Synergistic effect of ketone and hydroperoxide in Bronsted acid catalyzed oxidative coupling reactions

Article abstract of DOI:10.1002/anie.201306752

Waste not wasted: A mechanistic study of the autoxidative coupling of xanthene with cyclopentanone uncovered an autoinductive effect of the waste product hydrogen peroxide. It generates radicals in the presence of acid and ketones, which accelerate the reaction by providing an additional pathway to the reactive hydroperoxide intermediate. This discovery could be applied to achieve other Bronsted acid catalyzed oxidative coupling reactions.

Full text of DOI:10.1002/anie.201306752

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Communications
DOI: 10.1002/anie.201306752
Oxidative Coupling
Synergistic Effect of Ketone and Hydroperoxide in Brønsted Acid
Catalyzed Oxidative Coupling Reactions**
Bertrand Schweitzer-Chaput, Abhishek Sud, ꢀron Pintꢁr, Stefanie Dehn, Philipp Schulze, and
Martin Klussmann*
À
The functionalization of C H bonds by oxidative coupling has
attracted a lot of attention in recent years.[1] These reactions
offer several advantages, in particular the direct use of
unfunctionalized starting materials which can help to stream-
line synthesis. Often transition metal catalysts are used
together with peroxides, but metal-free and organocatalytic
examples of oxidative coupling reactions are known as
well.[2–4]
idation, hence the term “autoxidative coupling”.[6] Here, we
present results from a mechanistic study.
Even though we did not detect peroxides under standard
reaction conditions, we could characterize byproducts 4–8 in
some reaction mixtures, which indicate autoxidation process-
es (Scheme 1b). For example, peroxide 4 (CF3CO2H served as
the catalyst)[5a] and xanthone 5—which was seen in almost all
reactions using xanthene—are most likely secondary products
of 3. Up to a 25% yield of valerolactone 7 (based on 1) could
be observed, which is obviously formed from cyclopentanone
and a hydroperoxide by Baeyer–Villiger oxidation. When we
used acridanes, we could isolate 8, indicating rearrangement[7]
of the corresponding hydroperoxide 9. An involvement of
catalytic transition metal impurities was discounted as
unlikely based on trace element analysis.[5a]
We reported an oxidative coupling reaction of xanthene
(1) and related compounds with C-nucleophiles which only
requires a simple Brønsted acid as the catalyst and elemental
oxygen as the oxidant.[5] A representative example is the
coupling of xanthene with cyclopentanone using methanesul-
fonic acid (MsOH), which forms product 2 (Scheme 1a). The
reaction proceeds at ambient temperature, but was usually
performed at 408C for faster conversion. The lack of a redox-
active catalyst is remarkable and it was presumed that the
reaction proceeded via hydroperoxide 3 formed by autox-
For further investigations into the reaction mechanism, we
chose the coupling of xanthene (1) and cyclopentanone at
408C as a model reaction (Scheme 1a). When we subjected
1 to the reaction conditions but without acid we observed full
conversion to oxygenated products within 24 h (Scheme 2a).
Scheme 2. a) Autoxidation of xanthene in cyclopentanone. b) Conver-
sion of hydroperoxide 3 to coupling product 2.
Scheme 1. a) Autoxidative coupling reaction of xanthene and cyclo-
pentanone, the model reaction of the present study. b) Byproducts 4–8
isolated in this or related reactions.
Initially, hydroperoxide 3 is the major product, which over
time decomposes to the secondary products xanthone (5) and
xanthydrol (6) (see the Supporting Information, chap-
ter S2.4).
When hydroperoxide 3 was subjected to the reaction
conditions, coupling product 2 was formed in essentially
quantitative yield in a short time (Scheme 2b). As expected,
hydrogen peroxide was formed as well, as detected by UV/Vis
spectroscopy immediately after the reaction (see chap-
ter S2.14). Under the same conditions, xanthydrol 6 did not
form 2 but equal amounts of 1 and 5 by disproportionation,[8]
discounting it as key intermediate (see chapter S2.8). During
autoxidation as well as the oxidative coupling reaction, one
mole of oxygen was consumed per mole of converted
xanthene (see chapter S2.9).
