An excellent dual recycling strategy for the hypervalent iodine/nitroxyl radical mediated selective oxidation of alcohols to aldehydes and ketones

Article abstract of DOI:10.1039/c2gc16632a

Using a recyclable hypervalent iodine reagent 1, the authors have constructed versatile and green methods for the hypervalent iodine and nitroxyl radical-mediated selective oxidation of alcohols to aldehydes and ketones. The recyclable reagent 1 having a unique tetraphenyladamantane structure exhibited almost the same reactivity as the ordinary reagent, phenyliodine diacetate (PIDA), in the hypervalent iodine(iii)/2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-mediated oxidation. For recycling, the reagent 1 could be nearly quantitatively recovered as the reduced tetraiodide 2 by filtration after reaction completion, utilizing the insolubility of the formed 2 in a polar solvent, specifically, methanol. Based on the confirmed reactivity and excellent recycling operation of the reagent 1, the methodology has been further extended to a greener dual recycling strategy. By combining the recyclable iodine reagent 1 and silica-supported TEPMO catalyst, a variety of alcohols were effectively oxidized to the desired aldehydes, and the two types of used reagent and catalyst, the iodine 1 and immobilized TEMPO 5, could be separately recovered by an easy workup, and both repeatedly used without any loss of their original activities for at least four cycles.

Full text of DOI:10.1039/c2gc16632a

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PAPER
An excellent dual recycling strategy for the hypervalent iodine/nitroxyl
radical mediated selective oxidation of alcohols to aldehydes and ketones†
Toshifumi Dohi,a Kei-ichiro Fukushima,a Tohru Kamitanaka,a Koji Morimoto,a Naoko Takenagab and
Yasuyuki Kita*a
Received 20th December 2011, Accepted 6th March 2012
DOI: 10.1039/c2gc16632a
Using a recyclable hypervalent iodine reagent 1, the authors have constructed versatile and green methods
for the hypervalent iodine and nitroxyl radical-mediated selective oxidation of alcohols to aldehydes and
ketones. The recyclable reagent 1 having a unique tetraphenyladamantane structure exhibited almost the
same reactivity as the ordinary reagent, phenyliodine diacetate (PIDA), in the hypervalent iodine(III)/
2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-mediated oxidation. For recycling, the reagent 1 could be
nearly quantitatively recovered as the reduced tetraiodide 2 by filtration after reaction completion, utilizing
the insolubility of the formed 2 in a polar solvent, specifically, methanol. Based on the confirmed
reactivity and excellent recycling operation of the reagent 1, the methodology has been further extended
to a greener dual recycling strategy. By combining the recyclable iodine reagent 1 and silica-supported
TEPMO catalyst, a variety of alcohols were effectively oxidized to the desired aldehydes, and the two
types of used reagent and catalyst, the iodine 1 and immobilized TEMPO 5, could be separately recovered
by an easy workup, and both repeatedly used without any loss of their original activities for at least four
cycles.
The hypervalent iodine atoms can oxidize substrates, with the
accompanying formation of more stable monovalent iodine com-
Introduction
The hypervalent iodine reagent is now widely used as a safe and
user-friendly tool in synthetic chemistry for performing environ-
mentally benign oxidations which meet the standards of green
chemistry. The inherent low toxicity and ready availability of the
hypervalent iodine reagent, combined with its attractive reactiv-
ity—similar to those of toxic heavy metal-based oxidants, such
as lead, mercury, and thallium elements—have gained it con-
siderable interest among modern synthetic chemists. The initial
role of the hypervalent iodine reagent in 20th century organic
synthesis seemed to be a safer replacement of the toxic heavy
metal oxidants, but this situation rapidly changed with the recent
outstanding advances of hypervalent iodine chemistry in devel-
oping metal catalyst-free reactions. Consequently, a variety of
useful transformations utilizing the mild and selective oxidizing
properties of hypervalent iodine reagents have been frequently
reported for constructing complex organic molecules, which can
now partially replace classical methods that require highly toxic
heavy metals as well as other metal catalysts.1
pounds as the driving force. Despite the environmentally benign
characteristics of the hypervalent iodine reagent as a green
oxidant, a general problem in its use in chemical reactions
occurs from the co-production of stoichiometric amounts of
iodoarenes. Therefore, several types of recyclable alternatives to
hypervalent iodine reagents have already appeared to facilitate
the separation of the iodine co-product from the reaction mixture
as well as the reuse of the reagents (Fig. 1).2 Attaching the
reagent to an insoluble polymer support, such as polystyrene, is
a promising strategy for its recycling, and accordingly, the
polymer-supported hypervalent iodine reagents of type I, poly
(diacetoxyiodo)styrene (PDAIS) and poly[bis(trifluoroacetoxy-
iodo)]styrene (PBTIS), incorporating phenyliodine diacetate
(PIDA) and phenyliodine bis(trifluoroacetate) (PIFA) in their
polymer backbones were first independently introduced by Togo
et al.3 and Ley et al.4 for synthesis. With recent progress in
aCollege of Pharmaceutical Sciences, Ritsumeikan University, 1-1-1
Nojihigashi, Kusatsu, Shiga, 525-8577, Japan. E-mail: kita@ph.ritsu-
mei.ac.jp; Tel: +81 77 561 5829
bSagami Chemical Research Institute, 2743-1 Hayakawa, Ayase,
Kanagawa 252-1193, Japan
†Electronic supplementary information (ESI) available: General exper-
imental procedures and compound characterization data including 1H
and 13C NMR. CCDC 232058. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2gc16632a
Fig. 1 Representative types of recyclable iodine(III) reagents.
