Sc3+ (or HClO4) Activation of a Nonheme FeIII-OOH Intermediate for the Rapid Hydroxylation of Cyclohexane and Benzene

Article abstract of DOI:10.1021/jacs.8b01435

[Fe(β-BPMCN)(CH₃CN)₂]²⁺ (1, BPMCN = N,N′-bis(pyridyl-2-methyl)-N,N′-dimethyl-trans-1,2-diaminocyclo-hexane) is a relatively poor catalyst for cyclohexane oxidation by H₂O₂ and cannot perform benzene hydroxylation. However, addition of Sc³⁺ activates the 1/H₂O₂ reaction mixture to be able to hydroxylate cyclohexane and benzene within seconds at -40 °C. A metastable S = 1/2 Fe^III-(η¹-OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon addition of Sc³⁺ at rates independent of [substrate] but linearly dependent on [Sc³⁺]. HClO₄ elicits comparable reactivity as Sc³⁺ at the same concentration. We thus postulate that these additives both facilitate O-O bond heterolysis of 2 to form a common highly electrophilic Fe^VO oxidant that is comparably reactive to the fastest nonheme high-valent iron-oxo oxidants found to date.

Full text of DOI:10.1021/jacs.8b01435

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3+
4
III
Sc (or HClO) activation of a nonheme Fe–OOH intermediate
for the rapid hydroxylation of cyclohexane and benzene
Subhasree Kal, Apparao Draksharapu, and Lawrence Que
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01435 • Publication Date (Web): 05 Apr 2018
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Sc3+ (or HClO4) activation of a nonheme FeIII–OOH interme-
diate for the rapid hydroxylation of cyclohexane and benzene
9
Subhasree Kal, Apparao Draksharapu and Lawrence Que, Jr*
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Department of Chemistry and Center for Metals in Biocatalysis, 207 Pleasant St. SE, University of Minnesota,
Minneapolis, Minnesota 55455, United States
ABSTRACT: [Fe(β-BPMCN)(CH3CN)2]2+ (1, BPMCN = N,N’-bis(pyridyl-2-methyl)-N,N'-dimethyl-trans-1,2-diaminocyclo-
hexane) is a relatively poor catalyst for cyclohexane oxidation by H2O2 and cannot perform benzene hydroxylation. How-
ever, addition of Sc3+ activates the 1/H2O2 reaction mixture to be able to hydroxylate cyclohexane and benzene within se-
conds at -40 °C. A metastable S = ½ FeIII–(η1-OOH) intermediate 2 is trapped at -40 °C, which undergoes rapid decay upon
addition of Sc3+ at rates independent of [substrate] but linearly dependent on [Sc3+]. HClO4 elicits comparable reactivity as
Sc3+ at the same concentration. We thus postulate that these additives both facilitate O–O bond heterolysis of 2 to form a
common highly electrophilic FeV=O oxidant (Ox) that is comparably reactive to the fastest nonheme high-valent iron-oxo
oxidants found to date.
INTRODUCTION
subsequently found evidence for the formation of a Sc3+-
bound MnV(O)(TAML) complex (TAML = tetraamido macro-
cyclic ligand) with the Lewis acid coordinated to the carbonyl
oxygen of the macrocyclic ligand.17 Similarly, Lau demon-
strated activation of polyoxo anions such as chromate, per-
manganate and ferrate by Lewis acids for the oxidation of C–
H bonds.18–21 In a more recent effort, Lau and coworkers
found that Lewis acids could activate [OsVI(N)(Cl)4]– by in-
teracting with the nitride ligand to catalyze alkane oxidation
with H2O2 or ROOH.22
More relevant to our effort was the finding that Sc3+ and Y3+
induce cleavage of the O–O bond in [FeIII(η2-O2)(TMC)]+
(TMC = tetramethylcyclam) to form the corresponding ox-
oiron(IV) complex,23,24 which led us to explore whether Fe-
based intermediates involved in nonheme iron oxidation
catalysis with peroxides as oxidants might be similarly acti-
vated. We were encouraged by recent papers that demon-
strated positive effects of adding Sc3+ to metal-catalyzed oxi-
dations at room temperature. Yin and coworkers found that
adding Sc3+ tripled the olefin epoxidation yield by
FeII(BPMEN)/H2O2,25 while Nodzewska and Watkinson
showed that Sc3+ accelerated epoxidation rates of electron-
deficient styrenes with the [Mn2(μ-O)3(TMTACN)2]2+/H2O2
combination (TMTACN = 1,4,7-trimethyl-1,4-7-triazacyclo-
nonane).26 Chatterjee and Paine reported that Sc3+ promoted
the stoichiometric hydroxylation of cyclohexane in the reac-
tion of a nonheme iron(II)-benzilate complex with O2.27 In all
these cases, some interaction of Sc3+ with the putative metal-
