Enantioselective organocatalytic epoxidation driven by electrochemically generated percarbonate and persulfate

Article abstract of DOI:10.1002/adsc.200800064

An organocatalytic asymmetric epoxidation reaction using iminium salt catalysts is described, in which the stoichiometric persulfate oxidant is generated electrochemically. This system offers comparable ees to the use of commercially available persulfate.

Full text of DOI:10.1002/adsc.200800064

FULL PAPERS
DOI: 10.1002/adsc.200800064
Enantioselective Organocatalytic Epoxidation Driven by
Electrochemically Generated Percarbonate and Persulfate
Philip C. Bulman Page,a,* Frank Marken,b,* Craig Williamson,c Yohan Chan,a
Benjamin R. Buckley,c and Donald Bethelld
a
School of Chemical Sciences & Pharmacy, University of East Anglia, Norwich, Norfolk NR4 7TJ, U.K.
Fax : (+44)-(0)1603-592-003; e-mail: p.page@uea.ac.uk
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
Department of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, U.K.
Department of Chemistry, University of Liverpool, P O Box 147, Liverpool L69 3BX, U.K.
b
c
d
Received: January 29, 2008; Published online: April 9, 2008
Dedicated to Prof Andreas Pfaltz on the occasion of his 60th birthday
Abstract: An organocatalytic asymmetric epoxida- percarbonate ion is also a successful and novel oxi-
tion reaction using iminium salt catalysts is de- dant system for use with iminium salts.
scribed, in which the stoichiometric persulfate oxi-
dant is generated electrochemically. This system
offers comparable ees to the use of commercially Keywords: catalysis; electrochemistry; epoxidation;
available persulfate. Electrochemically generated Oxone; percarbonates; persulfates
Introduction
nature of the reaction intermediates by NMR spec-
troscopy.[7]
We have developed a highly enantioselective organo-
Use of such a stoichiometric oxidizing reagent, with
catalytic asymmetric epoxidation process.[1] In this the concomitant production of inorganic by-products,
system, an iminium salt catalyst, for example 1 or 2 necessarily brings inherentenvironmenatl disadvan-
(Figure 1), is oxidized to the corresponding oxaziridi- tages and other hazards, and we were therefore inter-
nium salt, which is active in the transfer of oxygen to ested in seeking a ꢀgreenꢁ variant of this reaction, in
olefins. We have observed ees of up to 97%.[2] The use which the requirement for addition of an inorganic or
of iminium salts as pre-catalysts for asymmetric epoxi- organic stoichiometric oxidant would be eliminated.
dation reactions was first observed in 1987,[3] and has We conjectured that this aim might be achievable if
subsequently been reported by several groups.[4] As the iminium salt-catalysed asymmetric epoxidation
with other catalytic epoxidations, this system of process could be driven by an electrochemical means,
course requires the presence of a stoichiometric, ter- with water as the source of the epoxide oxygen atom.
minal oxidant, usually the water-soluble triple salt Electrochemically driven catalytic epoxidations were
[8]
Oxone. We have demonstrated that tetraphenylphos- firstreporetd in 1986;
in two independent reports,
phonium monoperoxysulfate (TPPP), which can be metalloporphyrins were used to mediate oxygen
prepared from Oxone and is soluble in organic sol- transfer to unfunctionalized olefins. Since these re-
vents,[5] is also a compatible terminal oxidant,[6] and ports, there have been several further instances of cat-
use of this oxidant has allowed us to investigate the alytic electroepoxidations,[9] butin nearly all cases the
oxygen transfer step has been reliant on a metal
atom, and very few have been enantioselective.[9b,d]
The formulation of an enantioselective organocatalyt-
ic variant of this reaction appears to be an attractive
proposition. Herein we report such an approach,
aided by the recently developed boron-doped dia-
mond (BDD) electrode-based direct conversion of
water into reactive peroxo intermediates.[10]
Figure 1. Iminium salt catalysts.
Adv. Synth. Catal. 2008, 350, 1149 – 1154
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1149

Philip C. Bulman Page etal.
FULL PAPERS
Table 1. Percarbonate-mediated epoxidation of 1-phenylcy-
clohexene.
