Ruthenium(I)-catalyzed cyclopropanation reactions with (trimethylsilyl)diazomethane and aryldiazomethanes

Article abstract of DOI:10.1016/S0040-4039(01)01221-7

The polymeric ruthenium(I) complex [Ru₂(CO)₄(μ-OAc)₂]_n is a suitable catalyst for the cyclopropanation of mono-, 1,1- as well as 1,2-disubstituted, and trisubstituted alkenes with (trimethylsilyl)diazomethane, phenyl-diazomethane, and (4-cyanophenyl)diazomethane. Trisubstituted alkenes are cyclopropanated with a remarkable degree of syn-selectivity.

Full text of DOI:10.1016/S0040-4039(01)01221-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6137–6140
Ruthenium(I)-catalyzed cyclopropanation reactions with
(trimethylsilyl)diazomethane and aryldiazomethanes
Gerhard Maas* and Ju¨rgen Seitz
Division of Organic Chemistry I, Albert-Einstein-Allee 11, D-89081 Ulm, Germany
Received 8 July 2001; accepted 9 July 2001
Abstract—The polymeric ruthenium(I) complex [Ru2(CO)4(m-OAc)2]n is a suitable catalyst for the cyclopropanation of mono-, 1,1-
as well as 1,2-disubstituted, and trisubstituted alkenes with (trimethylsilyl)diazomethane, phenyl-diazomethane, and (4-
cyanophenyl)diazomethane. Trisubstituted alkenes are cyclopropanated with a remarkable degree of syn-selectivity. © 2001
Elsevier Science Ltd. All rights reserved.
The transition-metal-catalyzed carbene transfer from
diazo compounds to appropriate substrates provides an
access to a variety of compounds with different struc-
tural motifs and functionalities.1 The most efficient and
versatile catalysts are based on Rh(II), Cu, and Pd(II),
in that sequence. Other catalytically active metals have
not arrived at general attention and use (for a survey of
catalysts, see Refs. 1–4). Although it is known since
1981 that Ru3(CO)12 catalyzes both cyclopropanation
and ylide-forming reactions with diazoacetates,5 the
potential usefulness of ruthenium catalysts for carbene
transfer reactions has started to emerge only recently.
Among the promising candidates for cyclopropanation
and insertion reactions are RuCl2(PPh3)3,6 certain
[RuCl2(PR3)(h6-arene)]7 and [RuCl(p-cymene)(TsN-R-
NH2)]8 complexes, ruthenium porphyrins,9 and several
Ru(II) complexes with chiral ligands.10–12 The poly-
analogy to the isoelectronic rhodium(II) complex
Rh2(OAc)4. In fact, 1 is similarly efficient as the
rhodium complex in catalytic cyclopropanation reac-
tions with diazoacetates13,14 and carbonyl ylide forming
reactions with (trimethylsilyl)diazoacetates.15
We report now that 1 is also able to catalyze the
cyclopropanation
of
alkenes
with
(trimethyl-
silyl)diazomethane and aryldiazomethanes. In contrast
to diazocarbonyl compounds, catalytic carbene-transfer
reactions with these diazomethane derivatives have
been studied far less frequently and their cyclopropana-
tion reactions are in most cases less effective.
(Trimethylsilyl)diazomethane: When a hexane solution
16
of (Me3Si)CHN2 (2) was added during 12 h to ruthe-
nium catalyst 1 (3 mol%) suspended in CH2Cl2, the
formal carbene dimer, 1,2-bis(trimethylsilyl)ethene (4),
was obtained in 80% yield and with a E/Z ratio of
meric
dicarbonylruthenium(I)
acetate
complex
[Ru2(CO)4(m-OAc)2]n (1)13 is so far the only ruthen-
ium(I) catalyst and represents, therefore, the closest
H
N2
(2,5,8)
R2
R2
Z
R1
Z
R1
H
R2
H
R1
R3
H
cat. 1
Z
+
+
-N2
R3
syn- 3,6,9
H
R3
anti- 3,6,9
Z
Z
Z,E- 4,7,10
2-4: R = SiMe3; 5-7: Z = Ph; 8-10: Z = C6H4-4-CN
Scheme 1. See Table 1 for individual compounds.
Keywords: catalysts; cyclopropanation; diazo compounds; ruthenium and compounds; silicon and compounds.
* Corresponding author. Fax: +0049 731 502 2803; e-mail: gerhard.maas@chemie.uni-ulm.de
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01221-7