based oxidant was postulated, but no direct evidence for such
an interaction was demonstrated. In this paper we focus on
The emergence of nonheme iron enzymes as excellent bio-
catalysts for C–H bond functionalization1,2 has spurred the
investigation of synthetic nonheme iron catalysts that per-
form hydrocarbon oxidations with H2O2 as the oxidant.3–5
Spectroscopic and mechanistic studies on some of these syn-
thetic catalysts have provided evidence for an S = 1/2 FeIII–
OOH intermediate that then undergoes O–O bond heteroly-
sis to generate the actual oxidant. An electrophilic ox-
oiron(V) species derived therefrom is proposed to be respon-
sible for substrate oxidation.6–9 For FeII(TPA) and
FeII(BPMEN) (TPA = tris(pyridyl-2-methyl)amine; BPMEN =
N,N’-bis(pyridyl-2-methyl)-1,2-diaminoethane), this cleavage
is promoted by a proton, delivered by a water or carboxylic
acid ligand that is proposed to bind to the iron center cis to
the HOO moiety. However, [Fe(β-BPMCN)(CH3CN)2]2+ (1,
Figure 1) shows a reactivity pattern different from the well-
studied FeII(TPA) and FeII(BPMEN) catalysts, and has previ-
ously been found to be a sluggish hydroxylation catalyst with
H2O2 as the oxidant.10 We have thus investigated strategies
by which to enhance the catalytic performance of 1.
Extensive work by Fukuzumi and Nam showed that addition
of Sc3+ and other Lewis acids can significantly enhance the
oxidative reactivity of high-valent metal-oxo species, mainly
by boosting the rate of electron transfer from substrate to a
proposed Lewis-acid adduct of the metal-oxo center, and the
electron transfer rate increased with the strength of the Lew-
is acid.11,12 In other work, Yin, Goldberg and Collins inde-
pendently showed Lewis-acid activation of high-valent
Mn(OH)2 or Mn(O) centers,.13–16 while Fukuzumi and Nam
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the sluggish catalyst 1 and report our findings that addition
generated after the initial H-atom abstraction must be short-
lived and immediately rebounds to the oxygen attached to
the metal center, which is also consistent with the absence of
any significant effect of O2 on the product TONs (Tables 1
and S1). The competitive oxidation of c-C6H12 and c-C6D12 in
the presence of Sc3+ shows a product kinetic isotope effect
(PKIE) of 2.5(2) versus 5(1) in the absence of Sc3+ (Table 1),
suggesting the generation of a more powerful oxidant in the
presence of Sc3+. Taken together, the high A/K ratio, the ab-
sence of a significant effect of O2 and a PKIE ≥ 2 point to a
metal-based oxidant that forms in the presence of Sc3+.
of Sc3+ or HClO4 to the 1/H2O2 combination generates a pow-
erful hydroxylating agent. We have trapped a transient S = ½
FeIII–(η1-OOH) intermediate 2 at cryogenic temperatures and
demonstrate its activation by interaction with Sc3+ or HClO4
to form a species that hydroxylates cyclohexane and benzene
at -40 °C within a few seconds.
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RESULTS AND DISCUSSION
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Effect of Sc(OTf)3 on the reactivity of 1/H2O2. The oxida-
tion of cyclohexane using 1 and 90% H2O2 (10 eq relative to 1)
in CH3CN at 20 °C affords 0.5 eq cyclohexanol and 0.6 eq
cyclohexanone as products (Figure 1 and Table 1). The appar-
ent stoichiometric oxidation of cyclohexane and the low al-
cohol-to-ketone ratio (A/K) of 0.8 together indicate an un-
promising catalyst/oxidant combination. These values do not
change much upon addition of 2 eq Al3+, Y3+, Yb3+, or Zn2+
relative to 1 (Figure S1), but treatment with 2 eq Sc3+ elicits a
10-fold increase in both the cyclohexanol yield and the A/K
ratio (Figure 1). This observation can be rationalized by not-
ing that H2O2 solutions contain water, and Lewis acidic metal
salts such as Al(OTf)3 and Zn(OTf)2 are known to react with
water and decompose. However, Sc(OTf)3 is a relatively sta-
ble Lewis acid in water and a much stronger Lewis acid than
the other water-stable Lewis acids tried like Y(OTf)3 and
Yb(OTf)3.12,28 The increase in the alcohol TON (turnover
number) is dependent on [Sc3+] and this effect starts to plat-
eau at ca. 2 eq Sc3+. These results suggest that addition of Sc3+
changes the nature of the active oxidant formed in the reac-
tion (Figure 1).
