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A. Kejriwal et al. / Journal of Molecular Catalysis A: Chemical 413 (2016) 77–84
oxone as the terminal oxidant [12]. Apart from these examples, a
vast majority of catalysts based on inexpensive first-row transi-
tion metal catalysts have exhibited only modest reactivity towards
olefin cis-hydroxylation [13].
dimethyl tartarate as the major product with high diol selectivities,
viz., diol/epoxide ratio of 7.5 which shoots up to 22 in presence of
cis-diol and epoxides with higher selectivity for the diol (D/E of 4.0,
Entry 5, Table 2). In presence of acetic acid, a clear preference for the
formation of cis-diol product, as evident from a D/E value of 14.3, is
one also exhibited similar product profile under identical reaction
reaction condition results in the formation of diols and epoxide
with somewhat lower yields (Entry 7 & 8, Table 2). In this case,
preliminary studies have indicated a competitive oxidation of the
phenyl ring of the substrate by 1/H2O2(AcOH) [32], which, in turn,
The results encouraged us to explore the catalytic reactivity of
the diiron(III) catalyst (1) towards electron-poor olefins. The cat-
alytic reactions have been performed in acetonitrile medium by
both syringe-pump as well as all-at-once addition of H2O2 (Table 1).
Oxidation of tert-butyl acrylate in presence of catalytic amount of
Fe(NO3)3. 9H2O is found to yield only a trace amount of epoxide
under the present experimental condition. Moreover, no oxidation
any metal catalyst. However, as shown in Table 1, using 5 mol%
catalyst 1, oxidation of tert-butyl acrylate affords the 17% syn-
diol and 6% epoxide in acetonitile at room temperature (298 K)
(Entry 1, Table 1). When the reaction is carried out in presence
of one-equivalent acetic acid (w.r.t. 1), modest improvement of
further improve the catalyst turnovers and substrate conversion,
H2O2 (1.5 equiv. w.r.t. the substrate) has been delivered over a
period of 15 min at a rate of 0.6 mL/h. Under the syringe pump
addition protocol, significant improvement substrate conversion
(Entry 4, Table 1) is observed and the corresponding syn-diol is
obtained as the major product. However, increasing the amount
of substrate under identical reaction condition has been found to
result in poor substrate conversion. As shown in Table 1, employing
iterative addition protocol, yield of the diol increases up to 64%
(Entry 7 & 8, Table 1). It is noteworthy in this regard that oxidation
of tert-butyl acrylate catalysed by the monomeric iron(II) com-
plex of bpmen [(bpmen)FeII(OTf)2] [19,28,36]. and H2O2 has been
shown to afford both epoxide and diol with a slight preference for
the diol product over epoxide (epoxide/diol = 1:1.5). Furthermore,
addition of acetic acid dramatically improved epoxide selectivity
(epoxide/diol = 5:1). Therefore, in order to have further insight into
the catalytic reactivity of complex 1, attempts have also been made
to assess the amount of catalytically reactive species at the end of
the reaction. Thus, after delivering H2O2 (75.0 mol, 1.5 equiv. w.r.t.
of 1 and HOAc, followed by stirring the reaction mixture for an
additional 15 min, a second aliquot of both the substrate, acid and
oxidant have been added in an identical fashion. Interestingly, the
catalytic system has been found to exhibit almost identical catalytic
reactivity (Entry 9, Table 1). The reactivity remains unchanged even
after the third addition of substrate, acid and oxidant indicating
almost complete regeneration of the oxidizing species at the end of
each catalytic cycle.
rationalizes the lower yield of the products derived from the C
C
oxidation. Oxidation of electron-rich olefins by 1/H2O2 exhibits a
complete reversal in product profile with epoxides as the major
products with minor amounts of cis-diols. The electron-rich cis-
cyclooctene has been converted to the cyclooctane oxide in 60%
yield. A small amount of the corresponding cis-diol product (10%)
has also been obtained (Entry 13, Table 2).
Addition of 1 equiv. of acetic acid prior to the addition of the
substrate results in considerable increase in the product yield (87%)
case of oxidation of 1-octene by 1/H2O2, overall yield of oxygenates
reaches 50% with an epoxide/cis-diol ratio of 4.0. Moreover, com-
bined yields of epoxide and cis-diol as well as the epoxide selectivity
have been found to increase in presence of one equivalent of AcOH
(Entries 15 & 16, Table 2). Olefin epoxidation catalysed by 1/H2O2
has exhibited a modest degree of retention of configuration in case
under identical reaction condition yielded the corresponding epox-
ide with 50% retention of configuration. However, in presence of
one equivalent AcOH, the epoxide product was formed with more
than 80% retention of configuration (Table 2). Such a high degree of
stereo-retention further supports the involvement of a metal-based
oxidant in the present oxidizing system [37].
The epoxidizing ability of 1/H2O2 has been compared with the
best known iron-based epoxidation catalysts in order to assess its
potential for the application in preparative scale organic synthesis.
The catalytic reactions were performed in acetonitrile medium
using a large excess of the olefin (1000 equiv./catalyst) and by
tion condition cyclooctene has been found to be converted into a
mixture of epoxide and cis-diol in 40% and 5% yields respectively.
erably upon addition of 1 equiv. acetic acid prior to the addition of
H2O2 (Table 3). In presence of acid turnover number (TON) based on
complex 1 reached 65 and only a trace amount (2%) of cis-diol was
obtained. As shown in Table 3, epoxidizing ability of 1/H2O2/AcOH
is comparable to that obtained with the (-oxo) diiron(III) com-
plex of a dinucleating pyridyl ligand (6-HPA) reported recently by
Kodera et al. under similar reaction condition. In this case, 70% of the
oxidant has been accounted for the products. In comparison, 65% of
the total H2O2 has been shown to be converted into mainly epox-
ides during oxidation of cis-cyclooctene by the present catalytic
system.
The role of the acidic additives in the catalytic olefin oxidation
by 1/H2O2 is also evaluated. The formation of cis-diol product has
been found to get suppressed in oxidation of tert-butyl acrylate by
1/H2O2 in presence of mineral acids such as HCl and HNO3 (Table
the catalytic reactivity of 1/H2O2 and pKa of the additives can be
highlighted, it is believed that the catalytic reactivity is augmented
due to the binding of acetic acid in equilibrium condition.
The substrate scope of the present catalytic system has also been
examined and the results are summarised in Table 2. In all catalytic
reactions, the ratio of catalyst:substrate:HOAc:H2O2 of (1:20:1:30)
has been maintained and the oxidant is delivered over a period of
15 min (see Section 2 for details). For the electron deficient olefins,
cis-diols are obtained as the major product with modest yields.
Oxidation of dimethyl fumarate afforded the cis-diol product, d/l-
In order to evaluate the nature of oxidant in the present cat-
alytic system, competitive olefin oxidation reactions under similar
reaction condition were performed. Equimolar amounts of a pair of
olefin substrates have been oxidised by 1/H2O2 either in presence
& in absence of AcOH. The results are presented in Fig. 1 and Table
S1. For instance, when cis-cyclooctene and tert-butyl acrylate are
used as substrates, cycloctene oxidation yields 55% products (epox-
ide/diol = 4.5:1) while only 4% oxygenates (epoxide/diol = 1:3) are