Ruthenium(II)-Catalyzed Oxidative Annulation Reactions of Arylimidazolium Salts via N-Heterocyclic Carbene-Directed C-H Activation

Article abstract of DOI:10.1002/adsc.201500788

An efficient ruthenium(II)-catalyzed oxidative annulation reaction of various arylimidazolium salts with alkynes via N-heterocyclic carbene-directed C-H activation to obtain substituted benzo[ij]imidazo[2,1,5-de]quinolizinium salts is reported. This catalytic reaction proceeds in an excellent regioselective manner when using unsymmetrical alkynes as reactants. The intermediate mono-annulated products can be obtained by reducing the amount of catalyst. Two catalytically competent N-heterocyclic carbene-based cyclometallated ruthenium(II) complexes have been isolated and characterized, which represent the key intermediates in the catalytic cycle. Moreover, most of the products show a strong fluorescent property, indicating their potential for making new light-emitting materials.

Full text of DOI:10.1002/adsc.201500788

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DOI: 10.1002/adsc.201500788
Ruthenium(II)-Catalyzed Oxidative Annulation Reactions of
Arylimidazolium Salts via N-Heterocyclic Carbene-Directed
À
C H Activation
a
a
a
a
a
a,b,
Renhe Li, Yang Hu, Ran Liu, Ruofang Hu, Bin Li, and Baiquan Wang *
a
State Key Laboratory of Elemento-Organic Chemistry, Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin 300071, Peopleꢀs Republic of China
Fax: (+86)-22-2350-4781; e-mail: bqwang@nankai.edu.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, Shanghai 200032, Peopleꢀs Republic of China
Received: August 22, 2015; Revised: September 30, 2015; Published online: December 4, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500788.
Abstract: An efficient ruthenium(II)-catalyzed oxi-
dative annulation reaction of various arylimidazoli-
um salts with alkynes via N-heterocyclic carbene-di-
rected CÀH activation to obtain substituted ben-
sive and efficient catalytic systems for oxidative annu-
lations reactions with alkynes.
[2,3]
As well-known ligands in organometallic chemistry,
N-heterocyclic carbenes (NHCs) can also perform as
a directing group which undergoes facile CÀH activa-
zo[ij]imidazo[2,1,5-de]quinolizinium salts is report-
ed. This catalytic reaction proceeds in an excellent
regioselective manner when using unsymmetrical al-
kynes as reactants. The intermediate mono-annulat-
ed products can be obtained by reducing the
amount of catalyst. Two catalytically competent N-
heterocyclic carbene-based cyclometallated ruthe-
nium(II) complexes have been isolated and charac-
terized, which represent the key intermediates in
the catalytic cycle. Moreover, most of the products
show a strong fluorescent property, indicating their
potential for making new light-emitting materials.
tion with ruthenium complexes and then generates
[4,5]
stable NHC-based cyclometallated species.
Our
group recently developed a new and efficient protocol
for the synthesis of NHC-based cycloruthenium com-
plexes, which can then react with alkynes to obtain
a new kind of imidazolium salts through the insertion
of alkynes into the RuÀC bond and subsequent reduc-
[5f]
tive elimination [Scheme 1, Eqs. (1) and (2)]. How-
ever, although the stoichiometric reaction between
NHC-based cycloruthenium complexes and alkynes
has been achieved, the transition metal-catalyzed oxi-
dative annulation reaction of NHCs or imidazolium
[6]
Keywords: annulation; arylimidazolium salts; CÀH
salts has remained elusive until now, and there is no
report on the ruthenium(II)-catalyzed oxidative annu-
lation reaction of NHCs or imidazolium salts with al-
kynes. Based on our previous work on NHC-Ru com-
plexes and Ru(II)-catalyzed CÀH activation reac-
activation; N-heterocyclic carbenes; ruthenium
[3b,d,r,5a,f,7]
tions,
herein, we report an efficient Ru(II)-
Recently, the transition metal-catalyzed oxidative an- catalyzed cascade oxidative annulation reaction of
nulation reaction via CÀH bond activation has been various arylimidazolium salts with alkynes to obtain
shown to play a very important role in the prepara- substituted
tion of pharmaceuticals and natural products. Due salts in good yields [Scheme 1, Eq. (3)].
