Palladium-catalyzed cross-coupling reaction of azides with isocyanides

Article abstract of DOI:10.1039/c5cc05981j

An efficient palladium-catalyzed cross-coupling reaction of azides with isocyanides is developed, providing a general synthetic route to unsymmetric carbodiimides with excellent yields. This method shows a broad substrate scope, including not only aryl azides, but also unactivated benzyl and alkyl azides. Furthermore, from readily available substrates, Pd-catalyzed coupling with a tandem amine insertion cascade to obtain unsymmetric trisubstituted guanidines has been achieved in a one-pot fashion.

Full text of DOI:10.1039/c5cc05981j

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Palladium-Catalyzed Cross-Coupling Reaction of Azides with
Isocyanides
Received 00th January 20xx,
Accepted 00th January 20xx
Zhen Zhang, Zongyang Li, Bin Fu and Zhenhua Zhang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient palladium-catalyzed cross-coupling reaction of azides
with isocyanides is developed, providing a general synthetic route
to unsymmetric carbodiimides with excellent yields. This method
shows broad substrates scope, including not only aryl azides, but
also unactivated benzyl and alkyl azides. Furthermore, from
readily available substrates, Pd-catalyzed coupling with tandem
amine insertion cascade to obtain unsymmetric trisubstituted
guanidines has been achieved in a one-pot fashion.
With the rapid development of nitrene chemistry, organic
azides, as convenient amino sources and efficient nitrene
precursors, have been explored for C-N bond formations.1
Conventional transformations involve C-H amination and
aziridination, which are usually catalyzed by Rh2, Ru3, Fe4, Ir5,
etc6 (Scheme 1). However, though acting as the stable and
common catalysts for various cross-coupling reactions,
palladium complexes have seldom been used in nitrene
transformations.7 On the other hand, Pd-catalyzed coupling
reaction of carbene/nitrene precursors with σ-donor/π-
acceptor ligands is an efficient route to active unsaturated
intermediates. Although Pd-catalyzed carbene transfer
reactions with carbon monoxide8, alkynes9, and isonitriles10 to
form ketenes, allenes, and ketenimines have been developed,
direct Pd-catalyzed nitrene transformation with σ-donor/π-
acceptor ligands has rarely been explored.7a,b,e Thus, we have
paid much attention to Pd-catalyzed cross-coupling reaction of
azides with isonitriles, which would be a high efficient strategy
to access unsymmetric carbodiimides.
Scheme 1 Transition metal-catalyzed nitrene transformation.
Conventional synthetic approaches to unsymmetric
carbodiimides14 mainly involve dehydration of ureas15
,
desulfurization of thioureas16, and aza-Wittig reaction of
phosphinimines with isocyanates,17 which either need
specialized substrates or produce much waste. An alternative
strategy involving transition-metal-catalyzed oxidative
coupling of amines with isocyanides has been studied.18
However, the requirement of stoichiometric external oxidants
limits reaction efficiency and substrate scope. Transition-metal,
such as Fe19, Ni20, Cu21, and Co22, catalyzed carbodiimides
formation via nitrene transformation has also been
sporadically documented. For example, Fe(CO)3 was employed
to catalyzed alkyl carbodiimides formation in an early
report.19a In recent years, N-heterocyclic carbenes
/ β-
Functionalized unsymmetric carbodiimides are one of the
most common precursors to multi-substituted guanidines11
and N-containing heterocycles11c,12, which serve as important
building blocks for compounds of high utility in
pharmaceuticals, agrochemicals and materials chemistry.13
diketiminates stabilized Fe and Ni species catalyzed reactions
of azides with isocyanides have also been reported.19-20
Nonetheless, these methods tend to require rigorous
experimental environment or suffer from limited the substrate
scope due to the instability and sensitivity of metal catalyst
bearing bulky or complicated ligands. Herein we report a
general and efficient Pd(PPh3)4-catalyzed expeditious synthesis
of unsymmetric carbodiimides from azides via nitrene transfer
to isocyanides under redox-free and environmentally friendly
conditions (N2 as the only byproduct). It is worth mentioning
that this protocol covers not only common aryl azides, but also
Department of Applied Chemistry, China Agricultural University, West
Yuanmingyuan Rd. 2, Beijing 100193, People’s Repubic of China. E-mail:
zhangzhh@cau.edu.cn; Fax: +86-010-62732873; Tel: +86-010-62732873
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1 Examination of reaction conditionsa
Table 2 Substrate generality of the reactions of aryl azides and isocyanides.a
Entry [Pd] (mol%)
Solvent
PhMe
PhMe
PhMe
PhMe
THF
t (h)
8
T (oC)
50
50
50
50
rt
Yield (%)b
1
2
3
4
5
6
7
8
Pd2(dba)3(2.5)
Pd(PPh3)4(5)
PdCl2 (5)
85
91
trace
15
99
90
89
0
5
10
10
5
Pd(OAc)2 (5)
Pd(PPh3)4(5)
Pd(PPh3)4(5)
Pd(PPh3)4(5)
----
DCE
14
14
24
5
rt
MeCN
THF
rt
50
rt
9
Pd(PPh3)4(2.5)
THF
98 (90)c
a
b
Reaction conditions: 1a (0.15 mmol), 2a (0.18 mmol), 2 mL of solvent. Determined
by 1H NMR using mesitylene as internal standard. c Isolated yield.
