Oxidation of alcohols and activated alkanes with lewis acid-activated tempo

Article abstract of DOI:10.1021/ic5018888

The reactivity of MCl₃(η¹O) (M = Fe, 1; Al, 2; TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) with a variety of alcohols, including 3,4-dimethoxybenzyl alcohol, 1-phenyl-2-phenoxyethanol, and 1,2-diphenyl-2-methoxyethanol, was investigated using NMR spectroscopy and mass spectrometry. Complex 1 was effective in cleanly converting these substrates to the corresponding aldehyde or ketone. Complex 2 was also able to oxidize these substrates; however, in a few instances the products of overoxidation were also observed. Oxidation of activated alkanes, such as xanthene, by 1 or 2 suggests that the reactions proceed via an initial 1-electron concerted proton-electron transfer (CPET) event. Finally, reaction of TEMPO with FeBr₃ in Et₂O results in the formation of a mixture of FeBr₃(η¹OH) (23) and [FeBr₂(η¹OH)]₂(μ-O) (24), via oxidation of the solvent, Et₂O.

Full text of DOI:10.1021/ic5018888

Article
pubs.acs.org/IC
Oxidation of Alcohols and Activated Alkanes with Lewis Acid-
Activated TEMPO
Thuy-Ai D. Nguyen, Ashley M. Wright, Joshua S. Page, Guang Wu, and Trevor W. Hayton*
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
S
* Supporting Information
ABSTRACT: The reactivity of MCl3(η1-TEMPO) (M = Fe,
1; Al, 2; TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) with
a variety of alcohols, including 3,4-dimethoxybenzyl alcohol, 1-
phenyl-2-phenoxyethanol, and 1,2-diphenyl-2-methoxyethanol,
was investigated using NMR spectroscopy and mass
spectrometry. Complex 1 was effective in cleanly converting
these substrates to the corresponding aldehyde or ketone.
Complex 2 was also able to oxidize these substrates; however, in a few instances the products of overoxidation were also
observed. Oxidation of activated alkanes, such as xanthene, by 1 or 2 suggests that the reactions proceed via an initial 1-electron
concerted proton−electron transfer (CPET) event. Finally, reaction of TEMPO with FeBr3 in Et2O results in the formation of a
mixture of FeBr3(η1-TEMPOH) (23) and [FeBr2(η1-TEMPOH)]2(μ-O) (24), via oxidation of the solvent, Et2O.
systems, oxidizing alcohols within minutes at room temperature.
While the MCl3(η1-TEMPO) system appears to have some
advantages over other TEMPO protocols, there are still several
mechanistic questions that remain unanswered. In particular,
previous work on the Cu/TEMPO system suggests that the
reaction proceeds via a concerted 2e− oxidation, wherein a Cu-
bound alcohol is simultaneously oxidized by TEMPO and
Cu(II).15 In contrast, preliminary mechanistic experiments with
MCl3(η1-TEMPO) suggest that the reaction proceeds via an
initial 1e− hydrogen atom transfer event, which apparently makes
this system unique among TEMPO-containing oxidants. As a
result, we wanted to solidify our proposed mechanism by
exploring the reactivity of MCl3(η1-TEMPO) with a variety of
mechanistic probes, including activated alkanes and radical
clocks. Herein, we report the reactivity of 1 and 2 toward a variety
of alcohols and activated alkanes, including xanthene.10,16−24
The latter substrate is significant, because its reactivity confirms
that these oxidations can proceed via a concerted proton coupled
electron transfer (CPET) step, as was previously surmised.14 To
test the role of the Lewis acid in activating the TEMPO moiety,
we also explored the reactivity of TEMPO with FeBr3 in Et2O.
INTRODUCTION
■
N-Oxyl radicals, such as TEMPO (TEMPO = 2,2,6,6-
tetramethylpiperidine-N-oxyl) and ABNO (ABNO = 9-
azabicyclo[3.3.1]nonane N-oxyl), are widely used in a variety
of organic oxidations. In particular, they have proven excellent
reagents for the selective oxidation of primary alcohols,1−5
secondary alcohols,6,7 primary amines,8 and the α-oxyamination
of aldehydes.9 More recently, TEMPO has also found use in the
depolymerization of lignin. For example, Stahl and co-workers
demonstrated that efficient oxidation of the alcohol function-
alities in lignin10 can be achieved with the 4-acetamido-TEMPO/
HNO3/HCl system, wherein the active oxidant is likely [4-
acetamido-TEMPO]+. Stephenson and co-workers report a
similar lignin oxidation process, in which the benzylic alcohol
functionalities are oxidized by [4-acetamido-TEMPO][BF4].11
Hanson and co-workers have also had considerable success in
effecting the degradation of lignin model compounds by using a
TEMPO-based oxidant.12,13 For example, they reported that the
CuCl/TEMPO and CuOTf/2,6-lutidine/TEMPO catalyst sys-
tems could oxidatively cleave both a β-O-4 lignin model and β-1
lignin model, using oxygen as the terminal oxidant. While
promising, these Cu/TEMPO protocols required harsh
conditions and long reaction times, which is significant because
several lignin functional groups are not stable at elevated
temperatures.11
RESULTS AND DISCUSSION
■
Exploration of Substrate Scope. Reaction of MCl3(η1-
TEMPO) (M = Fe, 1; Al, 2) with 3,4-dimethoxybenzyl alcohol
(3), in Et2O (for 1) or CD2Cl2 (for 2), results in the complete
consumption of the alcohol within 10 min at room temperature
and formation of 3,4-dimethoxybenzaldehyde (4) in good yields
(Table 1, entry 1). For both reactions, compound 4 is the only
organic product observable in the reaction mixture by 1H NMR
spectroscopy.25 Similarly, oxidation of 1-phenyl-2-phenoxyetha-
Previously, our research group reported the use of the Lewis
acids FeCl3 and AlCl3, to activate TEMPO toward the oxidation
of alcohols.14 The resulting MCl3(η1-TEMPO) (M = Fe, 1; Al, 2)
adducts were observed to quickly oxidize both 1° and 2° alcohols,
forming the corresponding carbonyl compounds under mild
conditions. Complexes 1 and 2 are also capable of oxidizing 9,10-
dihydroanthracene, although this oxidation is much slower than
those performed with alcohol substrates. Importantly, complexes
1 and 2 appear to be more reactive than other TEMPO-based
Received: April 23, 2014
© XXXX American Chemical Society
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a
Table 1. Oxidation of Lignin Models by Complexes 1 and 2
a
1
Yields determined by H NMR spectroscopy.
nol (5), which has been previously employed as a β-O-4 lignin
model compound,21−23 with complex 1 results in complete
consumption of the alcohol within 3 h, and formation of 2-
phenoxyacetophenone (6) in 65% yield (Table 1, entry 2).
Complex 2 also oxidizes 5 to 6 (75% yield); however, a small
amount of a new product is also observed in this transformation,
namely, 2-(2,2,6,6-tetramethylpiperidine-N-oxyl)-2-phenoxya-
cetophenone (7), which is produced in 5% yield (Table 1,
entry 2). We suggest that this product is formed through further
oxidation of compound 6 by complex 2, which results in
formation of an α-keto radical, via H atom abstraction. The α-
keto radical is subsequently quenched by coupling to free
TEMPO, resulting in the formation of the new C−O bond. The
formation of 7 is perhaps not surprising considering that the
strength of the C−H bond abstracted in 6 (80.6 kcal/mol in
DMSO)26 is identical to the benzylic C−H bond enthalpy of
9,10-dihydroanthracene (80.6 kcal/mol in DMSO),14,27 which
both complexes 1 and 2 can readily oxidize. Interestingly, a
related α-oxyamination using TEMPO has been previously
described for enamines, and likely occurs via a similar
mechanism.9
We also explored the reaction of complexes 1 and 2 with 1,2-
diphenyl-2-methoxyethanol (8), a common β-1 lignin model
compound.10,12,21 Oxidation of 8 with complex 1 results in
complete consumption of the starting material and formation of
2-methoxy-1,2-diphenylethanone (9) in 75% yield (Table 1,
entry 3). No other oxidation products were observed in the
reaction mixture, according to 1H NMR spectroscopy. In
contrast, reaction of 2 with 8 does not result in the formation
of 9. Instead, the major organic product formed in the reaction is
benzil (10) in 54% yield (77% conversion; Table 1, entry 3). We
suggest that formation of benzil occurs via hydrogen abstraction
of the transiently formed 9 by complex 2, which results in the
formation of an α-keto radical stabilized by the captodative
effect.28 The α-keto radical is then quenched by TEMPO,
resulting in the formation of a new C−O bond. This species then
forms benzil by release of methoxy and piperidyl radicals.
