Sodium percarbonate as an oxygen source for mto catalyzed epoxidations

Full text of DOI:10.1021/jo000191o

4210
J . Org. Chem. 2000, 65, 4210-4212
Sod iu m P er ca r bon a te a s a n Oxygen Sou r ce
for MTO Ca ta lyzed Ep oxid a tion s
Andrew R. Vaino*
Department of Chemistry and the Skaggs Institute for
Chemical Biology, The Scripps Research Institute, 10550
North Torrey Pines Rd., La J olla, California 92037
Received February 10, 2000
Since its introduction by the Herrmann group in 1991,1
the singularly efficient epoxidation of olefins using
methyltrioxorhenium (MTO) in the presence of hydrogen
peroxide has elicited much interest. MTO is of particular
benefit for the epoxidation of notoriously unreactive
monosubstituted olefins.2 One of the initial difficulties
with the MTO-H2O2 reaction was decomposition of acid-
sensitive epoxides exposed to the system’s inherent
acidity. This difficulty was addressed by the introduction
of a substoichiometric amount of pyridine3 into the
reaction mixture, which served not only to stabilize acid-
susceptible epoxides, but also to enhance the rate of
epoxidation. Subsequently, it was reported that either
pyrazole (pz)4 or 3-cyanopyridine2,5 was superior as
an accelerant.
In the MTO epoxidation, aqueous hydrogen peroxide
is typically added dropwise to a dichloromethane solution
of the olefin, accelerant, and MTO.3 Sources of the
peroxide other than aqueous hydrogen peroxide, for
example, urea-hydrogen peroxide adduct6 or bis(tri-
methylsilyl)peroxide,7 have been successfully applied to
the MTO epoxidation. Sodium percarbonate (SPC), to-
gether with sodium perborate (SPB), represent two of the
most powerful, yet underused, oxidants available.8 SPC
and SPB are particularly advantageous owing to their
ease of handling and storage. Epoxidation of R,â-unsatur-
ated ketones has been achieved using either SPC9 or
SPB.10 In addition, there have been reports of epoxidation
by SPB in the presence of acetic anhydride11 or acetic
acid,12 or by SPC in the presence of trifluoroacetic
F igu r e 1. Mode of activation of MTO by SPC/TFA for
epoxidation of olefins.
Sch em e 1
anhydride.13 SPC has been used in chromium-catalyzed
oxidation in organic solvents, although this method
requires a phase-transfer catalyst.14
* Phone: (858) 784-2517. Fax: (858) 784-2590. E-mail: Andrew@
scripps.edu.
(1) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 1638.
(2) Cope´ret, C.; Adolfsson, H.; Sharpless, K. B. J . Chem. Soc., Chem.
Commun. 1997, 1565.
(3) Rudolph, R.; Reddy, K. L.; Chiang, J . P.; Sharpless, K. B. J . Am.
Chem. Soc. 1997, 119, 6189.
(4) Herrmann, W. A.; Kratzer, R. M.; Ding, H.; Thiel, W. R.; Glas,
H. J . Organomet. Chem. 1998, 555, 293.
(5) Adolfsson, H.; Converso, A.; Sharpless, K. B. Tetrahedron Lett.
1999, 40, 3991.
(6) Adam, W.; Mitchell, C. M. Angew. Chem., Int. Ed. Engl. 1996,
35, 533.
(7) Yudin, A. K.; Sharpless, K. B. J . Am. Chem. Soc. 1997, 119,
11556.
A means to combine the tremendous epoxidizing power
of MTO with the mildness of either SPB or SPC was
sought. SPB is known to exist as a hydrate of the
disodium salt of a 1,4-diboratetroxane having discrete
peroxo-boron bonds.15 An X-ray study of the crystal
structure of SPC has revealed that H2O2 is found whole,
encapsulated by hydrogen bonds in a Na2CO3 matrix.16
SPC is known to decompose both in water17 and in
organic solvents18 to release H2O2, albeit very slowly. It
was hypothesized that the use of an acid should aid in
the release of H2O2 from both SPB and SPC. Upon
(8) For reviews on SPB and SPC, see: (a) McKillop, A.; Sanderson,
W. R. Tetrahedron 1995, 51, 6145. (b) Muzart, J . Synthesis 1995, 1325.
(9) Allen, J . V.; Drauz, K-.D.; Flood, R. W.; Roberts, S. M.; Skidmore,
J . Tetrahedron Lett. 1999, 40, 5417.
