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The mechanism of C-H bond oxidation by aqueous permanganate Jens Blotevogel, Anthony K. Rappe, Arthur N. Mayeno, Tom Sale, and Thomas Borch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03157 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018
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Environmental Science & Technology
The mechanism of C-H bond oxidation by aqueous permanganate
Jens BlotevogelA,*, Anthony K. RappéB,*, Arthur N. MayenoC, Tom C. SaleA, Thomas BorchA,B,D,*
A
Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO 80523, USA
B
Department of Chemistry, Colorado State University, Fort Collins, Colorado, CO 80523, USA
C
Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, CO 80523, USA
D
Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO 80523, USA
C─ H
C─ OH oxygen rebound
C=O
TOC Art
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ABSTRACT
2
The permanganate ion (MnO4-) has been widely used as reagent for water treatment for
3
over a century. It is a strong enough oxidant to activate carbon-hydrogen bonds, one of the most
4
important reactions in biological and chemical systems. Our current textbook understanding of the
5
oxidation mechanism in aqueous solution involves an initial, rate-limiting hydride abstraction by
6
permanganate followed by reaction of the carbocation with bulk water to form an alcohol. This
7
mechanism fits well into the classic oxidation sequence of alkane → alcohol → aldehyde →
8
carboxylate, the central paradigm for both abiotic and biotic alkane oxidation in aqueous
9
environments. In this study, we provide three lines of evidence through (1) a broken-symmetry
10
density functional theory approach, (2) isotope labeling experiments, and (3) kinetic network
11
modeling to demonstrate that aqueous permanganate can circumvent prior alcohol formation and
12
produce aldehydes directly via a reaction path that bifurcates after the initial transition state. In
13
contrast to classic transition state theory, the rate-limiting step is found to not determine product
14
distribution, bearing critical implications for pathway and rate predictions. This complex reaction
15
network provides new insights into the oxidation mechanisms of organic compounds involving
16
transition metal complexes as well as enzyme or metal oxide surface active sites.
17 18 19
INTRODUCTION
20
The permanganate anion MnO4- is a classic chemistry textbook oxidant that has been
21
widely used since the mid-19th century for drinking and wastewater treatment,1,2 water
22
disinfection,3 and groundwater remediation,4,5 as well as in chemical synthesis and medical
23
applications.6-8 Permanganate’s popularity is founded on the activation of rather unreactive carbon-
24
hydrogen bonds, a significant focus area of fundamental and industrial research for several
25
decades9 owing to its pivotal role in the functionalization of alkanes,10,11 metabolism of
26
endogenous and exogenous molecules,12 and destruction of environmental contaminants.13,14,15
27
Despite decades of intensive research, however, the exact mechanism by which
28
permanganate and many other transition metals oxidize C-H bonds is still a center of scientific
29
debate as the intermediates of these reactions are short-lived and thus not easily detected.13,16-17 In
30
1995, Gardner and Mayer18 reported that in aqueous solution the kinetics of toluene C-H bond
31
activation were consistent with an initial, rate-limiting hydride (H-) abstraction generating a water2 ACS Paragon Plus Environment
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stabilized carbocation intermediate, likely forming benzyl alcohol (Pathway 1 in Scheme 1). The
33
observed products were benzaldehyde and benzoic acid. These observations fit well into the classic
34
oxidation sequence of alkane → alcohol → aldehyde → carboxylate, the central paradigm for
35
abiotic and biotic alkane oxidation in aqueous environments.
36
37 38
Scheme 1.
39 40
In a previous study we investigated the oxidation of hexamethylphosphoramide (HMPA)
41
by aqueous-phase permanganate in excess under pseudo-first-order conditions.19 HMPA is an
42
industrial solvent and groundwater contaminant,20,21 and well suited for C-H bond oxidation
43
experiments as its six methyl substituents are located on the outside of the molecule, shielding
44
other potentially oxidizable functional groups.20 In an effort to establish a predictive model for this
45
reaction under environmentally relevant conditions, however, we were unable to obtain a
46
satisfactory kinetic network model fit for our experimental data under the assumption of the classic
47
oxidation sequence suggested above (see Fig. S1 in the Supporting Information). This led us to
48
hypothesize that there is an additional mechanism and pathway of C-H bond activation by aqueous
49
permanganate. As early as 1963, Wiberg & Fox22 had proposed oxygen rebound as possible
50
mechanism, in which the carbocation and hypomanganate (i.e., HMnVO42- after hydride
51
abstraction) would react to form an ester intermediate (Pathway 2 in Scheme 1), but failed to
52
provide conclusive experimental support. Furthermore, the potential fate of an ester intermediate 3 ACS Paragon Plus Environment
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remained unexplained. Thus, the objective of our study was to provide multiple lines of both
54
theoretical and experimental evidence in order to finally unravel the mechanism of C-H bond
55
oxidation by aqueous permanganate, and in a broader context, to advance our understanding and
56
predictive capabilities regarding organic chemical oxidation involving transition metals.
