Manganese(IV)-mediated hydroperoxyarylation of alkenes with aryl hydrazines and dioxygen from air

Article abstract of DOI:10.1002/chem.201400064

We report a new carbooxygenation-type version of the Meerwein arylation in which the introduction of oxygen is achieved by using dioxygen from the air. In this way, hydroperoxides were obtained from activated as well as non-activated alkenes by oxidizing aryl hydrazines with manganese dioxide. The best results were obtained with α-substituted acrylates. Importantly, the aryl hydrazine has to be added slowly to the reaction mixture to allow sufficient uptake of dioxygen from the air. Competition and labeling experiments revealed hydroperoxyl radicals as novel oxygen-centered radical scavengers. The introduction of oxygen into a Meerwein arylation has been achieved with dioxygen from air as an oxygen-centered radical scavenger. The slow addition of the aryl hydrazine is key to aryl radical formation through oxidation with manganese dioxide; whereby, hydroperoxides were obtained from activated as well as non-activated alkenes.

Full text of DOI:10.1002/chem.201400064

DOI: 10.1002/chem.201400064
Communication
&
Synthesis
Manganese(IV)-Mediated Hydroperoxyarylation of Alkenes with
Aryl Hydrazines and Dioxygen from Air
Stephanie Kindt, Hannelore Jasch, and Markus R. Heinrich*[a]
limited to 1,3-butadienes,[5] a far broader range of activated
Abstract: We report a new carbooxygenation-type version
and non-activated alkenes is tolerated in iron(II)-mediated reac-
of the Meerwein arylation in which the introduction of
tions with TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] as
oxygen is achieved by using dioxygen from the air. In this
a radical scavenger.[6] An improved metal-free version of this
way, hydroperoxides were obtained from activated as well
reaction has been reported by Studer and co-workers.[7,8] In ad-
as non-activated alkenes by oxidizing aryl hydrazines with
dition to TEMPO, dioxygen has recently emerged as an
manganese dioxide. The best results were obtained with
oxygen-introducing “reagent” in Meerwein arylations. Examples
a-substituted acrylates. Importantly, the aryl hydrazine has
of such reactions are ferrocyanide-catalyzed carbooxygenations
to be added slowly to the reaction mixture to allow suffi-
by Taniguchi and co-workers,[9,10] manganese(III) acetate-medi-
cient uptake of dioxygen from the air. Competition and la-
ated oxyarylations of alkenes with aryl boronic acids by Studer
beling experiments revealed hydroperoxyl radicals as
and Dickschat,[11] and carbohydroxylations with phenylazocar-
novel oxygen-centered radical scavengers.
boxylates studied by our group.[12] Other than one single ex-
ample with a-methylstyrene,[9] all of the dioxygen-based reac-
tions reported so far rely on the presence of a pure oxygen at-
Through many recent developments, especially those in the
field of photocatalysis,[1] the Meerwein arylation,[2] dating back
to 1939, has become a highly versatile radical multicomponent
reaction for the functionalization of alkenes (Scheme 1). Over
the last few decades, new aryl radical sources such as bromo-
mosphere. In this communication, we report a simple carboox-
ygenation reaction with dioxygen from air.
To generate the required aryl radicals under oxidative reac-
tion conditions, we used phenyl hydrazines in combination
with the strong oxidant manganese dioxide, which was used
in its commercially available form. Studies on aryl radical gen-
eration from aryl hydrazines with various oxidants have recent-
ly been reported by the research groups of Demir,[13] Tanigu-
chi,[9] Chen,[14] and our group.[15] To optimize the reaction con-
ditions, we started with an experiment in which the aryl hydra-
zine 4a was added over 20 min to a stirred mixture of alkene
5a, manganese dioxide, acetic acid, and acetonitrile in an
open flask at room temperature (Table 1, entry 1). The desired
hydroperoxide was first obtained in a low yield of 25%, but
the observation that 4,4’-dichloroazobenzene was formed as
the major product (43%) led us to increase the time over
which 4a was added. This modification successfully reduced
the undesired homocoupling of 4a (entry 2), which probably
occurs through aryl radical addition onto the intermediate
phenyldiazene (see structure 10, Table 4).[16] Having recognized
the remarkable dependence of the hydroperoxyarylation on
the slow addition of aryl hydrazine 4a, we kept the longer ad-
dition time of 1 h for all further experiments. Variations of the
reaction temperature (Table 1, entries 3, 4) did not lead to any
improvement. Acetic acid is expected to be beneficial for the
activation of the oxidant manganese dioxide, and the concur-
rent deactivation of the hydrazine through partial protonation
appears to play a minor role (Table 1, entry 5).[17] An attempt to
accelerate the oxidation of aryl hydrazine 4a by increasing the
solubility of MnO2 through the addition of water failed
(Table 1, entry 6), as did two attempts to decrease the amount
of MnO2 through prior activation of the oxidant.[18] After a con-
trol experiment under an argon atmosphere had unambigu-
Scheme 1. The Meerwein arylation.
