FeCl3-catalyzed tritylation of alcohols in ionic liquids

Article abstract of DOI:10.1080/00397910902838888

A simple and efficient protection of alcohols as trityl ethers is described using trityl chloride in the presence of 5mol% FeCl3 as catalyst in ionic liquids at room temperature in shorter reaction times. This mild and efficient method gives access to the

Full text of DOI:10.1080/00397910902838888

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FeCl3-Catalyzed Tritylation of
Alcohols in Ionic Liquids
B. Sreedhar a , P. Radhika a , B. Neelima a , D. V. D.
B. Chowdary a & M. V. Basaveswara Rao b
a Inorganic and Physical Chemistry Division, Indian
Institute of Chemical Technology (Council of
Scientific and Industrial Research), Hyderabad, India
b S.C. College of Arts and Science, Chikaluripet,
Guntur, Andhra Pradesh, India
Published online: 07 Oct 2009.
To cite this article: B. Sreedhar , P. Radhika , B. Neelima , D. V. D. B. Chowdary & M.
V. Basaveswara Rao (2009): FeCl3-Catalyzed Tritylation of Alcohols in Ionic Liquids,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 39:21, 3785-3795
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Synthetic Communications1, 39: 3785–3795, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902838888
FeCl3-Catalyzed Tritylation of Alcohols
in Ionic Liquids
B. Sreedhar,1 P. Radhika,1 B. Neelima,1 D. V. D. B. Chowdary,1
and M. V. Basaveswara Rao2
1Inorganic and Physical Chemistry Division, Indian Institute of Chemical
Technology (Council of Scientific and Industrial Research),
Hyderabad, India
2S.C. College of Arts and Science, Chikaluripet, Guntur,
Andhra Pradesh, India
Abstract: A simple and efficient protection of alcohols as trityl ethers is described
using trityl chloride in the presence of 5 mol% FeCl3 as catalyst in ionic liquids at
room temperature in shorter reaction times. This mild and efficient method gives
access to the protection of a wide range of alcohols in good yields. The use of
ionic liquid allows easy separation of the product and recycling of catalyst.
Keywords: Alcohols, FeCl3, ionic liquids, recyclability, tritylation
INTRODUCTION
The hydroxyl group is the most important and versatile functional group
in organic chemistry, the masking of which is often necessary in multistep
synthetic manipulations.[1,2] Therefore, access to mild and efficient proce-
dures for the installation of the desired protecting group followed by its
removal is an important target for synthetic chemists. Triphenylmethyl
(trityl) group is part of the most common, attractive approach for the
protection of alcohols because of its stability under neutral or basic con-
ditions. Further, the trityl group can be easily removed under mild acidic
Received October 24, 2008.
Address correspondence to B. Sreedhar, Inorganic and Physical Chemistry
Division, Indian Institute of Chemical Technology, Hyderabad 500 607, India.
E-mail: sreedharb@iict.res.in
3785

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B. Sreedhar et al.
Scheme 1. FeCl3-catalyzed tritylation of alcohols in ionic liquids.
conditions using 1% trifluoroaceticacid (TFA) in dichloromethane (DCM)
to regenerate the corresponding alcohol,[3] and it can also be employed in
orthogonality with other protecting groups as well as functional groups.[4]
Traditionally, tritylation is carried out with trityl chloride in the presence of
a base.[5,6] The other methods include AgOTf–TrCl, TrODT–Tr1ATC15,
tritylated pyridones, BnOTr–DDQ, TrOTMS–TMSOTf, p-methoxyben-
zyl trityl ether or prenyl trityl ether (PTE)–DDQ, and TrOH-B(C6-
F5)3.[7–13] Most of these tritylating reagents require extensive preparation
procedures from trityl chloride. Some of them also require harsh reaction
conditions, longer times, and tedious workup procedures.[14–16] Further-
more, to the best of our knowledge, there are no reports for the trityl
protection of alcohols using a recyclable catalytic system.
In recent years, ionic liquids (ILs) have received much attention as envir-
onmentally friendly alternatives to volatile organic solvents because of their
unique properties such as low vapor pressure, wide liquid range, high thermal
stability, and highly conductivity and solvation ability for a variety of
organic substrates and catalysts including Lewis acids and enzymes.[17–20]
The use of ILs offers the combined advantages of both homogeneous and
heterogeneous catalysts. Reactions in ILs exhibit the high activity and
selectivity of a homogeneous catalyst and also allow easy separation of the
products and reusability of the catalyst, analogous to a heterogeneous
catalyst.[21,22] Iron salts are well-known Lewis acid catalysts for various
organic transformations because of their low cost, easy availability, environ-
mentally benign nature, and exceptionally high reaction rates.[23]
Encouraged by the pioneering work of Tamura and Kochi on iron
salts and our studies on ILs as recyclable media for various organic trans-
formations,[24,25] we intend to study the protection of alcohols as trityl
ethers under mild conditions in shorter reaction times using FeCl3 as
catalyst in ILs (Scheme 1).
RESULTS AND DISCUSSION
Initially, the protection of p-methyl benzyl alcohol with trityl chloride
using various Lewis acid catalysts (5 mol%) in different ILs at room
temperature was studied, and the results are presented in Table 1.

