Catalytic epoxidation of cyclic vinylsilanes by ruthenium(II) complexes under aerobic conditions

Article abstract of DOI:10.1016/S0040-4020(03)01210-9

A new methodology has been developed for the catalytic epoxidation of cyclic vinylsilanes using a ruthenium(II) bisoxazoline complex 2 with molecular oxygen. An attempt has been made to understand the role of -SiMe₃ group on the rate of epoxidation process.

Full text of DOI:10.1016/S0040-4020(03)01210-9

Tetrahedron 59 (2003) 7761–7765
Catalytic epoxidation of cyclic vinylsilanes by ruthenium(II)
complexes under aerobic conditions
Abirami Srikanth,a,b Gopalpur Nagendrappab and Srinivasan Chandrasekarana
,
*
aDepartment of Organic Chemistry, Indian Institute of Science, C. V. Raman Avenue, Bangalore 560012, India
bDepartment of Chemistry, Bangalore University, Central College campus, Bangalore 560001, India
Received 13 May 2003; revised 1 July 2003; accepted 30 July 2003
Abstract—A new methodology has been developed for the catalytic epoxidation of cyclic vinylsilanes using a ruthenium(II) bisoxazoline
complex 2 with molecular oxygen. An attempt has been made to understand the role of –SiMe3 group on the rate of epoxidation process.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
ease of preparation from readily available starting materials
and the versatility of ligand design have made them
attractive to organic chemists.
Metal catalysed epoxidation of alkenes constitutes an
important tool in organic synthesis.1 The importance of
the epoxidation reaction lies in the fact that the formed
epoxides can be converted into a variety of useful products.
It is also attractive in asymmetric synthesis because it can
lead to two asymmetric centres in one step. Transition-metal
complexes as catalysts for epoxidation have been receiving
increased attention during the past three decades.2 The need
for functionalisation of lower alkenes formed as by-products
in the manufacture of gasoline, the need for partial selective
oxidation and the need for the preparation of compounds
with a specific spatial structure are some of the reasons for
the rapid growth in this field. Today a number of transition
metal complexes are available which use a variety of
oxygen sources for the epoxidation.3
The synthesis of ruthenium complexes 1 and 2 can be
accomplished by treating two equivalents of the correspond-
ing bisoxazoline with trans tetrakis(acetonitrile)dichloro
ruthenium in refluxing dichloroethane6 (Fig. 1). The catalyst
2 was preferred over 1 for a study of catalytic activity due
to its better solubility in common organic solvents although
the catalytic activity of both 1 and 2 was almost the
same. In an attempt to expand the scope of this new
methodology, we present the results of our investigations
on the aerobic epoxidation of cyclic vinylsilanes with
ruthenium catalyst, 2.
Ruthenium complexes are good epoxidation catalysts,4
which form high valent oxo-ruthenium species as active
intermediates in the oxygen transfer process. Ruthenium
complexes with nitrogen based ligands5 have been exten-
sively studied in recent years to develop new catalysts with
better selectivity and several co-oxidants have been
employed to get the best results. Clearly, the ideal oxidant
would be molecular oxygen since it is least expensive and
also environmental friendly. Recently we reported a novel
ruthenium(II) bisoxazoline complex 16 as a catalyst for a
highly stereo, regio and diastereoselective epoxidation of
simple olefins using molecular oxygen. The choice of
bisoxazolines as ligands was natural owing to the fact that in
combination with ruthenium, they form complexes analo-
gous to Schiff’s base derived complexes. Additionally, the
Figure 1.
2. Results and discussion
Epoxidation of cyclic vinylsilanes furnish a,b-epoxy
silanes,7 a class of compounds of considerable interest to
the organic chemists. The presence of the trialkylsilyl group
provides regioselective control in the opening of epoxide by
a variety of nucleophiles.8 Also it can be removed under
extremely mild conditions to afford epoxides9 which in
some cases are difficult targets by direct synthetic
transformations. Further, the a,b-epoxy silanes can be
used as “vinyl cation” equivalents10 which are very useful
Keywords: epoxidation; ruthenium; vinylsilanes; molecular oxygen.
*
Corresponding author. Tel.: þ91-80-293-2404; fax: þ91-80-360-0529;
e-mail: scn@orgchem.iisc.ernet.in
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)01210-9

