RhIII-catalyzed C-H activation: A versatile route towards various polycyclic pyridinium salts

Article abstract of DOI:10.1002/chem.201302290

An efficient and convenient method for the synthesis of highly substituted polycyclic pyridinium salts from the reaction of various 2-aryl-pyr-idines and 2-aryl-sp²-nitrogen-atom-containing heterocycles with alkynes through rhodium(III)-catalyzed C-H activation and annulation under an O₂ atmosphere is described. A possible mechanism that involves the chelation-assisted C-H activation of the 2-arylpyridine substrate, insertion of the alkyne, and reductive elimination is proposed. This mechanism was supported by the isolation of a five-mem-bered rhodacycle (I'). In addition, kinetic isotope studies were performed to understand the intimate reaction mechanism.

Full text of DOI:10.1002/chem.201302290

FULL PAPER
DOI: 10.1002/chem.201302290
III
À
Rh -Catalyzed C H Activation: A Versatile Route towards Various
Polycyclic Pyridinium Salts
Ching-Zong Luo, Parthasarathy Gandeepan, Jayachandran Jayakumar,
Kanniyappan Parthasarathy, Yu-Wei Chang, and Chien-Hong Cheng*[a]
Abstract: An efficient and convenient
method for the synthesis of highly sub-
stituted polycyclic pyridinium salts
from the reaction of various 2-aryl-pyr-
idines and 2-aryl-sp2-nitrogen-atom-
containing heterocycles with alkynes
À
atmosphere is described. A possible
mechanism that involves the chelation-
pyridine substrate, insertion of the
alkyne, and reductive elimination is
proposed. This mechanism was sup-
ported by the isolation of a five-mem-
bered rhodacycle (I’). In addition, ki-
netic isotope studies were performed
to understand the intimate reaction
mechanism.
À
assisted C H activation of the 2-aryl-
À
Keywords: alkynes · C H activa-
tion
·
fluorescence
·
pyridinium
through rhodiumACHTUNGTRENNUNG(III)-catalyzed C H
activation and annulation under an O2
salts · rhodium
Introduction
tion of benzaldimines (or aryl aldehydes and amines) as the
key step.[6] Earlier, Brennessel and Jones had shown that
isoquinolinium salts were formed from the reactions of
2-phenylpyridine and benzo[h]quinoline with dimethyl ace-
tylenedicarboxylate (DMAD), mediated by RhCp* com-
plexes.[7] In 2011, Sanford and co-workers developed a Pd-
catalyzed synthesis of cyclic pyridinium salts through the
Highly substituted pyridinium salts are important intermedi-
ates for the synthetic organic chemistry and are found in
many naturally occurring compounds that exhibit a variety
of biological activities.[1] Owing to their broad applications,
several traditional and metal-mediated methods have been
developed for the synthesis of pyridinium salts and related
compounds.[2] In addition, the transition-metal-catalyzed
syntheses of isoquinolines from N-tert-butyl o-halobenzald-
3
À
pyridine-directed olefination of the C
(sp ) H bond and in-
tramolecular Michael addition.[8] Our continued interest in
[9]
À
transition-metal-catalyzed C H activation and the synthe-
imines,[3a–c] N-tert-butyl benzaldimines,[3d] aryl ketoximes,[3e–h]
sis of N
N
À
and alkynes have been reported. In these reactions, isoqui-
nolinium salts were probably the intermediates. As part of
our program concerning the synthesis and chemistry of iso-
quinolinium salts, we reported an efficient nickel-catalyzed
synthesis of isoquinolinium salts from the reactions of
2-halobenzaldimines[4a,b] and ortho-iodoketimines with alky-
plore the formation of polycyclic pyridinium salts by C H
activation. Herein, we report the synthesis of various polycy-
clic pyridinium salts by using substituted 2-phenylpyridine
and NACHTNUGRTNEUNG
(sp2)-containing substrates with alkynes in good-to-ex-
cellent yields under very mild reaction conditions. During
the preparation of this manuscript, a report has appeared on
the synthesis of similar compounds.[10] The reported reaction
conditions required heating the reaction mixtures in MeOH
at 1208C for 22 h in the presence of O2 and one equivalent
nes.[4c] In most of these reactions, a carbon halogen moiety
À
was used as the reactive species.
