Selective intermolecular [2 + 2] cycloaddition reaction using platinum(II) catalyst with hollow-shaped triethynylphosphine

Article abstract of DOI:10.1246/cl.2012.786

Intermolecular [2 + 2] cycloaddition reaction of an allenyl silyl ether with vinyl ethers was realized on the basis of electrophilic activation of the allene by the platinumphosphine catalyst, affording the corresponding methylenecyclobutanes in good yield. The use of the bulky trialkynylphosphine as a ligand was indispensable to achieve highly selective [2 + 2] cycloaddition.

Full text of DOI:10.1246/cl.2012.786

doi:10.1246/cl.2012.786
786
Published on the web July 28, 2012
Selective Intermolecular [2 + 2] Cycloaddition Reaction Using Platinum(II) Catalyst
with Hollow-shaped Triethynylphosphine
Masaru Ebisawa, Hiroyuki Kusama, and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551
(Received May 24, 2012; CL-120449; E-mail: niwasawa@chem.titech.ac.jp)
Intermolecular [2 + 2] cycloaddition reaction of an allenyl
In the previous study, it was found that use of triisopro-
pylsilyl allenyl ether (1) instead of tert-butyldiphenylsilyl
(TBDPS) allenyl ether favored the formation of a methylene-
cyclobutane derivative, although the selectivity was not high.
Thus, when 2.5 mol % of [PtCl2(C2H4)]2 was added to a mixture
of the allenyl ether 1 and 2-methoxypropene (2a) in CH2Cl2 at
room temperature, the reaction proceeded smoothly to give a
mixture of methylenecyclobutane 3a and cyclopentene 4a in
the combined yield of 73% in a ratio of 56:44. Then we first
examined the effect of phosphine ligand on this reaction to
realize the selective [2 + 2] cycloaddition reaction. The reaction
was examined as follows. To a CH2Cl2 solution of 1 and 2a
was added a CH2Cl2 solution of 2.5 mol % of [PtCl2(C2H4)]2
(5 mol % based on Pt) and 5 mol % of a phosphine or phosphite
ligand at room temperature. After appropriate reaction time, the
reaction was quenched with N,N,N¤,N¤-tetramethylethylenedi-
amine (TMEDA), and the results are summarized in Table 1. As
shown here, most of the standard ligands gave the products in
moderate selectivity (Entries 2-5). But when the reaction was
carried out in the presence of Sawamura’s phosphine 5 (bulky
trialkynylphosphine) (Figure 1),10 the selectivity was improved
in favor of the formation of the cyclobutane derivative 3a
without lowering the yield (66% yield, 3a:4a = 83:17)
silyl ether with vinyl ethers was realized on the basis of
electrophilic activation of the allene by the platinum-phosphine
catalyst, affording the corresponding methylenecyclobutanes in
good yield. The use of the bulky trialkynylphosphine as a ligand
was indispensable to achieve highly selective [2 + 2] cyclo-
addition.
Use of allenes as a component of [2 + 2] cycloaddition
reactions has been extensively studied, such as photochemical
or thermal reactions with alkenes and alkynes, Lewis acid-
promoted reactions of electron-rich allenes with electron-
deficient alkenes, etc.1 In these reactions, combination of
substrates is restricted depending on the kind of the reaction.
For example, standard photochemical reactions usually employ
the combination of simple allenes and cyclic enones,1,2 and
most of the thermal reactions are restricted to intramolecular
reaction.1,3 More recently, transition-metal-catalyzed [2 + 2]
cycloaddition reactions of allenes have become a topic of
interest of many research groups, and several novel reactions
