Rhodium(III) porphyrin-catalyzed reactions via activation of alkynes

Article abstract of DOI:10.1246/cl.140810

The cycloisomerization of 1,6-enynes catalyzed by rhodium(III) porphyr in under mild reaction conditions successfully afforded a five-membered ring system. The rhodium porphyrin was found to be a strong π-Lewis acid that could activate alkynes. Thus, rhod

Full text of DOI:10.1246/cl.140810

Received: August 30, 2014 | Accepted: September 9, 2014 | Web Released: December 5, 2014
CL-140810
Rhodium(III) Porphyrin-catalyzed Reactions via Activation of Alkynes
Makoto Hasegawa,1 Takuya Kurahashi,*1,2 and Seijiro Matsubara*1
1Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510
2ACT-C, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
(E-mail: kurahashi.takuya.2c@kyoto-u.ac.jp, matsubara.seijiro.2e@kyoto-u.ac.jp)
Table 1. Optimization of the reaction conditionsa
The cycloisomerization of 1,6-enynes catalyzed by rhodi-
um(III) porphyrin under mild reaction conditions successfully
afforded a five-membered ring system. The rhodium porphyrin
was found to be a strong π-Lewis acid that could activate
alkynes. Thus, rhodium porphyrin-catalyzed intramolecular
Friedel-Crafts-type reactions of alkynes with arenes were also
accomplished. Furthermore, rhodium porphyrin-catalyzed inter-
molecular cyclization of alkynes with styrenes afforded the
indene derivatives.
Ph
E
Rh catalyst (2.5 mol%)
E
E
E
solvent, T °C, 1 h
Ph
E = COOEt
2a
1a
Entry Catalysts
Solvent
Temp./°C Yield/%b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
[Rh(TPP)]TFPB DCE
0
0
0
80
0
80
0
80
0
0
0
0
0
80
0
80
0
80
>99
62
[Rh(TPP)]SbF6
[Rh(TPP)]BF4
[Rh(TPP)]BF4
[Rh(TPP)]PF6
[Rh(TPP)]PF6
[Rh(TPP)]Cl
[Rh(TPP)]Cl
DCE
DCE
DCE
DCE
DCE
DCE
DCE
<1
>99
<1
>99
<1
<1
52
An enyne cyclization reaction is a useful strategy for
preparing molecules with complicated structures in a single
operation.1 Moreover, the reaction does not produce any waste
and is atom economic. Many transition-metal catalysts such as
Pt, In, Ga, Ru, Au, and other metals have been reported. Further,
there are several reports on Rh-catalyzed enyne cycloisomeriza-
tion.2 In most cases, the active species for Rh-catalyzed enyne
cycloisomerization are Rh(I) or Rh(II) complexes, which can
coordinate to alkynes as π-Lewis acids.
In this study, cationic rhodium(III) porphyrin was success-
fully synthesized and used to efficiently catalyze the cyclo-
isomerization of 1,6-enynes under extremely mild reaction
conditions because of its strong Lewis acidity. First, tetraphe-
nylporphyrin (TPP) was prepared according to the reported
procedure.3 [Rh(TPP)]Cl was synthesized by the reaction of
RhCl3¢3H2O with TPP in benzonitrile at 210 °C for 12 h.4,5
[Rh(TPP)]Cl was treated with NaTFPB (TFPB: tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate) in CH2Cl2 at ambient temper-
ature for 12 h. After the treatment, the solution was filtered to
remove NaCl, and the solvent was evaporated to afford a new
cationic rhodium(III) porphyrin, [Rh(TPP)]TFPB. This high-
oxidation-state cationic Rh(III) species was protected and
stabilized by the rigid porphyrin ligand. The metalloporphyrin
has cationic properties because the counter anion (TFPB) weakly
coordinates to the metal center (Rh), thus enhancing its reactivity
toward enyne cycloisomerization (Figure 1).
[Rh(TPP)]TFPB toluene
[Rh(TPP)]TFPB acetonitrile
[Rh(TPP)]TFPB THF
54
<1
<1
<1
<1
<1
<1
<1
<5
[Rh(TPP)]TFPB pyridine
[Rh(cod)]TFPB
[Rh(cod)]TFPB
[Rh(cod)]SbF6
[Rh(cod)]SbF6
DCE
DCE
DCE
DCE
[Rh(salen)]SbF6 DCE
[Rh(salen)]SbF6 DCE
aReactions were carried out using the catalyst (2.5 mol %) and
enyne 1a (0.2 mmol) in 1.0 mL of the solvents. NMR yields.
b
We investigated the yields of 2a by changing the counter
anions. When SbF6 was used as the counter anion, the yield
of 2a was 62% (Entry 2). In the cases of BF4 and PF6, the
cycloisomerization did not proceed at 0 °C; however, the
reaction proceeded smoothly and quantitatively when heated to
80 °C (Entries 3-6). When [Rh(TPP)]Cl was used in the
reaction, the cycloisomerization did not occur at all both at 0
¹
and 80 °C, because the Cl anion strongly coordinated to the
The cycloisomerization of 1a with [Rh(TPP)]TFPB in 1,2-
dichloroethane (DCE) afforded the corresponding diene product,
2a, in >99% yield at 0 °C for 1 h (Table 1, Entry 1).
metal center, thus making the Rh complex less cationic and less
electrophilic (Entries 7 and 8). We also investigated the effect of
several solvents on the yield of 2a. Toluene affected the result of
the reaction, decreasing the yield of 2a. Apparently, the catalyst
did not solubilize sufficiently in toluene at 0 °C. The cyclo-
isomerization did not proceed satisfactorily in coordinating
solvents such as acetonitrile, THF, and pyridine because the
solvent molecules coordinated to the metal center of the catalyst,
making the catalyst less cationic and less electrophilic (Entries
10-12). Other Rh complexes did not catalyze this cycloisome-
rization at all (Entries 13-18).
R
H
N
+
R
H
Ph
Ph
N
+
N
N
Ph
Ph
Rh
Rh
Ph
Ph
N
N
N
N
We investigated the cycloisomerization of various types of
enynes under the optimized reaction conditions (Table 2). The
reaction of substrate 1a afforded the desired product, 2a,
Ph
Ph
Figure 1. Activation of an alkyne by [Rh(TPP)]+.
Chem. Lett. 2014, 43, 1937–1939 | doi:10.1246/cl.140810
© 2014 The Chemical Society of Japan | 1937

