Dicationic platinum porphyrin catalyzed cycloisomerization of enynes

Article abstract of DOI:10.1016/j.tetlet.2013.08.128

Cycloisomerization of 1,6-enynes is successfully carried out in the presence of a dicationic platinum(IV) catalyst to afford five-membered ring systems. The use of a weakly coordinating axial ligand is the key to bringing out the catalytic activity of platinum porphyrin for the reaction.

Full text of DOI:10.1016/j.tetlet.2013.08.128

Tetrahedron Letters 54 (2013) 6196–6198
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Tetrahedron Letters
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Dicationic platinum porphyrin catalyzed cycloisomerization
of enynes
a,
Makoto Hasegawa a, Takuya Kurahashi a,b, , Seijiro Matsubara
⇑
⇑
a Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
b JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Cycloisomerization of 1,6-enynes is successfully carried out in the presence of a dicationic platinum(IV)
catalyst to afford five-membered ring systems. The use of a weakly coordinating axial ligand is the key to
bringing out the catalytic activity of platinum porphyrin for the reaction.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 23 July 2013
Revised 26 August 2013
Accepted 30 August 2013
Available online 7 September 2013
Keywords:
Platinum
Porphyrin
Cycloisomerization
Transition-metal-catalyst
Transition-metal-catalyzed enyne cycloisomerization is a useful
synthetic methodology for the one-step, atom-economical con-
struction of highly functionalized carbocyclic or heterocyclic com-
pounds.1–12 This reaction has therefore been used for the synthesis
of various kinds of pharmaceuticals, agrochemicals, and function-
alized materials.13–17 The development of enyne cyclizations is
therefore a research topic of great interest. Enyne cycloisomeriza-
tion is generally triggered by the coordination of an alkyne moiety
to an electron-deficient metal catalyst to afford key intermediates
such as vinyl-metal and metal-alkylidene species. Over the dec-
ades, several metal catalysts for the cycloisomerization of enynes
have been investigated intensively; for example, Pt,18 In,19 Ga,20
Ru,21 Rh,22 Au,23 and other metal catalysts have been reported.24,25
valences and charges. In order to solve this problem, we proposed
structurally rigid tetradentate porphyrin ligands with a large
p-
conjugated planar aromatic structure; such ligands have a signifi-
cant stabilizing effect on the metal center, so dicationic plati-
num(IV) species would become manageable metal catalysts, even
under reaction conditions.33 And thus, we supposed that a cationic
high-valent platinum–porphyrin complex would be a promising
catalyst for this reaction.34 In this Letter, we report that a porphy-
rin ligand can stabilize dicationic platinum(IV) species, and that a
dicationic platinum(IV) complex catalyzes the cycloisomerization
of enynes to afford cyclic compounds. This is the first example of
the use of a dicationic platinum(IV) porphyrin complex in the
cycloisomerization of enynes.
In terms of the Lewis acidities of the catalysts toward
p
-bonds, we
Benzaldehyde and pyrrole were stirred in propionic acid at
160 °C for 30 min to afford tetraphenylporphyrin (TPP).35 [Pt(TPP)]
was synthesized by the reaction between PtCl2 and TPP in benzo-
nitrile at 210 °C for 12 h (Fig. 1).36 [Pt(TPP)] was quantitatively oxi-
dized to [Pt(TPP)]Cl2 using H2O2 and HCl.37,38
The oxidation number of platinum increased from II to IV as a
result of this oxidation. [Pt(TPP)](BF4)2 was synthesized by treating
[Pt(TPP)]Cl2 with AgBF4 in CH2Cl2 at ambient temperature for 12 h.
After the reaction, the solution was filtered in order to remove the
Ag salt, and the solvent was evaporated. [Pt(TPP)](BF4)2 was
obtained in 65% yield. Because this platinum species had a high
oxidation state and the counter-anion weakly coordinated with
the metal center, we presumed that this platinum–porphyrin
complex would behave as a strong Lewis acid, and that it would
be effective for activation of unsaturated carbon–carbon triple
bonds of enynes in cycloisomerizations.
presumed that higher-valent redox-stable complexes would be
more effective for the cycloisomerization than lower-valent ones.
Moreover, we supposed that cationic catalysts would be more
effective for the reaction than non-cationic ones, because the posi-
tive charge and vacant coordination site on the catalyst allow coor-
dination to the alkyne moiety. A number of platinum(II) complexes
have been reported to be powerful catalysts for cycloisomeriza-
tion.26–32 In the light of this knowledge, we assumed that dication-
ic high-valent platinum(IV) species would have a significant effect
on the process. However, dicationic platinum(IV) species are rela-
tively unstable under the reaction conditions because of their high
⇑
Corresponding authors. Tel.: +81 75 383 2440; fax: +81 75 383 2438.
E-mail addresses: kurahashi.takuya.2c@kyoto-u.ac.jp (T. Kurahashi), matsubara.
seijiro.2e@kyoto-u.ac.jp (S. Matsubara).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.08.128