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A. Srikanth et al. / Tetrahedron 59 (2003) 7761–7765
(2.5 mol%) and isobutyraldehyde (1.5 mmol) under an
oxygen atmosphere in dichloromethane at room temperature
(288C, 4.5–20 h). Solid NaHCO3 was added to buffer the
isobutyric acid formed during the reaction (Scheme 1).
The results of epoxidation of a number of cyclic vinylsilanes
are summarized in Table 1. All the cyclic vinylsilanes
except 5 produced the corresponding a,b-epoxysilanes in
moderate to high yields (62–94%) along with 8–10% of the
corresponding 3-trimethylsilyl-2-ene-1-ones. In the case of
5, we observe the formation of the silylated enones to the
extent of 26%. Similar results have been documented in the
literature using dimethyldioxirane as the oxidizing agent.13
Interestingly in the reaction of 11, the epoxide 12 is the
exclusive product. Similarly in the reaction of 9 with
catalyst 2, the epoxide 10 is the sole product formed. An
interesting feature of this methodology is that the primary
Scheme 1.
synthetic intermediates. They can be readily transformed
into b-hydroxy silanes11 and also into a-hydroxy carbonyl
compounds. The cyclic vinylsilanes needed for the present
study were readily synthesized from the cyclic vinyl halides
and chlorotrimethylsilane by the Wurtz type coupling
methodology.12
A typical aerobic epoxidation of cyclic vinylsilane involves
treatment of the vinylsilane (1 mmol) with the catalyst 2
Table 1. Epoxidation of cyclic vinylsilanes using 2 under aerobic conditions at room temperature (288C)
Entry
1
Substrate
Time (h)
7
Yieldg (Conversion)d
Product(s)e
72 (98)
2
3
11
14
56 (90)
65 (90)
4
8
80 (97)
5
6
9
4.5
20
92 (98)
73 (88)
10c
7
8
6.5
80 (90)
17
70 (95)
aIn the case of 3, 7, 13 and 15, 8–10% of silylated enones (allylic oxidation) were also isolated. bIn the case of 5, 26% of diastereomeric mixture of the silylated
enones was formed. cThe reaction was performed at 608C. dThe conversion was determined by means of gas chromatography using a SE 30 capillary column.
eAll products exhibited spectroscopic data identical to that reported in the literature. fA mixture of 42% cis and 58% trans. gRefers to isolated yields.