and iodobenzenes (X=Br, I) have been reported in addition to
the traditionally used diazonium salts 1 (X=N2+). Non-activat-
ed alkenes are now tolerated as well as the activated alkenes
(e.g., acrylates, 2, R=COOR’; styrene, 2, R=Ph) originally pre-
ferred. In addition, a variety of novel radical scavengers has
been discovered for the introduction of a broad range of func-
tional groups or atoms, Y.[3]
Particularly useful are nitrogen- and oxygen-substituted aryl-
ation products 3 (Y=NR1R2, OR1).[3a] Whereas early examples of
carbooxygenations[4] with diazonium and copper(II) salts were
[a] S. Kindt, Dr. H. Jasch, Prof. Dr. M. R. Heinrich
Department fꢀr Chemie und Pharmazie, Pharmazeutische Chemie
Friedrich-Alexander-Universitꢁt Erlangen-Nꢀrnberg
Schuhstrasse 19, 91052 Erlangen (Germany)
E-mail: markus.heinrich@fau.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400064.
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tion of the alkene.[20] As shown by the spectra of the crude
products, in which only 6 f and 6g appeared, fragmentation
to the corresponding ketones 7 f and 7g (Table 3) does
occur during purification on silica gel.[21] The acrylates 5c–
e almost always gave good to high yields of clean hydro-
peroxides before purification (Table 2, entries 8–10, 12–15).
On silica gel, significant reduction of the hydroperoxides
6h–n to the corresponding alcohols was observed. Reduc-
tion of 6i occurred to a much lower extent (only 3%) when
Al2O3 was used for purification. However, product separa-
tion from byproducts is likely to be more difficult with this
sorbent. The hydroperoxide 6p, obtained from aryl hydra-
zine 4a and a-methylstyrene (5 f) in moderate yield, re-
mained stable during column chromatography (Table 2,
entry 16), whereas the product of 2-methacrylonitrile (5g)
gave 6q in good yield, but clean and quantitative fragmen-
tation was observed during purification (Table 3). Monosub-
stituted alkenes like acrylonitrile (5h) and methyl acrylate
(5i) are less suited for hydroperoxyarylations owing to the
formation of unstable products, such as cyanohydrins
(Table 2, entry 18),[22] or competing alkene oligomerization
(Table 2, entry 19). Accordingly, an experiment with only
two equivalents of alkene 5i gave a slightly improved yield of
6s (34% vs. 29% with 6 equiv of 5i).
Table 1. Optimization experiments.
Entry
Variation compared with standard conditions[a]
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
–
25
51, 47[c]
3
40
26
–
addition of 4a over 1 h
08C, 1 h[d]
408C, 1 h[d]
absence of acetic acid[d]
solvents: CH3CN/H2O (10:1)[d]
argon atmosphere[d]
stirring vigorously[d]
addition of KO2 (1 equiv)[d]
addition of 2 equiv of 4a[d]
–
49
54
32
[a] Standard conditions: 4a (1.00 mmol in 4 mL CH3CN, added over 20 min),
5a (6.00 mmol), MnO2 (5.00 mmol), acetic acid (2.00 mmol), CH3CN (5 mL), rt.
[b] Yield determined by addition of dimethyl terephthalate as an internal
standard. [c] Yield after purification by column chromatography. [d] Addition
of 4a over 1 h.
ously proved the importance of dioxygen from air (Table 1,
entry 7), we tried to improve the dioxygen saturation of the re-
action mixture through vigorous stirring (Table 1, entry 8);
however, this did not increase the yield. Slightly better yields
were obtained with potassium superoxide as an addi-
Regarding the reduction of hydroperoxides to their corre-
sponding alcohols, we found sodium thiosulfate to be particu-
tive (Table 1, entry 9), whereas a run with double the
Table 2. Hydroperoxyarylation—scope and limitations.[a]
amount of aryl hydrazine (2 mmol) showed that the
alkene and manganese dioxide should be used in
excess of six and five equivalents, respectively.
