Controlling Selectivity in Alkene Oxidation: Anion Driven Epoxidation or Dihydroxylation Catalysed by [Iron(III)(Pyridine-Containing Ligand)] Complexes

Article abstract of DOI:10.1002/cctc.201901045

A highly reactive and selective catalytic system comprising Fe(III) and macrocyclic pyridine-containing ligands (Pc-L) for alkene oxidation by using hydrogen peroxide is reported herein. Four new stable iron(III) complexes have been isolated and characterized. Importantly, depending on the anion of the iron(III) metal complex employed as catalyst, a completely reversed selectivity was observed. When X=OTf, a selective dihydroxylation reaction took place. On the other hand, employing X=Cl resulted in the epoxide as the major product. The reaction proved to be quite general, tolerating aromatic and aliphatic alkenes as well as internal or terminal double bonds and both epoxides and diol products were obtained in good yields with good to excellent selectivities (up to 93 % isolated yield and d.r.=99 : 1). The catalytic system proved its robustness by performing several catalytic cycles, without observing catalyst deactivation. The use of acetone as a solvent and hydrogen peroxide as terminal oxidant renders this catalytic system appealing.

Full text of DOI:10.1002/cctc.201901045

Accepted Article
Title: Controlling selectivity in alkene oxidation: anion driven
epoxidation or dihydroxylation catalysed by [Iron(III)(Pyridine-
Containing Ligand)] complexes
Authors: Giorgio Tseberlidis, Luca Demonti, Valentina Pirovano, Marco
Scavini, Serena Cappelli, Silvia Rizzato, Rubén Vicente, and
Alessandro Caselli
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Controlling selectivity in alkene oxidation: anion driven epoxidation
or dihydroxylation catalysed by [Iron(III)(Pyridine-Containing
Ligand)] complexes
Giorgio Tseberlidis,a Luca Demonti,a Valentina Pirovano,b Marco Scavini,a Serena Cappelli,a Silvia
Rizzato,a Rubén Vicentec and Alessandro Casellia*
a Department of Chemistry and ISTM-CNR-Milano, Università degli Studi di Milano, via Golgi 19 –
20133 Milano, Italy. Email: alessandro.caselli@unimi.it
b Department of Pharmaceutical Sciences, General and Organic Chemistry Section “A. Marchesini”,
University of Milan, Via Venezian 21 – 20133, Milano, Italy.
c
Departamento de Química Orgánica e Inorgánica, and Instituto Universitario de Química
Organometálica “Enrique Moles”, Universidad de Oviedo. c/ Julián Clavería 8, 33007, Oviedo,
Spain. Email: vicenteruben@uniovi.es.
ABSTRACT:
A highly reactive and selective catalytic system comprising Fe(III) and macrocyclic pyridine-
containing ligands (Pc-L) for alkene oxidation by using hydrogen peroxide is reported herein. Four
new stable iron(III) complexes have been isolated and characterized. Importantly, depending on the
anion of the iron(III) metal complex employed as catalyst, a completely reversed selectivity was
observed. When X = OTf, a selective dihydroxylation reaction took place. On the other hand,
employing X = Cl resulted in the epoxide as the major product. The reaction proved to be quite
general, tolerating aromatic and aliphatic alkenes as well as internal or terminal double bonds and
both epoxides and diol products were obtained in good yields with good to excellent selectivities (up
to 93% isolated yield and d.r. = 99:1). The catalytic system proved its robustness by performing
several catalytic cycles, without observing catalyst deactivation. The use of acetone as a solvent and
hydrogen peroxide as terminal oxidant renders this catalytic system appealing.
