Denitrogenative palladium-catalyzed coupling of aryl halides with arylhydrazines under mild conditions

Article abstract of DOI:10.1039/c8ob00305j

The development of a method for the Pd(ii)-catalyzed denitrogenative coupling of arylhydrazines to give functionalized biaryls in good yield, using aryl bromides or aryl iodides as convenient and inexpensive aryl sources, is reported. High functional group tolerance is demonstrated for electronically distinct arylhydrazines as well as aryl halides. The desired products were isolated in good to excellent yields for 58 examples. Control experiments and mechanism studies revealed that the transformation undergoes a base-promoted Pd-catalyzed process.

Full text of DOI:10.1039/c8ob00305j

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Denitrogenative palladium-catalyzed coupling of
aryl halides with arylhydrazines under mild
conditions†
Cite this: DOI: 10.1039/c8ob00305j
a
b
a
a
Sheng Chang, * Lin Lin Dong, Hai Qing Song and Bo Feng*
The development of a method for the Pd(II)-catalyzed denitrogenative coupling of arylhydrazines to give
functionalized biaryls in good yield, using aryl bromides or aryl iodides as convenient and inexpensive aryl
sources, is reported. High functional group tolerance is demonstrated for electronically distinct arylhydra-
zines as well as aryl halides. The desired products were isolated in good to excellent yields for 58
examples. Control experiments and mechanism studies revealed that the transformation undergoes a
base-promoted Pd-catalyzed process.
Received 5th February 2018,
Accepted 4th April 2018
DOI: 10.1039/c8ob00305j
rsc.li/obc
in cross-coupling reactions under transition metal catalysis.
However, the existing methods still suffer from harsh reaction
Introduction
Biaryls have attracted considerable attention due to their wide conditions, limited substrate scope and the poor stability of
application in organic synthesis, advanced functional the substrates. Environmentally friendly aryl halide surrogates
1
materials, and pharmaceuticals. Enormous efforts have been that function under mild reaction conditions with excellent
devoted to developing efficient methods for building such a chemo- and regioselectivities are still in great demand. On this
structure. Transition-metal-catalyzed cross-coupling reactions basis, there is still a great requirement for developing new
of aryl halides with arylmetallic reagents, such as Suzuki– methods to cleave C–N bonds. Due to the stability and high
Miyaura, Migita–Kosugi–Stille, Negishi, Kumada–Tamao– dissociation energy of the C–N bond, arylhydrazines have
7
Corriu, and Hiyama reactions, constitute one of the most recently become a hot topic as potential coupling partners.
important and attractive research areas in both academia and
Arylhydrazine salts have been widely used for organic syn-
industry, and provided powerful protocols to construct thesis because of their stability, high reactivity, and avail-
2
8
biaryls.
ability. On the other hand, despite the fact that arylhydrazines
The area of transition-metal-catalyzed cross-coupling reac- as readily available and extremely valuable reagents are widely
tions has experienced considerable growth in the past few used to prepare numerous nitrogen containing compounds
9
decades. This strategy has created powerful and indispensable and produce radical species via oxidation, little attention has
methods for selective construction of carbon–carbon/carbon– been paid to their use as arylating reagents via denitrogena-
heteroatom bonds, as well as in synthesizing functional mole- tion. Recently, pioneering work by Loh et al. demonstrated
3
cules and natural products. To expedite synthetic endeavors, that an aryl hydrazine could be transferred into palladium
chemists have mainly focused on the development of new cata- species under aerobic oxidative reaction conditions, with N
2
1
0
lysts for various coupling reactions. On the other hand, the and H O as the only byproducts. As appealing environmen-
2
developments toward novel coupling partners also represent tally benign surrogates for aryl halides, aryl hydrazines have
the major challenges in organometallic research area.
been employed in a number of Pd-catalyzed coupling reactions
1
1
12
13
14
Recently, transition-metal-catalyzed cross-coupling via C–N (Heck-type,
Suzuki-type,
addition,
homocoupling,
1
5
bond cleavage has attracted considerable attention and versa- etc. ) via C–N bond cleavage, although the use of these com-
tile activated C–N bond containing partners, such as diazo- pounds in the coupling with aryl halides in organic transform-
4
5
6
nium salts, triazoles and azaheterocycles have been explored ations is still in its infancy.
The current coupling methods of arylhydrazines mainly
relied on the application of acids as the promoter. The
common catalytic course initially began with L Pd(II)A (A)
which is formed by the oxidation of Pd(0)L and an acid.