Table 2. Cycloisomerization of enynesa
Table 3. Cycloisomerization of alkynylarenesa
[Rh(TPP)]TFPB (2.5 mol%)
Entry
1
Substrate
Product
Yield/%b
>99
E
E
E
E
DCE, 80 °C, 5 h
E
E
Ph
R
R
E
E
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
E = COOEt
2
Ph
1
Entry
1
Substrate
Product
Yield/%b
81
E
E
E
E
85c
50
37
69
39
E
E
2
3
4
5
6
E
E
1g
1h
2g
E
E
E
E
E
E
E
E
2
3
>99
2h
2i
E
E
E
E
E
E
E
E
91 (1/1)c
1i
E
E
E
E
Pr
Pr
E
E
2i′
Pr
E
E
E
E
E
E
4
5
80
<5
1j
2j
E
E
Pr
E
E
E
E
aReactions were carried out using [Rh(TPP)]TFPB (2.5 mol %)
and enyne 1 (0.2 mmol) in 1.0 mL of 1,2-dichloroethane
(DCE) at 0 °C for 1 h. E = COOEt. NMR yields. Yield at
1k
2k
b
c
Br
Br
25 °C.
aReactions were carried out using [Rh(TPP)]TFPB (2.5 mol %)
and enyne 1 (0.2 mmol) in 1.0 mL of 1,2-dichloroethane
(DCE) at 80 °C for 5 h. NMR yields. Ratio of regioisomers.
quantitatively because the benzylic cation intermediate was
extremely stable. The reaction of the substrate with a terminal
alkene moiety, 1b, afforded the desired product, 2b, in 46%
yield at 0 °C. The relatively low yield was attributable to the low
nucleophilicity of the terminal alkene. However, at 25 °C the
reaction successfully afforded the product, 2b, in 85% yield. The
reaction of the substrate with a crotyl group, 1d (a mixture of E-
and Z-forms), afforded the corresponding product, 2d, stereo-
selectively (only E-form). Furthermore, we investigated the
stereochemistry of the alkene moiety. The reactions of both the
E- and Z-substrates afforded only the E-products (Entries 5 and
6). Thus, this cationic rhodium(III) porphyrin catalyzed the
cycloisomerization of enynes to selectively afford the corre-
sponding E-products.
We also found that the cationic rhodium(III) porphyrin
catalyst could activate C-C triple bonds selectively and
efficiently. We assumed that the cationic vinyl-metal species
generated by alkynes and the Rh(III) catalyst would undergo
nucleophilic attacks of not only alkenes, but also arenes.6-8 With
this assumption, we investigated the rhodium(III) porphyrin-
catalyzed intramolecular Friedel-Crafts type reactions of alky-
nylarenes. First, the reaction of substrate 1g under the given
reaction conditions (Table 3) afforded the corresponding dihy-
dronaphthalene derivative, 2g, in 81% yield. Compared to 0 °C,
a higher temperature condition, 80 °C, was essential for the
cycloisomerization of alkynylarenes probably because higher
activation energy was needed for their dearomatization. This
b
c
[Rh(TPP)]SbF6 (2.5 mol%)
+
DCE, 80 °C, 12 h, 62%
3
4
5
Scheme 1. The intermolecular reaction between an acetylene
and a styrene derivatives.
type of Rh-catalyzed reaction has not been reported yet.
Encouraged by this result, we investigated other substrates
possessing tolyl groups. In the case of the o-tolyl group, the
product, 2h, was obtained quantitatively; thus, the electron-
donating group (methyl) worked efficiently in this Friedel-
Crafts reaction (Entry 2). The substrate with a m-tolyl group, 1i,
afforded the corresponding o-substituted and p-substituted
products in an equal ratio (Entry 3). For 1j, the yield was 80%
(Entry 4). The reaction of the substrate with an electron-
withdrawing group on the aromatic ring, 1k, hardly proceeded
(Entry 5).
We applied this strong π-Lewis acid catalyst to the
intermolecular reaction between an acetylene and a styrene
derivative (Scheme 1). The reaction of phenylacetylene (3) with
1938 | Chem. Lett. 2014, 43, 1937–1939 | doi:10.1246/cl.140810
© 2014 The Chemical Society of Japan

