K. Mohammadiannejad-Abbasabadi et al. / Tetrahedron 72 (2016) 1433e1439
1437
As depicted in Scheme 8, a series of novel 9,10-disubstituted an-
thracenes was produced in 58e87% yields in a single reaction vessel.
Bis-(dihexyloxyphenyl)arylmethane 3a in the reaction with acylal 6a
generated 9,10-diaryl-2,3,6,7-tetrahexyloxyanthracene 7a in 79%
yield. In addition, 9-(4-chlorophenyl)-2,3,6,7-tetrakis(hexyloxy)-10-
phenylanthracene 7b was prepared through two different pathways.
Although, the reaction of bis-(dihexyloxyphenyl)arylmethane 3a
with acylal 6b proceeded smoothly to furnish anthracene 7b in 71%
yield, this compound was obtained in 58% yield using bis-(dihex-
yloxyphenyl)arylmethane 3c with acylal 6a as starting material.
These facts confirm that the reactivity of TRAM has a determining
role on the progress and the yield of reaction. However, bis-(dihex-
yloxyphenyl)arylmethanes are suitable precursors for this protocol
and it discloses truly the route to synthesis a novel class of multi-
substituted anthracene derivatives. Yet, to the best of our knowl-
edge, the present protocol to prepare of 9,10-diaryl-2,3,6,7-
tetrahexyloxyanthracenes have never been attempted.
To estimate the generality of this procedure, a series of alde-
hydes 2 and diveratrylmethanes 3 were also submitted to the re-
action to prepare other derivatives of anthracene. Once DVM 3k
was treated with acylal 6a, anthracene 7c was isolated in 87% yield
(Scheme 8). This is the first report of contribution of DVMs bearing
an aliphatic group in the synthesis of 9,10-disubstituted-2,3,6,7-
tetramethoxyanthracenes. Interestingly, DVMs bearing an aryl
group involving 3-Bromophenyl 3i, biphenyl 3j, and 3,4-
difluorophenyl 3q, in the reaction with favorite acylals gave the
Fig. 1. X-ray crystal structure of 3h.
clean procedures, simple work-up processes, milder reaction con-
ditions, and broad substrate scope.
corresponding
9,10-diaryl-2,3,6,7-tetramethoxyanthracenes
(7def) in high isolated yields.
Nevertheless, the present method was not compatible with bis-
(dihexyloxyphenyl)arylmethane derivatives bearing Nitro- or
Cyano-groups. Treatment of bis-(dihexyloxyphenyl)arylmethanes
3b with acylal 6a, whereby the reaction was worked-up after one
hour, yielded starting material 3b. The same experiment with bis-
(dihexyloxyphenyl)arylmethane 3e made available a complicated
mixture of reaction (Scheme 8).
Of these results, it can be hypothesized that sufficiently stable
and reactive bis-(dihexyloxyphenyl)arylmethanes or DVMs can be
reacted with acylals to obtain the corresponding 9,10-
disubstituted-2,3,6,7-tetraalkoxyanthracene derivatives.
4. Experimental part
4.1. General considerations
Bi(OTf)3 was prepared using reported procedures.80,81 Other
chemicals were purchased from commercial sources and used as
received. All reactions were magnetically stirred and their progress
was monitored by TLC using aluminum sheets precoated with silica
gel 60, F 252. Chromatography columns were performed on silica
gel 60 (230e400 mesh) using n-hexane/ethyl acetate as eluent. 1H
NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded on
a Bruker-AC 500 MHz spectrometer in CDCl3 at 25 ꢀC, using TMS as
an internal standard. Melting points are uncorrected and were
determined on a Stuart Scientific SMP2 apparatus. IR spectra of
products were measure on Nicolet-Impact 400D instrument by
transmittance using KBr optics. Micromass Platform II spectrome-
ter was used to record Mass spectra of products. X-ray diffraction
data of compound 3h was recorded on a Rigaku Mercury CCD area
detector with graphite monochromated Mo-Ka radiation.
The use of Bi(OTf)3/O2 reagent system in the place of
30
H3PW12O40
,
to promote the reactions of anthracene synthesis
benefits of promising features such as shorter reaction times,
higher yields, milder reaction conditions, cleaner reactions, easier
work-up, and wider substrate scope.
The structures of products were deduced from their elemental
analyses and by IR, mass, 1H NMR and 13C NMR spectra. Formation
of known compounds was confirmed by matching their resulting
28e32
spectra with cited references.
Furthermore, the structure of
3h was confirmed by X-ray crystallographic analysis (CCDC 833544,
Fig. 1).
4.2. General procedures
3. Conclusion
4.2.1. General procedure for solvent-free synthesis of triarylmethanes
(Schemes 1e7). To a magnetically stirred mixture of arene 1
(3 mmol) and the corresponding aldehyde 2 (1 mmol) was added
Bi(OTf)3 (8 mol %). The resulting mixture was stirred at 70 ꢀC for the
appropriate time (Schemes 1e7), and then was allowed to cool to rt.
The crude product was diluted twice by addition CH2Cl2 (5 mL) and
catalyst was separated by simple filtration. The solvent of combined
organic layers was evaporated in vacuo and the pure product 3 was
obtained by recrystallization from EtOH or EtOH/H2O (10:2) or by
silica-gel column chromatography using n-hexane/ethyl acetate
(5:1) as eluent. The same procedure were used for synthesis of
tetrakis(veratryl) adduct 4.
In summary, a versatile and highly efficient protocol for the
synthesis of triarylmethanes including bis-(dihexyloxyphenyl)
arylmethanes, diveratrylmethanes, etc. was described via the re-
action of arenes with aldehydes catalysed by Bi(OTf)3 under
solvent-free conditions. Synthesis of bis-(dihexyloxyphenyl)aryl-
methanes is detailed for the first time. The reaction of veratrole
with aromatic dialdehydes gave different results based on the
molar ratio of reactants and the nature of aromatic dialdehyde. In
addition, tandem three-step reaction of acylals with bis-(dihex-
yloxyphenyl)arylmethanes or diveratrylmethanes promoted by
Bi(OTf)3/O2 reagent system was utilized for the synthesis of 9,10-
disubstituted-2,3,6,7-tetraalkoxyanthracenes. The present pro-
tocols offer advantages such as short reaction times, high yields,
The recovered catalyst, after washing with CH2Cl2, dried at 50 ꢀC
and reused successfully at least three successive runs.