A. Srikanth et al. / Tetrahedron 59 (2003) 7761–7765
7763
Scheme 2.
product of allylic oxidation, i.e. the allylic alcohol was not
detected even in trace amounts and no desilylation was
observed in any of the substrates indicating the mildness of
the procedure.
electronic factor in the present case of the transition metal
based epoxidation process.
The mechanism of the reaction probably involves high
valent ruthenium-oxo species during the oxo transfer
process to the alkene. Although perisobutyric acid is
known to be formed15 under the given reaction conditions,
it is more likely that the active oxo transfer step involves the
ruthenium-oxo species. Several attempts towards isolation
of these intermediates remain futile. Since the ruthenium-
bisoxazoline catalyst 2 is analogous to ruthenium por-
phyrins, a similar mechanistic pathway involving the
formation of Ru2þ, Ru4þ and Ru6þ oxidation states has
been proposed (Scheme 3).16
The optimal loading of the catalyst 2 was found to be
2.5 mol% with respect to the alkene. However, the reaction
takes place even with 1 mol% of catalyst 2 with longer
reaction time. When 1-trimethylsilyl cyclooctene (9) was
subjected to epoxidation using 1 mmol of the catalyst, the
reaction went to completion in 12 h whereas with 2.5 mol%
of 2, the same conversion was accomplished in about 8 h.
Next, the effect of temperature on the reaction was also
examined. With vinylsilane 9 as the substrate, the reaction
with 2 was completed in 8 h (288C, entry 4). However,
when the reaction was performed at 608C, keeping other
experimental conditions identical, the reaction went to
completion in 4.5 h (entry 5, 94%). A further increase in
temperature seems to be detrimental resulting in a mixture
of products.
3. Conclusion
In summary, we have reported a new methodology using a
novel ruthenium(II) complex 2 for the catalytic epoxidation
of cyclic vinylsilanes under effectively neutral conditions
with molecular oxygen and isobutyraldehyde as a sacrificial
reductant.
To understand the effect of the trimethylsilyl substituent at
the double bond in the rate of the transition metal catalyzed
aerobic epoxidation, a competitive experiment between
trimethylsilyl cyclooctene and cyclooctene was performed.
Treatment of cyclooctene and silylsubstituted cyclooctene 9
(1:1) with the ruthenium catalyst 2 (2.5 mol%, CH2Cl2,
288C, 1 h), it was found that cyclooctene was oxidized faster
compared to the corresponding silyl substituted alkene, 9
(Scheme 2). The relative ratio of consumption of the two
olefins was found to be 84:16, after a period of 1 h. This is in
contrast to the results of epoxidation by peracids wherein
the vinylsilanes were found to be more reactive than their
unsilylated analogues.14 The relative sluggishness of the
vinylsilanes can be attributed to the steric hindrance exerted
by the trimethylsilyl substituent for the approach of the
relatively large sized ruthenium catalyst 2 during the
oxygen transfer process. The difference in reactivity seem
to indicate that steric factor is more important than the
4. Experimental
4.1. General
4.1.1. Preparation of the ruthenium complex 1.6
A
solution of trans-tetrakis(acetonitrile)dichlororuthenium17
(0.336 g, 1 mmol) and 4,4,40,40-tetrahydro-2-20-bisoxazole
(0.308 g, 2.2 mmol) was refluxed in dry ethanol (10 mL) for
6 h under nitrogen. The red solution was concentrated to a
minimum volume (1 mL) under reduced pressure and the
red solid was precipitated by addition of dry ether (15 mL).
The solid was collected by filtration through a sintered
funnel under nitrogen atmosphere. Recrystallization of the
solid from dichloromethane/diethyl ether (1:1) gave the
Scheme 3.