In recent years, Rh and Ru catalysts have stood out
among the various transition-metal complexes for the func-
of strong acid (HOTf) by using [RhCp*ACTHNUTRGNEUNG
(H2O)3]2+ as a cata-
À
tionalization of C H bonds, owing to the high efficiency, se-
lyst. In contrast, we carried out similar reactions in 1,2-dime-
thoxyethane (DME) at ambient temperature under 1 atmo-
lectivity, and functional-group tolerance of these catalysts.[5]
À
Prompted by the various examples of C H activation cata-
lyzed by Rh and Ru complexes, we successfully demonstrat-
ed that isoquinolinium salts could also be synthesized
ACHTUNGRTNENsUNG phere of O2 by using [(RhCl2Cp*)2] as the catalyst and Cu-
ACHTNUGERTN(NUGN BF4)2 as the co-catalyst and BF4 source. No acid was
added during the reaction (see below for details). In addi-
tion, to understand the intimate mechanism of this rhodium-
catalyzed reaction, we studied the kinetic isotope effects of
À
À
through the rhodium- or ruthenium-catalyzed C H activa-
À
the catalytic reaction to confirm that cleavage of the C H
bond was the product-determining step.
[a] C.-Z. Luo, Dr. P. Gandeepan, J. Jayakumar, Dr. K. Parthasarathy,
Y.-W. Chang, Prof. Dr. C.-H. Cheng
Department of Chemistry
National Tsing Hua University
Hsinchu, 30013 (Taiwan)
Fax : (+86)3572-4698
Results and Discussion
E-mail: chcheng@mx.nthu.edu.tw
The treatment of 2-phenylpyridine (1a) with diphenylacety-
lene (2a) in the presence of 1 mol% [(RhCl2Cp*)2] and Cu-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302290.
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Table 1. Reactions of 2-phenylpyridines 1 with alkynes 2.[a]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
which contained a pyridoACTHNUTRGENUGNisoquinolinium cation and
a tetrafluoroborate anion, was confirmed by 1H,
13C, 19F, and 11B NMR spectroscopy and by mass
spectrometry.
To evaluate the scope of this catalytic formation
of pyridoisoquinolinium salts, we examined the re-
actions of several substituted 2-phenylpyridines
(1b–1n) with diphenylacetylene (2a) under the
same reaction conditions (Table 1). Thus, 4-methyl-
, 4-tert-butyl-, 4-methoxy-, 4-acyl-, and 4-fluorophe-
À
nylpyridines (1b–1 f) effectively underwent C H
activation and annulation reactions with compound
2a, thereby affording their corresponding pyrido-
ACHTUNGTRENNUNGisoquinolinium salts (3ba–3 fa) in 97–88% yield
(Table 1, entries 2–6). These results indicated that
both electron-withdrawing and electron-donating
arylpyridines worked well in the catalytic reaction.
Similarly, the reactions of 2-(2-methylphenyl)pyri-
dine (1g) and 2-(2-isopropylphenyl)pyridine (1h)
with compound 2a proceeded smoothly to give
compounds 3ga and 3ha in 89 and 91% yield, re-
spectively (Table 1, entries 7 and 8). In addition,
2-(3-methylphenyl)pyridine (1i) reacted nicely with
compound 2a to give compound 3ia in 91% yield
(Table 1, entry 9). In the reaction of 3-methylaryl-
1
2
Product 3
3aa: R1 =H
Yield [%][b]
1
2
3
4
5
6
1a
1b
1c
1d
1e
1 f
2a
2a
2a
2a
2a
2a
94
91
89
97
93
88
3ba: R1 =Me
3ca: R1 =tBu
3da: R1 =OMe
3ea: R1 =COMe
3 fa: R1 =F
7
8
1g
1h
2a
2a
3ga: R1 =Me
89[c]
3ha: R1 =iPr
91[d]
À
pyridine, there were two possible C H activation
sites, that is, at the C2 and C6 positions of com-
À
pound 1i, but the C H activation only occurred at
the C6 position, likely owing to the steric effect of
the methyl group at the C3 position. In a similar
manner, 2-(3,5-dimethylphenyl)pyridine (1j) and 2-
(2,5-dimethoxyphenyl)pyridine (1k) gave pyrido-
9
1i
1j
2a
2a
2a
3ia
3ja
3ka
91
ACHTUNGTRENNUNGisoquinolinium salts 3ja and 3ka in 88 and 91%
10
11
88[d]
yield, respectively (Table 1, entries 10 and 11). To
evaluate the scope of the present catalytic reaction,
we examined the reactions of substrates 1 that con-
tained substituents on the pyridine ring (1l–1n)
with compound 2a under the standard reaction
conditions. Thus, 5-methyl- (1l), 5-methoxy- (1m),
and 3-methoxy-substituted pyridines (1n) effective-
1k
91[d]
12
13
1l
1m
2a
2a
3la: R1 =Me
90
95
À
ly underwent C H activation and annulation reac-
3ma: R1 =OMe
tions with compound 2a, thereby affording salt
products 3la, 3ma, and 3na in 90, 95, and 95%
yield, respectively (Table 1, entries 12–14). Ben-
zo[h]quinoline (1o) and 4,6-dimethyldibenzo-
ACHTUNGTRENNUNG[f,h]quinoline (1p) also reacted well with com-
pounds 2c and 2a to give compounds 3oc and 3pa
in 89 and 87% yield, respectively (Table 1, en-
tries 15 and 16).