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Fig. 2 Adamantane-based recyclable hypervalent iodine reagent.
fluorous chemistry and use of ionic liquid mediums, Gladysz,
Qian, and Handy reported the preparation and use of hypervalent
iodine compounds bearing type II fluorous tags and ionic sup-
ports, such as type III imidazolium salt, as recyclable.5,6 Other
various useful recyclable reagents have also been systematically
reported by Zhdankin and other research groups.7,8
Scheme 1 Hypervalent iodine/TEMPO mediated greener oxidation of
alcohols to aldehydes and ketones.
synthetic methods using less-toxic oxidants. A survey of the
metal-free conditions for the selective oxidation of alcohols has
been significantly studied in recent decades, as it appears to
surely reduce the undesirable environmental and economical
impact caused by the use of a metal oxidant and catalyst.13 One
of the attractive routes to the metal-free selective oxidation of
alcohols to aldehydes and ketones includes a method utilizing
stable nitroxyl radicals, e.g., 2,2,6,6-tetramethyl-piperidine 1-
oxyl (TEMPO), as a mediator.14a In the presence of a stoichio-
metric oxidant, the strategy requires only catalytic amounts (typi-
cally, <10 mol%) of TEMPO as an organocatalyst.14
Usually, these recyclable hypervalent iodine reagents were
solely employed during the reactions, and the combined use with
other types of recyclable reagents was rarely considered. This is
mainly due to the reasons based on the following two issues:9
(1) the significant change in reactivity, and (2) difficult separ-
ation from each other in the multiple recycling systems. When
combining one reagent with another recyclable component, the
reactivity change was sometimes too critical to smoothly repro-
duce the reactions of conventional hypervalent iodine reagents.
Furthermore, contamination of the two used reagents frequently
occurred during their recovery. As a result, either the hypervalent
iodine reagent or the other recyclable component cannot be
effectively reused. A recyclable hypervalent iodine reagent satis-
fying the reactivity requirements and recovery has never been
suggested for use in a combined reagent system.
We have previously developed a structurally new recyclable
hypervalent iodine reagent 1, namely, 1,3,5,7-tetrakis[4-(diace-
toxyiodo)phenyl]adamantane (Fig. 2), which has a very similar
reactivity to that of the conventional PIDA and an easily accom-
plishable recovery by precipitation, utilizing the insolubility of
the tetraiodide 2, stoichiometrically produced from 1 after the
reactions, in common polar solvents.10–12 The adamantane
reagent 1 has high thermal stability (mp (decomp.) 195–196 °C)
like PIDA, and is not sensitive to air and moisture. As a continu-
ation of our research studies, a dual recycling strategy which
enables effective recovery and reuse of the two types of the
reagent and catalyst is now reported based on the advantageous
use of the adamantane reagent 1, for performing the hypervalent
iodine and nitroxy radical-mediated green and selective oxi-
dation of alcohols to aldehydes and ketones in high yields
(Scheme 1).
For the transformation of natural products and other complex
molecules, the combination of hypervalent iodine(III) reagents,
that is, PIDA, and TEMPO15a (or its derivatives)15b,c is specifi-
cally known to promote the efficient and chemoselective oxi-
dation of alcohols for access to highly functionalized carbonyl
compounds. The oxidative process is mild enough to even be
compatible with easily racemized compounds and substrates
having many oxidation-sensitive functional groups, such as
ethers, sulfides, selenides, allyl, furyl, epoxides, and alkenes,
which makes it the promising choice among the other known
methods. In addition, this protocol shows a perfect selectivity for
the oxidation of primary alcohols to aldehydes, without any
over-oxidation to carboxylic acids, in preference to primary alco-
hols over secondary ones. Upon reaction completion, the only
quantitative waste was iodobenzene, a PIDA-derived co-product.
To facilitate isolation of the carbonyl product as well as removal
and recovery of the stoichiometric organoiodide, several kinds of
recyclable hypervalent iodine reagents have already been envi-
sioned for use.16 The polymer-supported iodine reagent was first
selected as the recyclable alternative to PIDA in the TEMPO-
mediated oxidation of alcohols.16a However, as claimed by
Zhdankin et al.7a and other groups,5,6 including our own,10 the
polymer reagents are usually much less reactive than PIDA due
to steric hindrance of the iodine(III) sites in the polymer struc-
tures and the low solubility in various solvents. Moreover, degra-
dation of the resin frequently occurred when it was repeatedly
used, which made the maintenance of the original reactivity of
the polymer reagents difficult, thus resulting in the limited avail-
ability for their recycling and the somewhat unsatisfactory
overall recycling yield. Hence, the efforts to develop more
Results and discussion
Selective conversions of alcohols to the corresponding partially-
oxidized aldehydes and ketones are important and fundamental
chemical processes in accordance with the broad utilization of
carbonyl compounds in organic synthesis. The recent direction
toward environmentally friendly chemical processes has strongly
motivated chemists to develop milder, safer, and cleaner
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suitable types of recyclable hypervalent iodine reagents have
recently appeared.16b,c
equivalent of PIDA was supported even for this type of alcohol
oxidation by this reactivity check and comparison of the
reagents. Nonetheless, the reaction hardly occurred in the
absence of the TEMPO catalyst for both the adamantane reagent
1 and PIDA.18
Based on these criteria, we also aimed to further establish a
greener hypervalent iodine and TEMPO-mediated alcohol oxi-
dation strategy with an improved overall recycle yield based
on the sustainable use of our recyclable hypervalent iodine
reagent 1.
On the other hand, the recovery of 1 could be conducted by
utilizing the insolubility of the co-produced tetraiodide 2 in a
polar solvent, specifically, methanol (see Scheme 3).19 The oper-
ation usually started with the removal of the used solvent. Thus,
the flask containing the homogeneous reaction mixture was
attached to a rotary evaporator, and the solvent was removed. To
the resulting oily mixture, methanol was added to extract the
aldehyde 4a, while the highly symmetrical and hardly soluble
tetraiodide 2 simultaneously precipitated as a powder. The
heterogeneous solution was then filtered, and the solid on the
filter was washed several times with small amounts of methanol
to nearly quantitatively recover 2. From the filtrate, a crude 4a
with small amount of impurities and TEMPO was obtained,
which was further purified by short column chromatography on
silica gel. The pure 4a was thus obtained, and the co-produced 2
could be easily recovered from the reaction mixture by a simple
solid/liquid separation technique (i.e., filtration). The latter was
then recycled for a second run as the initial adamantane reagent
1 after the treatment of the recovered 2 with m-chloroperbenzoic
acid in acetic acid (see Experimental section).10 Consequently,
the present method should be a facile, simple, clean, and con-
venient method for the recycling of the hypervalent iodine
reagent 1 as well as for the isolation of the aldehyde 4a.