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a
Table 1: Hydroxylation of c-C6H12 and C6H6 by 1/H2O2
%
TON
alco-
hol
TON
ke-
tone
(K)
H2O2 TON
A/K
PKIEc
con-
ver-
tedd
PhOH
(P)
b
(A)
no Sc3+ 0.5(1)
2000
0.6(1)
0.4(1)
0.8
5
5(1)
11
0
0
eq
2.0(1)
4.2(4)
24
AcOH
2
eq
4.2(4) 0.3(1)
4.0(2) 0.1(1)
14
2.5(2)
2.0(1)
45
40
4.2(2)
4.0(3)
Sc3+
2
eq
40
HClO4
a
All reactions were performed at room temperature under
air; TON (Turnover Number) = moles of product/moles of 1.
A/K = TON alcohol/TON ketone. PKIE = kinetic isotope
effect based on yields of cyclohexanol and cyclohexanol-d11
(For PKIE experiments, H2O2 was added by syringe pump).
b
c
d
% conversion of H2O2 = (A + K)/H2O2 x 100 for c-C6H12 oxi-
dation.
Interestingly, the 1/H2O2/Sc3+ combination can also catalyze
benzene hydroxylation, affording phenol in amounts compa-
rable to cyclohexanol in cyclohexane oxidation (Figure 1).
Benzene hydroxylation is observed only when Sc3+ is present
with as much as 5 TON or 50% conversion of H2O2 into phe-
nol (Figure 1), a yield comparable to or better than the other
iron systems reported so far under similar conditions (Table
S2). As found for cyclohexane hydroxylation, the phenol yield
depends on [Sc3+] and begins to plateau at ~2 eq Sc3+ w.r.t 1
(Figure 1). No over-oxidation products are found, unlike for
other catalytic systems in which p-benzoquinone and/or
catechol were observed as byproducts.9,29,30 The oxidant
formed is also able to attack electron-poor benzene deriva-
tives such as bromobenzene, trifluoromethylbenzene, nitro-
benzene (Figure S2), and even 1-chloro-2-nitrobenzene. For
nitrobenzene, 3-nitrophenol and 2-nitrophenol products are
obtained, with respective TONs of 1.1(1) and 0.5(1). These
results point to the formation of a powerful and highly elec-
trophilic oxidant. An inverse KIE of 0.9 is found from GC-MS
analysis based on product peak intensity ratios in the oxida-
tion of a 1:1 mixture of C6H6 and C6D6. Such values are typi-
cally found for electrophilic aromatic substitution reactions,31
Figure 1. Yields in the hydroxylation of C6H6 (100 eq relative
to 1) or c-C6H12 (1000 eq) by 1 (0.7 mM) and 10 eq 90% H2O2
in CH3CN at 20 °C under air as a function of [Sc3+]. Xn+ re-
flects the averaged results from adding 2 eq Al3+, Y3+, Yb3+, or
Zn2+ (for individual results see Figure S1). (90% H2O2 was
used as oxidant to minimize Lewis-acid deactivation by water
present in the H2O2 solution.)
In the presence of Sc3+, 50% of H2O2 can be converted into
products versus only 11% conversion in the absence of Sc3+
(Figure 1). The high alcohol-to-ketone ratio (A/K) of 14 found
for the Sc-activated reaction indicates that the alkyl radical
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implicating a metal-based electrophilic oxidant, rather than a
radical-based oxidant for which the KIE is typically greater
than 1.30,32,33
(vide infra). These results together suggest the formation of a
common oxidant for HClO4 and Sc3+. In preliminary
experiments, we have found that adding HNO3 or H2SO4
instead of HClO4 to the 1/H2O2/benzene mixture also leads
to formation of 3 but either at a slower rate than HClO4 or at
a lower spectroscopic yield of 3, respectively. This reactivty
difference may derive from the higher pKa’s of these acids
and/or possible interactions of the NO3 or SO4
counteranion with the active oxidant. The acid dependence
will be further investigated in subsequent efforts.