14
15
1n
1o
2a
2c
3na
95[d]
In addition to compound 2a, other symmetrical
À
alkynes (2b–2 f) were also tested in the C H acti-
3oc: R3, R4 =4-OMeC6H4
89
vation and annulation reaction. Thus, methyl- (2b),
methoxy- (2c), bromo- (2d), and fluoro-substituted
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III
À
Rh -Catalyzed C H Activation
FULL PAPER
À
diphenylacetylenes (2e) underwent C H activation
and annulation with compound 1a to afford the
corresponding pyridoisoquinolinium salts (3ab–
3ae) in good-to-excellent yields (Table 1, en-
tries 17–20). Likewise, treatment of 4-octyne (2 f)
with compound 1a provided the corresponding salt
(3af) in 88% yield (Table 1, entry 21).
Table 1. (Continued)
1
2
Product 3
Yield [%][b]
16
1p
2a
3pa
87
17
18
19
20
21
2b
2c
2d
2e
2 f
3ab: R3, R4 =4-MeC6H4
85
92
91
71
88
To understand the regioselectivity of this reac-
tion, unsymmetrical alkynes 2g–2j were also inves-
tigated. Thus, 1-phenyl-1-propyne (2g) and
1-phenyl-1-butyne (2h) reacted efficiently with
compound 1a to give two regioisomeric products
in excellent yields with high regioselectivities
(Table 1, entries 22 and 23). Similarly, two re-
gioisomers were observed in the reaction of 3-(p-
methylphenyl)propargyl alcohol (2i) with com-
pound 1a, thus forming regioisomeric products 3ai
and 3ai’ in 79% yield in a 95:5 ratio (Table 1,
entry 24). The other internal alkyne that we tested
in the reaction with compound 1a, 4,4-dimethyl-2-
pentyne (2j), afforded regioisomers 3aj and 3aj’ in
a 7:1 ratio in a combined 78% yield (Table 1,
entry 25). The reaction of 2,4-diphenylquinoline
(1q) with compound 2a afforded pyridoisoquinoli-
nium salt 3qa in 93% yield. Similarly, 2-phenyl-
4,5-dihydrooxazole (1r), 1,2-diphenyl-1H-benzo[-
d]imidazole (1s), and 2-(benzofuran-2-yl)benzo[d]-
thiazole (1t) reacted with compound 2a at higher
reaction temperatures to form their corresponding
polycyclic pyridinium salts (3ra–3ta) in 84, 95, and
69% yield, respectively. The presence of five-mem-
bered heterocycles, including oxazoline, imidazole,
and thiazole groups in these substrates (1r–1t) ap-
peared to be the cause of the higher reaction tem-
peratures. The presence of five-membered rings
would require a higher activation energy in the re-
ductive elimination step (Scheme 1) to give the
final products, owing to the presence of angular
strain. It is noteworthy that no such products (3ra–
3ta) have been synthesized previously.[10] The reac-
tion of 2-(naphthalen-1-yl)pyridine (1u) with com-
pound 2a provided compound 3ua in excellent
yield under similar reaction conditions (Table 1,
entry 30). In a similar manner, 2-(naphthalen-2-yl)-
pyridine (1v) also reacted with compound 2a to
form the corresponding pyridinium salt (3va) in
92% yield (Table 1, entry 31). There were two pos-
3ac: R3, R4 =4-OMeC6H4
3ad: R3, R4 =4-BrC6H4
3ae: R3, R4 =4-FC6H4
3af: R3, R4 =(CH2)2CH3
1a
22
23
24
1a
1a
1a
2g
2h
2i
3ag
90 (6:1)[e]
89 (7:1)[e]
79 (95:5)[e]
3ah
3ai: R4 =4-MeC6H4
25
1a
2j
3aj
78 (7:1)[e]
26
27
28
29
1q
1r
1s
1t
2a
2a
2a
2a
3qa
3ra
3sa
3ta
93[d]
84[d,f]
95[g]
69[h,f]
30
31
1u
1v
2a
2a
3ua
3va
97[d]
92[d]
À
sible C H activation sites in substrate 1v, that is, at
the C1 and C3 positions. However, the activation
only occurred at the C3 position, owing to the
steric effect of the fused aromatic ring.
