Angewandte
Chemie
Table 2: Oxidation of cyclohexane with oxone catalyzed by 1 and 2.[a]
(Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), which is
[12]
=
Product TON[b]
Alcohol Ketone
Total
TON
known to support highly oxidizing {Ru O} complexes.
Entry Catalyst Substrate
In initial studies, we found that that propane and ethane
could be oxidized within five minutes at room temperature in
the presence of [FeIII(Tp)2]+ (1; Tp = hydrotris(1-pyrazolyl)-
borate)) and oxone (potassium peroxymonosulfate,
2 KHSO5·KHSO4·K2SO4), albeit in a stoichiometric manner.
We subsequently turned our attention to the [FeIII(Me3tacn)-
(R-acac)Cl]+ system (2, R-acac = 3-R-acetylacetonate; R =
H: 2a, R = Cl: 2b; Figure 1). Herein, we report the oxidation
of light alkanes that was mediated or catalyzed by the iron
complexes 1 and 2. Remarkably, the oxidation of propane
with oxone that was catalyzed by 2 afforded a total turnover
number (TON) of approximately 14 for a five-minute
reaction at room temperature.
1
1
2a
2b
6.4
13.0
17.5
6.3
28.1
17.6
12.7
41.1
35.1
2[c]
3[c]
[a] Reaction conditions: cyclohexane (5 mmol), catalyst (0.03 mmol),
oxone (1.5 mmol), NaHCO3 (4.5 mmol), acetone (5 mL), H2O (5 mL),
RT, 5 min. [b] Number of micromoles of product (determined by GC)
divided by number of micromoles of catalyst. [c] Cyclohexane (7 mmol),
catalyst (0.004 mmol), oxone (0.4 mmol), NaHCO3 (1.2 mmol).
a mixture of ethanol and acetic acid; under the same
conditions, propane was oxidized to a mixture of isopropanol,
acetone, and propanoic acid (Scheme 1).[16] These reactions,
Complexes 1[13] and 2[14] were prepared as their ClO4
À
salts; the structures of 1 and 2a were determined by X-ray
crystallography (Supporting Information, Figure S1 and
S2).[15] The cyclic voltammograms of aqueous solutions of
1 (pH 7) and 2c ([FeIII(Me3tacn)(acac)(CH3CN)]2+; pH 1)
gave FeIII/II reduction potentials at Epa/Epc = 0.27/0.17 V (1)
and 0.54/0.1 V (2c), and a large catalytic oxidation current,
which is indicative of an electrochemical oxidation reaction,
at approximately 1.5 V relative to the saturated calomel
electrode (SCE); the latter is tentatively assigned to oxidation
of the reaction medium, presumably water (Figure S3 and
S4). 2c also gave an irreversible two-electron oxidation wave
at 1.46 V (vs. SCE).
The catalytic oxidation of styrene, cyclohexene, cyclo-
octene, and 1-decene in the presence of 1 or 2 (4 mol%) and
oxone in CH3CN/H2O or acetone/H2O at room temperature
yielded the corresponding epoxides in up to 96% yield within
five minutes (alkene as the limiting reagent; Table 1).
Importantly, cyclohexane was oxidized to cyclohexanol and
cyclohexanone within five minutes with a total TON of up to
approximately 41 (Table 2; catalyst loading: 0.6 or
0.06 mol%).
Scheme 1. Oxidation of ethane and propane with oxone mediated by
1 or catalyzed by 2.
however, each gave a total TON of 1.0 and were thus not
catalytic; presumably, the Fe complex decomposed during
oxidation. The oxidation was assisted by H2O, as a control
experiment for the oxidation of propane in the absence of
H2O, but in the presence of 1 and tetrabutylammonium
oxone, with a reaction time of five minutes gave very little
acetone (10% yield based on 1), and no other oxidation
products were detected. Using 2a or 2b (1 mol%) as the
catalyst, the oxidation of propane with oxone in CH3CN/H2O
Treatment of ethane (6.9 bar) with 1 and oxone in
CH3CN/H2O at room temperature for five minutes afforded
Table 1: Epoxidation of alkenes with oxone catalyzed by 1 and 2.[a]
Entry Catalyst Substrate Conv.[b] [%]
Product
Yield[b,c] [%]
À
(1:1, v/v) at room temperature gave the C H oxidized
products with a total TON of 13.7 after five minutes
(Table S1); the product distribution (isopropanol/acetone/
propanoic acid ꢁ 10:23:1) was different from that of the
previously reported FeCl3-catalyzed oxidation of propane
with H2O2 (258C, 1 h; isopropylhydroperoxide as the major
product).[9b] Overoxidation likely results in the formation of
acetone and propanoic acid, as oxidation of isopropanol and
propanol in the presence of 2 and oxone under similar
conditions gave acetone and propanoic acid in 63% and 52%
yield, respectively. Oxidation of ethane and propane with iron
complex 2 and oxone gave the oxidized products with total
TONs of up to 3.4 and 20.4, respectively after 30 min
(Scheme 1).
1
2
1
1
85
81
72[d] (85[e])
75 (93[e])
3
4
5
1
2a
2b
97
99
97
96 (99[e])
79 (80[e])
87 (90[e])
6
1
77[f]
67 (87[e])
[a] Reaction conditions: substrate (0.5 mmol) in CH3CN (3 mL) for 1 or
in acetone (3 mL) for 2, aqueous solution of oxone (0.75 mmol, 3 mL),
NaHCO3 (2.25 mmol), catalyst (0.02 mmol), RT, 5 min. [b] Determined
by GC analysis. [c] Based on the starting substrate. [d] Benzaldehyde was
also formed (9.4% yield). [e] Selectivity (yield of epoxide product divided
by substrate conversion). [f] Reaction time: 15 min.
=
The involvement of {Fe O} intermediates V (Figure 1) in
the hydrocarbon oxidation with oxone catalyzed by 1 or 2 is
supported by the following observations: First, high-resolu-
Angew. Chem. Int. Ed. 2014, 53, 798 –803
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