As the improvement obtained by addition of po-
tassium superoxide was not decisive (Table 1,
entry 9), and as our main objective was to use dioxy-
gen from air as the oxygen-centered scavenger, the
optimized conditions (Table 1, entry 2) were em-
ployed to explore the scope and limitations of the
hydroperoxyarylation reaction (Table 2).
Entry
Aryl hydrazine 4
R1 =
Alkene 5
R2 =
Product
Yield[b]
[%]
Yield[c]
[%]
R3 =
1
2
3
4
5
6
7
8
4-Cl (4a)
H (4b)
4-F (4c)
(CH2)2OAc (5a)
(CH2)2OAc (5a)
(CH2)2OAc (5a)
(CH2)2OAc (5a)
(CH2)2OAc (5a)
CH2OAc (5b)
CH2OAc (5b)
CO2Me (5c)
CO2Me (5c)
CO2Me (5c)
CO2Me (5c)
CO2Et (5d)
CO2Et (5d)
CO2Et (5d)
CO2tBu (5e)
Ph (5 f)
CN (5g)
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
6a
6b
6c
6d
6e
6 f
6g
6h
6i
6j
6k
6l
6m
6n
6o
6p
6q
6r
51
58
52
40
53
23
37
67
quant.
75
41+6[e]
58
quant.
74
55
43
70
19
34+7[e]
47
55
46
37
Owing to the synthetic value of hydroperoxides in
many transformations,[19] we combined our study
with an investigation of the stability of the hydroper-
oxides with regard to purification by column chroma-
tography. In earlier studies, the primarily obtained hy-
droperoxides could either not be isolated[11] or were
reduced to the corresponding alcohols[9] prior to
chromatography. From all the reactions with butenyl
acetate 5a (Table 2, entries 1–5), the desired hydro-
peroxides were obtained in moderate to good yields
with volatile chlorobenzene as the only significant
byproduct. Purification of 6a–e by column chroma-
tography on deactivated silica gel was possible with-
out noticeable decomposition and the reliability of
the yields determined by comparison with an internal
standard could be verified in this way. Significantly
lower yields were produced with 2-methallyl acetate
(5b; Table 2, entries 6 and 7), probably owing to in-
creased hydrogen abstraction from the allylic posi-
4-Br (4d)
4-CN (4e)
4-Cl (4a)
4-CN (4e)
4-Cl (4a)
H (4b)
4-CN (4e)
3-OMe (4 f)
4-Cl (4a)
H (4b)
4-CN (4e)
H (4b)
4-Cl (4a)
4-Cl (4a)
4-Cl (4a)
4-Cl (4a)
51
7 f[d]
7g[d]
26+30[e]
65+22[e]
67+5[e]
29+6[e]
36+10[e]
n.d.
9
10
11
12
13
14
15
16
17
18
19
47+22[e]
n.d.
Me
Me
H
40
7 f[d]
CN (5h)
CO2Me (5i)[f]
10[e]
H
6s
27+14[e]
[a] Standard conditions: 4a–f (1.00 mmol in 4 mL CH3CN, added over 1 h), 5a–
i (6.00 mmol), MnO2 (5.00 mmol), acetic acid (2.00 mmol), CH3CN (5 mL), rt. [b] Yield
determined by addition of an internal standard. [c] Yield(s) after purification by
column chromatography. [d] Hydroperoxides 6 f, 6g, and 6q underwent clean frag-
mentation to ketones 7 f and 7g upon purification by column chromatography (see
also Table 3). [e] Yield of corresponding alcohol. [f] Methyl acrylate (5h) (2.00 mmol).[27]
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larly useful (Table 3).[23] In the case in which a labile cyanohy-
drin, 8q, is obtained from a hydroperoxide such as 6q, the pH
value of the reaction mixture had to be controlled by a citrate
buffer.[24] A possible explanation for the exceptionally high
yields of the experiments with phenyl hydrazine (4b; Table 3,
products 6i, 6m, 8i, 8m) could be that this was the only hy-
drazine that could be submitted to the reactions without prior
treatment of its hydrochloride salt with aqueous sodium hy-
droxide and subsequent extraction.
