S. M. S. Ló et al. / Tetrahedron Letters 52 (2011) 1441–1443
1443
also included this catalyst in our study (see Table 1, entries 13 and
Supplementary data
14). These reactions, which were conducted in exactly the same
manner as defined with BF3O(Et)2, indicated that it is possible to
synthesize 1 by using combined I2 (first step) and SeO2 (second
step). Reaction performed with 0.012 mmol of I2 gave 35% yield.
In the reaction using higher amounts of iodine, TPP (1) could not
be properly isolated after standard workup due to the appearance
of unidentified side products with similar chromatographic behav-
ior to 1.
The two-step, one-flask synthetic process can also employ
dipyrromethanes instead of pyrrole to produce a wide variety of
meso-substituted porphyrins.16 In order to verify if SeO2 was useful
in a different condition, a reaction performed with 5-phenyldipyr-
romethane17 and benzaldehyde (Scheme 2) was carried out as
shown in Table 1 (entry 15). This reaction gave 65% yield for TPP
(1), which extends the utility of SeO2 for this kind of procedure.
The higher yield obtained is attributed to the increasing of por-
phyrinogen formation in the first step, due to a simpler cyclocon-
densation reaction that probably occur with dipyrromethanes
and aldehydes.
Supplementary data associated with this article can be found, in
References and notes
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General reaction conditions14 were then successfully applied to
four other benzaldehydes yielding the corresponding meso-tetra-
arylporphyrins (Table 2). There are conflicting literature findings
describing the correlation of meso-tetraarylporphyrins yields to
the effect of electron-donating and withdrawing groups attached
to the parent benzaldehyde. For example, Sharghi and Nejad7 and
Liu et al.10 achieved higher yields using benzaldehydes bearing
electron-donating groups while Boëns et al.9b had opposite results.
We found that the lower were the influence of the benzaldehyde
substituents the higher were the yields. This can be noted by
observing the higher yield for benzaldehyde (1, 56%) followed by
benzaldehydes substituted with weakly donating (4, 51%) or
weakly withdrawing (5, 34%), and then with moderately (3, 28%)
or strongly (2, 18%) donating groups, as shown in Table 2.
In conclusion, we have demonstrated the applicability of SeO2
to the oxidative step of meso-tetraarylporphyrins synthesis in com-
bination with improved BF3O(Et)2 catalysis, or with I2 catalyst. Un-
der our reaction conditions, SeO2 works as a heterogeneous
oxidant, which facilitates its removal from reaction media. Further
studies involving SeO2 with different conditions and using a wide
variety of aldehydes are currently under investigation in our
laboratories.
11. Duan, L.; Wang, Y. L.; Fan, X. S.; Wang, J. Y. Chem. Lett. 2008, 37, 112–113.
12. Nascimento, B. F. O.; d’A. Rocha Gonsalves, A. M.; Pineiro, M. Inorg. Chem.
Commun. 2010, 13, 395–398.
13. (a) Astin, S.; Moulas, L. de V.; Riley, H. L. J. Chem. Soc. 1935, 901–904; (b)
Rabjohn, N. In Organic Reaction; Adams, R., Ed.; John Wiley and Sons: New York,
1949; Vol. 5, pp 331–386; (c) Waitkins, G. Chem. Rev. 1945, 36, 235–289; (d)
Toshima, K.; Ohta, K.; Ohtake, T.; Tatsuta, K. J. Chem. Soc., Chem. Commun. 1991,
694–695; (e) Larock, R. C. Comprehensive Organic Transformations, 2nd ed.;
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Chem. 2005, 83, 273–275; (g) Paul, S.; Sharma, S.; Gupta, M.; Choudhary, D.;
Gupta, R. Bull. Korean Chem. Soc. 2007, 28, 336–338; (h) Maity, A. C. Synlett
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14. General procedure: Under room temperature, aryl aldehyde (1 mmol), BF3O(Et)2
(70 lL from a freshly prepared 1 M solution in CH2Cl2) and then pyrrole (70 lL,
1 mmol) were added successively to 10 mL CH2Cl2; reaction was then stirred
for 30 min. Finely powdered SeO2 (1670 g, 15 mmol) was sequentially added
under vigorous stirring and reaction was then kept for additional 60 min. The
resulting mixture was directly filtered through a Celite pad, evaporated on
Silica 60 and purified by flash chromatography (Silica 60, 20 Â 150 mm) with
hexanes/CH2Cl2 (2:1) for 1, CHCl3/hexanes (3:2) for 2, CHCl3/hexanes (1:2) for
5, CHCl3/(Et)2O/hexanes (5:2:4) for 3 and CHCl3/hexanes (1:1) for 4 with yields
shown in Table 2. Compounds spectrometric properties coincided with
literature data (for details, see Supplementary data).
15. Compound 1 was purified by crystallization as follows: after the given reaction
time, reaction media were filtered through a Celite pad. Filtrate was then
evaporated to dryness to give a dark solid residue that was resuspended in the
minimal amount of CH2Cl2. Methanol was added to the resulting solution and
the mixture was kept overnight under room temperature. The dark purple
crystals were then filtered, washed with methanol, and vacuum-dried (1, 12%).
16. (a) Littler, B. J.; Ciringh, Y.; Lindsey, J. S. J. Org. Chem. 1999, 64, 2864–2872; (b)
Geier, G. R., III; Littler, B. J.; Lindsey, J. S. J. Chem. Soc., Perkin Trans. 2 2001, 701–
711; (c) Lindsey, J. S. Acc. Chem. Res. 2010, 43, 300–311.
Acknowledgments
The authors are thankful to Fundação Araucária, CNPq and PRO-
NEX-Carboidratos for financial support. S.M.S.L. thanks the scholar-
ship from REUNI program. M.E.D. and M.D.N. are Research
Members of the National Research Council of Brazil (CNPq).
17. Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea, D. F.; Boyle, P. D.;
Lindsey, J. S. J. Org. Chem. 1999, 64, 1391–1396.