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INTRODUCTION
Iron, the most abundant transition metal on earth, and its complexes are knowing an increasing
interest in organic synthesis.[1] The reasons why homogeneous iron catalysis has become a focal
research area after the turn of the millennium are brilliantly explained by Fürstner in a recent
Outlook.[2] In particular, the field of iron-catalysed oxidation reactions is of great importance not only
in synthetic organic chemistry, but also in biochemistry and industrial applications. As a result,[3] the
research in this area had an impressive boost, as witnessed by several timely reviews.[4] Synthetic
chemists have developed nonheme iron based systems that are capable of mimicking the high
efficiency in selective oxidation of non-activated substrates possessed by natural metalloenzymes
(mono- and di-oxygenases).[5] Despite these efforts, and the exponential growing number of scientific
papers appeared since 1960 to now, artificial catalyst systems that can compete with Nature are still
challenging.[6] In order to design catalysts capable of performing high regio- and/or stereo-selective
C-H or C=C bond oxidations, it is important the choice of critical components of iron coordination
sphere, namely the donor atoms and their geometry. Moreover, the need for sustainability imposes a
strict control of the reaction conditions and the choice of the oxidant in this respect is of fundamental
importance to assure the green principles of the overall process. In terms of atom-economy and E-
factor,[7] dioxygen, O2, and hydrogen peroxide, H2O2, are by far the reagents of choice since they
possess the highest oxygen content.[8] From a synthetic point of view, however, the usage of H2O2
seems to be more favourable since it is easier to handle, whilst molecular oxygen is always associated
with potential risk of explosion and very often requires the presence of a stoichiometric organic
sacrificial coreductant.[9] With hydrogen peroxide, instead, water is the only by-product formed.
Amongst nonheme catalysts, iron complexes of polyamine and aminopyridine ligands are the most
used.[10] In particular, iron(II) complexes of tetradentate N4 donor-ligands have proven to act as
efficient homogeneous catalysts in epoxidation[11] or syn-dihydroxylation[12] of alkenes with aqueous
hydrogen peroxide as terminal oxidant.
In the past few years, we have reported the synthesis of a series of pyridine-based 12-membered
tetraaza-macrocyclic ligands (Pc-L)[13] and the application of their copper(I)[14] and silver(I)[15]
complexes in catalysis. Spurred by the very recent reports on the generation of an highly reactive iron
species by reaction of peracetic acid with a ferrous complex ([FeII(CF3SO3)2(PyNMe3)], Figure
1a),[16] we became interested in exploring the coordination properties of our Pc-L ligands with iron.
We report herein that robust ferric complexes can indeed be obtained in excellent yields and that these
complexes are competent catalysts for alkene oxidation by aqueous hydrogen peroxide even in the
absence of any additives (Figure 1b). Remarkably, we observed a dependency on the reaction outcome
with respect to the anion employed for the synthesis of the iron(III) metal complex. Thus, when X =
2
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OTf, a selective alkene dihydroxylation reaction occurred. On the other hand, employing X = Cl,
alkene epoxidation was the preferred product.
Figure 1. a) OAT reaction catalysed by ferrous complex [FeII(CF3SO3)2(PyNMe3)] reported by Serrano-Plana and others;
b) controllable epoxidation or dihydroxylation reactions catalysed by ferric complex reported in this work.
RESULTS AND DISCUSSION
Preparation of the iron complexes. The selected pyridine-based 12-membered tetraaza macrocyclic
Pc-L 2 was obtained in useful overall yield by a modified Richman-Atkins procedure according to
Scheme 1a. We found that for this particular ligand, almost quantitative yields were obtained by the
mild hydrolysis of ortho-nosyl (Ns) protecting groups of ligand 1 (see Supporting Information for
details). On the other hand, macrocycle 5 could be easily synthesized by treatment of an N-tritosyl-
diethylenetriamine with pyridine-2,6-diylbis(methylene) dimethanesulfonate, in the presence of
K2CO3 as a base under heterogeneous conditions.[16c] After the hydrolysis in HBr of the Ts protecting
groups, the bromohydrated salt 4 has been directly perbenzylated in the presence of excess of DIPEA
(DIPEA = N,N-diisopropylethylamine) as a base (Scheme 1b).
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Scheme 1. Synthetic route used to obtain ligands 2 and 5 and their corresponding ferric complexes 3 and 6. Ns= ortho-
nosyl, DIPEA = N,N-diisopropylethylamine.