With this generalized procedure, a number of alcohols includ-
ing the representative substrates reported in the conventional
PIDA/TEMPO-mediated oxidation14,15 were evaluated using the
present recycle method with 1. The same experimental procedure
for all the substrates in Table 1 led to the successful production
of the expected partially oxidized aldehydes 4a–m from the cor-
responding primary alcohols 3a–m or the ketones 4n–r from the
secondary alcohols 3n–r at room temperature. Less than trace
amounts of carboxylic acids were produced in all cases. The sub-
strate versatility and availability of the functional groups were
not dramatically changed to those of the ordinary method using
the PIDA/TEMPO combination.14,15a Thus, the adamantane
reagent 1 could promote the formation of the target aldehydes
4b and 4c from geraniol 3b15a and an aliphatic alcohol 3c14 in
comparable efficiencies to that of PIDA (entries 2 and 3).
Several other aliphatic and benzylic alcohols having electron-
Selective oxidation of alcohols to aldehydes and ketones using
the recyclable adamantane reagent 1
We first confirmed the reactivity with regard to the recyclable
hypervalent iodine reagent 1 for the selective oxidation of
primary alcohol 3a to the corresponding aldehyde 4a under the
regulatory conditions using the TEMPO catalyst (Scheme 2).
The test reaction was carried out in dichloromethane since the
chlorinated solvent is most popularly employed for the PIDA/
TEMPO-mediated oxidation of alcohols in previous studies that
excludes the solubility problem of some substrates, otherwise
polar solvents, such as acetonitrile, are potentially applicable
(see the results in ESI†).14,15 Thus, after dissolving the adaman-
tane reagent 1 (1.1 × 1/4 equiv. relative to the alcohol 3a, which
corresponds to 110 mol% I(III)) in dichloromethane,17 the oxi-
dation of 3a was demonstrated in the presence of the TEMPO
catalyst (10 mol%) at room temperature in an open flask without
any particular precautions and optimization, producing the
desired partially-oxidized aldehyde 4a in 90% yield. No over-
oxidized carboxylic acid was detected during the reaction. At the
same concentration of the substrate 3a (0.2 M), the PIDA/
TEMPO combination14a produced 4a in a similar yield (92%)
for the same reaction time (3 h). Similar to other investigations,
our consensus of the high reactivity of 1 as the recyclable
Scheme 2 TEMPO-mediated selective oxidation of primary alcohol 3a
to aldehyde 4a with the adamantane reagent 1.
Scheme 3 Recycling procedure for the adamantane reagent 1.
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Table 1 Green and selective oxidation of alcohols 3 using a recyclable hypervalent iodine reagent 1 and TEMPO catalysta
Entry
1
Alcohols (3)
Carbonyl compound (4), yield
Entry
10
Alcohols (3)
Carbonyl compound (4), yield
2
3
11
12
4
5
13
14
6
15
7
8
9
16
17
18
a The reactions were performed overnight using the adamantane 1 (1.1 × 1/4 equiv. relative to alcohol 3) in dichloromethane at room temperature in
the presence of TEMPO catalyst (10 mol%). See the general procedure in Experimental section. The provided yields are determined based on the
alcohols 3 used.
rich and deficient aromatic moieties were converted to the corre-
sponding aldehydes 4c–j in satisfactory yields (entries 3–10). As
the functional groups, several oxidation-sensitive alkenes (entries
1 and 2), electron-rich aromatic and heteroaromatic rings (entries
8, 11, and 12), and alkyne (entry 13) were well tolerated. Both
the aliphatic and aromatic secondary alcohols 3n–r could be
smoothly oxidized (entries 14–18). In comparison to PIDA, no
significant decrease in the reaction rate and product yield were
observed in each substrate case when using 1, except for entry
12 (for PIDA, 72% of the product 4l was obtained). In addition,
95–100% of the reagent was constantly recovered as the tetra-
iodide 2 from the reaction mixtures. The reactions were performed
overnight to completely decompose 1 to 2, although most of the
starting alcohols 3 disappeared within a few hours in each case.
The conventional hypervalent iodine/nitroxyl radical-mediated
oxidation protocol is known to be mild enough to provide pro-
nounced chemoselectivities of the functional groups under the
standard reaction conditions.14,15 The almost perfect preference
of primary alcohols in the competitive oxidation, even in the
presence of structurally close secondary alcohols, is quite note-
worthy.15a It became clear that a similar chemoselective nature
was now maintained under the conditions with our recyclable
iodine reagent 1 (Scheme 4). In fact, a cross-over experiment for
a 1 : 1 mixture of 3-phenylpropanol 3j and 4-phenyl-2-butanol
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Regarding recycling of nitroxyl radical
To facilitate the removal and reuse of the nitroxyl radicals, con-
tinuous efforts have been made to provide a recyclable alterna-
tive to TEMPO and its derivatives.23 These approaches for the
oxidation of alcohols using a hypervalent iodine reagent as a
stoichiometric oxidant include the nitroxyl radicals attached to
the soluble or insoluble polymers,9,24a,b the fluorous tags,24c,d
and the ionic supports.24e,f The many successes of the construc-
tion of practical and ecological reaction systems with these
recyclable nitroxyl radicals have recently received considerable
attention. Such examples might be a good counterpart for PIDA,
but these were hardly employed for the recyclable hypervalent
iodine reagent. In a combination of the polymer-supported
iodine reagent, PDAIS,9 the immobilized TEMPO catalyst
exhibited a significant decrease in reactivity, and an excess
amount (∼2 equiv.) of PDAIS was needed to complete oxi-
dation. Moreover, the separation of the employed polymer-sup-
ported iodine reagent and nitroxyl radical was quite difficult
once mixed. Hence, the release of the stoichiometric amounts of
iodobenzene as waste or the poorer recovery of the reagent and
catalyst are serious drawbacks when using the recyclable nitroxyl
radicals as catalysts in this methodology.25
Scheme 4 Cross-over experiment: selectivity between primary and
secondary alcohols.
Scheme 5 Oxidation of optically active alcohol 3t to an enolizable
ketone 4t; AZADO = 2-azaadamantane-N-oxyl.
3s using 1 in the presence of 10 mol% TEMPO resulted in the
exclusive consumption of the primary alcohol 3j leading to the
corresponding aldehyde 4j, whereas the ketone 4s from the sec-
ondary alcohol 3s was produced in less than a measurable
amount (it was also judged by confirming a 97–98% recovery of
the secondary alcohol 3s). The conventional PIDA could also
exclusively deliver a 79% yield of the aldehyde 4j.
These facts certainly inspired us to consider the opportunity to
use the adamantane reagent 1 as a specific counterpart of the
recyclable TEMPO catalyst for realizing a more efficient and
greener strategy based on the dual recycling of the hypervalent
iodine reagent as well as the nitroxyl radical in the selective oxi-
dation of alcohols 3 to aldehydes 4.
