Nitrous oxide oxidation of olefins catalyzed by ruthenium porphyrin complexes

Article abstract of DOI:10.1246/cl.2001.268

In the presence of a catalytic amount of dioxo(tetramesitylporphyrinato)ruthenium(VI) complex, nitrous oxide (N₂O) oxidized trisubstituted olefins into the corresponding epoxides in good-to-high yields with high selectivities.

Full text of DOI:10.1246/cl.2001.268

268
Chemistry Letters 2001
Nitrous Oxide Oxidation of Olefins Catalyzed by Ruthenium Porphyrin Complexes
Tohru Yamada, Kentaro Hashimoto, Yasunori Kitaichi, Kyosuke Suzuki, and Taketo Ikeno
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama 223-8522
(Received December 28, 2000; CL-001169)
In the presence of a catalytic amount of dioxo(tetramesityl-
porphyrinato)ruthenium(VI) complex, nitrous oxide (N2O) oxi-
dized trisubstituted olefins into the corresponding epoxides in
good-to-high yields with high selectivities.
Nitrous oxide (N2O) is one of the most popular nitrogen
oxides.1 It has been medically employed as an anesthetic gas,
while a vast amount of nitrous oxide is emitted as a byproduct
from the industrial manufacturing of nylon2 and it has been
recently implicated with global warming as a greenhouse gas.3
In the field of synthetic organic chemistry, only several trials
involving nitrous oxide were reported as practical reactions.4
Since the decomposition of nitrous oxide into dinitrogen and
oxygen atoms requires a high activation energy, nitrous oxide
oxidation was usually performed under extremely drastic condi-
tions, i.e., high temperature4 and/or high pressure.5 Nitrous
oxide oxidation under mild conditions should provide a clean
oxidation reaction because dinitrogen is the only waste. Hence,
we attempted to employ transition-metal complexes as catalysts
in order to lower the activation energy for the decomposition of
nitrous oxide. The expected catalytic cycle is depicted as
Figure 1; that is, a transition-metal complex receives an oxygen
atom from nitrous oxide while releasing dinitrogen to generate
the metal oxo complex, which oxidizes a substrate to regenerate
the metal complex. Based on this working hypothesis, it was
already found and reported that low-valent nickel complexes
effectively activated nitrous oxide to oxidize phosphine(III) to
phosphine oxide(V),6 although this system was limited within
the oxidation of phosphine derivatives. In this communication,
we would like to describe the first success for catalytic nitrous
oxide epoxidation of olefins using ruthenium porphyrin com-
plexes as catalysts.
The transfer of an oxygen atom from nitrous oxide to Ru(TMP)
and stoichiometric epoxidation of β-methylstyrene by the gen-
erated dioxoruthenium complex has already been reported by
Groves et al.,8 however, catalytic nitrous oxide epoxidation has
not yet been reported, in spite of the fact that the possibility of a
catalytic cycle was recognized.9 This preliminary observation
prompted us to examine the nitrous oxide oxidation of choles-
teryl benzoate in the presence of a 5 mol% Ru(TMP)(O)2 com-
plex (Table 1).
When polar solvents, such as THF, EtOH, and acetonitrile,
were used, no product was obtained (Entries 1–3). On the con-
trary, the nitrous oxide oxidation in toluene afforded the corre-
sponding epoxide in moderate yield. A variety of aromatic sol-
vents were examined and fluorobenzene was then found to be a
suitable solvent (Entries 4–6).10 Also, the concentration of the
reaction was crucial; When the reaction was performed under
diluted conditions, the yield of the epoxide attained 99% (Entry
7). It is assumed that the more nitrous oxide dissolved in the
solution accelerated the reproduction of the dioxoruthenium
complex in the catalytic cycle. Chlorobenzene was also a suit-
able solvent at 140 °C (Entry 8). Control experiments revealed
that no reaction proceeded without the complex catalyst.
Although a 9% yield of the product was obtained when the
reaction was performed under an argon atmosphere in the pres-
ence of 5 mol% dioxoruthenium complex, it was considered