(10) (a) Savizky, R. M.; Suzuki, N.; Bove´, J . L. Tetrahedron:
Asymmetry 1998, 9, 3967. (b) Straub, T. S. Tetrahedron Lett. 1995,
36, 663. Dehmlow, E. V.; Vehre, B. New J . Chem. 1989, 13, 117.
(11) (a) Tao, F.-G.; Xu, L.-X.; Lu, Y.-Z.; Ma, S.-M.; Sun, G.-Z.; Xie,
G.-Y. Acta Chim. Sinica Engl. Ed. 1989, 463. (b) Xie, G.; Xu, L.; Hu,
J .; Ma, S.; Hou, W.; Tao, F. Tetrahedron Lett. 1988, 29, 2967.
(12) Zarraga, M.; Zunza, H.; Perez, C.; Franco, C.; Miranda, A. Bol.
Soc. Chil. Quı´m. 1998, 43, 213.
(13) Kang, H.-J .; J eon, H. S. Bull. Korean Chem. Soc. 1996, 17, 5.
(14) Delaval, N.; Bouquillon, S.; He´nin, F.; Muzart, J . J . Chem. Res.
Synop. 1999, 286 and references therein.
(15) (a) Hansson, A. Acta Chem. Scand. 1961, 15, 934. (b) Carrondo,
M. A. A. de C. T.; Skapski, A. C. Acta Crystallogr. 1978, B34, 3551.
(16) de C. T. Carrondo, M. A. A.; Griffith, W. P.; J ones, D. P.;
Skapski, A. C. J . Chem. Soc., Dalton Trans. 1977, 2323.
(17) Galwey, A. K.; Hood, W. J . J . Chem. Soc., Faraday Trans. 1
1982, 78, 2815.
(18) Rocha Gonsalves, A. M. d’A; J ohnstone, R. A. W.; Pereira, M.
M.; Shaw, J . J . Chem. Res., Miniprint 1991, 2101.
10.1021/jo000191o CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/31/2000

Notes
J . Org. Chem., Vol. 65, No. 13, 2000 4211
Ta ble 1. Exa m p les of SP C-MTO ep oxid a tion of olefin s
a
b
Products were identified by 1H NMR and 13C NMR spectroscopy. Reactions were performed on a 10 mmole scale. THF as solvent.
c CH2Cl2 as solvent. Yields are isolated yields.
d
addition of either acetic or trifluoroacetic acid to a
mixture of SPC, pyrazole, cyclooctene, and MTO in THF
a vibrant yellow colorscharacteristic of the peroxo-
activated MTO speciesswas noted. After being stirred
for 2 h, the TFA/SPC reaction resulted in complete
conversion of the olefin to the epoxide, while the yield
for the AcOH reaction was markedly lower. No reaction
was observed when SPB was used as the oxygen source,
nor was the distinct yellow color observed. In the absence
of MTO no epoxide was formed, consistent with the
inability of SPC to form peracids directly from organic
acids.8a Inorganic acids, namely 4 N HCl in 1,4-dioxane
or concentrated H2SO4 were tried in the reaction; no
epoxide formation was noted.
A combination of 2.5 equiv of SPC, 2.5 equiv of TFA, 1
equiv of olefin, 12 mol % pyrazole, and 1 mol % MTO in
either THF or CH2Cl2 was found to effectively convert a
wide range of olefins to epoxides. The overall process is
depicted in Scheme 1. Results are sumarized in Table 1.
Using less than 2.5 equiv of SPC/TFA resulted in lower
yields. Further, it was noted that addition of the MTO
in two portions, the second addition approximately
halfway through the reaction, improved the yield of
epoxide.
slow trans-phasic interaction between SPC and the acid
ensures the slow release of H2O2. In contrast to the
aqueous H2O2 reaction no external cooling is required.
Monitoring the pH of the reaction mixture19 revealed that
while the pH was initially low, ca. 2.5, it rose to a
constant value of 10.5 after 15 min.