We choose Fe(III) as preferred metal for the oxidation of alkenes due to its well known great activity
towards the generation of metal-oxo complexes and related oxygen transfer reactions.[17]
The complexes were obtained by treating an acetonitrile solution of the selected ligand with an
acetonitrile solution of aqueous iron(III) chloride or iron(III) triflate salts at room temperature in a
1:1 ligand to metal ratio, although the triflate complexes could be prepared conveniently by anion
exchange from complex 6a·2H2O and silver triflate as well (see Supporting Information).[18] The
structures of complexes 3a-b and 6a-b have been characterized by means of mass and elemental
analysis as detailed in the Supporting Information. Elemental analyses for the iron chloride complexes
3a·2H2O and 6a·2H2O are consistent with the presence of two molecules of water. Based on the
ionization observed, together with literature data, we propose for the metal complexes the structure
depicted in Scheme 1, with the iron placed in an octahedral environment and only two X groups
directly bound to the metal (X = Cl or OTf). Magnetic moments, eff, of 3.73 B and of 3.88 B were
measured by the Evans’ method for complexes 6a·2H2O and 6b, respectively. These room
temperature magnetic moments are in agreement with a mixture low spin and high spin sites.[19]
In order to have a better characterization of the spin state of the complexes we decided to perform
EPR experiments on 6a·2H2O and 6b complexes in powder form. Figure 2a shows the EPR spectrum
of 6a·2H2O collected at RT (black curve). Beside a broad asymmetric line around g≈2.06 (line
centred at ≈3312 Gauss, ΔHpp≈300 Gauss), a tiny one appear around g≈ 4.38, corresponding to 1560
Gauss (see inset, Figure 2a). Moving to 77 K (red curve), the intense resonance peak increases its
anisotropy turning into an axial g tensor with the perpendicular g⊥ components around ≈ 2.120
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(≈3215 G) and the parallel g∥ ≈ 1.77 ( ≈ 3850 G). The broad (ΔHpp≈370 Gauss) low field feature
growths in intensity in respect to RT.
Tang et al. observed similar features in {[Fe(mph)2](ClO4)(MeOH)0.5]}2 at 149 K and attributed them
to rhombic high spin (g=4.69, S=5/2) and low spin (g=2.06 S=1/2) Fe3+ contributions.[20] Peak
broadening of the latter peak was related to spin-spin interaction. Differently from figure 2a, the
intensity of the low field feature decreased on decreasing T.
EPR spectra of complex 6b (see Figure 2b) shows absorption phenomena at low field and in the 3000-
4000 G zone, with several differences in respect to the previous sample. First of all the low field line
seems to be more intense than in 6a·2H2O and more structured with a broad component with g≈7.75
around 880 Gauss and an intense peak at g≈4.32 (≈1575 G, ΔHpp≈170 Gauss). Their positions do not
change on cooling, while their intensities increase and the g=4.32 component sharpen (ΔHpp≈85
Gauss). In the high field region a broad bump is followed by a low intensity peak around g≈2.07
(≈3290 G), that seems to be almost unaffected by temperature (see inset of Figure 2b). Brewer et al.
observed almost identical spectra in Fe(III) coordination compounds and attributed the absorption
phenomena to a dominant transition arising from ms=±1/2 doublet of a Fe3+ S=5/2 spin state with
nearly axial symmetry coupled to a component originated from the ms=±3/2 doublet in the same
spin/charge state.[21]
Figure 2. a) EPR spectra for complex 6a·2H2O collected at RT (black curve) and 77 K (red curve), the inset highlights
the signal at RT in the 1000-2500 Gauss interval; b) EPR spectra for complex 6b collected at RT (black curve) and 77 K
(red curve), the inset highlights the signal in the 100-4900 Gauss interval.