[a] All of the reactions were carried out by using 2-arylpyridine 1 (0.34 mmol), alkyne
2 (0.28 mmol), [(RhCl2Cp*)2] (1.0 mol%), and Cu(BF4)2·6H2O (0.14 mmol) in DME
AHCTUNGTRENNUNG
(3 mL) at 258C for 24 h under 1 atm O2. [b] Yield of isolated product. [c] The reaction
was carried out for 48 h. [d] The reaction was carried out at 608C. [e] The ratio of the
regioisomers was determined by 1H NMR analysis and is given in parentheses; the
major isomer is shown. [f] 10 mol% AgBF4 was used. [g] The reaction was carried out
in tAmylOH at 1008C. [h] The reaction was carried out with 2-ethoxyethanol at
1208C for 48 h.
On the basis of the known metal-catalyzed, di-
À
recting-group-assisted C H activation and annula-
tion reactions,[5–11] a plausible mechanism for the
rhodium-catalyzed annulation of 2-phenylpyridine
(1a) with compound 2a is proposed in Scheme 1.
The first step likely involves the coordination of
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C.-H. Cheng et al.
Scheme 1. Proposed mechanism for the formation of pyridoisoquinolini-
um salt 3aa.
the pyridine group of compound 1a to the rhodium metal
À
center, followed by cyclometalation of an ortho-phenyl C H
bond to the rhodium
A
rhodacycle I. The coordination of alkyne 2a to complex I
gives intermediate II and subsequent insertion of the coordi-
À
nated alkyne to the Rh C bond affords seven-membered
rhodacycle III. Finally, reductive elimination from structure
III gives the pyridoisoquinolinium salt product (3aa) and a
rhodium(I) species. Oxidation of the latter species regener-
ates the active rhodium
To support this proposed mechanism, we tried to isolate
key intermediate (Scheme 1). Thus, 2-phenylpyridine
ACHTUNGTREN(NGNU III) catalyst.
I
(0.40 mmol) was heated in the presence of [(RhCl2Cp*)2]
(0.16 mmol) and AgBF4 (0.80 mmol) in MeOH at 608C for
8 h, which led to the isolation of five-membered rhodacycle
I in 75% yield. However, attempts to crystalize intermediate
I by using various solvents under different conditions failed.
Based on our previous experiments,[6] intermediate I was
treated with NaI in MeOH at room temperature for 0.15 h
to afford rhodium intermediate I’, which was isolated in
70% yield (Scheme 2). Single crystals of complex I’ were
readily obtained from its solution in MeOH and its structure
was determined by X-ray diffraction. As shown in Scheme 2,
the rhodium complex is an 18-electron system that contains
a cyclometalated 2-phenylpyridine moiety and a pentame-
thylcyclopentadienyl ligand, in addition to a directly coordi-
nated iodide ligand. The reaction of complex I’ with com-
pound 2a in DME did not afford any pyridoisoquinolinium
product (3) at room temperature; the starting materials
were recovered in almost-quantitative yield after 15 h at
room temperature.[7,12a] However, complex I’ reacted with
compound 2a in MeOH at 608C to afford compound 3aa’ in
95% yield with an IÀ counteranion; the structure was con-
Scheme 2. Structure and reactivity of intermediate I’.
duced compound 3aa in 90% yield, but with no incorpora-
tion of deuterium into the product, thus indicating that no
deuterium/hydrogen exchange had occurred during the cata-
lytic reaction (Scheme 3). To validate this result, we treated
compound 1a under our standard reaction conditions for
24 h and compound 1a was recovered in quantitative yield,
with no deuterium/hydrogen exchange observed. These ex-
periments indicated that the cyclometalation of RhIII with
substrate 1a was irreversible. To further understand the in-
herent nature of the mechanism of this catalytic reaction,
we determined the inter- and intramolecular kinetic isotopic
effects (KIEs) in the catalytic reaction of compound 1a with
compound 2a. An intermolecular KIE (kH/kD) of 2.03 was
observed for the competition reaction between compound
1a and deuterium-labeled compound [D5]-1a with com-
pound 2a (Scheme 3). In addition, an intramolecular compe-
tition experiment between compounds [D1]-1a and 2a
showed a kH/kD of 2.84. The difference between the intermo-
lecular and intramolecular KIE values indicated that the
1
firmed by H and 13C spectroscopy, HRMS, and X-ray dif-
fraction. On the other hand, the addition of 0.5 equivalents
of CuACHTUNGTRENNUNG(BF4)2·6H2O to the reaction of complex I’ with com-
pound 2a in DME led to the corresponding pyridoisoquino-
linium salt (3aa) in 93% yield at room temperature.
