Variable pathways for oxygen atom insertion into metal-carbon bonds: The case of Cp*W(O)2(CH2SiMe3)

Article abstract of DOI:10.1021/ja309755g

CpW(O)₂(CH₂SiMe₃) (1) (Cp* = η⁵-pentamethylcyclopentadienyl) reacts with oxygen atom donors (e.g., H₂O₂, PhIO, IO₄^-) in THF/water to produce TMSCH₂OH (TMS = trimethylsilyl). For the reaction of 1 with IO₄^-, the proposed pathway for alcohol formation involves coordination of IO₄^- to 1 followed by concerted migration of the -CH₂TMS ligand to the coordinated oxygen of IO₄^- with concomitant dissociation of IO ₃^- to produce Cp*W(O)₂(OCH ₂SiMe₃) (3), which undergoes protonolysis to yield free alcohol. In contrast to the reaction with IO₄^-, the reaction of 1 with H₂O₂ results in the formation of the η²-peroxo complex Cp*W(O)(η²-O ₂)(CH₂SiMe₃) (2). In the presence of acid (HCl) or base (NaOH), complex 2 produces TMSCH₂OH. The conversion of 2 to TMSCH₂OH catalyzed by Br?nsted acid is proposed to occur through protonation of the η²-peroxo ligand, which facilitates the transfer of the -CH₂TMS ligand to a coordinated oxygen of the η²-hydroperoxo ligand. In contrast, the hydroxide promoted conversion of 2 to TMSCH₂OH is proposed to involve hydroxide coordination, followed by proton transfer from the hydroxide ligand to the peroxide ligand to yield a κ¹-hydroperoxide intermediate. The migration of the -CH₂TMS ligand to the coordinated oxygen of the κ¹-hydroperoxo produces an alkoxide complex, which undergoes protonolysis to yield free alcohol.

Full text of DOI:10.1021/ja309755g

Article
pubs.acs.org/JACS
Variable Pathways for Oxygen Atom Insertion into Metal−Carbon
Bonds: The Case of Cp*W(O)2(CH2SiMe3)
Jiajun Mei,† Kurtis M. Carsch,‡ Cody R. Freitag,‡ T. Brent Gunnoe,*,† and Thomas R. Cundari*,‡
†Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United States
‡Department of Chemistry and Center for Advanced Scientific Computing and Modeling, University of North Texas, Denton, Texas
76203-5017, United States
S
* Supporting Information
ABSTRACT: Cp*W(O)2(CH2SiMe3) (1) (Cp* = η5-pen-
tamethylcyclopentadienyl) reacts with oxygen atom donors
−
(e.g., H2O2, PhIO, IO4 ) in THF/water to produce
TMSCH2OH (TMS = trimethylsilyl). For the reaction of 1
−
with IO4 , the proposed pathway for alcohol formation
−
involves coordination of IO4 to 1 followed by concerted
migration of the −CH2TMS ligand to the coordinated oxygen
−
−
of IO4 with concomitant dissociation of IO3 to produce
Cp*W(O)2(OCH2SiMe3) (3), which undergoes protonolysis
−
to yield free alcohol. In contrast to the reaction with IO4 , the
reaction of 1 with H2O2 results in the formation of the η2-
peroxo complex Cp*W(O)(η2-O2)(CH2SiMe3) (2). In the
presence of acid (HCl) or base (NaOH), complex 2 produces
TMSCH2OH. The conversion of 2 to TMSCH2OH catalyzed by Brønsted acid is proposed to occur through protonation of the
η2-peroxo ligand, which facilitates the transfer of the −CH2TMS ligand to a coordinated oxygen of the η2-hydroperoxo ligand. In
contrast, the hydroxide promoted conversion of 2 to TMSCH2OH is proposed to involve hydroxide coordination, followed by
proton transfer from the hydroxide ligand to the peroxide ligand to yield a κ1-hydroperoxide intermediate. The migration of the
−CH2TMS ligand to the coordinated oxygen of the κ1-hydroperoxo produces an alkoxide complex, which undergoes
protonolysis to yield free alcohol.
formation involves net oxygen atom insertion into metal−alkyl
bonds. For example, one possible catalytic cycle incorporates
oxygen atom insertion into a M−R bond followed by C−H
activation via net 1,2-addition across a M−OR bond to convert
an alkane to an alcohol.10,15−18 Scheme 1 shows two distinct
pathways for oxygen atom insertion into M−R bonds from the
INTRODUCTION
■
Hydrocarbon oxidation is a fundamentally important process
for the petrochemical industry.1−3 The development of new
catalysts for the selective oxidation of hydrocarbons, especially
alkanes, has the potential to enhance the efficiency of
production of chemicals and fuels.4−11 However, the selective
functionalization (e.g., direct partial oxidation) of alkanes is
among the most challenging catalytic processes.
Transition-metal catalysts for the partial oxidation of alkanes
to form alcohols must be able to perform two key steps: C−H
bond activation and C−O bond formation. The Pt-based Shilov
system, developed in the 1960s, was among the first
homogeneous catalysts to successfully activate alkanes to
form alcohols or alcohol precursors. It has been proposed
that C−H bond activation occurs at PtII followed by C−X (X =
OH or Cl) bond formation at PtIV−alkyl by reductive (i.e.,
reduction of PtIV to PtII) nucleophilic addition to an
electrophilic PtIV−alkyl ligand.12,13 The use of expensive PtIV
as a stoichiometric oxidant to convert PtII−alkyl to PtIV−alkyl
limits the utility of this catalyst.12−14
Scheme 1. Proposed Pathways for Partial Oxidation of
Hydrocarbon Involving Oxygen Atom Insertion into a M−R
Bond and 1,2-CH-Addition across a M−OR Bond
While C−X (X = OH or Cl) bond formation in the Shilov-
type catalysts has been proposed to involve nucleophilic
addition of water or halide to an electrophilic PtIV−alkyl
ligand,12 an alternative strategy for metal-mediated C−O bond
Received: October 2, 2012
© XXXX American Chemical Society
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reaction of a M−R moiety with an oxygen atom delivery
reagent (YO). One pathway involves a migration of the ligand
R to a metal-oxo intermediate. The second pathway
incorporates direct oxy-insertion from M(OY)(R) without
the formation of a metal-oxo intermediate. To be generally
useful for selective oxidations at high conversion, ideally, the
C−O bond forming step should proceed without the formation
of radical intermediates and can be facilitated by catalysts
including organocatalysts, as recently reported with flavinium-
s,19and by metal-oxo complexes as reported by Goldberg and
co-workers.20
reported to yield the η2-peroxo complex Cp*W(O)(η2-
O2)(CH2SiMe3) (2);39 however, the reaction of 1 with 1
equiv of NaIO4 in 1:1 THF-d8/D2O or 1:1 1,4-dioxane-d8/D2O
(v/v) does not produce complex 2. Rather, TMSCH2OH
(TMS = SiMe3, trimethylsilyl) is formed without observation of
2 as an intermediate. The reaction is complete within two hours
at room temperature and produces TMSCH2OH in almost
100% yield by 1H NMR spectroscopy (eq 1). To determine the
The 1,2-addition of C−H bonds across M−NHR and M−
OR bonds is known. In 2003, our groups studied and reported
intramolecular C−H activation by a PCP−RuII amido complex
and commented on the potential utility of this transformation
in catalytic transformations.21 Then, in 2005, we reported
intermolecular benzene C−H activation by RuII hydroxide and
anilide complexes.17,22 Related chemistry includes an IrIII
complex and RuII complexes by Periana, Goddard, and co-
workers,16,23 a RhI complex reported by Heinekey, Goldberg,
and co-workers,24 and a RhI complex reported by Bercaw,
Labinger, and co-workers.25
pathway for the conversion of 1 and NaIO4 to TMSCH2OH,
we first considered the possibility that the η2-peroxo complex
Cp*W(O)(η2-O2)(CH2SiMe3) (2) is formed as an intermedi-
ate, followed by oxy-insertion into the W−C bond and
subsequent protonolysis to give free TMSCH2OH. Complex
2 was reacted with NaIO4 under the same conditions as the
alcohol release from 1 and NaIO4, and no alcohol was observed
1
by H NMR spectroscopy after 24 hours. Furthermore, 2 does
While insertions of oxygen atoms into M−C bonds are
known, examples that occur by non-radical routes are rare. In a
1988 publication that focused on oxygen atom insertion into
TaV hydrocarbyl bonds, Bercaw and co-workers stated, “The
details of the actual oxygen-transfer step in controlled metal-
mediated oxidations are still poorly understood...examples of
clean carbon−oxygen bond formation for well-characterized
compounds are rarer still.”26 Despite a few recent exam-
ples,15,19,27−31 we believe that this statement remains accurate.
