Novel and highly efficient one pot protocol for the synthesis of diversely functionalized triarylmethanes

Article abstract of DOI:10.1016/j.tetlet.2014.11.139

An efficient, convenient, and novel one pot approach is described for the synthesis of triarylmethanes by in situ oxidation of benzylalcohols followed by the Friedel-Crafts alkylation of di/trimethoxybenzenes in the presence of BF₃·OEt₂. The generality of the present method is strengthened by screening a variety of aromatic, aliphatic, and heteroaromatic alcohols with methoxy arenes. Shorter reaction time, mild reaction condition, good yields, and wide scope of substrates are the significant features of this protocol.

Full text of DOI:10.1016/j.tetlet.2014.11.139

Accepted Manuscript
Novel and highly efficient one pot protocol for the synthesis of diversely func-
tionalized triarylmethanes
B. Madhu Babu, Pramod B. Thakur, Vikas M. Bangade, H.M. Meshram
PII:
S0040-4039(14)02053-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.11.139
TETL 45523
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
2 October 2014
27 November 2014
30 November 2014
Please cite this article as: Madhu Babu, B., Thakur, P.B., Bangade, V.M., Meshram, H.M., Novel and highly efficient
one pot protocol for the synthesis of diversely functionalized triarylmethanes, Tetrahedron Letters (2014), doi:
http://dx.doi.org/10.1016/j.tetlet.2014.11.139
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Novel and highly efficient one pot protocol
for the synthesis of diversely functionalized
triarylmethanes
B. Madhu Babu, Pramod B. Thakur, Vikas M. Bangade and H. M. Meshram*

Tetrahedron Letters
1
Novel and highly efficient one pot protocol for the synthesis of diversely
functionalized triarylmethanes
B. Madhu Babu, Pramod B. Thakur, Vikas M. Bangade and H. M. Meshram*
Medicinal Chemistry and pharmacology division
Indian Institute of Chemical Technology, Hyderabad – 500 007, India
Tel.: +91-40-27191640, Fax: +91-40-27193275, E-mail address- hmmeshram@yahoo.com
Abstract— An efficient, convenient and novel one pot approach is described for the synthesis of triarylmethanes by in situ oxidation of
benzylalcohols followed by the Friedel–Crafts alkylation of di/trimethoxybenzenes in the presence of BF3.OEt2. The generality of the
present method is strengthened by screening a variety of aromatic, aliphatic, heteroaromatic alcohols with methoxy arenes. Shorter reaction
time, mild reaction condition, good yields and wide scope of substrates are the significant features of this protocol.
Keywords: Triarylmethanes; Friedel–Crafts reaction; Oxidation; BF3.OEt2, Lewis acid.
Triarylmethanes (TRAMs) are valuable scaffolds which
possess promising pharmacological properties such as
antiviral,1 antitumor,2 antifungal,3 anti-inflammatory,3
antitubercular,4 antioxidant5 as well as anti-diabetic6 (Fig. 1).
Some of the triarylmethane also constitute the core structure
in natural products such as cassigarol B.7 In addition to this,
triarylmethanes are used as a protective groups,8 leuco dyes,9
photochromic agents,10 building blocks for dendrimers,11 and
in material science.12 Due to such widespread applications of
TRAMs, there is a continuous research interest to develop an
efficient and convenient protocol for the synthesis of these
molecules in mild condition.
catalytic amount of iodine which required longer reaction
times (72 h).20 Recently, palladium catalyzed arylation of
methyl phenyl sulfone was reported to yield
triarylmethanes.21 Although these reported methods are
satisfactory, some of these methods also suffer from certain
disadvantages such as harsh reaction conditions,13p,16,17,21
longer reaction time,13o,20 and unsatisfactory yield13a of the
desired products. Moreover, a few of methods involves the
catalyst which required multi-step synthesis.13b Therefore,
there is a scope to develop a general, convenient and high
yield method by addressing the shortcomings of reported
methods to afford triarylmethanes.
H
R2
N
OMe
N
R3
During our extensive literature survey it was found that, most
of the reported methods for the synthesis of triarylmethanes
were achieved from aldehydes and methoxy arenes. As we
are involved in the development of new methodologies, we
envision the synthesis of triarylmethane from alcohol and
arenes by one pot oxidation and Friedel–Crafts reaction using
Lewis acid with suitable oxidizing reagent (Scheme 1). In this
regard, we focused our attention on the use of TEMPO for
synthesis of triarylmethanes from alcohol and arenes.
TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) has been
effectively used as an oxidizing reagent for oxidation of
alcohols with suitable co-oxidant in organic synthesis.22
Recently, we have reported a new series of diversely
functionalized triarylmethane derivatives using BF3.OEt2 as a
catalyst.23b To the best of our knowledge, there is no report
for the direct synthesis of triarylmethanes starting from
alcohol. In the continuation of our work,23 in the development
of useful methodology for the synthesis of biologically active
molecules, herein we wish to report an operationally simple,
novel and an efficient one pot protocol for the direct synthesis
of triarylmethanes by the reaction of activated aromatic
compounds with substituted benzyl alcohols using
TEMPO/PhIO and BF3.OEt2 catalyst system.
R1
MeO
OMe
S
HO2C
N
R1
OH
Charge transportingmaterial
for electrographic photo
receptor
GPR40 modulor
(diabetes treatment)
anti-tuberculosis agent
Fig. 1: Representative examples of biologically active
triarylmethane derivatives.
Various protocols for the synthesis of triarylmethanes have
been reported so far which include the Friedel–Crafts reaction
of electron-rich arene on aldehyde using various Lewis or
Bronsted acids.13 Inorganic solid supported montmorillonite
K-10,14 organolithium,15 organotin,16 microwave assisted17
and acidic zeolites18 have also been employed to accomplish
this transformation. Olah19 and coworkers are reported the
hydroxyalkylation of weakly deactivated aromatics by
aromatic mono- and dicarboxaldehydes under super
electrophilic activation using BF3–H2O in good yields.
Friedel-Crafts alkylation has been also accomplished with a