Another important benefit of the present methodology is
exemplified by the non-racemic oxidation of an optically active
alcohol 3t to the enolizable ketone 4t (Scheme 5). With the ada-
mantane reagent 1, oxidation of 3t could proceed in good yield
without any decrease in the enantiomeric excess values of the
product. Due to the low reactivity of TEMPO toward 3t, the less
hindered class of nitroxyl radicals, 2-azaadamantane-N-oxyl
(AZADO),15b,c was used at this point as a specific catalyst for
3t. Under such conditions, no evidence of racemization was
found during the course of the reaction.20
As stated above, the adamantane reagent 1 is a promising
recyclable alternative to PIDA that can provide valuable advan-
tages over the conventional hypervalent iodine/nitroxyl radical
mediated oxidation of alcohols. We suppose that the remarkably
high reactivity of 1 among the recyclable reagents should be
responsible for the well-defined tetrahedral structure at which the
introduction sites of the four iodine(III) functions are fully con-
trolled. Based on an X-ray crystallographic analysis,21 the intera-
tomic distance between the iodine(III) atoms of 1 is about 13 Å,
and thus, these iodine sites are sufficiently separated from each
other and do not interfere during the reactions. Based on this
molecular design and its high solubility toward organic solvents,
1 showed an excellent ability that is compatible with PIDA in
the reactions. Furthermore, no degradation of the molecules of 1
and 2 was observed in the overall single recycling (recovery,
reactivation, and reuse) in Scheme 3, because the reagent 1 and
tetraiodide 2 are very stable under oxidative conditions.22
Nevertheless, recovery of the nitrosyl radical, i.e., TEMPO,
still remained untackled at this point, which is the next challenge
to be addressed in our methodology.
Combination of the recyclable hypervalent iodine reagent 1 and
silica-supported TEMPO catalyst 5: a dual recycling strategy for
greener oxidation
The adamantane reagent 1 and its recoverable form, the tetra-
iodide 2, are both soluble during the reactions, and thus the
removal and recovery of the catalyst nitroxyl radical based on
the solid/liquid phase separation from the reaction mixture was
anticipated. The catalyst separation can be easily accomplished
by simple filtration by utilizing the insoluble nitroxyl radical,
while the recovery of 1 from the filtrate is followed by the
described workups in Scheme 3. For this purpose, we have been
particularly interested in the use of the silica-supported TEMPO
526,27 as a suitable heterogeneous counterpart of the soluble
reagent 1 (Fig. 3). The immobilized silica reagents having
nitroxyl radical sites on the silica surface areas were originally
introduced as more reactive and stable alternatives to the corre-
sponding polystyrene analogues of TEMPO. These types of
insoluble nitroxyl radicals, to our knowledge, were first intro-
duced by Tsubokawa et al.27a during the stage of alcohol oxi-
dation, and those with commercial applications, having a
relatively high degree of loading (>0.7 mmol g−1), are now
readily available.
As we supposed, the replacement of regular TEMPO with the
insoluble silica-supported TEMPO was successful. The yield of
the products was maintained—in most cases under the same
reaction conditions—since the reactivity of 1 is similar to that of
the conventionally used PIDA. Nonetheless, the reaction usually
took a longer time than the soluble TEMPO due to the
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heterogeneous nature of 5 when the same molar amount of the
catalyst was used. The reaction rates for 1 and PIDA when using
the immobilized 5 were nearly identical. Thus, we decided to
use 20 mol% of the catalyst 5 which would not negatively affect
the net reaction outcomes for the dual use of the recycling of 1
and 5.28 Considering the excellent reusability of the immobilized
TEMPO 5 (see below) (recyclability, selectivity, etc.) and
efficiencies of the alcohol oxidation, rather the reactions under
such conditions including the high quantity of the catalyst some-
times faced the obvious enhancement of the product yields. The
reactions of the alcohols 3 in Table 1 proceeded smoothly
without problems, and Table 2 represents the results of the exam-
ined oxidations to the aldehydes. No tedious pre-treatment of the
silica TEMPO 5 in the solvent for swelling was required. The
reactions could thus be initiated by just adding 5 to the homo-
geneous mixture of 1 and 3 or adding the readily soluble 1 to the
heterogeneous solution of 5 and 3.
rinsing. From the homogeneous liquid phase, the used 1 could
be recovered as the tetraiodide 2 according to the already men-
tioned general procedure in Scheme 3 involving the second
solid/liquid separation. Indeed, the use of the silica-supported
TEMPO catalyst and the adamantane reagent satisfactorily met
the purpose of the dual recycling of the hypervalent iodine
reagent and catalyst nitroxyl radical with a high degree of recycl-
ability (95–100% recovery of 1 and quantitative recovery of 5).
Also, it should be noted that we can save the effort of isolation
of the aldehyde 3 in the dual recycling system compared to the
method of using the soluble TEMPO. In addition, the procedure
can be carried out without the difficulty of removing the
TEMPO.
To our delight, the reuse of both the adamantane reagent 1
and immobilized TEMPO 5 was successful using the efficient
Table 2 A combined use of two recyclable reagents, 1 and silica-
supported TEMPO 5: selected examples of greener aldehyde synthesisa
We could readily develop a facile operation procedure based
on the solid/liquid separation strategy for isolation of 1 and inso-
luble 5 from the reaction mixtures. Scheme 6 schematically illus-
trates the experimental procedure. First, the used insoluble 5
could be absolutely removed from the heterogeneous reaction
mixtures by filtration, and directly reused for the next run after
Entry
Alcohol (3)
Aldehyde (4)
Yield (%)
1
2
3
4
5
6
7
8
9
3a
3b
3c
3f
3g
3h
3i
4a
4b
4c
4f
4g
4h
4i
88
83
76
78
85
96
75
72
93
3j
3k
4j
4k
a The results were obtained from the reaction using the adamantane 1
(1.1 × 1/4 equiv.) and calcd 20 mol% of the silica-supported TEMPO
catalyst 5 in dichloromethane overnight at room temperature.
Fig. 3 Silica-supported recyclable TEMPO catalyst 5.
Scheme 6 Schematic drawing of dual recycling strategy of hypervalent iodine reagent 1 and silica-supported TEMPO 5.