Results and Discussion
Inorganic peracids such as peroxysulfates have previ-
ously been prepared electrochemically at both con-
ventional and boron-doped diamond electrodes.[11] We
reasoned that electrosynthetic monoperoxysulfate, the
active oxidizing constituent of Oxone, could be a
directanalogue of our successful Oxone sysetm. The
electrosynthetic process, however, is effective only at
very low pH,[10a] which we deemed unsuitable for
practical epoxidation reactions, and we therefore
sought an alternative oxidant.
Saha demonstrated that solutions containing
sodium percarbonate (i.e., Na2CO4), in equilibrium
with a mixture of hydrogen peroxide in carbonate
electrolyte, could be readily generated electrochemi-
cally from solutions of the corresponding carbonate[12]
when boron-doped diamond was used as the working
electrode. This process was selected as a suitable
starting point as it more closely matched the pH
range at which our epoxidation system had been de-
veloped. Percarbonate for epoxidation reactions has
also been generated from hydrogen peroxide pre-
pared electrochemically in ionic liquids.[9g]
Entry Carbonate source Co-solvent Conversion[a] [%]
(Isolated yield [%])
a
b
c
d
e
f
g
h
i
0.1M Na2CO3
0.5M Na2CO3
1.0M Na2CO3
1.0M NaHCO3
1.0M KHCO3
1.0M K2CO3
1.0M Cs2CO3
1.0M Na2CO3
1.0M Na2CO3
1.0M Na2CO3
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DCM
<5
<5
100 (99)
<5
<5
100 (95)
99 (83)
<5
<5
<5
MeOH
Acetone
j
k
1.0M (NH4)HCO3 MeCN
10
[a]
1
Conversions were determined by H NMR spectroscopy,
by integration of alkene and epoxide signals.
Electrolyses of solutions of 1M aqueous Na2CO3
were performed with vigorous stirring in a two-elec- (entries d and e); use of the more basic ammonium
trode undivided cell, using a BDD working electrode bicarbonate was also unsuccessful (entry k). Quantita-
(area 3 cm2) and a small Pt wire counter electrode tive voltammetric determination at the Pt electrode
(area 0.2 cm2), with an applied potential of 10.0ÀV2, using 1M aqueous sodium bicarbonate indicated that
producing an anodic currentdensiyt of 100 mAcm
.
percarbonate is not generated by the electrolysis pro-
In this process, the small counter electrode causes cess at this pH. Changing the reaction solvent com-
high current density hydrogen evolution, and diffu- pletely impeded the reaction: No epoxide was detect-
sion-controlled peroxide losses are minimized. Elec- ed when the reaction was conducted in dichlorome-
trolysis and chemical reactions were carried out in an thane, methanol, or acetone (entries h–j).
ice bath to dissipate heat. The presence of peroxy in-
Having proven the potential of the electrochemical-
termediate was subsequently confirmed by quantita- ly generated percarbonate system for epoxidation re-
tive hydrodynamic voltammetric determination (see actions, we were eager to investigate the potential
Experimental Section) at the Pt electrode using a asymmetric induction of this process. Analyses of the
commercial percarbonate standard. Following a 1 ees of epoxides prepared using catalyst 1 showed
hour electrolysis, the peroxide concentration reached near-racemic material in most cases (Table 2). This is
30–40 mM (approx. 30% currentefficiency), a similar
puzzling when it is considered that under both our
concentration to that used in our reactions with stoi- original Oxone- and TPPP-mediated conditions this
chiometric oxidants.
conversion proceeds with ees of between 17 and 49%
When an acetonitrile solution of catalyst 1 and 1- for these substrates.[13] As a further test, catalyst 2 was
phenylcyclohexene was emulsified with this percar- also evaluated. In this instance, low to moderate ees
bonate solution (approx. 2–4 equiv.) using vigorous were observed.
