Platinum(II)-catalyzed intermolecular [3 + 2] cycloaddition of propadienyl silyl ethers and alkenyl ethers

Article abstract of DOI:10.1021/ja907633b

(Chemical Equation Presented) A Pt(II)-catalyzed [3 + 2] cycloaddition reaction of silyl propadienyl ethers and alkenyl ethers has been developed as the first example of the utilization of allenes as a three-carbon unit in a transition-metal-catalyzed int

Full text of DOI:10.1021/ja907633b

Published on Web 10/27/2009
Platinum(II)-Catalyzed Intermolecular [3 + 2] Cycloaddition of Propadienyl
Silyl Ethers and Alkenyl Ethers
Hiroyuki Kusama, Masaru Ebisawa, Hideaki Funami, and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan
Received September 8, 2009; E-mail: niwasawa@chem.titech.ac.jp
The use of allenes as a three-carbon unit for [3 + n] cycloaddition
reactions is relatively unexplored compared with [2 + n]-type
reactions using allenes as a 2π unit.1 Representative examples of
such transformation are the Lewis acid-promoted [3 + 2] cycload-
dition reactions of allenylsilanes2 and the phosphine-triggered
reactions of 2,3-butadienoates3k with electron-deficient alkenes,3a-h
imines,3c,i aziridines,3j etc. On the contrary, transition-metal-
catalyzed [3 + n] cycloaddition reactions had not been reported
until recent reports of intramolecular [3 + 2]4 and [3 + 4]5
cycloaddition reactions, and to date there is still no report of an
intermolecular reaction using the 1,2-propadiene moiety of allene
derivatives as a three-carbon unit.6
attack of the silyl enol ether at the oxonium carbon to give the
cyclobutylplatinum(II) intermedate E (path b), which eliminated
the platinum catalyst to give 4a.9
Scheme 1. Proposed Reaction Mechanisms
We previously reported that 1,2,4-trienes undergo PtCl2-catalyzed
cyclization to give cyclopentadiene derivatives.7 In this reaction,
PtCl2 electrophilically activates the 1,2-propadiene moiety, which
is expected to behave like a metal-containing 1,3-dipole I, as shown
in eq 1. Then we thought of the possibility of carrying out
intermolecular [3 + 2] cycloaddition reactions of 1,2-propadienes
and appropriate dipolarophiles by using a Pt(II) catalyst. This paper
describes the successful realization of such a concept, that is, the
Pt(II)-catalyzed intermolecular [3 + 2] cycloaddition reaction of
silyl 1,2-propadienyl ether and alkenyl ethers to give functionalized
cyclopentene derivatives.
To improve the chemoselectivity toward cyclopentene derivative
3a, several reaction conditions were examined (Table 1). It was
found that the phosphine ligand [equimolar amount per Pt(II)] had
some effect on the ratio of the two products, and the rather bulky
tri(o-tolyl)phosphine gave the products in a 57:43 ratio (entry 3).10
It is likely that increasing the bulkiness around the platinum moiety
disfavored cyclobutane formation as a result of steric repulsion.
The addition of 2 equiv of phosphine ligand for platinum suppressed
the reaction, suggesting that the catalytically active species was the
platinum(II) monophosphine complex. In regard to the reaction
solvent, ethyl acetate was the solvent of choice, giving the products
in 95% yield with a ratio of 72:28 (entry 6).
To further improve the chemoselectivity toward the cyclopentene
derivative, we thought of lowering the nucleophilicity of the silyl
enol ether moiety by using the less electron-donating tert-butyl-
diphenylsilyl (TBDPS) group instead of the TIPS group. In fact,
the corresponding cyclopentene derivative was obtained in a 92:8
ratio in the reaction of TBDPS 1,2-propadienyl ether 1b (entry 7),
and the selectivity was further improved to 98:2 without decreasing
the yield by carrying out the reaction at 0 °C (entry 8).
As summarized in Table 2, this reaction showed wide generality
