Tetrahedron Letters
Aniline–terephthalaldehyde resin p-toluenesulfonic acid (ATRT) salt
as efficient mild polymeric solid acid catalyst
⇑
Kiyoshi Tanemura , Tsuneo Suzuki
School of Life Dentistry at Niigata, Nippon Dental University, Hamaura-cho, Niigata 951-8580, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 July 2013
Revised 24 September 2013
Accepted 30 September 2013
Available online 5 October 2013
Aniline–terephthalaldehyde resin p-toluenesulfonic acid (ATRT) salt was easily prepared by the reaction
of aniline with 1.25 equiv of terephthalaldehyde in the presence of 1.0 equiv of p-toluenesulfonic acid at
75 °C for 24 h in EtOH. ATRT efficiently catalyzed the tetrahydropyranylation of alcohols and deprotection
of tetrahydropyranyl (THP), triethylsilyl (TES), and tert-butyldimethylsilyl (TBDMS) ethers. Deprotection
of dodecyl THP ether and dodecyl TBDMS ether catalyzed by ATRT proceeded faster than those by pyrid-
inium p-toluenesulfonate (PPTS). ATRT was reused without significant loss of activities.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Solid acids
Tetrahydropyranylation
Aniline–aldehyde resin
Deprotection
Silyl ethers
Use of solid acid catalysts in organic syntheses and industrial
purposes provides some merits such as simple work-up procedure,
repeated use of the catalyst, and less corrosion of the reaction ves-
sel. A number of strongly acidic resin catalysts such as Amberlyst
alcohols and deprotection of tetrahydropyranyl (THP) and silyl
ethers catalyzed by ATRT are described.
First, we prepared aniline–formaldehyde resin p-toluenesul-
fonic acid (AFRT) salt. AN was treated with various amounts of for-
malin in the presence of 1.0 equiv of TsOH at room temperature in
water. Since the produced solids were soluble to hot DMSO and
DMF, the resins were post-cured at 160 °C for 6 h. The results are
summarized in Table 1. Acid density of AFRT was determined by
acid titration. When the ratio of AN:FA is 1.0:1.5, tetrahydropyr-
5, Dowex 50 series, Nafion NR50,3 and Nafion SAC-13 have
1
2
4
1
5
,6
been developed. These solid acids have limitations when applied
to compounds involving acid-sensitive groups. On the other hand,
a survey of literature revealed that commercially available weakly
acidic resin catalysts seem to be limited to a few examples such as
7
poly(4-vinylpyridinium p-toluenesulfonate) (polyPPTS)
and
anylation of 1-dodecanol at room temperature for 8 h in CH
3
CN
8
Poly(4-vinylpyridinium chloride) (polyPCL). In addition, these cat-
alysts are expensive and their frequency of use has been limited.
Aniline–aldehyde resins are one of the important synthetic
gave dodecyl THP ether (1b) in 91% yield. However, AFRT was par-
1
1
tially soluble in hot DMSO and DMF (entry 1). When the ratio of
AN:FA is 1.0:3.0, the yield of THP ether 1b decreased (10%)
although it was insoluble in hot DMSO and DMF (entry 4).
We examined the formation of aniline–benzaldehyde resin p-
toluenesulfonic acid (ABRT) salt. However, the reaction of AN with
1.5 equiv of benzaldehyde in the presence of 1.0 equiv of TsOH at
75 °C for 24 h in EtOH did not give the solid.
9
polymers. Formaldehyde (FA), furfural, and benzaldehyde have
been employed as aldehyde components. For example, aniline–
formaldehyde resins have been prepared by the treatment of
aniline (AN) with formalin in the presence of hydrochloric acid
10
followed by neutralization with NaOH. They are superior to
elasticity, impact resistance, and electric insulation, and have been
employed for electrical insulators.9
Next, we conducted the synthesis of ATRT. AN was treated with
various amounts of TPA in the presence of 1.0 equiv of TsOH at
75 °C for 24 h in EtOH. The results are shown in Table 2. Soft mate-
rial was obtained from the case of AN:TPA = 1.0:0.75. When the ra-
tio of AN:TPA is 1.0:1.0, THP ether 1b was obtained in 97% yield
after 2.5 h, but the obtained resin was somewhat wet (entry 1).
When the ratio of AN:TPA is 1.0:1.25, THP ether 1b was obtained
We found that aniline–terephthalaldehyde resin p-toluenesul-
fonic acid (ATRT) salt acted as a mild polymeric solid acid catalyst.
The resin can be synthesized from the condensation of AN with
terephthalaldehyde (TPA) in the presence of p-toluenesulfonic acid
(
TsOH) by only one step. In this Letter, we wish to report the results
1
2
for the synthesis of ATRT. In addition, tetrahydropyranylation of
in 98% yield after 2.5 h in CH
AN:TPA = 1.0:1.25) for the reactions described below. ATRT was
quite insoluble to hot water and hot organic solvents such as hex-
ane, benzene, CH Cl , CHCl , THF, acetone, CH CN, MeOH, DMSO,
3
CN (entry 2). We employed ATRT
(
⇑
2
2
3
3