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Summarizing, in 6a·2H2O both low (S=1/2) and high (S=5/2) spin Fe3+ species are present. Lowering
T the intensities related to the S=5/2 state grow up suggesting that it should be the ground electronic
state of iron. In 6b only S=5/2 phenomena appear at both temperature. Moreover, the slow field
signals of 6b are sharper than the ones for 6a·2H2O, suggesting some interatomic spin-spin relaxation
phenomena in the latter sample as a consequence of reduced average Fe-Fe distance.
Unfortunately, all attempts to grow crystals suitable for X-ray structural determination of the iron
complexes meet with failure. However, we discovered that by changing the molar ratio between
ligand 5 and the iron chloride source to 1:2 in the synthesis of the metal complex, a new species
characterized as ([FeIII(Cl)2(3BnPc-L)](FeCl4), 6a’, could be obtained. Single crystals suitable for X-
Ray crystallography were grown by slow evaporation from acetone/ether solution (Figure 3).
Figure 3. Molecular structure of the complex 6a’ with (FeCl4)- as counteranion. The acetone solvent is omitted for clarity.
Selected bond lengths (Å): Fe(1)-N(1) 2.102(14), Fe(1)-N(2) 2.231(14), Fe(1)-N(3) 2.285(15), Fe(1)-N(4) 2.223(13),
Fe(1)-Cl(1) 2.234(5), Fe(1)-Cl(2) 2.278(6). Full crystal structure data of 6a’·(ace) are reported in the Supporting
Information.
In the complex 6a’ the iron(III) exhibits a largely distorted octahedral geometry (see caption figure
3 and table S3). The coordination sphere consist of four N-donors from the tetra-azamacrocycle,
which adopts the expected cis-folded configuration, and two cis chloride ions. The pyridine ring is
responsible for the large deviation from linearity of the axial N(2)-Fe(1)-N(4) bond angle that is
comparable to the values found in the literature for complexes containing analogous cyclic
chelators.[21-22] Other structural parameters, such as Fe-N bond distances, are similar to those reported
for related systems.[21][23] The equatorial Fe(1)-N(1) (pyridine nitrogen) bond is the shortest and thus
the strongest metal-ligand bond, although all Fe-N bond lengths are longer than 2.0 Å, typical of high-
spin Fe(III)-complexes.[24]
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Iron-catalyzed alkene oxidations. At the outset, oxidation of trans-stilbene 7a by H2O2 served as
benchmark for the optimization of the reaction conditions (Table 1). The reaction proceeded well
using acetone, in presence of 5 mol% of 3a·2H2O, at 30 °C leading to trans epoxide 8a with almost
full conversion and a remarkable 95% of selectivity in 48 h (entry 1, Table 1). Higher temperatures
(60 °C) resulted in a faster reaction but in a loss of selectivity (77%, entry 2, Table 1). The use of
acetonitrile provided full conversion yet lower selectivity compared to acetone (entry 3, Table 1). On
the other hand, branched alcohols, known for being suitable solvents often used in oxidations, showed
good selectivities but with poor conversions (entries 4-5, Table 1). In other commonly used reaction
media, the reaction did not take place (entries 6-8, Table 1).
When we moved to complex 6a·2H2O, we were pleased to find that even higher selectivities in
epoxide formation could be obtained (entry 9, Table 1). This is most probably due to the better
solubility of complex 6a·2H2O in the reaction medium. Moreover, with this catalyst, higher
temperatures were allowed together with a decrease in the excess of oxidant used (6 instead of 9
equiv.), and the trans epoxide was selectively formed in 24 h (95% selectivity, 99% conversion, entry
10, Table 1). A lower catalyst loading of 1 mol% resulted in lower conversion and selectivity (entry
11, Table 1). The rest of the reaction mass balance is represented by benzaldehyde (10% selectivity)
and traces of 2-phenylacetophenone.
Remarkably, when triflate iron complexes were used as catalysts under otherwise identical reaction
conditions, we observed a different reaction outcome. Indeed, by using iron(III) complex 3b, acetal
9a, arising from the corresponding dihydroxylation product derived from trans-stilbene was obtained
with 65% of selectivity (trans/cis ratio 5:1, epoxide was not detected, while benzaldehyde accounted
for the rest of the mass balance, entry 12, Table 1). Using iron(III) complex 6b, the selectivity was
improved and product 9a was isolated in 86% yield, confirming the data for conversion and
1
selectivities obtained by GC analysis with internal standard (entry 13, Table 1). The H NMR
spectrum confirmed the correct assignment of the major isomer as trans.