2
a
College of Pharmacy, Jilin Medical University, Jilin, 132013, China.
2
2
E-mail: jmu_changsheng@126.com, fengbo2@sina.com; Tel: +86 432 6456 0532
b
College of Pharmacy, Yanbian University, Yanji, Jilin, 133002, China
Palladiaziridine complex B was easily generated in the pres-
ence of arylhydrazine and complex A. Oxidative insertion of
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8ob00305j
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Scheme 1 The theoretical analysis of Pd-catalyzed coupling with arylhydrazine and aryl halide.
Pd(0)L into complex B would generate the two palladium(II)-cen- Table 1 Optimization of the Pd-catalyzed denitrogenative cross-coup-
2
a
tered complex D via C–N bond cleavage. The following proto- ling of phenyl halide with phenylhydrazine
nolysis of D resulted in the generation of the aryl palladium
complex E and the palladiaziridine complex C that was sub-
sequently decomposed into Pd(0)L , N , and H O by O .
2 2 2 2
Intermediate E could undergo transmetallation with many
coupling partners. However, the reaction of aryl palladium
complex E with aryl halides was unachievable because further
oxidative addition to the Pd(IV) complex was unrealistic. So we
turned our attention towards the possibility of coupling with
aryl halides via a base-promoted Pd-catalyzed mechanism.
First, arylhydrazine is dehydrogenated in the presence of the
base to produce diazene F. Meanwhile, the oxidative addition
of Pd(0)L to aryl halides took place to produce the aryl palla-
2
dium(II) complex G. Transmetallation of diazene F to complex
G with the assistance of a base results in the formation of
intermediate H. Next, the aryl palladium complex H could be 11
transformed to a diaryl palladium(II) intermediate I via the
b
c
Entry
Ligand
Yield (%)
Yield (%)
1
2
3
4
5
6
7
8
9
—
13
45
55
33
62
38
69
68
77
88
89
92
67
74
61
66
68
52
71
21
52
50
39
66
34
75
80
88
86
95
85
64
79
73
77
67
45
74
P(t-Bu)
PCy3
3
P(n-Bu)
3
P(OPh)
P(OMe)
P[O(o-tol)]3
PPh
P(p-tol)
P(m-tol)
3
3
3
3
10
3
P(o-tol)3
TFP
Dppp
Dppe
Dppm
Dppb
Dppf
1
1
1
2
3
4
release of N molecules. Pd(0)L was then regenerated to com-
2
2
plete the Pd(0)–Pd(II) cycle by reductive elimination. It might 15
be an achievable process for the coupling of arylhydrazines
with aryl halides (Scheme 1).
1
1
1
6
7
8
Dmpe
BINAP
As part of our ongoing study on new coupling methods for 19
1
6
the construction of C–C bonds, herein we disclose the novel
palladium-catalyzed C–C bond-forming reaction of various
arylhydrazines with readily available aryl halides (aryl bromide
and aryl iodide) using air as the green oxidant under basic
reaction conditions via unactivated aryl C–N bond cleavage.
a
b
Isolated yields. Reaction conditions A: Phenylhydrazine (1 mmol),
bromobenzene (1.1 mmol), Pd(OAc)
CO (2.0 mmol), NMP (2 mL), 3 h. Reaction conditions B:
Phenylhydrazine (1 mmol), iodobenzene (1.1 mmol), Pd(OAc)
2 mol%), ligand (3 mol%), K CO (2.0 mmol), NMP (2 mL), 3 h.