FeCl3-Catalyzed Tritylation of Alcohols
3787
Table 1. Tritylation of p-methyl benzyl alcohol with different acid catalystsa
Entry
Catalyst
Time (min)
Conv. (%)b
1
2
3
4
5
6
7
8
9
10
HAuCl4
AlCl3
CuCl2
FeCl3
MgCl2
ZnCl2
FeCl3
FeCl3
FeCl3
––
135
210
180
35
210
240
240
40
60
94
98
90
—
—
c
—
90d
50e
135
240
f
—
aReaction conditions: catalyst (5 mol%), [bmim]BF4 (1 mL), p-methyl benzyl
alcohol (1 mmol), and trityl chloride (1 mmol) at room temperature.
1
bConversions determined by H NMR.
cUsing [bmim]Cl and [bmim]OTf in place of [bmim]BF4.
dUsing [bmim]PF6.
eCH2Cl2 was used as solvent in place of [bmim]BF4.
fNo reaction in the absence of catalyst.
Among the different Lewis acids screened, HAuCl4 gave the corre-
sponding trityl ethers in moderate yield (entry 1), whereas AlCl3, CuCl2,
and FeCl3 displayed better activity with the formation of trityl ethers in
good yields (entries 2–4). However, the reaction using AlCl3 and CuCl2 as
catalysts in [bmim]BF4 required longer reaction times when compared to
the reaction using FeCl3. On the other hand, other Lewis acids such as
MgCl2 and ZnCl2 were ineffective for the reaction (entries 5 and 6).
Further efforts were focused on the use of different ILs for the reaction.
Three different imidazolium-based ILs, [bmim]Cl, [bmim]OTf, and
[bmim]PF6, were tested using FeCl3 as catalyst. The reaction in ILs with
chloride and triflate anions, [bmim]Cl and [bmim]OTf, gave no product,
whereas [bmim]PF6 gave conversions comparable to that of [bmim]BF4
(entries 7 and 8). Therefore, changing the anion also has an influence
on the reaction. To ascertain the superior performance of ILs over

3788
B. Sreedhar et al.
organic solvents, experiments were conducted using FeCl3 as the catalyst
separately in DCM (entry 9). It is interesting to note that the correspond-
ing trityl ether was obtained in poor yield. Therefore, the reaction was
significantly faster and more efficient in ILs. However, there was no reac-
tion in the absence of catalyst (entry 10).
By employing the optimized reaction conditions, a variety of
alcohols have been subjected to trityl protection. The tritylation is quite
simple, fast, and efficient, with the formation of corresponding trityl
ethers in good yields. The reaction of electronically and structurally
diverse aliphatic primary benzylic alcohols such as p-fluoro, p-chloro,
p-methyl, and p-methylthio benzyl alcohols with trityl chloride gave
the desired products in quantitative yields (entries 1–5). A lignin-like
structure containing 2,5-dimethoxy benzyl alcohol could be easily pro-
tected into corresponding trityl ethers in 95% yield, whereas 3-phenoxy
benzyl alcohol afforded the product in 55% yield (entries 6 and 7). It
should be noted that the heteroatom-containing substrate, thiophene-2-
ethanol, could also be smoothly converted into tritylated product
(entry 8). Natural acyclic and cyclic monoterpenoids such as b-citronellol,
perillyl alcohol, and L-menthol were equally effective as substrates for
the reaction with trityl chloride to give the corresponding trityl ethers
in excellent yields (entries 9–11). Tritylation of hydroxyl group present
in steroidal alcohols such as cholesterol resulted in the formation of
corresponding product in 83% yield but required a longer reaction
time (entry 12). Similarly, the reaction succeeded well with aliphatic
cyclic alcohols such as cyclopentanol and norborneol (entries 13 and
14). The aliphatic diols and secondary alcohols reacted slowly at room
temperature; however, at slightly elevated temperature (40ꢀC), the
resulting diether products were obtained in good yields (entries
15–19). Nevertheless, diols required 2 equivalents of trityl chloride
for complete conversion. Further, to check the compatibility and to
confirm the mildness of the reagent, we studied the protection of
amino alcohols such as L-lucinol and DL-alaninol. These substrates
underwent selective O-tritylation in shorter reaction times and in good
yield (entries 20 and 21).
The plausible catalytic cycle may be explained by the more stable tri-
tyl cation formed in the reaction of trityl chloride with FeCl3 in IL, which
in the presence of alcohol facilitates the SN1 reaction to form the desired
trityl ethers under mild reaction conditions.
Finally, upon completion of the reaction, the IL phase containing
FeCl3 was almost quantitatively recovered by simple extraction of the
product with organic solvent. The recovered IL phase containing the
catalyst was reused for five cycles with consistent activity (Table 2,
entry 4).

