B.L.A. Prabhavathi Devi et al. / Journal of Molecular Catalysis A: Chemical 345 (2011) 96–100
97
◦
from ambient temperature to 250 C for 20 min to facilitate in situ
partial carbonization and sulfonation under static air condition at
atmospheric pressure without any nitrogen/air flow. The reaction
mixture was allowed at that temperature for about 10 min till the
foaming ceased to obtain the carbon compound. The resultant prod-
uct was cooled to ambient temperature and washed with hot water
under agitation till the wash water became neutral to pH. The par-
Catalyst, DCM RT
,
ROH
+
RO
Catalyst, MeOH, RT
O
O
R = alky, aryl
Scheme 1. Carbon-based solid acid catalyzed tetrahydropyranylation and
depyranylation of alcohols and phenols.
◦
tially crystalline product was filtered and dried in the oven at 120 C
for 2 h till it was moisture-free to obtain carbon catalyst (4.0 g).
protective groups of choice in peptide, nucleotide, carbohydrate
and steroid chemistry [15–17]. This transformation has been gen-
erally achieved using both Brønsted acid as well as Lewis acid
catalysts [18–20]. Recently the tetrahydropyranylation process
2.2. General procedure for tetrahydropyranylation of hydroxy
compounds: tetrahydropyranylation of 1-decanol (Table 1, Entry
1)
(
THP ether synthesis) has been performed under environmen-
tally friendly conditions with concomitant minimum purification
requirements [21–37]. Deprotection of THP ethers for the regen-
eration of hydroxyl compounds usually entails rather harsh acidic
conditions, which are rarely compatible with sensitive substrates
A mixture of 1-decanol (0.790 g, 5 mmol), DHP (0.420 g, 5 mmol)
and carbon catalyst (0.080 g, 10 wt% of substrate) was stirred in
dichloromethane (5 mL) at room temperature for 2 h. The progress
of the reaction was monitored by TLC (eluant, n-hexane/ethyl
acetate, 9:1). After completion of the reaction, the catalyst was fil-
tered out and the product was passed through a column of silica gel
eluting with hexane/ethyl acetate (9:1) to afford pure 2-decyloxy-
tetrahydro-pyran (1.18 g, 98% yield). IR 2927, 2856, 1462, 1075,
[
38,39]. Heterogeneous catalysts [40] and ion-exchange resins like
amberlyst, nafion and dowex [41–44], that are used extensively in
protection/deprotection processes in organic synthesis, are of con-
siderable help in this direction. Taking into account the increasing
demand for new and cleaner chemical processes, here we report a
novel carbon-based solid acid catalyzed methodology for effective
tetrahydropyranylation of alcohols and phenols and their deprotec-
tion (Scheme 1) at room temperature by changing solvent medium
from dichloromethane to methanol.
−
1 1
1031 cm
; H NMR ı 0.88 (3H, t); 1.25–1.90 (22H, m); 3.30–3.35
(1H, m), 3.4–3.5 (1H, m), 3.62–3.7 (1H, m), 3.75–3.8 (1H, m); 4.55
+
(1H, br, s); GC–MS: m/z [M ] 242. The recovered catalyst was
◦
washed with water, dried at 120 C in oven and reused for eight
cycles of THP protection reaction of 1-decanol for determining its
stability and activity. Control reaction of tetrahydropyranylation of
1-decanol was carried out employing glycerol-based sulfonic acid
functionalized carbon catalyst [13] for comparing the activity with
glycerol pitch based catalyst. In both the cases the reaction was
found to be completed within 2 h resulting the product in 95% yield.
2
. Experimental
Chemicals were purchased from S.d. Fine Chemicals Ltd., Mum-
bai, India. All other reagents and solvents used were of analytical
grade. Elemental composition of the sulfonic acid functionalized
carbon catalyst was estimated using the equipment Elementar
2.3. 2-(4-Methoxy benzyloxy)-tetrahydro-pyran (Table 1, Entry
10)
(
Model; Vario EL). TG/DTA was carried out on a Mettler-Toledo 851E
instrument. XRD pattern was recorded on a Rigaku, D-5000 diffrac-
tometer, using Ni-filtered Cu-K( radiation (ꢀ = 1.5405 A˚ ). X-Ray
IR 2943, 1248, 1030, 756 cm 1; 1H NMR ı 1.05–1.90 (6H, m);
−
Photoelectron Spectroscopy (XPS) measurements were conducted
with a KRATOS AXIX 165 with a DUAL anode (Mg and Al) apparatus
using the MgK( anode. The specific surface area of the catalyst was
3.42–3.55 (1H, m); 3.77 (3H, s); 3.80–3.90 (1H, m); 4.35–4.40 (2H,
m); 4.60 (1H, br, s); 6.8–7.25 (4H, m); GC–MS: m/z [M ] 222.
+
◦
estimated using N2 adsorption at −196 C by the single point BET
2.4. General procedure for deprotection of THP ethers
13
method using Quantachrome Antosorb-1. C Magic angle spin-
ning (MAS) nuclear magnetic resonance (NMR) spectra of solid
catalyst was recorded on a Bruker Spectrometer. The acidity of
the carbon catalyst was measured by the potentiometric titration
method using an automatic titrator, Schott GmbH, Germany. The
acid strength of the catalyst was measured by employing a satu-
rated calomel electrode. Catalyst (about 0.1 g) was suspended in
acetonitrile and stirred for about 3 h. Later, the suspension was
titrated with a solution of 0.05 N n-butyl amine in acetonitrile at a
flow rate of 0.5 mL/min. The variation in electrode potential, while
adding the n-butyl amine solution, was recorded using a double
THP ether (5 mmol) and carbon catalyst (10 wt% of substrate)
were stirred in methanol (5 mL) at room temperature for 30 min.
The cleavage of THP ethers was monitored by TLC (eluant, n-
hexane/ethyl acetate, 9:1). After completion of the reaction, the
catalyst was filtered and washed with methanol. The filtrate was
concentrated under reduced pressure to obtain the correspond-
ing pure hydroxy compound in quantitative yield. The recovered
catalyst was washed with water, dried at 120 C in an oven and
reused for eight cycles of reaction for determining its stability and
activity.
◦
1
junction electrode. H NMR spectra were recorded on a 300 (Varian,
Palo Alto, USA) spectrometer in CDCl3 solution with tetramethyl-
silane as the internal standard. Chemical shift values (ı) are given
in parts per million. GC-MS spectra were recorded on Agilent 5973
GC–mass spectrometer (Agilent, USA) in the EI mode and are given
in mass units (m\z). IR spectra were recorded on a Perkin Elmer
3. Results and discussion
3.1. Preparation and characterization of sulfonic acid
functionalized carbon catalyst from glycerol pitch
(
model: spectrum BX) FT-IR Spectrometer using CH Cl or KBr.
Sulfonic acid functionalized polycyclic aromatic carbon catalyst
was prepared by employing in situ partial carbonization and sul-
fonation protocol by heating the mixture of glycerol pitch (procured
from a local industry) and sulfuric acid in the ratio of 1:4 (wt/vol)
2
2
2.1. General procedure for the preparation of sulfonated carbon
catalyst from glycerol pitch
◦
to 250 C for 20 min. The yield of the carbon catalyst obtained was
The hard mass of glycerol pitch obtained from a local fat splitting
industry was used as such without any purification. A mixture of
glycerol pitch (10 g) and conc. sulfuric acid (40 g) was gently heated
about 40% by the weight of glycerol pitch taken for the reaction.
A thorough characterization of the carbon catalyst was carried out
to establish its physico-chemical characteristics. The catalyst was