P. Kumari et al. / Catalysis Communications 29 (2012) 15–20
19
TACFe(III)
O
TACFe(III)
O
TACFe(IV)
O
H
H
H
H
H
TACFe(V)=O
(19)
CH3
CH3
CH3
-H +
CH3
1
22
21
20
H+
TACFe(IV)
OH
OH
O
OH
CH3
CH3
CH3
CH3
2e- oxidation
H+
TACFe(V)=O
(atposition4)
TACFe(IV)
O
TACFe(III)
23
OH
O
9
24
7
Scheme 4. Proposed mechanism of oxidation.
excess of BF4- is added (Fig. 26S). The broadening of the peaks of catalyst
and ILs in 1H NMR spectra can attribute to π–π interaction of ILs with
catalyst.
The Fe\O bond present in 23 cleaves to afford 24 which is quickly ox-
idized to quinone (7). The rapid oxidation of 11 to 10 under the given
reaction condition (results not shown) also justifies the reaction
pathway. The oxidation of aromatic ring of other PAHs (2, 3 and 4)
and the side chain in 1 can also be explained on this basis. Similar
mechanism has been proposed for the oxidation of PAHs using
iron–porphyrins [33] and iron–phthalocyanines [34]. On going from
neat IL to IL co-solvent mixtures, the enhancement in the electron
transfer rate could be responsible for the better catalytic activity [35].
The effect of nature of substituent present on the ring of 17 on the
ease of oxidation of 1 was also examined (Table 2). The presence of
electron withdrawing groups favoured the oxidation of 1 and 82%
conversion of 1 was observed with 17e catalyst. The reaction using
17f gave poor yields of products (results not shown) and the decom-
position of 17f was also observed. The oxidation of 1 was also com-
pared using 17c and structurally similar analogue 5,10,15,20-tetra
(4-chlorophenyl) porphyrinatoiron (III) chloride [TCPPFe(III)Cl] as
catalyst under the experimental conditions (Table 2, entries 2–3).
The results explicit the higher catalytic activity of 17c than
TCPPFe(III)Cl which may be ascribed to much larger rate of oxygen
atom transfer from oxo-metal corroles to alkanes than that of analo-
gous oxo-metal porphyrins [29]. The catalytic system 17-H2O2 also
worked well for the oxidation of 2, 3 and 4 (Scheme 1, Table 3).
To investigate the generation of reactive intermediate in the reaction,
time-dependent UV–vis studies of 17a with H2O2 were performed. The
addition of H2O2 in the solution of 17a in CH2Cl2 and [bmim][PF6] initially
enhanced the absorption intensity of the Soret band (Fig. 2). This could be
attributed to the attack of H2O2 at the iron centre of (TACFe)+ produced
by the H-bonding effect of IL, to give hydroperoxy intermediate (18)
(Scheme 3). Further, the decay of the Soret band with time indicates
the homolytic cleavage of –OOH bond to give transient oxo-iron (V)
intermediate (19) [30]. The ESI-MS (+) spectra of 17d on addition
of H2O2 also exhibited peaks at m/z 785.89, 818.89 and 801.88
corresponding to [MH+\Cl], [(MOOH)H+\Cl] and [(MO)H+\Cl]
(Figs. 3 and 27S–29S), supporting the generation of hydroperoxy and
iron-oxo corrole intermediates in the reactions. Recently, the formation
of iron (V)oxo reactive intermediate in an iron-complex catalyzed oxi-
dation process has been proposed on the basis of mass spectroscopic
and cyclic voltametric data [31].
4. Conclusion
The oxidation of PAHs (1–4) with H2O2 catalyzed by 17 in IL
co-solvent mixture gives the oxidized products, particularly quinones
in high yields. The presence of electron-withdrawing groups at the ar-
omatic ring of 17 and, the use of non-coordinating IL and coordinating
organic solvent improves the yields. The present method offers vari-
ous advantages such as high yield of product, milder conditions and
reusability of the catalyst.
Acknowledgements
The authors are thankful to CSIR (Council of Scientific and Indus-
trial Research) for financial assistance.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
References
‡The part of this work has been presented in 3rd International Conference
on Heterocyclic Chemistry, Jaipur, India (Dec. 2011).
The oxidation of PAHs with H2O2 catalyzed by 17 in ILs is believed
to proceed with the dissociation of Fe\Cl bond to give (TACFe)+ spe-
cies which on reaction with H2O2 gives 18 and then 19 intermediate
(Scheme 3). In presence of methanol and ACN, (TACFe)+ could be
[1] J.M. Delgado-Saborit, C. Stark, R.M. Harrison, Environment International 37 (2011)
383–392.
[2] K. Saurabh, A. Sharma, S. Yadav, D. Parmar, Biochemical Pharmacology 79 (2010)
1182–1188.
[3] N.G. Giri, S.M.S. Chauhan, Catalysis Communications 10 (2009) 383–387.
[4] N. Jain, A. Kumar, S. Chauhan, S.M.S. Chauhan, Tetrahedron 61 (2005) 1015–1060.
[5] P. Kumari, N. Sinha, P. Chauhan, S.M.S. Chauhan, Current Organic Synthesis
8 (2011) 393–437.
[6] I. Aviv, Z. Gross, Chemical Communications (2007) 1987–1999.
[7] A.N. Biswas, A. Pariyar, S. Bose, P. Das, P. Bandyopadhyay, Catalysis Communications
11 (2010) 1008–1011.
present in the form of [(TAC)Fe(ACN)]+ and [(TAC)Fe(CH3\OH)]+
.
The species 19 attacks at the aromatic ring of 1 at position 1 creating
a new carbon-oxygen bond forming σ-adduct (20) (Scheme 4). The
e--transfer leads to the carbocation 21 and then to 22 after elimina-
tion of the proton. The cleavage of Fe\O-aromatic bond in 22 gives
rise to 9 and (TACFe)+ species regenerates [32]. The addition of a
second iron-oxo species (19) at position 4 gives 23 in several steps.