Mono- and diformylation of 4-substituted phenols: A new application of the duff reaction

Article abstract of DOI:10.1055/s-1998-2110

Careful control of the reaction conditions in a modified Duff reaction using anhydrous trifluoroacetic acid as the solvent enables the selective synthesis of either 4-substituted 2-formylphenols (5-substituted salicylaldehydes) or 4-substituted 2,6-diformylphenols in one step starting from commercially available 4-substituted phenols in moderate to good yield.

Full text of DOI:10.1055/s-1998-2110

July 1998
SYNTHESIS
1029
Mono- and Diformylation of 4-Substituted Phenols: A New Application of the Duff
Reaction
a,b
a
a,1
Leonard F. Lindoy, George V. Meehan, Niels Svenstrup*
a
Department of Chemistry and Chemical Engineering, James Cook University, Townsville Qld. 4811, Australia
School of Chemistry, University of Sydney, Sydney NSW 2006, Australia
b
Received 27 October 1997; revised 22 December 1997
Abstract: Careful control of the reaction conditions in a modified
Duff reaction using anhydrous trifluoroacetic acid as the solvent
enables the selective synthesis of either 4-substituted 2-
formylphenols (5-substituted salicylaldehydes) or 4-substituted
A difficulty with most common formylation reactions is
that they involve electrophilic attack on the aromatic ring
to introduce a functionality than can be easily converted to
a formyl group in a few synthetic steps, or during workup.
2
,6-diformylphenols in one step starting from commercially avail-
12
13
The Vilsmeier reaction, the Reimer–Tiemann reac-
tion, and traditional Duff14 reactions are all of this type.
Since the introduction of a formyl group on to an aromatic
ring significantly reduces its electron density, the intro-
able 4-substituted phenols in moderate to good yield.
Key words: 4-substituted phenols, modified Duff reaction, mono-
formylation, diformylation, 4-substituted 2-formylphenols, 4-sub-
stituted 2,6-diformylphenols
15
duction of a second formyl group is rarely possible. This
effect is well known for Friedel–Crafts acylation reactions
where clean monoacylation can normally be expected.16
Aromatic aldehydes, being one of the most synthetically
versatile functional groups in organic chemistry, are often
chosen as the starting point in synthetic sequences, espe-
cially since a number of such aldehydes are available
commercially. Additionally, many aromatic aldehydes are
easily prepared in a few steps from simple aromatic pre-
In view of the above, we were surprised to find that
formylation of 4-tert-butylphenol (3a) employing a
modified17 Duff reaction, and using neat trifluoroacetic
acid as the solvent instead of glycerol, produced small
amounts of 4-tert-butyl-2,6-diformylphenol (2a) along
with the expected 5-tert-butylsalicylaldehyde (1a).18 We
have investigated the scope of this reaction, since it ap-
peared to be an attractive synthetic route to 2,6-diformyl-
ated phenols, provided the reaction could be optimized. 4-
tert-Butylphenol (3a) was chosen as the substrate for ini-
tial study for both mono- and diformylation, since both 5-
tert-butylsalicylaldehyde (1a) and 4-tert-butyl-2,6-di-
formylphenol (2a) were of interest to us as starting mate-
rials in other investigations.
2
cursors. Of these, mono- and diformylated aromatic phe-
nols have proven to be especially important in the
synthesis of macrocyclic compounds due to their difunc-
tional nature; as such, they have been of interest to us.3
We wish to report here an improved synthesis of mono-
formylated phenols 1 (salicylaldehydes) and diformylated
phenols 2. The new strategy replaces traditionally tedious
multistep procedures with a straightforward one-pot syn-
thesis.
As expected, monoformylation of 4-tert-butylphenol was
achieved using one equivalent of hexamethylenetetramine
in refluxing trifluoroacetic acid to give pure 5-tert-butyl-
salicylaldehyde (1a) in 29% yield after purification by fil-
tration through a short silica gel column (Scheme 1).