Some early transition-metal complexes, such as group IV
complexes, initiate oxygen atom insertion into M−R bonds;
however, these reactions commonly proceed by radical
pathways.32,33 Brown and Mayer reported oxy-insertion into
M−Ar (Ar = aryl) bonds with ReVII via the migration of the Ar
group to an oxo ligand.29,31 Similar mechanisms that involve
the formation of metal−oxo have also been proposed for the
oxygen atom insertion of Pd complexes, but mechanistic studies
have not been disclosed.28,34 Hillhouse and co-workers have
reported net oxygen insertion into a series of Ni−R bonds and
a Hf−Ph bond upon reaction with N2O.30,35 Espenson and co-
workers reported that methyltrioxorhenium (MTO) reacts with
oxidants to release methanol.36 Later, Periana, Goddard, and
co-workers investigated the mechanism of this reaction. A
pathway that involves migration of the methyl ligand to the
oxygen of the coordinated oxidant was proposed, and an
analogy between this reaction and the Baeyer−Villiger reaction
(conversion of ketones to esters) was made.15,37,38
not react with NaIO3 or D2O to produce TMSCH2OH under
the same conditions. Thus, the evidence suggests that the
formation of TMSCH2OH from 1 and NaIO4 does not likely
proceed via complex 2.
The reaction of 1 and NaIO4 (5 equiv) in 1:1 THF-d8/D2O
1
(v/v) was monitored at −1.3 °C by H NMR spectroscopy.
During the conversion, the disappearance of 1, the emergence
of an intermediate, and the appearance of TMSCH2OH were
1
observed. The formation of TMSCH2OH occurs with t / ∼40
2
1
min in approximately 100% yield. On the basis of H NMR
spectroscopy, the intermediate is proposed to be the tungsten
alkoxide complex Cp*W(O)2(OCH2SiMe3) (3); however, we
1
were not able to isolate 3. The H NMR resonances of the
intermediate 3 are assigned as 4.17 (CH2, s), 2.04 (CH3, s),
−0.05 (SiMe3, s) ppm. Mo(O)2(OEt)2 exhibits a resonance due
to the OCH2Me at 4.65 ppm,40 and the similar complex
(bpy)Mo(O)2(OEt)2 (bpy = 2,2′-bipyridine) exhibits a
methylene resonance at 3.86 ppm.41 In contrast, the CH2
groups of 1 and 2 resonate at 0.43 and 3.16 ppm, respectively.
In addition to NaIO4, complex 1 reacts with iodosobenzene
(PhIO) in 1,4-dioxane at room temperature to produce 3 in
1
∼20% yield in 20 min by H NMR spectroscopy (eq 2);
however, at prolonged reaction times, an intractable mixture of
products is formed.
Given the importance of metal-mediated C−O bond
formation, we have been interested in understanding
mechanisms and strategies to facilitate these transformations.
To our knowledge, detailed studies of reactions that give clean
oxygen atom insertion into metal−hydrocarbyl bonds are
limited to the studies of ReVII complexes.15,29,31,35,36 Herein, we
present studies of oxygen atom insertion into the W−
CH2SiMe3 bond of Cp*W(O)2(CH2SiMe3) (1) and Cp*W-
(O)(η2-O2)(CH2SiMe3) (2). Complexes 1 and 2 have been
previously prepared and studied by Legzdins and co-workers.39
Figure 1 depicts the concentration versus time plot for all
species observed in the conversion of 1 and NaIO4 to
TMSCH2OH, based on the integrations of the methylene
resonances of 1, the TMSCH2OH product, and complex 3. The
sum of concentrations of these three species (black X’s, Figure
1) remains constant over the course of the reaction.
Scheme 2 depicts a proposed organometallic Baeyer−Villiger
(OMBV) pathway for the formation of TMSCH2OH from the
reaction of 1 with NaIO4. Periodate coordinates to complex 1,
followed by concerted migration of the alkyl ligand to the
RESULTS AND DISCUSSION
Reaction of Cp*W(O)2(CH2SiMe3) (1) with NaIO4. The
reaction of Cp*W(O)2(CH2SiMe3) (1) with H2O2 has been
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−
Figure 2. Plot of kobs1 vs [IO4 ] for the reaction of Cp*W-
(O)2(CH2SiMe3) (1) with NaIO4 showing a first-order dependence
−
Figure 1. Plot of concentration versus time during the conversion of
Cp*W(O)2(CH2SiMe3) (1) and NaIO4 (5 equiv) to TMSCH2OH
(−1.3 °C in 1:1 THF-d8/D2O (v/v)) including complex 1 (blue,
circles), Cp*W(O)2(OCH2SiMe3) (3) (red, squares), TMSCH2OH
(green, triangles), and the sum of all three species (black, X’s).
on [IO4 ] (R2 = 0.99).
Scheme 2. Proposed Mechanism for the Formation of
TMSCH2OH from the Reaction of Cp*W(O)2(CH2SiMe3)
(1) and NaIO4 in 1:1 THF-d8/D2O (v/v)
Figure 3. Plot of ln(k/T) vs 1/T for the conversion of Cp*W-
(O)2(CH2SiMe3) (1) and NaIO4 (5 equiv) to Cp*W-
(O)2(OCH2SiMe3) (3) (R2 = 0.99).
reaction indicates the possibility of substantial solvent ordering
in the transition state.
Density functional theory (DFT) calculations were carried
out to probe the mechanism for the reaction of 1 with IO4− to
form TMSCH2OH. Scheme 3 shows the energetics for the
−
lowest energy pathway for 1 + IO4 → ROH that was
calculated. The calculated free energy in 1,4-dioxane and water
(1,4-dioxane/water) are given in Scheme 3. In the following
text, we use the calculated numbers in 1,4-dioxane.
