirred at 24–26°C. 3-Nitrobenzenediazo-
nium tetrafluoroborate (IVc), 7.1 g (0.03 mol), was
added in portions, and the mixture was stirred until
nitrogen no longer evolved and was left to stand for
(
0.0125 mol) of CuSO , 3 g (0.05 mol) of NaCl, 5 ml
4
2
4 h. The organic phase was separated, washed with
of (0.06 mol) furfurol, and 40 ml of DMSO. The mix-
ture was left to stand for 24 h and diluted with water,
and the precipitate was filtered off, washed with water,
alcohol, and diethyl ether, and recrystallized from car-
bon tetrachloride. Yield of aldehyde IIc 6.2 g (57%).
water, and evaporated, and the residue was recrystal-
lized from carbon tetrachloride. Yield 5.0 g (77%),
mp 158–159°C. Run nos. 5, 7, and 14–16 followed
an analogous procedure.
At Vc–CuSO –NaCl ratios of 0.05:0.0125:0.025 and
4
Run no. 3. A solution of 3-nitrobenzenediazonium
chloride (Ic), prepared from 3.45 g (0.025 mol) of
0
.05:0.0125:0.006 the yield was 5.2 g (48%) and 2.2 g
(
20%), respectively.
3
-nitroaniline, 6.5 ml of concentrated hydrochloric
acid, 10 ml of water, and 1.8 g (0.026 mol) of sodium
nitrite, was added dropwise to a mixture of 0.7 g of
CuCl ·2H O, 2.5 ml (0.03 mol) of furfurol, and 25 ml
The reactions of diazonium salts Ia, Id–If, IVa,
IVd–IVf, Va, and Vd–Vf with furfurol were carried
out according to analogous procedures.
2
2
of water. The mixture was left to stand for two days
and diluted with water, and the precipitate was filtered
off, washed with water, alcohol, and diethyl ether, and
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Run no. 4. A solution of 3-nitrobenzenediazonium
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3
-nitroaniline, 13 ml of concentrated hydrochloric
2
1
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985, p. 5.
acid, 10 ml of water, and 3.6 g (0.052 mol) of sodium
nitrite, was added dropwise under stirring at 23–26°C
to a mixture of 0.7 g of CuCl ·2H O, 5 ml (0.06 mol)
2
3
. Hosoya, T., Aoyama, H., Ikemoto, T., Kihara, Y., Hira-
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2
2
of furfurol, and 50 ml of DMSO. The mixture was
stirred until nitrogen no longer evolved and diluted
with water, and the precipitate was filtered off, washed
with water, alcohol, and diethyl ether, and recrystal-
lized from carbon tetrachloride. Yield 5.6 g (52%).
Run nos. 2, 6, and 8–10 were carried out following
an analogous procedure.
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Reaction of 3-nitrobenzenediazonium hydrogen
sulfate with furfurol. Run no. 12. A solution of 3-ni-
trobenzenediazonium hydrogen sulfate (Ic), prepared
from 6.9 g (0.05 mol) of 3-nitroaniline, 6 ml of con-
centrated sulfuric acid, 35 ml of water, and 3.6 g
(
0.052 mol) of sodium nitrite in 15 ml of water, was
cooled and added dropwise under stirring to a mixture
of 0.8 g of CuSO , 5 ml (0.06 mol) of furfurol, and
4
5
5 ml of DMSO. The mixture was stirred for 3–4 h
(
slow evolution of nitrogen was observed), left over-
night, and dilted with water, and the precipitate was
filtered off and washed with water, alcohol, and diethyl
ether. The product contained no aldehyde IIc. Run
nos. 11 and 13 followed an analogous procedure.
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