2
(2 mol%), ligand (3 mol%),
c
K
2
3
2
(
2
3
bromobenzene and phenylhydrazine were reacted under the
standard conditions commonly employed in the base-pro-
Results and discussion
We commenced our studies by exploring reaction conditions moted Pd-catalyzed system using 2 mol% Pd(OAc)
for the envisioned Pd-catalyzed denitrogenative cross-coupling lyst and K CO as a base in NMP (N-methyl pyrrolidine) at
as a cata-
2
2
3
of bromobenzene with phenylhydrazine (Table 1, condition A). 60 °C (Table 1, entry 1). As expected, the desired coupling
As expected, even after heating up to 120 °C, the desired coup- product could be obtained in a yield of 13%. We initially
ling product could not be detected with an acid-promoted Pd- screened the combination of Pd(OAc)
catalyzed system, using TFA as an acid in DMF under monodentate phosphine ligands. It should be noted that
Pd(OAc) /dppp catalytic conditions. At the outset of our studies, trialkyl phosphine (P(t-Bu) , PCy , P(n-Bu) ) (Table 1, entries
2
and some common
2
3
3
3
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2
–4) and trioxy phosphine (P(OPh)
3
, P(OMe)
3
, P[O(o-tol)]
3
)
Table 2 Further optimization of the Pd-catalyzed denitrogenative
cross-coupling of phenyl halide with phenylhydrazine
a
(Table 1, entries 5–7) were less effective than triaryl phosphine
for this reaction (Table 1, entries 8–12). Among the monoden-
tate phosphine ligands tested, TFP (trifurylphosphine) was the
most effective affording the cross-coupled product in 92%
yield (Table 1, entry 12). We also tried bidentate phosphine
2
ligands with Pd(OAc) as the Pd source to determine the reac-
tion efficiency. The ligands Dppp (1,3-bis(diphenylphosphino) Entry Solvent
propane), Dppe (1,2-bisdiphenylphosphinoethane), Dppm
b
c
Catalyst
Pd(OAc)
Base
CO
Yield (%)
Yield (%)
1
2
3
4
5
6
7
8
9
NMP
DMSO
DMF
DMA
2
K
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
92
71
66
69
55
59
47
51
42
—
90
86
81
75
65
58
83
86
71
74
68
61
35
38
95
77
72
67
69
58
58
52
63
—
97
90
84
77
62
65
88
85
(
1,1-bisdiphenylphosphinomethane), and Dppb (1,4-bisdiphe-
nylphosphinobutane) instead of TFP were not suitable
Table 1, entries 13–16). The use of Dppf (1,1′-bisdiphenylphos-
phinoferrocene), Dmpe (1,2-bisdimethylphosphinoethane), or
BINAP ((+/−)-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl)
resulted in poor yields (Table 1, entries 17–19). To test the
Pd(OAc)2
2
K CO
Pd(OAc)
Pd(OAc)
2
2
2
K
K
K
2
CO
2
CO
2
CO
(
Toluene Pd(OAc)
Xylene
CH CN
Pd(OAc)2
2
K CO
3
Pd(OAc)
Pd(OAc)
Pd(OAc)
—
2
2
2
K
K
K
2
CO
2
CO
2
CO
THF
DME
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
feasibility of this process in an iodobenzene system, we investi- 10
gated the optimization of the reaction using iodobenzene as
an aryl halide source with a phenylhydrazine substrate as a
model substrate (Table 1, condition B). Subsequent attempts 14
at lower temperature showed that an excellent yield can be
achieved at 40 °C, and not much longer reaction time was
2
K CO
11
12
13
PdCl
2
K
K
K
2
CO
2
CO
2
CO
Pd(TFA)
Pd(OTs)
2
2
Pd(dba)2
Pd (dba)
2
K CO
1
1
1
5
6
7
2
3
K
K
K
2
CO
2
CO
2
CO
Pd(PPh
Pd(PPh
3
)
4
3
)
3
Cl
2
required. Further optimization revealed that the reaction 18
Pd(dppf)Cl2
2
K CO
d
19
20
21
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
Cs
Na
2
CO
CO
3
70
78
76
75
reached completion furnishing the desired product in 95%
yield when P(o-tol) was employed.
d
2
3
3
d
KOAc
NaOAc
NaOH
KOH
d
Optimization with respect to solvents, catalysts, and bases 22
Pd(OAc)2
d
2
2
3
4
Pd(OAc)
Pd(OAc)
2
44
was explored under these reaction conditions (Table 2, con-
dition A). Subsequent studies of bromobenzene in the pres-
ence of potassium carbonate (2 equiv.), TFP (3 mol%) and pal-
ladium acetate (2 mol%) showed that N-methyl pyrrolidine
d
2
42
a
b
Isolated yields. Reaction conditions A: Phenylhydrazine (1 mmol),
bromobenzene (1.1 mmol), catalyst (2 mol%), TFP (3 mol%), base
c
(
2.0 mmol), solvent (2 mL), 60 °C, 3 h. Reaction conditions B:
Phenylhydrazine (1 mmol), iodobenzene (1.1 mmol), catalyst
desired product was obtained (Table 2, entry 1). This improve- (2 mol%), P(o-tol) (3 mol%), base (2.0 mmol), solvent (2 mL), 40 °C,
(NMP) effectively facilitated the reaction and 92% of the
3
d
3
2
h. Using PdCl as the catalyst.