Interestingly, TEMPO is known to function as an O atom source
via N−O bond cleavage and release of the piperidyl radical.29 For
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a
Table 2. Oxidation of Selected Substrates by Complexes 1 and 2
a
1
Yields determined by H NMR spectroscopy.
comparison, Hanson and co-workers reported that oxidation of 8
with O2, in the presence of 10 mol % CuCl and 30 mol %
TEMPO at 100 °C in pyridine, resulted in formation of
benzaldehyde (84%) and methyl benzoate (88%).12 We do not
observe either of these products, which suggests that different
mechanisms are operative in the two systems.
of 1 in CH2Cl2 yields bixanthenyl33 (15) and xanthone34 (16), in
18% and 33% yield, respectively, after 2 d at room temperature
(Table 3, entry 7). Similarly, oxidation of 14 with 2 equiv of 2 in
CH2Cl2 yields 15 and 16, in 51% and 4% yield, respectively, after
3 d at room temperature (Table 3, entry 7). The presence of
bixanthenyl in each reaction mixture can be rationalized by
invoking the formation of the xanthenyl radical, which
subsequently dimerizes to give the final product. Importantly,
its presence provides evidence for an initial one-electron CPET
event upon oxidation of xanthene.35 The presence of xanthone in
the reaction mixture can be similarly rationalized. However,
instead of coupling to another xanthenyl radical, the xanthenyl
radical instead reacts with TEMPO, forming the C−O bond.
This TEMPO-xanthenyl intermediate then forms xanthone by
release of the piperidyl radical.29 The reactivity of complexes 1
and 2 with fluorene (17) and triphenylmethane (18), substrates
with slightly stronger C−H bonds than xanthene, was also
examined (Table 3, entries 8 and 9). However, no reaction was
observed upon addition of 1 equiv of 1 or 2 to either substrate in
CD2Cl2, even after prolonged reaction times (4 d). Finally, a
control reaction between TEMPO and 1,4-cyclohexadiene
reveals some reactivity. However, the reaction is extremely
slow, only reaching 22% conversion after 4 d at room
temperature. Similarly, TEMPO will react with xanthene in the
absence of a Lewis acid, but the reaction is slow, only achieving
9% conversion after 4 d. In line with this observations, Gunnoe
and co-workers reported that TEMPO will oxidize 1,4-
cyclohexadiene at elevated temperatures.36
The viability of ketones 6 and 9 to act as the substrates for the
formation of 7 and 10 was confirmed by their independent
reaction with complexes 1 and 2. Thus, reaction of 2 equiv of 2
with 6 in CH2Cl2 results in almost complete consumption of the
starting material (96% conversion) after only 3 h, and production
of the α-oxyamination product in 48% yield (Table 2, entry 4). In
contrast, reaction of 6 with 2 equiv of 1 in Et2O for 18 h left the
starting material largely unreacted and produced 7 in only 3%
1
yield, according to H NMR spectroscopy (Table 2, entry 4).
Oxidation of 9 by complex 2 in CH2Cl2 at room temperature for
15 h results in the formation of benzil (10) (53% yield) and 2,2-
dimethoxy-2-phenylacetophenone (11) (16% yield), demon-
strating that 2-methoxy-1,2-diphenylethanone (9) is a viable
precursor to compound 10 (Table 2, entry 5). Compound 11 is
probably formed by a Lewis acid-catalyzed reaction of 10 with
methanol.30−32 Notably, there is no reaction observed between
complex 1 and 9 in CH2Cl2 after 15 h of stirring (Table 2, entry
5), which is consistent with the reactivity observed between 1
and 8 (Table 1, entry 3).
Mechanistic Studies. Previously, we argued that the first
step in the oxidation of substrate by complexes 1 and 2 was a
concerted proton−electron transfer (CPET) event.14 To further
evaluate this hypothesis, and better understand the reactivity of
complexes 1 and 2 toward alcohols, we explored the reactivity of
MCl3(η1-TEMPO) with a variety of activated alkanes. Reaction
of 1,4-cyclohexadiene (12) with 2 equiv of 1 in toluene-d8 results
in almost complete consumption of 12 within 2 h, and formation
of benzene in 82% yield (Table 3, entry 6). Similarly, reaction of
12 with 2 equiv of 2 in CD2Cl2 results in the complete conversion
of 12 into benzene within 15 min (Table 3, entry 6), according to
1H NMR spectroscopy. Oxidation of xanthene (14) with 2 equiv
The experiments outlined in Table 3 reveal a clear correlation
between the BDE (or BDFE) of the cleaved C−H bond in the
substrate and its ability to react with 1 and 2. The strongest C−H
bonds that 1 and 2 are able to activate appear to be those of
xanthene (BDE = 77.9 kcal/mol in DMSO) and 9,10-
dihydroanthracene (BDE = 80.6 kcal/mol in DMSO),14,27
which we tested in an earlier study. For comparison, other
TEMPO/Lewis acid systems appear to be more reactive. For
example, the TEMPO/Co(OAc)2/NaOCl system is capable of
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Table 3. Oxidation of Activated Alkanes, Cyclobutanol, and Cyclopropylcarbinol by Complexes 1 and 2
a
b
c
1
BDE and BDFE from ref 27. Yields determined by H NMR spectroscopy. Concentrations of 1 and 2 were 385 and 200 mM, respectively.
d
e
Concentrations of 1 and 2 were 260 and 190 mM, respectively. Concentrations of 1 and 2 were 6.3 mM and 4.4 mM, respectively.
oxidizing a variety of benzylic C−H bonds, including those of
toluene (BDE = 92 kcal/mol in DMSO).27,37 While the
mechanism in the Co(OAc)2 system is not entirely clear, it is
possible that these oxidations proceed via the formation of a
[TEMPO]+ intermediate and not a TEMPO−Lewis acid adduct,
as is likely the case for our system.
Finally, the reactions of complexes 1 and 2 with cyclobutanol
(19) and cyclopropylcarbinol (21) were investigated. Both
reagents are common mechanistic probes used to distinguish
between oxidations that proceed in one- or two-electron redox
steps.38,39 This discrimination is possible because one-electron
oxidants favor the formation of ring-opened products, such as
butyraldehyde, or 2- and 3-butenaldehyde, while two-electron
oxidants convert 19 and 21 into cyclobutanone (20) and
cyclopropanecarboxaldehyde (22), respectively.39−42 All the
available evidence suggests that complexes 1 and 2 react via an
initial 1-electron CPET step. Accordingly, we hypothesized that
reaction of complexes 1 or 2 with 19 and 21 would result in
formation of ring-opened products. Thus, addition of 19 to 2
equiv of 1 or 2 in C6D6 results in complete consumption of the
alcohol within 10 min at room temperature, according to H
1
NMR spectroscopy. Surprisingly, the only oxidation product
observed in the reaction mixture is the ring-closed product 20
(Table 3, entry 10). The reactions of 21 with 2 equiv of 1 or 2 in
CD2Cl2 also yielded the ring-closed product, 22, as the only
oxidation product (100% conversion; 84 and 91% yield,
respectively; Table 3, entry 11). Performing the oxidation of
21 at lower concentrations also only resulted in formation of the
ring-closed product, 22 (Table 3, entry 12). These observations
are puzzling for several reasons. For one, complex 2 does not
contain a redox-active metal center and should only be capable of
a one-electron oxidation. Second, this selectivity is at odds with
the reactivity observed for the other substrates investigated in
this study, such as 2-phenoxyacetophenone (6) and xanthene
(14). These data suggest that multiple pathways could be
operative upon reaction of substrate with MCl3(η1-TEMPO),
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including a two-electron pathway involving the intermediacy of
[TEMPO]+ (which can function as a 2e− oxidant). In this regard,
we have recently demonstrated that coordination of TEMPO to
BBr3 can generate [TEMPO]+ via disproportionation of the
neutral TEMPO radical.43 In addition, [TEMPO]+ is known to
oxidize substituted cyclobutanols to cyclobutanone, without the
formation of ring-opened products.44 Alternately, it is possible
that the ring-opening rate constants for cyclobutanol and
cyclopropylcarbinol are not large enough to allow for the
discrimination between the 1e− and 2e− oxidation pathways in
our system. In support of this suggestion, we note that the
oxidation of cyclobutanol by Fe(aq)2+/O3 results in the
formation of both ring-closed and ring-opened products,
demonstrating that the rates of oxidation and ring-opening are
comparable in magnitude.45 Therefore, it is apparent that care
must be taken in interpreting results derived from radical clock
experiments, and on balance, we still suggest that for our system
the 1e− mechanism is most consistent with available evidence.