concerning alkenyl ethers. 1,1-Disubstituted and 1,1,2-trisubstituted
alkenyl ethers, 1-methoxycycloalkenes, dihydrofuran, and dihy-
dropyran could be employed for this reaction, giving the corre-
sponding cyclopentene derivatives in good yield with high selec-
tivity. Methylenetetrahydrofuran generated in situ from pent-4-yn-
1-ol 2m reacted with 1b to give the spirocyclic compound.
Furthermore, indole 2n could be employed for this reaction,
providing a synthetically useful tricyclic indole derivative, albeit
in moderate yield.
We chose the combination of silyl 1,2-propadienyl ether and
alkenyl ether with the expectation that selective electrophilic
activation of the silyl 1,2-propadienyl ether would be possible in
the presence of alkenyl ether because of its higher inherent reactivity
and that the alkenyl ether would attack the activated Pt(II)-
containing 1,3-dipole in a [3 + 2] cycloaddition manner. In fact,
when a mixture of triisopropylsilyl (TIPS) 1,2-propadienyl ether
1a and 2-methoxypropene 2a was treated with 2.5 mol %
[PtCl2(CH2dCH2)]2 in CH2Cl2 at room temperature, the reaction
proceeded smoothly to give the desired cyclopentene derivative 3a
along with methylenecyclobutane derivative 4a in a combined yield
of 73% in a 44:56 ratio.
The reaction was thought to proceed as follows (Scheme 1). TIPS
propadienyl ether 1a was activated by Pt(II) electrophilically to
give the corresponding π complex A or π-allyl cationic intermediate
B first.8 Next, 2-methoxypropene attacked this intermediate at the
nonsubstituted terminus to give Pt(II)-containing zwitterionic
intermediate C. Ring closure with electron flow from the anionic
platinum to the oxonium carbon at the position ꢀ to the metal (path
a) would give the cyclopentylidene Pt(II) complex intermediate D,
which finally underwent 1,2-hydrogen shift to give cyclopentene
derivative 3a with regeneration of the platinum catalyst. Methyl-
enecyclobutane derivative 4a was thought to be produced by the
9
16352 J. AM. CHEM. SOC. 2009, 131, 16352–16353
10.1021/ja907633b CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Optimization of Reaction Conditionsa
In conclusion, we have developed a Pt(II)-catalyzed [3 + 2]
cycloaddition reaction of silyl propadienyl ethers and alkenyl ethers
as the first example of the utilization of allenes as a three-carbon
unit in a transition-metal-catalyzed intermolecular cycloaddition
reaction. This reaction gave synthetically useful functionalized
cyclopentanone derivatives with wide generality, and this process
would be complementary to the well-precedented phosphine-
catalyzed [3 + 2] cycloaddition of electron-deficient allenes with
electrophilic 2π components.
entry
R3Si
ligand
solvent time (h) yield (3 + 4) (%) ratio (3:4)
1
2
3
4
5
6
7
8
TIPS (1a)
-
CH2Cl2
3
0.5
1
1
1
0.5
1
1
73
79
80
78
78
95
93b
44:56
38:62
57:43
37:63
59:41
72:28
92:8
PPh3
P(o-tolyl)3
Acknowledgment. This research was partly supported by a
Grant-in-Aid for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
toluene
THF
EtOAc
TBDPS (1b)
97b c
98:2
,
Supporting Information Available: Preparative methods and
spectral and analytical data for compounds 1-6. This material is
available free of charge via the Internet at http://pubs.acs.org.
a Yields and product ratios were determined by 1H NMR analyses.
b Isolated yield. c At 0 °C.
References
Table 2. Generality of the Pt(II)-Catalyzed Intermolecular [3 + 2]
Cycloaddition Reaction
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a NMR yield. b The product was isolated as a ketone after hydrolysis
of the silyl enol ether moiety. c At -40 °C. d At room temperature.
e The product 5 was obtained as an inseparable mixture of diastereomers
(1.2:1). f A mixture of unidentified products was obtained, and the
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