−
coordinated oxygen atom of IO4− and loss of IO3 . Protonation
of complex 3 generates the alcohol product. Assuming that the
−
Through a combination of entropy and solvation effects (free
oxygen atoms of coordinated IO4 do not exchange with the
IO4− is more heavily solvated than when coordinated to 1), the
oxo ligands of the putative adduct [Cp*W(O)2(OIO3)(R)]− in
the proposed pathway, the oxygen atom in the TMSCH2OH
product should be derived from the oxygen donor (i.e., IO4 )
−
Scheme 3. Calculated Mechanism for the Release of
TMSCH2OH in the Reaction of Cp*W(O)2(CH2SiMe3) (1)
and not from complex 1. Consistent with this hypothesis, gas
chromatography/mass spectrometry (GC/MS) analysis of the
reaction of Cp*W(16O)2(CH2SiMe3) with NaI18O4 shows that
only TMSCH218OH is formed.
a
with Periodate
Using kinetic simulation (Kinetica98 software;42 see the
Supporting Information and the Experimental Section), rate
constants for the formation of 3 (kobs1 = 8.46(1) × 10−4 s−1)
and the conversion of 3 to TMSCH2OH (kobs2 = 1.01(1) ×
10−3 s−1) were determined. In addition, under pseudo first-
−
order conditions (i.e., excess IO4 ), complex 1 was treated with
a series of concentrations of NaIO4 in 1:1 THF-d8/D2O (v/v)
at −1.3 °C, and a rate constant kobs1 for the disappearance of
complex 1 was determined for each reaction. A plot of kobs1 as a
−
−
function of [IO4 ] shows a first-order dependence on [IO4 ]
(Figure 2). Thus, the experimentally derived rate law for the
−
−
conversion of 1 and IO4 to 3 is rate = k1[1][IO4 ], and the
slope of the plot in Figure 2 gives k1 = 1.9(3) × 10−2 M−1·s−1
that corresponds to a ΔG‡ = 18.0(1) kcal·mol−1 at −1.3 °C.
1
The conversion of 1 and NaIO4 to 3 was monitored by H
NMR spectroscopy at −1.3, 10.7, 22.7, and 34.7 °C. An Eyring
plot using k1 (determined from the rate of disappearance of 1
divided by the [IO4 ]; (Figure 3) was used to calculate ΔH‡ =
−
8.5(2) kcal·mol−1 and ΔS‡ = −35.2(7) cal·mol−1·K−1 for the
a
Numbers are free energies (kcal·mol−1) for 1,4-dioxane (top, bold)
conversion of 1 to 3. The relatively large |ΔS‡| for a bimolecular
and water (bottom, italics) and are relative to complex 1 and IO4 .
−
C
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−
formation of the adduct 1−OIO3 is computed to be
assertion is also supported by the calculated C−O distances for
the carbon−oxygen bond being formed in the Baeyer−Villiger
TS: 2.23 Å (for Cp*W complex) compared to 2.067 Å (MTO)
TS.15
endergonic by ≥25 kcal·mol−1 in both solvents relative to
separated 1 and periodate. Presumably, the inclusion of
counterion effects would reduce the endergonicity of the
reaction to form an anionic adduct and thus also the
corresponding overall activation barrier of the formation of
complex 3. Periodate has been implicated in electron transfer
reactions as well as two-electron oxygen atom transfer (OAT)
chemistry.43 To model the possibility of one-electron
chemistry, we investigated the thermodynamics of the following
electron transfer reaction: 1 + [IO4]− → 1•− + [IO4]•. This
reaction is decidedly endergonic in 1,4-dioxane (ΔGcalc = +120
kcal·mol−1) and water (ΔGcalc = +122 kcal·mol−1). In the
absence of corroborating experimental data, quantitative
significance should not be ascribed to the computed electron
transfer thermodynamics. However, the significant computed
endergonicity suggests that, for these Cp*WVI complexes, the
periodate anion is not likely acting as a single electron transfer
reagent.
The reaction of complex 1 with [Bu4N][IO4] (Bu = n-butyl)
(3 equiv) in rigorously dried THF-d8 results in no reaction even
upon heating at 80 °C for 24 h, while the same starting material
produced the intermediate 3 and ultimately TMSCH2OH in
1:1 THF-d8/D2O (v/v) at room temperature in hours. The
−
failure of 1 and IO4 to produce free alcohol in the absence of
water is perhaps not surprising, since the conversion of 3 to free
alcohol requires a proton source. However, the lack of
−
formation of 3 for the reaction of 1 and IO4 in the absence
of water is less readily rationalized, especially since calculations
−
show that the formation of 3 is favorable from 1 and IO4 .
Water apparently facilitates the oxy-insertion reaction. Possible
roles for water in the conversion of 1 and IO4− to 3 include the
following: (1) as a solvent, water helps the ionization of the
−
IO4 anion for metal coordination. (2) As an electron donor,
The DFT calculations support the hypotheses that oxygen
atom insertion into the W−CH2TMS bond upon reacting with
water can coordinate the metal center resulting in a more
electron-rich metal center and more nucleophilic −CH2TMS
ligand, which is analogous to the role of pyridine in the
conversion of MTO and pyridine-N-oxide to the oxy-insertion
−
IO4 occurs by an OMBV pathway. The calculated energy
barrier for oxy-insertion from 1−OIO3− is 28 kcal·mol−1 (PCM
water). Part of the difference between the computed and
experimental barriers is, as delineated below, due to the neglect
of explicit solvation of the periodate oxidant and particularly its
iodate leaving group. Multiple attempts (see the Supporting
Information) to isolate alternative transition states (TSs) (e.g.,
a TS for formation of a peroxo leading to 2, a [3 + 2] addition
of periodate, etc.) either led to already found stationary points
or the OMBV TS depicted in Figure 4 (the TS is modified
product.15,44 (3) Water interacts with coordinated IO4 to
−
−
facilitate the dissociation of IO3 .
Under pseudo first-order conditions, the reaction of 1 and
excess [Bu4N][IO4] (15 equiv) with various amounts of D2O in
1
THF-d8 was monitored at 50 °C by H NMR spectroscopy.
Kinetic plots reveal a first-order decay of 1, and a plot of kobs
−
(divided by the [IO4 ]) as a function of [D2O] shows that the
decay of 1 with [Bu4N][IO4] has a second-order dependence
on [D2O] (Figure 5). Thus, the overall rate law for the
Figure 5. Plot of kobs vs [D2O] for the reaction of Cp*W-
(O)2(CH2SiMe3) (1) and excess [Bu4N][IO4] (15 equiv) in THF-
d8 with various amounts of D2O showing a second-order dependence
on [D2O] (R2 = 0.99).
−
conversion of 1 and IO4 to TMSCH2OH is rate =
k[1][IO4 ][D2O]2. A fit of the plot in Figure 5 gives k =
−
5.8(2) × 10−5 M−3·s−1 after dividing by [IO4 ] that
−
Figure 4. DFT-calculated TS for the oxy-insertion step of the overall
corresponds to a ΔG‡ = 25.2(1) kcal·mol−1 at 50 °C. The
participation of water in the reaction is consistent with the
relatively large |ΔS‡|.