57 58 59
EXPERIMENTAL SECTION
60
Isotope Labeling Experiments. For the experiment determining pathway, kinetics, and
61
origin of oxygen in the aldehyde (i.e., formyl-PMPA), 1 mL of H218O (97.2%, Cambridge Isotope
62
Laboratories) was transferred into an HPLC vial. KMn16O4 (certified ACS, Fisher Scientific) and
63
HMPA (99%, MP Biomedicals) were added to yield final concentrations of 60 mM and 2.5 mM,
64
respectively. The reaction was carried out under pseudo-first-order kinetics19 and non-agitated at
65
22 °C. Samples (15 μL) were quenched by adding 1.485 mL of 5 mM Na2S2O3 and filtered (0.2
66
µm, nylon).
67
To validate the findings of the experiment above and determine the source of oxygen in the
68
alcoholic species, we synthesized KMn18O4 by dissolving 50 mg KMn16O4 in 1 mL H218O (97.7%,
69
Cambridge Isotope Laboratories) within a sealed 2-mL amber glass ampule. The ampule was
70
heated in an oven at 115 °C for 11 days. The 18O-content of the permanganate after evaporation of
71
the solvent water was determined to be 91.4% (see below). Subsequently, the synthesized
72
KMn18O4 and HMPA were dissolved in 1.5 mL H216O to yield final concentrations of 60 mM and
73
2.5 mM, respectively. The reaction was carried out non-agitated at 22 °C. Samples (15 μL) were
74
quenched by adding 1.485 mL of 5 mM Na2S2O3 and filtered (0.2 µm, nylon).
75
To quantify the natural exchange of oxygen isotopes between the alcoholic / aldehydic
76
oxidation products and the solvent water under conditions of the kinetic experiment, the sample
77
taken after five minutes during the isotope experiment described above was analyzed after
78
quenching, KMn18O4 precipitation and filtration over the course of 65 minutes, the duration of the
79
kinetic experiments described above. The decreasing
80
dissolved in H216O revealed only minor oxygen exchange of 1.0% in the alcohol (HM-PMPA) and
81
0.5% in the aldehyde (formyl-PMPA).
18
O-content in the two oxidation products
82
To quantify the natural exchange of oxygen isotopes between permanganate and the solvent
83
water under conditions of the kinetic experiment, KMn16O4 was added to 1 mL of H218O (97.5%, 4 ACS Paragon Plus Environment
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Cambridge Isotope Laboratories) at a final concentration of 60 mM. Over the course of 65 minutes,
85
the 18O-content in KMn16O4 increased by only 0.21%.
86
Kinetic Network Model. To determine pseudo-first-order rate constants k’ for HMPA and
87
its reaction products, an analytical solution for networks of irreversible first-order reactions23 was
88
implemented in Mathcad (Version 14.0.0.163, PTC). This algorithm involves the solution of a
89
coefficient matrix, in which the individual (pseudo-)first-order rate constants for all reactions
90
within the network are the coefficients, for the direct calculation of species concentrations at any
91
time of interest. The matrix coefficients were iterated to fit the experimental data until the pooled
92
unweighted non-linear R2 as well as the non-linear R2 for each individual compound, both of which
93
based upon the use of the total corrected sum of squares, were maximized.
94
Chemical Analyses. HMPA and its phosphoramide-based oxidation products were
95
analyzed with an Agilent 1100 Series liquid chromatograph equipped with a 150 mm × 2.1 mm
96
XTerra phenyl column, 3.5 µm particle size (Waters) coupled to an Agilent G3250AA MSD TOF
97
system with an electrospray ionization (ESI) source in positive ion mode (LC/ESI+-TOF-MS).
98
Separation was carried out isocratically with 0.01% formic acid in water / acetonitrile (98:2).24
99
Reference masses for mass spectrometric calibration were 121.050873 and 922.009798. Further
100
details are provided in the Supporting Information.