2
Tetrahedron Letters
OCH3 OCH3
OCH3
Although we observed the formation of triarylmethane 3aa in
H3CO
OCH3
CH2OH
good yield by this sequential oxidation and Friedel–Crafts
reaction process the longer reaction time was taken to
complete both reactions. Conversely drastic reduction in
reaction time was observed when we performed the reaction
of alcohol 1a and arenes 2a in one pot in the presence of
PhIO, TEMPO and BF3.OEt2. The reaction proceeded
efficiently using 1a (1 mmol) and 2a (2 mmol) in the
presence of PhIO (1.2 equiv), TEMPO (0.13 equiv) with
BF3.OEt2 (2 equiv) and resulted in high yields (94 %) of the
desired product in 30 min. (Table 1, entry 2). It is worthy to
Oxidant
OCH3
OCH3
H3CO
H3CO
OCH3
OCH3
H3CO
OCH3
Lewis acid, Solvent
OH
OH
1a
2a
3aa
Scheme 1: Optimization of the reaction conditions.
Initially, the reaction of 4-(hydroxymethyl)-2,6-
dimethoxyphenol 1a (1 mmol) with 1,2,4-trimethoxybenzene
2a (2 mmol) was performed using BF3.OEt2 (2 equiv) and
PhIO (1.2 equiv) in the presence of catalytic amount of
TEMPO in 5mL DCM(CH2Cl2) at room temperature. The
progress of the reaction was monitored by TLC. After work-
up, triarylmethane was obtained as a sole product in high
isolated yield (94%) (Table 1, entry 2). The structure of
Table 2: Synthesis of new substituted triarylmethanes.a
MeO
MeO
MeO
OMe
H/OMe
CH2OH
R1
TEMPO/PhIO
H/OMe
1
BF3.OEt2, DCM, 30 min, rt
obtained triarylmethane 3aa was confirmed by means of H
R1
MeO
R1
MeO
NMR, 13C NMR, IR and Mass spectrometry analysis. We
presumed that the reaction might have proceeded by the in
situ formation of aldehydes which undergoes Friedel–Crafts
alkylation reaction with arene to afford triarylmethane
(Scheme 1). To support our assumption, an independent
experiment was performed. When we performed the reaction
of alcohol 1a with catalytic amounts of TEMPO in
combination with a stoichiometric amount of reoxidant
iodosylbenzene (PhIO) in 5mL DCM at rt, we observed the
formation of the corresponding aldehyde in good yield in 24
hours. The formed aldehydes in the reaction vessel, was
subjected to undergo Friedel–Crafts alkylation with 1,2,4-
trimethoxybenzene in conjugation with BF3.OEt2 which
afforded triarylmethane 3aa in 88% yield (Table 1, entry 1).
Yieldb(%)
Product
Entry
Arene
OCH3 OCH3
OCH3
H3CO
OCH3
1
94
OCH3
OCH3
H3CO
H3CO
H3CO
OCH3
OCH3
OH
OCH3
OCH3
2a
3aa
1a
OH
H3CO
OCH3
OCH3
H3CO
94
2
H3CO
OCH3
H3CO
OCH3
H3CO
H3CO
OCH3
OH
1a
2b
3ab
OCH3
OH
OCH3
H3CO
OCH3
3
96
Table 1: Optimization of the reaction conditions.a
OCH3
H3CO
H3CO
OCH3
H3CO
OCH3
OH
1a
2c
3ac
OCH3
OH
Oxidant
Lewis acid
(equiv)
(0.13equiv)/
Co-oxidant
(1.2equiv)
Time
Yieldb
(%)
H3CO
Entry
Solvent
OCH3
OCH3
H3CO
OCH3
4
5
6
7
95
1
2
TEMPO/PhIO
TEMPO/PhIO
TEMPO/PhIO
TEMPO/PhIO
TEMPO/PhIO
TEMPO/PhIO
TEMPO/PhIO
TEMPO/PhI(OAc)2
TEMPO/I2
BF3.OEt2 (1.0)
BF3.OEt2 (1.0)
Sc.(OTf)3 (2.0)
Bi(OTf)3 (2.0)
Bi(OTf)3 (2.0)
BF3.OEt2 (2.0)
BF3.OEt2 (2.0)
TfOH (2.0)
DCM
DCM
DCM
DCM
Toluene
THF
24 h
30 min
5 h
88c
94
10
18
68
78
51
19
37
21
-
H3CO
OCH3
OCH3
OCH3
OCH3
OCH3
2a
OCH3
1b
3ba
OCH3
3
H3CO
OCH3
OCH3
H3CO
OCH3
4
5 h
93
H3CO
5
30 min
30 min
30 min
30 min
30 min
30 min
24 h
OCH3
OCH3
H3CO
OCH3
OCH3
OCH3
6
1b
2b
3bb
OCH3
7
DCM
EtOH
DCE
OCH3
H3CO
OCH3
8
96
OCH3
OCH3
OCH3
H3CO
9
Yb(OTf)3 (2.0)
FeCl3 (2.0)
OCH3
2c
1b
OCH3 3bc
OCH3
10
11
TEMPO/NaOCl
-
MeCN
DCM
BF3.OEt2 (2.0)
OCH3 OCH3
OCH3
OCH3
H3CO
92
a
All the reactions were conducted one pot with 4-(hydroxymethyl)-
2,6-dimethoxyphenol 1a (1 equiv) and 1,2,4-trimethoxybenzene 2a
OCH3
OCH3
2a
H3CO
OCH3
OCH3
OCH3
OH
1c
3ca
OH
(2 equiv) in the presence of PhIO (1.2 equiv), TEMPO (0.13 equiv)
b
with BF3.OEt2 (2.0 equiv) in 5 mL DCM at room temperature.
c
Isolated yield.
reaction.
in the sequential oxidation and Friedel–Crafts
Table 2 cont.