Table 4. Competition experiments.
Table 3. Transformation of hydroperoxides on silica gel and through re-
duction with sodium thiosulfate.
Entry Variation compared with standard conditions[a]
Ratio 13:14[c]
1
2
3
4
5
6
7
8
9
10
–
4:3
1:2
5:1
1:1
6:1
addition of 4a over 15 min
under O2 atmosphere
addition of MnCl2 (2 equiv)[b]
addition of MnCl2 and under O2 atmosphere[b]
addition of KO2 (0.5 equiv)
addition of KO2 (1 equiv)
addition of KO2 (2 equiv)[d]
addition of KO2 (2 equiv)
Entry
Ketone 7
Hydroperoxide 6
Alcohol 8
[%][b]
[% yield from 4 and 5][a] [% yield over 2 steps][b]
4:1
10:1
>20:1[e]
1:1
1
2
3
4
5
6
7
8
7 f [quant.]
7g [quant.] 6g [37]
6h [70]
6 f [23]
–
–
8h [55]
8i [91]
8j [54]
8l [39]
8m [quant.]
8n [46]
8o [55]
8q [63]
8s [41]
addition of KO2 (1 equiv) and under O2 atmosphere >20:1[e]
6i [quant.]
6j [67]
6l [41]
6m [quant.]
6n [58]
6o [55]
6q [70]
6s [34+7[c]]
[a] Standard conditions: 4a (1.00 mmol in 4 mL CH3CN, added over 1 h), b-
pinene (9; 6.00 mmol), MnO2 (5 mmol), acetic acid (2.00 mmol), CH3CN
(5 mL), rt. [b] Addition of H2O (0.1 mL) required for solubility of MnCl2.
[c] Ratios determined by 1H NMR analysis. Products further characterized
through reduction to the corresponding alcohols with NaBH4 (see the Sup-
porting Information). [d] Standard conditions with 3.00 mmol acetic acid.
9
10
11
7 f [quant.]
1
[e] Hydroperoxide 14 not detected by H NMR spectroscopy.
[a] Yield determined by internal standard. [b] Yield after purification by
column chromatography. [c] Yield of corresponding alcohol 8s.
entry 3). Attempts to further accelerate the trapping of 12 by
the addition of manganese(II) ions[27] did not lead to significant
improvements (Table 4, entries 4, 5 cf. entries 1, 3). Remarkably
improved ratios of 13/14 were, however, observed upon addi-
tion of increasing amounts of potassium superoxide (Table 4,
entries 6–8), an observation that suggests that the hydroper-
oxyl radicals formed in step 10!11 might play an important
role in the trapping of 12 to give 13 or 14. Comparison of the
experiments in entries 8 and 9 shows that sufficient acetic acid
has to be present to maintain the beneficial effect of KO2,
probably by ensuring protonation of the superoxide ions to
hydroperoxyl radicals. Unsurprisingly, the combination of su-
peroxide and a dioxygen atmosphere led to hydroperoxide 13
without detectable amounts of 14 (Table 4, entry 10).
The origin of the oxygen atoms in the reaction products was
determined by hydroperoxyarylation experiments under an at-
mosphere of 18O-labeled dioxygen and subsequent reduction
(Table 5). The exclusive formation of 15* under 18O2 suggests
that the dioxygen atmosphere is the only source of the
oxygen atom(s) in the hydroperoxides and corresponding alco-
hols (Table 5, entry 2). However, if hydroperoxyl radicals are
present (for example, from the addition of potassium superox-
ide under slightly acidic conditions), these radicals can com-
pete with the labeled 18O2 to give a mixture of 15 and 15*
(Table 5, entry 3). Interestingly, hydroperoxyl radicals have not
so far been described as oxygen-centered radical scavengers.