−
reaction Cp*W(O)2(CH2SiMe3) (1)
(O)2(OCH2SiMe3) (3) + IO3 . Bond lengths are given in Å and
+
IO4
→ Cp*W-
−
bond angles in degrees.
The role of water in the reaction of 1 and periodate was
investigated computationally. Attempts to model a four-legged
piano stool complex with inner-coordination sphere water,
Cp*W(O)2(OH2)R, led instead to an outer-coordination
sphere aqua complex in which water is hydrogen bonded to
an oxo ligand. The possibility that water enhances the oxidizing
based on studies of the impact of water, see below). The
formation of 3 is calculated to be favorable, which is consistent
with the observation of putative 3 as an intermediate in the
overall reaction. The calculated I−O bond distance in the TS
for oxygen atom insertion is 2.16 Å, which is much shorter than
the 2.399 Å reported for the corresponding TS for MTO,
implying an earlier TS for the Cp*W complex. The latter
−
potential of IO4 was probed computationally in several ways.
−
−
The calculation of the OAT free energy for IO4 → IO3
+
1/2O2 is exergonic by −17 kcal·mol−1 in the gas phase.
D
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Inclusion of continuum solvent effects (CPCM, water solvent)
increases the exergonicity of this reaction to −26 kcal·mol−1.
The calculations indicate that the increased driving force results
from more favorable solvation of the smaller iodate ion in
relation to periodate. Thus, the calculations predict that water
should enhance the thermodynamics of oxygen atom transfer
[OH−] (Figure 7). The slope of the plot in Figure 7 gives k =
1.30(6) × 10−2 M−1·s−1, which corresponds to ΔG‡ = 19.1(1)
kcal·mol−1 at 10.7 °C.
−
from IO4 , but the extent to which this would enhance the oxy-
insertion kinetics is uncertain without calculation of the
corresponding hydrated OMBV transition state.
Explicit solvation effects on periodate-mediated OMBV
reactions were modeled. Hydrogen bonding a water molecule
to each oxo of the iodate leaving group in the OMBV TS results
in a reduction of the calculated energy barrier from 28
kcal·mol−1 (Scheme 3) to 24 kcal·mol−1 (Figure 6), which is
Figure 7. Plot of kobs vs [OH−] for the reaction of Cp*W(O)(η2-
O)2(CH2SiMe3) (2) with NaOH showing a first-order dependence on
[OH−] (R2 = 0.99).
The rate of the reaction between 2 and NaOH was
1
monitored by H NMR spectroscopy at −1.3, 10.7, 22.7, and
34.7 °C. An Eyring plot (using k values that were corrected for
[HO−], Figure 8) was used to calculate ΔH‡ = 13.6(4)
kcal·mol−1 and ΔS‡ = −20(1) cal·mol−1·K−1.
Figure 6. DFT-calculated TS with water for the oxy-insertion step of
−
the overall reaction Cp*W(O)2(CH2SiMe3) (1) + IO4 → Cp*W-
−
(O)2(OCH2SiMe3) (3) + IO3 . Bond lengths are given in Å and bond
angles in degrees.
Figure 8. Plot of ln(k/T) vs 1/T for the reaction of Cp*W(O)(η2-
O)2(CH2SiMe3) (2) with NaOH (5 equiv) (R2 = 0.99).
closer to the experimental value of 18 kcal·mol−1 (see above).
Bond lengths within the active site of the OMBV TS (Figure 4)
are little changed upon hydrogen bonding with three water
molecules. What is more noticeable is the shortening of the O−
H−O hydrogen bonds by 0.05 Å from 2.00 Å ([IO4(OH2)3]−)
to 1.95 Å in the oxy-insertion TS. In conjunction with the
implicit solvation results above, these data lend credence to the
proposal that preferential solvation of the iodate leaving group
enhances oxy-insertion of periodate into the W−C bond of 1
both kinetically and thermodynamically, effectively making the
iodate a better leaving group and the periodate a more potent
oxidant.
Three possible pathways for the reaction of complex 2 with
NaOH are shown in Scheme 4. In pathway A, the hydroxide
undergoes direct nucleophilic addition to the TMSCH2 ligand.
For this pathway, the oxygen atom in the alcohol would
Scheme 4. Possible Pathways for Alcohol Release from the
Reaction of Cp*W(O)(η2-O)2(CH2SiMe3) (2) with NaOH
in 1:1 THF-d8/D2O (v/v)
Reaction of Cp*W(O)(η2-O2)(CH2SiMe3) with NaOH.
Complex 2 reacts with NaOH to produce TMSCH2OH in 1:1
THF-d8/D2O or 1,4-dioxane-d8/D2O (v/v). The reaction
produces TMSCH2OH in quantitative yield (1H NMR) after
3 h at room temperature (eq 3).
Under pseudo first-order conditions, the reaction of 2 with
NaOH in 1:1 THF-d8/D2O (v/v) was monitored at 10.7 °C by
1H NMR spectroscopy. Kinetic plots reveal a first-order decay
of 2, and a plot of kobs as a function of [OH−] shows that the
reaction of 2 with NaOH has a first-order dependence on
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a
Scheme 5. Calculated Free Energy for the Reaction of Cp*WO(η2-O)2(CH2SiMe3) (2) with OH−
a
Numbers are free energies (kcal·mol−1) for 1,4-dioxane (top, bold) and water (bottom, italics) and are relative to complex 2 (Cp* = κ1-Cp*).
originate from hydroxide. Pathway B involves hydroxide
coordination to W followed by C−O reductive elimination.
In pathway B, the oxygen atom of the alcohol would also
originate from hydroxide. Pathway C involves hydroxide
coordination to W, which would facilitate transfer of the
−CH2TMS ligand to an η2-peroxo oxygen atom. Protonation
with water generates alcohol. The oxygen atom of the alcohol
in pathway C originates from complex 2. MeLi was reacted with
18O-labeled water to generate the 18O-labeled Li18OH in H218O.
The alcohol product from the reaction of 2 with Li18OH was
then analyzed by GC/MS. Only R16OH was observed in the
MS spectrum, which is consistent with pathway C and
inconsistent with pathways A and B in Scheme 4.
Scheme 6. Two Possible Pathways for the Alcohol Release
Reaction of Cp*WO(η2-O)2(CH2SiMe3) (2) with HCl
The role of hydroxide for the conversion of 2 to
TMSCH2OH was probed computationally. Several mechanisms
were investigated (Scheme 5). The very large calculated free
energy barriers (≥67 kcal·mol−1) led us to discount
nucleophilic substitution (pathway A) and reductive elimi-
nation (pathway B). The calculations suggest that the
hydroxide coordination-assisted alkyl migration to an oxygen
atom of the η2-peroxo ligand (34 and 35 kcal·mol−1 in 1,4-
dioxane and water) has almost the same energy barrier as the
non-assisted alkyl migration (35 and 32 kcal·mol−1 in 1,4-
dioxane and water) (see Scheme 7 below). What emerged as
the most reasonable pathway was a H atom transfer pathway in
which hydroxide coordinates to 1, followed by proton transfer
(ΔG‡ = 25 and 24 kcal·mol−1 in 1,4-dioxane and water,
respectively) to yield a hydroperoxide intermediate, [(κ1-
Cp*)W(O)2(R)(OOH)]− (Scheme 5). The OMBV TS from
the latter is calculated to be 24 and 23 kcal·mol−1 in 1,4-dioxane
and water above the starting complex 1, in reasonable
agreement with the experimental measurement. After oxy-
insertion, hydroxide loss (to yield 3 + OH−) or proton transfer
(to yield [Cp*W(O)3]− + ROH) was determined to be facile
(ΔG‡ = 12 kcal·mol−1 in 1,4-dioxane and water, respectively).