Tetrahedron Letters
3

4
Tetrahedron Letters
alcohols with the TEMPO/PhIO in terms of yields and
mildness.
Next the effect of solvent is also tested by screening various
solvents like THF, toluene, DCM, CH3CN, EtOH, DCE and
the results are incorporated in Table 1. The study showed that
DCM is the best choice of solvent for model reaction. The
model reaction was also performed in the absence of
TEMPO/PhIO but formation of the desired product was not
observed even after stretching the reaction time up to 24
hours (Table 1, entry 11). With this study, we selected
reaction of 1a (1 mmol) and 2a (2 mmol) in the presence of
PhIO (1.2 equiv), TEMPO (0.13 equiv) with BF3.OEt2 (2
equiv) in DCM at rt as an optimized reaction condition to
afford 3aa (Table 1, entry 2).
With this established optimum condition, we were keen to
explore the generality of the reaction with respect to various
electron-rich arenes and substituted aromatic or aliphatic
alcohols for the synthesis of the corresponding TRAMs and
the results are outlined in the Table 2. We have studied the
electronic effects of the substituents on the rate of the reaction
and the mode of formation of triarylmethanes. To our delight,
we found that, an electron-rich substituents (OH, OMe and
Me) on the benzylalcohol proceeded smoothly with arenes to
afford desired products 3aa-3gb in excellent yields under
optimized reaction condition (Table 2, entries 1-13). For
example, the reaction of 2-hydroxybenzylalcohol with 1,2,4-
trimethoxybenzene gave desired triarylmethane 3ea in 93%
isolated yield (Table 2, entry 9). Analogously 2-
methylbenzylalcohol
also
reacted
with
1,2,4-
trimethoxybenzene and 1,3,5- trimethoxybenzene in the same
fashion under standard reaction condition to give
corresponding triarylmethanes 3ga, 3gb in very good yields
(Table 2, entries 12 and 13 respectively). Whereas the
presence of electron-withdrawing substituents such as halides,
cyano, nitro on the benzylalcohol ring decreases the yield of
the corresponding products. For example the reaction of
halogenated benzylalcohols with arenes also proceeded same
way and afforded the corresponding products 3hb-3kb in
good yields (Table 2, entries 14-17). Other substituted
a
benzylalcohols
like
3-nitro,
3-cyano
and
4-
All the reactions were conducted one pot with alcohol 1a-1v (1
cyanobezylalcohols also participated under same reaction
conditions to furnish corresponding products 3la-3nb in good
to moderate yields (Table 2, entries 18-22).
equiv) and methoxybenzene 2a-2c (2 equiv) in the presence of PhIO
(1.2 equiv), TEMPO (0.13 equiv) with BF3.OEt2 (2.0 equiv) in 5 mL
DCM at room temperature. b Isolated yield.
note that, under our condition two processes, oxidation and
Friedel–Crafts reactions were taking place rapidly in one pot
which resulted into the high isolated yield of triarylmethane
in short reaction time without isolation of the intermediates.
The efficiency of the reaction was further strengthened by the
participation of hindered alcohol like naphthalen-1-
ylmethanol 1o and 4-methoxynaphthalen-1-ylmethanol 1v
which also underwent smooth reaction with arenes and
resulted in high yield of corresponding products 3oa, 3ob,
3vb and 3vc (Table 2, entry 23, 24, 34 and 35). The synthesis
of heteroaryldiarylmethane was also explored. As can be seen
in Table 2, the 2-(bis(2,4-dimethoxyphenyl)methyl)thiophene
