An organocatalytic method for the synthesis of some novel xanthene derivatives by the intramolecular Friedel-Crafts reaction

Article abstract of DOI:10.1039/c6ra27094h

An efficient organocatalytic method for the synthesis of new substituted 9-arylxanthenes (2a-2u) starting from diarylcarbinol compounds with an arenoxy group (1a-1u) has been developed using the intramolecular Friedel-Crafts reaction. The substrates were

Full text of DOI:10.1039/c6ra27094h

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An organocatalytic method for the synthesis of
some novel xanthene derivatives by the
intramolecular Friedel–Crafts reaction†
Cite this: RSC Adv., 2017, 7, 16644
*
Tulay Yıldız and Hatice Baspinar Kuçuk
¸
¨
¨ ¨
An efficient organocatalytic method for the synthesis of new substituted 9-arylxanthenes (2a–2u) starting from
diarylcarbinol compounds with an arenoxy group (1a–1u) has been developed using the intramolecular Friedel–
Crafts reaction. The substrates were prepared in two steps by Ullmann-type coupling and then Grignard
reaction. Some organic Brønsted acids were studied as catalysts (3a–3g) in the intramolecular Friedel–Crafts
alkylation reaction for the first time. N-triflylphosphoramide (3g) provided the synthesis of some novel
substituted 9-arylxanthenes with excellent yields at room temperature within 15 minutes.
Received 21st November 2016
Accepted 27th February 2017
DOI: 10.1039/c6ra27094h
rsc.li/rsc-advances
We have determined that the most suitable method to obtain
9-arylxanthenes is the intramolecular Friedel–Cras alkylation
Introduction
(FCA) of proper alcohols because the literature on arylation using
an FCA protocol has recently started to see new developments. In
particular, arylation that does not use any metals has became
a very popular topic for avoiding the disadvantages of organo-
metallic chemistry, which poses high cost, uses toxic materials,
and poses difficulties during purication.14 These new methods
have opened new doors using mild reaction conditions and
employing cheap and less toxic catalysts.15
In this eld, the development of new p-activated alcohols
has increasing importance over the development of organo-
halides, which are more toxic and require harsh conditions.16
Thus, Lautens et al. reported an efficient synthesis of the tar-
geted cis-hexahydrobenzophenanthridines from tetralins,
which are very easy to obtain. The reaction included p-activated
alcohols produced using intramolecular FCA in the presence of
Lewis acids.17 An important work which realised cyclization of
p-activated alcohols by Bi(OTf)3-catalyzed intramolecular FCA
was reported by Rueping et al.18 To synthesise substituted 9-
arylxanthenes, another Lewis acid-catalysed intramolecular FCA
procedure was carried out by Panda and co-workers in 2009.19 In
this study, they used well-known FCA catalysts and some Lewis
acids like conc. H2SO4, anhyd. AlCl3, FeCl3, Sc(OTf)3 and TfOH.
Among the catalysts studied, FeCl3 gave the best result with 91%
yield in dry CH2Cl2 at room temperature.
Xanthenes and their derivatives have biological characteristics;
they are used against inammatory diseases and to treat
illnesses caused by viruses and bacteria.1 Thus, xanthenes are
pharmaceutically active and used in photodynamic therapy.2
They are used also as uorescent materials or dyes and molec-
ular switches, which occur in natural products.3 The structure of
xanthenes being straight and strict is advantageous; they are
used as linkers in peptide syntheses and in the synthesis of
unnatural amino acids.4 The HIV-1 nucleocapsid protein has
around 2000 small molecules (the NCI Diversity Set), in which
26 of them are active inhibitors and ve of them were reported
to have a xanthenyl ring.5
Many synthetic routes to obtain xanthene derivatives have
been reported.6 The most important examples are the iron-
catalysed cascade benzylation–cyclisation of phenols,7 Pd-
catalyzed cyclisation of polycyclic aryltriate esters,8 Lewis
acid-catalysed cyclisation of salicylaldehydes and cyclo-
hexenones or tetralones,9 and the condensation of 2-naphthol
and numerous aryl aldehydes.10 The synthesis of substituted 9-
arylxanthenes using the tandem arylation/Friedel–Cras reac-
tion of O-hydroxy bisbenzylic alcohols with diaryliodonium
salts was recently reported by Wang et al.11 Another study con-
cerning the synthesis of substituted 9-arylxanthenes reported
an Sc(OTf)3-catalyzed domino reaction.12 Additionally some
nitrogen or sulphur-containing xanthene derivatives, which
may be used especially as antitumor agents, were synthesized in
previous works.13
According to the literature, although there are reports of
hydroarylation reactions of activated alkenes using FCA with
Brønsted acids such as diphenyl hydrogen phosphate as the
catalysts,20 there are no reports of intramolecular FCA using
organic Brønsted acids as the catalysts instead of Lewis acids.