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Despite being quite different transformations, cyclohexane
and benzene oxidation by 1/H2O2/Sc3+ show a similar de-
pendence on [Sc3+] (Figure 1), allowing the comparison of the
two distinct and relatively difficult reactions. Competitive
oxidation of cyclohexane and benzene shows the oxidation of
the latter to be favored by 10:1 on a per mole basis (Figure 2).
On the other hand, oxidation of a 1:1 mixture of cyclohexane
and nitrobenzene shows cyclohexane oxidation to be favored
3:1 over that of nitrobenzene. These comparisons give rise to
a reactivity order of benzene > cyclohexane > nitrobenzene
and, to the best of our knowledge, represent a unique oppor-
tunity for such a reactivity comparison for a first-row transi-
tion metal oxidation catalyst.
-
2–
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Figure 3. Yields in the hydroxylation of C6H6 (100 eq relative
to 1) or c-C6H12 (1000 eq) by 1 (0.7 mM) and 10 eq 90% H2O2
in CH3CN at 20 °C under air as a function of [HClO4].
Characterization of the [FeIII(β-BPMCN)(OOH)]2+ inter-
mediate 2. To gain insight into this chemistry of 1/H2O2 with
Sc3+ or HClO4, the reaction of 1 with H2O2 was initially inves-
tigated in the absence of Sc3+ and HClO4 at -40 °C in CH3CN.
A transient purple intermediate, 2, is observed with a λmax at
545 nm, an EPR signal with g = 2.22, 2.17, and 1.96, and a res-
onance Raman spectrum with bands at 613 and 802 cm-1 as-
signed to Fe–O and O–O vibrations, respectively (Figure 4
and S3). Taken together, all the spectroscopic evidence char-
acterizes 2 as a low-spin (S = ½) FeIII–η1-OOH intermediate,
based on literature precedents (Table S3).35,36 The corre-
sponding [(TPA)FeIII–OOH]2+ species has been implicated as
the precursor to the electrophilic oxidant responsible for
Fe(TPA)-catalyzed alkane hydroxylation and olefin epoxida-
tion and cis-dihydroxylation.6–8 However, despite their spec-
troscopic similarities, 2 is much less reactive than its TPA
analog6,10 and requires activation by Sc3+ or HClO4 to carry
out electrophilic oxidations.
Figure 2. Competitive hydroxylations of cyclohexane (c) and
benzene (b, left) or nitrobenzene (n, right) with 0.7 mM 1, 10
eq H2O2, 2 eq Sc3+ and 600 eq total substrate. The numbers on
the x-axis represent the substrate ratio in the competition
experiments.
Effect of HOAc and HClO4 on the reactivity of 1/H2O2. In
our previous work with 1 and olefins, we showed that addi-
tion of 2000 eq HOAc to 1/H2O2 switched its reactivity pref-
erence from cis-dihydroxylation of electron-poor olefins to
epoxidation of electron-rich olefins, involving formation of a
putative FeV(O)(OAc) oxidant.34 Here, we found that addition
of 2000 eq HOAc to 1/H2O2 only doubled the amount of cy-
clohexanol from cyclohexane but did not convert benzene to
phenol (see Table 1). Additionally, the PKIE observed for
HOAc addition is quite similar to the value observed without
additive but different from that obtained in the presence of
Sc3+ (Table 1). These results suggest that the oxidant pro-
duced by adding HOAc is less effective and quite distinct
from that formed with Sc3+.
We have also investigated replacing Sc3+ with HClO4 and
found HClO4 to have the same effect as Sc3+ on 1-catalyzed
oxidations (Table 1). At the same concentration as Sc3+,
HClO4 affords similar yields of cyclohexanol and phenol
(Figure 3). Furthermore, essentially the same PKIE value is
found for cyclohexane oxidation by 1/H2O2/HClO4 as
1/H2O2/Sc3+, showing comparable selectivity for attacking the
cyclohexane C–H over C–D bonds. Additionally, addition of
Sc3+ or HClO4 affects the lifetime of intermediate 2 similarly
Figure 4. (a) UV-vis spectrum of 2 formed at -40 °C in
CH3CN from 1 mM 1 and 20 eq H2O2; (b) X-band EPR spec-
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trum of 2 obtained at 40 dB at 2 K; (c) resonance Raman
spectrum of 2 formed with 2.5 mM 1 and 20 eq H2O2 at -30 °C
(λexc 561 nm).