Activation of TEMPO with FeBr3. Previously, we
speculated that activation of TEMPO with stronger Lewis
acids would allow us to expand the substrate scope to unactivated
alkanes.14,43 To test this hypothesis we explored the reaction of
FeBr3 with TEMPO. While a quantitative evaluation of the Lewis
acidity of FeBr3 has not been performed,46 it is likely to be a
stronger Lewis acid than FeCl3. We based this conclusion on the
knowledge that bromide salts are often better Lewis acids that
their chloride congeners. For example, it is well-established that
BBr3 is a stronger Lewis acid that BCl3.47−49 Likewise,
thermochemical data suggest that AlBr3 is a stronger Lewis
acid than AlCl3.46 Given these considerations, we rationalized
that FeBr3 would likely follow the same trend as the group 13
Lewis acids. Accordingly, addition of 1 equiv of TEMPO to an
Et2O solution of FeBr3 results in the immediate formation of an
orange precipitate. A subsequent 1H NMR analysis of this solid
reveals the presence of two major species, identified as FeBr3(η1-
TEMPOH) (23) and [FeBr2(η1-TEMPOH)]2(μ-O) (24) (eq
1). These species are present in an approximate 3:2 ratio,
respectively.
the analogous TEMPO β-H resonance observed for FeCl3(η1-
TEMPOH).14 Unfortunately, we have been unable to cleanly
separate complex 23 from 24, and so are unable to complete its
characterization. However, we have been able to isolate a few X-
ray quality crystals of 23, which has permitted its characterization
by X-ray crystallography. Complex 23 crystallizes in the
orthorhombic space group Pnma, and its solid state molecular
structure is shown in Figure 1. Complex 23 is isostructural with
Figure 1. ORTEP diagram of FeBr3(η1-TEMPOH) (23) with 50%
probability ellipsoids. All hydrogen atoms, except the N−H hydrogen
atom, are omitted for clarity. Atoms labeled with an asterisk are
generated by symmetry. Selected bond lengths (Å) and angles (deg):
Fe1−Br1 = 2.3184(3) Å, Fe1−Br2 = 2.3396(5) Å, Fe1−O1 = 1.882(2)
Å, N1−O1 = 1.406(3) Å, ∑(E−N1−E) = 339.2°.
MCl3(η1-TEMPOH) (M = Fe, Al).14 It features an N1−O1 bond
length of 1.406(3) Å, consistent with the presence of the reduced
[TEMPO]− moiety. In addition, the sum of the angles around
N1 is 339.2°, while a hydrogen atom was located in the difference
map and successfully refined on N1. Finally, the Fe1−O1 bond
length in 23 is 1.882(2) Å, which is similar to that exhibited by
FeCl3(η1-TEMPOH).14 Interestingly, addition of 1 equiv of
TEMPOH to an Et2O solution of FeBr3 also generates a mixture
of 23 and 24. Under these conditions, 23 and 24 are formed in a
1
1:1 ratio, according to H NMR spectroscopy (Supporting
Information Figure S32).
Complex 24 can be generated in higher yield by reaction of 1
equiv of TEMPO with FeBr3 in Et2O, in the presence of 1 equiv
of 1,4-cyclohexadiene. When formed in this manner, it can then
be separated from the small amount of 23 that is also generated in
the reaction by recrystallization twice from CH2Cl2/Et2O.
Generated in this fashion, 24 can be isolated as an orange
crystalline solid in 32% yield. Its 1H NMR spectrum in CD2Cl2
exhibits two broad resonances at δ 3.72 and 1.86 ppm, assignable
to the methyl protons of TEMPOH, and three broad resonances
at δ 6.39, 2.36, and 1.64 ppm, assignable to the methylene
protons of TEMPOH. The resonance assignable to the NH
proton is observed at δ 47.21 ppm. We suggest that the bridged
oxo ligand in complex 24 is derived from the TEMPO moiety,
The identity of complex 23 was determined by comparison of
1
its H NMR spectral parameters with those of its chloride
congener, FeCl3(η1-TEMPOH).14 In particular, 23 features a
diagnostic resonance at δ 59.20 ppm, assignable to a TEMPO β-
1
H resonance, in its H NMR spectrum in CD2Cl2 (Supporting
Information Figure S31). This chemical shift is nearly identical to
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based on the well-established ability of TEMPO to act as an
oxygen atom source in both transition metal and actinide
systems.29,50−53 For example, reaction of Fe(hfac)2(H2O)2 with
TEMPO results in O atom transfer and formation of an Fe(III)
oxo-bridged dimer, along with formation of the piperidinium
ion.54
ethane, were not observed in the GC trace, likely because of their
greater volatility versus that of Et2O. Ethanol, another possible
oxidation product,57 was also not observed. Interestingly,
however, unreacted TEMPO was observed in the GC/MS
trace. The GC/MS analysis is significant because it confirms that
FeBr3/TEMPO can oxidize the methylene C−H bond in Et2O,
which is a stronger C−H bond (BDE = 92.5 kcal/mol) than the
other substrates that we have probed.27,60
Complex 24 crystallizes in the orthorhombic space group Pccn,
and its solid state molecular structure is shown in Figure 2. In the
CONCLUSIONS
■
In summary, we have shown that the Lewis acid-TEMPO
adducts, MCl3(η1-TEMPO) (M = Fe, 1; Al, 2), can oxidize the
lignin models 3,4-dimethoxybenzyl alcohol (3), 1-phenyl-2-
phenoxyethanol (5), and 1,2-diphenyl-2-methoxyethanol (8) at
room temperature. While these simple molecules are not very
accurate models of lignin, they nonetheless represent a good
starting point for further studies with lignin itself. Their
oxidations likely proceed via an initial 1-electron CPET event,
a hypothesis that is supported by the isolation of 2-(2,2,6,6-
tetramethylpiperidyl-N-oxyl)-2-phenoxyacetophenone (7) upon
oxidation of either 5 or 2-phenoxyacetophenone (6) by complex
2. The isolation of bixanthenyl (15) upon oxidation of xanthene
(14) by complexes 1 or 2 also supports this premise. The
reaction of TEMPO with FeBr3 in Et2O results in the formation
of a mixture of FeBr3(η1-TEMPOH) and [FeBr2(η1-TEM-
POH)]2(μ-O), via oxidation of the solvent, Et2O. This result
further confirms the hypothesis that the strength of the Lewis
acid can modulate the oxidation potential of TEMPO and result
in an expanded substrate scope. We will continue to probe the
reactivity of TEMPO with Lewis acids of varying strengths to
further our understanding of the mechanisms of TEMPO-
mediated oxidations, and search for N-oxyl radicals that are less
sensitive to N−O bond cleavage.
Figure 2. ORTEP diagram of [FeBr2(η1-TEMPOH)]2(μ-O) (24) with
50% probability ellipsoids. All hydrogen atoms, except the N−H
hydrogen atom, are omitted for clarity. Atoms labeled with an asterisk
are generated by symmetry. Selected bond lengths (Å) and angles (deg):
Fe1−Br1 = 2.3565(4) Å, Fe1−Br2 = 2.3312(5) Å, Fe1−O1 = 1.7702(7)
Å, Fe1−O2 = 1.889(2) Å, N1−O2 = 1.396(3) Å, ∑(E−N1−E) =
330.0°.
EXPERIMENTAL SECTION
solid state, complex 24 exists as a bimetallic oxo-bridged
complex. The Fe−O(oxo) bond length in 24 is 1.7702(7) Å,
which is consistent with the presence of a Fe(III)-bridged oxo
linkage. For comparison, the closely related Fe(III) complex,
[FeBr2(HNP(tBu)3)]2(μ-O), features Fe−O(oxo) bond lengths
of 1.768(7) and 1.760(7) Å.55 The Fe−O(TEMPOH) bond
length in 24 (Fe1−O2 = 1.889(2) Å) is comparable to that of 1
(Fe−O = 1.8996(12) Å),14 while the metrical parameters of the
TEMPOH ligand in 24 are consistent with the presence of
[TEMPO]−, indicating that reduction of this moiety has
occurred.14,56 In particular, the N1−O2 bond length in 24 is
1.396(3) Å, while the sum of the angles around N1 is 330.0°.
Additionally, a hydrogen atom was located in the difference map
and successfully refined on N1. Finally, the Fe−Br bond lengths
in 24 (av 2.34 Å) are consistent with those of 23 and other Fe3+
bromide complexes.55
We have also attempted to identify the organic products
formed during the formation of 23 and 24. Thus, the supernatant
formed upon addition of 1 equiv of TEMPO to an Et2O solution
of FeBr3 was filtered through basic alumina to remove the metal
containing products. A GC/MS trace of the resulting filtrate
reveals the presence of 1,1-diethoxyethane (25) (Supporting
Information Figure S41). Its formation can be rationalized by
invoking the formation of an Et2O methylene radical, which
subsequently reacts with a molecule of Et2O, resulting in C−O
bond cleavage, formation of 1,1-diethoxyethane, and ejection of
an ethyl radical. It is unlikely that 25 is the only oxidation product
generated in the reaction; however, other possible Et2O
oxidation products, such as acetaldehyde,57−59 ethylene, or
■
General Procedures. All reactions and subsequent manipulations
were performed under anaerobic and anhydrous conditions under an
atmosphere of nitrogen. Hexanes, diethyl ether, and toluene were dried
by passage over activated molecular sieves using a Vacuum Atmospheres
solvent purification system, while C6D6, CD2Cl2, and toluene-d8 were
dried over 3 Å molecular sieves for 24 h prior to use. Methylene chloride,
mesitylene, 3,4-dimethoxybenzyl alcohol (3), cyclobutanol (19), and
cyclopropylcarbinol (21) were degassed and dried over 3 Å molecular
sieves for 24 h prior to use. AlCl3(η1-TEMPO) (2),14 FeCl3(η1-
TEMPO) (1),14 1-phenyl-2-phenoxyethanol (5),61 2-phenoxyaceto-
phenone (6),62 1,2-diphenyl-2-methoxyethanol (8) (mixture of 85:15 u
(R,S + S,R): l (R,R + S,S) diastereomers),21 and anhydrous TEMPOH63
were prepared according to the previously reported procedures. NMR
spectral data for these compounds were consistent with those reported
in the literature (note that the 1H NMR spectral data for TEMPOH in
C6D6 differ substantially between refs 63 and 64; our data were
consistent with those in ref 64).14,61,38,64 Compound 8 was recrystallized
from a concentrated CH2Cl2 solution at −25 °C and washed with cold
Et2O (3 × 1 mL) before use. All other reagents were purchased from
commercial suppliers and used as received.