unactivated benzyl and alkyl azides, which remain a challenge
in nitrene transformations.23 In addition, the tandem amine
insertion achieved in a one-pot fashion also provides a
valuable route to unsymmetric trisubstituted guanidines.
At the outset of our investigation, we selected the model
a Reaction conditions: 1 (0.2 mmol), 2 (0.24 mmol), 2.5 mol% Pd(PPh3)4, 2 mL of THF, rt,
reaction of azidobenzene
(
1a)
and 2-isocyano-2- 5 h. Yields were determined by 1HNMR using mesitylene as internal standard. b Isolated
yield. c 40 oC for 12 h, 5 mol% Pd(PPh3)4.
methylpropane (2a) as substrates under N2. To our delight,
when the reaction was performed using Pd2(dba)3 as catalyst
at 50 °C in PhMe, 3aa was directly obtained in excellent yield.
This result encouraged us to screen diverse Pd catalysts.
Consequently, comparison experiments were implemented
between Pd(II) and Pd(0) for their efficiency, showing the
superior activity of Pd(0) complex (Table 1, entries 3 and 4). In
further explorations, we examined various solvents. When
reactions were carried out in DCE and MeCN, comparable
yields were obtained (90% and 89%, respectively, Table 1,
entries 6 and 7); almost equivalent yield was obtained when
the experiment was carried out in THF by using Pd(PPh3)4 as
catalyst, even at ambient temperature for 5 h (Table 1, entry
5). Finally, the catalyst loading was reduced to 2.5 mol%, which
provided 98% NMR yield and 90% isolated yield (Table 1, entry
9). Notably, the expected product couldn’t be accessed in the
absence of Pd catalysts (Table 1, entry 8).
wide range of heterocycles, including thiophene, pyridine,
quinoline, indole, etc., proceed smoothly to give the
corresponding products in high yields (3ma−3ta). For further
inspection of the reaction, the scope of isocyanides was then
examined in the transformation with phenyl azide. 1-Isocyano-
butane, isocyanocyclohexane and (isocyanomethyl)-benzene
worked equally well
(3ab-3ad). Regarding different aryl
isocyanides, N,N’-diarylsubstituted carbodiimides were
obtained in moderate to good yields.
Although we have presented broad substrate generality of
aryl azides with a high level of functional group tolerance,
further applications of this method in organic synthesis require
even broader generality. The installation of benzyl- and alkyl-
substituted imino groups was envisioned to be highly
desirable. Compared with aryl azides, benzyl and alkyl azides
demonstrated poorer reactivity.23 However, the reaction could
furnish the desired carbodiimides in acceptable to high yields,
when properly prolonging reaction time to 10 h and increasing
the loading of Pd(PPh3)4 to 5 mol% and the reaction
temperature to 50 oC.24 Encouraged by this result, the scope of
benzyl azides was subsequently investigated (Table 3). To our
delight, the anticipated products were obtained in high yields
when a trifluoromethyl group was substituted at the 2-, 3- or
4-position of the arene moiety. Reactions of benzyl azides
bearing both electron-donating substituents (Me, OMe), and
electron- withdrawing substituents (CO2Me) or halogens on
Under the optimized reaction conditions described above,
an array of functionalized aryl azides and isocyanides were
subjected to this reaction in order to investigate the substrate
scope (Table 2). Generally, the transformation of substrates
bearing electron-donating groups resulted in good yields,
albeit slightly decreased due to steric hindrance of 1d
.