benzo[ij]imidazo[2,1,5-de]quinolizinium
[1]
to the potential to streamline the organic synthesis by
In order to evaluate the reaction conditions for the
avoiding the prior preparation of activated substrates, oxidative annulation reaction, we initiated our study
CÀH activation reactions successfully render shorter by treating the phenylimidazolium salt (1a) with di-
synthetic routes and make them become more step- phenylacetylene (2a) in the presence of [(p-cym-
and atom-economic. Compared to other transition ene)RuCl ] (5 mol%) and Cu(OAc) ·H O (4.0 equiv.)
2
2
2
2
metals catalysts including rhodium and palladium, in MeOH at 808C for 12 h under argon (Table 1). The
their characteristics of being easy to prepare and ex- reaction afforded mono-annulated product 4aa in
hibiting good stability help ruthenium(II) species 55% yield, but bis-annulated product 3aa was not de-
make a great contribution to the design of less-expen- tected under these conditions (entry 1). Switching the
solvent to t-AmOH and CF CH OH would lower the
3
2
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Renhe Li et al.
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1
0 mol%, both 3aa and 4aa were obtained in 30%
and 45% yields, respectively (entry 4). Raising the re-
action temperature and prolonging the reaction time
would significantly increase the overall yield and the
selectivity for 3aa (entries 5–7). To our delight, when
AgSbF was added to the reaction system as an addi-
6
tive product 3aa was obtained in nearly quantitative
yield (99%) (entry 8). Nevertheless, the bis-annulated
product 3aa could not be obtained when the loading
of catalyst was reduced to 5 mol%, even though
2
0 mol% of AgSbF6 was added as an additive
(
entry 9), or when the amount of Cu(OAc) ·H O was
2
2
reduced to 2.2 equiv. , even under an oxygen atmos-
phere (entry 10), or when using water as solvent
(
entry 11). The solvent screening disclosed that
MeOH was the most efficient medium for this reac-
tion (entries 11 and 12). Furthermore, no product was
generated when the reaction was carried out in the
absence of [(p-cymene)RuCl2]2 catalyst (entry 13).
Therefore, we chose the reaction conditions in entry 8
as the optimal reaction conditions {10 mol% [(p-cym-
ene)RuCl ] as catalyst, 4.4 equiv. of Cu(OAc) ·H O
2
2
2
2
as oxidant, 40 mol% AgSbF as additive, MeOH as
6
Scheme 1. Stoichiometric reaction of cycloruthenium com- solvent at 1308C for 24 h}. The structures of 3aa and
1
13
plexes with alkynes and Ru(II)-catalyzed oxidative annula- 4aa were confirmed by H and C NMR spectroscopy
tion reaction.
and high-resolution mass spectrometry (HR-MS).
With the optimized reaction conditions in hand, we
yield of 4aa (entries 2 and 3). Interestingly, when the first surveyed the reaction scope using a series of
loading of [(p-cymene)RuCl2]2 was increased to para-substitued phenylimidazolium salts (Table 2).
[a]
Table 1. Optimization of the reaction conditions.