[*] B. Schweitzer-Chaput, Dr. A. Sud, Dr. ꢀ. Pintꢁr, S. Dehn,
Dr. P. Schulze, Dr. M. Klussmann
Max-Planck-Institut fꢂr Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mꢂlheim an der Ruhr (Germany)
E-mail: klusi@mpi-muelheim.mpg.de
Homepage: http://www.kofo.mpg.de
[**] Financial support from the DFG (Heisenberg scholarship to M.K.;
KL 2221/4-1; KL 2221/3-1), the MPI fꢂr Kohlenforschung, and Prof.
Benjamin List is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306752.
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It seemed that all these experiments adequately explained
the mechanism: rate-limiting aerobic autoxidation of xan-
thene generates hydroperoxide 3, which forms 2 and H2O2 by
a fast acid-catalyzed nucleophilic substitution of the hydro-
peroxide group with cyclopentanone. However, we could not
detect hydrogen peroxide during and at the end of the
reaction. Additionally, when we took a closer look at the
reaction rates, we were surprised to see that the autoxidation
is actually slower than the overall reaction (Scheme 3). This
Although hydrogen peroxide usually requires transition
metal catalysis for oxidative coupling reactions,[3] we won-
dered whether it could enter another reaction pathway. Using
aqueous or anhydrous H2O2 was unsuccessful due to problems
with solubility and reproducibility, respectively. To stay as
close as possible to our reaction conditions, we studied the
effect of H2O2 generated in situ. Hydroperoxide 3, substituted
xanthene 10,[12] and MsOH were mixed in cyclopentanone
under argon to exclude aerobic autoxidation. After several
hours, roughly 50% of 10 was converted to 11 (Scheme 4).
Scheme 4. Oxidative coupling of 10 by hydrogen peroxide generated
in situ as a byproduct in the reaction of 3 with cyclopentanone.
While the general outcome of this experiment was
reproducible, the yield of the oxidatively formed product
varied between 15 and 46% after three to five hours. In the
presence of 2,6-di-tert-butyl-4-methylphenol (BHT) as a rad-
ical inhibitor, substitution of the hydroperoxide in 3 by
cyclopentanone occurred, but oxidative coupling of xanthene
did not. These experiments clearly demonstrate that the
byproduct H2O2 induces an alternative radical pathway, which
could explain the accelerated rate of the reaction compared
with the autoxidation (see chapter S2.13, for details). While it
has been shown that the oxidative power of H2O2 can be
enhanced under acidic conditions, the suggested mechanisms
were ionic.[13]
For further experiments, we chose tert-butylhydroperox-
ide in decane as an easily available and safe source of a non-
aqueous hydroperoxide. Stirring xanthene, cyclopentanone,
and two equivalents of tBuOOH under an atmosphere of
argon resulted in low conversion of xanthene. Product 2 was
not formed but small amounts of mixed peroxide 12 as well as
the dimer 13 arose instead (Scheme 5). In the presence of
7 mol% of MsOH, full conversion of xanthene was achieved,
giving 80% yield of the coupling product 2 and 14% yield of
13.
Scheme 3. Representative conversion profile of the coupling reaction
(squares) and of the autoxidation (triangles, sum of oxygenated
products 3, 5, and 6).
result suggested either that another factor influences the
kinetics of the autoxidation process, that we overlooked
a secondary pathway or that a completely different mecha-
nism is in effect.