stirring, complete conversion into the epoxide was ob-
served within one hour. The results of these experi- metric induction with catalyst 2, t heees obtained are
ments are presented in Table 1. lower than we had expected, based on our previous
While it was satisfying to observe reasonable asym-
It can be seen that the concentration and precise results using commercial Oxone (up to 60% ee).[13] As
nature of the percarbonate precursor are crucial to percarbonate is a novel oxidant for this catalyst
the success of the reaction. Carbonate concentrations system, the possibility exists that significant back-
less than 1.0M appear to be too low for successful ep- ground epoxidation is occurring, which would be ex-
oxidation (entries a and b). While the nature of the pected to reduce the observed ees significantly. Entry
metal cation appears to have no effect on the success i shows that this is unlikely: a control experiment run
of the system (entries c, f, g), the use of a bicarbonate in the absence of iminium salt showed only a 1.9%
anion clearly prevents the reaction from proceeding conversion into epoxide in the same time period,
1150
asc.wiley-vch.de
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1149 – 1154

Enantioselective Organocatalytic Epoxidation
FULL PAPERS
Table 2. Percarbonate-mediated asymmetric epoxidation.
Entry Substrate
Catalyst Conversion[a] [%] Isolated yield [%] Absolute configuration[f] ee [%]
a
b
c
d
e
f
g
h
i
1-Phenylcyclohexene
Styrene
1
1
1
1
2
2
2
2
2
2
100
100
91
96
43
98
100
79
28
21
60
89
91
0
3[b]
9[b]
1,2-Dihydronaphthalene
1-Phenyl-3,4-dihydronaphthalene
trans-a-Methylstilbene
1-Phenyl-3,4-dihydronaphthalene
1,2-Dihydronaphthalene
1-Phenylcyclohexene
<1[c]
8[b]
(À)-(1S,2S)
14[b]
3[c]
97
100
100
1.9
96
(À)-(1S,2R)
(À)-(1S,2S)
35[b,c]
32[b,c]
-
1-Phenylcyclohexene[d]
1-Phenylcyclohexene[e]
j
84
(À)-(1S,2S)
32[c]
[a]
[b]
[c]
[d]
[e]
[f]
1
Conversions were determined by H NMR spectroscopy, by integration of alkene and epoxide signals.
Determined by chiral shift reagent H NMR spectroscopy.
Determined by chiral stationary phase gas chromatography using a Chiralcel OD column.
Experiment performed in the absence of catalyst.
Experiment performed with commercially available sodium percarbonate.
Established by comparison with literature data.
1
demonstrating that the background epoxidation pro-
Serrano reported the presence in low concentration
cess does not make a significant contribution to the of monoperoxysulfuric acid (H2SO5) as a side product
observed conversions, and ruling outhte significant
involvementof peroxyimidic acid derived from hte
in the electrosynthesis of peroxydisulfuric acid
(H2S2O8) from solutions of concentrated sulfuric
acetonitrile solvent. Indeed, we were unable to detect acid.[10a] Monoperoxysulfuric acid is thought to stem
any benzamide, the by-product of oxidation by per- from the hydrolysis of peroxydisulfuric acid, which ul-
ACHTREUNG
A further control experiment was performed using oxysulfuric acid and hydrogen peroxide are believed
a commercially available sample of sodium percar- to decompose rapidly, giving oxygen. It appears that
bonate (Aldrich).[14] Significantly, in this instance the rate of oxidation of the monoperoxysulfuric acid
(entry j), the conversion and ee were essentially iden- is higher than its rate of formation under the electro-
tical to those obtained using the electrochemical chemical reaction conditions, which would explain the
system (entry h). These data support the suggestion low concentration.[11b]
that the ees obtained using the electrochemically
driven system are a direct consequence of percarbon- clohexene under our standard oxone-mediated condi-
ate-driven, iminium ion-mediated epoxidation.
tions[15] using commercially available ꢀpotassium per-
We attempted the oxidation reaction of 1-phenylcy-
This interesting difference in the ees obtained by sulfateꢁ (peroxydisulfate, K2S2O8). This commercial
two distinct oxidant systems, monoperoxysulfate from material does not appear to activate iminium salts,
Oxone, and commercial percarbonate, could be inter- and we were unable to isolate the corresponding ep-
preted as evidence that iminium ion-mediated epoxi- oxide. We were therefore pleased to find that a suita-
dation with percarbonate as the terminal oxidant pro- ble oxidant could be conveniently prepared by the
ceeds through a different mechanism from the reac- electrolysis of concentrated sulfuric acid solution, fol-
tion in which monoperoxysulfate is the oxidant. In lowed by subsequentadditon of solid K 2CO3 until
order to investigate this hypothesis further, we felt it the cloudy suspension reached basic pH (pH 7–9).
