Ruthenium porphyrin catalyzed friedel-crafts type reaction of arenes with imines

Article abstract of DOI:10.3987/COM-12-12556

Cationic ruthenium porphyrin complex was found to be an efficient catalyst for the Friedel-Crafts type reaction of arenes with imines. The use of a structurally rigid tetradentate porphyrin as the equatorial ligand and weakly coordinating axial ligand is the key to bring out the catalytic reactivity of ruthenium for the reaction.

Full text of DOI:10.3987/COM-12-12556

HETEROCYCLES, Vol. 85, No. 10, 2012
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HETEROCYCLES, Vol. 85, No. 10, 2012, pp. 2415 - 2419. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 26th July, 2012, Accepted, 29th August, 2012, Published online, 6th September, 2012
DOI: 10.3987/COM-12-12556
RUTHENIUM PORPHYRIN CATALYZED FRIEDEL–CRAFTS TYPE
REACTION OF ARENES WITH IMINES
Takuma Terada, Takuya Kurahashi,* and Seijiro Matsubara*
Department of Material Chemistry, Graduate School of Engineering, Kyoto
University, Kyoto 615-8510, Japan; tkuraha@orgrxn.mbox.media.kyoto-u.ac.jp;
matsubar@orgrxn.mbox.media.kyoto-u.ac.jp
Abstract – Cationic ruthenium porphyrin complex was found to be an efficient
catalyst for the Friedel–Crafts type reaction of arenes with imines. The use of a
structurally rigid tetradentate porphyrin as the equatorial ligand and weakly
coordinating axial ligand is the key to bring out the catalytic reactivity of
ruthenium for the reaction.
In the past few decades, many transition metal complexes have emerged as powerful catalyst for the
Friedel-Crafts type reaction. The transformation is considered to be triggered by activation of the
electrophiles through coordination to electrophilic metal complex to provide key intermediates such as
alkyl cations and acyl cations.1,2 Thus, taking into account the Lewis acidity of the metal complexes, we
supposed that higher-valent complexes that do not oxidize the substrate are favorable catalysts for the
Friedel-Crafts type reaction. In this context, we presumed that a cationic high-valent metalloporphyrin
would be an ideal catalyst because the structurally rigid tetradentate porphyrin ligand with a large
!-conjugated planar aromatic structure can help in maintaining the high oxidation state of the metal
throughout the catalytic process.3 Herein, we report that the cationic ruthenium porphyrin complex
catalyzes the Friedel-Crafts type reaction of arenes with imines.4
Our investigation began with attempted Friedel-Crafts type reaction of trimethoxybenzene (1a) with
aldimine (2a) in the presence of 1 mol % of cationic ruthenium porphyrin catalyst [Ru(TBPP)](SbF6)2 in
MeCN at ambient temperature for 1 h. This resulted in formation of 3aa in 97% yield (Table 1, entry 1).
Of note, non-ruthenium cationic metalloporphyrin complexes such as [Mn(TPP)]SbF6, [Fe(TPP)]SbF6,
and [Co(TPP)]SbF6 did not show any catalytic activity (entries 2 and 4). We also examined the effect of
various counteranions such as BF4– and Cl–, which would act as axial ligands, on the cationic nature of
ruthenium (entries 5–6).5 The desired product 3aa was not obtained when using other ruthenium
catalysts such as [Ru(TBPP)]Cl2 and [Ru(TBPP)](BF4)2. Among various reaction solvents examined,

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HETEROCYCLES, Vol. 85, No. 10, 2012
toluene gave the best yield of the product (entries 7–9). It was found that the use of mono cationic
ruthenium complex [RuCl(TPPB)](SbF6) as a catalyst resulted in formation of 3aa in slightly lower yield
than that of [Ru(TPPB)](SbF6)2 (entry 10).
Table 1. Metalloporphyrin catalyzed reaction of 1a with 2a
Ts
MeO
MeO
NH
Ph
OMe
Ts
Metalloporphyrin (1 mol %)
Solvent, 25 °C, 1 h
N
+
Ph
MeO
OMe
MeO
1a
2a
3aa
3aa (Yield /%)a
Entry
Metalloporphyrin
Solvent
1
2
3
4
5
[Ru(TBPP)](SbF6)2
[Mn(TPP)]SbF6
[Fe(TPP)]SbF6
[Co(TPP)]SbF6
[Ru(TBPP)]Cl2
MeCN
MeCN
MeCN
MeCN
MeCN
97
<1
<1
<1
<1
6
[Ru(TBPP)](BF4)2
MeCN
<1
7
8
[Ru(TBPP)](SbF6)2
[Ru(TBPP)](SbF6)2
CH2Cl2
92
96
1,4-dioxane
9
[Ru(TBPP)](SbF6)2
toluene
98
97
10
[RuCl(TBPP)](SbF6)
toluene
c
aNMR yields are given. TPP, meso-tetraphenylporphinato.
bTBPP, meso-tetra(4-tert-butyl)phenylporphinato.
With the optimized conditions in hand, we next investigated the scope of the reaction using various
imines and the results are summarized in Table 2. Imines 2 possessing electron-withdrawing and
–donating substituents on the phenyl group also reacted with 1a to afford the correspondingly substituted
3 in excellent yields. It is worth to note that acetoxy, cyano, and nitro substituents were well tolerated
under the present reaction conditions and the corresponding adducts were isolated in excellent yields.
Naphthyl aldimines reacted with 1a in the presence of the ruthenium catalyst to furnish the
correspondingly substituted products 3ag and 3ah respectively. Notably, aliphatic aldimine 2i also
reacted with 1a to afford 3ai in 96% isolated yield.
Imine 2 bearing a nosyl (Ns:

