Gold-Catalyzed Oxidative [2+2+1] Annulations of Aryldiazo Nitriles with Imines To Yield Polyarylated Imidazolium Salts

Article abstract of DOI:10.1002/anie.201701157

Gold-catalyzed oxidative [2+2+1] annulations between two imines and one α-cyano arylgold carbene afforded polyarylated imidazolium salts and molecular hydrogen efficiently. Control experiments suggest that the gold catalyst alone facilitates the main annulation, whereas Ag⁺ avoids the formation of inactive LAuCN. DFT calculations suggest that the success of this annulation relies on a 6 π-electrocyclization of cyano-free intermediates with cis-configured imines as initial reagents.

Full text of DOI:10.1002/anie.201701157

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International Edition: DOI: 10.1002/anie.201701157
German Edition: DOI: 10.1002/ange.201701157
Annulations
Gold-Catalyzed Oxidative [2+2+1] Annulations of Aryldiazo Nitriles
with Imines To Yield Polyarylated Imidazolium Salts
Samir Kundlik Pawar, Ming-Chung Yang, Ming-Der Su,* and Rai-Shung Liu*
Abstract: Gold-catalyzed oxidative [2+2+1] annulations
between two imines and one a-cyano arylgold carbene
afforded polyarylated imidazolium salts and molecular hydro-
gen efficiently. Control experiments suggest that the gold
through a novel oxidative [2+2+1] annulation involving one
diazo nitrile (1) and two imines (2) to release H [Eq. (3)].
2
This annulation is in sharp contrast with the well-known
reactions of diazo esters with imines to afford a-carbonyl
+
[9]
catalyst alone facilitates the main annulation, whereas Ag
aziridines (VI) [Eq. (1); Tf = trifluoromethanesulfonyl].
avoids the formation of inactive LAuCN. DFT calculations
suggest that the success of this annulation relies on a
One notable exception is a gold-catalyzed [4+1] annulation
of triazines with diazo esters, thus yielding the imidazolidines
VII [Eq. (2)]. This process excludes a [2+2+1] annulation
because existing imine species do not participate in the
6
p-electrocyclization of cyano-free intermediates with cis-
configured imines as initial reagents.
[
10]
reactions. Besides synthetic value, our new synthesis has
two novel aspects: 1) it is the first [2+2+1] annulation
between diazo species and imines in a novel and unexpected
mechanism, and 2) the first use of aryldiazo nitriles as
arylmethine “ArC” precursors. Our DFT calculations exclude
imidazolidine intermediates (VII’), from a direct [2+2+1]
annulation. Instead, the construction of this five-membered
azacyclic framework relies on 6p-electron cyclization of
cyano-free intermediates.
A
ll aryl-substituted benzenes and heteroarenes, such as I–IV
(
Figure1), have found diverse applications in materials and
[
1–4]
pharmaceuticals.
well known as nonlinear optical materials, discotic liquid
crystals, and OLEDs, whereas the heteroarene derivatives
Hexaphenylbenzene derivatives (I) are
[
1]
Figure 1. All aryl-substituted benzenes and heteroarenes.
II-IV possess a key role in dyes, pharmaceuticals, materials,
[2–4]
and natural products.
Chemical syntheses of these poly-
arylated species I–IV have been achieved with efficient
[5,6]
methods.
In the context of ionic aromatic compounds, an
imidazolium salt is one prominent representative because of
its great potential as an electrolyte, ligand, ionic liquid, and
[
7]
liquid crystal. The synthesis of their polyarylated derivatives
V was reported for only one particular compound (Ar= Ph)
[
8]
by a tedious procedure. The development of a one-pot
synthesis of new polyarylated imidazoliums using readily
available reagents is highly desirable and significant in
[
7]
imidazolium chemistry.
Diazo esters cannot serve as arylmethine precursors
because the elimination of an ester is difficult, and aryldiazo
To achieve this goal, we describe in this work a gold-
catalyzed synthesis of polyarylated imidazolium salts (3)
[11]
halides are too unstable to generate carbene species.
