Palladium catalyzed aryl C-H amination with O2via in situ formation of peroxide-based oxidant(s) from dioxane

Article abstract of DOI:10.1039/c4cy00764f

(DAF)Pd(OAc)₂ (DAF = 4,5-diazafluorenone) catalyzes aerobic intramolecular aryl C-H amination with N-benzenesulfonyl-2-aminobiphenyl in dioxane to afford the corresponding carbazole product. Mechanistic studies show that the reaction involves i

Full text of DOI:10.1039/c4cy00764f

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Palladium catalyzed aryl C–H amination with O2
via in situ formation of peroxide-based oxidant(s)
from dioxane†
Cite this: DOI: 10.1039/c4cy00764f
*
Adam B. Weinstein and Shannon S. Stahl
(DAF)Pd(OAc)2 (DAF = 4,5-diazafluorenone) catalyzes aerobic intramolecular aryl C–H amination with
N-benzenesulfonyl-2-aminobiphenyl in dioxane to afford the corresponding carbazole product. Mechanistic
studies show that the reaction involves in situ generation of peroxide species from 1,4-dioxane and O2,
and the reaction further benefits from the presence of glycolic acid, an oxidative decomposition product
of dioxane. An induction period observed for the formation of the carbazole product correlates with the
formation of 1,4-dioxan-2-hydroperoxide via autoxidation of 1,4-dioxane, and the in situ-generated
peroxide is proposed to serve as the reactive oxidant in the reaction. These findings have important
implications for palladium-catalyzed aerobic oxidation reactions conducted in ethereal solvents.
Received 11th June 2014,
Accepted 1st August 2014
DOI: 10.1039/c4cy00764f
www.rsc.org/catalysis
into in situ generation of the peroxide-based oxidant is
Introduction
provided.12,13
Palladium-catalyzed oxidation reactions that form aryl C–N
bonds from aryl C–H bonds and amines could have signifi-
cant value in organic synthesis, and such transformations
could complement other widely used C–N bond-forming
reactions, such as Ullmann-type and Buchwald–Hartwig cross-
coupling reactions.1 Palladium-catalyzed C–H amination
methods typically require strong stoichiometric oxidants,
Results
In an effort to discover mild conditions for aryl C–H amination
reactions, we evaluated the various ligand-supported palla-
dium catalysts for the transformation of N-benzenesulfonyl-2-
aminobiphenyl into the corresponding carbazole with molec-
ular oxygen as the sole oxidant. This reaction was selected
based on the ease of product analysis as well as the prece-
dence for this reaction to be compatible with aerobic catalytic
turnover (albeit at high temperature, 120 °C, and with limited
scope).7a,b During the course of screening studies, 44 turnovers
such as diacetoxyiodobenzene,2 oxone,3 or [F+] sources.4,5
A
common reaction pathway associated with these reactions
involves oxidation of an aryl-PdII intermediate to a high-valent
PdIII or PdIV species,6 which then undergoes facile reductive
elimination of the C–N bond and circumvents the inter-
mediacy of an unstable Pd0 species (Scheme 1).7,8
The requirement for strong stoichiometric oxidants
impacts the appeal of the reactions, because the advantage of
direct C–H functionalization in terms of step economy are
partially offset by reduced overall atom economy. To address
this deficiency, we have been interested in the development
of C–H amination methods compatible with molecular
oxygen as the oxidant.9–11 Here, we report the identification
of a unique mechanistic pathway to achieve aerobic aryl
C–H amination in the conversion of N-benzenesulfonyl-2-
aminobiphenyl to N-benzenesulfonylcarbazole. The reaction
involves in situ generation of a peroxide-based oxidant from
O2 and the solvent, 1,4-dioxane, and mechanistic insight
Department of Chemistry, University of Wisconsin–Madison, 1101 University
Avenue, Madison, Wisconsin 53706, USA. E-mail: stahl@chem.wisc.edu
† Electronic supplementary information (ESI) is available. See DOI: 10.1039/
c4cy00764f
Scheme 1 Aryl C–H amination via PdII/PdIV catalysis.
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Table 1 Investigation of additive effects on aerobic carbazole synthesis
with a (DAF)Pd(OAc)2 catalyst system
were achieved with a 4,5-diazafluorenone (DAF)-ligated Pd(OAc)2
catalyst in 1,4-dioxane at 80 °C under 1 atm O2 (Scheme 2a).
