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T.-H. Chen et al. / Inorganica Chimica Acta 451 (2016) 65–72
Table 2
3.3. Substrate scope
Catalytic oxidation of thioanisole by iron(III) corrole (1a) with various oxygen sources
in the absence of H2O.a
Subsequently, the substrate scope of the catalytic sulfoxidations
were further explored under optimized conditions. Table 3 lists the
oxidized products and corresponding selectivities using 1a and 1b
as the catalyst, respectively. In analogy with what we observed for
2a, all catalytic oxidation of substituted thioanisoles proceeded
with a quantitative conversion into the corresponding sulfoxides
as the only identifiable products (>99% by GC). Again, in all cases,
no traces of sulfones were detected. Significant for preparative
purposes, comparable yields in isolated products were obtained.
Some sulfoxidation reactions were scaled up to 5.0 mmol and to
our delight, 100% conversion and over 90% isolated yield
exclusively for sulfoxides were achieved (entries 1, 2 and 6).
As evident in Table 3, we have demonstrated that the highly
chemoselective and efficient oxidation of sulfides to sulfoxides
was achieved by iron(III) corroles with PhI(OAc)2. The slow and
steady-state formation of PhIO in the presence of small amount
of water may contribute the enhanced catalytic activity, the stabil-
ity of the corrole catalyst against degradation, and even improved
solubility of the oxygen source. Its efficacy for sulfoxidation was
further investigated in the oxidation of vinyl sulfides (Table 4).
The oxidation of sulfides in the presence of electron-rich double
bonds is often problematic with many traditional oxidants and
catalytic systems because of interference with epoxidations. With
catalysts (1a and 1b) and PhI(OAc)2 in the presence of H2O, we
found that no epoxidation took place and only sulfoxidations were
observed in all cases. However, moderate conversions were
observed in some cases (entries 1, 2 and 5) mainly due to the cat-
alyst bleaching under those catalytic conditions, as indicated by
UV–vis spectroscopic spectra in the end of reactions (see Fig. S2
in the Supporting information). Similarly, the presence of the
hydroxyl group did not disturb the chemoselective oxidation of
sulfide and no alcohol oxidation was observed (entry 3–4 and 7–8).
O
S
FeIIITPC (0.2 mol%)
O
O
S
S
Ph
Me
Ph
Me
Ph
Me
PhI(OAc)2 (1.5 eqiv.)
CD3OD, 25 o
C
2a
3a
4a
Entry
Oxygen source
Convn.b (%)
Mbb (%)
Selectivity (3a:4a)b
1
2
3
PhI(OAc)2
PhIO
60
5
>95
92
>95
93
>99:1
>99:1
>99:1
77:23
>99:1
TBHP
17
36
12
4
m-CPBA
H2O2
5c
90
a
Unless otherwise noted, all reactions were performed in CD3OD (2 mL) at ca.
23 °C with a 1:1.5 molar ratio of thioanisole versus PhI(OAc)2 and 0.2 mol% catalyst
at an initial substrate concentration of 0.25 M in the absence of H2O and the
mixture was stirred for 1 h.
b
Conversions (Convn.), mass balance (Mb) and product ratios were determined
by 1H NMR (JEOL 500 M) and by quantitative GC–MS analysis with an internal
standard (1,2,4-trichlorobenzene) on the crude reaction mixture after the reaction
is quenched by sodium hydroxide solution (consuming all oxygen source).
c
30% aqueous solution of H2O2 was used.
generation of the active oxidizing species. With respect to the
sulfoxidation of 2a under identical reaction conditions, FeIII(TPC)
or FeIII(TPFC) performs as well as one of the best porphyrin cata-
lysts known FeIII(TPFPP)Cl [TPFPP = tetrakis(pentafluorophenyl)-
porphinato] (entry 11) [36]. Control experiments showed that no
sulfoxide was formed in the absence of either the catalyst or the
PhI(OAc)2 even at elevated temperature (50 °C), or in the presence
of PhI(OAc)2 and water without catalyst (data not shown).
3.2. Comparison of various oxygen sources in the catalytic oxidation of
thioanisole
3.4. Spectral studies for probing the active intermediate
The promising results with the PhI(OAc)2 in Table 1 prompted
us to screen other commonly used oxygen sources in the iron(III)
corrole-catalyzed oxidation of thioanisole under identical
experimental conditions for the intended purpose of comparison.
The representative results shown in Table 2 demonstrated that
the mild oxygen source PhI(OAc)2 was especially effective in the
iron(III) corrole-catalyzed selective oxidation of sulfide to sulfoxide
(entry 1). In particular, PhI(OAc)2 shows great advantage in
enhancing corrole catalyst stability against the catalytic
degradation owing to its mild oxidizing ability. Monitoring
catalytic reactions by UV–vis spectroscopy showed no significant
catalyst bleaching in the end of reactions. Although PhIO or organic
peroxide is a common oxygen source generally used in metal-
catalyzed oxidations, it was found that the use of PhIO or tert-butyl
hydroperoxide (TBHP) under the same conditions led to a lower
catalytic activity (entries 2 and 3). m-Chloroperoxybenzoic acid
(m-CPBA) gave a moderate conversion albeit with undesirable
sulfone formation (entry 4). H2O2 as the oxygen source gave only
12% conversion (entry 5) and the catalyst was also found to be
bleached during the reaction (>90% bleaching in the end of reaction
as indicated by UV–vis spectroscopy). The most likely explanation
is that these oxygen sources with greater oxidizing abilities might
accelerate the degradation of the iron(III) corrole catalysts as well
as facilitate the over oxidation of sulfide to sulfone. Similar results
were also observed in our previous work of selective epoxidation
catalyzed by iron corrole complexes and PhI(OAc)2 [38].
Apparently, the choice of oxygen sources is crucial regarding
catalyst stability and reactivity in metallocorrole-mediated
oxidations.
High-valent metal-oxo corroles have been proposed as the key
intermediates in many metallocorrole-catalyzed reactions
[21,43]. Of note, the possible formation of the highly reactive
iron(V)-oxo corrole (5) was reported in previous transient
absorption studies [44,45]. To this end, we conducted the chemical
oxidation of FeIII(Cor) by PhI(OAc)2 in CH3OH in the absence of
sulfides to probe the nature of the active oxidizing intermediate.
If involvement of a high-valent iron-oxo is operative in the current
study, one can expect that the formed iron-oxo corroles may
behave differently by considering the different electronic environ-
ments of the two corrole ligands. In view of its electrophilic nature,
the reactivity of 5 follows TPFC > TPC. On the other hand, the for-
mation of the transient 5 in the electron-deficient TPFC system is
apparently less favorable because of the expected high oxidation
or energy barrier.
As shown in Fig. 2A, addition of 20 equivalents of PhI(OAc)2
resulted in instant conversion of 1a to a short-lived transient (5)
that decayed rapidly to an unknown product (6) with a weaker
Soret band at 410 nm and a stronger Q band at 720 nm, character-
istic of an iron(IV)-oxo corrole radical cation. Direct spectroscopic
evidence for such a high-valent iron-oxo corrole radical cation
has been reported [46]. It is noteworthy that the UV–vis signal of
the transient 5 that was instantly formed closely resembled that
of the putative corrole-iron(V)-oxo species, which apparently also
was formed when the neutral corrole-iron(III) complex was mixed
with excess m-CPBA [44]. As thermodynamically favored in the
less electron-demanding TPC system, iron(V)–oxo species 5 might
relax to radical cation species 6 by internal electron transfer (ET)
from the corrole ligand to the iron [47]. In contrast, the