ment is in agreement with the positive effect of NMP observed
in a number of organic reactions. Interestingly, none of the
other high boiling point solvents examined afforded the
product in significant yield (Table 2, entries 2–6). The screen- hydroxide, and potassium hydroxide afforded only modest
ing of other solvents, such as CH
CN, THF, and DME (di- amounts of products (Table 2, entries 21–24). Then further
3
methoxyethane), gave modest yields (Table 2, entries 7–9). The optimization with respect to solvents, catalysts, and bases was
reaction failed to proceed in the absence of the palladium cata- explored with iodobenzene (Table 2, condition B). Thus, the
lyst (Table 2, entry 10), suggesting that the palladium catalyst optimized conditions can be summarized as Pd(OAc)
is very crucial for the formation of biaryl compounds. As the catalyst, additive K CO , using bromobenzene as the sub-
detailed in Table 2, the optimal conditions for the denitro- strate, and PdCl /P(o-tol) as the catalyst, and K CO as the
genative coupling can be applied toward the use of other palla- base using iodobenzene as the substrate (Table 2, entry 26).
dium catalysts. It was observed that most of the palladium cat-
Having gained preliminary insight into this novel reaction
alysts could successfully promote the reaction. Therefore, and identified the optimized reaction conditions, we next
PdCl , Pd(TFA) and Pd(OTf)
in combination with TFP were explored the scope and generality of this process. With the
applied affording biphenyl in yields varying between 81% and best optimized reaction conditions in hand (Pd(OAc) /TPE/
0% (Table 2, entries 11–13). It should be noted that Pd(dba)2,
CO and PdCl /P(o-tol) /K CO ), we began to the evaluation
Pd (dba) and Pd(PPh were less effective than Pd(OAc)
for of the substrate scope of the denitrogenative Pd-catalyzed
2
/TFP as
2
3
2
3
2
3
2
2
2
2
9
K
2
3
2
3
2
3
2
3
3
)
4
2
this reaction (Table 2, entries 14–16). The introduction of coupling of bromobenzenes and various arylhydrazines
various palladium catalysts with phosphine ligands (Pd (Table 3, condition A). Using arylhydrazines with either elec-
(
PPh
entries 17 and 18). Finally, the influence of a base was studied (Table 3, 3d–3f) at the para-position led to the formation of
Table 2, entries 19–24). Potassium carbonate proved superior, the corresponding biaryl derivatives in good to excellent yields
3 3 2 2
) Cl and Pd(dppf)Cl
) didn’t increase the yields (Table 2, tron-donating (Table 3, 3a–3b) or electron-withdrawing groups
(
though cesium carbonate and sodium carbonate afforded low in all the cases. Interestingly, phenylhydrazine substituted
yields of the coupled product (Table 2, entries 19 and 20). with a hydroxy or amino substituent furnished exclusively the
Reactions with sodium acetate, potassium acetate, sodium desired product without the detection of the by-product
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Table 3 Scope of the Pd-catalyzed denitrogenative cross-coupling of phenyl halides with various arylhydrazines
Reaction conditions A: Arylhydrazine (1 mmol), bromobenzene (1.1 mmol), Pd(OAc)
h. Reaction conditions B (in brackets): Arylhydrazines (1 mmol), iodobenzene (1.1 mmol), PdCl
2.0 mmol), NMP (2 mL), 3 h.