1
All NMR spectra were collected at 25 °C. H NMR spectra were
recorded on a Varian UNITY INOVA 400 MHz spectrometer, an
Agilent Technologies 400-MR DD2 400 MHz spectrometer, a Varian
UNITY INOVA 500 MHz spectrometer, or an actively shielded Varian
UNITY INOVA 600 MHz spectrometer. 13C{1H} NMR spectra were
recorded on a Varian UNITY INOVA 500 MHz spectrometer. 1H and
13C{1H} NMR spectra are referenced to external SiMe4 using the
residual protio solvent peaks as internal standards (1H NMR
experiments) or the characteristic resonances of the solvent nuclei
(13C{1H} NMR experiments). All 1H NMR spectra containing
mesitylene as an internal standard were recorded with a relaxation
F
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Inorganic Chemistry
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delay time (d1) of 60 s. IR spectra were recorded on a Nicolet 6700 FT-
IR spectrometer with a NXR FT Raman Module. GC/MS traces were
collected on a Hewlett-Packard 5890A GC coupled to a Hewlett-
Packard 5970B Mass Selective Detector (MSD), equipped with a J&W
DB-5 ms capillary column (30 m × 0.25 mm i.d. with 0.25 μm film
thickness). The MSD has a dedicated electron ionization (EI) source
and a quadrupole mass analyzer. All other mass spectra were collected by
the Mass Spectrometry Facility at the University of California, Santa
Barbara. Elemental analyses were performed by the Micro-Mass Facility
at the University of California, Berkeley.
Synthesis of 2-Methoxy-1,2-diphenylethanone (9). 2-Me-
thoxy-1,2-diphenylethanone was prepared according to a previously
reported procedure65 for using Dess-Martin periodinane (DMP) to
oxidize alcohols: To a stirring CH2Cl2 (3 mL) solution of 1,2-diphenyl-
2-methoxyethanol (149.8 mg, 656.2 μmol) was added dropwise a
CH2Cl2 (10 mL) solution of DMP (337.5 mg, 795.7 μmol). The
reaction mixture was allowed to stir for 2.5 h, whereupon it was
transferred to a separatory funnel containing 5% NaOH (aq) (10 mL)
and Et2O (20 mL). The organic layer was separated and rinsed with H2O
(2 × 30 mL), dried with anhydrous MgSO4, and filtered to give a
colorless solution. The solvent was then removed in vacuo to provide a
white solid that was recrystallized from a concentrated CH2Cl2/Et2O
(1:1) solution at −25 °C and washed with cold Et2O (3 × 1 mL): 133.2
mg, 90% yield of 9. 1H NMR (500 MHz, 25 °C, CD2Cl2): δ 3.43 (s, 3H,
OCH3), 5.52 (s, 1 H, CH), 7.30 (m, 1H, p-Ph), 7.35 (m, 2H, m-Ph),
7.39−7.45 (m, 4H, Ph), 7.53 (m, 1H, p-Ph), 7.97 (m, 2H, o-Ph).
13C{1H} NMR (126 MHz, 25 °C, CD2Cl2): δ 57.91 (CH3), 87.00
mg, 59.3 μmol) was added to a stirring Et2O solution (0.5 mL) of 1-
phenyl-2-phenoxyethanol (5.2 mg, 24.3 μmol), resulting in the
immediate conversion of the deep purple solution to a red-brown
suspension. The reaction mixture was allowed to stir for 18 h,
whereupon it was filtered through a basic alumina column (1.5 cm × 0.5
cm) supported on glass wool. The column was then rinsed with Et2O (6
mL) to provide a colorless filtrate. The solvent was removed in vacuo,
and the resulting white solid was dissolved in CD2Cl2 (700 μL). This
solution was transferred to an NMR tube, mesitylene (5 μL, 35.9 μmol)
1
was added via microsyringe, and a H NMR spectrum was recorded.
Analysis of the chemical shifts and comparison of the product peak
integrations with those of the internal standard revealed complete
consumption of the alcohol and formation of 2-phenoxyacetophenone
in 65% yield. This was confirmed by comparison of the NMR spectral
data to that of an authentic sample of 2-phenoxyacetophenone.61 1H
NMR (600 MHz, 25 °C, CD2Cl2): δ 5.32 (s, 2H, CH2), 6.94 (d, J = 8.2
Hz, 2H, o-Ph), 7.00 (t, J = 7.4 Hz, 1H, p-Ph), 7.31 (t, J = 7.8 Hz, 2H, m-
Ph), 7.54 (t, J = 7.7 Hz, 2H, m-Ph), 7.65 (t, J = 7.5 Hz, 1H, p-Ph), 8.01 (d,
J = 7.6 Hz, 2H, o-Ph).
Oxidation of 1-Phenyl-2-phenoxyethanol by AlCl3(η1-
TEMPO). AlCl3(η1-TEMPO) (15.1 mg, 52.1 μmol) was added to a
solution of 1-phenyl-2-phenoxyethanol (5.0 mg, 23.3 μmol) dissolved in
CH2Cl2 (1 mL). The yellow solution was allowed to stand without
stirring for 3 h. The color of the solution gradually lightened over this
time. The solvent was removed in vacuo, and the resulting solid was
extracted into Et2O (3 mL) and filtered through a basic alumina column
(1.5 cm × 0.5 cm) supported on glass wool. The column was then rinsed
with Et2O (6 mL) to provide a colorless filtrate. The solvent was
removed in vacuo, and the resulting white solid was dissolved in CD2Cl2
(700 μL). This solution was transferred to an NMR tube, mesitylene (5
μL, 35.9 μmol) was added via microsyringe, and a 1H NMR spectrum
was recorded. Analysis of the chemical shifts and comparison of the
product peak integrations with those of the internal standard revealed
complete consumption of 1-phenyl-2-phenoxyethanol, and formation of
2-phenoxyacetophenone (6) and 2-(2,2,6,6-tetramethylpiperidine-N-
oxyl)-2-phenoxyacetophenone (7) in 75% yield and 5% yield,
respectively. The identities of these products were confirmed by
comparison of the NMR spectral data to that of authentic material. 1H
NMR (500 MHz, 25 °C, CD2Cl2): for 6, δ 5.31 (s, 2H, CH2), 6.93 (d, J =
8.8 Hz, 2H, o-Ph), 6.99 (t, J = 6.9 Hz, 1H, p-Ph), 7.30 (m, 2H, m-Ph),
7.53 (t, J = 7.8 Hz, 2H, m-Ph), 7.65 (t, J = 7.4 Hz, 1H, p-Ph), 7.99 (d, J =
8.0 Hz, 2H, o-Ph); for 7, δ 1.03 (s, 3H, CH3), 1.23 (s, 3H, CH3), 1.36 (s,
3H, CH3), 6.02 (s, 1H, CH), 8.26 (d, J = 7.98 Hz, 2H, o-Ph), missing
methyl resonance overlaps with solvent, missing aryl resonances overlap
with those of compound 6.
(HCOMe), 128.21 (Ar), 129.01 (Ar), 129.04 (Ar), 129.31 (Ar), 129.44
(Ar), 133.75 (Ar), 135.78 (Ar), 136.95 (Ar), 197.46 (CO). FI-MS: m/z
226.10 [M]+, 121.08 [C6H5CHOCH3 fragment]+, 105.04 [C6H5CO
fragment]+.
Oxidation of 3,4-Dimethoxybenzyl Alcohol by FeCl3(η1-
TEMPO). 3,4-Dimethoxybenzyl alcohol (5 μL, 34.4 μmol) was added
via microsyringe to a stirring Et2O solution (2 mL) of FeCl3(η1-
TEMPO) (23.9 mg, 75.1 μmol). The purple solution immediately
lightened upon addition of the alcohol, concomitant with the deposition
of a reddish tan precipitate. The reaction mixture was allowed to stir for
10 min, whereupon it was filtered through a basic alumina column (1 cm
× 0.5 cm) supported on glass wool. The column was subsequently
rinsed with Et2O (5 mL) to provide a colorless filtrate. The solvent was
removed in vacuo, and the resulting residue was dissolved in C6D6 (700
μL) and transferred to an NMR tube. Mesitylene (5 μL, 35.9 μmol) was
added via microsyringe, and a 1H NMR spectrum was recorded. Analysis
of the chemical shifts and comparison of the product peak integrations
with those of the internal standard revealed complete consumption of
the alcohol and formation of 3,4-dimethoxybenzaldehyde in 75% yield.