Substrates containing strongly electron-withdrawing groups
such as COMe, CO2Me (while direct CO2H was not succeeded),
CN and CF3 groups in 1i–l are also compatible. In addition,
halogen, amino, alkenyl and alkynyl substituents were well-
tolerated on the aromatic ring (3ea-3ga, 3ua-3wa), thus
the aryl ring afforded the desired products (5ea-5ha),
offering a handle for further synthetic transformations.
Importantly, reactions of azide-substituted naphthalene and a
respectively, in good yields (80%, 72%, 84%, 70%). More
2 | J. Name., 2012, 00, 1-3
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Table 3 Substrate generality of the reactions of ArCH(R)N3 and isocyanides.a
As mentioned above, unsymmetric carbodiimide is an
important partner in a two-component synthetic route to
guanidines. Although a large body of works have been
published for the preparation of the guanidine scaffold,9,23 the
discovery of a facile method to synthesize unsymmetric
N,N’,N’’-trisubstituted guanidines from simple and available
substrates through concise and convenient steps is still
needed. In the following investigation, we successfully
N
C
R'
N3
N
Pd(PPh3)4 (5 mol %)
THF, 50 oC, 10 h
N
R
R
R'
N
C
+
4a-4k
2a-2d
5
N
C
t-Bu
C
t-Bu
N
N
N
R' = t-Bu
F3C
5aa (88%) (79%)b
5ba (82%)
CF3
N
N
N
F3C
t-Bu
C
t-Bu
C
C
t-
Bu
implemented
a one-pot Pd–catalyzed three-component
N
N
reaction of azides, isocyanides and amines (Scheme 2).
Generally, phenyl azide as well as different substituted
5ca (90%)
5da (89%)
5ea (80%)
N
N
C
t-Bu
N
C
t-
Bu
isocyanides
(2a, 2d, 2e) were selected to give desired
N
C
C
t-Bu
MeO2C
N
N
unsymmetric carbodiimides. Then, diverse amines as another
nitrogen source were added for the next transformation at 90
oC for another 18 h. Interestingly, a diversity of symmetric and
unsymmetric N,N’,N’’-trisubstituted guanidines were obtained
in 51%-73% overall yields from the one-pot synthesis.
Furthermore, when 1-azido-2-bromobenzene (1t) was treated
with (isocyanomethyl)benzene (2d) and piperidine in the
presence of Pd and Cu complexes, 1-benzyl-2-(piperidin-1-yl)-
Cl
MeO
5fa (72%)
5ga (84%)
5ha (70%)
N
N
N
C
t-Bu
O
C
t-Bu
t-Bu
S
N
N
N
5ia (56%)
5ja (72%)
5ka (88%)
R = H
N
N
N
C
Cy
C
n-Bu
C
N
N
N
1H-benzo[d]imidazole (
9) was produced in 72% total yield
5ab (73%)
5ac (74%)
5ad (40%)
without isolation of the carbodiimide intermediate. Derivatives
a
of 9 were widely studied for potential antistaphylococcal
activities.25
Reaction conditions: 4 (0.2 mmol), 2 (0.24 mmol), 5 mol% Pd(PPh3)4, 2 mL of THF, 50
oC, 10 h. Yields were determined by 1HNMR using mesitylene as internal standard.
b
Isolated yield.
On the basis of palladium nitrene intermediate as stated
earlier7, a plausible mechanism was proposed in Scheme 3.27
First, the probable palladium nitrene species
A
is formed from
importantly, substrates containing heterocycles and other alkyl
isocyanides were found to be compatible with this protocol.
However, presumably owing to steric effect and weak
reactivity of benzyl isocyanides, the carbodiimides 5ia and 5ad
were obtained in moderate yields.