To gain more-detailed information on the mechanism of
this catalytic reaction, we carried out the reaction between
compounds 1a and 2a under standard conditions with the
addition of 10 equivalents of CD3OD. The reaction pro-
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III
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Rh -Catalyzed C H Activation
FULL PAPER
idinium functional group. We have been interested in the
search for highly efficient materials for application in organ-
ic light-emitting diodes (OLEDs).[14] It is known that im-
proved OLED device performance has been achieved by
using ionic materials as the dopants.[15] Moreover, pyridini-
um salts are also found to be potential candidates for use in
the electron-injection layer in organic light-emitting diodes,
owing to their relatively low HOMO and LUMO levels.[16]
The ionic character of these materials also make them excel-
lent candidates as OLED materials for solution processing.
Investigations in this direction are underway.
Conclusion
We have successfully developed a convenient method for
the synthesis of highly substituted isoquinolinium salts
À
through a rhodium-catalyzed C H activation/cyclization se-
quence in one pot under very mild reaction conditions. Our
proposed mechanism was strongly supported by the isola-
tion of a five-membered rhodacycle intermediate. Kinetic
À
isotope labeling studies implied that the C H activation
step followed the irreversible cyclometalation step and that
À
the initial C H activation may be the rate-limiting step. Pri-
mary photophysical studies were also performed on the new
pyridinium salt compounds. Further studies towards intra-
Scheme 3. Kinetic isotope study of the RhIII-catalyzed formation of the
pyridoisoquinolinium salts.
À
molecular C H activation and the application of these com-
pounds in OLED materials are underway in our laboratory.
pre-binding of substrate 1a (likely through the pyridine-ni-
trogen atom) to the RhIII center took place prior to an irre-
Experimental Section
À
versible cleavage of the C H bond to form intermediate I.
General procedure for the synthesis of pyridinium salts: A sealed tube
À
The observed primary KIE of 2.84 suggests that direct C H
that contained [(RhCl2Cp*)2] (1.0 mol%) and Cu
(0.14 mmol) was evacuated and purged with oxygen gas three times.
Then, solution of 2-arylpyridine (0.34 mmol) and alkyne 2
ACHTNUGRTNE(NUGN BF4)2·6H2O
bond cleavage is involved in the cyclometalation step and is
the product-determining step.[13]
a
1
As shown in Figure 1, the synthesized polycyclic pyridini-
um salts showed deep-blue-to-green fluorescence emission,
depending on the structures of the salts. These salts appear
to be a new class of fluorescent materials that contain a pyr-
(0.28 mmol) in 1,2-dimethoxyethane (3.0 mL) was added to the system by
syringe under an oxygen atmosphere and the reaction was stirred at
258C for 24 h under an O2 atmosphere. At the end of the reaction, the
mixture was diluted with CH2Cl2 (10 mL) and filtered through a pad of
celite, which was washed several times with CH2Cl2 (50 mL). The com-
bined filtrate was concentrated in vacuo and the mixture was purified by
column chromatography on silica gel (CH2Cl2/MeOH, 95:5) to afford the
desired pure product (3).
Acknowledgements
We thank the National Science Council of the Republic of China (NSC-
101-2628-M-007-005) for support of this research.
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Received: June 17, 2013
Published online: && &&, 0000
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

III
À
Rh -Catalyzed C H Activation
FULL PAPER
À
C H Activation
C.-Z. Luo, P. Gandeepan,
J. Jayakumar, K. Parthasarathy,
Y.-W. Chang, C.-H. Cheng* &&&& — &&&&
RhIII and easy: A convenient synthesis
atmoACTHNGUETRNNUsG phere at ambient temperature is
III
À
Rh -Catalyzed C H Activation: A
of polycyclic pyridinium salts in excel-
described (see scheme). These salts
showed strong fluorescence in CH2Cl2.
DME=1,2-dimethoxyethane.
lent yields through RhIII-catalyzed
Versatile Route towards Various Poly-
cyclic Pyridinium Salts
II
À
C H activation under an O2/Cu
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!