Inspection of the crude 1H NMR spectrum of the reaction
mixture, however, revealed the presence of numerous
byproducts, which were eventually traced to the use of an
aged bottle of dioxane that had been used in the experi-
ments. Significant quantities of 1,4-dioxan-2-hydroperoxide
were detected in the bottle of dioxane by 1H NMR spectro-
scopic analysis of a partially concentrated sample of the
solvent, and the same hydroperoxide and various oxidative
Solvent
%
Dioxane
Entry Additive (equiv.)
(0.1 M) yield decomposition?
1
None
Dioxane
0
No
2
3
t-Butylhydroperoxide (1.1) Toluene 45
t-Butylperoxybenzoate (1.1) Toluene 82
n/a
n/a
1
fragmentation products were identified in the crude H NMR
spectrum of the completed catalytic reaction (Scheme 2b).
When the reaction was repeated with a fresh bottle of
1,4-dioxane, no carbazole product was obtained (Table 1,
entry 1). This result prompted us to carry out a number of con-
trol experiments to probe the origin of successful reactivity with
the old bottle of dioxane depicted in Scheme 2. Upon changing
the solvent to toluene and adding tert-butylhydroperoxide
(TBHP), a yield comparable to that obtained with the
decomposed dioxane was observed (45%, entry 2). Addition of
a more activated peroxide oxidant, tert-butylperoxybenzoate
(TBPB), provided an even higher yield of carbazole (82%,
entry 3). These results implicate the involvement of 1,4-dioxan-
2-hydroperoxide as an oxidant in the reaction, and they
resemble previous observations of Alper14 and Sigman15 in their
studies of Wacker oxidations of alkenes in tetrahydrofuran
(THF). In these reactions, oxidative decomposition of THF was
observed, and TBHP was found to be effective as an oxidant.
Additional control experiments showed that successful
catalytic turnover could be achieved in fresh dioxane if specific
additives were included in the reaction mixture. The reaction
of N-benzenesulfonyl-2-aminobiphenyl was performed using
fresh dioxane in the presence of 1–2 equiv. of glycolic acid,
glyoxylic acid and oxalic acid (cf. Scheme 2b) (Table 1, entries
4–9). Of these additives, glycolic acid promotes the reaction. An
80% yield of carbazole was obtained when two equivalents of
glycolic acid were included in the reaction mixture (entry 6). In
contrast, no product was observed from reactions performed in
the presence of glyoxylic acid and oxalic acid (entries 8 and 9),
4
5
6
7
8
9
10
11
12
Glycolic acid (0.1)
Glycolic acid (1)
Glycolic acid (2)
Glycolic acid (2)
Glyoxylic acid (2)
Oxalic acid (2)
Methyl glycolate (2)
Ethylene glycol (2)
1-Propanol (2)
Dioxane 10
Dioxane 43
Dioxane 80
Trace
Yesa
Yesa
n/a
No
No
Trace
Nob
Nob
Toluene
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
0
0
0
4
0
0
a
1,4-Dioxan-2-hydroperoxide, 1,4-dioxan-2-one and unidentified
formate species were detected in the 1H NMR spectrum of the reaction
mixture. Co-solvent quantities (4 : 1 dioxane : alcohol) were also
ineffective.
b
nor was product formed in the presence of glycolic acid when
toluene was used as the solvent (entry 7). Analysis of the reac-
tion mixtures after 20 h showed that dioxane decomposition
products, similar to those observed when the reaction was
performed with aged dioxane, were observed in the successful
reactions that contained glycolic acid (entries 5 and 6). Dioxane
decomposition was not observed in the unsuccessful reactions
containing glyoxylic acid or oxalic acid. Inclusion of methyl
glycolate provided only trace reactivity (entry 10), while ethyl-
ene glycol and propanol were completely ineffective (entries
11–12). In none of the latter three reactions was dioxane
decomposition observed. To summarize, successful formation
of carbazole product correlates with the oxidative decomposi-
tion of dioxane. 1,4-Dioxan-2-hydroperoxide and 1,4-dioxan-2-
Scheme 2 a) Discovery of a positive effect of peroxide species in carbazole synthesis. b) Dioxane decomposition by autoxidation.
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one were isolated from the successful reaction mixtures, and
several unidentified formate species were evident in the
1H NMR spectrum of the crude reaction mixtures (see the ESI†
for details).