have been reported.1 These reactions are mostly carried out with
simple allenes, and use of electron-rich substrates is still rare in
spite of its utility for the preparation of highly functionalized
cyclobutane derivatives.1,4 Quite recently, another combination
of substrates, that is, the combination of electron-rich allenes and
electron-rich alkenes,5 has attracted attention with the rapidly
expanding chemistry of the electrophilic activation of carbon-
carbon multiple bonds.1a,6
Table 1. Screening of reaction conditions
n mol%
[PtCl2(C2H4)]2
SiO
SiO
OMe
2n mol% ligand
+
+
Me
SiO
•
MeO
MS4A, rt
Me
MeO
Me
1
2a
We previously reported the first example of the transition-
metal-catalyzed intermolecular [3 + 2] cycloaddition reaction of
an allenyl silyl ether with vinyl ethers utilizing electrophilic
activation of the allenyl ether by a platinum(II) catalyst.7 The
reaction showed wide generality and afforded synthetically
useful oxygenated cyclopentene derivatives in high yield (eq 1).
2.5 mol%
3a
4a
(Si = i-Pr3Si)
Time Yield Ratio
/h /%a 3a:4a
Entry
n
Ligand
Solvent
CH2Cl2
1
2
3
4
5
2.5
®
PPh3
P(o-tol)3
P(2-furyl)3
P(OEt)3
3
73 56:44
79 62:38
80 43:57
72 64:36
72 59:41
89 64:36
0.5
1
SiO
[PtCl2(C2H4)]2
5 mol% P(o-tol)3
OR1
1
2
R2
R1O
+
ð1Þ
SiO
•
R2
EtOAc, MS4A,
R3
R3
6
P(CCSiMe3)3
0.5
0 °C
(Si = TBDPS)
7
8
9
10
11
5
0.25 66 83:17
0.25 89 91:9
0.25 85 95:5
0.25 74 74:26
In the present study, we focused our attention on the
development of platinum(II)-catalyzed intermolecular [2 + 2]
cycloaddition reaction of electron-rich allenes and electron-rich
alkenes. It should be noted that there are several precedents for
the intramolecular [2 + 2] cycloaddition of the same concept
using gold(I) catalysts,5a,8 and quite recently, the first example of
the intermolecular [2 + 2] cycloaddition was reported by Chen
et al.,9 however, the combination of substrates is still limited and
expansion of this reaction is highly desirable in view of the high
synthetic utility of the reaction for the preparation of function-
alized cyclobutane derivatives.
toluene
hexane
EtOAc
toluene/hexaneb 0.25 86 93:7
12 0.5
13 0.05
toluene/hexanec 0.5
toluene/hexaned
90 95:5
95e 95:5
1
3
aYield and ratio of the products were determined by H NMR
analyses using 1,1,2,2-tetrachloroethane as an internal
standard. bToluene:hexane = 1:2. cToluene:hexane = 1:14.
dToluene:hexane = 1:149. Isolated yield.
e
Chem. Lett. 2012, 41, 786-788
© 2012 The Chemical Society of Japan
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787
OMe
SiO
MeO
t-Bu
t-Bu
MeO
t-Bu
t-Bu
SiO
OMe
t-Bu
t-Bu
OMe
cat. [Pt]
+
+
Me
SiO
•
MeO
MS4A, rt
Me
P
MeO
Me
Si
Ar
Si
Ar
1
2a
3a
4a
t-Bu
t-Bu
[Pt]
[Pt]
Si
t-Bu
t-Bu
a
MeO
OMe
SiO
OSi
t-Bu
t-Bu
t-Bu
t-Bu
[Pt]
•
path a
[2+2]
Me
[Pt]
OMe
SiO
5 (Ar = 3,5-(t-Bu)2-4-MeO-C6H2-)
MeO
OMe
[Pt]
2a
6
Me
and/or
[Pt]
7
Figure 1. Structure of the Sawamura’s phosphine 5.
[Pt]
b
SiO
Me
H
path b
[3+2]
SiO
SiO
(Entry 7). As the use of tris(trimethylsilylethynyl)phosphine
gave nearly the same ratio as the reaction using other typical
ligands (Entry 6), the bulkiness of the ligand should play an
important role in the selective preparation of the cyclobutane
derivative 3a.
MeO
OMe
Me
Scheme 1. Proposed reaction mechanisms.
Further examination of the reaction solvent revealed that use
of less polar solvent favored the formation of the cyclobutane
derivative. Thus when the reaction was carried out in toluene,