The calculated transition state for oxy-insertion from [(κ1-
Cp*)W(O)2(R)(OOH)]− (Figure 9) is structurally similar to
the corresponding TS for the transition state involving oxygen
atom insertion from periodate (Figure 4). The C−O bond
being formed is longer (2.23 vs 2.10 Å) and the W−C bond
F
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movement away from d0 motifs will disfavor the OMBV
pathway. Thus, while functionalization of alkyl ligands through
processes that formally reduce the metal center can be facile for
late transition metal complexes, similar reactions with middle
transition metals may have inherently and prohibitively high
activation barriers.
Scheme 7. Calculated Free Energy for the Reaction of
a
Cp*WO(η2-O)2(CH2SiMe3) (2) with H+
Reaction of Cp*W(O)(η2-O2)(CH2SiMe3) (2) with
Brønsted Acid. The formation of alcohol is also observed
when complex 2 is treated with HCl in 1,4-dioxane. The
reaction is complete in 40 h at room temperature and produces
1
Cp*W(O)2Cl and TMSCH2OH in ∼90% yield by H NMR
spectroscopy (eq 4). The acid-promoted reaction of 2 is slower
than the hydroxide-promoted conversion. For example,
complex 2 with 3 equiv of NaOH quantitatively forms
TMSCH2OH in less than 1 h at room temperature.
Table 1 shows the conditions and results for the reactions of
complex 2 with various proton sources. Without a proton
Table 1. Reaction Conditions and Yields for Reaction of
Cp*W(O)(η2-O)2(CH2SiMe3) (2) with Brønsted Acid
a
+
The cation (H(diox)2 (diox =1,4-dioxane)) was used to model the
proton. Numbers are free energies (kcal·mol−1) for 1,4-dioxane (top,
bold) and water (bottom, italics) and are relative to complex 2.
a
T
yield
(%)
no.
solvent
benzene
acid
(°C) t (h)
1
2
3
none
80
23
23
24
40
32
0
b
dioxane
dioxane
3 equiv of HCl (Et2O)
∼90
∼70
3 equiv of HCl
(dioxane)
4
5
6
7
8
9
dioxane
3 eq HCl (dioxane)
50
23
23
23
75
23
2
100
100
12
∼60
∼80
>90
dioxane/water 3 equiv of HCl (35%)
dioxane/water 5 equiv of H2SO4 (98%)
dioxane
dioxane/water none
dioxane HOTf (99%)
5 equiv of H2SO4 (98%)
decomp.
∼70
decomp.
24
12
a
1
Yields of TMSCH2OH are based on integration of H NMR spectra
b
versus an internal standard. 1,4-dioxane.
source, when 2 is heated in benzene at 80 °C for 24 h, no
alcohol is observed, and the starting material decomposes to
form complex 1 (entry 1). Heating increases the rate of
conversion but decreases the yield (entries 3 and 4). The use of
strong acid in 1,4-dioxane results in decomposition of the
starting material with no alcohol production (entries 7 and 9).
Adding [nBu4N][Cl] does not accelerate the reaction, which is
consistent with the acceleration by HCl resulting from the
addition of protons rather than chloride. Also, weaker acids,
such as lutidinium, do not facilitate this reaction at room
temperature.
Figure 9. DFT calculated organometallic Baeyer−Villiger (OMBV)
transition state for oxy-insertion of hydroperoxide into W−C bond of
Cp*W(O)2(CH2SiMe3) (1). Bond lengths are given in Å and bond
angles in degrees.
We have considered two roles for the acid in the conversion
of 2 and HCl to Cp*W(O)2Cl and TMSCH2OH (Scheme 6):
(1) the proton serves as a catalyst by protonating the η2-peroxo
ligand, which would increase oxygen electrophilicity and
facilitate oxy-insertion. A similar mechanism was proposed for
the acid-catalyzed rearrangement of an η2-peroxo Ta alkyl to
form oxo-alkoxide derivatives.26 (2) The proton is not a
catalyst, but rather, the acid serves to generate free alcohol from
Cp*W(O)2(OCH2SiMe3).
being broken is shorter (2.44 vs 2.61 Å) for the periodate TS,
implying an earlier TS for the OMBV oxy-insertion with
periodate in relation to the hydroperoxide congener. The
tremendous kinetic and thermodynamic free energy preference
for the OMBV pathway in relation to the pathways that lead to
reduced WIV intermediates (nucleophilic substitution and
reductive elimination, Scheme 5) is interesting in connection
with the highly endergonic single-electron transfer reaction
modeled above, and supports the notion of the important role
of metal d orbital occupation in oxy-insertion, and further that
To investigate rate dependence of the reaction of 2 and HCl,
we attempted to use excess HCl to achieve pseudo first-order
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conditions. However, when more than 5 equiv of HCl are used,
complex 2 decomposed with little production of TMSCH2OH.
Reaction coordinates for conversion of 1 to 3 in the absence
and presence of a proton were evaluated computationally to
help assess the impact of Brønsted acids upon the proposed
mechanisms of oxy-insertion. Various basic sites were evaluated
in the starting materials, products, intermediates, and transition
sites (e.g., protonation of oxo versus peroxo ligands). The
discussion focuses on the most stable tautomers (Scheme 7). In
the absence of a proton, the migration of R to the peroxide
moiety has a calculated activation barrier of 35 or 32 kcal·mol−1
in 1,4-dioxane or water, respectively. Upon protonation (the
protonated bis-dioxane cation, [(dioxane)2H]+, was used to
model HCl in dioxane), the corresponding barriers were
reduced to 17 kcal·mol−1 (1,4-dioxane) or 22 kcal·mol−1
(water). The results thus support the observation of
acceleration of the oxy-insertion reaction upon the introduction
of Brønsted acids.
ment;36 Periana, Goddard, and co-workers studied the
mechanism for the reaction of MTO with H2O2 in the
presence of hydroxide.37 Calculations show that the OMBV
pathway in which OOH− coordinates to rhenium followed by
the migration of the methyl to the coordinated oxygen atom of
Re−OOH has the lowest energy barrier. To determine whether
the reaction of 1 with H2O2 in the presence of OH− proceeds
via an OMBV pathway to form the alkoxide complex 3, as the
−
reaction of 1 and IO4 does (see above), the reaction of
complex 1 with a mixture of 5 equiv of H2O2 and 5 equiv of
NaOH in 1:1 THF-d8/D2O (v/v) was monitored at −1.3 °C by
1H NMR spectroscopy. Under these conditions, the starting
material 1 converts to complex 2 in 10 min. Complex 2 then
undergoes slow transformation to TMSCH2OH over a period
of 8 h. Consequently, alcohol release from complex 1 and H2O2
in the presence of hydroxide proceeds by an η2-peroxo pathway
that involves two steps: the formation of the η2-peroxo complex
2 and the conversion of 2 to TMSCH2OH. Compared to the
reaction with H2O2 under acidic conditions, the reaction rates
of both steps (i.e., formation of 2 and release of TMSCH2OH)
are faster with hydroxide. For example, the reaction of complex
1 with a mixture of H2O2 (3 equiv) and HCl (3 equiv)
produces complex 2 in hours at room temperature, which then
converts to TMSCH2OH in 48 h (Scheme 8).