Very interestingly, upon addition of Sc3+ to 2, the rate of 3
formation in the presence of benzene matches the decay rate
of 2 in the presence of cyclohexane (Figure 5b and 6). At the
same concentration as Sc3+, HClO4 causes 2 to decay (in the
presence of cyclohexane) or 3 to form (in the presence of
benzene) at essentially the same rates as Sc3+ (Figure S5 and
Table S4), supporting our earlier inference that Sc3+ and
HClO4 activate 2 in a similar fashion to form a common ac-
tive oxidant. Importantly, the rates of 2 decay and 3 for-
mation both increase as a function of [Sc3+]/[HClO4] (Figure
6b and Figure S5) but do not depend on either [cyclohexane]
or [benzene] (Table S4). Taken together, these observations
suggest that 2 is not the actual oxidant, but instead interacts
with Sc3+/HClO4 to form Ox, the species responsible for sub-
strate oxidation, and the substrate oxidation step must be
faster than the oxidant forming step.
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Kinetic analysis of the reaction of 2 with Sc3+ or HClO4 in
the presence of benzene and cyclohexane. Upon addition
of excess H2O2 to 1 in the presence of cyclohexane, 2 forms
and reaches a pseudo-steady-state phase and then decays
over the course of 90 min at -40 °C (Figure S4a). However,
addition of Sc3+ to this mixture upon maximum accumulation
of 2 accelerates the decay of 2 by a 1000-fold (Figures 5b
(green) and S4b). Similarly, 2 accumulates upon addition of
H2O2 in the presence of benzene within the same time period
as for cyclohexane. However, addition of Sc3+ to 2 in the pres-
ence of benzene instead of cyclohexane generates an intense
blue chromophore characteristic of an FeIII-phenolate species
3 (λmax 620 nm),37 which is conveniently monitored at 800
nm where there is no contribution from 2 (Figures 5a and
5b). No formation of 3 was observed until Sc3+ was added to
the reaction mixture containing benzene (Figure 5b). These
observations concur with the catalytic results for 1/H2O2
where phenol is formed only in the presence of Sc3+ (Table 1).
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Figure 6. (a) Time traces monitoring decay of 2 at 545 nm in
the presence of 185 eq cyclohexane (■) and formation of 3 at
800 nm with 100 eq benzene (○) showing the effect of in-
creasing [Sc3+] (green: 1 eq; red: 2 eq; black: 8 eq Sc3+). (b)
[Sc3+] dependence of kobs for 2 decay with cyclohexane as
substrate (■) or 3 formation (○) with benzene as substrate.
Data for the effect of HClO4 addition are shown in Figure S5.
Addition of Sc3+ or HClO4 allows 1/H2O2 to perform previous-
ly unachievable reactions, such as aromatic hydroxylations,
as well as accelerates the reaction step responsible for the
formation of the active oxidant. FeIII–OOH intermediates
have also been trapped for other nonheme iron catalysts and
found to serve as precursors to the actual oxidant in their
reactions, where the step to form the actual oxidant can be
accelerated by addition of water/acids (Table 2). For
[FeIII(TPA)(OOH)]2+, we showed that it underwent water-
Figure 5. (a) Spectral changes in the visible region upon re-
action of 0.5 mM 1 (dashed black line) in CH3CN at -40 °C
with 20 eq H2O2 to form 2 (dotted purple lines). Formation of
3 is observed upon subsequent addition of 1 eq Sc3+to 2 (solid
blue lines). (b) Time traces monitoring nearly instantaneous
changes in absorbance at 545 and 800 nm after addition of 1
eq Sc3+. ■: 545 nm and ○: 800 nm in the presence of C6H6; ▲:
545 nm in the presence of C6H12.
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assisted O–O bond cleavage to generate an epoxidation
[FeIV(O)(Me3NTB)]2+ and [FeIV(O)(TQA)]2+, and
S = 2
agent,8 wherein substitution of H2O with D2O resulted in a
2.5-fold decrease in the rates of intermediate decay and epox-
ide product formation. Addition of 200 eq HOAc increased
the reaction rate about 100-fold.38 Similarly, Rybak-Akimova
concluded that for FeII(BPMEN)/H2O2 the observed FeIII-
OOH intermediate serves as precursor to the actual oxidant.9
Hydroxylation of benzene to form FeIII-OPh occurs at a [ben-
zene]-independent rate of 0.03 s-1 at 20 °C, which increases to
[FeIV(O)(TDCPP)]+ (a synthetic analog for Compound I).40–43
The decay rate of 2 is 10-fold slower than that of the fastest
cyclohexane oxidation rate reported thus far, which is found
for the putative [FeV(O)(O2CR)(PyNMe3)]2+ oxidant generat-
ed from the reaction of [FeII(PyNMe3)(OTf)2] with excess
peracid;44,45 however increasing the amount of added Sc3+ or
HClO4 to 8 eq makes the rates of the two systems compara-
ble. Interestingly, the fastest rates in this series match, or
perhaps even exceed, the 13 s−1 rate for oxidation of the C3-H
bond of taurine by the S = 2 FeIV=O intermediate of taurine
dioxygenase at 5 °C, after correction for the 45° temperature
difference.46 Thus, the 1/H2O2/(Sc3+ or HClO4) combination
can generate a highly reactive oxidant for the hydroxylation
of challenging hydrocarbon substrates like cyclohexane and
benzene.
