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References and notes
O
O
Bi3+
N O
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OH Bi
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HO
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1a
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Figure 3. Proposed mechanism for ipso-nitration of aryl boronic acids by
Bi(NO3)3ꢀ5H2O.
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14. Solvents such as toluene, benzene, acetonitrile, acetone, DMF, DCM, methanol,
isopropanol, THF, and dioxane were investigated in the presence of bismuth
(III) nitrate as nitrating agent for ipso-nitration of aryl boronic acids. Benzene
and toluene were found to be the most suitable solvents. In other solvents,
such as ACN, acetone, DMF, and DCM, reaction did not proceed at all. No
product was formed in isopropanol and dioxane, however small amount (<5%)
of product formation was observed in MeOH and THF. Thus study indicated
that polar solvents are not suitable. Non-polar solvents such as benzene and
toluene were the best among all solvents investigated. Thus, the choice of
solvent was restricted to toluene or benzene.
acid, and cyclohexyl boronic acid (not shown) have not partici-
pated in this reaction. All the synthesized nitroarenes are known
in the literature and were characterized by comparison of their
melting point, 1H NMR, GC–MS, and IR data with literature val-
ues.16,17,2,18–23,7
Next, we sought to determine the mechanism of ipso-nitration
by Bi(NO3)3ꢀ5H2O. The plausible mechanism of ipso-nitration is de-
picted in Figure 3. Maiti and coworkers7 propose a free-radical
mechanism for Bi(NO3)3/K2S2O8 catalyzed ipso-nitration. To check
whether the present catalyst-free ipso-nitration also occurs via
free-radical mechanism, reaction of phenyl boronic acid 1a was
performed in the presence of free-radical scavengers TEMPO and
thiourea. The reaction took place smoothly in the presence of TEM-
PO and thiourea, thus ruling out the possibility of free radical
mechanism. The fact that aliphatic boronic acid (e.g. cyclohexyl
boronic acid) did not participate in this reaction, indicates that
the aromatic ring plays an important electronic role in the ipso-
nitration. We presume that bismuth nitrate produces in situ
Bi–O–NO2 species. Since, boron is known to be highly oxophilic
in nature, it is likely that through electronic interaction between
boronic acid group and the Bi–O–NO2 species, there is formation
of ionic species which helps the nitration to occur at ipso-position
as depicted in Figure 3.
15. Bose, A.; Sanjoto, W. P.; Villarreal, S.; Aguila, H.; Banik, B. K. Tetrahedron Lett.
2007, 48, 3945–3947.
In conclusion, we have developed a simple, efficient, and eco-
nomical protocol for ipso-nitration of aryl and heteroaryl boronic
acids in shortest reaction times. Most importantly, the present pro-
tocol is catalyst-free, making it more ecofriendly, economical, and
feasible for large scale synthesis of nitroarenes.
16. Aridoss, G.; Laali, K. K. J. Org. Chem. 2011, 76, 8088–8094.
17. Joseph, P. J. A.; Priyadarshini, S.; Kantam, M. L.; Maheswaran, H. Tetrahedron
Lett. 2012, 53, 1511–1513.
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H. J. Org. Chem. 2011, 76, 6356–6361.
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Acknowledgments
R.R.Y. is thankful to CSIR for the award of Junior Research Fel-
lowship. Authors thank analytical department, IIIM for NMR and
MS analysis of our compounds.
22. Tromelin, A.; Demerseman, P.; Royer, R. Synthesis 1985, 1074–1076.
23. Panchenko, P. A.; Fedorov, Y. V.; Fedorova, O. A.; Perevalov, V. P.; Jonusauskas,
G. Russ. Chem. Bull., Int. Ed. 2009, 58, 1233–1240.
24. General procedure for ipso-nitration of aryl and heteroaryl boronic acids: The
mixture of aryl boronic acid (50 mg, 1 equiv) and bismuth (III) nitrate (2 equiv)
in toluene or benzene (2 mL) was refluxed for 1.5–2 h. Reaction mixture was
allowed to cool to room temperature and was filtered through Whatman filter
paper. Residue was washed with ethyl acetate followed by DCM. Combined
organic layers were evaporated on a rotary evaporator. Crude product was
purified by silica gel column chromatography (#100–200) using EtOAc/hexane
(1:99 to 5:95) as mobile phase.
Supplementary data
Supplementary data (spectral data of all compounds) associated
with this article can be found, in the online version, at http://