Considering the recent reports, non-chiral, organic Brønsted
“super-acids” are comparable or even better than Lewis or
inorganic Brønsted acids.21 Furthermore, the organic Brønsted
acids are similar to Lewis acids as they are usually stable against
Istanbul University, Faculty of Engineering, Department of Chemistry, 34320, Istanbul,
Turkey. E-mail: tulayyil@istanbul.edu.tr
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra27094h
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Table 1 Screening of catalysts for intramolecular FCA of 1aa
Fig. 1 Structures of used organic Brønsted acids.
oxidizing ambience or humidity, and they can be easily handled
and are stable over a long period of time.22 Recently, in order to
create new types of super acids or super acid catalysts, new
Brønsted acids including ]NSO2CF3 or ]NSO2F groups were
synthesized because these groups increase the acidity
substantially.23 It is understood that triate groups in partic-
ular, among all acidic compounds, increase the acidity signi-
cantly. Due to N-triate groups having lower pKa values than
normal organic acids,24 the more acidic N-triate groups can
catalyze the reactions with generally better yields and in less
time. As an example of an organic Brønsted acid, N-tri-
ylphosphoramide (3g) (Fig. 1) was used in some previous
reports. A study on this subject includes the preparation of 1,3-
dioxolane-4-ones starting from a-hydroxy acids and aldehydes
with the use of 3g.25 Another example reports a transformation,
which has merged the Ugi reaction and Houben–Hoesch cycli-
sation in the presence of 3g.26 In that report, aer several
Brønsted and Lewis acids were examined, they found that the
acid-promoted reaction is effective in the synthesis of many 3-
aminoindole molecules using 3g with high yields and under
mild conditions.
Entry
Catalyst (10% equiv.)
Time
Yieldb (%)
1
2
3
4
5
6
7
8
None
3a
3b
3c
3d
3e
3f
24 h
24 h
2 h
24 h
24 h
2 h
nr
2
65
nr
nr
70
5
24 h
2 h
3g
78
a
Condition: 1a (0.1 mmol) and catalyst (10% equiv.) in CH2Cl2 (2.5 mL)
b
were stirred at room temperature. Yield of isolated product.
the starting material was consumed. In this way, the different
Brønsted acids were compared against each other for the
intramolecular FCA reaction.
Moderate yields were obtained using 3b and 3e catalysts
(Table 1, entries 3 and 6). 3a and 3f gave very low yields (Table 1,
entries 2 and 7) despite the long reaction times. Benzoic acid (3c)
and L-lactic acid (3d) did not react at all (Table 1, entries 4 and 5).
As expected, no conversion was observed in the absence of the
catalyst (Table 1, entry 1). Interestingly, the best result was
achieved when 10 mol% of 3g was used to obtain the desired
product 2a in 78% yield within 2 hours (Table 1, entry 8).
In the second phase, we examined the effect of the solvent on
the intramolecular FCA reaction 1a using the best catalyst 3g
In this study, to develop an effective and economic organo-
catalytic FCA protocol to synthesize some novel substituted
arylxanthenes under mild conditions, we explored some organic
Brønsted acids as catalysts (3a–3g) in the intramolecular FCA
reaction for the rst time.
We obtained arylxanthenes with excellent yields using N-
triylphosphoramide (3g). Among all synthesised substituted
arylxanthenes, seventeen of the compounds are novel, and the
others, 2i,27 2j,27 and 2k,19 have been synthesised previously with
different methods.