to be desirable to run organocatalytic electroepoxida- This technique is reminiscent of the reported proce-
tions using a monoperoxysulfate system, to allow a dure for the industrial preparation of the Oxone
direct comparison with the original oxone and percar- triple salt.[16] Agitating this suspension with an organic
bonate systems. As indicated above, this is problemat- solventconatining an olefin and iminium saltcaatlyst
ic because, unlike percarbonate generation, electro- 2 was found to promote acceptable conversion of 1-
chemical generation of peroxymonosulfuric acid phenylcyclohexene into the corresponding (À)-
occurs only atvery low pH. [10a]
(1S,2S)-epoxide (Table 3). Quantitative voltammetric
Adv. Synth. Catal. 2008, 350, 1149 – 1154
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1151

Philip C. Bulman Page etal.
FULL PAPERS
Table 3. Persulfate-mediated asymmetric epoxidation of 1-phenylcyclohexene.
Entry
Cat. Loading [mol%]
Sulfate source
Co-solvent
Conversion[a] [%]
Isolated yield [%]
85
ee [%]
a
b
c
d
e
f
g
h
i
20
10
10
10
10
10
10
10
10
5
2.5M H2SO4
2.5M KHSO4
0.6M Na2SO4
2.5M (NH4)HSO4
1.0M H2SO4
2.0M H2SO4
2.5M H2SO4
5.0M H2SO4
2.5M H2SO4
2.5M H2SO4
2.5M H2SO4
2.5M H2SO4
5.0M H2SO4
5.0M H2SO4
2.5M H2SO4
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN[f]
MeCN
MeCN
MeCN
MeCN
DCM
100
0
0
13
16
64[b]
-
-
-
31
31
80
62
54
93
44
100
100
81
95
47
58[b]
54[c]
53[b]
59[c]
33[c]
-
j
k
l[e]
m[e]
n
o
10[d]
10
10
10
10
11
100
100
47
100
86
44
64[c]
34[b]
39[c]
MeOH
[a]
1
Conversions were determined by H NMR spectroscopy, by integration of alkene and epoxide signals.
[b]
[c]
[d]
[e]
[f]
1
Determined by chiral shift reagent H NMR spectroscopy.
Determined by chiral stationary phase gas chromatography using a Chiralcel OD column.
Catalyst 1 used.
Suspension was filtered prior to addition of organics.
100 mL used.
determination at the Pt electrode using 2.5M sulfuric 1H NMR spectroscopy (entry n). Performing the reac-
acid indicated that the total peroxide concentration tion in methanol (entry o) allowed a moderate con-
reached 60–70 mM following a one-hour electrolysis.
version to epoxide, although the observed ee was
When 5M sulfuric acid is used during the electroly- somewhat reduced. It is possible that transport of the
sis (see Experimental Section), it is convenient to oxidizing species across the aqueous/organic interface
remove by vacuum filtration the K2SO4 that precipi- occurs following reaction of the inorganic oxidant
tates during basification, and to perform epoxidations with the iminium salt to give a neutral adduct incor-
by emulsifying an organic solvent with the resulting porating a sulfate or carbonate residue; transport
persulfate solution. The use of more dilute acid properties would be expected to be different for the
(2.5M) also allows quantitative conversion into epox- two species. An alternative scenario is that the inor-
ides, but in this instance it was found that filtering the ganic oxidant is transported across the aqueous/organ-
precipitated solids from the oxidant reduced the con- ic interface as an ion pair by the iminium salt acting
version (entry l vs. entry g); it is possible that mono- as a phase-transfer agent; again, transport would be
peroxysulfate is co-precipitated under these condi- expected to be different for the percarbonate- and
tions.
It is interesting to note that, unlike the carbonate
persulfate-derived species.