FeCl3-Catalyzed Tritylation of Alcohols
3789
a
Table 2. Tritylation of various alcohols using FeCl3 in [bmim]BF4
Entry
Alcohol
Time (min)
Yield (%)b
99
1
10
2
3
4
30
60
35
95
95
98, 95c
5
6
120
120
97
95
7
195
55
8
9
195
150
92
90
10
11
180
230
92
93
(Continued )

3790
B. Sreedhar et al.
Table 2. Continued
Entry
Alcohol
Time (min)
Yield (%)b
12
300
83
13
14
220
255
92
95
15
16
255
190
99d,e
99d,e
17
18
150
180
89d
85d
19
195
98d
20
21
180
170
80
69
aReaction conditions: catalyst (5 mol%), [bmim]BF4 (1 mL), p-methyl benzyl
alcohol (1 mmol), and tritylchloride (1 mmol) at room temperature.
bIsolated yield.
cIsolated yield after fifth cycle.
dReaction temperature at 40ꢀC.
eTwo equivalents of trityl chloride.

FeCl3-Catalyzed Tritylation of Alcohols
3791
EXPERIMENTAL
General Remarks
All chemicals were purchased from Aldrich Chemicals and S.D. Fine
Chemicals Pvt. Ltd., India, and used as received. All the solvents used
were laboratory-reagent (LR) grade and used as received from S.D. Fine
Chemicals Pvt. Ltd, India. ACME silica gel (100–200 mesh) was used for
column chromatography, and thin-layer chromatography (TLC) was per-
formed on Merck precoated silica-gel 60-F254 plates. Melting points
were recorded on a Barnstead electrothermal 9200 instrument and are
uncorrected. CHNS analysis was performed on a Vario EL analyzer.
The infrared (IR) spectra of all compounds were recorded on a Nexus
670 Fourier transform (FT)-IR spectrometer (Nicolet Corporation
Ltd., USA) using KBr pellets or neat. The IR values are reported in reci-
1
procal centimeters (cmꢁ1). The H and 13C NMR spectra were recorded
on Varian-Gemini 200-MHz and Bruker-Avance 300-MHz spectro-
meters. Mass spectra were obtained at an ionization potential of 70 eV
[scanned on VG 70–70H (micromass)]. Thermogravimetric (TG) and dif-
ferential thermal analysis (DTA) were studied on a TGA=SDTA Mettler
Toledo 851e system using open alumina crucibles containing samples
weighing about 8–10 mg with a linear heating rate of 10ꢀC minꢁ1. Nitro-
gen was used as purge gas for all these measurements.
Synthesis of Ionic Liquids
ILs synthesized according to the procedures reported in the literature,
1
and the purity was confirmed by H NMR and TGA–DTA analysis.[20]
General Procedure for the Preparation of Trityl Ethers
Alcohol (1 mmol) and trityl chloride (1 mmol) were added to a solution of
FeCl3 (0.008 g, 5 mol%) in IL (1 mL). The mixture was stirred at room
temperature for the given time (see Tables 1 and 2). After completion
of the reaction, as monitored by TLC, the crude product was extracted
with organic solvent (3 ꢂ 10 mL). The combined organic extracts were
concentrated in vacuo, and the resulting product was purified by column
chromatography on silica gel with ethyl acetate–n-hexane (2:8) as eluent
to afford the desired product. The products were characterized by com-
parison of their nuclear magnetic resonance (NMR) and mass spectra
with those reported in the literature.[7,12,13] The IL containing FeCl3