NMR spectral data of the product are consistent with those reported in
the literature for 3,4-dimethoxybenzaldehyde.10 1H NMR (400 MHz, 25
°C, C6D6): δ 3.21 (s, 3H, OCH3), 3.25 (s, 3H, OCH3), 6.34 (d, J = 8.1
Hz, 1H, Ph), 7.08 (dd, J = 8.1, 1.9 Hz, 1H, Ph), 7.35 (d, J = 1.9 Hz, 1H,
Ph), 9.75 (s, 1H, CHO).
Oxidation of 2-Phenoxyacetophenone by FeCl3(η1-TEMPO).
An Et2O solution (1.5 mL) of FeCl3(η1-TEMPO) (49.2 mg, 154.5
μmol) was added to a stirring Et2O solution (0.5 mL) of 2-
phenoxyacetophenone (15.1 mg, 71.1 μmol). The reaction mixture
was allowed to stir for 18 h, whereupon a small amount of dark solid
precipitated. The solution remained dark purple. This reaction mixture
was filtered through a basic alumina column (2 cm × 0.5 cm) supported
on glass wool. The column was then rinsed with Et2O (6 mL) to provide
a pale orange filtrate. The solvent was removed in vacuo, and the
resulting pale orange solid was dissolved in CD2Cl2 (700 μL). This
solution was transferred to an NMR tube, mesitylene (5 μL, 35.9 μmol)
Oxidation of 3,4-Dimethoxybenzyl Alcohol by AlCl3(η1-
TEMPO). AlCl3(η1-TEMPO) (26.3 mg, 90.8 μmol) was dissolved in
CD2Cl2 (700 μL) and transferred to an NMR tube. 3,4-Dimethox-
ybenzyl alcohol (5 μL, 34.4 μmol) was then added via microsyringe.
Within 3 min, the clear yellow solution lightened to a very faint yellow.
After 2 h, mesitylene (5 μL, 35.9 μmol) was added via microsyringe, and
a 1H NMR spectrum was recorded. Analysis of the chemical shifts and
comparison of the product peak integrations with those of the internal
standard revealed complete consumption of the alcohol and formation
of 3,4-dimethoxybenzaldehyde in 99% yield. NMR spectral data of the
product are consistent with those reported in the literature for 3,4-
dimethoxybenzaldehyde.10 1H NMR (600 MHz, 25 °C, CD2Cl2): 3,4-
dimethoxybenzaldehyde: δ 3.90 (s, 3H, OCH3), 3.93 (s, 3H, OCH3),
7.01 (d, J = 8.2 Hz, 1H, Ph), 7.39 (s, 1H, Ph), 7.48 (d, J = 7.9 Hz, 1H,
Ph), 9.83 (s, 1H, CHO). AlCl3(η1-TEMPOH): δ 1.43 (s, 6H, CH3), 1.55
(s, 6H, CH3), 1.63−1.85 (m, 4H, CH2), 1.99 (d, J = 14.5 Hz, 2H, CH2),
7.15 (br s, 1H, N-H).
1
was added via microsyringe, and a H NMR spectrum was recorded.
Analysis of the chemical shifts and comparison of the product peak
integrations with those of the internal standard revealed the formation of
7 (3% yield) and the presence of unreacted 2-phenoxyacetophenone
(73% yield). 1H NMR (600 MHz, 25 °C, CD2Cl2): for 6, δ 5.32 (s, 2H,
CH2), 6.95 (d, J = 7.63 Hz, 2H, o-Ph), 7.01 (t, J = 6.86 Hz, 1H, p-Ph),
7.32 (t, J = 7.12 Hz, 2H, m-Ph), 7.54 (t, J = 7.03 Hz, 2H, m-Ph), 7.66 (t, J
= 6.74 Hz, 1H, p-Ph), 8.01 (d, J = 7.2 Hz, 2H, o-Ph); for 7, δ 1.05 (s, 3H,
CH3), 1.25 (s, 3H, CH3), 1.38 (s, 3H, CH3), 6.04 (s, 1H, CH), 8.28 (d, J
= 7.45 Hz, 2H, o-Ph), missing aryl resonances overlap with those of
compound 6, and unassigned CH2 and CH3 resonances of 7 overlap
with CH3 resonances and solvent.
Oxidation of 2-Phenoxyacetophenone by AlCl3(η1-TEMPO).
AlCl3(η1-TEMPO) (22.7 mg, 78.4 μmol) was added to a solution of 2-
Oxidation of 1-Phenyl-2-phenoxyethanol by FeCl3(η1-
TEMPO). An Et2O solution (1.5 mL) of FeCl3(η1-TEMPO) (18.9
G
dx.doi.org/10.1021/ic5018888 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
phenoxyacetophenone (7.0 mg, 33.0 μmol) dissolved in CH2Cl2 (1
mL). The yellow solution was allowed to stand without stirring for 3 h.
The solvent was removed in vacuo, and the resulting solid was extracted
into Et2O (4 mL) and filtered through a basic alumina column (1.5 cm ×
0.5 cm) supported on glass wool. The column was then rinsed with Et2O
(6 mL) to provide a colorless filtrate. The solvent was removed in vacuo,
and the resulting colorless oil was dissolved in CD2Cl2 (700 μL). This
solution was transferred to an NMR tube, mesitylene (5 μL, 35.9 μmol)
product). ESI-MS: m/z 251.11 [8 + Na]+. FI-MS: m/z 228.11 [8]+,
210.07 [10]+.
Attempted Oxidation of 2-Methoxy-1,2-diphenylethanone
by FeCl3(η1-TEMPO). FeCl3(η1-TEMPO) (22.8 mg, 71.6 μmol) was
added to a solution of 2-methoxy-1,2-diphenylethanone (7.1 mg, 31.4
μmol) dissolved in CH2Cl2 (1 mL). This dark purple reaction mixture
was allowed to stand without stirring for 15 h. No color change or
formation of precipitate was observed over this time frame. Et2O (2 mL)
was added to the reaction mixture. This solution was then filtered
through a basic alumina column (2 cm × 0.5 cm) supported on glass
wool and rinsed with Et2O (10 mL) to provide a pale orange filtrate. The
solvent was removed in vacuo, and the resulting oil was dissolved in
CD2Cl2 (700 μL). This solution was transferred to an NMR tube,
1
was added via microsyringe, and H and 13C{1H} NMR spectra were
recorded. Analysis of the chemical shifts and comparison of the product
peak integrations with those of the internal standard revealed the
formation of 7 in 48% yield and the presence of unreacted 2-
1
phenoxyacetophenone in 4% yield. For compound 7, H NMR (400
1
mesitylene (5 μL, 35.9 μmol) was added via microsyringe, and a H
MHz, 25 °C, CD2Cl2): δ 1.02 (s, 3H, CH3), 1.15 (s, 3H, CH3), 1.22 (s,
3H, CH3), 1.32 (s, 2H, γ-CH2 overlapping with CH3 resonance), 1.36 (s,
3H, CH3), 1.43−1.67 (m, 4H, β-CH2 overlapping with solvent), 6.01 (s,
1H, CH), 6.95−7.05 (m, 3H, Ph), 7.25 (t, J = 8.0 Hz, 2H, m-Ph), 7.50 (t,
J = 7.6 Hz, 2H, m-Ph), 7.60 (t, J = 7.4 Hz, 1H, p-Ph), 8.26 (d, J = 7.9 Hz,
2H, o-Ph). 13C{1H} NMR (126 MHz, 25 °C, CD2Cl2): δ 17.62 (γ-CH2),
20.46 (CH3), 21.19 (CH3), 33.41 (CH3), 34.14 (CH3), 40.35 (β-CH2),
40.73 (β-CH2), 60.54 (α-C), 61.71 (α-C), 110.35 (CHO), 117.31 (o-
CH), 122.70 (p-CH), 128.85 (m-CH), 130.02 (m-CH), 130.86 (o-CH),
133.69 (i-C), 134.06 (p-CH), 157.30 (i-C), 192.98 (CO). ESI-MS: m/z
390.21 [M + Na]+, 757.44 [2 M + Na]+.
NMR spectrum was recorded. Analysis of the chemical shifts and
comparison of the product peak integrations with those of the internal
standard revealed a 98% recovery of the starting 2-methoxy-1,2-
diphenylethanone and no formation of oxidation products.