Gratifyingly, analyses of epoxides produced by this
system, when the sulfate system was used it was possi- system using acetonitrile as the organic phase showed
ble to observe good conversions in solvents other ees of a similar magnitude to those observed in our
than acetonitrile. This is similar to the pattern of cata- original Oxone-driven system.[13] This provides evi-
lytic activity we have observed when our original dence that the active oxidant in this instance is indeed
Oxone system is used in these solvents. With the sul- monoperoxysulfate. As described above, the percar-
fate system, dichloromethane was successful when bonate-driven system provides lower ees.
using the more active catalysts, for example 2, particu-
There are two distinct mechanistic steps that could
larly when using emulsification by Ultraturrax agita- conceivably contribute to the control of enantioselec-
tion. When the reaction was performed in dichloro- tivity in a subsequent epoxidation reaction. These are
methane,
a 76% conversion was observed by the transfer of active oxygen from the peroxy salt to
1152
asc.wiley-vch.de
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1149 – 1154

Enantioselective Organocatalytic Epoxidation
FULL PAPERS
the iminium cation, generating an oxidizing species, Conclusions
and the subsequent oxygen transfer from this oxidiz-
ing species to the alkene substrate. The fact that the In summary, we have further developed our organoca-
percarbonate-driven system provides lower ees than talytic asymmetric epoxidation reaction such that the
the persulfate-driven system suggests that one of terminal, stoicheiometric oxidant can now be sepa-
these steps is less selective for percarbonate.
rately generated electrochemically as required. The
It is possible that the percarbonate and persulfate system generates persulfate on an as-required basis,
systems generate different concentrations of hydrogen and offers comparable ees to commercially available
peroxide in the respective equilibria, but, in the ab- persulfate. Further to this, we have identified electro-
sence of carbonate or sulfate, hydrogen peroxide does chemically generated percarbonate ion as a novel oxi-
not induce epoxidation in the presence of the iminium dant system for use with iminium salts, although at
salt catalysts.
this stage it appears that, despite permitting good con-
In each system, the active peroxy agent could add versions to epoxides, this system is not able to provide
to either face of the iminium ion, producing a pair of high asymmetric induction.
diastereoisomeric oxidizing intermediates, which
would each be expected to provide different enantio-
selectivities upon reaction with alkene substrates. The Experimental Section
difference in ionic Gibbs free energy of transfer be-
tween percarbonate and persulfate could affect the Epoxidation using Commercially Available Sodium
transition state reactivity during oxygen transfer and Percarbonate
therefore the ratio of oxaziridinium diastereoisomers.
Potassium carbonate (276 mg, 2 mmol) was dissolved in a
The additional role, if any, of the counter-ion of the
water:acetonitrile mixture (2:1, 30 mL) and the solution was
oxaziridinium cation is unclear, but if, as we believe,
cooled to 08C. Sodium percarbonate (314 mg, 2 mmol) was
the enantioselective oxidizing agent is in both percar-
bonate and persulfate systems the same pair of diaste-
reoisomeric oxaziridinium species, and the inorganic
components have no great effect on the epoxidation
process, then the oxygen transfer step to the alkene
substrate occurs from the same pair of diastereoiso-
meric oxidizing species in both systems. This points to
a different diastereofacial selectivity between the per-
carbonate and persulfate systems in the reaction of
the oxidant with the iminium cation, so generating a
different ratio of the diastereoisomeric oxaziridinium
intermediates, and consequently different ees in t he
epoxidation process (Figure 2).
then added to the solution. Catalyst 2 (35 mg, 10 mol%) was
dissolved in acetonitrile (1 mL) and added to the reaction
mixture at 08C. 1-Phenylcyclohexene (80 mL, 0.5 mmol) was
added, neat or in solution, and the reaction mixture stirred
at0 8C for one hour. Diethyl ether (20 mL) was then added
and the organic phase separated, washed with brine, dried
over sodium sulfate and concentrated under reduced pres-
sure. The residue was then placed on a column of silica gel
and eluted with ethyl acetate:petroleum ether (1:4) to
afford the title compound as a yellow oil; yield: 73 mg
(84%).
Epoxidation using Electrosynthesis of a Solution of
SodiumCarbonate
Aqueous sodium carbonate (1M, 100 mL) was placed in a
two-electrode undivided cell fitted with a boron-doped dia-
mond (Diafilm, Windsor Scientific, UK) electrode (3 cm2) as
the working electrode and a small platinum wire electrode
(0.2 cm2) as the counter electrode. The cell was placed in an
ice bath. With vigorous stirring, a 10.0 V current at 0.3 A
(Thurlby power source, anodic currentdensyit approx.