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2
3
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8
0.5 s-1 with
1 eq HOAc. Serrano-Plana et al. found
[FeIII(PyNMe3)(OOH)]2+ to be quite unreactive but could be
activated by adding 1.1 eq HOTf to generate an oxidant capa-
ble of hydroxylating cyclohexane.39 When adjusted to a
common temperature of -40 °C, the decay rate constants of
these (L)FeIII–OOH species increase in the order: L =
BPMEN,9 TPA,8 BPMEN + 1 eq HOAc,9 PyNMe3 + 1.1 eq
HOTf,39 TPA + 200 eq HOAc,38 and β-BPMCN + 1-8 eq Sc3+ or
HClO4) (Table 2). This comparison places the 1/H2O2/(Sc3+ or
HClO4) combination among the fastest in the conversion of
an FeIII-OOH intermediate into a highly reactive oxidant for
oxidation catalysis.
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Table 3. Cyclohexane oxidation rates by high-valent non-
heme iron species at -40 °C.
k2 (M-1s-1)
0.00026
0.011
Ref
40
41
[FeV(O)(TAML)]–
Table 2. Rates of decay of FeIII–OOH intermediates at -40 °C,
except where noted.
[FeIII(13-TMC)(OIAr)]2+
[FeIV(O)(TDCPP)]+
obs (s-1)
0.03
Ref
9
0.11
42
42
43
44
Iron complex + additive
k
S = 1 [FeIV(O)(Me3NTB)]2+
S = 2 [FeIV(O)(TQA)]2+
[FeV(O)(O2CR)(PyNMe3)]2+
0.25
FeIII(BPMEN)(OOH) (20 °C)a
FeIII(BPMEN)(OOH) + 1 eq HOAc (20 0.5
°C)a
9
0.37
2.8
FeIII(TPA)(OOH)b
0.002
8
[FeIII(β-BPMCN)(OOH)]2+ (2) kobs (s-1) = This work
+ 1 – 8 eq Sc3+ or HClO4
0.3-2.6
FeIII(TPA)(OOH) + 200 eq HOAc
FeIII(PyNMe3)(OOH) + 1.1 eq HOTf c
0.17
38
39
Abbreviations used: TAML = tetraaza macrocyclic ligand;
H2TDCPP = meso-tetrakis(2,6-dichlorophenyl)porphin; 13-
0.02
FeIII(β-BPMCN)(OOH)
+ 1-8 eq Sc3+/H+ d
0.3–2.6
This
work
TMC
Me3NTB
tris(quinolyl-2-methyl)amine.
=
1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane;
=
tris(benzimidazolyl-2-methyl)-amine; TQA
=
a
c
b
In the presence of benzene. In the presence of 1-octene.
PyNMe3 = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclo-
decaphane. d In the presence of cyclohexane or benzene.
Mechanistic insights and the nature of Ox. 18O-labeling
experiments have been shown to be the key to shedding
mechanistic light on the action of oxygenases since 1955,47–50
and have also proven useful in providing insight into the
nature of the oxidants formed upon O–O cleavage of FeIII–
OOH intermediates in bio-inspired metal-catalyzed reac-
tions.6,7,51 For the well-studied Fe(BPMEN) and Fe(TPA) cata-
lysts, the accumulated evidence supports the rate determin-
ing H2O-assisted cleavage of the O–O bond of the cis-H2O–
FeIII–OOH intermediate to form an (HO)–FeV=O oxidant.