Oxidation of 2-Methoxy-1,2-diphenylethanone by AlCl3(η1-
TEMPO). AlCl3(η1-TEMPO) (32.6 mg, 112.6 μmol) was added to a
solution of 2-methoxy-1,2-diphenylethanone (11.4 mg, 50.4 μmol)
dissolved in CH2Cl2 (1 mL). The dark yellow reaction mixture was
allowed to stand without stirring for 15 h, whereupon the solvent was
removed in vacuo. The resulting solid was extracted into Et2O (3 mL)
and filtered through a basic alumina column (2 cm × 0.5 cm) supported
on glass wool. The column was then rinsed with Et2O (6 mL) to provide
a pale yellow filtrate. The solvent was removed in vacuo, and the
resulting solid was dissolved in CD2Cl2 (700 μL). This solution was
transferred to an NMR tube, mesitylene (5 μL, 35.9 μmol) was added via
Oxidation of 1,2-Diphenyl-2-methoxyethanol by FeCl3(η1-
TEMPO). A CH2Cl2 (0.5 mL) solution of FeCl3(η1-TEMPO) (23.7 mg,
74.4 μmol) was added to a CH2Cl2 (0.5 mL) solution of 1,2-diphenyl-2-
methoxyethanol (6.4 mg, 28.0 μmol). The deep purple solution
immediately lightened to maroon. This solution was allowed to stand
without stirring for 2 h. The reaction mixture was filtered through a basic
alumina column (2 cm × 0.5 cm) supported on glass wool. The column
was then rinsed with Et2O (6 mL) to provide a pale orange filtrate. The
solvent was removed in vacuo, and the resulting light orange oil was
dissolved in CD2Cl2 (700 μL). This solution was transferred to an NMR
tube, mesitylene (5 μL, 35.9 μmol) was added via microsyringe, and a 1H
NMR spectrum was recorded. Analysis of the chemical shifts and
comparison of the product peak integrations with those of the internal
standard revealed complete consumption of the alcohol and formation
of 2-methoxy-1,2-diphenylethanone in 75% yield. The identity of this
product was confirmed by comparison of the NMR spectral data to that
of authentic material.10 1H NMR (600 MHz, 25 °C, CD2Cl2): δ 3.43 (s,
3H, CH3), 5.52 (s, 1H, CH), 7.31 (t, J = 7.3 Hz, 1H, Ph), 7.36 (t, J = 7.4
Hz, 2H, Ph), 7.39−7.45 (m, 4H, Ph), 7.53 (t, J = 7.4 Hz, 1H, Ph), 7.97
(d, J = 7.4 Hz, 2H, Ph). FI-MS: m/z 226.10 [M]+, 121.08
[C6H5CHOCH3 fragment]+, 105.04 [C6H5CO fragment]+.
1
microsyringe, and a H NMR spectrum was recorded. Analysis of the
chemical shifts and comparison of the product peak integrations with
those of the internal standard revealed the presence of 2-methoxy-1,2-
diphenylethanone (9) in 11% yield, and the formation of benzil (10)
and 2,2-dimethoxy-2-phenylacetophenone (11) in 53% and 16% yields,
respectively. The identities of the products were confirmed by
comparison of the NMR and mass spectral data to those of authentic
sample.66,67 1H NMR (600 MHz, 25 °C, CD2Cl2): δ 3.20 (s, 3H, OCH3,
11), 3.44 (s, 3H, OCH3, 9), 5.52 (s, 1H, CH, 9), 7.27−7.48 (m,
overlapping aryl CH of 9 and 11), 7.54 (t, J = 7.7 Hz, Ar of 10,
overlapping aryl CH of 9), 7.59 (d, J = 7.7 Hz, 2H, o-Ph, 11), 7.69 (t, J =
7.5 Hz, 2H, p-Ph, 10), 7.97 (d, J = 7.8 Hz, o-Ph of 10, overlapping aryl
CH of 9), 8.04 (d, J = 7.9 Hz, 2H, o-Ph, 11). ESI-MS: m/z 249.09 [9 +
Na]+, 279.11 [11 + Na]+. FI-MS: m/z 226.10 [9]+, 210.07 [10]+.
Oxidation of 1,4-Cyclohexadiene by FeCl3(η1-TEMPO).
FeCl3(η1-TEMPO) (36.4 mg, 114.3 μmol) was dissolved in toluene-
d8 (700 μL), and 1,4-cyclohexadiene (5 μL, 52.9 μmol) was added via
microsyringe. The dark purple solution immediately lightened. The
reaction was allowed to stand for 2 h without stirring. The solution was
then filtered through a basic alumina column (1 cm × 0.5 cm) supported
on glass wool into an NMR tube. The column was rinsed with toluene-d8
(0.5 mL), mesitylene (5 μL, 35.9 μmol) was added to the NMR tube via
Oxidation of 1,2-Diphenyl-2-methoxyethanol by AlCl3(η1-
TEMPO). AlCl3(η1-TEMPO) (58.6 mg, 202.4 μmol) was added to a
CH2Cl2 (1 mL) solution of 1,2-diphenyl-2-methoxyethanol (22.0 mg,
96.4 μmol). This orange reaction mixture was allowed to stand without
stirring for 3 h. The solvent was then removed in vacuo, and the resulting
solid was extracted into Et2O (3 mL) and filtered through a basic
alumina column (2 cm × 0.5 cm) supported on glass wool. The column
was rinsed with Et2O (6 mL) to provide a pale yellow filtrate. The
solvent was removed in vacuo, and the resulting light yellow oil was
dissolved in CD2Cl2 (700 μL). This solution was transferred to an NMR
tube, mesitylene (5 μL, 35.9 μmol) was added via microsyringe, and a 1H
NMR spectrum was recorded. Analysis of the chemical shifts and
comparison of the product peak integrations with those of the internal
standard revealed the presence of unreacted 1,2-diphenyl-2-methox-
yethanol (8) in 23% yield, the formation of benzil (10) in 54% yield
(based on 2), and the formation of a minor unidentified product, as
evidenced by a methoxy resonance at 3.51 ppm. The identity of benzil
was confirmed by comparison of the NMR spectral data to that of
authentic material.66 1H NMR (600 MHz, 25 °C, CD2Cl2): δ 3.21 (s,
3H, OCH3, 8), 3.51 (s, 3H, OCH3, unknown minor product), 4.35 (s,
1H, CH, 8), 4.88 (s, 1H, CH, 8), 7.14−7.32 (m, Ar, 8, overlapping with
unknown minor product), 7.55 (t, J = 7.83 Hz, 4H, m-Ph, 10,
overlapping with unknown minor product), 7.70 (t, J = 7.1 Hz, 2H, p-Ph,
10), 7.97 (d, J = 8.22 Hz, 4H, o-Ph, 10, overlapping with unknown minor
1
microsyringe, and a H NMR spectrum was recorded. Analysis of the
chemical shifts and comparison of the product peak integrations with
those of the internal standard revealed the presence of unreacted 1,4-
cyclohexadiene in 1% yield, and formation of benzene in 82% yield. 1H
NMR (600 MHz, 25 °C, toluene-d8): benzene: δ 7.14 (s, 6H, CH); 1,4-
cyclohexadiene: δ 2.52 (s, 4H, CH2), 5.59 (s, 4H, CH).
Oxidation of 1,4-Cyclohexadiene by AlCl3(η1-TEMPO).
AlCl3(η1-TEMPO) (35.8 mg, 123.6 μmol) was dissolved in CD2Cl2
(700 μL) and transferred to an NMR tube. An initial 1H NMR spectrum
was recorded. 1,4-Cyclohexadiene (5 μL, 52.9 μmol) was then added via
microsyringe, whereupon the yellow solution immediately lightened.
After 15 min, mesitylene (5 μL, 35.9 μmol) was added via microsyringe,
and a 1H NMR spectrum was recorded. Analysis of the chemical shifts
and comparison of the product peak integrations with those of the
internal standard revealed the complete consumption of 1,4-cyclo-
1
hexadiene and the formation of benzene in 96% yield. H NMR (400
MHz, 25 °C, CD2Cl2): benzene, δ 7.36 (s, 6H, CH); AlCl3(η1-
TEMPOH), δ 1.43 (s, 6H, CH3), 1.55 (s, 6H, CH3), 1.63−1.85 (m, 4H,
CH2), 1.99 (d, J = 14.29 Hz, 2H, CH2), 7.14 (br s, 1H, N-H).
H
dx.doi.org/10.1021/ic5018888 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Oxidation of Xanthene by FeCl3(η1-TEMPO). FeCl3(η1-
TEMPO) (109.6 mg, 344.2 μmol) was added to a CH2Cl2 (3 mL)
solution of xanthene (28.5 mg, 156.4 μmol). After 48 h, the orange
reaction mixture was filtered through a basic alumina column (2.5 cm ×
0.5 cm) supported on glass wool, and the column was rinsed with Et2O
(6 mL) to provide a nearly colorless filtrate. The solvent was removed in
vacuo, and the resulting white solid was dissolved in CD2Cl2 (700 μL).
This solution was transferred to an NMR tube, mesitylene (5 μL, 35.9
via microsyringe resulted in an immediate color change to orange, which
then lightened to a very pale yellow within 30 min. Mesitylene (5 μL,
35.9 μmol) was added via microsyringe, and a 1H NMR spectrum was
recorded. Analysis of the chemical shifts and comparison of the product
peak integrations with those of the internal standard revealed complete
consumption of cyclobutanol and formation of cyclobutanone in 100%
yield. The NMR spectral data of the product were consistent with that
previously reported for cyclobutanone.68 1H NMR (500 MHz, 25 °C,
CD2Cl2): cyclobutanone, δ 1.98 (quintet, J = 8.2 Hz, 2H, CH2
overlapping with AlCl3(η1-TEMPOH)), 3.05 (t, J = 8.2 Hz, 4H,
CH2); AlCl3(η1-TEMPOH), δ 1.42 (s, 6H, CH3), 1.54 (s, 6H, CH3),
1.63−1.86 (m, 4H, CH2), 1.95−2.02 (m, 2H, CH2 overlapping with
cyclobutanone), 7.14 (br s, 1H, N-H).
