1,6-Hexanediamine methanesulfonate: A mild and efficient catalyst for the tetrahydropyranylation of alcohols under solvent-free conditions

Article abstract of DOI:10.1080/10826068.2010.488491

Various alcohols react with 3,4-dihydro-2 H-pyran under mild conditions using a catalytic amount of 1,6-hexanediamine methanesulfonate. It affords the corresponding tetrahydropyranyl ethers in good yields at a faster rate in the absence of solvent. Taylor & Francis Group, LLC.

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1,6-HEXANEDIAMINE
METHANESULFONATE: A MILD AND
EFFICIENT CATALYST FOR THE
TETRAHYDROPYRANYLATION OF
ALCOHOLS UNDER SOLVENT-FREE
CONDITIONS
Rui Wang a & Heng Jiang a
a School of Chemistry and Materials Science, Liaoning Shihua
University , Fushun Liaoning, P.R. China
Published online: 08 Jul 2010.
To cite this article: Rui Wang & Heng Jiang (2010) 1,6-HEXANEDIAMINE METHANESULFONATE:
A MILD AND EFFICIENT CATALYST FOR THE TETRAHYDROPYRANYLATION OF ALCOHOLS UNDER
SOLVENT-FREE CONDITIONS, Preparative Biochemistry and Biotechnology, 40:3, 171-176, DOI:
10.1080/10826068.2010.488491
To link to this article: http://dx.doi.org/10.1080/10826068.2010.488491
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Preparative Biochemistry & Biotechnology, 40:171–176, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1082-6068 print/1532-2297 online
DOI: 10.1080/10826068.2010.488491
1,6-HEXANEDIAMINE METHANESULFONATE: A MILD AND
EFFICIENT CATALYST FOR THE TETRAHYDROPYRANYLATION
OF ALCOHOLS UNDER SOLVENT-FREE CONDITIONS
Rui Wang and Heng Jiang
School of Chemistry and Materials Science, Liaoning Shihua University, Fushun
Liaoning, P.R. China
& Various alcohols react with 3,4-dihydro-2H-pyran under mild conditions using a
catalytic amount of 1,6-hexanediamine methanesulfonate. It affords the corresponding
tetrahydropyranyl ethers in good yields at a faster rate in the absence of solvent.
Keywords alcohols, catalyst, 1,6-hexanediamine methanesulfonate, protecting group,
solvent-free, tetrahydropyranylation
INTRODUCTION
The importance of selective introduction and removal of protecting
groups in synthetic organic chemistry is well established. Tetrahydropyrany-
lation is one of the most frequently used processes to protect hydroxyl
groups, due to the remarkable stability of tetrahydropyranyl (THP) ethers
under a variety of conditions such as alkaline media, Grignard reagents,
alkyllithiums, metal hydrides, oxidative reagents, and alkylating and acylat-
ing reagents.[1] A number of catalysts have been used for this conversion,
such as protic acids,[2,3] Lewis acids,[4] ion exchange resins,[5] K5CoW12O4 Á
3H2O,[6] PdCl2(CH3CN)2,[7] solid silica-based sulfonic acid,[8] La(NO3)3 Á
6H2O,[9] ionic liquids,[10] zeolites,[11] Fe(NO3)3 Á 9H2O=NaI,[12] and
Cu(NO3)2 Á 3H2O=HOAc,[13] as well as miscellaneous catalysts. Although
these methods are suitable for many synthetic conditions, there are still sev-
eral drawbacks, which include long reaction time, refluxing temperature,
harsh and strongly acidic conditions, complicated procedure for the prep-
aration of catalyst, the presence of some solvents such as CH2Cl2, and also
Address correspondence to Heng Jiang, School of Chemistry and Materials Science, Liaoning
Shihua University, Fushun Liaoning 113001, P.R. China. E-mail: hjiang78@hotmail.com

172
R. Wang and H. Jiang
SCHEME 1 Tetrahydropyranylation of alcohols catalyzed by 1,6-hexanediamine methanesulfonate.
unsatisfactory product yields. So it is interesting to develop new catalysts
with great efficiency, convenient procedures, and better yields.
Methanesulfonic acid is a strong acid that has been widely used as
polymer solvents and various syntheses (e.g., esterification, condensation
and alkylation)[14–16] catalysts. Methanesulfonic acid has a low tendency to
oxidize organic compounds and is far less corrosive and toxic than the usual
mineral acids. Moreover, it is readily biodegradable, ultimately forming car-
bon dioxide and sulfate, making it an environmentally friendly material.[17]
Based on this idea, methanesulfonic acid has been applied in catalyzing tet-
rahydropyranylation of alcohols. However, in this work, methanesulfonic
acid is found, like many strongly acidic catalysts such as Nafion-H[18], to
produce polymeric by-products of 3,4-dihydro-2H-pyran (DHP). The yield
of the corresponding THP ether is very low and most of alcohol remains
unreacted. Sulfonates-mediated Lewis acid catalysis has been reported as
excellent and effective to perform tetrahydropyranylation under mild con-
ditions.[19,20] However, the reaction was carried out using acetic acid-assisted
sulfonate as a synergistic catalyst. This is ‘‘Brønsted-assisted Lewis acid
catalysis.’’[19,20] Now, we wish to report tetrahydropyranylation catalyzed by
1,6-hexanediamine methanesulfonate, which acted as a Brønsted acid with
high activity and selectivity under mild conditions (Scheme 1).
EXPERIMENTAL
General
Gas chromatography (GC) analysis was carried out with a Perkin Elmer
Auto System XL gas chromatograph. Infrared (IR) spectra were recorded
on a Spectrum GX series Fourier-transform instrument (Perkin Elmer).
1H-Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker
AVANCE 600 spectrometer in CDCl3 using tetramethylsilane (TMS) as an
internal standard.
Typical Procedure for Preparation of 1,6-Hexanediamine
Methanesulfonate
Methanesulfonic acid (0.1 mol, 9.6 g) was added to 1,6-hexanediamine
(0.05 mol, 5.8 g) in a mortar and was ground with a pestle for 0.5 hr at room