101 102 103
COMPUTATIONAL DETAILS
104
The majority of quantum chemical calculations were conducted via Gaussian 09 (Rev.
105
B.01). All optimization and frequency calculations were performed without symmetry constraints
106
for 295.15 K (i.e., the temperature of our experimental system) at the B3LYP level of theory25,26
107
with the 6-31+G(d) basis set27 for O, N, C and H as well as the effective core potential (ECP)-type
108
LANL2DZ basis set28,29 for P, Mn and K (K added to balance the negative charge on the oxyanion).
109
To account for solvation effects on reaction pathways, mechanisms and kinetics, a cluster-
110
continuum model based on the SMD solvation model was applied.20,30 One potassium ion was
111
added as counterion to balance each anionic manganese species in the respective calculation. Wave
112
function stability/symmetry was tested using the stable=opt keyword and reoptimized using a
113
corrected wave function as needed. To test the broken symmetry computational model (see Results
114
and Discussion for more details on broken-symmetry wave functions), a complete active 5 ACS Paragon Plus Environment
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space/perturbation theory model was used to compute the barrier height. For this study the ORCA
116
electronic structure suite31 was used, and the 6-31 basis set was applied for all “non-active” centers.
117
For Mn, the four oxo oxygens, and the reactive C-H, a set of p (H) and d (O, C) polarization
118
functions were added. For both the ground state and transition state, a CAS(14,12) wave function
119
optimization was followed by a NEVPT2 perturbation treatment.32-34 The SMD solvation model
120
was used here as well. A detailed description of computational methods is provided in the
121
Supporting Information.
122 123 124
RESULTS AND DISCUSSION
125
To generate a first line of evidence for our hypothesis of an additional C-H bond oxidation
126
mechanism by aqueous permanganate, we performed a B3LYP density functional theory (DFT)
127
study on HMPA (compound GS1 in Figs. 1-3). Beyond the transition state (TS(1-2/4) in Figs. 1-
128
3) for initial hydride abstraction from any of the methyl (-CH3) functional groups of HMPA by
129
permanganate, the intrinsic reaction coordinate (IRC) suggested oxygen rebound as the exclusive
130
mechanism (magenta circles, Fig. 1), in contradiction to the previously suggested reaction of the
131
carbocation with bulk water to form an alcohol.18 Furthermore, the addition of explicit solvent
132
water molecules and hydroxide ion in our calculations did not stabilize the intermediate
133
carbocation. To validate this surprising finding, we conducted a spin stability study of the transition
134
state wave function, which indicated that the closed shell DFT wave function did not accurately
135
describe this system, and thus failed to provide either the lowest-energy transition state or the
136
lowest-energy product ground state(s). Bond pairs, described by molecular orbitals, are a mixture
137
of covalent and ionic contributions. As a bond pair is stretched during the course of a reaction, at
138
a certain point known as the Coulson-Fischer point,35,36 on-site repulsion due to the ionic
139
contribution to the wave function is no longer compensated for by favorable kinetic energy and
140
nuclear attraction terms due to the ionic contribution. At this point a wave function emerges as the
141
ground state wherein different spins occupy different orbitals. This unrestricted wave function is
142
referred to as a broken-symmetry wave function. As shown in Fig. S2 in the Supporting
143
Information, β spin density builds up on the Mn center in the broken symmetry wave function
144
while α spin density delocalizes over the hydrogen, carbon, and adjacent phosphoramide oxygen
145
centers. The density-mapped electrostatic potential surface highlights build-up of positive charge 6 ACS Paragon Plus Environment
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on potassium and negative charge on the hydroxide ion. The DFT spin-natural orbitals and CAS
147
natural orbitals in Fig. S2 support a model for the wave function consisting of diradical character
148
(occupations of roughly 1.5 and 0.5) but with significant ionic character as well. Given this
149
diradical ionic mixture, it is not surprising that the computed kinetic isotope effect of kCH3/kCD3=5.6
150
for the methyl group(s) in HMPA falls between the experimental carbocationic (small enthalpy of
151
activation, large entropy of activation, and large kinetic isotope effect, kC7H8/kC7D8=9.7) and
152
hydrogen atom transfer (larger enthalpy of activation, smaller entropy of activation, and moderate
153
kinetic isotope effect, kC7H8/kC7D8=2.5) range for the methyl substituent in toluene.18
154
Relative Energy (kcal/mol)
20
0
156 157 158 159 160 161 162
GS1
-10
hydride abstraction
hydride abstraction
oxygen rebound
-20 -30
Hydride Abstraction IRC (BS-B3LYP) PES Scan Water Stabilization (BS-B3LYP) PES Scan Oxygen Rebound (BS-B3LYP) Hydride Abstraction IRC (B3LYP)
-40 -50 -60
155
TS(1-2/4)
10
0
20
40
60
80 100 Coordinate 120 140 160 Reaction
INT2 GS4 180
200
220
Figure 1. Potential energy surface (PES) for C-H bond activation by aqueous permanganate. The spin-restricted intrinsic reaction coordinate (IRC) falsely implies exclusive oxygen rebound after initial hydride abstraction from HMPA, leading to manganate ester formation only. The total energy for the corrected IRC levels off after hydride abstraction. Relaxed PES scans show that either oxygen rebound or (solvent) water addition proceed without energy barrier before a carbocation ground state is produced. Transition state and bifurcation point geometries are provided in the Supporting Information.