Table 1. Cyclopropanation of alkenes with diazo compounds 2, 5, and 8 catalyzed by 1 (see Scheme 1)
Entry
Alkene
R1
R2
R3
Me3SiCHN2 (2)
PhCHN2 (5)
Ratiob (anti:syn)
(4-CN-C6H4)CHN2 (8)
Yield of 9 (%)a Ratiob (anti:syn)
/
Yield of 3 (%)a Ratiob (anti:syn) Yield of 6 (%)a
a
b
c
d
e
f
Styrene
Ph
EtO
Ph
–(CH2)4–
Me
Me
H
H
H
H
H
64
68
67.5:32.5
63:37
56:44
35
45
21
19
22
27
22.5:77.5
19:81
45:55
45:55
5:95
42
66
18:82
23:77
Ethyl vinyl ether
a-Methyl-styrene
Cyclohexene
2-Me-2-butene
2,5-Dimethyl-2,4-hexadiene
Me 61
H
Me
Me
48
91:9
c
Me
CHꢀCMe2
c
73:27
8:93
4
)
a Yields of isolated products are given.
b Determined by 1H NMR integration; anti/syn corresponds to E/Z for entries a–c,e,f and to exo/endo for entries d.
6
c Cyclopropanes 3e,f could not be separated from several other unidentified products and were formed in estimated yields of 30%; determination of the anti/syn ratio from the 1H NMR spectrum of
the product mixture was possible only for 3f.
6

G. Maas, J. Seitz / Tetrahedron Letters 42 (2001) 6137–6140
6139
>99:1.17 Encouraged by this efficient transformation, we
carried out the same procedure in the presence of an
excess of an alkene and obtained cyclopropanes 3
(Scheme 1 and Table 1). Although the stationary con-
centration of the diazo compound was kept low, forma-
tion of carbene dimers 4 could not be suppressed
completely (yields: 20–25%).18
demand of a phenyl group in the transition state of a
reaction may be not much different from that of a
methyl group.
Catalyst 1 is so far the only one, which has been
applied to cyclopropanation of the same set of alkenes
with methyl diazoacetate (MDA),13,14 Me3SiCHN2 (2),
and PhCHN2 (5). A comparison of the diastereoselec-
tivities shows that cyclopropanations are anti-selective
with 2 but syn-selective with 5, while MDA gives
syn-cyclopropanes preferentially only with trisubsti-
tuted alkenes. In other words, while catalyst 1 provides
an exceptional syn-selectivity for cyclopropanation of
trisubstituted alkenes with MDA and 5, this is not the
case with Me3SiCHN2. This difference may be due to a
higher steric demand of the SiMe3 group as compared
to CO2Me or Ph. It also shows that it may be difficult
to make stereochemical predictions based on
existing1,14,22,26 mechanistic proposals.
Little is known about metal-mediated cyclopropanation
reactions of unactivated alkenes with 2. The stoichio-
metric reaction between an isolable trimethylsilylcar-
bene–ruthenium(II) complex and styrene gave
cyclopropane 3a in only 34% yield.19 CuCl was used as
a catalyst for cyclopropanation of a number of alkenes;
the comparison for styrene20 (46% yield, E/Z=4.8) and
cyclohexene21 (72% yield, exo/endo=9.3) with our
results reveals the similar performance of catalyst 1.
The trisubstituted alkenes (entries e and f) afforded a
complex mixture (NMR, GC) of highly volatile prod-
ucts from which the expected cyclopropanes could not
be separated in pure form. It appears that some of the
products are formed from cationic intermediates, and
further investigations are needed to clarify this. In
terms of diastereoselectivity, we note that the E-isomers
of cyclopropanes 3 prevail in all cases and that 1 is less
E-selective than CuCl for cyclopropanation of styrene,
but shows also an expressed exo selectivity for
cyclohexene.
Acknowledgements
Financial support of this work by the Fonds der
Chemischen Industrie is gratefully acknowledged.
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