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Table 3 Repeated uses of the two recyclable reagentsa
Upon recycling, 1 can be typically recovered in 95–100%
quantities as a fine solid via filtration by utilizing the insolubility
of the co-produced tetraiodide 2 that was formed from 1 after the
reactions in the polar solvent, methanol. The tetraiodide 2 was
then recycled for a second run as the initial adamantane form 1
after the treatment with m-chloroperbenzoic acid in acetic acid
(see Experimental section). No degradation of the molecule was
detected for the reagent 1 during the reactions and the regener-
ation step. Thus, the repeated use of 1 as well as 5 was possible
for at least four cycles since the recovered materials maintained
high reactivity.
As highlighted in the dual recycling strategy, the utility of 1
for the hypervalent iodine/nitroxyl radical-mediated selective
oxidation of alcohols to aldehydes and ketones has been first
demonstrated in this study. It is beneficial to use 1 in hypervalent
iodine-mediated oxidation in terms of reactivity, solubility,
removal, recovery, and recycling, for extending the methodology
to dual recycling. Considering the versatility and salient features
of 1,10,11 the dual recycling strategy is not only a reliable choice
for alcohol oxidation, but also a model case for the construction
of other potential multi-component recyclable reagent systems in
further extended research studies. Hence, we believe that this
demonstration provides a useful concept and general guideline of
green chemistry for oxidation using recyclable hypervalent
iodine reagents.29
ð1Þ
Entry
Reuse
Yield (%)
1
2
3
4
5
—
1
2
3
4
85
93
90
88
86
a The reactions were repeated using the (reused) adamantane 1 (1.1 × 1/4
equiv. relative to the primary alcohol 3g) and (reused) silica-supported
TEMPO catalyst 5 (calcd 20 mol%) at room temperature for 12 hours.
Isolated yield of the aldehyde 4g is indicated for each run.
dual recycling strategy. Reuse of the recovered 1 and 5 was eval-
uated for at least four cycles using the alcohol 3g as a model sub-
strate (eqn (1), Table 3), which suggests no remarkable loss in
the activity of the reagent and catalyst during alcohol oxidation
and their recycling. The yields and reaction rates after the sub-
sequent recyclings were almost constant for both the reagent and
catalyst, proving the structure of the adamantane reagent 1 and
the amine linkage of the immobilized TEMPO catalyst 5 are
sufficiently stable to withstand repeated uses. The reused reagent
and catalyst did not add any harmful impurities to the reaction.
The aldehyde 4g was thus selectively produced by the reused
materials, and over-oxidation of the product 4g was neglectable
even after the prolonged stirring of the reaction mixtures for
24 h.
Experimental
Preparation of the recyclable hypervalent iodine reagent 1: the
regeneration step from the recovered tetraiodide 2
To a stirred solution of the (recovered) tetraiodide 230 (1.42 g,
1.5 mmol) in dichloromethane (150 mL) and acetic acid
(150 mL) was added m-chloroperbenzoic acid (mCPBA)
(ca. 69% purity, 3.12 g, 18 mmol) at room temperature. The
mixture was stirred for 12 h under the same reaction conditions
until the cloudy solution became clear. The resultant mixture
was filtered, and dichloromethane was removed from the filtrate
using a rotary evaporator. Hexanes were added to the residue to
precipitate 1,3,5,7-tetrakis[4-(diacetoxyiodo)phenyl]adamantane
1. After filtration, the crude 1 was washed several times with the
hexanes, and then dried in vacuo to give 1 (2.09 g, 97%) as a
white solid.
At this stage, the aim to realize a dual recycling strategy for
the selective oxidation of alcohols to aldehydes and ketones that
can separately recover the hypervalent iodine reagent and
nitroxyl radical has been accomplished along with high levels of
recycling efficiency.
Conclusions
1,3,5,7-Tetrakis[4-(diacetoxyiodo)phenyl]adamantane (1).10a
In this paper, we have clarified the excellent ability of the ada-
mantane reagent 110 as a recyclable hypervalent iodine reagent
in the universally used nitroxyl radical-catalyzed oxidation of
alcohols. The series of versatile and green oxidation systems
using 1, that consist of combination of (1) TEMPO catalyst, and
(2) insoluble silica-supported TEMPO 5, have been developed
for the selective oxidation of alcohols 3 to aldehydes or ketones
4, in which the recyclable reagent 1 having the well-defined
tetrahedral structure showed a promising reactivity similar to the
classically used phenyliodine diacetate, PIDA. In the latter com-
bined case, notably, both 1 and 5 could be recovered and isolated
on the basis of practical operation techniques, i.e., successive
solid/liquid separations, and the only waste material released
into the environment from the reactions was then the nontoxic
acetic acid.
Colorless crystal; mp (decomp.) 195–196 °C (from AcOH–
dichloromethane-hexanes by vapor diffusion method); H NMR
1
(400 MHz, CDCl3): δ 2.01 (s, 24H), 2.20 (s, 12H), 7.56 (d, J =
8.7 Hz, 8H), 8.09 (d, J = 8.7 Hz, 8H) ppm; 13C NMR
(100 MHz, CDCl3): δ = 20.4, 39.6, 46.4, 119.5, 127.7, 135.2,
151.9, 176.5 ppm; elemental analysis: calcd for
C50H52I4O16·2H2O: C 41.34, H 3.89, I 34.95; found: C 41.30, H
3.70, I 35.03.
Typical procedure for TEMPO catalyzed oxidation of alcohols 3
to aldehydes 4 using the recyclable hypervalent iodine reagent 1
(Table 1)
The reagent 1 (77.9 mg, 0.22 × 1/4 mmol) was added to a
dichloromethane solution (1 mL) including the alcohol 3
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(0.20 mmol) and catalytic amount of TEMPO (3.1 mg,
0.02 mmol). The reaction mixture was stirred until the alcohol 3
was no longer detectable by TLC. After removal of the solvent
using a rotary evaporator, methanol (5 mL) was added to the
residue to precipitate the tetraiodide 2. After filtration, the crude
2 on the filter was washed several times with small amounts of
methanol, and the residue was nearly quantitatively recovered as
the pure tetraiodide 2. From the filtrate, the crude product 4 was
obtained after evaporation, which was purified by short column
chromatography on silica gel to give the pure aldehyde or
ketone.
which was purified by short column chromatography on silica
gel to give the pure aldehyde or ketone if required.