HETEROCYCLES, Vol. 85, No. 10, 2012
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2-nitrobenzenesulfonyl) substituent, which is more easily removable than the tosyl group, reacted with 1a
to yield 3aj in 67% yield. Furthermore, the reaction of imine possessing Boc and Cbz protection group
with 1a provided 3ak and 3al in 90% and 71% isolated yields, respectively.
Table 2. [Ru(TBPP)](SbF6)2 catalyzed reaction of 1a with imines 2
R1
MeO
MeO
NH
R2
OMe
R1
[Ru(TBPP)](SbF6)2 (1 mol %)
toluene, 25 °C, 1 h
N
+
R2
MeO
OMe
MeO
Ts
1a
2
3
Product (Yield /%)a
Ts
Ts
MeO
NH
MeO
NH
MeO
NH
MeO
OMe
MeO
OMe
Cl
MeO
OMe
OAc
3aa 86%
3ab 85%
3ac 83%
Ts
Ts
Ts
MeO
NH
MeO
NH
MeO
NH
MeO
OMe
CN MeO
OMe
CF3 MeO
OMe
NO2
3ad 94%
3ae 72%
3af 71%
Ts
Ts
Ts
MeO
NH
MeO
NH
MeO
NH
MeO
OMe
MeO
OMe
MeO
OMe
3ag 60%
3ah 70%
3ai 96%
Boc
Cbz
Ns
MeO HN
MeO HN
MeO HN
MeO
OMe
3ak 90%
MeO
OMe
3al 71%
MeO
OMe
3aj 67%
aIsolated yields are given.
During the course of our study, we found that the reaction of imine 2a with less stericaly hindered arene
such as 1b in the presence of [Ru(TBPP)](SbF6)2 catalyst furnish 4ba in 59% yield via double arylation

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HETEROCYCLES, Vol. 85, No. 10, 2012
and elimination of tosyl amide (Table 3). Further examination of reaction condition revealed that the
use of [RuCl(TBPP)](SbF6) in place of [Ru(TBPP)](SbF6)2 gave better result and 4ba was obtained in
69% yield. Imines 2 possessing electron-withdrawing and –donating substituents on the phenyl group
also reacted with 1b to afford the correspondingly substituted triarylmethanes 4 in excellent yields.
Naphthyl aldimines 2g and 2h reacted with 1b to furnish 4bg and 4bh in 47% and 49% isolated yields,
respectively. Aliphatic aldimine 2i also reacted with 1b to afford 4bi in 44% yield. It was also found
that the [RuCl(TBPP)](SbF6) catalyzed reaction of 2-methylfuran (1c) in place of 1b with 2a also
afforded 5ca in 77% isolated yield via elimination of tosyl amide (Scheme 1). However, the reaction of
less electron-rich arenes such as toluene and anisole did not afford any products.
Table 3. [RuCl(TBPP)](SbF6) catalyzed reaction of 1b with imines 2
MeO
MeO
R2 OMe
Ts
[RuCl(TBPP)](SbF6) (1 mol %)
toluene, 25 °C, 1 h
N
+
R2
MeO
MeO
OMe
1b
2
4
Product (Yield /%)a
Cl
CN
MeO
OMe
OMe
OMe
MeO
OMe
MeO
OMe
MeO
OMe MeO
OMe MeO
OMe
4ba 69%
4bb 65%
4bd 54%
CF3
NO2
OMe
MeO
MeO
OMe
MeO
MeO
OMe
OMe
MeO
OMe MeO
4bg 47%
4be 52%
4bf 71%
MeO
OMe
MeO
MeO
OMe
OMe
MeO
4bi 44%
4bh 49%
aIsolated yields are given.

HETEROCYCLES, Vol. 85, No. 10, 2012
2419
Scheme 1. [RuCl(TBPP)](SbF6) catalyzed reaction of 1c with imines 2a
Ts
[RuCl(TBPP)](SbF6) (1 mol %)
toluene, 25 °C, 1 h
O
N
+
O
O
1c
2a
5ca 77% yield
In conclusion, we demonstrated the catalytic efficiency of the ruthenium-porphyrin complexes
[Ru(TBPP)](SbF6)2 and [RuCl(TBPP)](SbF6) for the Friedel–Crafts type reaction of imines with arenes.
The high activation ability of the catalyst that competes with the high chemoselectivity could be attributed
to the cationic ruthenium, which in turn was due to the rigid tetradentate planar porphyrin ligand and the
weakly coordinating axial SbF6 ligand. Efforts toward elucidation of the unique reactivity of the cationic
ruthenium-porphyrin complex are underway.
ACKNOWLEDGEMENTS
This work was supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science, and
Technology, Japan. T. K. acknowledges Asahi Glass Foundation, Tokuyama Science Foundation, Kurata
Memorial Hitachi Science and Technology Foundation.
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