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Consequently, we used a broken-symmetry wave function (BS-B3LYP)37 to revisit the C-
165
H bond oxidation reaction by permanganate. Here, the IRC leveled off after hydride/hydrogen
166
abstraction without indication of oxygen rebound (blue diamonds in Fig. 1), despite the close
167
proximity of opposite charges on the forming hypomanganate anion and carbocation centers. The
168
barrier height of the initial hydride/hydrogen abstraction was calculated to be 7.1 kcal/mol (Fig.
169
2), compared to 8.1 kcal/mol from the CAS/NEVPT2 calculation (see the Supporting Information).
170
As described in the Supporting Information, the broken-symmetry approach has been shown to
171
produce results similar to those obtained from multi-reference ab initio calculations for molecular
172
systems with spin instability,38 yet at much lower computational cost. Relaxed scans of the
173
potential energy surface (PES) starting from the downward energy slope of the IRC toward
174
hypomanganate and solvent waters were conducted, in which the distances between the
175
carbocation center and the nearest oxygen atom in hypomanganate as well as between the
176
carbocation center and the nearest oxygen atom in a solvent water molecule were decremented.
177
These suggested that both water stabilization (yellow squares, Fig. 1) and oxygen rebound (green
178
triangles, Fig. 1) pathways proceed without an additional energy barrier.
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MnO3 O H H2CR
H 2O -
Relative Energy (kcal/mol)
TS(1-2/4) (7.1)
Mn(OH)O2
MnO3
O
O
H2CR
H
GS1 (0.0)
(-37.9)
OH
TS(2-4) INT2 Mn(OH)O2
(-47.1)
H2CR
TS(2-3) (-38.4)
(-41.9)
GS4
O
O
(-90.2)
H2CR
HCR
(-95.3)
TS(3-5) -
OH
H
INT3
O
Mn(OH)O2
Mn(OH)O2
O
O
-
O HCR
GS5 (-119.1)
HCR
181 182 183 184 185
TS(4-5)
(-50.5)
Mn(OH)O2
MnO3
180
HC(OH)R
HCR
Reaction Coordinate Figure 2. BS-B3LYP reaction profile for C-H bond oxidation by aqueous permanganate. Orange: Solvent water binding after initial hydride abstraction producing an alcohol. Red: Oxygen rebound leading to alcohol production. Blue: Oxygen rebound circumventing alcohol production. Complete ground state, intermediate and transition state geometries are provided in the Supporting Information.
186 187
Thus, the basic DFT approach without consideration of broken symmetry wrongfully
188
provided an asymmetric PES wherein the IRC diverted according to the mass-weighted steepest
189
descent path to one specific product ground state,39 leaving the parallel pathway hidden. The BS-
190
B3LYP-corrected three-dimensional PES now reveals a post-transition state bifurcation on a 9 ACS Paragon Plus Environment
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191
symmetric coordinate (Fig. 3), in which a single transition state (TS(1-2/4)) leads to formation of
192
more than one product without additional energy minima or barriers in between, generally referred
193
to as "two-step (or two-event), no-intermediate" mechanism.40,41
194
195 196 197
Figure 3. Reaction potential energy surface. The computed minimum energy paths are plotted in red; the remainder of the surface is conceptual.