that the oxygen atom of the epoxide was stoichiometrically
It was reported that cholesteryl esters were preferentially
epoxidized on their α-face using peroxy acid as an oxidant,
while their β-epoxides were selectively obtained when transi-
tion-metal oxo species were used as oxidants.7 Hence, choles-
teryl esters were adopted as the probe for the transition-metal
catalyzed nitrous oxide oxidation. After various transition-
metal complexes were examined as catalysts for the nitrous
oxide oxidation of cholesteryl benzoate, a β-selective epoxi-
dized product was obtained in low yield using dioxo(tetrame-
sitylporphyrinato)ruthenium(VI) (Ru(TMP)(O)2) as a catalyst.
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
269
transferred from the starting dioxoruthenium complex. When
employing the dioxo(tetraphenylporphyrinato)ruthenium(VI),
dioxo(octaethylporphyrinato)ruthenium(VI), and dioxo-
[tetrakis(4-methoxyphenyl)porphyrinato]ruthenium(VI) com-
plexes as catalysts, no product was obtained with almost total
Cholesteryl acetate was epoxidized in quantitative yield
(Entry 1), and other substrates with a steroidal framework were
also oxidized in 97–99% yield (Entries 2 and 3). Various
trisubstituted acyclic olefins were smoothly epoxidized with
good-to-high yield. The isolated carbon-carbon double bond in
neryl acetate was predominantly oxidized to afford the corre-
sponding mono-epoxide in good yield (Entry 9). Although
diepoxidation was observed during the epoxidation of tert-
butyldimethylsilyl(TBDMS)ether of nerol, the isolated carbon-
carbon double bond was preferentially oxidized compared with
that of the allylic alcohol (Entry 10). Epoxidation of the
TBDMS ether of citronellol proceeded to afford the correspon-
ding epoxide in high yield (Entry 11). An activated disubstitut-
ed olefin could be easily epoxidized to afford the corresponding
product in high yield (Entry 12).
recovery of the cholesteryl benzoate.
When the
dioxo[tetrakis(2,6-dichlorophenyl)porphyrinato]ruthenium(VI)
complex was used as a catalyst, the reaction proceeded to
obtain the epoxidized product in 78% yield (Entry 9). These
observations suggested that porphyrin complexes with sterically
hindered aryl groups were effective catalysts for the nitrous
oxide oxidation.9,11 In all cases mentioned above, β-epoxides
were predominantly obtained, and these selectivities indicated
that the dioxoruthenium complexes reacted with the
carbon–carbon double bonds in the cholesteryl benzoate during
the oxidation step. This reaction would be the first success for
transition-metal catalyzed epoxidation using nitrous oxide as an
oxidant.
In conclusion, the catalytic version of nitrous oxide epoxi-
dation was achieved for the first time using 5 mol%
Ru(TMP)(O)2 as a catalyst to obtain the epoxides in good-to-
high yield. A mechanistic study, the scope, and the limitations
of the present oxidation system are currently in progress.
The catalytic nitrous oxide oxidation by ruthenium com-
plexes was successfully applied to various olefinic compounds
(Table 2).12,13
References and Notes
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10 The reactions in fluorobenzene also gave slightly better yields than in
benzene for the other substrates.
11 J. T. Groves and R. Quinn, J. Am. Chem. Soc., 107, 5790 (1985).
12 The nitrous oxide oxidation of olefins less reactive than cholesteryl ben-
zoate, such as 2-methyl-2-dodecene and the TBDMS ether of nerol, did
not complete at 100 °C in fluorobenzene, but better yields were obtained
at 140 °C. Chlorobenzene was the suitable solvent compared with fluo-
robenzene for the reaction at 140 °C.
13 Typical procedure is as follows: To a chlorobenzene solution of
Ru(TMP)(O)2 (21 mg, 0.023 mmol) in an autoclave, 2-methyl-2-
dodecene (50 mg, 0.27 mmol) was added under a nitrous oxide atmos-
phere. After the solvent amount was fixed at 14 mL, the reaction mix-
°
ture was heated at 140 C under 10 atm of N2O for 5 h. The desired
epoxide was purified through by silicagel column chromatography
(Hexane:EtOAc 20:1, 86% yield).