Table 2 Screening of solvents for intramolecular FCA of 1aa
Results and discussion
Our starting materials were synthesised in two steps involving
metal-free Ullmann coupling28 and then Grignard reaction. Finally,
we obtained diarylcarbinol compounds including an arenoxy
group as the starting material with very high yields (95–100%). All
starting compounds (diarylcarbinols) are novel except 1k.19
Entry Catalyst Solvent
Cat. amount (mol%) Time
Yieldb (%)
1
2
3
4
5
6
7
8
9
3g
3g
3g
3g
3g
3g
3g
3b
3e
CH2Cl2
Benzene 10
Toluene 10
CHCl3
THF
CH3CN
CH3CN
CH3CN
CH3CN
10
2 h
2 h
2 h
2 h
2 h
78
13
17
91
11
We have chosen carbinol 1a in order to examine the catalysts
and carry out initial optimisation experiments of the intra-
molecular FCA. Therefore, we explored the different organic
Brønsted acids as catalysts for the intramolecular FCA reaction
(Table 1). The catalysts we used were acetic acid (3a), triuoro-
acetic acid (3b), benzoic acid (3c), L-lactic acid (3d), p-toluene-
sulfonic acid (3e), diphenyl hydrogen phosphate (3f), and N-
triylphosphoramide (3g). Firstly, the solution of 1a and acid
10
10
10
5
10
10
15 min >99
15 min 85
15 min 85
15 min 87
a
Condition: 1a (0.1 mmol) and catalyst (10% mmol) in solvent (2.5 mL)
b
ꢀ
were stirred at room temperature. Yield of isolated product.
catalyst (3a–3g) in dichloromethane was stirred at 25 C until
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Table 4 The effect of the functional groups on Ar2 for intramolecular
(Table 2). Performing the reaction in several solvents gave
interesting results. The intramolecular FCA reaction of 1a with
CH2Cl2 and CHCl3 showed good yields with side products.
While THF, benzene and toluene gave very low yields, CH3CN
provided the best result with 99% yield and no side reaction
when using 10 mol% of 3g (Table 2, entry 6). Also we observed
a lower yield when using 5 mol% of the catalyst (Table 2, entry
7). Additionally, the other best catalysts 3b and 3e were also
tested in CH3CN. However, lower yields of 85 and 87% were
obtained in these reactions (Table 2, entries 8 and 9).
FCAa
Entry
Product
Ar1
Ar2 (or R)
Yieldb (%)
In parallel to our work, it was shown in previous studies that
CH3CN is a good discriminating solvent for strong acids,
because of its weak basicity and weak solvating power to
anions.23 Therefore we found that performing the cyclisation by
intramolecular FCA with 10 mol% 3g in CH3CN at room
temperature for 15 min showed excellent results for obtaining
substituted 9-arylxanthene derivatives. Aer these optimisation
reactions, we looked at the differences when we used different
aromatic hydroxy compounds (Ar1) in the Ullmann coupling
reaction. We used different substituent aryl groups including
Me, Br, Cl, F, CN, NO2 etc. (Ar1) during this reaction step and
synthesized diaryl compounds that have these substituents on
the arenoxy group in order to investigate the effect of the
different functional groups on the intramolecular FCA. The
other aryl group p-methoxy phenyl (Ar2) was chosen for use in all
substrates for these experiments. The effect of the functional
groups on Ar1 screening reactions are summarized in Table 3.
As can be seen in Table 3, electron donating groups like
methyl and naphthyl groups (entries 1 and 2) increase the rate
of the electrophilic substitution. Therefore, their yields are
excellent during intramolecular FCA. When we look at halogen
containing electron withdrawing groups, although they are
weakly deactivating, their yields are very good, especially for the
3-Br substituent (Table 3, entry 3). As expected, the other elec-
tron withdrawing groups nitro and cyano gave very low yields
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
4-MeC6H4
3-MeC6H4
2-MeC6H4
2-MeC6H4
2-MeC6H4
2-MeC6H4
3-MeC6H4
4-MeC6H4
2-MeC6H4
4-MeC6H4
2-MeC6H4
4-MeC6H4
4-MeOC6H4
2-Np
>99
>99
95
96
96
88
73
79
2m
2n
C15H31
2-Thienyl
a
Condition: carbinol (0.1 mmol) and 3g (10% mmol) in CH3CN (2.5 mL)
b
were stirred at room temperature. Yield of isolated product.