The crucial role of the ligand was further demonstrated since iron(III) triflate, as well as iron(III)
chloride in the absence of any ligand, did not provide any oxidation product, under otherwise identical
reaction conditions after 5 days (entry 14, Table 1).
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Table 1. Optimization of [FeIII(X)2(Pc-L)]X catalysed alkene oxidationa
Epoxide (8a) % Acetal (9a) %
Entry
Catalyst
Solvent
Temp
Time
Select (conv)b
Select (conv)b
1
2
acetone
acetone
CH3CN
t-BuOH
30 °C
60 °C
30 °C
60 °C
48h
36h
48h
48h
48h
48h
48h
48h
48h
24h
24h
48h
24h
95 (97)
-
77 (95)
-
3
75 (99)
-
4
96 (45)
-
3a·2H2O
5
t-amylOH 30 °C
88 (32)
-
6
TFE
30 °C
30 °C
30 °C
30 °C
60 °C
60 °C
60 °C
60 °C
-
-
7
H2O
-
-
8
PhCl
-
-
9
acetone
acetone
acetone
acetone
acetone
acetone
acetone
97 (95)
-
10c
11d
12c
13c
14f
15c,g
95 (99)
-
6a·2H2O
85 (58)
-
-
-
65 (99)
87 (99)e
traces
60 (83)
3b
6b
60 °C 5 days
60 °C 24h
-
Fe(X)3
6b
34
a Reactions were performed with [FeIII] (2.5 x 10-2 mmol) in the solvent (10 mL) at a cat/stilbene/H2O2 ratio of 1:20:180;
H2O2 (30% sol) was added in three portions every 12 h. b Conversions and selectivities were calculated by GC (dodecane
c
d
as internal standard). cat/stilbene/H2O2 ratio of 1:20:120; H2O2 (30% sol) was added in two portions every 12 h.
cat/stilbene/H2O2 ratio of 1:100:600. e Isolated in 86% yield as a mixture of trans and cis acetonide in 5:1 ratio. f X = Cl
or Tf. g BHT (2.5 x 10-2 mmol) was added to the reaction mixture.
Dihydroxilation versus epoxidation. Epoxides can be readily converted into acetonides in the
presence of catalytic amounts of Lewis acids in acetone as solvent.[25] Among the catalysts used, both
[26]
[27]
Fe(OTf)3 and anhydrous FeCl3 salts were reported to promote this reaction with good yields.
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On the other hand, it is well known that strong acids can readily open epoxides. Hintermann and co-
workers have elegantly shown that HOTf generated from metal triflate precursors can indeed act as a
hidden Brønsted acid catalyst.[28] Treatment of the isolated epoxide 8a in the presence of catalytic
amounts of triflic acid in acetone with H2O2 yielded to the corresponding acetal, 9a (trans/cis ratio
4:1,), in 8 hours, as expected, the same results can be obtained employing complex 6b instead of
HOTf (see Supporting Information). It is reasonable to assume that, under our reaction conditions,
hidden HOTf might act as a non-innocent species in the epoxide ring opening (as well as in the acetal
formation). It should be noticed that, to the best of our knowledge, the possible role of hidden
Brønsted acid catalysis in dihydroxylation reactions promoted by FeLn(OTf)m complexes reaction has
not been investigated in previous reports.