We first studied the autoxidation of 1 separately and
realized its strong solvent dependence. The fastest rates and
highest yields were obtained in ketones, with cyclopentanone
being one of the most efficient solvents. Low or moderate
yields and rates of autoxidation were observed for reactions in
all other solvents tested (see chapter S2.10). This explains our
difficulties in achieving high yields with nucleophiles other
than ketones[5] and the solvent effects seen in a related
reaction.[6a] The autoxidation of organic compounds is obvi-
ously a known phenomenon[9] and acceleration by ketones has
been reported for cyclohexane at 1458C,[10] but the rate at
which 1 is fully converted by autoxidation in cyclopentanone
at ambient temperature is still remarkable.
There are only a few reports of acid effects on the rate of
autoxidation reactions.[11] To investigate whether the autox-
idation of Scheme 2a is accelerated by the Brønsted acid
catalyst, we employed weaker acids that do not or only barely
promote the nucleophilic substitution of the hydroperoxide,
because the effect of strong acids on the autoxidation cannot
be deconvoluted from the coupling step of Scheme 2b. These
experiments gave no indication that catalytic amounts of acid
accelerate the autoxidation of 1 under our conditions (see
chapters S2.7, S2.11, and S2.12). Moreover, the question of
why no H2O2 is detectable remains unexplained.
The presence of 13 suggests the intermediate formation of
a xanthenyl radical, once more indicating the radical nature of
the hydro(gen)peroxide-mediated oxidation pathway. Inter-
estingly, we could see the mixed peroxide 12 as a transient
Scheme 5. Anaerobic reaction of 1 with tBuOOH in cyclopentanone.
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1
intermediate when this reaction was monitored by H NMR
spectroscopy, unlike the more reactive hydroperoxide 3, and
full conversion of tBuOOH to tert-butanol. If instead of
cyclopentanone, dichloromethane was used as a solvent,
hardly any conversion of xanthene was observed. In other
solvents, mostly low conversion to unidentified products
occurred (see chapter S2.16).
To separate the aspects of the solvent and the nucleophile,
we used 14 as a nucleophilic additive (Table 1). The reactions
were performed for six hours in closed vials but without strict
Figure 1. Potential radical initiators formed in situ.
1458C, but no acid was present in this study.[10] Perketals or
perhemiketals are said to be labile compounds which
decompose into radicals.[14] Solyanikov and co-workers
actually investigated the synergistic effect of ketone and
strong acid in the radical decomposition of hydroperoxides.[15]
They suggested the involvement of peroxy hemiketals, but the
mechanism was not further investigated and their results were
not utilized for any application in synthesis.
Table 1: Investigating the solvent effect.[a]
To further evaluate the potential occurrence of such
compounds, we synthesized 17 and 19 from cyclopentanone
by acid catalysis.[16] No reaction occurred when xanthene and
cyclopentanone were stirred with these under strict exclusion
of oxygen (Table 2, entries 1 and 2). In the presence of
Entry
Solvent
1 [%][b]
15 [%][b]
13 [%][b]
1
2
3
4
5
6
7
8
9
10
acetone
cyclopentanone
CH3CN
MeOH
EtOAc
CHCl3
CH3NO2
DMSO
toluene
MTBE
7
92
48
27
21
17
18
15
7
<1
<1
<1
1
1
1
1
0
1
1
52
73
78
79
81
84
90
90
93
Table 2: Evaluating 17 and 19 as oxidants.[a]
Entry
mol% MsOH
t [h]
oxidant
2 [%][b]
13 [%][b]
8
5
1
2
3
4
0
0
7
7
12
12
1
17
19
17
19
0
0
4
25
0
0
5
35
[a] 1 (0.25 mmol), 14 (0.25 mmol) solvent (0.25 mL), tBuOOH
(0.5 mmol, in decane), MsOH (0.025 mmol). [b] Analysis by H NMR
spectroscopy.
