212
T.S. Rivera et al. / Applied Catalysis A: General 443–444 (2012) 207–213
Table 5
green process for the synthesis of aryl-14H-dibenzo[a,j]xanthenes
by condensation of various aromatic aldehydes with 2-naphthol
using ZrTPA as a heterogeneous acid catalyst in solvent-free con-
ditions, using conventional heating at 130 ◦C, has been developed.
Additionally, short reaction times, excellent yields, straightforward
procedure, and relative nontoxicity of the catalyst are other note-
worthy advantages of this method. Finally, these solid acid catalysts
can be recovered and reused at least four times with negligible loss
in their activity.
Effect of reusability of ZrTPA60BT100 catalyst on the xanthene yield.
Entry
Cycle
Yielda (%)
1
2
3
4
0
1
2
3
99
98
98
96
Reaction conditions: benzaldehyde, 1.2 mmol; 2-naphthol, 2 mmol; catalyst amount,
170 mg; temperature, 130 ◦C; time, 1 h.
a
Isolated yield.
Acknowledgements
Table 6
Synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes with ZrTPA60BT100 as catalyst.
The authors thank ANPCyT, CONICET and Universidad Nacional
de La Plata, for financial support, and L. Soto, L. Osiglio, D. Pen˜a and
N. Firpo for their collaboration in the experimental measurements.
Entry
Aromatic aldehyde
Time (h)
Yielda (%)
1
2
3
4
5
6
7
Benzaldehyde
1
1
1
2
2
5
3
99
94
93
98
96
60
78
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
4-Florobenzaldehyde
4-Chlorobenzaldehyde
2,4,6-Trimethoxybenzaldehyde
2-Naphthaldehyde
Appendix A. Supplementary data
Supplementary data associated with this article can be
j.apcata.2012.08.001.
Reaction conditions: aldehyde, 1.2 mmol; 2-naphthol, 2 mmol; catalyst amount,
85 mg; temperature, 130 ◦C.
a
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After the best reaction conditions were achieved, the general-
ity of these conditions to other substrates was studied, employing
ZrTPA60BT100 as catalyst. Using this method, different kinds of aro-
matic compounds were reacted with 2-naphthol to produce the
corresponding 14-aryl-14H-dibenzo[a,j]xanthenes at 130 ◦C under
solvent-free conditions. The results are summarized in Table 6.
Several aromatic aldehydes with different functional groups were
subjected to the condensation reaction, and the desired products
were synthesized in good to excellent yields. The substituted func-
tional groups in the aromatic ring of the aldehyde affect the yield
of electron-donating groups in the aldehyde decreased both the
reaction rate and the yield of products (Table 6, entries 6 and 7).
A mechanistic rationale portraying the probable sequence of
events is given in Scheme 2. The reaction likely proceeds via the
initial formation of the carbocation (1). The oxonium species (2) is
then formed on reaction with 2-naphthol, which then undergoes
dehydration to afford the desired product (3).
It must be remarked that for all the studied reactions the
products were selectively obtained, without the formation of by-
products.
4. Conclusions
Tungstophosphoric acid/zirconia composite materials were pre-
pared using PEG as pore-forming agent, via sol–gel reactions.
FT-IR and 31P MAS-NMR results indicated that the main species
3−
present in the samples is the [PW12O40
]
anion, which was par-
6−
7−
tially transformed into [P2W21O71
]
and [PW11O39
]
anions
during the synthesis and drying steps. The prepared catalysts
presented acid and textural properties adequate for their use as
catalysts in heterogeneous acid reactions.
ZrTPA composites have proved to be a very efficient catalyst
for the synthesis of dibenzoxanthene derivatives. A convenient and
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