1
μmol) was added via microsyringe, and a H NMR spectrum was
recorded. Analysis of the chemical shifts and comparison of the product
peak integrations with those of the internal standard revealed the
absence of xanthene, and formation of bixanthenyl (15) and xanthone
(16) in 18% yield and 33% yield, respectively. The NMR spectral data of
the products are consistent with those previously reported for
bixanthenyl and xanthone.33,34 1H NMR (500 MHz, 25 °C, CD2Cl2):
for 16, δ 7.40 (m, 2H, Ar), 7.50−7.54 (m, 2H, Ar), 7.75 (m, 2H, Ar),
8.30 (dd, J = 7.9, 1.8 Hz, 2H, Ar); for 15, δ 4.26 (s, 2H, CH), 6.70 (dd, J =
7.6, 1.6 Hz, 4H, Ar), 6.83 (dd, J = 8.1, 1.2 Hz, 4H, Ar), 6.95 (td, J = 7.4,
1.2 Hz, 4H, Ar), 7.21 (m, 4H, Ar). 13C{1H} NMR (126 MHz, 25 °C,
CD2Cl2): for 16, δ 118.56 (CH), 122.36 (C(CO)C), 124.46 (CH),
126.99 (CH), 135.39 (CH), 156.77 (COC), 177.36 (CO); for 15, δ
49.99 (CH), 116.23 (Ar CH), 122.43 (C), 123.23 (Ar CH), 128.63 (Ar
CH), 129.74 (Ar CH), 153.56 (COC).
Oxidation of Cyclopropylcarbinol by FeCl3(η1-TEMPO) (260
mM Concentration). FeCl3(η1-TEMPO) (41.6 mg, 130.6 μmol) was
dissolved in C6D6 (500 μL) to provide a purple solution. This
corresponded to a FeCl3(η1-TEMPO) concentration of 260 mM.
Addition of cyclopropylcarbinol (5 μL, 61.7 μmol) via microsyringe
resulted in the immediate lightening of the solution, concomitant with
the deposition of a maroon precipitate. The reaction mixture was
allowed to stand without stirring for 30 min, whereupon it was filtered
into an NMR tube through a basic alumina column (1 cm × 0.5 cm)
supported on glass wool. The column was rinsed with C6D6 (0.5 mL),
mesitylene (5 μL, 35.9 μmol) was added to the NMR tube via
Oxidation of Xanthene by AlCl3(η1-TEMPO). AlCl3(η1-TEMPO)
(77.3 mg, 266.9 μmol) was added to a CH2Cl2 (700 μL) solution of
xanthene (24.1 mg, 132.3 μmol) and transferred to a J. Young NMR
tube. Mesitylene (5 μL, 35.9 μmol) was added via microsyringe, and the
1
microsyringe, and a H NMR spectrum was recorded. Analysis of the
chemical shifts and comparison of the product peak integrations with
those of the internal standard revealed the complete consumption of
cyclopropylcarbinol and formation of cyclopropanecarboxaldehyde in
84% yield. The NMR spectral data of the product were consistent with
that previously reported for cyclopropanecarboxaldehyde (Supporting
Information Figure S27).69 1H NMR (400 MHz, 25 °C, C6D6): δ 0.28
(m, 2H, CH2), 0.47 (m, 2H, CH2), 1.27 (m, 1H, CH), 8.53 (d, J = 5.5
Hz, 1H, CHO).
1
yellow solution was monitored by H NMR spectroscopy for 72 h,
during which time it gradually darkened to orange. The solvent was
removed in vacuo, and the resulting solid was extracted into toluene (2
mL) and filtered through a basic alumina column (1.5 cm × 0.5 cm)
supported on glass wool. The column was then rinsed with toluene (6
mL) to provide a colorless filtrate. The solvent was removed in vacuo,
and the white solid was dissolved in CD2Cl2 (700 μL). This solution was
transferred to an NMR tube, mesitylene (5 μL, 35.9 μmol) was added via
Oxidation of Cyclopropylcarbinol by FeCl3(η1-TEMPO) (6.3
mM Concentration). FeCl3(η1-TEMPO) (1.0 mg, 3.14 μmol) was
dissolved in C6D6 (500 μL) to provide a purple solution. This
corresponded to a FeCl3(η1-TEMPO) concentration of 6.3 mM.
Addition of cyclopropylcarbinol (1 μL of a 1.23 M stock solution, 1.23
μmol) via microsyringe resulted in the immediate lightening of the
solution. The reaction mixture was allowed to stand without stirring for
30 min, whereupon it was filtered into an NMR tube through a basic
alumina column (1 cm × 0.5 cm) supported on glass wool. The column
was rinsed with C6D6 (0.5 mL), mesitylene (1 μL of a 0.72 M stock
solution, 0.72 μmol) was added to the NMR tube via microsyringe, and a
1H NMR spectrum was recorded. Analysis of the chemical shifts and
1
microsyringe, and a H NMR spectrum was recorded. Analysis of the
chemical shifts and comparison of the product peak integrations with
those of the internal standard revealed the presence of unreacted
xanthene (14) in 1% yield, and formation of bixanthenyl (15) and
xanthone (16) in 51% and 4% yields, respectively. The NMR spectral
data of the products are consistent with those previously reported for
bixanthenyl and xanthone.33,34 1H NMR (600 MHz, 25 °C, CD2Cl2):
for 15, δ 4.26 (s, 2H, CH), 6.71 (dd, J = 7.4, 1.6 Hz, 4H, Ar), 6.83 (d, J =
8.1 Hz, 4H, Ar), 6.95 (t, J = 7.4 Hz, 4H, Ar), 7.20−7.23 (m, 4H, Ar
overlapping with Ar from xanthene starting material); for 16, δ 7.40 (t, J
= 7.5 Hz, 2H, Ar), 7.53 (d, J = 8.5 Hz, 2H, Ar), 7.76 (m, 2H, Ar), 8.30
(dd, J = 7.9, 1.7 Hz, 2H, Ar).
comparison of the product peak integrations with those of the internal
standard revealed the complete consumption of cyclopropylcarbinol
and formation of cyclopropanecarboxaldehyde in 98% yield. The NMR
spectral data of the product were consistent with that previously
reported for cyclopropanecarboxaldehyde.69 No evidence for the
Oxidation of Cyclobutanol by FeCl3(η1-TEMPO) (385 mM
Concentration). FeCl3(η1-TEMPO) (85.9 mg, 269.7 μmol) was
dissolved in C6D6 (700 μL) to provide a purple solution. This
corresponded to a FeCl3(η1-TEMPO) concentration of 385 mM.
Addition of cyclobutanol (10 μL, 127.7 μmol) via microsyringe resulted
in the immediate lightening of the solution, concomitant with the
deposition of a maroon precipitate. The reaction mixture was allowed to
stand without stirring for 10 min, whereupon it was filtered into an NMR
tube through a basic alumina column (1 cm × 0.5 cm) supported on
glass wool. The column was rinsed with C6D6 (0.5 mL), mesitylene (5
μL, 35.9 μmol) was added to the NMR tube via microsyringe, and a 1H
NMR spectrum was recorded. Analysis of the chemical shifts and
comparison of the product peak integrations with those of the internal
standard revealed the complete consumption of cyclobutanol and
formation of cyclobutanone in 75% yield. The NMR spectral data of the
product were consistent with that previously reported for cyclo-
butanone.68 1H NMR (600 MHz, 25 °C, C6D6): δ 1.20 (quintet, J = 8.2
Hz, 2H, CH2), 2.47 (t, J = 8.2 Hz, 4H, CH2).
1
formation of any ring-opened product was observed in the H NMR
spectrum of the reaction mixture. 1H NMR (400 MHz, 25 °C, C6D6) δ
0.26 (m, 2H, CH2), 0.45 (m, 2H, CH2), 1.25 (m, 1H, CH), 8.52 (d, J =
5.5 Hz, 1H, CHO).
Oxidation of Cyclopropylcarbinol by AlCl3(η1-TEMPO) (190
mM Concentration). AlCl3(η1-TEMPO) (38.7 mg, 133.6 μmol) was
dissolved in CD2Cl2 (700 μL) and transferred to an NMR tube to
provide a yellow solution. This corresponded to a AlCl3(η1-TEMPO)
concentration of 190 mM. Addition of cyclopropylcarbinol (5 μL, 61.7
μmol) via microsyringe resulted in an immediate color change to orange,
which slowly lightened to a pale yellow over the course of 45 min.