1
1
[a] 1 (0.5 mmol), 17/19 (0. 5 mmol) cyclopentanone (2.0 mmol), MsOH
(0.035 mmol), under Ar. [b] Yields determined by 1H NMR spectroscopy.
exclusion of oxygen. When we stirred xanthene, tBuOOH, 14,
and catalytic amounts of MsOH in acetone in a closed vial,
nearly full conversion to the coupling product 15 was achieved
(Table 1, entry 1). In cyclopentanone, the reactivity was
decreased and 15 was formed in about 50% yield (Table 1,
entry 2). For comparison, the aerobic formation of 15 can only
be performed under more forcing conditions (708C, 10 bar
partial pressure of oxygen, 24 h).[5b] In all other solvents
investigated, yields reached 5–27% only (Table 1, entries 3–
10). While some conversion could be observed in all cases, the
solvent effects cannot be rationalized based on polarity; for
example, both DMSO and toluene give very low conversions
(Table 1, entries 8 and 9).
Instead, the results of Scheme 5 and Table 1 indicate that
the combination of acid, hydroperoxide, and ketone as
a solvent are optimal to achieve high conversion of xanthene
to the coupling products 2 and 15. Potentially, peroxy
hemiketals, perketals, or related compounds like 16–19
(Figure 1) form by Brønsted acid catalysis in the presence
of hydrogen peroxide or tert-butylhydroperoxide and undergo
MsOH, however, conversion to coupling product 2 and dimer
13 was observed within hours, with 19 being more efficient
than 17 (Table 2, entries 3 and 4). Once more, stronger acids
are required: nitric acid was only effective with 17 and
trifluoroacetic acid was ineffective with both 17 and 19 (see
chapters S2.19 and S2.20).
While these experiments do not unequivocally determine
whether 17 and 19 are truly intermediates in the reactions
studied, they support the suggested role of acid, ketone, and
hydroperoxide. The role of the acid is twofold, catalyzing the
formation of perketals or related compounds and their
decomposition to radicals. Further studies are needed to
elucidate the exact mechanism of this Brønsted acid catalyzed
radical formation and the structure of the radicals involved.
Consistent with the results discussed above, we propose
a mechanism for the autoxidative coupling as shown in
Scheme 6. In the presence of oxygen, autoxidation of 1 is
initiated and proceeds via radicals 20 and 21 to hydroperoxide
3, which under strong Brønsted acid catalysis generates H2O2
and a stabilized carbocation 22. Reaction with the nucleo-
philic enol form of the ketone forms reaction product 2. A
second pathway is opened by the release of H2O2: in the
presence of ketone and Brønsted acid, radicals of a yet
unknown structure (“23”) are formed, probably via inter-
À
homolytic O O bond cleavage. This is further supported by
the lack of reactivity with di-tert-butyl peroxide, which cannot
form such compounds, while aqueous tBuOOH or cumyl
hydroperoxide are effective (see chapter S2.18).
À
Cyclohexanone is suggested to assist the O O bond
cleavage of cyclohexylhydroperoxide by H-atom transfer at
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pathway. The byproduct of the reaction, hydrogen peroxide,
shows an autoinductive effect, inducing its own re-formation
and accelerating the formation of the final product. This
radical-forming process is mediated by the presence of
ketones and strong Brønsted acids and presumably proceeds
via perketals or related compounds. Inspired by this, we could
facilitate the Brønsted acid catalyzed oxidative coupling of
other substrates using tert-butyl hydroperoxide. These dis-
coveries indicate the potential to develop further acid-
catalyzed oxidative radical reactions and could have implica-
tions for other mechanisms.[18]
Received: August 1, 2013
Published online: October 22, 2013
Keywords: autoxidation · organocatalysis · oxidative coupling ·
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peroxides · reaction mechanisms
Scheme 6. Mechanistic proposal for the autoxidative coupling of
xanthene 1 with cyclopentanone to 2, showing the autoinductive effect
of waste product H2O2 on the formation of hydroperoxide 3.
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Scheme 7. Application of the Brønsted acid catalyzed radical forma-
tion. New bonds are indicated by a dashed line. brsm: based on
recovered starting material. [a] Cyclopentanone as solvent/nucleophile.
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