Reusability of the recovered adamantane reagent 1 and silica-
supported TEMPO catalyst 5 for the alcohol oxidation was eval-
uated at least four times using the alcohol 3g as a model sub-
strate in the dual recycling strategy (see Table 3).
Acknowledgements
This work was partially supported by a Grant-in-Aid for Scien-
tific Research (A) from the Japan Society for the Promotion of
Science (JSPS), a Grant-in-Aid for Scientific Research on Inno-
vative Areas “Advanced Molecular Transformations by Organo-
catalysts” from The Ministry of Education, Culture, Sports,
Science and Technology (MEXT), and Ritsumeikan Global
Innovation Research Organization (R-GIRO) project. T. D. also
acknowledges financial support from the Research Foundation
for the Electrotechnology of Chubu (REFEC) and the Industrial
Technology Research Grant Program from the New Energy and
Industrial Technology Development Organization (NEDO) of
Japan.
Cross-over experiment: selective synthesis of aldehyde 4j from
primary alcohol 3j in the presence of a secondary alcohol 3s
(Scheme 4)
The reagent 1 (88.5 mg, 0.25 × 1/4 mmol) was added to a
dichloromethane solution (1.2 mL) containing the alcohol 3j
(34.0 mg, 0.25 mmol), 4-phenyl-2-butanol 3s (37.5 mg,
0.25 mmol), and a catalytic amount of TEMPO (3.9 mg,
0.025 mmol) at 0 °C. The reaction mixture was stirred until the
alcohol 3j was no longer detectable by TLC. Workup of the reac-
tion mixture and recovery of the tetraiodide 2 were conducted
according to the typical procedure already described. From the
filtrate, the crude product 4j and remaining 4-phenyl-2-butanol
3s were obtained as a mixture after evaporation, which was sep-
arated by short column chromatography on silica gel to give the
pure aldehyde 4j in 73% yield with a 97% recovery of the 4-
phenyl-2-butanol 3s.
Notes and references
1 For recent reviews and accounts, see: (a) V. V. Zhdankin and P. J. Stang,
Chem. Rev., 2002, 102, 2523; (b) T. Wirth, M. Ochiai, V. V. Zhdankin, G.
F. Koser, H. Tohma and Y. Kita, in Top. Curr. Chem.: Hypervalent Iodine
Chemistry – Modern Developments in Organic Synthesis, ed. T. Wirth,
Springer-Verlag, Berlin, 2003; (c) H. Tohma and Y. Kita, Adv. Synth.
Catal., 2004, 346, 111; (d) T. Wirth, Angew. Chem., Int. Ed., 2005, 44,
3656; (e) V. V. Zhdankin and P. J. Stang, Chem. Rev., 2008, 108, 5299;
(f) V. V. Zhdankin, ARKIVOC, 2009, (i), 1; (g) T. Dohi and Y. Kita,
Chem. Commun., 2009, 2073; (h) L. F. Silva Jr. and B. Olofsson, Nat.
Prod. Rep., 2011, 28, 1722; (i) Y. Kita, T. Dohi and K. Morimoto,
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Non-racemic synthesis of enolizable ketone 4t (Scheme 5)
2 Reviews and accounts: (a) H. Togo and K. Sakuratani, Synlett, 2002,
1966; (b) E. W. Kalberer, S. R. Whitfield and M. S. Sanford, J. Mol.
Catal. A: Chem., 2006, 251, 108; (c) T. Dohi, Yakugaku Zasshi, 2006,
126, 757; (d) M. Ochiai and K. Miyamoto, Eur. J. Org. Chem., 2008,
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3 (a) H. Togo, G. Nogami and M. Yokoyama, Synlett, 1998, 534;
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(d) Y. Suzuk and H. Togo, Synthesis, 2010, 2355; (e) A. Moroda and
H. Togo, Tetrahedron, 2006, 62, 12408.
The enolizable ketone 4t was obtained in the optically pure form
from the corresponding alcohol 3t by a procedure similar to that
described for the alcohols in Table 1 using AZADO instead of
TEMPO. The optical purity of the obtained ketone 4t was
measured by HPLC (Daicel OD column, hexane–i-PrOH =
95 : 5, flow rate 0.8 mL min−1
(–)-enantiomer].20
, tR = 10.0 min. [(S)-
4 (a) S. V. Ley, A. W. Thomas and H. Finch, J. Chem. Soc., Perkin Trans.
1, 1999, 669; (b) S. V. Ley, O. Schucht, A. W. Thomas and P. J. Murray,
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J., 2003, 9, 88; (b) V. Tesevic and J. A. Gladysz, Green Chem., 2005, 7,
833; (c) V. Tesevic and J. A. Gladysz, J. Org. Chem., 2006, 71, 7433;
(d) A. Podgorsek, M. Jurisch, S. Stavber, M. Zupan, J. Iskra and
J. A. Gladysz, J. Org. Chem., 2009, 74, 3133.
6 Ion-supported reagents: (a) W. Qian, E. Jin, W. Bao and Y. Zhang,
Angew. Chem., Int. Ed., 2005, 44, 952; (b) W. Qian and L. Pei, Synlett,
2006, 709; (c) S. T. Handy and M. Okello, J. Org. Chem., 2005, 70,
2874.
7 (a) M. S. Yusubov, L. A. Drygunova and V. V. Zhdankin, Synthesis,
2004, 2289; (b) U. Ladziata, J. Willging and V. V. Zhdankin, Org. Lett.,
2006, 8, 167; (c) M. S. Yusubov, M. P. Gilmkhanova, V. V. Zhdankin and
A. Kirschning, Synlett, 2007, 563; (d) M. S. Yusubov, L. A. Drygunova
and V. V. Zhdankin, Synthesis, 2004, 2289; (e) R. R. Karimov, Z.-G.
M. Kazhkenov, M. J. Modjewski, E. M. Peterson and V. V. Zhdankin, J.
Org. Chem., 2007, 72, 8149; (f) M. S. Yusubov, T. V. Funk, K.-W. Chi,
E.-H. Cha, G. H. Kim, A. Kirschning and V. V. Zhdankin, J. Org. Chem.,
2008, 73, 295; (g) J.-M. Chen, X.-M. Zeng and V. V. Zhdankin, Synlett,
2010, 2771; (h) J.-M. Chen, X.-M. Zeng, K. Middleton and V.