Finally, we investigated the conversion of the now readily
available a-hydroperoxycarboxylic acids to chemoluminescent
To get some initial insights into the factors that influence
the trapping of the intermediate alkyl radicals by dioxygen or
other related species, b-pinene (9) was employed as the alkene
(Table 4). After aryl radical 11 is formed from diazene 10,[16] the
addition of 11 to 9 leads to alkyl radical 12. Depending on the
rate of trapping by the oxygen atom-centered radical scaveng-
er, 12 will undergo ring opening to give 13 or 14 as the final
product.[25] Under our standard conditions (see Table 2), a 4:3
ratio of ring-closed to ring-opened product 13/14 was found
and, in accordance with the effect observed in the optimiza-
tion study (Table 1, entry 1), trapping becomes less efficient
when aryl hydrazine 4a is added in only 15 min (Table 4,
entry 2).
Taking into account the low solubility of dioxygen in air-
equilibrated acetonitrile (2.42 mm),[26] this observation is not
surprising, as the average volume of 7 mL of acetonitrile used
in the experiments can only dissolve 0.017 mmol of dioxygen
at a time. This value corresponds to 0.9% or 1.7% of the stoi-
chiometrically required amount, depending on whether dioxy-
gen is required for two or only one of the mechanistic steps
(10!11 and/or 12!13/14). It therefore appears to be a crucial
element for the success of such hydroxyperoxyarylation reac-
tions that the reaction mixture is given enough time to equili-
brate and replace used dioxygen by taking up more from the
air. A control experiment under pure dioxygen gas led to the
expected predominance of the ring-closed product 13 (Table 4,
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Experimental Section
Table 5. Experiments under 18O2 atmosphere.
General procedure for carbohydroxylations with
MnO2
A solution of aryl hydrazine (1.00 mmol) in acetonitrile
(4 mL) was added dropwise over a period of 1 h to
a stirred suspension of alkene (6.00 mmol), acetic acid
(115 mL, 2.00 mmol), and MnO2 (435 mg, 5.00 mmol) in
acetonitrile (5 mL) at 23 oC. The reaction mixture was fil-
tered and the filter cake was further washed with diethyl
ether. The organic layer was washed with water (30 mL)
and brine, then dried over Na2SO4. The solvents were re-
moved under reduced pressure and the products were
purified by flash column chromatography on silica gel.
Entry
Conditions[a]
Ratio 15:15*[b]
1
2
3
control experiment under O2 atmosphere
under 18O2 atmosphere
addition of KO2 (1 equiv) and under 18O2 atmosphere
only 15
only 15*
3:1
[a] Standard conditions: 4a (1.00 mmol in 4 mL CH3CN, added over 1 h), b-pinene (9;
6.00 mmol), MnO2 (5 mmol), acetic acid (2.00 mmol), CH3CN (5 mL), rt. [b] Determined
by mass spectrometry.
Acknowledgements
The authors are grateful for financial support from
the Deutsche Forschungsgemeinschaft (DFG).
dioxetanones (Scheme 2).[28] Starting from crude hydroperoxy-
ester 6h (Table 2, entry 8), the acid 16 could be obtained in
59% yield through saponification with aqueous sodium hy-
droxide. Ring closure with dicyclohexyl carbodiimide at low
temperature gave the unstable dioxetanone 17,[29] which was
added in solution to the sensitizer 9,10-diphenylanthracene
(18)[30] in benzene. Cycloreversion of 17 under simultaneous
liberation of photochemically excited carbon dioxide led to
ketone 7 f as shown by 1H NMR spectroscopy and TLC.
Keywords: aryl hydrazines · carbooxygenation · dioxygen ·
hydroperoxyl radicals · radical reactions
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[27] Based on the assumption of an equilibrium between MnII/dioxygen and
III
´
´
Mn /superoxide. For related effects, see: I. Ivanovic-Burmazovic, M. R.
´
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Received: January 7, 2014
[21] For an alternative radical access to a-aryl methyl ketones from diazoni-
um salts and propenyl acetate, see: C. Molinaro, J. Mowat, F. Gosselin,
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Synthesis
S. Kindt, H. Jasch, M. R. Heinrich*
&& – &&
Manganese(IV)-Mediated
Hydroperoxyarylation of Alkenes with
Aryl Hydrazines and Dioxygen from
Air
The introduction of oxygen into
radical formation through oxidation
with manganese dioxide; whereby, hy-
droperoxides were obtained from acti-
vated as well as non-activated alkenes.
a Meerwein arylation has been achieved
with dioxygen from air as oxygen-cen-
tered radical scavenger. The slow addi-
tion of the aryl hydrazine is key to aryl
&
&
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!