702
Published on the web May 29, 2010
Cyclopropanation of Olefins with Diazo Compounds Catalyzed
by a Dicopper-substituted Silicotungstate [£-H2SiW10O36Cu2(®-1,1-N3)2]4¹
Keigo Kamata,1,2 Toshihiro Kimura,1 and Noritaka Mizuno*1,2
1Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
2Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST),
4-1-8 Honcho, Kawaguchi 332-0012
(Received April 13, 2010; CL-100360; E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp)
The dicopper-substituted £-Keggin silicotungstate (TBA)4-
CuO39] showed low catalytic activity and selectivity to 3a.
The dilacunary silicotungstate (TBA)4[£-SiW10O34(H2O)2] and
saturated silicotungstate (TBA)4[£-SiW12O40] were almost in-
active, suggesting that the tungsten species are not involved in
the present catalytic system. The chemoselectivity to 3a could be
increased by the slow addition of 2 into the reaction solution of I
and 1a. Upon the addition of 2 in five portions, the cyclo-
propanation proceeded efficiently and chemoselectively to give
3a in 83% yield.
The cyclopropanation of 1a with 2 with low catalyst loading
of I (0.01 mol % with respect to 2) chemoselectively proceeded
and 0.26 g of analytically pure 3a was isolated (eq 1). In this
case, the turnover number (TON) reached 6926 and the value
was much higher than those for CH3C(CH2NPCp3)3Cu(OTf)2
(175, Cp = cyclopentyl), [Cu(dppipa)2]ClO4 (425, dppipa =
(Ph2P)2N(iPr)), TpMsCu (485, TpMs = hydrotris[3-(2,4,6-tri-
[£-H2SiW10O36CuII (®-1,1-N3)2] (I, TBA = tetra-n-butylammo-
2
nium) could act as an efficient precatalyst for the chemoselective
cylopropanation of olefins with diazo compounds. Various kinds
of olefins were efficiently converted to the corresponding
cyclopropane derivatives in good yields.
Cyclopropanes are important chemical compounds because
they have been used as starting materials and intermediates in
organic synthesis.1 Many efficient systems have been developed
for the copper-catalyzed efficient diastereo- and enantio-selec-
tive synthesis of cyclopropanes.1,2 An electrophilic copper(I)
carbene intermediate LnCu=C(R)R¤ has been proposed on the
basis of mechanistic, kinetic, and computational studies.3 Very
recently, the syntheses and structures of the mono- and dinuclear
copper carbene species with neutral iminophosphanamide and/
or anionic ¢-diketiminate ligands have been reported and the
reactivities of these compounds are discussed.4 Although some
dinuclear Ru- and Rh-based compounds are efficient catalysts
for the cyclopropanation,1,2,5 the cyclopropanation of olefins by
dinuclear copper catalysts has scarcely been reported.6
methylphenyl))pyrazolylborate],
[Rh(C7H15CO2)2]2
(168),
Fe(TDCPP)/CoCp2 [970, TDCPP = meso-tetra(2¤,6¤-dichloro-
phenyl)porphyrinato], Rh(NCTMP)I2 [1860, NCTMP = N-con-
fused tetrakis(mesityl)porphyrin] systems.10,11
COOEt
N2
I (0.01 mol%)
+
+ N2
ð1Þ
1,2-dichloroethane
COOEt
2
(2 mmol)
The catalysis of metal-substituted POMs (polyoxometalates),
which are synthesized by the introduction of substituent metal
cations into the vacant site(s) of lacunary POMs, have attracted
much attention because of the unique reactivity that results from
the composition and structure of the catalytically active sites.7
Recently, we have reported the cooperative activation of organic
substrates such as alkynes and azides by a dicopper-substituted £-
Keggin silicotungstate with bis-®-1,1-azido ligands (TBA)4[£-
Ph
333 K, 17 h
Ar (1 atm)
1a
3a
(10 mmol)
0.26 g (69% yield)
cis/trans = 29/71
The scope of the present catalytic cyclopropanation with
diazo compounds (2 and tert-butyl diazoacetate (5)) was
investigated with regard to a range of structurally diverse olefins
(I:1:2 or 5 = 1:1000:100, Table 1). Various kinds of olefins
could efficiently and chemoselectively be converted to the
corresponding cyclopropanes in high yields. The cyclopropana-
tion of styrenes 1a-1e, which contain electron-donating as well
as electron-withdrawing p-substituents, proceeded selectively to
afford the corresponding cyclopropanes 3a-3e in good yields
(Entries 1-7). The reaction of 1a with 2 proceeded at ambient
temperature under the stoichiometric conditions (I:1a:2 =
1:100:100) (Entry 2). The reaction rates were dependent on
the electronic effects of the substituents on the aromatic rings of
styrenes. The Hammett plots (log(kX/kH) versus ·+) for the
competitive cyclopropanation of 1a and p-substituted styrenes
1b-1e are shown in Figure S2.9 The negative µ+ value (¹0.19)
indicates the electrophilic active copper carbene species and
partial positive charge on the styrene in the transition state.12
The reaction of internal cis-¢-methylstyrene 1f proceeded
stereospecifically to form the corresponding cyclopropane 3f
(Entry 8). Not only aryl olefins 1a-1f but also alkyl olefins 1g-
1i were efficiently converted to the corresponding cyclopropanes
(Entries 9-12). The cyclopropanation of cyclohexene 1g and
H2SiW10O36CuII (®-1,1-N3)2] (I, Figure S1).8,9 Compound I
2
shows high catalytic activity for oxidative alkyne-alkyne
homocoupling, 1,3-dipolar cycloaddition of organic azides to
alkynes, and three-component reaction of organic halides, NaN3,
and alkynes to produce 1,4-disubstituted-1,2,3-triazole deriva-
tives.8 In this communication, we report that I acts as a precatalyst
for the chemoselective cyclopropanation of various kinds of
olefins with diazo compounds.
First, the cyclopropanation of styrene (1a) with ethyl
diazoacetate (2) was carried out under various conditions
(Table S1).9 In the present system, diastereomeric mixture of
the corresponding cyclopropane 3a was obtained with the co-
production of diethyl maleate and diethyl fumarate (4) by the
dimerization of 2. The cyclopropanation of 1a with 2 in the
presence of I efficiently proceeded in dichloromethane and 1,2-
dichloroethane solvents (I:1a:2 = 1:250:100) and the cis/trans
ratios of 3a were 44/56 and 45/55, respectively. In the absence
of 2, 4 was formed in 85% yield with ²99% selectivity. The
mono-copper-substituted silicotungstate (TBA)4[¡-H2SiW11-
Chem. Lett. 2010, 39, 702-703
© 2010 The Chemical Society of Japan