Although the yield is moderate, this procedure compares
favorably with the standard procedure for the preparation
of 5-tert-butylsalicylaldehyde (1a) published by Suckling
and co-workers.19
2
,6-Diformylated phenols have been widely used as pre-
4
–7
and acyclic8
cursors to binucleating macrocyclic
ligands for use as model systems in bioinorganic chemis-
try and for fundamental inorganic chemistry studies. Sev-
eral synthetic strategies have been developed for the
synthesis of 2,6-diformylated phenols; however, the pub-
lished preparations of diformylated phenols are often low
9
–11
yielding and/or involve multistep syntheses.
The
8
synthesis of 4-tert-butyl-2,6-diformylphenol (2a) in
which the product is obtained from 4-tert-butylphenol in
four steps (bis-hydroxymethylation of the phenol under
basic conditions, selective tosylation of the phenolic hy-
droxy group, oxidation of the two hydroxymethyl groups
Scheme 1
to formyl groups using Na Cr O , followed by hydrolytic Clean diformylation of 4-tert-butylphenol (3a) was
2
2
7
cleavage of the tosyl group using concentrated sulfuric ac- achieved by reaction with two equivalents of hexamethyl-
id) in an overall yield of less than 4% is representative of enetetramine in refluxing trifluoroacetic acid for 24 hours;
a number of these traditional synthetic strategies.
after this time TLC of an aliquot of the reaction solution

1
030
Papers
SYNTHESIS
no longer showed the presence of 5-tert-butylsalicylalde-
hyde (1a). The crude product was subjected to aqueous
workup, and then the resulting orange oil was filtered
through a short silica gel column (eluent: dichlo-
romethane). Evaporation of the filtrate gave 4-tert-butyl-
2
,6-diformylphenol (2a) as essentially the only product as
1
13
evidenced by TLC, melting point, H NMR, and
C
Scheme 3
NMR. Analytically pure material was prepared by recrys-
tallization (cyclohexane) to give the product in 65% yield.
We took advantage of this selectivity in a synthesis of 1,2-
bis(4-tert-butyl-2-formylphenoxy)ethane (7) which was
required as the central starting material in the synthesis of
a series of cage molecules. 1,2-Bis(4-tert-butylphen-
oxy)ethane (6) was refluxed in trifluoroacetic acid for 24
hours with two equivalents of hexamethylenetetramine to
give the required product 7 in 58% yield after filtration
through silica gel. No ‘over-formylated’ products were
detected (Scheme 4).
Scheme 2
The above reactions were also performed with the follow-
ing 4-substituted phenols: 4-methylphenol (3b), 4-meth-
oxyphenol (3c), 4-bromophenol (3d), and 4-nitrophenol
(3e) (Scheme 2). These reactions generally proceeded in
moderate to good yield, except for the diformylation of
the nitro derivative. Apparently, the electron-withdrawing
effect of the nitro group is so strong, that, in combination
with the similar effect of the first formyl group (or per-
haps, more accurately, of the intermediate 5-tert-butyl-
salicylaldehyde N-methylimine), the second formylation
reaction is greatly retarded. This hypothesis is supported
by the fact that 5-nitrosalicylaldehyde was isolated in
good yield in this reaction, as well as in the deliberate
monoformylation reaction of 4-nitrophenol. Incidentally,
the actual yield of 2,6-diformyl-4-nitrophenol (2e) is
Scheme 4
In conclusion, we have demonstrated that the mono- and
diformylation of suitably activated aromatic compounds
under modified Duff reaction conditions is a convenient
and straightforward synthetic route to 5-substituted sali-
cylaldehydes and 4-substituted 2,6-diformylphenols. To-
gether with the selectivity exemplified by the
transformation 6 → 7, this procedure makes a range of im-
portant aromatic building blocks more readily available
for synthesis.