α-phenylstyrene (4) in the presence of [Rh(TPP)]SbF6 afforded
an indene derivative, 5, in 62% yield.9
i) A. M. Echavarren, C. Nevado, Chem. Soc. Rev. 2004, 33,
431. j) I. J. S. Fairlamb, Angew. Chem., Int. Ed. 2004, 43,
1048. k) C. Aubert, O. Buisine, M. Malacria, Chem. Rev.
2002, 102, 813.
In conclusion, we applied rhodium(III) porphyrin to the
cycloisomerization of enynes. The desired cycloisomerization
of enynes under extremely mild reaction conditions smoothly
afforded the corresponding products containing a five-membered
ring. This reactivity can be attributed to the high Lewis acidity
of the Rh catalyst. Thus, this strong π-Lewis acid catalyst also
catalyzed the intramolecular Friedel-Crafts type reactions of
alkynes with arenes. Although a higher reaction temperature was
necessary for the dearomatization of the arenes, the correspond-
ing products were obtained in high yields. Moreover, the Rh-
catalyzed intermolecular reaction of phenylacetylene with α-
phenylstyrene smoothly afforded the indene derivative.
2
A massive number of examples involving cycloisomeriza-
tion and related skeletal rearrangement of 1,n-enynes
have been reported in the past decade. For a few selected
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This work was supported by the JST, ACT-C, and Grants-in-
Aid from the Ministry of Education, Culture, Sports, Science
and Technology, Japan. T.K. acknowledges the Asahi Glass
Foundation.
3
4
5
6
7
8
9
Supporting Information is available electronically on J-STAGE.
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Chem. Lett. 2014, 43, 1937–1939 | doi:10.1246/cl.140810
© 2014 The Chemical Society of Japan | 1939