General experimental procedure for green oxidation of alcohols
using a combination of the recyclable hypervalent iodine reagent
1 and supported TEMPO catalyst: a dual recycling strategy
(Tables 2 and 3)
The reagent 1 (77.9 mg, 0.22 × 1/4 mmol) was added to a stirred
solution of the alcohol 3 (0.20 mmol) and catalytic amount of
silica-supported TEMPO 526 (65.6 mg, 0.04 mmol) in dichloro-
methane (1 mL) at room temperature. After consumption of the
alcohol 3, the insoluble TEMPO 5 was recovered in a quantitat-
ive amount by filtration after washing several times with di-
chloromethane. After removal of the solvent from the filtrate,
methanol (5 mL) was added to the residue to precipitate the tetra-
iodide 2. After filtration, the crude 2 was washed several times
with small amounts of methanol, and the residue on the filter
was nearly quantitatively collected as the pure 2. From the
filtrate, the crude product 4 was obtained after evaporation,
1500 | Green Chem., 2012, 14, 1493–1501
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V. Zhdankin, Tetrahedron Lett., 2011, 52, 1952; (i) J.-M. Chen, X.-
M. Zeng, K. Middleton, M. S. Yusubov and V. V. Zhdankin, Synlett,
2011, 1613.
adamantane analogue also worked well as an effective oxidant in terms
of the yield of the product 4a as well as the recovery, however, a high
amount of solvent was needed to dissolve the alternative reagent and to
make the homogeneous conditions. Thus, we specifically selected the
adamantane reagent 1 in this study.
8 (a) M. Mülbaier and A. Giannis, Angew. Chem., Int. Ed., 2001, 40, 4393;
(b) G. Sorg, A. Mengel, G. Jung and J. Rademann, Angew. Chem., Int.
Ed., 2001, 40, 4395; (c) P. Lecarpentier, S. Crosignani and B. Linclau,
Mol. Diversity, 2005, 9, 341; (d) Z. Lei, C. Denecker, S. Jegasothy, D.
C. Sherrington, N. K. H. Slater and A. J. Sutherland, Tetrahedron Lett.,
2003, 44, 1635; (e) W.-J. Chung, D.-K. Kim and Y.-S. Lee, Tetrahedron
Lett., 2003, 44, 9251; (f) J. Zhang, G. Jin, T. Qiu, Y. Wang and
H. Kuang, J. Chem. Res., 2010, 34, 194; (g) J. Zhang, D. Zhao, Y. Wang,
H. Kuang and H. Jia, J. Chem. Res., 2011, 35, 333; (h) T. Miura,
K. Nakashima, N. Tada and A. Itoh, Chem. Commun., 2011, 47, 1875.
9 See: T. Y. S. But, Y. Tashino, H. Togo and P. H. Toy, Org. Biomol. Chem.,
2005, 3, 970. The employed polymer-supported hypervalent iodine
reagent and nitroxyl radical should be recovered as a mixture. In addition,
an excess amount of the polymer reagent (2 equiv.) was necessary for
this oxidation due to the low reactivity of the reagents.
19 The measured solubilities of the tetraiodide 2, a reduced form of the ada-
mantane reagent 1, in several (mixed) solvents at 25 °C are as follows:
0.04 mg mL−1 (methanol); 0.08 mg mL−1 (isopropanol); 0.26 mg mL−1
(acetonitrile); 5.3 mg mL−1 (ethyl acetate); 6.0 mg mL−1 (acetone);
0.06 mg mL−1 (methanol–ethyl acetate = 10:1); 0.08 mg mL−1 (metha-
nol–ethyl acetate = 5:1); 0.42 mg mL−1 (methanol–ethyl acetate = 1:1).
20 We have confirmed the lack ofracemization of the product by comparing
the obtained chiral 4t to the authentic sample in ref. 11g.
21 CCDC 232058 contains crystallographic data of the adamantane
reagent 1.
22 The polymer-supported reagents of type I in Scheme 1 are not stable
under various oxidative conditions. The degradation of the resin often
occurred when they were repeatedly used, which caused a loss of activity
and lower overall recycling efficiency. See ref. 7a and 10.
23 (a) S. Weik, G. Nicholson, G. Jung and J. Rademann, Angew. Chem., Int.
Ed., 2001, 40, 1436.; For a review, see:R. A. Sheldon, I. W. C. S. Arends,
G.-J. ten Brink and A. Dijksman, Acc. Chem. Res., 2002, 35, 774.
24 For the use of recyclable TEMPO catalysts in alcohol oxidation with
PIDA, see ref. 9 and the following papers: (a) G. Pozzi, M. Cavazzini,
S. Quici, M. Benaglia and G. Dell’Anna, Org. Lett., 2004, 6, 441; (b) M.
A. Subhani, M. Beigi and P. Eilbracht, Adv. Synth. Catal., 2008, 350,
2903; (c) G. Pozzi, M. Cavazzini, O. Holczknecht, S. Quici and
I. Shepperson, Tetrahedron Lett., 2004, 45, 4249; (d) O. Holczknecht,
M. Cavazzini, S. Quici, I. Shepperson and G. Pozzi, Adv. Synth. Catal.,
2005, 347, 677; (e) W. Qian, E. Jin, W. Bao and Y. Zhang, Tetrahedron,
2006, 62, 556; (f) A. Fall, M. Sene, M. Gaye, G. Gomez and Y. Fall, Tet-
rahedron Lett., 2010, 51, 4501.
25 A combination of the ion-supported hypervalent iodine reagent and
TEMPO catalyst was reported for the oxidation of the alcohols in ref.
24e. However, the perfect separation of the two ion-supported reagent
and catalyst was difficult in principle as seen in ref. 9, and such a method
typically caused partial contamination of the reagent and catalyst during
their recovery. The recycling efficiency regarding the quantity of the
recovered iodine reagent, a result of its repeated use, and isolation of the
used TEMPO catalyst were not clearly determined. In addition, re-prep-
aration of the ion-supported iodine reagent could proceed in only a 78%
yield, which corresponds to a 22% loss of the reagent for the next overall
recycling step.