The reaction of N-benzenesulfonyl-2-aminobiphenyl in
the presence of 2 equiv. of glycolic acid in dioxane was moni-
1
tored by H NMR spectroscopy, and the results reveal a sig-
nificant induction period for product formation. As shown in
Fig. 1a, the appearance of carbazole product correlates with
the appearance of dioxane oxidation products 1,4-dioxan-2-
hydroperoxide and 1,4-dioxan-2-one. The transformation of
glycolic acid into oxidized products was not monitored, but
glycolic acid was consumed by the end of the reaction. This
time course may be compared with a time course for the
same reaction, using tert-butylperoxybenzoate (TBPB) in
toluene (Fig. 1b), which exhibits a negligible induction period
but proceeds in similar yield.
A screen of common (bi)pyridyl ligand derivatives in the
presence of glycolic acid revealed that DAF is especially effec-
tive as a ligand for the reaction (Chart 1).16 Simple pyridine,
bipyridine, and phenanthroline derivatives did not promote
the reaction, and a control experiment in which no ligand
was added gave only trace carbazole product. Different diaza-
fluorene derivatives and 6,6′-dimethylbipyridine also pro-
moted the reaction, but not as effectively as DAF.
Discussion
The results described above suggest that in situ formation of
alkyl peroxides from 1,4-dioxane solvent enable Pd-catalyzed
C–H oxidation with O2 as the terminal oxidant. The principal
observations described above may be summarized as follows:
(1) 1,4-dioxan-2-hydroperoxide is directly observed in the
reaction mixture, (2) product formation is observed in diox-
ane only when solvent oxidative decomposition products are
also observed, (3) formation of 1,4-dioxan-2-hydroperoxide
(and other dioxane-based decomposition products) tempo-
rally correlates with the formation of the carbazole product,
(4) glycolic acid promotes the oxidative decomposition of
fresh dioxane under the catalytic conditions, and (5) the reac-
tion carried out with dioxane and glycolic acid exceeds the
performance of reactions that employ simple alkylperoxide-
based oxidants.
Although various mechanistic aspects of these reactions
are not fully understood, a number of reasonable hypotheses
can be offered to explain the experimental observations
(Schemes 3, 4 and 5). The disappearance of glycolic acid under
the catalytic conditions is best explained by (DAF)Pd(OAc)2-
mediated oxidation of the primary alcohol to afford glyoxylic
acid via aerobic PdII/Pd0 catalysis. Aerobic alcohol oxidation
by PdII generates hydrogen peroxide,17 which could form an
adduct with glyoxylic acid (the aldehyde generated from
glycolic acid oxidation; Scheme 3a). Either hydrogen peroxide
or the corresponding glyoxylic acid adduct could serve as an
initiator for autoxidation of dioxane via a radical chain path-
way (Scheme 3b). Trapping of alkyl radicals by O2 eventually
Fig. 1 a) Reaction time course: (DAF)Pd(OAc)2 catalyst with glycolic
acid in dioxane and b) reaction time course: (DAF)Pd(OAc)2 catalyst
with t-butylperoxybenzoate in toluene. Yields were calculated with
respect to 0.05 mmol of starting substrate and were determined by
1H NMR spectroscopic analysis using phenyltrimethylsilane as the
internal standard. DAF = 4,5-diazafluorenone.
affords 1,4-dioxan-2-hydroperoxide and other associated
oxidative decomposition products.18
Our results suggested that 1,4-dioxan-2-hydroperoxide and
TBHP are capable of promoting the aryl C–H amination
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Chart 1 Comparison of neutral donor ligands for the Pd(OAc)2/glycolic
acid mediated carbazole synthesis in dioxane.
Scheme 3 a) Generation of activated peroxide by reduction of O2 and
in situ trapping. b) Proposed mechanism for autoxidation of dioxane.
reaction, but the more activated peroxide TBPB gave a
higher yield, comparable to that obtained when glycolic
acid was used in dioxane (cf. Table 1). The 1,4-dioxan-2-
hydroperoxide species generated from the autoxidation of
dioxane accumulates during the reaction (cf. Fig. 1a), and
various options exist to generate a more reactive peroxide
(Scheme 4a). Any of the activated peroxide species could
potentially contribute to the catalytic aryl C–H amination
reaction, and these oxidants also could be consumed in the
oxidative decomposition of dioxane (Scheme 4b) and glycolic
acid (Scheme 4c).