Entry [(p-Cymene)RuCl2]2
mol%]
Additive
[mol%]
Solvent
Temperature
[8C]
Time
[h]
Yield of 3aa
[%]
Yield of 4aa
[%]
[
[
[
[
[
[
b]
b]
b]
b]
b]
1
2
3
4
5
6
7
8
9
1
1
1
1
5
5
5
–
–
–
–
–
-
MeOH
t-AmOH
80
80
12
12
12
12
12
12
24
24
24
24
24
24
24
–
–
–
30
57
83
92
99
–
–
–
87
–
55
12
31
45
CF CH OH 80
3
2
10
10
10
10
10
5
10
10
10
–
MeOH
80
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
100
130
130
130
130
130
130
130
130
41
trace
trace
trace
51
78
86
–
AgSbF (40)
AgSbF (20)
AgSbF (40)
AgSbF (40)
AgSbF (40)
AgSbF (40)
6
6
[c]
0
1
2
3
6
H O
6
2
EtOH
MeOH
–
–
6
6
[a]
Reaction conditions: 1a (0.2 mmol), 2a (0.44 mmol), catalyst, Cu(OAc) ·H O (4.4 equiv.), additive, solvent (2.0 mL) at the
2
2
indicated temperature for 12 h or 24 h, under argon; isolated yields are reported.
[
[
b]
1
a (0.2 mmol), 2a (0.4 mmol), Cu(OAc) ·H O (4.0 equiv.).
Cu(OAc) ·H O (2.2 equiv.) under oxygen.
2
2
c]
2
2
3
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Ruthenium(II)-Catalyzed Oxidative Annulation Reactions
Table 2. Substrate scope of the oxidative annulation reac- phenylacetylenes 2b and 2d the reaction proceeded
[
a]
tion.
smoothly to afford the corresponding products 3ab
and 3ad in good yields (83% and 78%, respectively).
Notably, the electronic nature of the phenyl moiety in
2
also has an obvious effect on the efficiency of the
reaction. The electron-rich aryl-substituted alkyne 3c
could only give the bis-annulated product 3ac in 36%
yield, and the electron-poor aryl-substituted 3f even
could not provide the desired bis-annulated product.
Only the mono-annulated product 4af was obtained
in 40% yield. Additionally, 3-hexyne 2g could react
with imidazolium salt 1a to give the mono-annulated
product 4ag in 60% yield. To further understand the
regioselectivity of the present reaction, 1-phenyl-1-
propyne 2e and 1-phenyl-1-butyne 2h were investigat-
ed. To our surprise, both two unsymmetrical alkynes
underwent the oxidative annulation reaction with imi-
dazolium salt 1a to give a single isomer in good yields
(
3ae and 4ah), even though for 2h only the mono-an-
nulated product was obtained.
It is worth mentioning that most of the benzo[ij]i-
midazo[2,1,5-de]quinolizinium salts 3 exhibit strong
fluorescence emission at the wavelength range of
4
64–475 nm. The fluorescence spectra of 3aa, 3ga,
3
ha, 3oa, 4ia and 3ac recorded for dichloromethane
solutions are shown in Figure 1. Compared to the
mono-annulated compound 4ia, the bis-annulated
products 3 show a significant intense fluorescence
emission. In addition, the signal intensity of 3aa, 3ga
and 3ha follows the order 3ga>3aa>3ha, suggesting
that the electron-donating group on the para-position
of the phenyl ring can increase the fluorescence inten-
sity, while the electron-withdrawing group on the
same position will decrease the fluorescence intensity.
Moreover, the electron-donating group on the diphe-
nylacetylene has little impact on the fluorescence in-
[
[
a]
b]
Reaction conditions: 1a (0.2 mmol), 2a (0.44 mmol),
Cu(OAc) ·H O (0.88 mmol), [(p-cymene)RuCl2]2
10 mol%), AgSbF6 (40 mol%), MeOH (2.0 mL) at
2
2
(
1308C for 24 h under argon; isolated yields are reported.
1
a (0.2 mmol), 2a (0.66 mmol), Cu(OAc) ·H O
2
2
(1.32 mmol).
Phenylimidazolium salts with halo, alkyl, alkoxy, and tensity (Figure 1, 3ac), and the N-alkyl group can also
ester groups at the para-position of the phenyl ring increase the fluorescence intensity (Figure 1, 3oa).