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A. Srikanth et al. / Tetrahedron 59 (2003) 7761–7765
complex, 1 as a dark red powdery solid, which was dried
under vacuum for 2 h and stored in a desiccator.
silica gel (2% ethylacetate–hexanes). Yield: 0.098 g, 56%;
bp: 90–938C/30 mm (lit.,19 bp: 112–1148C/73 mm).
Yield 0.380 g, 82%. Mp: 280–2818C; FT-IR (Nujol): 2950,
1610, 335 cm21; 1H NMR (300 MHz, CDCl3): d 3.49 (8H,
m), 3.79 (8H, m); 13C NMR (D2O): d 53.6, 68.8, 155.7;
Mass (MALDI): 453 (Mþþ1); UV (lmax) (MeOH): 222
(1.98), 467 (0.81); Elemental Analysis: calcd for
C12H16Cl2N4O4Ru (C 31.86, H 3.56, N 12.38); Found (C
31.81, H 3.48, N, 12.40).
4.2.3. Trimethyl(8-oxa-bicyclo[5.1.0]oct-1-yl)silane 8.18
Colourless liquid. Purified by column chromatography on
silica gel (2% ethylacetate–hexanes). Yield: 0.123 g, 65%;
bp: 84–868C/10 mm (lit.,18 bp: 778C/5 mm).
4.2.4. Trimethyl(9-oxa-bicyclo[6.1.0]non-1-yl)silane
10.19 Colourless liquid. Purified by column chromatography
on silica gel (1% ethylacetate–hexanes). Yield: 0.163 g,
80%; bp: 58–608C/0.5 mm (lit.,19 bp: 52–548C/0.5 mm).
4.1.2. Preparation of the ruthenium complex 2. A
solution of trans-tetrakis(acetonitrile)dichlororuthenium
(0.336 g, 1 mmol) and 4,4,40,40-tetramethyl-2,20-bisoxazole
(0.392 g, 2 mmol) was refluxed in dry dichloroethane
(10 mL) for 8 h under N2 atmosphere. The reaction mixture
was concentrated to half its volume and then addition of
dry ether (15 mL) precipitated the complex. The reddish
brown complex was collected by filtration through a
sintered funnel. The solid was washed with dry ether and
dried under high vacuum for 3 h. Recrystallization from
dichloromethane/diethyl ether (1:1) gave the ruthenium
complex 2 as a brownish red powder, which can be stored in
a desiccator indefinitely.
4.2.5. Trimethyl(3-oxa-tricyclo[2.2.1.02,4]octan-2-yl)-
silane 12.20 Colourless liquid. Purified by column chroma-
tography on silica gel (2% ethylacetate–hexanes). Yield:
0.133 g, 73%; bp: 82–888C/10 mm (lit.,20 bp: 82–858C/
10 mm); 1H NMR (CDCl3, 300 MHz): d 0.13 (9H, s), 1.0–
1.52 (6H, m, complex), 2.37 (1H, m), 2.47 (1H, m), 3.04
(1H, s).
4.2.6. Trimethyl(3-oxa-tricyclo[4.2.1.02,4]non-2-yl)silane
14.21 Colourless liquid. Purified by column chromatography
on silica gel (2% ethylacetate–hexanes). Yield: 0.157 g,
1
80%; bp: 65–678C/7 mm; H NMR (CDCl3, 300 MHz): d
Yield 0.401 g, 71%. FT-IR (cm21) (KBr): 3126 (br), 1618,
1507, 1400, 1207, 751; 1H NMR (CDCl3, 300 MHz): d 1.36
(24H, s), 4.12 (8H, s); 13C NMR (CDCl3, 75 MHz): d 27.9,
0.03 (9H, s), 1.0–1.08 (1H, m), 1.16–1.34 (2H, m), 1.53–
2.01 (6H, m), 2.47 (1H, m), 2.83 (1H, d, J¼3.6 Hz); 13C
NMR (CDCl3, 75 MHz): d 24.4, 26.5, 29.5, 30.4, 32.0,
34.4, 35.2, 51.7, 60.1; Mass: m/z 196 (Mþ).
68.2, 79.6, 153.1; Mass (MALDI): 564 (Mþ); UV (lmax
)
(CHCl3): 325 (2.02), 383 (2.90), 468 (1.57), 641 (0.45);
Elemental Analysis: calcd for C20H32N4O4ClRu·2H2O (C
39.93, H 5.99, N 9.32); Found (C 40.45, H 5.94, N 9.08).
4.2.7. Trimethyl(13-oxa-bicyclo[10.1.0]tridecan-1-yl)-
silane 16.18 Colourless liquid. Purified by column chroma-
tography on silica gel (2% ethylacetate–hexanes). Yield
0.178 g, 70%; bp: 132–135 8C/1 mm (lit.,18 bp: 127–
4.2. General procedure for the epoxidation of cyclic
vinylsilanes
1
1348C/1 mm); H NMR (CDCl3, 300 MHz): d 0.016 (9H,
s), 1.0–2.35 (20H, complex multiplets), 2.98 (1H, d,
J¼2.7 Hz); 13C NMR (CDCl3, 75 MHz): d 24.3, 19.6,
20.0, 25.6, 54.1, 55.3.
To a magnetically stirred solution of the cyclic vinylsilane
(1 mmol) in dry dichloromethane (4 mL) was added
ruthenium complex 2 (0.014 g, 2.5 mol%), isobutyralde-
hyde (0.18 mL, 1.5 mmol) and solid sodium bicarbonate
(0.168 g, 1.5 mmol). The resulting homogenous solution
was stirred at room temperature under an atmosphere of
oxygen for 7–20 h. The progress of the reaction was
monitored by following the disappearance of the starting
olefin.
Acknowledgements
A. S. thanks the Council of Scientific and Industrial
Research, New Delhi for the grant of a Senior Research
Fellowship.
The reaction mixture was diluted with diethyl ether (25 mL)
and was filtered through a pad of Celite. The clear filtrate
was washed with water (2£15 mL) and saturated brine
solution (20 mL) and finally dried over anhydrous Na2SO4.
After removal of the solvent on a rotary evaporator, the
crude product was purified by column chromatography on
silica gel using 1–2% ethylacetate–hexanes. The pure
epoxysilanes gave physical constants and spectroscopic data
concordant with the literature values.
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