These mechanistic considerations provide a rationale for
the role of glycolic acid in the generation of a strong peroxide
oxidant under Pd-catalyzed aerobic oxidation conditions in
dioxane. A hypothetical PdII/PdIV catalytic cycle that incorpo-
rates a peroxide-based oxidant in intramolecular aryl C–H
amination is shown in Scheme 5. In 2013, Jiao and coworkers
described the use of N-hydroxyphthalimide to mediate autoxi-
dation of toluene to generate a reactive oxidant in chelate-
directed Pd-catalyzed arene hydroxylation.19 Although certain
details of their proposed mechanism differ from Scheme 5,
both reactions exploit autoxidation of a weak C–H bond in
the solvent to generate an O2-derived peroxide capable of oxi-
dizing an arylpalladium(II) intermediate to a higher oxidation
state, resulting in facile carbon-heteroatom bond formation.
Overall, this concept represents an unusual, but valuable
strategy to use O2 as a stoichiometric oxidant in Pd-catalyzed
oxidation reactions.20
Conclusions
In summary, we have identified a palladium catalyst system
that takes advantage of glycolic acid as a primary alcohol
additive and 1,4-dioxane as a solvent for in situ generation of
strong peroxide oxidants that promote efficient Pd-catalyzed
intramolecular aryl C–H amination of a 2-aminobiphenyl
derivative. Monitoring of the reaction time course demon-
strates a direct correlation between generation of the reactive
hydroperoxide and formation of the C–H amination product.
These observation highlight a valuable strategy to use O2 as a
stoichiometric oxidant in challenging C–H oxidation reac-
tions, but also highlight potential complexities that could
arise when performing aerobic oxidation reactions in the
presence of solvents that have weak C–H bonds.
Experimental
General considerations
All commercially available compounds were purchased
and used as received, unless otherwise noted. Anhydrous
1,4-dioxane in 100 ml sure-seal bottles was purchased from
1
Aldrich. H and 13C NMR spectra were recorded on Bruker
400 MHz or 500 MHz spectrometers and chemical shifts are
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Scheme 4 Mechanistic pathways for a) peroxide activation b) decomposition of dioxane and c) decomposition of glycolic acid.
chromatography was carried out with SiliaFlash® P60
(Silicycle, particle size 40–63 μm, 230–400 mesh) or by using a
CombiFlash Rf® automated chromatography system with
reusable high performance silica columns (RediSep® Rf Gold
Silica, 20–40 μm spherical particles).
Caution: Although no explosions or other safety incidents
were encountered in the course of this work, the experiments
described here involve the formation of potentially explosive
peroxide intermediates. All reactions were performed on
small scale behind a blast shield. Appropriate safety mea-
sures should be taken into consideration in the reproduction
or extension of this work.
General procedure for reactions set up in a custom
parallel reactor
A heavy wall 13 × 100 mm culture tube was charged with
N-benzenesulfonyl-2-aminobiphenyl (15.5 mg, 0.05 mmol)
and other solid additives, such as glycolic acid, as desired.
Separate stock solutions of Pd(OAc)2 and ligand were pre-
pared such that the correct quantity of each (0.01 equiv.)
could be delivered and the total volume would reach 0.5 ml
(0.1 M). The culture tube was loaded onto a custom 48-well
parallel reactor that allows for heating under a reflux con-
denser and 1 atm of O2 with orbital shaking in the absence
of ambient light. The reaction vessel was purged with O2 after
Scheme 5 Proposed mechanism for substrate oxidation via high-valent
PdII/PdIV catalysis with an in situ generated peroxide as the oxidant.
given in parts per million relative to internal tetramethyl-
1
silane (0.00 ppm for H) or CDCl3 (77.16 ppm for 13C). Flash
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being loaded onto the shaker apparatus. The reaction mix-
tures were heated to 80 °C and allowed to shake vigorously.
At the desired time point, the reaction mixtures were allowed
to cool to room temperature, removed from the parallel reac-
tor and concentrated to oil. The mixtures were analyzed by
1H NMR spectroscopy using CDCl3 containing phenyltri-
methylsilane as the internal standard.
3 (a) H. Y. Thu, W. Y. Yu and C. M. Che, J. Am. Chem. Soc.,
2006, 128, 9048; (b) S. W. Youn, J. H. Bihn and B. S. Kim,
Org. Lett., 2011, 13, 3738.
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133, 1466; (d) G. B. Boursalian, M. Y. Ngai, K. N. Hojczyk and
T. Ritter, J. Am. Chem. Soc., 2013, 135, 13278.
5 For examples of palladium-catalyzed aryl C–H amination
that are less likely to involve high-valent palladium interme-
diates, but are likely to involve heterobimetallic species
based on the requirement of stoichiometric silver or copper
oxidants, see: (a) K. Inamoto, T. Saito, M. Katsuno,
T. Sakamoto and K. Hiroya, Org. Lett., 2007, 9, 2931; (b)
E. J. Yoo, S. Ma, T. S. Mei, K. S. L. Chan and J. Q. Yu, J. Am.
Chem. Soc., 2011, 133, 7652; (c) M. Wasa and J. Q. Yu, J. Am.