(1b–h) reacted with 2a to yield bis-annulated products
3
ba–ha in good to excellent yields (68–99%). It is
worth noting that electronic effects of the substituents
have a significant impact on this reaction. No desired
bis-annulated products were detected when substrates
with strong electron-donating (1i) or electron-with-
drawing groups (1j) were used. Instead, mono-annu-
lated products 4ia and 4ja were obtained in 78% and
44% yields, respectively. This is probably due to the
fact that electron-withdrawing and electron-donating
groups may destabilize the cycloruthenium intermedi-
ate in the second step oxidative annulation reaction.
In addition, substrates bearing N-para-substituted
benzyl and N-alkyl groups (1k–o) could also provide
the corresponding bis-annulated products 3ka–oa in
good yields (79–89%).
To further demonstrate the generality and practica-
bility of this reaction, an array of alkynes 2b–h was Figure 1. Fluorescence spectra of 3aa, 3ga, 3ha, 3oa, 4ia, and
À5
also tested. For 4-fluoro- and 4-methyl-substituted di- 3ac (2”10 M in CH Cl ).
2
2
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Scheme 2. Kinetic isotope effect experiments.
To gain some mechanistic insights into the current whole oxidative annulation reaction was found to be
protocol, KIE (kinetic isotope effect) experiments 2.8, which is consistent with the conclusion drawn
were performed (Scheme 2). For the first oxidative from the KIE in the second step [Scheme 2, Eqs. (8)
annulation reaction to form the mono-annulated and (9)].
product 4aa, the KIE value of 1.2 indicates that the
In addition, the H/D exchange reactions were also
cleavage of the CÀH bond on the phenyl ring in carried out (Scheme 3). When the reaction was per-
which NHC acts as a directing group is not involved formed in the absence of 2a, for the recovered 1a
in the rate-determining step [Scheme 2, Eqs. (4) and both the C(Ph)ÀH and C(5-im)ÀH were deuterated
(
5)]. Interestingly, the KIE value of 3.3 in the second [Scheme 3, Eq. (10)], which indicates that the activa-
oxidative annulation reaction where an abnormal tions of these two CÀH bonds are reversible in
NHC acts as a directing group indicates that the nature. To our surprise, no deuteration of the C(2-
cleavage of CÀH bond on the phenyl ring in this step im)ÀH was detected, which may due to the fact that
is actually involved in the rate-determining step once the RuÀC(2-im) bond was formed, it was too
[
8]
[
Scheme 2, Eqs. (6) and (7)]. The KIE value of the strong to be cleaved. Furthermore, the C(Ph)ÀH and
3
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Ruthenium(II)-Catalyzed Oxidative Annulation Reactions
Scheme 3. H/D exchange reaction.
C(5-im)ÀH on the recovered 4aa were also deuterat- tion in the presence of Cu(OAc) ·H O gives the
2
2
ed, indicating that the CÀH activation in the second mono-annulated product 4aa and regenerates the cat-
oxidative annulation reaction is also reversible alytic species I. The next annulation begins with a re-
[
Scheme 3, Eq. (11)].
Fortunately, two cycloruthenium intermediates A followed by the similar coordination and insertion of
and B were isolated through the reactions of 1a and alkyne 2a to give the seven-membered intermediate
aa with [(p-cymene)RuCl ] and Cs CO , respectively III or III’. The subsequent reductive elimination pro-
versible cycloruthenation affording intermediate B,
4
2
2
2
3
[
Scheme 4, Eqs. (12) and (15)]. It should be noted vides the final bis-annulated product 3aa and regener-
that the synthesis and molecular structure of inter- ates the catalytic species I for next catlytic cycle. In
mediate A has already been reported in our previous addition, it is worth noting that although the additive
[
5f]
work.
The structure of intermediate B was fully AgSbF may help generate the cationic [Ru] catalytic
6
1 13
À
À
characterized by H and C NMR spectra and HR- species, the SbF6 anion will be exchanged by Cl in
MS. Since intermediate B is unstable in the air, we the catalytic cycle, which can be proved by the
1
9
failed to obtain a single crystal. Then intermediates
A and B were used as catalyst in place of SbF6 anion could be found in the F NMR spectra
(p-cymene)RuCl ] in the reactions of 1a and 4aa of products 3 and 4.