(Table 3, entries 6 and 7), because of their strongly deactivating
effect in the electrophilic aromatic substitution reactions.
Following this, various arylmagnesium bromides (Ar2) were
used in the Grignard reaction step. When Ar1 was kept as an Me
group with different positions and Ar2 (or R) was changed with
different groups and positions (Table 4), we observed that CH3
and CH3O groups showed almost the same yield with 99–95%
(entries 1–5). In addition, the position of the Me group on the
Ar1 did not signicantly affect the reaction. The naphthyl and
thienyl groups resulted in slightly lower yields of 88 and 79%
(Table 4, entries 6 and 8). When a long chain alkyl group was
used instead of Ar2, the yield was only 73% (Table 4, entry 7).
Consequently, the Ar1 group is more effective than the Ar2 group
in this reaction (FCA) because the Ar1 group is part of the
mechanism of the reaction.
The well known SN1 type intramolecular Friedel–Cras
alkylation mechanism can be shown for our compounds
(Scheme 1).
Table 3 The effect of the functional groups on Ar1 for intramolecular
FCAa
Entry
Product
Ar1
Ar2
Yieldb (%)
1
2
3
4
5
6
7
2e
2k
2p
2r
2s
2t
2-MeC6H4
2-Np
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
96
>99
98
95
75
5
3-BrC6H4
4-ClC6H4
4-FC6H4
4-CNC6H4
4-NO2C6H4
2u
4
a
Condition: carbinol (0.1 mmol) and 3g in CH3CN (2.5 mL) were stirred
b
Scheme 1 Proposed Brønsted acid-mediated intramolecular Friedel–
Crafts alkylation mechanism of 1a.
at room temperature. Yield of isolated product.
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In the proposed mechanism, aer protonation of the start- cyclisation is completed to give the desired product 2a. The new
ing material 1a, a water molecule leaves and the carbocation ring formed is very stable because it is six membered.
anion is formed. Following this, the carbocation anion attacks
In addition, although the mechanism that we have proposed
the other aryl functional group and the arenium ion is is an SN1 type Friedel–Cras mechanism, an ortho-quinone-
produced. Finally, a proton is lost from the arenium ion and the methide (o-QM) intermediate may also be composed in the
presence of the acidic media. Recently, Rueping et al. studied
Table 5 Scope of substrates for intramolecular FCA of 1a–1u and their yieldsab
a
Condition: 1a–1u (0.1 mmol) and 3g (10% equiv.) in CH3CN (2.5 mL) were stirred at room temperature for 15–60 min. b Yield of isolated product.
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this mechanism with asymmetric Brønsted acids.29 In these compared to previous intramolecular FCA reactions and the
works, a possible transition-state model was proposed in order synthesis of 9-arylxanthenes.
to describe the mechanism of some Brønsted acid catalysed
Finally, we determined a new organocatalytic FCA protocol,
reactions via multiple hydrogen-bond interplay between the which does not require inert reaction conditions or any
chiral catalyst and substrate.
protection, to synthesise some novel substituted arylxanthenes
using 3g with excellent yields. The twenty newly synthesised
substituted 9-arylxanthenes 2a–2u (Table 5) may be used as
natural and bio-active compounds, dyes or uorescent mate-
rials, and their biochemical properties and activities will be
investigated in the future.
Experimental
General information
The majority of the chemicals used in this work were
commercially available from Merck or Aldrich. CH3CN was
purchased from Merck with catalog number 114291. It has
99.8% purity and was not additionally dried. The starting
carbinols 1a–1u were prepared by Ullmann coupling of 2-uo-
robenzaldehyde and substituted phenols, and then by the
Grignard reaction of 2-arenoxybenzaldehydes and some aryl-
magnesium bromides. All substrates were puried by crystalli-
zation or column chromatography and characterized by IR, 1H-
NMR, and 13C-NMR spectroscopy, elemental analysis and GC-
Acknowledgements
This work was supported by Istanbul University, Scientic
Research Projects, project numbers BYP 41636 and BYP 53627.
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