M. Hasegawa et al. / Tetrahedron Letters 54 (2013) 6196–6198
6197
low yields (entries 7 and 8). Also, the desired product 2a was not
obtained in 1,4-dioxane and acetonitrile, which could coordinate
to the platinum–porphyrin complex (entries 9 and 10). Other me-
tal–porphyrin complexes, namely [Cr(TPP)]BF4, [Mn(TPP)]BF4, and
[Fe(TPP)]BF4, showed lower catalytic activities for the cycloisomer-
ization than did the cationic platinum–porphyrin complexes (en-
tries 11–13), because their metal centers were monocationic and
had lower Lewis acidities than dicationic platinum(IV) species.
In order to demonstrate the scope of platinum–porphyrin-com-
plex-catalyzed cycloisomerization of enynes, we carried out the
reaction with various types of enynes and diynes under the opti-
mized reaction conditions (Table 2.). The reaction using a substrate
with a prenyl group afforded 2c in low yield; this was attributed to
the steric effect of the prenyl group (entry 2). The reaction with 1d,
bearing a terminal alkene moiety, was performed successfully to
furnish 2d in 72% yield (entry 3). The reaction with 1e afforded
the corresponding product 2e in 89% yield (entry 4). In order to
determine whether the stereochemistry of the alkene moiety of
Figure 1. Preparation of platinum–porphyrin complex
Table 2
We next briefly examined catalytic activity of the porphyrin
catalyst with cycloisomerization of enynes. The cycloisomerization
of 1a with [Pt(TPP)](BF4)2 in 1,2-dichloroethane (DCE) afforded the
five-membered product 2a in 84% yield as the sole product (Table 1,
entry 1). We then investigated the reaction conditions extensively,
and found that the counter-anions and solvents had a profound ef-
fect on the reactivities of the catalysts. In the presence of
[Pt(TPP)](SbF6)2, 2a was obtained in 74% yield (entry 2). However,
when [Pt(TPP)]Cl2 was used as the catalyst, sufficient product was
not obtained, probably because strong coordination of the chloride
ion made the platinum center less cationic and less electrophilic.
The effects of hexafluorophosphate, which is a weaker coordinat-
ing counter-anion than BF4 and SbF6, were also examined. The
reaction with [Pt(TPP)](PF6)2 did not afford product 2a (entry
4).39 Furthermore, when [Pt(TPP)] was used as the catalyst, the de-
sired product was not obtained (entry 5). The use of AgBF4 in place
of the cationic platinum complex afforded a trace amount of 2a
(entry 6). Solvent effects were also investigated. The reaction in
aromatic solvents, that is, benzene and toluene, afforded 2a in
Scope of the cycloisomerizationa
Entry
Substrate
Product
Yieldb (%)
1
70
2b
2c
1b
2
34
1c
3
4
5
72c
89
2d
1d
2e (1/1)
2f (E)
1e (1/1)
1f (E)
Table 1
Optimization of reaction conditionsa
76c
6
7
91c
2g (Z)
1g (Z)
Entry
Catalyst
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
[Pt(TPP)](BF4)2
[Pt(TPP)](SbF6)2
[Pt(TPP)]Cl2
[Pt(TPP)](PF6)2
[Pt(TPP)]
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
Benzene
84
74
<1
<1
<1
<5
28
43
<1
<1
19
<1
10
—
—
1h (1/1)
AgBF4
8
9
—
–
—
—
[Pt(TPP)](BF4)2
[Pt(TPP)](BF4)2
[Pt(TPP)](BF4)2
[Pt(TPP)](BF4)2
[Cr(TPP)]BF4
[Mn(TPP)]BF4
[Fe(TPP)]BF4
1i
Toluene
1,4-Dioxane
Acetonitrile
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
1j
a
Reactions were carried out using [Pt(TPP)](BF4)2 (2.5 mol %) and enyne
1
a
(0.2 mmol) in 1.0 mL of 1,2-dichloroethane (DCE) at 80 °C for 24 h.
Reactions were carried out using the catalyst (2.5 mol %) and enyne 1a
b
NMR yields.
(0.2 mmol) in 1.0 mL of 1,2-dichloroethane (DCE) at 80 °C for 24 h.
c
b
Reactions were carried out for 12 h.
NMR yields.