On the other hand, it is commonly accepted in literature that tetradentate iron complexes with cis-
labile coordination sites allows for a facile formation of a Fe(III)-hydroperoxo species in situ and the
mechanism of the catalytic O-O cleavage has been the object of several studies.[4a,b][6a][29] It has been
reported that there is a correlation between the iron spin state in octahedral Fe(III) hydroperoxo
complexes of tetradentate ligands in cis geometry[30] and the epoxide/diol ratio provided in alkene
oxidation. Low-spin complexes (S = 1/2) tend to be epoxide selective, while high spin complexes
lead mainly to diols. In particular, product distribution can be controlled by modification of the ligand
steric properties or complex geometry.[31] As demonstrated by our EPR analysis (see above) changing
the counteranion from chloride to triflate causes an increase of high spin population of the iron metal
centre. Thus, these analyses provide a rationale for the exquisite product selectivity observed.
Anyway, a possible role also of complex 6b and/or hidden triflic acid in the opening of the epoxide
cannot be completely ruled out at the present stage (see Supporting Information).
Scope of the alkene oxidations. With these preliminary studies in our hands, we decided to study
the scope of the reaction by using catalysts 6a·2H2O and 6b (Table 2). The catalytic system was
found to tolerate a wide range of substrates. When using complex 6a·2H2O, the reaction proceeds
with excellent yields and selectivities for internal tri- and disubstituted aromatic alkenes (Table 2,
products 8a,b,c). A preference towards the formation of trans products was observed (determined by
GC-MS and NMR analysis, see Supporting Information), even starting from cis substrates (Table 2,
product 8a). A similar result was reported also when using FeCl3·6H2O in the presence of 1-
methylimidazole using hydrogen peroxide as terminal oxidant.[32] Starting from trans-stilbene, the
observed trans/cis epoxide ratio was 99:1, while starting form cis-stilbene trans/cis epoxide ratio was
9:1. Since a radical mechanism can be proposed to explain this results, we have run two additional
test reactions by using catalysts 6a·2H2O and 6b under the optimized reaction conditions in the
presence of BHT (BHT = dibutylhydroxytoluene) as radical trap (see Supporting Information). In
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both cases, we observed a lower conversion, although the reaction is not completely inhibited. More
interestingly, also the diol/epoxide ratio is influenced by the presence of the radical trap, in particular
when employing complex 6b as catalyst, where 9a is obtained in 60% selectivity, accompanied by
the formation of epoxide 8a (34%), which is never observed in the standard reaction conditions (entry
15, Table 1).
Table 2. Scope of alkene oxidations catalysed by complexes 6a·2H2O and 6ba
a
Reactions were performed with [FeIII] (2.5 x 10-2 mmol) in acetone (10 mL) at a cat/stilbene/H2O2 ratio of 1:20:120
H2O2 (30% sol) was added in two portions every 12 h. Conversions and selectivities were calculated by GC and GC-MS
b
(dodecane as internal standard, n.d. = not detected). trans diastereoisomer was obtained as major product. Isolated in
c
93% yield for 8a, 75% for 8b, 85% yield for 9a and 90% for 9b (relative stereochemistry of the substituents shown).
Obtained as a mixture of the acetonide and the non-protected diol (see Supporting Information for details). d 9m was also
e
detected with a 22% selectivity (d.r = 9:1.). Only one diastereoisomer was observed, no traces of the acetonide were
f
detected. 9n was also detected with a 50% selectivity as a mixture of four diastereoisomer in relative ratio 26:4:1:1. g
Only two diastereoisomers were observed in 9:1 relative ratio, no traces of the acetonide were detected.
Electron-poor styrenes showed complete conversions but the epoxide was obtained with moderate
selectivities (Table 2, products 8g,h). In both cases formation of other oxidation products such as the
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aldehyde and the acetophenone were observed. On the other hand, electron donating substituents on
the aromatic ring are not well tolerated and, for example, 4-methoxystyrene was completely converted
but in a number of different oxidation products (Table 2, product 8i). Other terminal styrenes, -
methylstyrene and indene underwent the ring opening of the formed epoxide, which was not detected,
to yield a mixture of the acetonide and the unprotected diol (Table 2, products 9d,e,f,j’). A lower
control of the diastereoselectivity in those case was observed and, for example, diol 9j’ was obtained
with d.r. = 1:1.3. More challenging substrates such as methyl cinnamate, despite the modest
conversion, gave excellent results in terms of product and diastereoselectivity (Table 2, product 8k,
only trans isomer was detected).