3792
B. Sreedhar et al.
was dried under vacuum and preserved for the next run. The physical
data and the spectroscopic data of selected cyanohydrins reported in
Table 2 are given here.
Data
1-Fluoro-4-[(trityloxy)methyl]benzene (Entry 2, Table 2)
Solid; mp 138–140ꢀC; yield 95%; 1H NMR (CDCl3) d 7.65–6.91 (m,
19H), 4.21 (s, 2H); MS (EI) m=z 259, 243, 137, 125, 109, 95, 77; IR
(KBr): t 3058, 2924, 2882, 2853, 1583, 1489, 1447, 1373, 1223, 1078,
1031, 932, 837, 762, 702, 629 cmꢁ1. Anal. calcd. for C26H21OF: C,
80.62; H, 5.426. Found: C, 80.55; H, 5.40.
1-(Methylsulfanyl)-4-[(trityloxy)methyl]benzene (Table 2, Entry 5)
Solid; mp 144–145ꢀC; yield 97%; 1H NMR (CDCl3) d 7.50–7.20 (m,
19H), 4.10 (s, 2H), 2.47 (s, 3H); MS (EI) m=z 243, 165, 137, 122, 91,
47; IR (KBr): t 3475, 3061, 3021, 2923, 2854, 1958, 1891, 1733, 1596,
1490, 1443, 1372, 1325, 1213, 1154, 1073, 1009, 895, 801, 757, 699,
635 cmꢁ1. Anal. calcd. for C27H24OS: C, 81.81; H, 6.06; S, 8.08. Found:
C, 81.74; H, 5.99; S, 8.12.
1,4-Dimethoxy-2-[(trityloxy)methyl]benzene (Table 2, Entry 6)
Solid; mp 158–160ꢀC; yield 95%; 1H NMR (CDCl3) d 7.50–6.60 (m,
18H), 4.13 (s, 2H), 3.79 (s, 3H), 3.64 (s, 3H); MS (EI) m=z 243, 165,
137, 121, 105, 91, 77; IR (KBr): t 3476, 3061, 2937, 2835, 1679, 1594,
1441, 1276, 1217, 1154, 1006, 933, 888, 756, 696, 635, 581 cmꢁ1. Anal.
calcd. for C28H26O3: C, 81.95; H, 6.34. Found: C, 81.89; H, 6.21.
3-(2-Trityloxy-ethyl)-thiophene) (Table 2, Entry 8)
Solid; mp 130–132ꢀC; yield 92%; 1H NMR (CDCl3) d 7.40–7.18 (m,
15H), 7.10 (d, 1H), 7.40–6.89 (d, 1H), 6.79 (d, 1H), 3.32 (t, 2H,
J ¼ 6.6 Hz), 3.07 (t, 3H, J ¼ 6.6 Hz); MS (EI) m=z 243, 139, 111, 97,
77 cmꢁ1; IR (KBr): t 3466, 3058, 3022, 2920, 2865, 1953, 1594, 1488,
1445, 1374, 1268, 1216, 1150, 1079, 929, 896, 756, 697, 631, 588 cmꢁ1
.
Anal. calcd. for C25H22OS: C, 81.08; H, 5.94; S, 8.64. Found: C, 81; H,
5.82; S, 8.51.

FeCl3-Catalyzed Tritylation of Alcohols
3793
1-[Phenyl(trityloxy)methyl]benzene (Table 2, Entry 19)
1
Solid; mp 96–98ꢀC; yield 98%; H NMR (CDCl3) d 7.40–7.26 (m, 25H),
5.36 (s, 1H); MS (EI) m=z 183, 167, 105, 91, 77; IR (KBr): t 3468, 3060,
2923, 2853, 1954, 1741, 1595, 1489, 1443, 1326, 1154, 1079, 1007, 889,
758, 698, 635, 579 cmÀ1. Anal. calcd. for C32H26O: C, 90.14; H, 6.10.
Found: C, 90.2; H, 6.15.
CONCLUSION
In conclusion, a facile, recyclable, and clean protection of alcohols as tri-
tyl ethers using a catalytic amount of FeCl3 in ILs at room temperature in
shorter reaction times has been demonstrated. The simplicity of opera-
tion, mild conditions, easy recyclability of the catalyst, and the environ-
mentally benign nature of the reaction allows easy application to a wide
range of substrates.
ACKNOWLEDGMENTS
P. R. thanks the director, Indian Institute of Chemical Technology
(IICT), Hyderabad, India, for financial support under the project
CNP-0222-18. B. N. thanks the Council of Scientific and Industrial
Research (CSIR), New Delhi, India, for a research fellowship.
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