Catalyst for Tetrahydropyranylation of Alcohols
173
temperature. After completion of the reaction, alcohol was added and the
solid was filtered off and dried in an oven at 353 K.
General Procedure for the Tetrahydropyranylation of
Alcohols (2p)
A solution of benzyl alcohol (2.1 g, 20 mmol) and 3,4-dihydro-2H-pyran
(2.0 g, 24 mmol, 1.2 equiv.) were stirred as 1,6-hexanediamine methanesul-
fonate (2.4 g, 0.8 mmol) was added at room temperature. After an appro-
priate time (monitored by GC), the reaction mixture was diluted with
benzene (10 mL). The catalyst was removed by filtration and washed with
benzene (2 Â 10 mL). The filtrate was washed with 1 M NaOH, dried
(anhydrous Na2SO4), and evaporated. The product was purified further
by column chromatography on silica gel and the corresponding THP ether
of benzyl alcohol was obtained in 93% yield. The THP ethers prepared
are known compounds[21–26] and were characterized by comparison with
1
authentic samples by means of IR and H-NMR.
RESULTS AND DISCUSSION
Our initial efforts focused on identifying the optimum conditions
for the tetrahydropyranylation of benzyl alcohol as model substrate in
the presence of various solvents and solvent-free at room temperature.
The results are shown in Table 1. The yields of the reaction under solvent-
free conditions are higher and the reaction time is shorter.
Under optimal condition, alcohols in a series were easily converted
to THP ethers by the treatment of DHP with catalytic amounts of 1,6-
hexanediamine methanesulfonate at room temperature (Table 2). Primary,
secondary, and tertiary alcohols, benzyl alcohol (Entry 2p), cyclic saturated
alcohol (Entry 2r), furfuryl alcohol (Entry 2s), and optically active alcohol
(–)-menthol (Entry 2t) undergo facile tetrahydropyranylation to form THP
ethers in high yields. No polymeric by-product of DHP was observed,
TABLE 1 Comparative Conversion Study of Tetrahydropyranylation of Benzyl Alcohol Under
Different Conditions in the Presence of 1,6-Hexanediamine Methanesulfonate
Entry
Solventa
Time (h)
Yieldb (%)
1a
1b
1c
1d
1e
Dichloromethane
Acetonitrile
Benzene
Tetrahydrofuran
None
0.8
1
1
0.5
0.2
90
84
90
88
93
aThe reaction was carried out in 10 mL of solvent.
bYields based on the isolated products.

174
R. Wang and H. Jiang
TABLE 2 Tetrahydropyranylation of Alcohols Catalyzed by 1,6-Hexanediamine Methanesulfonate
Under Solvent-Free Conditions
Entry
Alcohols
Cat: DHP (mmol: mmol)
Time (h)
Yield (%)a
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
2s
CH3OH
C2H5OH
n-C3H7OH
i-C3H7OH
n-C4H9OH
s-C4H9OH
t-C4H9OH
i-C4H9OH
n-C5H11OH
(CH3)2CHCH2CH2OH
n-C6H13OH
n-C8H17OH
i-C8H17OH
n-C12H25OH
n-C16H33OH
PhCH2OH
PhCH2CH2OH
c-C6H11OH
Furfuryl alcohol
(–)-menthol
0.8:24
0.8:24
0.8:24
1.3:24
1.3:24
0.8:24
0.8:24
1.3:24
1.3:24
1.3:24
1.3:24
1.3:24
0.8:24
0.8:24
0.8:24
0.8:24
0.8:24
2:24
0.2
0.2
0.7
0.8
0.8
8
95
96
90
89
87
74
80
92
86
90
89
86
90
86
83
5
0.3
0.8
0.8
1
1.2
1
1.5
4
0.2
0.2
3
93, 92, 90, 87
95
80
86
88
0.8:24
0.8:24
0.7
0.7
2t
aYields based on the isolated pure products and confirmed by comparison with authentic samples
(GC, IR, and 1H NMR).
and the THP ether was isolated as the only product. The recovered 1,6-
hexanediamine methanesulfonate could be used directly in next run
without any treatment. Even if 1,6-hexanediamine methanesulfonate has
been reused four times, no considerable activity loss was observed in the
tetrahydropyranylation of benzyl alcohol (Entry 2p).
CONCLUSIONS
In conclusion, this work shows that 1,6-hexanediamine methanesulfo-
nate, prepared with relatively cheap raw materials with a simple operation,
efficiently catalyzes the tetrahydropyranylation of a variety of alcohols.
Solvent-free condition, very short reaction time, and highly active and
environmentally benign catalysts offer improvements over many existing
methods. Work on other applications of the catalysts to other types of trans-
formations is ongoing in our laboratories.
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