100 mAcmÀ2) was applied to the solution for one hour at
08C. A flow of nitrogen ensured that hydrogen gas evolving
at the cathode was removed and safely vented. The concen-
tration of peroxide was monitored by hydrodynamic voltam-
metry at the 0.2 cm2 platinum wire electrode in the stirred
solution. Under these conditions, the anodic limiting current
is mass transport controlled and directly proportional to the
peroxide concentration. Catalyst 1 or 2 (10 mol%) was dis-
solved in the corresponding organic solvent (50 mL), and
the solution cooled to 08C and added to the solution of per-
carbonate. The alkene (1.25 mmol), neat or in solution, was
added, and the reaction mixture stirred for one hour at 08C.
Diethyl ether (50 mL) was added, and the organic phase
separated, washed with brine, dried over sodium sulfate, and
concentrated under reduced pressure. The residue was
Figure 2. Catalytic reaction scheme showing the duality of
the overall enantioselection.
Adv. Synth. Catal. 2008, 350, 1149 – 1154
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1153

Philip C. Bulman Page etal.
FULL PAPERS
placed on a column of silica gel and eluted with ethyl acet-
liet, Tetrahedron Lett. 1988, 29, 3941; c) L. Bohꢃ, G.
Hanquet, M. Lusinchi, X. Lusinchi, Tetrahedron Lett.
1993, 34, 7271; d) L. Bohꢃ, M. Lusinchi, X. Lusinchi,
Tetrahedron 1999, 55, 141.
A
Epoxidation using Electrosynthesis of a Solution of
Sulfuric Acid
[4] a) V. K. Aggarwal, M. F. Wang, Chem. Commun. 1996,
191; b) A. Armstrong, G. Ahmed, L. Garnett, K.
Gioacolou, Synlett 1997, 1075; c) A. Armstrong, G.
Ahmed, L. Garnett, K. Gioacolou, J. S. Wailes, Tetrahe-
dron 1999, 55, 2341; d) L. Bohꢃ, M. Kammoun, Tetra-
hedron Lett. 2002, 43, 803; e) A. Gluszynska, I. Mack-
owska, M. D. Rozwadowska, W. Sienniak, Tetrahedron:
Asymmetry 2004, 15, 2499; f) M. R. Biscoe, R. Breslow,
J. Am. Chem. Soc. 2005, 127, 10812.
Aqueous sulfuric acid (2.5M, 100 mL) was placed in a two-
electrode undivided cell fitted with a boron-doped diamond
(Diafilm, Windsor Scientific, UK) electrode (3 cm2) as t he
working electrode and a small platinum wire electrode
(0.2 cm2) as the counter electrode. The cell was placed in an
ice bath. With vigorous stirring, a 5.0 V potential resulted in
a
1.0 A current(anodic currentdenysit approx.
330 mAcmÀ2) applied to the solution for one hour at 08C. A
flow of nitrogen ensured that hydrogen gas evolving at the
cathode was removed and safely vented. The concentration
of peroxide was monitored by hydrodynamic voltammetry
atthe 0.2 cm 2 platinum wire electrode in the stirred solution.
Under these conditions, the anodic limiting current is mass
transport controlled and directly proportional to the perox-
ide concentration. The solution was basified using potassium
carbonate until the pH of the mixture reached pH 7–9. Cat-
alyst 1 or 2 (10 mol%) was dissolved in the corresponding
organic solvent (50 mL), and the solution cooled to 08C and
added to the solution of peroxymonosulfate. 1-Phenylcyclo-
hexene (1.25 mmol), neat or in solution, was added, and the
reaction mixture stirred for one hour at 08C. Diethyl ether
(50 mL) was added, and the organic phase separated,
washed with brine, dried over sodium sulfate, and concen-
trated under reduced pressure. The residue was placed on a
column of silica gel and eluted with ethyl acetate:petroleum
ether (1:4) to yield the epoxide.
[5] S. Campestrini, F. Di Furia, G. Labat F. Novello, J.
Chem. Soc. Perkin Trans. 2 1994, 10, 2175.
[6] P. C. B. Page, D. Barros, B. R. Buckley, A. Ardakani,
B. A. Marples, J. Org. Chem. 2004, 69, 3595; P. C. B.
Page, B. R. Buckley, D. Barros, A. J. Blacker, B. A.