1
probably significantly higher than 4% as evidenced by H
NMR spectroscopy of the crude reaction mixture. Howev-
er, due to difficulties experienced in the chromatographic
separation of 2,6-diformyl-4-nitrophenol (2e) and 5-nitro-
salicylaldehyde, related to the poor solubility of the two
compounds, only a fraction of the ‘initial’ product was ob-
tained in a pure state. 2,6-Diformyl-4-nitrophenol (2e)
could also be prepared from 5-nitrosalicylaldehyde, albeit
with no increase in yield compared to the synthesis from
All the reagents used were purchased from Aldrich, and used without
prior purification, except CH Cl , which was distilled from CaCl ,
and EtOAc, which was distilled from K CO . Petroleum ether refers
2
2
2
2
3
to the fraction boiling from 60–80˚C. All the chromatographic sepa-
rations were performed using vacuum assisted elution on Merck silica
gel 60H F254, 60 mesh (63–230 µm). Elemental analyses were per-
formed at the Microanalytical Laboratory, University of Copenhagen,
and at the Microanalytical Facility, James Cook University, Towns-
4-nitrophenol (vide supra). It is noted that 5-bromosalicyl-
aldehyde and 5-nitrosalicylaldehyde are more easily pre-
pared by bromination or nitration of salicylaldehyde,
respectively.
1
ville. H NMR spectra were recorded at 300 MHz on a Bruker AM300
spectrometer, while 13C spectra were recorded on the same instrument
at 75 MHz, in both cases using the solvent as internal reference.
Chemical shifts in ppm are relative to TMS. EI-MS were recorded on
a Kratos M25RFA instrument, while PD-MS was performed on a
Bio-Ion 20R instrument from Applied Biosystems. Mps were record-
ed on a Büchi melting point apparatus, and are uncorrected.
Further investigation of the scope of this reaction demon-
strated that the presence of a free phenolic group is essen-
tial for diformylation to take place. By subjecting 4-tert-
butylanisole (4) to formylation under similar conditions to
those described above, we were able to synthesize 5-tert-
butyl-2-methoxybenzaldehyde (5) in 70% yield. No trace
of 4-tert-butyl-2-methoxyisophthalaldehyde was detect-
ed, even after prolonged reflux using a large excess of
hexamethylenetetramine (Scheme 3).
5-tert-Butylsalicylaldehyde (1a):
4-tert-Butylphenol (10.20 g, 67.9 mmol) was dissolved in anhyd TFA
(65 mL) under N , and hexamethylenetetramine (9.52 g, 67.9 mmol)
was added in one portion. The yellow solution was refluxed until all
2

July 1998
SYNTHESIS
1031
the starting phenol was converted (TLC control, ca. 90 min); the mix-
ture was then cooled to r.t. The cooled solution was poured into 4 M
HCl (200 mL) and stirred for 15 min, the product was extracted with
CH Cl (2 × 150 mL). The combined organic extracts were washed
4-Bromo-2,6-diformylphenol (2d):
This preparation closely followed the preparation of 2a, starting from
p-bromophenol (6.77 g, 37.3 mmol) and hexamethylenetetramine
(11.17 g, 79.6 mmol) in anhyd TFA (50 mL), but utilized a different
workup procedure. After refluxing the mixture for 24 h, the mixture
was poured into 4 M HCl (300 mL), and left to crystallize overnight.
The resulting yellow crystals were collected by filtration, dried and
2
2
with 4 M HCl (2 × 200 mL), sat. brine (200 mL), then dried (Na SO )
2
4
and the solvent removed in vacuo. The yellow residue was purified by
column chromatography (silica gel, CH Cl /petroleum ether 2:1) to
2
2
remove baseline impurities. After evaporation 1a was isolated as a
recrystallized (EtOH/H O). The resulting yellow needles were fil-
2
1
9
pale yellow oil; yield: 3.52 g (29%).
tered off, and dried in a vacuum oven at 70°C to give pure 2d; yield:
1
9
H NMR (CDCl ): δ = 10.87 (s, 1H, -OH), 9.89 (s, 1H, -CHO), 7.58
3.71 g (41%); mp 136°C (lit. mp 137–138°C).
3
1
(
dd, 1H, J = 3 Hz, J = 9 Hz, Ar-H6), 7.51 (d, 1H, J = 3 Hz, Ar-H4),
H NMR (CDCl ): δ = 11.52 (s, 1H, -OH), 10.16 (s, 2H, -CHO), 8.03
1
2
3
6
.94 (d, 1H, J = 9 Hz, Ar-H3), 1.28 [s, 9H, -C(CH ) ].
(s, 2H, Ar-H).