2
(2 mol%), TFP (3 mol%), K
2
CO
3
(2.0 mmol), NMP (2 mL),
(3 mol%), K CO
3
(
2
(2 mol%), P(o-tol)
3
2
3
(Table 3, 3g–3h). Remarkably, a number of synthetically useful phenylhydrazines proceeded smoothly with a series of para-/
functionalities, including aryl fluoride, nitrile and aldehyde meta-substituted aryl bromides under the same reaction con-
groups, were tolerated under the reaction conditions (Table 3, ditions as mentioned above, thus affording the corresponding
3
d–3g). Notably, when the chloro phenylhydrazine is used, the coupling products in good yields (Table 4, 4a–4f). Diverse aryl
corresponding denitrogenative coupling product is obtained in bromides with electron-withdrawing and electron-donating
high yield, suggesting that the chloride leaving group in the groups can undergo the reaction smoothly, with the corres-
arylhydrazine could survive (Table 3, 3k–3l). In the case of a ponding products isolated in good to excellent yields (Table 4,
phenylhydrazine substituted with an aldehyde, chloro and tri- 4a–4d). The carbonyl groups acetyl, and formyl survived
fluoromethyl group at the meta-/para-position, the yields of (Table 4, 4e–4f); the functional groups –Me, –OMe, and –NO
substrates with varied positions had no obvious differences at the p-/m-position in arenes imparted moderate reactivity fur-
Table 3, 3i–3n). When a series of nitro-substituted arylhydra- nishing 4a–4d in acceptable yields. Some heterocycles are also
2
(
zines were introduced, the corresponding products were compatible with our conditions, with 5-(4-methoxyphenyl)pyri-
obtained in moderate to good yields (Table 3, 3o–3r). Next, this midine and 2-(4-methoxyphenyl)pyridine being produced in
reaction was extended with naphthalen-1-ylhydrazine and modest yields (Table 4, 4g–4h). p-Cyan phenylhydrazines were
found to produce the corresponding coupling derivative in thus chosen for the reaction instead of 4-methoxy phenyl-
good yield (Table 3, 3s). Gratifyingly, this reaction could also hydrazines (Table 4, 4i–4l). Thus the scope was broad, and we
be applied to the synthesis of 2-phenyl pyridine, albeit in a found that 4-nitro phenylhydrazine substrates with para-
moderate yield (Table 3, 3t). After having observed the general functionalization furnished products in high yields, and with
reactivity of arylhydrazines with bromobenzenes, we turned moderate to good yields (Table 4, 4m–4p).
our attention to the reactivity of arylhydrazines with iodo-
benzene in lieu of bromobenzene for comparison (Table 3). was further verified by a series of control experiments
The corresponding yields are shown in brackets. (Scheme 2). Both base and ligand were indispensable to this
After achieving the coupling of different arylhydrazines transformation, which indicated that the reaction proceeded
with bromobenzene/iodobenzene, we further investigated the under base-promoted ligand-assisted Pd(II) catalysis
The possible mechanism of the palladium catalytic system
direct reaction of various aryl bromides under similar reaction (Scheme 2A and B). Otherwise, no desired products were
conditions (Table 4). The reaction of electron-rich 4-methoxy detected under nitrogen and no better result was obtained
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Table 4 Scope of the Pd-catalyzed denitrogenative cross-coupling of various aryl bromides with arylhydrazines
Reaction conditions: Arylhydrazines (1 mmol), aryl bromides (1.1 mmol), Pd(OAc)
2
(2 mol%), TFP (3 mol%), K
CO
2 3
(2.0 mmol), NMP (2 mL), 3 h.
Scheme 2 Control experiments of the coupling reaction.
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and mild conditions has been developed. This process pro-
vides concise and highly practical access to biaryls, aryl bro-
mides and aryl iodides and serves as cost-effective and con-
venient aryl sources for a broad range of substrates in up to
97% yield. Mechanism studies and control experiments
revealed that the transformation undergoes a base-promoted
Pd-catalyzed process.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Scheme 3 Possible mechanism.
This work was supported by Scientific and technological
innovation development plan of Jilin Science and Technology
Bureau (201750237), The State Key Laboratory of Medicinal
under pure oxygen (Scheme 2C and D). In the absence of Chemical Biology (NanKai University) (2017001), National
oxygen, the application of 1 equiv. of Cu(OAc) as an oxidant Natural Science Foundation of China (81773692) and the
2
had no influence on the reaction and afforded the product in National Undergraduate Training Programs for Innovation and
similar yields under ambient atmosphere (Scheme 2E). Entrepreneurship (201713743012).
Notably, both bromo- and iodo-substituents on the same sub-
strates were all reactive under the standard conditions. The
direct coupling reaction between 2 equiv. of phenylhydrazine Notes and references
and 1-bromo-4-iodobenzene could afford terphenyl in 79%
1
2
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2
5
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A plausible reaction mechanism is outlined in Scheme 3;
this mechanism is based on the procedure of the reported
base-promoted coupling and the results of our experiments.
First, arylhydrazine is dehydrogenated in the presence of the
base in air to produce diazene product A. Oxidative addition of
aryl halide to the in situ generated Pd(0) catalyst results in the
formation of intermediate B. The transmetallation of inter-
mediate B with diazene A by the assistance of a base cleaves
the C–N bond to give palladium(II)-centered complex C.
Complex C collapses to give diarylpalladium species D and
nitrogen gas in the presence of oxygen. Reductive elimination
of D results in the formation of the biaryl product and regener-
ates the palladium catalyst.
(
(
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