10 (a) H. Tohma, A. Maruyama, A. Maeda, T. Maegawa, T. Dohi, M. Shiro,
T. Morita and Y. Kita, Angew. Chem., Int. Ed., 2004, 43, 3595; Analogues
of 1: (b) T. Dohi, A. Maruyama, M. Yoshimura, K. Morimoto, H. Tohma,
M. Shiro and Y. Kita, Chem. Commun., 2005, 2205.
11 For reactions using the recyclable hypervalent iodine reagent 1, see:
(a) T. Dohi, A. Maruyama, M. Yoshimura, K. Morimoto, H. Tohma and
Y. Kita, Angew. Chem., Int. Ed., 2005, 44, 6193; (b) T. Dohi,
K. Morimoto, N. Takenaga, A. Maruyama and Y. Kita, Chem. Pharm.
Bull., 2006, 54, 1608; (c) T. Dohi, Yakugaku Zasshi, 2006, 126, 757;
(d) T. Dohi, K. Morimoto, N. Takenaga, A. Goto, A. Maruyama,
Y. Kiyono, H. Tohma and Y. Kita, J. Org. Chem., 2007, 72, 109;
(e) T. Dohi, A. Maruyama, Y. Minamitsuji, N. Takenaga and Y. Kita,
Chem. Commun., 2007, 1224; (f) T. Dohi, K. Morimoto, C. Ogawa,
H. Fujioka and Y. Kita, Chem. Pharm. Bull., 2009, 57, 710;
(g) N. Takenaga, A. Goto, M. Yoshimura, H. Fujioka, T. Dohi and
Y. Kita, Tetrahedron Lett., 2009, 50, 3227; (h) T. Dohi, Yakugaku Zasshi,
2009, 129, 1187; (i) T. Dohi, Chem. Pharm. Bull., 2010, 58, 135.
12 1,3,5-7-Tetrakis[4-(diacetoxyiodo)phenyl]adamantane 1 is now commer-
cially supplied from Wako Co. Ltd. (code No. 204-18473).
13 For reviews and accounts of organocatalytic oxidation of alcohols, see:
(a) S. Caron, R. W. Dugger, S. G. Ruggeri, J. A. Ragan and
D. H. B. Ripin, Chem. Rev., 2006, 106, 2943; (b) Future Trends for
Green Chemistry in the Pharmaceutical Industry, in Green Chemistry in
the Pharmaceutical Industry, eds. P. J. Dunn, A. S. Wells and
M. T. Williams, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim,
2010 Chapter 16 and references cited therein; (c) M. Uyanik and
K. Ishihara, Chem. Commun., 2009, 2086.
26 Commercially available from Aldrich Co. Ltd (catalog No. 576344,
120–230 mesh, extent of labeling: 0.7 mmol g−1 loading).
14 For reviews, see: (a) N. Merbouh, J. M. Bobbitt and C. Brueckner, Org.
Prep. Proced. Int., 2004, 36, 1; (b) Y. Iwabuchi, J. Synth. Org. Chem.
Jpn., 2008, 66, 1076; (c) R. Ciriminna and M. Pagliaro, Org. Process
Res. Dev., 2010, 14, 245; Recent reports: (d) C. Zhu, L. Ji and Y. Wei,
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Commun., 2010, 40, 2057.
27 (a) N. Tsubokawa, T. Kimoto and T. Endo, J. Mol. Catal. A: Chem.,
1995, 101, 45; (b) A. Heeres, H. A. van Doren, K. F. Gotlieb and
I. P. Bleeker, Carbohydr. Res., 1997, 299, 221; (c) C. Bolm and T. Fey,
Chem. Commun., 1999, 1795; (d) R. Ciriminna, M. Pagliaro, J. Blum and
D. Avnir, Chem. Commun., 2000, 1441; (e) D. Brunel, F. Fajula,
J. B. Nagy, B. Deroide, M. J. Verhoef, L. Veum, J. A. Peters and H. van
Bekkum, Appl. Catal., A, 2001, 213, 73.
28 An alternative solution for improving the reaction rate is the use of the
more reactive nitroxyl radical instead of the silica-supported TEMPO 5.
Other insoluble TEMPO catalysts onto silica, such as SiliaCat® TEMPO
(M. Pagliaro, D. Avnir, J. Blum and G. Deganello, U.S. Patent, 6,797,773
B1, 2004; A. Michaud, G. Gingras, M. Morin, F. Beland, R. Ciriminna,
D. Avnir and M. Pagliaro, Org. Proc. Res. Dev., 2007, 11, 766.), was
applicable for the adamantane oxidant 1 to reduce the amount of the
catalyst.
29 Recently, an elegant method for catalytic use of the hypervalent iodine
reagent using a TEMPO-hybrid oxidant has been reported to produce car-
boxylic acids from primary alcohols. This method only produced car-
boxylic acids, rather than aldehydes. See: T. Yakura and A. Ozono, Adv.
Synth. Catal., 2011, 353, 855.
30 (a) H. Newman, Synthesis, 1972, 692; (b) E. B. Merkushev, N.
D. Simakhina and G. M. Koveshnikova, Synthesis, 1980, 486; (c) V.
R. Reichert and L. J. Mathias, Macromolecules, 1994, 27, 7015; (d) L.
J. Mathias, V. R. Reichert and A. V. G. Muir, Chem. Mater., 1993, 5, 4.
15 The PIDA/TEMPO combination: (a) A. De Mico, R. Margarita,
L. Parlanti, A. Vescovi and G. Piancatelli, J. Org. Chem., 1997, 62, 6974;
the
PIDA/2-azaadamantane
N-oxyl
(AZADO)
combination:
(b) M. Shibuya, M. Tomizawa, I. Suzuki and Y. Iwabuchi, J. Am. Chem.
Soc., 2006, 128, 8412 and references cited therein (c) M. Shibuya,
M. Tomizawa, Y. Sasano and Y. Iwabuchi, J. Org. Chem., 2009, 74,
4619. See also ref. 1.
16 For the TEMPO mediated oxidation of alcohols to aldehydes using
recyclable hypervalent iodine reagents, see: (a) K. Sakuratani and
H. Togo, Synthesis, 2003, 21; (b) C. I. Herrerias, T. Y. Zhang and
C.-J. Li, Tetrahedron Lett., 2006, 47, 13; (c) X.-Q. Li and C. Zhang, Syn-
thesis, 2009, 1163. See also ref. 3e.
17 The adamantane reagent 1 is readily soluble in chlorinated solvents
(dichloromethane, chloroform, etc.) as well as many polar solvents (for
examples, methanol, acetonitrile, and fluorinated alcohols). See our ref.
11.
18 We also evaluated the reactivity of a related reagent having the tetraphe-
nylmethane core in ref. 10b for the oxidation of the alcohol 3a. This
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