3qc was obtained in excellent yields on treatment of
thiophen-2-ylmethanol 1q with 1,3-dimethoxybenzene 2c
(Table 2, entry 26). To further extend the scope of the
reaction, we intended to study the reaction of cyclic alcohols
We then examined the catalytic activity of several Lewis
acids such as BF3.OEt2, Sc(OTf)3 (entry 3), Bi(OTf)3 (entry
4), TfOH (entry 8), Yb(OTf)3 (entry 9), FeCl3 (entry 10)
with co-oxidants like PhIO, PhI(OAc)2, FeCl3, NaOCl, I2,
tBuOCl and the results are incorporated in Table 1. These
preliminary results indicated that BF3.OEt2 was the catalyst of
choice for the oxidation as well as Friedel–Crafts reactions of

Tetrahedron Letters
with arenes. In this regard, we treated cyclohexylmethanol
5
5. Mibu, N.; Sumoto, K. Chem. Pharm. Bull. 2000,
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with 1,3,5- trimethoxybenzene to afford the diarylalkanes 3pb
in acceptable yield (Table 2, entry 25). It was found that even
butan-1-ol 1r, 2-methylpropan-1-ol 1s, propan-1-ol 1t and
Octan-1-ol 1u as fairly sterically demanding precursors
reacted with corresponding arenes in similar fashion to afford
the respective diarylalkanes in acceptable yields (Table 2,
entries 27-33). It is important to note that, the developed
method is amenable for the synthesis of different
functionalized triarylmethanes and all the compounds
synthesized in this work are not prepared earlier. We believe
that this protocol may be useful for the rapid preparation of
library of new compounds desirable in drug discovery
program and material sciences.
6. Ellsworth, B. A.; Ewing, W. R.; Jurica, E.; U.S.
Patent Application 2011/0082165A1, Apr 7, 2011.
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Katrizky, A. R.; Sabongi, G. J.; Eds.; Plenum: New
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In conclusion, we have demonstrated efficient synthesis of
new diversely functionalized triarylmethanes from alcohol by
one pot oxidation, Friedel–Crafts reactions process using
PhIO-TEMPO with BF3.OEt2 in DCM at rt under mild
reaction conditions. The developed protocol is a novel and
operationally simple which provide direct access for the
synthesis of triarylmethane scaffolds from alcohol and arene.
Moreover the developed method is applicable for variety of
aromatic, aliphatic, heteroaromatic and cyclic alcohols and
arenes. Shorter reaction time, mild reaction condition, good
yields and wide scope of substrates are the significant features
of this protocol. Further investigations on the scope,
applications and limitations of this method are in progress in
our laboratory and results will be published elsewhere in due
course.
Acknowledgments
The authors B M B, V M B thank CSIR for the award of
senior research fellowship and to Dr. Ahmed Kamal,
Outstanding Scientist and Head, MCP Division, IICT, for his
support and encouragement.
The authors thank CSIR, New Delhi for financial support as
part of XII Five Year plan program under title ACT (CSC-
0301).
13. (a) Barbero, M.; Cadamuro, S.; Dughera, S.; Rucci,
M.; Spano, G.; Venturello, P. Tetrahedron 2014,
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C.; Cui, Y.; Zheng, X.; Yin, J.; Yang, G. Applied
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Tangestaninejad,
S.;
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V.;
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