the reaction completed within 15 min at room temperature and
the selectivity of the reaction was improved to 91:9 in the
combined yield of 89% (Entry 8). Use of hexane as solvent
further improved the selectivity up to 95:5 (Entry 9), but the
reproducibility of the product yield is not very good probably
due to the low solubility of the platinum catalyst. In order to
solve this problem, a small amount of toluene was employed as
a solvent to prepare the platinum-phosphine complex, which
was added to a hexane solution of the two reactants. By this
protocol high yield was achieved with good reproducibility
without lowering the selectivity (Entry 11). Furthermore, the
catalyst loading could be reduced even to 0.05 mol % (0.1 mol %
based on Pt) without lowering the yield of the products
(Entry 13).
The reaction was thought to proceed in a similar manner as
in the previous [3 + 2] cycloaddition reaction (Scheme 1). Thus,
zwitterionic intermediate 6 was generated by the attack of
2-methoxypropene on the platinum-activated allenyl ether and
then the silyl enol ether attacked the oxonium carbon to give the
cyclobutylplatinum intermediate 7 (path a), which finally gave
the product 3 with regeneration of the platinum catalyst. The
Sawamura’s bulky tris(triarylsilylethynyl)phosphine 5 would
make the conformation of the zwitterionic intermediate 6
favorable for the cyclobutane formation.
Table 2. Generality of the intermolecular [2 + 2] cycloaddition
reaction
0.5 mol%
[PtCl2(C2H4)]2
SiO
SiO
1 mol% 5
vinyl
+
+
R
SiO
•
ether
toluene/hexane
(1:14), MS4A, rt
R'O
3
R'O
1
2
4
R
(Si = i-Pr3Si)
Entry
Vinyl ethera
Major product Yield/%b Ratio (3:4)c
SiO
OBn
1
2b
3b
86
>97:<3
BnO
Me
Me
SiO
OMe
Ar = p-BrC6H4 2c
p-NO2C6H4 2d
2
3d
3c
3d
94
77
96:4
MeO
Ar
>97:<3
Ar
OSi
OMe
OMe
n = 1 2e
4
5
3e
3f
98
99
94:6
n
2 2f
>97:<3
n
OSi
O
O
n = 1 2g
2 2h
6
7
3g
3h
76
69
>97:<3
>97:<3
n
n
The generality of the reaction concerning the vinyl ethers
is summarized in Table 2.11 Several 1,1-disubstituted alkenyl
ethers 2b-2d reacted to give the cyclobutane derivatives 3b-3d
in good yield with high selectivity (Entries 1-3). Cyclic alkenyl
ethers 2e and 2f could also be employed without problem to give
bicyclic cyclobutanes 3e and 3f in good yield (Entries 4 and 5).
Dihydrofuran 2g and dihydropyran 2h also reacted to give the
products 3g and 3h in highly selective manner (Entries 6 and 7).
In conclusion, we have developed the platinum-catalyzed
intermolecular [2 + 2] cycloaddition reaction of triisopropylsilyl
allenyl ether and vinyl ethers. This reaction affords a concise
method for the preparation of functionalized methylenecyclo-
butane derivatives in high yields. Combined with our previous
a1.1-5 equiv. bCombined yield of 3 and 4. cThe ratio was
determined by H NMR analysis. Toluene:hexane = 1:6.5.
1
d
report,7 either of the [2 + 2] or [3 + 2] cycloaddition products
could be selectively prepared mainly by choosing the appro-
priate phosphine ligand.12
The authors thank Prof. Masaya Sawamura (Hokkaido
Univ.) for the generous gift of ligand 5 and helpful discussions
on this subject. This research was supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
Chem. Lett. 2012, 41, 786-788
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

788
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Chem. Lett. 2012, 41, 786-788
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/