3
3
1
3
13
C NMR (CDCl ): δ = 196.94 (-CHO), 159.55 (arom. C-1), 142.76
C NMR (CDCl ): δ = 190.81 (-CHO), 162.25 (arom. C-1), 141.92
3
3
(
arom. C-4), 134.69 (arom. C-5), 129.76 (arom. C-3), 120.21 (arom.
(arom. C-3,5), 139.75 (arom. C-2,6), 124.60 (arom. C-4).
MS (EI): m/z (%) = 228 (M , 45).
+
C-2), 117.19 (arom. C-6), 33.91 [-C(CH ) ], 31.07 [-C(CH ) ].
MS (EI): m/z (%) = 178 (M , 35).
3
3
3 3
+
2,6-Diformyl-4-nitrophenol (2e):
4-Nitrophenol (3.53 g, 25.4 mmol) and hexamethylenetetramine
4
-tert-Butyl-2,6-diformylphenol (2a); Typical Procedure:
(10.89 g, 77.7 mmol) were dissolved in anhyd TFA (65 mL) under dry
4
-tert-Butylphenol (5.14 g, 34.2 mmol) and hexamethylenetetramine
N , and the resulting yellow solution refluxed for 7 d. The orange so-
2
(9.60 g, 68.5 mmol) were dissolved in anhyd TFA (60 mL) under N2,
lution was poured into 4 M HCl (300 mL) and extracted with CH Cl2
2
and the resulting yellow solution was refluxed for 24 h (color change
to orange). The mixture was poured into 4 M HCl (200 mL) and
stirred for 10 min, after which it was extracted with CH Cl (2 × 150
(4 × 75 mL). The combined organic extracts were washed with 4 M
HCl (200 mL), sat. brine (200 mL) and then dried (Na SO ). Evapo-
2
4
ration of the solvent gave an orange residue, which was redissolved
in CH Cl and filtered through a short silica gel column (5% MeOH/
2
2
mL). The combined organic extracts were washed with 4 M HCl (2 ×
00 mL), water (200 mL), sat. brine (200 mL), then dried (Na SO )
2
2
2
CH Cl ). The filtrate was evaporated, and the residue purified by re-
2
4
2 2
and evaporated to give a yellow crystalline residue. The product was
purified by chromatography (short column, silica gel, CH Cl ) to re-
peated chromatography on silica gel (EtOAc) to give 2e as an orange
microcrystalline solid; yield: 197 mg (4%); mp 124–126°C.
2
2
move baseline impurities and trace amounts of 5-tert-butylsalicylal-
dehyde. This gave 2a as light yellow crystals sufficiently pure for
further reactions. Analytically pure material could easily be prepared
by recrystallization (cyclohexane) to give large yellowish needles;
C H NO
5
calcd
found
C
49.24
49.10
H
2.58
2.74
N
7.18
7.06
8
5
(195.13)
1
H NMR (CDCl ): δ = 12.23 (s, 1H, -OH), 10.28 (s, 2H, -CHO), 8.83
3
2
0
(s, 2H, Ar-H).
MS (EI): m/z (%) = 195 (M , 50).
yield: 4.58 g (65%).; mp 102–103°C (lit. mp 105°C).
+
1
H NMR (CDCl ): δ = 11.45 (s, 1H, -OH), 10.21 (s, 2H, -CHO), 7.95
3
(
s, 2H, Ar-H), 1.32 [s, 9H, -C(CH ) ].
3 3
13
Monoformylation of 4-Substituted Alkoxybenzenes; General
Procedure:
C NMR (CDCl ): δ = 192.34 (-CHO), 161.69 (arom. C-1), 143.09
3
(
[
arom. C-4), 134.62 (arom. C-3,5), 122.62 (arom. C-2,6), 34.28
-C(CH ) ], 31.07 [-C(CH ) ].
The alkoxybenzene (30 mmol) and hexamethylenetetramine
(30 mmol) were dissolved in anhyd TFA (60 mL) under N2, and the
resulting solution refluxed for 24 h. The mixture was concentrated in
vacuo and the residue partitioned between CH Cl (200 mL) and H O
3
3
3 3
+
MS (EI): m/z (%) = 206 (M , 40).