Analysis of the calculated oxy-insertion TS geometries from 2
with and without an added proton is revealing (Figure 10). The
Scheme 8. Comparison of Alcohol Release from
Cp*W(O)2(CH2SiMe3) (1) with H2O2 in the Presence of H+
and OH−
Figure 10. Calculated TS for oxy-insertion of Cp*W(O)(η2-
O)2(CH2SiMe3) (2) with (red) and without (black) an added proton.
Bond lengths are given in Å.
various bond distances in the active site point to an earlier TS
upon the introduction of a proton, which is consistent with a
lower barrier according to the Hammond postulate. Addition-
ally, one may hypothesize that the protonation of the incipient
oxide group in the peroxo TS yields a better leaving group
(hydroxide). The greater exergonicity for neutral Cp*W(O)-
(η2-O)2R → Cp*W(O)2(OR) versus the protonated variant
(−69 kcal·mol−1 in 1,4-dioxane (Scheme 5) versus −80
kcal·mol−1 (Scheme 7)) provides additional support for these
proposals and the role of Brønsted acids in catalyzing oxy-
insertion from 2.
DFT was used to probe the conversion of dioxo complex 1
and hydrogen peroxide to the η2-peroxo complex 2 (Scheme
9). A [2 + 2] addition of the OH bond of hydrogen peroxide
across the WO bond of 1 (red line in Scheme 9) has a
calculated barrier of 28 and 30 kcal·mol−1 for 1,4-dioxane and
water, respectively. This [2 + 2] TS leads to the hydroxy/
hydroperoxy intermediate Cp*W(R)(O)(OH)(OOH), which
is 10 kcal·mol−1 above complex 1. From this intermediate, a
modest barrier of 12 kcal·mol−1 (1,4-dioxane) or 14 kcal·mol−1
(water) must be surmounted to transfer hydrogen from the
hydroperoxide to hydroxide ligand to dissociate water and yield
Reaction of Cp*W(O)2(CH2SiMe3) (1) with Hydrogen
Peroxide in the Presence of Hydroxide. The reaction of
complex 1 with H2O2 (3 equiv) in benzene generates complex
2 at room temperature after 24 h without production of
alcohol. However, alcohol production is observed at room
temperature when complex 1 is treated with a mixture of H2O2
(3 equiv) with NaOH (3 equiv) in 1:1 THF-d8/D2O (v/v).
The reaction is complete in ∼4 h and produces TMSCH2OH
complex 2. Overall, with HO2 /OH− as the oxygen atom
−
transfer couple, the transformation of 1 to 2 is calculated to be
mildly endergonic by 3 kcal·mol−1 in both 1,4-dioxane and
water. Deprotonation of hydrogen peroxide and coordination
of hydroperoxide gives Cp*W(O)2(R)(OOH), which is higher
in energy than 1 by 10 and 13 kcal·mol−1. Conversion of
Cp*W(O)2(R)(OOH) to complex 2 through an intramolecular
proton transfer and dissociation of hydroxide are calculated to
occur with overall activation barriers of 28 and 27 kcal·mol−1.
Thus, the calculations do not reveal any obvious advantage to
base-promoted conversion of 1 and hydrogen peroxide to
complex 2.
1
in >95% yield by H NMR spectroscopy (eq 5).
Espenson and co-workers observed faster decomposition and
methanol release from MTO with H2O2 in a basic environ-
H
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Scheme 9. Calculated Free Energy for the Conversion of Cp*W(O)2(CH2SiMe3) (1) and H2O2 to Cp*WO(η2-O)2(CH2SiMe3)
a
(2)
a
Numbers are free energies (kcal·mol−1) for 1,4-dioxane/water and are relative to complex 2.
peroxide complexes can be promoted by addition of hydroxide
or Brønsted acid. DFT calculations lead to the suggestion that
protonation of the η2-peroxide ligand facilitates alkyl migration
to the unprotonated oxygen atom. Our calculations suggest that
the addition of base leads to a similar TS in which the alkyl
group migrates to the unprotonated oxygen of an η2-
hydroperoxide ligand; however, the base-promoted TS is
overall anionic and has a second oxo ligand (Scheme 11).
SUMMARY AND CONCLUSIONS
■
Insertion of oxygen atoms into metal−hydrocarbyl bonds is a
potential key step in catalytic oxidation of hydrocarbons.
Despite the potential importance of such oxygen atom insertion
reactions, there are few examples of nonradical conversion of
M−R bonds and oxygen atom transfer reagents to M−OR, and
detailed studies of nonradical oxygen atom insertion into M−R
bonds are rare. We have established that Cp*WVI complexes
can undergo clean oxygen atom insertion reactions by at least
three different pathways (Scheme 10). Oxy-insertion from η2-
Scheme 11. Comparison of TSs Based on DFT Calculations
for Base- and Acid-Promoted Conversion of Cp*W(O)(η2-
O)2(CH2SiMe3) (2) to the Oxy-Insertion Products
Scheme 10. Summary of Pathways for the Oxygen Atom
Insertion into W−C Bonds of Cp*W(O)2(CH2SiMe3) (1)
and Cp*W(O)(η2-O)2(CH2SiMe3) (2)
Experiments clearly show that the base-promoted oxy-insertion
is faster than the acid-catalyzed reaction. Quantification of such
effects is important, since most successful transition-metal-
catalyzed alkane oxidations incorporate electrophilic late
transition metals that tolerate acidic conditions but are not
likely to be amenable to alkaline conditions.6,45 Some caution is
advised when comparing these calculated values, since most
computations use an implicit water solvation model and
calculations for the reaction of 1 and IO4 with explicit
water demonstrate that hydrogen bonding can be important.
The third oxy-insertion follows an OMBV pathway. To our
knowledge, this is only the second example of an oxygen atom
I
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Eyring Plot of Reaction of Cp*W(O)2(CH2SiMe3) (1) with
NaIO4. A representative kinetic experiment is described. Complex 1
(13.5 mg, 32.0 μmol) was dissolved in 4.8 mL of THF-d8. Two drops
insertion into a M−R bond that likely proceeds by this
concerted process.15 Importantly, the OMBV reaction with 1
occurs in neutral water/1,4-dioxane, suggesting that catalysts
with nucleophilic hydrocarbyl groups that are tolerant of water
should be amenable for this oxy-functionalization process.
1
of benzene were added to the solution as an internal standard for H
NMR integration. A 300 μL aliquot (0.0067 mol/L) was transferred to
an NMR tube by syringe. NaIO4 (44.8 mg, 0.200 mmol) was dissolved
in 6.0 mL of D2O (0.033 mol/L). The solution of complex 1 was
cooled in ice water. Then, 300 μL (5.0 equiv) of the NaIO4 solution
was added to the solution of complex 1. The reaction mixture was
monitored by array 1H NMR spectroscopy on a 500 MHz
EXPERIMENTAL SECTION
■
General Methods. Unless otherwise noted, all synthetic
procedures were performed under anaerobic conditions in a
nitrogen-filled glovebox or by using standard Schlenk techniques.
