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It is necessary to distinguish the mechanism of alkene oxida-
tions with the BAP system in CH3CN/H2O from that of Payne s
procedure8 in alcoholic solvent. Payne oxidations employ a slight
excess of stoichiometric acetonitrile in alkaline hydrogen peroxide
solution to produce a peroxycarboximidic acid, which oxidizes
alkenes. The byproduct acetamide is obtained stoichiometrically
from the reaction of peroxycarboximidic acid with alkene or
hydrogen peroxide. In our study, oxidation of 4-vinylbenzene-
sulfonate with the BAP system in the presence of a stoichiometric
1
amount of acetonitrile in D2O was investigated by using H NMR.
Over 90% of the alkene was converted to its epoxide product in
1
24 h, but no acetamide was detected in the H NMR spectrum.
In contrast, replacement of NaHCO3 (pH. 8.4) or NH4HCO3 (pH.
8.0) by Na2CO3 (pH. 10.5) gave no oxidation products of the
alkene after 24 h, but acetamide was formed. We conclude that
the role of acetonitrile in the BAP system is to provide for
substrate solubility and maintain high solvent polarity, favoring
epoxidation by HCO4-.20
The mechanism for HCO4- epoxidation may be closely related
a
Stoichiometry: alkene
to that for typical peracids, i.e., the generally accepted butterfly
b
ammonium bicarbonate
transition state,21 except that the proton transfer is to a carbonate-
bicarbonate gave negligible epoxide products after 24 h, except for
leaving group (A) rather than to a carboxylates. Since the BAP
3-methyl-2-buten-1-ol (10% conversion to epoxide in 24 h). Dibasic
reactions here are in aqueous or mixed aqueous solution, the
ammonium phosphate was employed in controls to maintain similar
c
ionic strength and pH of reaction media. All allylic epoxides rear- intramolecular proton transfer that reduces charge separation in
d
ranged to form terminal epoxides as the major product. The epoxide
the transition state could also occur by solvent participation (e.g.,
e
was not stable; decomposition products not identified. Mixture of
B). Further studies on the detailed mechanism are in progress.
statistically distributed epoxide products.
alkenes, internal alkenes, and allylic alcohols) if a mixed solvent
system is used. By using acetonitrile/water (3:2 v:v), epoxidations
of hydrophobic alkenes were accomplished with H2O2 and NH4-
HCO3 (
Oxidation of styrene was followed in CD3CN/D2O (3:2, v:v)
by using NMR. Addition of styrene (0.05 M) to a solution of
13
H2O2 (0.3 M) and NH4HCO3 (0.2 M) yielded styrene oxide (40%)
Our C NMR studies on H13CO4- formation from H13CO3-
as the only product after 24 h. Because of peroxide dispropor-
with 2 MH2O2 in CH3CN/H2O (3:2, v:v) indicate Keq (eq 1) a"
tionation, excess hydrogen peroxide is needed to give a high yield
[HCO4-][H2O]/[HCO3-][H2O2] H" 35 (25 °C) with a t1/2
of epoxide, and the epoxidation reaction was attempted prepara-
(pH ) 7.4). After 20 h,
tively in CH3CN/H2O (3:2, v:v). With 0.19 M NH4HCO3, 10 equiv
decomposition based on the integration ratio of H13CO3- and
of 30% aqueous H2O2 gave styrene oxide in 75% distilled yield.18 H13CO4- in the spectrum. Therefore, decomposition of hydrogen
Other unfunctionalized alkenes in Table 2 (R-methylstyrene and
peroxide in acetonitrile is relatively slow compared to the
norbornene) form epoxide as the major product. The rate of alkene
formation of HCO4-. Oxidation reactions of alkenes with moder-
oxidation decreases significantly by replacement of acetonitrile
ate reactivity can be achieved by forming HCO4- with a small
with alcohol, e.g., ethanol or tert-butyl alcohol. For example, only
excess of H2O2 despite the accompanying decomposition of H2O2
trace oxidation products were detected for styrene after heating
in the presence of CH3CN as a cosolvent.22 Catalyst lifetime is
to 45 °C for 2 days in d6-EtOH/D2O (3:2, v:v) with H2O2 (0.3
not a major concern given the low cost and high stability of
M) and NH4HCO3 (0.2 M).19
bicarbonate ion.
BAP oxidations of various allylic alcohols were also investi-
We believe BAP oxidations can be useful when a mild, low
gated. Allyl alcohol (0.1 M) and 2-cyclohexen-1-ol (0.1 M) have
environmental impact oxidation method is desirable.23,24 Some
the least reactive double bonds, and only trace oxidation products
limits to the utility of the method remain to be overcome (e.g.,
are observed for dilute H2O2 (0.3 M) with NH4HCO3 (0.2 M) in
low conversions for less nucleophilic substrates, hydrolysis of
CD3CN/D2O after 24 h. Allylic alcohols with more substituted
sensitive epoxides). Kinetic studies and development of optimal
double bonds are epoxidized by the BAP system under similar
catalysts and synthetic methods for alkenes and other substrates
conditions (Table 2). For all of the allylic alcohols epoxidation
are in progress.
is strongly preferred over alcohol oxidation. In the case of
JA993935S
geraniol, both allylic and remote alkene are oxidized with
comparable rates. A striking feature in the BAP oxidation of these
(20) In addition, the same reaction was carried out in mixed CD3CN/D2O
(18) Procedure: 6.30 g of NH4HCO3 (79 mmol) and 38 mL of H2O2 (30%, (1:7, v:v) solvent that was buffered with (NH4)2HPO4 to maintain similar pH
360 mmol) were dissolved in 130 mL water, mixed with 240 mL of and ionic strength compared to a bicarbonate solution, and only 5% of alkene
acetonitrile, and 4 mL of styrene (35 mmol) was added. The rt reaction was conversion was observed after 24 h. In contrast to the simplicity of the
allowed to proceed in the dark without stirring for 24 h. The reaction mixture homogeneous BAP procedure, the Payne procedure requires stirring and
was diluted with 200 mL of water and extracted with chloroform (5 × 200 continuous addition of peroxide and base,
mL). The filtrate was washed with water (2 × 40 mL), dried, and concentrated (21) See Bach, R. D.; Glukhovtsev, M. N.; Gonzalez, C. J. Am. Chem.
by removal of solvent. Fractional distillation of the crude product gave 3.1 g Soc. 1998, 120, 9902-9910 and references therein.
of styrene oxide (75%). (22) As was found in Payne s study, decomposition of H2O2 is significantly
(19) The observations of solvent dependence in the BAP epoxidations accelerated in the higher pH media of CH3CN/H2O with added Na2CO3, and
contrast with those for the H2O2/dicyclohexylcarbodiimide (DCC) system acetamide byproduct is observed.
reported by by Majetich and co-workers.9 The best solvents for the DCC- (23) Bolm, C.; Beckman, O.; Dabard, O. A. G. Angew. Chem., Int. Ed.
activated epoxidation are hydroxylic ones such as methanol, ethanol or 1999, 38, 907.
2-propanol (except pure water). (24) Dartt, C. B.; Davis, M. E. Ind. Eng. Chem. Res. 1994, 33, 2887. [ Pobierz całość w formacie PDF ]