198 199
Beyond the reaction path bifurcation (Fig. 2), oxygen rebound produces a hypomanganate
200
ester (INT2), alternatively water stabilization leads to the formation of an alcohol (GS4), which is
201
further oxidized through hydride abstraction by permanganate (TS(4-5)), with concurrent
202
deprotonation of the hydroxyl substituent) to an aldehyde (GS5). The symmetric nature of the
203
calculated PES would suggest a product ratio of hypomanganate ester to alcohol of 1:1.39,42
204
While the formation of a hypomanganate ester had been suggested previously,22 direct
205
experimental evidence for it has not been provided until now. The BS-B3LYP calculations suggest
206
two energetically similar reactions (Fig. 2). Backside hydrolysis of the Mn-O bond (TS(2-4)) leads 10 ACS Paragon Plus Environment
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to the generation of the same alcoholic species GS4 as after initial water stabilization. Slightly
208
more favorable is hydride abstraction from the carbon center by a second permanganate ion (TS(2-
209
3)), making the organic substituent a better leaving group (INT3). Subsequent (base-catalyzed)
210
backside hydrolysis of the Mn-O bond (TS(3-5)) then directly leads to aldehyde formation (GS5).
211
Furthermore, the reaction network suggests that all oxidations yield hypomanganate HMnVO42- as
212
product. Consequently, analyses of Mn speciation were not attempted as they would be
213
unsupportive of mechanistic investigations.
214
Thus, isotopic labeling studies were carried out to complement the theoretical predictions.
215
First, HMPA was oxidized with Mn16O4- in H218O. Two isotopes of the aldehydic species formyl-
216
pentamethylphosphoramide (formyl-PMPA) were detected via liquid chromatography coupled
217
with electrospray ionization time-of-flight mass spectrometry (LC/ESI+-TOF-MS). One aldehyde
218
contained 16O, while the other contained 18O in the formyl substituent (Table 1). Appearance of
219
both isotopes confirmed that both water and permanganate are sources of oxygen in the aldehydic
220
product.
221
pentamethylphosphoramide (HM-PMPA) product was hindered by the fact that the alcoholic
222
species underwent neutral loss of water ([M+H]+ - H216/18O) during electrospray ionization mass
223
spectrometry.19 In direct injection mode, however, the major adduct of the alcohol was [M+Na]+,
224
and the detection of 18O-HM-PMPA provided evidence that water was at least one of the sources
225
of oxygen in the alcohol. Secondly, the presence of
226
determined to support hypomanganate ester hydrolysis as a source of the alcohol (TS(2-4)), since
227
ESI+-TOF-MS was unable to resolve the
228
overlapping (sodiated) 18O-formyl-PMPA signal. The oxidation experiment was thus repeated with
229
freshly prepared Mn18O4- in H216O. Here,
230
confirmation that C-H bond activation by aqueous permanganate indeed proceeds through at least
231
three parallel pathways as shown in Fig. 2. Oxygen exchange between the detected oxygenated
232
reaction intermediates and the solvent water as well as between permanganate and water over the
233
experimental duration of 65 minutes was confirmed to be negligible (see the Supporting
234
Information).
Support
for
the
two
predicted
16
oxygen
16
sources
in
the
hydroxymethyl-
O-HM-PMPA could not be conclusively
O-HM-PMPA accurate mass at m/z of 218 from the
18
O-HM-PMPA was successfully detected, providing
235
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Table 1. Accurate mass spectrometric data for HMPA and its products detected during the isotope labeling experiments. Compound
Molecular ion peak
HMPA
[M+H]+
18
O-HM-PMPA
16
O-formyl-PMPA
16 18
Measured m/z
Calculated exact m/z
Error (ppm)
C6H19N3OP+
180.1256
180.1260
2.2
[M+Na]+
C6H18N3Na16O18OP+
220.1077
220.1071
2.7
[M+H]+
C6H17N316O2P+ +
O-formyl-PMPA
[M+Na]
O-formyl-PMPA
+
[M+H]
Molecular ion peak formula
194.1053
194.1053
0.0
16
+
216.0874
216.0872
0.9
18
+
196.1095
196.1095
0.0
C6H16N3Na O2P 16
C6H17N3 O OP
238 239 240
To determine observed rate constants k’ for the parallel reactions, we applied a first-order
241
kinetic network model23 based on the HMPA oxidation experiment with Mn16O4- in H218O, which
242
enabled discrimination between formyl-PMPA but not HM-PMPA isotopes (Fig. 4). The vastly
243
improved pooled unweighted R2 value of 0.9934 compared to the initial fit for water addition only
244
(R2 = 0.8950, Fig. S1) further supported the validity of the BS-B3LYP pathway prediction. The
245
kinetic network model revealed that the initial hydride abstraction by permanganate is the rate-
246
limiting step during C-H bond oxidation,18 supported by the experimental detection of HM-PMPA
247
and formyl-PMPA as the two first stable intermediates with the other five methyl substituents
248