Chem. Soc., 2008, 130, 14058.
6 For leading references, see: (a) A. J. Canty, M. C. Denney,
B. W. Skelton and A. H. White, Organometallics, 2004, 23,
1122; (b) A. R. Dick, J. W. Kampf and M. S. Sanford, J. Am.
Chem. Soc., 2005, 127, 12790; (c) N. R. Deprez and
M. S. Sanford, Inorg. Chem., 2007, 46, 1924; (d) D. C. Powers
and T. Ritter, Nat. Chem., 2009, 1, 302; (e) N. R. Deprez and
M. S. Sanford, J. Am. Chem. Soc., 2009, 131, 11234; ( f )
K. Muñiz, Angew. Chem., Int. Ed., 2009, 48, 9412; (g)
D. C. Powers, E. Lee, A. Ariafard, M. S. Sanford, B. F. Yates,
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(i) A. J. Hickman and M. S. Sanford, Nature, 2012, 484, 177.
7 For two examples of aryl C–H amination that use
PdII/Pd0 catalysis, see: (a) W. C. P. Tsang, N. Zheng and
S. L. Buchwald, J. Am. Chem. Soc., 2005, 127, 14560; (b)
W. C. P. Tsang, R. H. Munday, G. Brasche, N. Zheng and
S. L. Buchwald, J. Org. Chem., 2008, 73, 7603; (c) Y. C. Tan
and J. F. Hartwig, J. Am. Chem. Soc., 2010, 132, 3676.
8 For an example of aryl C–H amination that may involve
nitrene insertion into a Pd–aryl bond, see: (a) K. H. Ng,
A. S. C. Chan and W. Y. Yu, J. Am. Chem. Soc., 2010, 132, 12862;
(b) A. R. Dick, M. S. Remy, J. W. Kampf and M. S. Sanford,
Organometallics, 2007, 26, 1365.
9 For reviews of Pd-catalyzed aerobic oxidation reactions, see:
(a) S. S. Stahl, Angew. Chem., Int. Ed., 2004, 43, 3400; (b)
K. M. Gligorich and M. S. Sigman, Chem. Commun., 2009,
3854; (c) Z. Z. Shi, C. Zhang, C. H. Tang and N. Jiao, Chem.
Soc. Rev., 2012, 41, 3381; (d) A. N. Campbell and S. S. Stahl,
Acc. Chem. Res., 2012, 45, 851.
10 To our knowledge, the only examples of aerobic Pd-catalyzed
aryl C–H amination reactions are those described in ref. 7a, b.
11 For studies describing the use of O2 in Pd-catalyzed C–H
oxidation reactions believed to proceed via high-valent
(i.e., PdIII or PdIV) intermediates, see the following: (a)
Y. H. Zhang and J. Q. Yu, J. Am. Chem. Soc., 2009, 131,
14654; (b) K. J. Stowers, A. Kubota and M. S. Sanford, Chem.
Sci., 2012, 3, 3192; (c) J. Zhang, E. Khaskin, N. P. Anderson,
P. Y. Zavalij and A. N. Vedernikov, Chem. Commun., 2008,
General procedure for reactions employing peroxide additives
A 6 ml vial was charged with N-benzenesulfonyl-2-
aminobiphenyl(15.5mg,0.05mmol).Stocksolutionsofperoxide
additive (t-butylhydroperoxide or t-butylperoxybenzoate)
in toluene were prepared such that the correct quantity
(1.1 equiv.) could be added via syringe. Separate stock solu-
tions of Pd(OAc)2 and ligand were prepared such that the cor-
rect quantity of each (0.01 equiv.) could be delivered and the
total volume would reach 0.5 ml (0.1 M). The vial was sealed
with a Teflon cap and loaded into a heating block on a shaker
table, allowing for orbital shaking. The reaction mixture was
heated to 80 °C and allowed to shake vigorously. At the desired
time point, the reaction mixture was allowed to cool to room
temperature and concentrated to oil. The mixture was analyzed
by 1H NMR spectroscopy using CDCl3 containing with phenyl-
trimethylsilane as the internal standard.
Acknowledgements
We thank the NIH (R01 GM67173) for financial support of
this work. Spectroscopic instrumentation was partially funded
by the NSF (CHE-1048642, CHE-0342998, CHE-9208463).
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