F NMR. No characteristic peak belonging to the
À
19
[
2
2
with diphenylacetylene to form the corresponding
In summary, we have developed an efficient
products 4aa and 3aa in 99% and 83% yields, respec- Ru(II)-catalyzed oxidative annulation reactions of ar-
tively [Scheme 4, Eqs. (13) and (16)]. Intermediate B ylimidazolium salts with alkynes via NHC-directed
could also catalyze the reaction of 1a with 2a to form CÀH activation to give various benzo[ij]imidazo[2,1,5-
the final bis-annulated product 3aa in 79% yield de]quinolizinium salts in moderate to good yields.
[
Scheme 4, Eq. (17)]. In addition, the stoichiometric This catalytic reaction proceeds in an excellent regio-
reaction of intermediate A with alkyne in the pres- selective manner and the intermediate mono-annulat-
ence of oxidant was also performed, which gave the ed product can be obtained by reducing the amount
final bis-annulated product 3aa in 99% yield of ruthenium catalyst. The reaction mechanism was
[
Scheme 4, Eq. (14)]. Notably, under similar condi- fully studied through isolation of reaction intermedi-
tions without oxidant only the mono-annulated prod- ates, KIE and H-D exchange experiments. Moreover,
[
5f]
uct 4aa was obtained. The stoichiometric reaction the good fluorescence properties of the bis-annulated
of intermediate B with alkyne gave the final product products may help them find a wide application in the
3
aa in 82% yield [Scheme 4, Eq. (18)]. These results area of organic electronic materials.
confirmed that intermediates A and B are probably
two active species in the reaction.
Based on the above mechanism studies and known
Ru(II)-catalyzed oxidative annulation reactions,
a possible mechanism is proposed (Scheme 5). First,
[3]
Experimental Section
General Procedure for Ru(II)-Catalyzed Oxidative
Annulation Reaction of Arylimidazolium Salts with
Alkynes
catalytic species I [(p-cymene)Ru(OAc) ] is formed in
2
situ by reaction with Cu(OAc) ·H O. Then the NHC-
2
2
directed CÀH activation takes place, affording the
five-membered cycloruthenium intermediate A.
Alkyne 2a coordinates to the Ru atom and is inserted
A mixture of arylimidazolium salt 1 (0.2 mmol, 1.0 equiv.),
alkyne
2
(0.44 mmol, 2.2 equiv.), [(p-cymene)RuCl2]2
into the RuÀC bond to deliver the seven-membered (12.2 mg, 0.02 mmol, 10 mol%), Cu(OAc) ·H O (175.6 mg,
2
2
intermediate II or II’. Subsequent reductive elimina- 0.88 mmol, 4.4 equiv.), and AgSbF6 (27.5 mg, 0.08 mmol,
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Scheme 4. Synthesis and catalytic/stoichiometric reaction of intermediates A and B.
4
0 mol%) was added to an oven-dried Schlenk tube
Acknowledgements
equipped with a stir bar. Dry MeOH (2.0 mL) was added to
the tube and the mixture was stirred at 1308C for 24 h
under an argon atmosphere. Afterward, it was transferred to
a round-bottom flask. The solvent was evaporated under re-
duced pressure and the residue was absorbed onto small
amounts of alumina. The purification was performed by
flash column chromatography on alumina (dichlorome-
thane/methanol=50:1).
We are grateful to the National Natural Science Foundation
of China (Nos 21372121, 21372122, 21421062, and J1103306)
for financial support.
3
890
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Ruthenium(II)-Catalyzed Oxidative Annulation Reactions
Scheme 5. Proposed mechanism for the Ru(II)-catalyzed oxidative annulation reaction via double CÀH activation.
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Renhe Li et al.
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[
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