2
2
2
2
,6-Diformyl-4-methylphenol (2b):
(200 mL). The phases were separated, and the aqueous phase was
This preparation closely followed the preparation of 2a, starting from
p-cresol (4.03 g, 37.3 mmol) and hexamethylenetetramine (10.59 g,
acidified with conc. aq HCl (50 mL) and extracted with CH Cl2
(100 mL). The combined organic phases were washed with 4 M HCl
2
7
5.5 mmol) in anhyd TFA (50 mL). The product was purified by col-
(200 mL), sat. aq NaHCO (200 mL) water (200 mL), and then dried
3
umn chromatography (silica gel, CH Cl ) to give 2b as yellow nee-
(Na SO ). The solvent was evaporated in vacuo, and the residue pu-
rified by column chromatography (silica gel) to give the product after
evaporation of the solvent.
2
2
2
4
9
20
dles; yield: 4.15 g (68%); mp 130°C (lit. mp 132–133°C, lit. mp
30°C).
1
1
H NMR (CDCl ): δ = 11.43 (s, 1H, -OH), 10.18 (s, 2H, -CHO), 7.73
3
(
s, 2H, Ar-H), 2.35 (s, 3H, -CH ).
5-tert-Butyl-2-methoxybenzaldehyde (5):
3
13
C NMR (CDCl ): δ = 192.20 (-CHO), 161.72 (arom. C-1), 137.96
Prepared from 4-tert-butylanisole (4.80 g, 29.2 mmol), hexamethy-
lenetetramine (9.35 g, 66.7 mmol) and TFA (60 mL) to give crude 5
as a yellow oil after aqueous workup. Purification by column chroma-
tography (silica gel, CH Cl /petroleum ether 1:1 to 3:1) gave 5 as a
3
(arom. C-3,5), 129.48 (arom. C-4), 122.85 (arom. C-2,6), 20.05
(-CH3).
+
MS (EI): m/z (%) = 164 (M , 100).
2
2
21
slightly yellow oil; yield: 3.90 g (70%).
H NMR (CDCl ): δ = 10.44 (s, 1H, -CHO), 7.82 (d, 1H, J = 3 Hz,
1
2
,6-Diformyl-4-methoxyphenol (2c):
This preparation closely followed the preparation of 2a, starting from
-methoxyphenol (5.243 g, 42.2 mmol) and hexamethylenetetramine
11.95 g, 85.1 mmol) in anhyd TFA (60 mL). The product was puri-
3
Ar-H6), 7.57 (dd, 1H, J = 3 Hz, J = 9 Hz, Ar-H4), 6.91 (d, 1H, J =
1
2
4
(
9 Hz, Ar-H3), 3.89 (s, 3H, -OCH ), 1.29 [s, 9H, -C(CH ) ].
3 3 3
C NMR (CDCl ): δ = 190.14 (-CHO), 159.85 (arom. C-2), 143.46
3
13
fied by chromatography (short column, silica gel, gradient from
(arom. C-5), 133.14 (arom. C-6), 124.97 (arom. C-4), 124.05 (arom.
C-1), 111.33 (arom. C-3), 55.64 (-OCH ), 34.17 [-C(CH ) ], 31.26
CH Cl to CH Cl /MeOH 40:1) and evaporated to give 2c as yellow
2
2
2
2
3
3 3
9
needles; yield: 3.22 g (42%); mp 135–136°C (lit. mp 137–139°C).
H NMR (CDCl ): δ = 11.10 (s, 1H, -OH), 10.19 (s, 2H, -CHO), 7.48
[-C(CH ) ].
3 3
1
+
3
MS (EI): m/z (%) = 192 (M , 42).
(
s, 2H, Ar-H), 3.83 (s, 3H, -OCH ).
3
1
3
C NMR (CDCl ): δ = 191.84 (-CHO), 157.96 (arom. C-1), 152.60
1,2-Bis(4-tert-butyl-2-formylphenoxy)ethane (7):
3
Prepared from 1,2-bis(4-tert-butylphenoxy)ethane22 (13.05 g,
40.0 mmol), hexamethylenetetramine (11.23 g, 80.11 mmol) and
TFA (120 mL) to give crude 7 as a yellow oil after aqueous workup.