Marples, M. R. J. Elsegood, Tetrahedron 2007, 63, 5386.
[7] P. C. B. Page, D. Barros, B. R. Buckley, A. J. Blacker,
B. A. Marples, Tetrahedron: Asymmetry 2005, 16, 3488.
[8] a) J. T. Groves, J. A. Gilbert, Inorg. Chem. 1986, 25,
125; b) S. E. Creager, S. A. Raybuck, R. W. Murray, J.
Am. Chem. Soc. 1986, 108, 4225.
[9] a) Y. M. Lvoy, Z. Lu, J. B. Schenkman, X. Zu, J. F. Rus-
ling, J. Am. Chem. Soc. 1998, 120, 4073; b) H. Tanaka,
M. Kuroboshi, H. Takeda, H. Kanada, S. Torii, J. Elec-
troanal. Chem. 2001, 507, 75; c) Y. Shen, M. Atobe, W.
Li, T. Nonaka, Electrochim. Acta 2003, 48, 1041; d) P.
Guo, K.-Y. Wong, Electrochem. Commun. 1999, 1, 559;
e) C. M. Elliot, J. R. Dunkle, S. C. Paulson, Langmuir
2005, 21, 8605; f) L. Espinal, S. L. Suib, J. F. Rusling, J.
Am. Chem. Soc. 2004, 126, 7676; g) K. P. Ho, K. Y.
Wong, T. H. Chan, Tetrahedron 2006, 62, 6650.
[10] a) K. Serrano, P. A. Michaud, C. Comminellis, A.
Savall, Electrochim. Acta 2002, 48, 431; b) P. A. Mi-
chaud, E. Mahe, W. Haenni, A. Perret, C. Comninellis,
Electrochem. Solid-State Lett. 2000, 3, 77.
[11] a) C. Provent, W. Haenni, E. Santoli, P. Rychen, Elec-
trochim. Acta 2004, 49, 3737; b) J. Balej, M. Thumova,
M. Kaderavek, Collect. Czech. Chem. Commun. 1980,
45, 3254.
Acknowledgements
This investigation has enjoyed the support of the EPSRC
(Speculative Research Leading to Greener Chemical Technol-
ogies). We are indebted to The Royal Society for an Industry
Fellowship (to PCBP) and to the EPSRC Mass Spectrometry
Unit, Swansea.
[12] M. S. Saha, T. Furuta, Y. Nishiki, Electrochem. Solid-
State Lett. 2003, 6, D5; M. S. Saha, T. Furuta, Y. Nishi-
ki, Electrochem. Commun. 2004, 6, 201.
References
[1] a) P. C. B. Page, G. A. Rassias, D. Bethell, M. B. Schil-
[13] P. C. B. Page, G. A. Rassias, D. Barros, A. Ardakani, D.
ling, J. Chem. Soc. Perkin Trans.
1 2000, 3325;
Bethell, E. Merifield, Synlett 2002, 580.
b) P. C. B. Page, G. A. Rassias, D. Barros, A. Ardakani,
B. R. Buckley, D. Bethell, T. A. D.; Smith, A. M. Z.
Slawin, J. Org. Chem. 2001, 66, 6926.
[14] Commercial percarbonate (4.0 equiv.) was added be-
cause this quantity is the convention when using Oxone
as stoichiometric oxidant.
[15] Standard conditions: 5 mol% catalyst, 4 equiv. Na2CO3,
2 equiv. Oxone, 1:1 MeCN-H2O, 0 8C.
[2] P. C. B. Page, B. R. Buckley, A. J. Blacker, Org. Lett.
2004, 6, 1543; P. C. B. Page, H. Heaney, B. R. Buckley,
B. A. Marples, A. J. Blacker, Org. Lett. 2005, 7, 375.
[3] a) G. Hanquet, X. Lusinchi, P. Milliet, Tetrahedron
Lett. 1987, 28, 6061; b) G. Hanquet, X. Lusinchi, P. Mil-
[16] E I DuPontde Nemours and Company, U. S. Paetnt
2,802,722, 1957.
1154
asc.wiley-vch.de
ꢂ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1149 – 1154