intact. The ester hydrolysis steps are more than four orders of magnitude faster. Moreover, the
249
best-fit rate constants of 0.133 min-1 for both water stabilization and oxygen rebound confirmed
250
the predicted symmetric nature of the PES after reaction path bifurcation, leading to an equal ratio
251
of hypomanganate ester to alcohol generation. The overall observed rate constant for HMPA
252
oxidation of 0.133 min-1 + 0.133 min-1 = 0.266 min-1 matched well the previously determined rate
253
in unlabeled systems of 0.249 ± 0.022 min-1.19
254
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1.0
X
R2 = 0.9992
0.9
hydroxymethyl R2 = 0.9940
k* ≥ 1500
0.8 0.7 0.6
C/C0
alkyl
16O-formyl
R2 = 0.9892 18O-formyl
R2 = 0.9898
0.5 0.4 0.3 0.2
pooled R2 = 0.9931
0.1 0.0
0
10
20
255 256 257 258 259 260
30
Time (min)
40
50
60
Figure 4. C-H bond oxidation by Mn16O4- in H218O. Symbols represent experimental, lines represent pseudo-first-order kinetic network model data of normalized HMPA concentration over time. Insert shows setup of the kinetic network model with best-fit rate constants k’ (min-1). Rate constant k*, determined during statistical fitting, has a lower limit of 1500 min-1 (i.e., an increase of k* above 1500 did not improve or change the model fit).
261 262
Implications. Given that post-transition state bifurcations occur after the rate-limiting step, they
263
are difficult to establish experimentally. Advances in quantum chemical methods and
264
computational resources have permitted computational studies to suggest their presence.43 This
265
bears critical implications for classic two-dimensional transition state theory,44 which has been
266
traditionally used by organic and environmental chemists for the prediction of transformation rates,
267
pathways, and product distributions. As a shared transition state structure may lead to the formation
268
of more than one product after post-transition state bifurcation, the assumptions imposed by
269
transition state theory of one distinct transition state per product and the reaction pathway being
270
independent of PES shape fall apart.
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As implied by our findings, metal-mediated undirected C-H bond activation selectivities
272
do not necessarily depend exclusively on the rate-limiting step in their mechanisms,16 but at times
273
on post-transition state PES shape and dynamic effects.45 Given solvent (water) participation in
274
one of the bifurcated paths, and transition state diradical-ionic mixed character, production
275
distribution should be sensitive to the solvent. Other factors such as the relative concentration of
276
species and non-covalent interactions during reaction may affect product ratios in these systems as
277
well.41
278
Here, we have unraveled the modes of C-H bond oxidation by one metal oxo species after
279
over a century of extensive application and decades of scientific controversy. However, our
280
findings likely have far broader implications for oxidation mechanisms of organic compounds
281
involving transition metal complexes as well as enzyme or metal oxide surface active sites. Reports
282
of reaction path bifurcations in biological systems are emerging,46 and recognition of their role in
283
heterogeneous catalysis is anticipated.
284 285 286
AUTHOR INFORMATION
287
Corresponding Authors
288
* (J.B.) E-mail:
[email protected]. Phone: +1-970-491-8880. Fax: +1-970-491-8224.
289
* (A.K.R.) E-mail:
[email protected]. Phone: +1-970-491-6292. Fax: +1-970-491-
290
1801.
291
* (T.B.) E-mail:
[email protected]. Phone: +1-970-491-6235. Fax: +1-970-491-5676.
292
Notes
293
The authors declare no competing financial interest.
294 295 296
ACKNOWLEDGMENTS
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Funding for this work was provided by E.I. du Pont de Nemours and Company. We thank Y.
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Desyaterik for discussion of mass spectrometric analyses and K. Karimi Askarani for graphical
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assistance.
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ASSOCIATED CONTENT
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Supporting Information. The Supporting Information is available free of charge on the ACS
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Publications website at DOI: …
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Alternative kinetic fit without oxygen rebound, additional analytical details, additional
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computational details, as well as values and optimized structures of broken-symmetry ground
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states, transition states, intermediates, and the bifurcation point.
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