Glovebox purity was maintained by periodic nitrogen purges and was
monitored by an oxygen analyzer (O2 < 15 ppm for all reactions).
Tetrahydrofuran and 1,4-dioxane were dried by distillation from
sodium/benzophenone and P2O5, respectively. Diethyl ether was
distilled over CaH2. THF-d8, 1,4-dioxane-d8, D2O, and H218O were
used as received and stored under a N2 atmosphere over 4 Å molecular
1
spectrometer at 10.7 °C. A H NMR spectrum was acquired every 2
min. Integration of the methylene peak of complex 1 gave the variation
in concentrations. Similar reactions were set up at −1.3, 22.7, and 34.7
°C. The time between every 1H NMR spectrum in the array was
adjusted according to the rate of the reaction. To ensure
reproducibility, every concentration was repeated at least twice for a
minimum of three total experiments under each set of conditions.
Oxygen Labeling of Reaction of Cp*W(O)2(CH2SiMe3) (1)
with NaI18O4. NaIO4 (10.7 mg, 50.0 μmol) was dissolved in 300 μL
of H218O and allowed to equilibrate for 1 h at room temperature with
sonication (the exchange of 18O and 16O under such condition is
extremely fast).53 Complex 1 (4.2 mg, 10 μmol) was dissolved in 300
μL of THF, and the solution was transferred to the NaIO4 solution.
After 20 min at room temperature, the color of the solution faded from
pale yellow to colorless. A 3.0 μL aliquot of the reaction mixture was
analyzed by GC/MS for TMSCH2OH content. The fragmentation
pattern of TMSCH2OH produced from the reaction was compared to
patterns for TMSCH218OH and TMSCH216OH.
Reaction of Cp*W(O)2(CH2SiMe3) (1) with PhIO. Complex 1
(4.2 mg, 10 μmol) and PhIO (8.0 mg, 30 μmol) was dissolved in 400
μL of 1,4-dioxane-d8 in a J-Young tube. The tube was then taken out of
the box and sonicated for 30 min. 1H NMR spectroscopy was used to
monitor the reaction.
Reaction of Cp*W(O)2(CH2SiMe3) (1) with [Bu4N][IO4].
Complex 1 (4.3 mg, 10 μmol) and [Bu4N][IO4] (12.9 mg, 30.0
μmol) were dissolved in 300 μL of dry THF-d8. 1H NMR spectroscopy
was used to monitor the reaction at room temperature. No reaction
was observed after 24 h. D2O (300 μL) was added to the reaction
mixture. A 1H NMR spectrum was acquired after 2 h, and quantitative
formation of TMSCH2OH was observed.
1
sieves (except water). H NMR spectra were recorded on a Varian
1
Mercury 300 or Varian Inova 500 MHz spectrometer. All H spectra
are referenced against residual proton signals of the deuterated
solvents. GC/MS was performed using a Shimadzu GCMS-QP2010
Plus system with a 30 mm × 0.25 mm SHRXI-5MS column with 0.25
mm film thickness using negative chemical ionization (NCI), which
also allows for simulated electron impact (SEI) ionization. The
preparation, isolation, and characterization of Cp*W(O)2(CH2SiMe3)
and Cp*W(O)(η2-O2)(CH2SiMe3) have been previously reported.39
All other reagents were used as purchased from commercial sources.
Computational Methods. DFT calculations with the B3LYP46−48
functional employed the Gaussian 09 program49 in conjunction with
the pseudopotentials and valence basis sets of Stevens and co-
workers50,51 for W, Si, and I. All 2p elements plus hydrogen were
modeled with the 6-311++G(d,p) all-electron basis set. All species
were singlet spin states and optimized within the restricted Kohn−
Sham formalism with the exception of 1•−and [IO4]•, which are
doublets and optimized with unrestricted Kohn−Sham methods. For
the latter, spin contamination was minimal. All stationary points were
optimized in the gas phase without symmetry constraint and identified
as minima or TSs through the calculation of the energy Hessian.
Solvent effects were incorporated implicitly through the use of the
CPCM52 model for water and 1,4-dioxane. All quoted energetics are
free energies and are reported, assuming a temperature of 298.15 K
and 1 atm and were obtained using unscaled vibrational frequencies.
Reaction of Cp*W(O)2(CH2SiMe3) (1) with NaIO4. Complex 1
(2.1 mg, 5.0 μmol) was dissolved in 300 μL of THF-d8 in an NMR
tube. NaIO4 (3.3 mg, 15 μmol) was dissolved in 300 μL of D2O and
Kinetics of Water Dependence in the Reaction of Cp*W-
(O)2(CH2SiMe3) (1) with [Bu4N][IO4]. A representative kinetic
experiment is described. Complex 1 (32.0 mg 75.0 μmol) and
[Bu4N][IO4] (487 mg, 1.12 mmol, 15 equiv) were dissolved in 6.0 mL
of THF-d8. Two drops of benzene were added to the solution as an
internal standard for 1H NMR integration. A 400 μL aliquot was
transferred to a J-Young tube. D2O (5.0 μL, 50 equiv) was added to
the solution of 1 and [Bu4N][IO4] by syringe. The reaction mixture
1
transferred to the solution of 1. The reaction was monitored by H
NMR spectroscopy until completion. The production of TMSCH2OH
was confirmed using two methods. First, 5 μL of the reaction mixture
was analyzed by GC/MS, and TMSCH2OH was detected (and
compared with GC/MS of an authentic sample). Second, 1 μL of
TMSCH2OH was added to the reaction mixture. The intensity of the
product peaks increased in the 1H NMR spectrum. Cp*W-
1
was then monitored by array H NMR spectroscopy on a 500 MHz
1
spectrometer at 50 °C. A H NMR spectrum was acquired every 20
min. Integration of the methylene peak of complex 1 gave the variation
in concentrations. Similar procedures were used to set up the reaction
of 1 and 15 equiv of [Bu4N][IO4] with 10.0, 20.0, 30.0, 40.0, 60.0, and
100 μL of D2O (for the reaction with 60.0 and 100 μL of D2O, the
same amount of 1 and [Bu4N][IO4] were dissolved in 300 μL of THF-
d8 instead of 400 μL). To ensure reproducibility, every concentration
was repeated in triplicate.
1
(O)2(OCH2SiMe3) (3): H NMR (THF-d8/D2O 1:1): 4.17 (CH2,
s), 2.04 (CH3, s), −0.05 (Si(CH3)3, s) ppm.
Kinetics of Reactions of Cp*W(O)2(CH2SiMe3) (1) with NaIO4.
A representative kinetic experiment is described. Complex 1 (32.0 mg
75.0 μmol) was dissolved in 4.5 mL of THF-d8. Two drops of benzene
were added to the solution as an internal standard for 1H NMR
integration. A 300 μL aliquot (0.0166 mol/L) was transferred to an
NMR tube. NaIO4 (143 mg, 0.667 mmol) was dissolved in 2.0 mL of
D2O (0.33 mol/L). D2O (225 μL) was added to the solution of 1 by
syringe and cooled in ice water. Then, 75 μL (5.0 equiv) of the NaIO4
solution was added to the solution of complex 1. The reaction mixture
Reaction of Cp*W(O)(η2-O2)(CH2SiMe3) (2) with NaOH.