1
Mesitylene (5 μL, 35.9 μmol) was added via microsyringe, and a H
NMR spectrum was recorded. Analysis of the chemical shifts and
comparison of the product peak integrations with those of the internal
standard revealed complete consumption of cyclopropylcarbinol and
formation of cyclopropanecarboxaldehyde in 91% yield. The NMR
spectral data of the product were consistent with that previously
reported for cyclopropanecarboxaldehyde (Supporting Information
Figure S28).69 1H NMR (400 MHz, 25 °C, CD2Cl2): cyclo-
propanecarboxaldehyde, δ 1.04−1.11 (m, 4H, CH2), 8.84 (d, J = 6.0
Oxidation of Cyclobutanol by AlCl3(η1-TEMPO) (200 mM
Concentration). AlCl3(η1-TEMPO) (40.6 mg, 140.2 μmol) was
dissolved in CD2Cl2 (700 μL) and transferred to an NMR tube to
provide a yellow solution. This corresponded to a AlCl3(η1-TEMPO)
concentration of 200 mM. Addition of cyclobutanol (5 μL, 63.9 μmol)
I
dx.doi.org/10.1021/ic5018888 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Hz, 1H, CHO), unassigned CH resonance overlaps with CH2 resonance
from AlCl3(η1-TEMPOH); AlCl3(η1-TEMPOH), δ 1.42 (s, 6H, CH3),
1.54 (s, 6H, CH3), 1.61−1.87 (m, 4H, CH2 overlapping with
cyclopropanecarboxaldehyde), 1.94−2.04 (m, 2H, CH2), 7.15 (br s,
1H, NH).
974 (m), 945 (m), 870 (s), 744 (m), 712 (w), 635 (s), 565 (w), 499 (m),
427 (m), 412 (w).
Reaction of FeBr3 with TEMPOH in Et2O. An Et2O solution (1
mL) of TEMPOH (57.8 mg, 0.368 mmol) was added to a solution of
FeBr3 (102.0 mg, 0.345 mmol) in Et2O (3 mL). An orange precipitate
immediately formed. Storage of this mixture at −25 °C for 12 h resulted
in the deposition of more orange powder. The solid was then collected
by decanting off the supernatant (101.5 mg). Analysis of this material by
1H NMR spectroscopy revealed the presence of complexes 23 and 24, in
an approximate 1:1 ratio. 1H NMR (400 MHz, 25 °C, CD2Cl2): for 23, δ
−1.37 (s, 2H, β-H), 7.89 (s, 1H, γ-H), 10.69 (s, 1H, γ-H), 12.31 (br s,
6H, CH3), 15.75 (br s, 6H, CH3), 58.60 (s, 2H, β-H), a resonance
assignable to NH was not observed, likely due to paramagnetic
broadening; for 24, δ 1.70 (sh, 4H, CH2), 1.93 (s, 12H, CH3), 2.46 (s,
4H, CH2), 3.69 (s, 12H, CH3), 6.48 (s, 4H, CH2), 47.09 (s, 2H, NH).
X-ray Crystallography. Data for 23 and 24 were collected on a
Bruker KAPPA APEX II diffractometer equipped with an APEX II CCD
detector using a TRIUMPH monochromator with a Mo Kα X-ray
source (α = 0.710 73 Å). Crystals were mounted on a cryoloop under
Paratone-N oil, and all data were collected at 100(2) K using an Oxford
nitrogen gas cryostream system. X-ray data for both 23 and 24 were
collected utilizing frame exposures of 10 (low angle) and 15 s (high
angle). Data collection and cell parameter determination were
conducted using the SMART program.70 Integration of the data frames
and final cell parameter refinement were performed using SAINT
software.71 Absorption correction of the data was carried out using the
multiscan method SADABS.72 Subsequent calculations were carried out
using SHELXTL.73 Structure determination was done using direct
methods and difference Fourier techniques. All hydrogen atom
positions were idealized, and rode on the atom of attachment with the
exception of the NH hydrogen atom. Structure solution, refinement,
graphics, and creation of publication materials were performed using
SHELXTL.73 Further crystallographic details can be found in
Supporting Information Table S1.
Oxidation of Cyclopropylcarbinol by AlCl3(η1-TEMPO) (4.4
mM Concentration). AlCl3(η1-TEMPO) (0.9 mg, 3.11 μmol) was
dissolved in CD2Cl2 (700 μL) and transferred to an NMR tube to
provide a pale yellow solution. This corresponded to a AlCl3(η1-
TEMPO) concentration of 4.4 mM. Addition of cyclopropylcarbinol (1
μL of a 1.23 M stock solution, 1.23 μmol) via microsyringe resulted in a
color change to colorless over the course of 45 min. Mesitylene (1 μL of
a 0.72 M stock solution, 0.72 μmol) was then added via microsyringe,
and a 1H NMR spectrum was recorded. Analysis of the chemical shifts
and comparison of the product peak integrations with those of the
internal standard revealed complete consumption of cyclopropylcarbi-
nol and formation of cyclopropanecarboxaldehyde in 95% yield. The
NMR spectral data of the product were consistent with that previously
reported for cyclopropanecarboxaldehyde.69 No evidence for the
1
formation of any ring-opened product was observed in the H NMR
spectrum of the reaction mixture. 1H NMR (400 MHz, 25 °C, CD2Cl2):
cyclopropanecarboxaldehyde, δ 1.05−1.12 (m, 4H, CH2), 8.85 (d, J =
6.0 Hz, 1H, CHO), unassigned CH resonance overlaps with CH2
resonance from AlCl3(η1-TEMPOH); AlCl3(η1-TEMPOH), δ 1.43 (s,
6H, CH3), 1.55 (s, 6H, CH3), 1.62−1.88 (m, 4H, CH2 overlapping with
cyclopropanecarboxaldehyde), 1.95−2.05 (m, 2H, CH2), 7.14 (br s, 1H,
NH).
Reaction of FeBr3 with TEMPO in Et2O. An Et2O solution (1 mL)
of TEMPO (105.2 mg, 0.673 mmol) was added to a solution of FeBr3
(204.1 mg, 0.691 mmol) in Et2O (1 mL). An orange precipitate
immediately formed. The reaction mixture was allowed to stand at room
temperature for 5 h, whereupon the supernatant was filtered through a
basic alumina column (1 cm × 0.5 cm) supported on glass wool to
remove the metal containing products. The column was rinsed with
Et2O (1 mL). Analysis of the filtrate by GC/MS revealed the presence of
1,1-diethoxyethane and TEMPO (Supporting Information Figure S41).
The retention times of 1,1-diethoxyethane and TEMPO were consistent
with that of authentic material, recorded using the same conditions. The
orange precipitate was then dissolved in CH2Cl2 (3 mL), and the
resulting orange solution was filtered through a Celite plug supported on
glass wool (1 cm × 0.5 cm). The dark orange filtrate was then layered
with hexanes (5 mL), and subsequent storage at −25 °C for 12 h
resulted in the deposition of an orange powder (236.0 mg). Analysis of
this powder by 1H NMR spectroscopy revealed the presence of
complexes 23 and 24 in an approximate 3:2 ratio. 1H NMR (400 MHz,
25 °C, CD2Cl2): for 23, δ −1.14 (s, 2H, β-H), 8.18 (s, 1H, γ-H), 10.89
(s, 1H, γ-H), 12.47 (br s, 6H, CH3), 16.02 (br s, 6H, CH3), 59.20 (s, 2H,
β-H), a resonance assignable to NH was not observed, likely due to
paramagnetic broadening; for 24, δ 1.72 (sh, 4H, CH2), 2.02 (s, 12H,
CH3), 2.50 (s, 4H, CH2), 3.73 (s, 12H, CH3), 6.58 (s, 4H, CH2), 47.18
(s, 2H, NH). GC/MS: m/z 1,1-diethoxyethane, 117 [M − H]+, 103
[C5H11O2 fragment]+, 73 [C4H9O fragment]+, 45 [C2H5O fragment]+;
TEMPO, 156 [M]+.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic details (as a CIF file), spectral data, and
additional figures and tables. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: hayton@chem.ucsb.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Center for Sustainable Use of
Renewable Feedstocks (CenSURF), a National Science
Foundation (NSF) Center for Chemical Innovation (CCI).
J.S.P. thanks the UCSB MRL RISE program for a summer
internship. We also thank Prof. Eric Schelter for sharing
unpublished experimental data with us.
Synthesis of [FeBr2(η1-TEMPOH)]2(μ-O) (24). An Et2O solution
(1 mL) of TEMPO (131 mg, 0.838 mmol) and 1,4-cyclohexadiene (80
μL, 0.846 mmol) was layered on to an Et2O solution (2 mL) of FeBr3
(229 mg, 0.774 mmol) that was previously filtered through a Celite plug
supported on glass wool (1 cm × 0.5 cm). An orange precipitate
immediately formed. The reaction mixture was allowed to stand at room
temperature for 18 h, whereupon the resulting orange precipitate was
isolated by decanting off the supernatant. This solid was recrystallized
twice from CH2Cl2 solutions layered with Et2O, which were stored at
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dx.doi.org/10.1021/ic5018888 | Inorg. Chem. XXXX, XXX, XXX−XXX