Aryldiazo nitriles prove to be efficient carbene precursors
[
12]
in many catalytic reactions. As shown in Table 1, treatment
[
*] Dr. S. K. Pawar, Prof. Dr. R.-S. Liu
Department of Chemistry, National Tsing-Hua University
Hsinchu (Taiwan, ROC)
of phenyldiazo nitrile (2a) with the imine 1a (2a/1a = 2) in
dichloromethane (DCM; 258C, 12 h) with P(tBu) (o-biphe-
2
nyl)AuCl/AgNTf yielded the pentaphenylimidazolium 3a in
2
E-mail: rsliu@mx.nthu.edu.tw
1
5
8% yield together with unreacted 1a and PhCHO in 5 and
6%, respectively (entry 1). A similar distribution of products
Dr. M.-C. Yang, Prof. Dr. M.-D. Su
Department of Chemistry, National Chia-Yi, University
Chiayi (Taiwan, ROC)
was obtained for AgCl-free P(tBu) (o-biphenyl)AuNTf
2
2
E-mail: midesu@mail.ncyu.edu.tw
(entry 2). In this case, the recovered gold catalyst,
P(tBu) (o-biphenyl)AuCN, was inactive. The low yields of
3a in entries 1 and 2 are ascribed to an inefficient amount of
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201701157.
2
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Table 1: [2+2+1] Annulations with various catalysts.
Table 2: [2+2+1] Annulations with various imines.
[
b]
[c]
Entry
Catalyst (mol%)
Solvent
Yields [%]
1
a
3a
2a’
[
d]
1
2
3
4
5
6
7
8
9
LAuCl/AgNTf (10:10)
DCM
DCM
DCM
DCM
DCM
DCE
5
6
–
20
25
–
15
35
85
87
35
18
18
65
45
33
51
40
8
74
76
55
65
69
59
71
90
85
89
80
2
[
d]
LAuNTf (10)
2
LAuCl/AgNTf (10:50)
2
AuClPPh /AgNTf (10:50)
3
2
IPrAuCl/AgNTf (10:50)
2
LAuCl/AgNTf (10:50)
2
LAuCl/AgNTf (10:50)
THF
2
LAuCl/AgNTf (10:50)
ACN
DCM
DCM
DCM
2
AgNTf (50)
–
–
–
2
1
0
1
Rh (OAc) /AgNTf (2:50)
2
4
2
1
IPrCuCl/AgNTf (10:50)
[a] [substrate]=(0.13m). [b] L=P(tBu) (o-biphenyl), [c] Yields are
2
2
reported for compounds isolated after using silica chromatography.
[
a] 1a (0.13m). [b] L=P(tBu) (o-biphenyl), IPr=1,3-bis (diisopropyl-
2
phenyl)imidazol-2-ylidene. [c] Yields are reported after silica chroma-
tography. [d] PhCHO was isolated in 56 and 58% yield for entries 1 and
, respectively. ACN=acetonitrile, THF=tetrahydrofuran.
results are provided in Table 3. We prepared various para-
substituted phenyldiazo nitriles (2b–e; X = tert-butyl, Me, Cl,
2
Br and CF ) to afford the corresponding desired imidazolium
3
AgNTf to serve as a counter anion and thus avoid formation
salts 4a–e in 41–70% yields (entries 1–5), and the electron-
2
of P(tBu) (o-biphenyl)AuCN. A high loading of AgNTf2
2
(
0.5 equiv), accordingly, increased the yield of this desired
Table 3: [2+2+1] Annulations with various aryldiazo cyanides.
salt to 68%. Herein, insoluble AgCN was collected and
confirmed by its IR: u(CN) 2166 cm (entry 3). Other gold
ꢀ1
catalysts, PPh AuCl/AgNTf and IPrAuCl/AgNTf , with the
3
2
2
same Au/Ag proportions (10/50 mol%), yielded 3a in 45 and
5% yields, respectively (entries 4 and 5). 1,2-Dichloroethane
DCE), THF, and CH CN were less efficient solvents for
3
(
3
P(tBu) (o-biphenyl)AuCl/AgNTf (10/50 mol%) and yielded
2
2
3
a in 8–51% yields, with DEC giving the best yield (entries 6–
8
). We examined the reactions with AgNTf2 (50 mol%),
Rh (OAc) /AgNTf (10/50 mol%), and IPrCuCl/AgNTf (10/
2
4
2
2
5
0 mol%), and only diazo decompositions were observed
with these catalysts (entries 9–11). Under standard reaction
conditions (entry 3), H was detected in significant propor-
2
tions (ca. 35%) by GC using a 5 ꢀ M.S. column.