(
arom. C-4), 123.49 (arom. C-2,6), 122.43 (arom. C-3,5), 56.14
(-OCH3).
+
MS (EI): m/z (%) = 180 (M , 100).

SYNTHESIS
1
032
Papers
Purification by passage through a short silica gel column (CH Cl )
followed by evaporation of the solvent gave 7 as a pale yellow oil,
which solidified upon standing to give a white waxy solid; yield: 8.93
g (58%).
(6) Brychy, K.; Dräger, K.; Jens, K.-J.; Tilset, M.; Behrens, U.
Chem. Ber. 1994, 127, 465.
(7) Gelling, O. J.; Feringa, B. L. J. Am. Chem. Soc. 1990, 112,
7599.
2
2
(
8) Drago, R. S.; Desmond, M. J.; Corden, B. B.; Miller K. A.
J. Am. Chem. Soc. 1983, 105, 2287.
C H O
4
calcd
found
C
75.36
75.26
H
7.91
8.04
24
30
(382.50)
(9) Hu, Y.; Hu, H. Synthesis 1991, 325.
1
H NMR (CDCl ): δ = 10.40 (s, 2H, -CHO), 7.84 (d, 2H, J = 3 Hz,
3
(
(
10) Raju, B.; Krishna Rao, G. S. Synthesis 1985, 779.
11) Zondervan, C.; van den Beuken, E. K.; Kooijman, H.; Spek, A.
L.; Feringa, B. L. Tetrahedron Lett. 1997, 38, 3111.
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13) Wynberg, H. In Comprehensive Organic Synthesis, Vol. 2;
Trost, B. M., Ed-in-chief; Heathcock, C. H., Ed.; Pergamon
Press: Oxford, 1991; pp 769–775.
Ar-H3), 7.58 (dd, 2H, J = 3 Hz, J = 9 Hz, Ar-H5), 6.88 (d, 2H, J =
1
2
9
Hz, Ar-H6), 4.48 (s, 4H, -OCH -), 1.30 [s, 18H, -C(CH ) ].
C NMR (CDCl ): δ = 189.69 (-CHO), 158.79 (arom. C-1), 144.38
2 3 3
1
3
3
(
(
(arom. C-4), 133.11 (arom. C-3), 125.08 (arom. C-5), 124.52 (arom.
C-2), 112.53 (arom. C-6), 67.16 (-OCH CH O-), 34.28 [-C(CH ) ],
2
2
3 3
3
1.26 [-C(CH ) ].
3 3
+
MS (PD-MS): m/z (%) = 382 (M , 100).
Wynberg, H.; Meijer, E. W. Org. React. 1982, 28, 2.
14) Duff, J. C. J. Chem. Soc. 1941, 547.
15) See however Thoer, A.; Denis, G.; Delmas, M.; Gaset, A. Synth.
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Financial support from the Australian Research Council and the
Danish National Science Research Council (to N. S.) is gratefully
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(
(
(
(
(
16) Olah, G. A. Friedel Crafts Chemistry; Wiley: New York, 1973.
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18) The diformylation of 2-tert-butylphenol in a modified Duff re-
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Chem. 1994, 59, 1939.
(
1) Visiting Ph.D. student from the Department of Chemistry, Uni-
versity of Odense, Campusvej 55, DK-5230 Odense M, Den-
mark.
(
(
(
2) Stetter, H. in Houben-Weyl, 4th ed., Vol. E3; Falbe, J., Ed.;
Thieme: Stuttgart, 1982; pp 2–608.
(19) Kerr, J. M.; Suckling, C. J.; Bamfield, P. J. Chem. Soc., Perkin
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Matthews, K. D.; Kahwa, I. A.; Williams, D. J. Inorg. Chem.
Trans. 1 1990, 887.
(
(
20) Taniguchi, S. Bull. Chem. Soc. Jpn. 1984, 57, 2683.
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1951, 16, 586.
(22) Bradshaw, J. S.; Koyama, H.; Dalley, N. K.; Izatt, R. M.; Bier-
1
994, 33, 1382.
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3, 1071.
nat, J. F.; Bochenska, M. J. Heterocycl. Chem. 1987, 24, 1077.
(
3