Oxo-hydroxo tautomerism rationalizes the observed partial
incorporation of 18O from added H218O into the oxidation
products (Table 4).6–9
For the case of 1/H2O2 in the absence of additives, 18O from
H218O is partially incorporated into the products. However a
significant fraction of the O-atom incorporated into the
products is derived from O2, unlike for Fe(TPA) and
Fe(BPMEN) (Table 4).10 The observed incorporation of an O-
atom from O2 suggests that the alkyl radical formed after
initial hydrogen atom abstraction by the iron oxidant is suffi-
We also compare the oxidant generated by the 1/H2O2/Sc3+
combination with a number of high-valent nonheme iron-
oxo species that have been characterized and shown to di-
rectly hydroxylate cyclohexane (Table 3). For the latter com-
plexes, their reactivity is reflected by the magnitude of their
second order rate constants measured at -40 °C. Unlike these
complexes, Ox, the oxidant from our [FeII(β-
BPMCN)(OTf)2/(Sc3+ or HClO4) system cannot be observed,
because its formation upon decay of 2 is slower than the ac-
tual substrate oxidation step. Nevertheless, a comparison of
the rate of 2 decay to the cyclohexane oxidation rates from
the examples in the literature would still be informative, and
these data are collected in Table 3. With 1 eq Sc3+ or HClO4, 2
decays to form the active oxidant at 0.3 s-1, which is signifi-
cantly faster than the oxidation rates of 1 M cyclohexane by
[FeV(O)(TAML)]– and [FeIII(13-TMC)(OIAr)]2+ (which could
be considered to be a ‘masked FeV(O)’ species), but compara-
ble to those of the oxoiron(IV) complexes,
S = 1
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ciently long lived to be captured by O2. In contrast, no 18O-
incorporation from added H218O into the cyclohexanol and
phenol products is observed in the 1-catalyzed oxidation of
cyclohexane and benzene in the presence of Sc3+ or HClO4,
which is confirmed by complementary experiments with a
10% aqueous solution of H218O2 showing essentially quantita-
tive 18O-label incorporation into the cyclohexanol and phenol
products in the Sc3+ experiments (Table 4). Furthermore, the
absence of O-atom incorporation from O2 in the presence of
Sc3+ indicates the formation of short-lived alkyl radicals that
undergo fast rebound, as we deduced from the reactivity
studies mentioned earlier. These results thereby exclude the
H2O-assisted mechanism associated with the FeII(TPA) and
FeII(BPMEN) catalysts6,7 to account for the labeling results
observed for the 1/H2O2/(Sc3+ or HClO4) system.
1
2
3
4
5
6
7
8
Scheme 1. Proposed mechanism for the effect of Sc3+ or
HClO4 in cyclohexane and benzene hydroxylation by 1/H2O2.
9
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60
SUMMARY
In summary, we have shown that addition of a redox-inactive
strong Lewis acid like Sc(OTf)3 or a Brønsted acid like HClO4
to the reaction mixture of the nonheme iron catalyst 1 and
H2O2 leads to the formation of a highly electrophilic oxidant
capable of hydroxylating cyclohexane and benzene with
comparable catalytic efficiency, a combination of transfor-
mations rarely observed for synthetic nonheme iron cata-
lysts.3,4,36 This highly reactive species compares well in C–H
bond cleaving ability with the fastest nonheme iron-oxo oxi-
dants reported thus far (Table 3). Catalyst 1 is unique as the
nature of the active oxidant that is formed can be tuned by
various additives to perform four distinct reactions. The cis-
dihydroxylation of electron-deficient C=C bonds in the ab-
sence of any additive and the epoxidation of electron-rich
C=C bonds in the presence of acetic acid have been reported
previously.10,34 In this paper, we demonstrate its ability to
hydroxylate benzene and cyclohexane within seconds at -40
°C upon addition of Sc3+ or HClO4 and identify a new route
for accessing a powerful electrophilic oxidant in this fascinat-
ing landscape of high-valent nonheme oxoiron oxidants.
Table 4: Percentage of 18O-incorporation into products
from the oxidation of cyclohexane and benzenea
H218O2 H218O
Ref
Cyclohexanol
FeII(β-BPMCN) + Sc3+
FeII(β-BPMCN)
FeII(TPA)
97
34
70
84
1
This work
10
27
18
10
6
FeII(BPMEN)
6
Phenol
FeII(β-BPMCN) + Sc3+
FeII(BPMEN)
99
81
2
This work
9
22
a
All experiments reported in this work were performed in
the presence of air. The percentage incorporation values
were calculated based on the 18O-content of the reagents. The
H218O2 labeling experiments were performed with 10%
H218O2. The larger amount of H2O present in 10% H218O2 rela-
tive to that in 90% H216O2 reduced the product yield by 50%
but the effect of Sc3+ is still present (Figure S6).