We assessed the scope of these catalytic [2+2+1] annu-
lations with various imines. The results are presented in
Table 2. The imines 1b–d, bearing N-aryl groups (Ar=
[a] [substrate]=(0.13m). [b] L=P(tBu) (o-biphenyl), [c] Yields are
reported for compounds isolated after silica chromatography.
2
4
-XC H , X = Me, Cl and NO ) reacted with 2a to yield the
6
4
2
corresponding compounds 3b–d in 28–64 yields, with the
methyl derivative being the most productive (entries 1–3).
Other imines, 1e–h, bearing varied imino groups (Ar’ =
donating substituents (X = Me, tert-butyl) were more effec-
tive than their halo analogues. For the1,3-dioxolyl derivative
2 f, its resulting imidazolium 4 f was obtained in 55% yield
(entry 6). The meta-substituted phenyl derivatives 2g–i (X =
OMe, Cl, Br) were compatible with these [2+2+1] annula-
tions to yield their imidazolium salts 4g–i in 42–69% yields.
Again, a 4-methoxy substituent was more efficient than the
4-chloro and 4-bromo derivatives (entries 7–9).
Equation (4) shows the synthesis of new imidazolium salts
bearing five aryls of the same type. With suitable imines and
aryldiazo cyanides, these gold-catalyzed annulations allow the
productions of penta-methyl, penta-iodo, penta-bromo,
penta-chloro, and penta-fluoro imidazolium salts (5a–e) in
41–61% yields. Again, an electron-rich methyl is more
effective than their electron-deficient halo analogues. The
4
-XC H , X = tert-butyl, Me, Cl, and NO ) afforded the
6
4
2
expected imidazoliums 3e–h in 21–67% yields (entries 4–7).
The molecular structure of the chloro derivative 3g was
[
13]
confirmed with X-ray diffraction. The N-heteroaryl imines
i–k (Ar’ = 2,3-thienyl and 2-furyl) afforded the desired
1
imidazoliums 3i–k in 56–68% yields (entries 8–10). For
imines 1l,m bearing the same N-aryl and imino groups
(
3
Ar= Ar’ = Me, OMe), the corresponding reactions afforded
l,m in 58–61% yields (entries 9–10). Electron-rich aryls of
Ar and Ar’’ attained an annulation efficiency better than that
of their electron-deficient analogues.
The scope of this imidazolium synthesis was further
expanded with its application to various aryldiazo nitriles. The
2
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[13]
structure of 5a was characterized with X-ray diffraction.
These halo-containing imidazoliums can be elaborated into
other new derivatives of imidazoliums. The pentaiodo species
5
b was subjected to Sonogashira coupling with 1-octyne and
1
-pentyne, thus yielding the penta-alkynyl-containing imida-
zolium salts 5 f (85%) and 5g (45%), respectively [Eq. (5)].
Scheme 2. A plausible mechanism and Gibbs free energies of transi-
tion states and intermediates in DCM.
