Received: February 25, 2015 | Accepted: March 23, 2015 | Web Released: April 3, 2015
CL-150166
Highly Efficient Acetalization of Carbonyl Compounds Catalyzed by Aniline-Aldehyde Resin Salts
Kiyoshi Tanemura* and Tsuneo Suzuki
Chemical Laboratory, School of Life Dentistry at Niigata, Nippon Dental University, Hamaura-cho, Niigata 951-8580
(E-mail: tanemura@ngt.ndu.ac.jp)
A mild procedures for the syntheses of ethylene acetals and
dimethyl acetals from the corresponding aldehydes and ketones
catalyzed by 1 mol % of aniline-aldehyde resin salts are
described. This method is also useful for the synthesis of
dimethyl acetals of diaryl ketones.
Various aldehydes and ketones reacted with EG at reflux
temperature in toluene, catalyzed by DTRT (H), to give the
corresponding ethylene acetals in good to excellent yields
(Table 3). 1 mol % of the catalyst was sufficient to conduct the
reactions. The reactions of ketones were slower than those of
aldehydes. α,β-Unsaturated ketones 9a and 10a, and acetophe-
nones 11a-13a reacted more slowly than saturated ketones 6a-
8a. Especially, compounds 2a-4a, 9a, and 10a were cleanly
converted into the corresponding acetals without concomitant
double bond migration.2a,2b The reactions catalyzed by ATRT
proceeded more slowly than those catalyze by DTRT (H) (Entries
2, 7, 9, and 11).
Scheme 1 illustrates the results of acetalization of the acid-
sensitive Wieland-Miescher Ketone 15, which possesses both
saturated and α,β-unsaturated ketones.1c,2b The saturated ketone
moiety was acetalized predominantly without double bond
migration to give monoacetal 16 (83%) and bisacetal 17 (11%).
Based on the difference in reactivities between aldehydes and
ketones, we examined the chemoselective acetalization of alde-
hydes in the presence of ketones. Undecanal (1a) and p-
chlorobenzaldehyde (5a) were converted into the corresponding
acetals predominantly in the presence of 2-nonanone (6a) and p-
chloroacetophenone (12a), respectively (Scheme 2).
Acetals are employed for the protection of carbonyl groups
as well as synthetic intermediates and final products in organic
synthesis and industry. A large number of procedures using
protonic acids,1 Lewis acids,2 and other reagents3 have been
reported. Pyridinium p-toluenesulfonate (PPTS) is a commonly
used mild and general catalyst for acetalization.1a Recently, some
synthetic methods utilizing solid acids have been developed from
the ease of the work-up procedure and the environmental point
of view.4 Although poly(4-vinylpyridinium p-toluenesulfonate)
(polyPPTS)5 and poly(4-vinylpyridinium chloride) (polyPCL)4n,6
have been devised, these catalysts are expensive.
We have reported the syntheses of aniline-terephthalalde-
hyde resin p-toluenesulfonate (ATRT)7 and a more hydrophobic
resin salt diphenylamine-terephthalaldehyde resin p-toluene-
sulfonate with high sulfo group content [DTRT (H)].8 Preparation
of these resin salts is so simple. Especially, ATRT can be
synthesized by only one step.
Next, acetalization of compound 12a with methyl orthofor-
mate in MeOH at room temperature for 1.5 h was examined using
1 mol % of various acidic catalysts (Table 4). Polymeric solid
acids such as DTRT (H), ATRT, polyPPTS, and polyPCL
possessed considerably higher activities than PPTS. The reaction
promoted by ATRT proceeded slightly slower than those by
polyPPTS and polyPCL.
In this paper, we wish to report that ATRT and DTRT (H) are
mild solid acid catalysts for the acetalization of a wide range of
aldehydes and ketones. In addition, ATRT was more active than
PPTS for dimethyl acetalization, and was useful for the synthesis
of the dimethyl acetals of diaryl ketones, which were difficult to
prepare.
First, we examined the acetalization of 4-tert-butylcyclohex-
anone (7a) as the model compound with ethylene glycol (EG) at
reflux temperature for 1 h in toluene using 1 mol % of various
acidic catalysts. The activity of DTRT (H) was the highest among
the examined resin salts such as polyPPTS and polyPCL, which
was comparable to that of PPTS (Table 1).
The results of recycle experiments of 12a catalyzed by
5 mol % of the catalysts are given in Table 5. The yields
decreased gradually in the case of DTRT (H) because the catalyst
Table 2. Recycle experiments for acetalization of carbonyl com-
pounds with ethylene glycol in toluene
Recycle experiments of 7a using 5 mol % of DTRT (H) and
1 mol % of ATRT were carried out. We employed 5 mol % of
DTRT (H) because it was gradually decomposed by EG8 when
1 mol % of DTRT (H) was used. These catalysts could be used
repeatedly as shown in Table 2.
Yields/%a
Entry
Catalyst
1st
2nd
3rd
4th
5th
1b
2c
DTRT (H)
ATRT
93
93
91
98
88
98
88
98
86
96
aIsolated yields. b7a (10 mmol), EG (11 mmol), DTRT (H) (427 mg,
5 mol %), toluene (50 mL), reflux, 1 h. 7a (10 mmol), EG (30 mmol),
c
Table 1. Acetalization of compound 7a with ethylene glycol in
toluene using various acidic catalystsa
ATRT (99 mg, 1 mol %), toluene (50 mL), reflux, 6 h.
Entry
Catalyst
Yield/%b
O
1
2
3
4
5
DTRT (H)
ATRT
PolyPPTS
PolyPCL
PPTS
80
31
62
8
DTRT (H)
O
O
O
O
1.7 equiv EG
+
+
15
O
hexane
reflux, 6 h
O
O
5%
O
88
15
16
17
11%
83%
a7a (10 mmol), EG (11 mmol), catalyst (1 mol %), toluene (50 mL),
reflux, 1 h. Isolated yields.
b
Scheme 1. Chemoselective acetalization of 15a.
© 2015 The Chemical Society of Japan | 797