EXPERIMENTAL SECTION
Materials and Instrumentation. All materials were pur-
chased from Sigma-Aldrich and used as received unless noted
otherwise. H218O (97% 18O-enriched) and H218O2 (90% 18O-
enriched, 10% solution in H216O) were obtained from Berry &
Associates-ICON Isotopes. Cyclohexane, benzene and nitro-
benzene were passed through alumina and silica gel before
the reactions. 90% H2O2 was obtained from FMC Corpora-
tion. Caution: 90% H2O2 is potentially explosive and should
be handled with proper safety precautions.52,53 The ligand
BPMCN and the complex [FeII(β-BPMCN)](OTf)2 were syn-
thesized according to previously published procedures.10,54
Instead, we propose the mechanism shown in Scheme 1,
which rationalizes the similar reactivity patterns observed
upon addition of Sc3+ or HClO4, the equivalent kinetic effects
of adding Sc3+ or HClO4 to 2 and the 18O-labeling results.
Both these additives facilitate O–O bond heterolysis in the
FeIII-OOH intermediate 2 to form a distinct and highly elec-
trophilic oxidant Ox that hydroxylates cyclohexane and ben-
zene at high rates. As the hydroxide formed in this cleavage
combines with Sc3+ or the proton from HClO4, the nascent
FeV=O center in this case no longer has a bound hydroxide,
which should make it much more electrophilic than the pro-
posed FeV(O)(OH) oxidant associated with the water-assisted
mechanism. This mechanism parallels that proposed by Ser-
rano-Plana et al. for the activation of [FeIII(PyNMe3)(OOH)]2+
by strong acid,39 which is supported by DFT calculations.
Furthermore, in contrast to Ox, the presence of the anionic
acetate ligand in the putative FeV(O)(OAc) oxidant formed
from 1/H2O2 in the presence of HOAc presumably reduces
the electrophilicity of the FeV(O) unit, rationalizing its inabil-
ity to perform benzene hydroxylation.
Product analyses were performed on a Perkin-Elmer Sigma 3
gas chromatograph (AT-1701 column) with a flame-ionization
detector. GC mass spectral analyses were performed on a HP
6890 GC (HP-5 column) using an Agilent 5973 mass detector.
For chemical ionization analyses, NH3/CH4 (4%) was used as
the ionization gas. UV-visible absorption spectra were rec-
orded on a HP8453A diode array spectrometer equipped with
a cryostat from Unisoku, Scientific Instruments (Osaka, Ja-
o
pan). Resonance Raman spectra were obtained at -30 C with
excitation at 561 nm (100 mW at source, Cobolt Lasers)
through the sample in a flat bottom NMR tube using a 90o
backscattering arrangement (parallel to the slit direction).
The collimated Raman scattering was collected using two
Plano convex lenses (f = 12 cm, placed at an appropriate dis-
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tance) through appropriate long pass edge filters (Semrock)
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Reaction conditions. All experiments reported in this work
were performed in the presence of air. In a typical reaction,
70 μL of a 0.2 M H2O2 solution (diluted from 90% H2O2/H2O
solution) in CH3CN (10 eq H2O2 relative to 1(OTf)2) was add-
ed all at once to a vigorously stirred CH3CN solution (1.93
mL) containing the iron catalyst 1(OTf)2, the substrate and
Sc(OTf)3 or HClO4 and stirred for 30 min at room tempera-
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benzene/ 600 eq nitrobenzene and 0.5–8 eq Sc(OTf)3 or
HClO4. After the reaction was over, 0.1 mL 1-methylimidazole
and 1 mL acetic anhydride were added to the reaction solu-
tion to esterify the alcohol/phenol. An internal standard
(naphthalene) was added after this and then the products
were extracted into CH3Cl and the solution was then subject-
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10 eq or 70 μL of a 0.2 M H218O2 solution (diluted from the
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H218O solution in CH3CN (170 eq H218O relative to 1(OTf)2,
which is about the same amount of H2O that would be intro-
duced from a 10% H2O2 solution) was added to the reaction
mixture before adding H2O2.
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Supporting Information.
The Supporting Information is available free of charge via the
internet at http://pubs.acs.org.
Additional data and experimental details (PDF)
AUTHOR INFORMATION
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Corresponding Author
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*larryque@umn.edu ORCID 0000-0002-0989-2813
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