A disadvantage of the present route is the requirement
that TFA be used as the acid. As the acid neutralizes
the SPC, it chips away at the carbonate sarcophagus in
which the H2O2 is held (Figure 1). Thus, liberation of
H2O2 is accompanied by formation of NaHCO3 together
with carbonic acid (CO2 evolution is observed) and
F3COONa. The sodium salts produced in this way will
be negligibly soluble in organic solvents. Epoxidation of
R-methyl styrene was attempted using AcOH in place of
TFA. While the characteristic yellow color of the activated
rhenium species was indeed evident, at best 70% conver-
sion of the olefin was observed. Of note is that the
reaction with AcOH, either in CH2Cl2 or in THF, was
much more difficult to stir, suggesting that the inorganic
salts produced during the course of the reaction were
(19) Accurate monitoring of pH for nonaqueous systems is known
to be fraught with difficulties; see, for example: Gold, V.; Grist, S. J .
Chem. Soc., Perkin Trans. 2 1972, 89. In the present study, 200 µL
aliquots were removed at 5 min intervals and diluted into 5 mL of
H2O and the pH measured.
As a peroxide source SPC in concert with TFA has
some advantages over aqueous hydrogen peroxide. The

4212 J . Org. Chem., Vol. 65, No. 13, 2000
Notes
pyrazole (82 mg, 1.2 mmol), and MTO (12.5 mg, 0.05 mmol). The
mixture was stirred vigorously as TFA (1.9 mL, 25 mmol) was
added dropwise over the course of 2 min. After addition of a few
drops TFA, a vibrant yellow color was noted. After the mixture
was stirred for an additional 3 h, a further aliquot of MTO (12.5
mg, 0.05 mmol) was added, and stirring was continued for 3 h.
The mixture was introduced into a separatory funnel together
with CH2Cl2 and water. The organic phase was removed, and
the aqueous phase was further extracted with CH2Cl2. To the
combined organic extracts was added a trace of MnO2 to
decompose any remaining peroxide. The solution was dried over
MgSO4 and filtered, and the solvent was removed under vacuum.
Purification was achieved by Kugelrohr distillation to afford the
product as a clear oil (1.49 g, 96%). For diolefins, twice the
amounts of MTO, SPC, THF, and pyrazole were employed.
interfering with the release of H2O2; a fairly common
problem with solid phase reagents when the desired
reaction with a component in solution produces a new
solid phase which effectively covers the reactive surface,
precluding further reaction. Fortuitously, this does not
occur when F3COO-+Na is the new salt produced at the
surface.
Of note in Table 1 is the ready epoxide formation with
monosubstituted olefins (entries 6 and 7). For terminal
olefins the use of CH2Cl2 as solvent afforded better yields.
Both cis and trans olefins are readily epoxidized.
A novel means to harness the oxidizing power of
sodium percarbonate, a so-called “solid form” of H2O2 has
been demonstrated.16 The added safety and ease of
handling using SPC, commonly employed as an additive
in household washing detergent and toothpaste, in place
of aqueous hydrogen peroxide, together with the slow
release of the H2O2 due to the trans-phasic nature of its
release from SPC render this a useful method.
Ack n ow led gm en t. The National Institute of Gen-
eral Medical sciences, National Institutes of Health
(GM-28384), the National Science Foundation (CHE-
9531152), and the W. M. Keck Foundation are thanked
for financial support to the Sharpless laboratory. Pro-
fessor K. Barry Sharpless is warmly thanked for his
support and insightful discussions. Dr. Bruce R. Bender
is thanked for helpful discussions. Neal Read (Scripps)
and Austin Chen (University of New Brunswick) are
thanked for their meticulous review of the manuscript.
NSERC (Canada) is thanked for financial support in the
form of a postdoctoral fellowship.
Exp er im en ta l Section
All compounds in Table 1 have been previously isolated and
characterized.3,20
Typ ica l P r oced u r e. To a solution of cis-4-decene (1.8 mL,
10 mmol) in THF (7 mL) were added SPC (3.9 g, 25 mmol),
(20) Swern, D. In Organic Peroxides; Swern, D., Ed.; Wiley: New
York, 1970; Vol. 2, p 355.
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