Having successfully surveyed aryl and benzyl azides, we
considered expanding the methodology to aliphatic azides. As
shown in Table 4, the reaction could be carried out with a
series of substituted aliphatic azides, affording the
corresponding primary, secondary, and tertiary alkyl
substituted unsymmetric carbodiimides in moderate to good
yields. Even more, some special functional groups like
morpholine and TBS moiety were well-tolerated. Besides, the
transformation of 1-isocyanobutane and isocyanocyclohexane
(
7ab, 7ac) proceed smoothly in 50%, 67% yield, respectively.
Table 4 Substrate generality of the reactions of alkyl azides and isocyanides.a
H
N
Ref. 26
N3
cat.
Pd(PPh3)4
N
N
N
Ph
CuI / 1,10-Phen
Cs2CO3, 80 oC, 20 h
N
Pd(PPh3)4 (5 mol %)
THF, 50 oC, 14 h
+
N
C
R
C
R'
R
N3
6a-6f
Ph
R'
N
N
C
+
Br
N
Dioxane
2a-2c
7
1t
2d
9, 72%
Ph
N
Ph
O
C
O
C
Scheme 2 One-pot synthesis of N,N’,N’’-trisubstituted guanidines and
N-containing heterocycles.
N
N
7ab (50%)
7ac (67%)
R' = t-Bu
O
N
N
N
C
t-Bu
t-Bu
C
t-Bu
N
C
N
t-Bu
TBSO
N
Ph
Ph
N
N
7aa (74%) (65%)b
7ba (66%)
7ca (55%)
N
N
O
O
C
Ph
C
t-Bu
O
C
t-Bu
( )3
N
N
N
7da (74%)
7ea (54%)
7fa (48%)
a
Reaction conditions: 6 (0.2 mmol), 2 (0.26 mmol), 5 mol% Pd(PPh3)4, 2 mL of THF, 50
oC, 14 h. Yields were determined by 1HNMR using mesitylene as internal standard.
b
Isolated yield.
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Scheme 3 Proposed mechanism.
Párkányi, I. Foch, L. I. Simándi and A. Kálmán, Chem.
Commun., 1997, 1143. (d) S. Kamijo, T. Jin and Y. Yamamoto,
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Chem. Commun., 2014, 50, 3706; (f) D. L. J. Broere, B. de
Bruin, J. N. H. Reek, Martin L., S. Dechert and J. I. van der
Vlugt, J. Am. Chem. Soc., 2014, 136, 11574.
Z. Zhang, Y. Liu, L. Ling, Y. Li, Y. Dong, M. Gong, X. Zhao, Y.
Zhang and J. Wang, J. Am. Chem. Soc., 2011, 133, 4330.
Q. Xiao, Y. Xia, H. Li, Y. Zhang and J. Wang, Angew. Chem. Int.
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1a simultaneously with the release of N2. Subsequently,
insertion of isocyanide (2a) into Pd-nitrene species
to give intermediate . Finally, reductive elimination of
intermediate affords the product carbodiimide 3aa and
A occurred
B
B
8
9
regenerates the Pd(0)-catalyst.
In conclusion, under a robust and redox-free condition, we
have developed a novel Pd-catalyzed cross-coupling reaction
of azides with isocyanides, which is a general and efficient
route to unsymmetric carbodiimides. All of aryl, benzyl, and
alkyl azides/isocyanides were selectively transformed in high
yields with excellent functional group tolerance. Moreover,
based on this mild protocol, a series of unsymmetric N,N’,N’’-
trisubstituted guanidines were easily prepared in an efficient
one-pot fashion from simple and readily available starting
materials. This work amply demonstrates that Pd-catalyzed
nitrene reactions using organic azides as convenient
precursors show great potential in C-N bond construction.
Exploration of reaction mechanism and further applications of
this chemistry are currently under way in our laboratory.
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Acknowledgements
Financial support from the National Science Foundation of
China (No. 21302219), Chinese Universities Scientific Fund
(2014RC009), the National S&T Pillar Program of China
(2015BAK45B01), and the Beijing National Laboratory of
Molecular Sciences (BNLMS) is greatly appreciated.
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