feasibility is supported by DFT calculations (M06/
LANL2DZ). Imines might exist in cis and trans forms which
[
15]
As 1a might afford imidazolidine intermediates (VII)
Eq. (2)] by a dipolar [3+2] cycloaddition, we added reactive
reagents such as diethyl acetylenedicarboxylate or benzalde-
hyde to trap any possible 1,3-dipole intermediate. These
reaction conditions still yielded 3a in 61–63% yields, as
depicted in Scheme 1. The phenyldiazo ester 6a and ketone
can be isomerized with acid catalysts. Our DFT calculations
suggest that the cis-configured imine provides a low-energy
profile for possible intermediates and transition states. Only
[
[14]
ꢀ1
one transition state (TS-4) is about 14.3 kcalmol above that
of the gold carbene A and trans-imine 2a. We postulate an
attack of the cis-configured imine at A to yield the gold-
containing iminium species B with zero-activation energy. A
concerted-type 1,3-dipolar cycloaddition between B and 2a is
unlikely to occur because benzaldehyde and 1,3-diethyl
acetylenedicarboxylate do not affect the reactions. Instead,
the imine is superior to benzaldehyde as a nucleophile to
generate either a new iminium species C or its conformer C’,
which can proceed through two possible paths. In path
a (Scheme2), a gold-catalyzed cyclization of this species to
yield the key imidazolidine D is unlikely to occur because the
corresponding transition state TS-8 is too high in energy
ꢀ1
(
+ 40.9 kcalmol ). In path b, C’ loses HCN to form a stable
Fisher-type gold carbene (F), thus rendering a 6p-electro-
[
16]
cyclization feasible through a disrotatory mode. Notably,
these two steps have low energies for transition-states TS-4
ꢀ
1
and TS-5 (+ 14.3 and + 6.6 kcalmol ). The resulting inter-
mediate G is affected by a neighboring cation to release
Scheme 1. Experiments to elucidate the mechanism.
+
LAu , thus yielding the zwitterion species H, which is
6
b were tested in this gold-catalyzed reaction, but effected
subsequently oxidized with a proton to the desired 3a. In
contrast to cis-2a, trans-2a gives one transition state (TS-4’) of
only diazo decomposition. The reaction still proceeded well
with LAuCl/AgOTf [L = P(tBu) (o-biphenyl), 10:50 mol%],
but became inefficient with LAuCl/AgOTf/LiOTf
ꢀ
1
up to + 34.1 kcalmol in the Ctrans!E
conversion, and is
2
trans
thus unlikely to occur (see Scheme S1 in the Supporting
Information).
(
10:10:40 mol%). In the case of Cu(OTf) and Zn(OTf) , the
2
yields of 3a’’ were 50 and 41%, respectively.
LAu and Ag are indispensable for this [2+2+1]
annulation, presumably in a relayed catalysis sequence. A
This postulated mechanism rationalizes well our observa-
tions that electron-rich imines are more efficient for these
annulations because of their stabilizations of iminium inter-
mediates like B, C, F, and G. The final oxidations of E and H,
with a proton, also follow this trend. The significance of this
mechanism indicates that direct [2+2+1] annulations
between two imines and one gold carbene is not feasible,
+
+
gold catalyst alone is responsible for the main reaction
+
(
Table 1, entry 2), whereas Ag in excess avoids formation of
the inactive LAuCN. A plausible mechanism is depicted in
Scheme 2, and involves a stepwise ionic mechanism. Its
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polyarylated imidazolium salts lacks a short and efficient
synthesis. We report a new gold-catalyzed [2+2+1] annula-
tion of aryldiazo nitriles and two imines to achieve efficient
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Conflict of interest
The authors declare no conflict of interest.
Keywords: annulations · arylation ·
density functional calculations · diazo compounds · gold
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Manuscript received: February 1, 2017
506 – 516; b) M. R. Fructos, T. R. Belderrain, P. de Fremont,
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Annulations
S. K. Pawar, M.-C. Yang, M.-D. Su,*
R.-S. Liu*
&&&& — &&&&
Gold-Catalyzed Oxidative [2+2+1]
Annulations of Aryldiazo Nitriles with
Imines To Yield Polyarylated Imidazolium
Salts
Five rings for a ring of five: The title
reactions between two imines and one a-
cyano arylgold carbene efficiently afford
polyarylated imidazolium salts and
molecular hydrogen. Density functional
calculations suggest that the success of
this annulation relies on a 6p electro-
cyclization of cyano-free intermediates
with cis-configured imines as initial
reagents.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!