6198
M. Hasegawa et al. / Tetrahedron Letters 54 (2013) 6196–6198
pand the scope of the chemistry and studies of the detailed mech-
anism are currently underway in our laboratories.
Acknowledgments
This work was supported by 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, The Uehara
Memorial Foundation, Tokuyama Science Foundation, and Kurata
Memorial Hitachi Science and Technology Foundation.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.08.
128.
Scheme 1. Plausible reaction mechanism.
the substrate was preserved, 1f and 1g were reacted. Substrate 1f,
which had an E-form alkene moiety, afforded a product bearing an
E-form alkene moiety, in 76% yield (entry 5). Similarly, using a sub-
strate with a Z-form alkene moiety gave product 2g, with a Z-form
alkene moiety, in 91% yield (entry 6). The information on the E- or
Z-form of the alkene moiety of the substrate was preserved in the
alkene moieties of the products. As can be seen, substrates with an
internal alkyne moiety did not react at all (entries 7 and 9). The
platinum–porphyrin complexes essentially activated the terminal
alkyne group. This might be derived from the steric effect of the
substituent at the terminal alkynic carbon. A diyne substrate was
inactive in this reaction (entry 8). The cycloisomerization of 1,7-
enynes and 1,6-enynes possessing oxygen or nitrogen tether moi-
ety did not afford the products.
The reaction with substrates bearing substituents at the termi-
nal olefinic carbon afforded the corresponding products stereose-
lectively. Taking into account the fact that stereoselectivity was
preserved during the reaction, the reaction mechanism is assumed
to be very similar to the previously reported one.26,27 A plausible
mechanism for the platinum–porphyrin-catalyzed cycloisomeriza-
tion of enynes is shown in Scheme 1. Coordination of the terminal
alkyne of the substrate 1 to the platinum–porphyrin complex af-
fords the vinyl-metal species 3, which is stabilized by the alkene
moiety and the porphyrin ligand. Considering the results of entries
7 and 9 in Table 2, internal alkyne moieties of substrates have dif-
ficulty in forming intermediate 3, presumably because of the steric
effect of the substituent at the terminal alkynyl carbon.40 Then, a
cyclobutane annulated five-membered intermediate 4 is formed.
In the final step, the platinum–porphyrin complex is eliminated
from 4 to give product 2. In this catalytic process, the porphyrin li-
gand may play an important role in stabilizing not only the dicat-
ionic platinum(IV) species but also the cationic intermediates such
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as 3 and 4 with its large p-conjugated planar aromatic structure.
In summary, we developed dicationic platinum(IV)–porphyrin-
catalyzed cycloisomerization of enynes. Dicationic high-valent
platinum(IV) species were found to be effective in the catalytic
process. The key to the success of this reaction is the use of a por-
phyrin ligand along with a weakly coordinating axial counter-an-
ion ligand, which makes the platinum center cationic and
sufficiently electrophilic to activate carbon–carbon triple bonds
but also stable under the reaction conditions. Further efforts to ex-
40. For example of the use of platinum(IV) complex for cycloisomerization of
enyne, see: Oh, C. H.; Bang, S. Y.; Rhim, C. Y. Bull. Korean Chem. Soc. 2003, 24,
887.