Complex 2 (4.3 mg, 10 μmol) was dissolved in 300 mL of THF-d8
in an NMR tube. NaOH (1.2 mg, 30 μmol) was dissolved in 300 μL of
D2O and transferred to the solution of complex 1. The reaction
1
mixture was monitored by H NMR spectroscopy until completion.
The production of TMSCH2OH was confirmed using two methods.
First, 5 μL of the reaction mixture was analyzed by GC/MS, and
TMSCH2OH was detected (and compared with GC/MS of an
authentic sample). Second, 1 μL of TMSCH2OH was added to the
1
was then monitored by array H NMR spectroscopy on a 500 MHz
spectrometer at −1.3 °C. A 1H NMR spectrum was acquired every 26
s. Integration of the methylene peak of complex 1, TMSCH2OH, and
the intermediate 3 gave the variation in concentrations. Similar
reactions were set up for 10.0, 15.0, and 20.0 equiv of NaIO4 by
adjusting the amounts of the D2O and NaIO4 solution. To ensure
reproducibility, every concentration was repeated in triplicate.
1
reaction mixture. The intensity of the product peaks increased in H
NMR spectrum.
Kinetics of Cp*W(O)(η2-O2)(CH2SiMe3) (2) with NaOH. A
representative kinetic experiment is described. Complex 1 (33.0 mg,
J
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Journal of the American Chemical Society
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1
75.0 μmol) was dissolved in 4.5 mL of THF-d8. Several milligrams of
hexamethylbenzene were added to the solution as an internal standard
for 1H NMR integration. A 300 μL aliquot (0.0166 mol/L) was
transferred to an NMR tube. NaOH (26.7 mg, 0.667 mmol) was
dissolved in 2.0 mL of D2O (0.33 mol/L). D2O (225 μL) was added to
the solution of complex 1 by syringe and cooled in ice water. Then, 75
μL (5.0 equiv) of the NaOH solution was added to the complex 1
solution. The reaction mixture was then monitored by array 1H NMR
on a 500 MHz spectrometer at −1.3 °C. A H NMR spectrum was
acquired every 26 s.
Kinetics of Reaction of Cp*W(O)2(CH2SiMe3) (1) and Cp*W-
(O)(η2-O2)(CH2SiMe3) (2) with H2O2/OH−. A mixture of complex 1
(2.1 mg, 5.0 μmol) and complex 2 (2.2 mg, 5.0 μmol) was dissolved in
300 μL of THF-d8 in an NMR tube. NaOH (2.0 mg, 50 μmol) was
dissolved in 300 μL of D2O, and 5 μL (48 μmol) of a 30% H2O2
solution was added to the NaOH solution. Both starting materials
were cooled in ice water. The mixture of H2O2 and NaOH was then
added to the solution of 2. The reaction mixture was then monitored
1
spectroscopy on a 500 MHz spectrometer at 10.7 °C. A H NMR
spectrum was acquired every 2 min. Integration of the Cp* methyl
peak of complex 2 and the methylene peak of TMSCH2OH gave the
variation in concentrations. Similar reactions were set up for 10.0, 15.0,
and 20.0 equiv of NaOH by adjusting the amounts of the D2O and
1
by array H NMR spectroscopy on a 500 MHz spectrometer at −1.3
°C. A H NMR spectrum was acquired every 26 s.
1
1
ASSOCIATED CONTENT
* Supporting Information
NaOH solutions. The time between every H NMR spectrum in the
■
array was adjusted according to the rate of the reactions. To ensure
reproducibility, every concentration was repeated in triplicate.
Eyring Plot of Reaction of Cp*W(O)(η2-O2)(CH2SiMe3) (2)
with NaOH. A representative kinetic experiment is described.
Complex 2 (35.2 mg, 80.4 μmol) was dissolved in 4.8 mL of THF-
d8. Several milligrams of hexamethylbenzene were added to the
S
Kinetics and GC/MS data, additional experimental details, and
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
solution as an internal standard for H NMR integration. A 300 μL
AUTHOR INFORMATION
Corresponding Author
tbg7h@virginia.edu; t@unt.edu
■
aliquot (0.0166 mol/L) was transferred to an NMR tube by syringe.
NaOH (20.2 mg, 0.505 mmol) was dissolved in 6.0 mL of D2O (0.083
mol/L). The solution of complex 1 was cooled in ice water. Then, 300
μL (5.0 equiv) of the NaOH solution was added to the solution of
Notes
1
complex 1. The reaction mixture was then monitored by array H
The authors declare no competing financial interest.
1
NMR spectroscopy on a 500 MHz spectrometer at 10.7 °C. A H
NMR spectrum was acquired every 2 min. Integration of the Cp*
methyl peak of complex 2 and the methylene peak of TMSCH2OH
gave the variation in concentrations. Similar reactions were set up at
ACKNOWLEDGMENTS
■
This work was solely supported as part of the Center for
Catalytic Hydrocarbon Functionalization, an Energy Frontier
Research Center funded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under award
no. DE-SC0001298.
1
−1.3, 22.7, and 34.7 °C. The time between every H NMR spectrum
in the array was adjusted according to the rate of the reaction. To
ensure reproducibility, every concentration was repeated in triplicate.
Oxygen Labeling of Reaction of Cp*W(O)(η2-O2)(CH2SiMe3)
(2) with Li*OH. H218O (300 μL) was transferred to a vial and frozen
i
in an PrOH/dry ice bath. CH3Li in diethyl ether solution (1 M, 30
REFERENCES
μL) was added to the frozen H218O. The iPrOH/dry ice bath was then
removed, and the mixture was warmed up to room temperature to
generate the Li18OH in H218O solution. Complex 2 (4.3 mg, 10 μmol)
was dissolved in 300 μL of THF, and the solution was added to the
Li18OH solution. After 30 min at room temperature, a 3.0 μL aliquot
of the reaction mixture was analyzed by GC/MS for TMSCH2OH
content. The fragmentation pattern of TMSCH2OH from the reaction
was compared to patterns for TMSCH218OH and TMSCH216OH.
Reaction of Cp*W(O)(η2-O2)(CH2SiMe3) (2) with Brønsted
Acid. A representative reaction is described. Complex 2 (2.2 mg, 5.0
μmol) was dissolved in 400 μL of 1,4-dioxane-d8 in a J-Young tube,
and HCl in diethyl ether solution (1N, 15 μL) was added to the
solution of 2. The reaction was then monitored by 1H NMR
spectroscopy. The reaction was complete after approximately 2 days.
Reaction of Cp*W(O)2(CH2SiMe3) (1) with H2O2/H+. Complex
1 (4.2 mg, 10 μmol) was dissolved in 300 μL of 1,4-dioxane-d8 in an
NMR tube. Hydrochloric acid (35%, 3.0 μL, 33 μmol) and H2O2
(30%, 3.0 μL, 29 μmol) were added to 300 μL of D2O. The mixture of
■
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