Aliphatic alkenes worked well with this catalytic system. 1-Octene reacted to provide the
corresponding epoxide in almost quantitative selectivity and very good yield (Table 2, product 8l).
On the other hand, cyclohexene produced the corresponding epoxide 8m with only 20% of selectivity,
accompanied by the diol 9m (22%; d.r. = 9:1) (Table 2, product 8m). The rest of the yield was
attributed to the plethora of oxidation products of cyclohexene, amongst which the major product
formed was 2-cyclohexene-1-one (33%). This result is the evidence of a preference to diol formation
starting from endocyclic substrates and was confirmed performing the reaction on limonene, where,
together with epoxide 8n (35%), the corresponding diol 9n was formed as main product with 50%
selectivity (Table 2, product 8n). A complete regioselectivity towards the oxidation of the internal
double bond over the terminal one was observed and, more interestingly, a quite good stereocontrol
was observed. In fact, the trans-epoxide was obtained in a 4-fold excess with respect to the cis and
out of the four possible diastereoisomers that could be obtained upon ring opening, a 26:4:1:1 ratio
was determined by GC-MS analysis.
By exchanging the counter anion from chloride to triflate, using 6b, the reaction outcome was reverted
providing the acetonide as the main reaction product (Table 2, right columns). In few cases, traces of
the corresponding non-protected diols were also detected by GC-MS (see Supporting Information for
details). The two products are both the result of a dihydroxylation reaction, and for this reason,
selectivities are given as the sum of the two products.
The reaction proceeds with excellent yields and selectivities for most of the tested unbiased styrene
derivatives (Table 2, products 9a,b,c,d,e,f,j) and shows a preference towards the formation of trans
products, even starting from cis substrates, exactly like in the epoxidations previously reported. This
result is in contrast to what observed in the Fe(III) catalysed conversion of epoxides to acetonide,[27]
where diastereoisomeric mixtures of acetonides were obtained starting from pure trans-epoxides. It
is worth to note that, when employing complex 6b the acetonide 9j was obtained major compound
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and a single diastereoisomer has been detected by GC-MS, in contrast to what observed in the case
of the reaction catalysed by complex 6a·2H2O, where both diastereoisomers of 9j’ diol were formed.
Electron-poor styrenes as well as more electron-rich ones were tolerated but showed lower
selectivities (Table 2, products 9g,h,i), while cinnamate, gave quite good results in terms of
selectivity (although with no stereocontrol, d.r. = 1.2:1), but a low conversion (Table 2, product 9k).
Even with aliphatic substrates the reaction proceeded well, giving excellent selectivity in the case of
1-octene (Table 2, product 9l) and reasonable results for cyclohexene and limonene (Table 2,
products 9m,n). Again, in the case of limonene, a complete regioselectivity for the internal double
bond over the terminal one was observed. More interestingly, a single diastereoisomer was obtained
for 9m and out of the four possible diastereoisomers only two, in 9:1 ratio, were observed for 9n.
The robustness of the catalytic system was proved by performing repeated batches of the benchmark
reaction simply by adding the reagents after each cycle ends, avoiding the isolation of the metal
complex. Thus, with catalyst 6a·2H2O an isolated yield of 90% (TON = 54) of three cycles was
obtained (95% one single batch, GC yield). Similarly, catalyst 6b provided 84% (TON = 84) isolated
yield after five cycles (86% one single batch, GC yield). It should be noticed that product (and by-
product) accumulation generated along the consecutive cycles had a minor impact in catalyst
deactivation, which is stable and productive even after five days of reaction.
Conclusions
In summary, we have demonstrated that both epoxidation and dihydroxilation of terminal and internal
alkenes can be efficiently and selectively achieved by using a catalytic system comprising Fe(III) and
pyridine-containing ligands (Pc-L). This catalytic system takes advantage of an economic metal
source and an easy to assemble macrocyclic ligand. Remarkably, a complete reversal of selectivity
between the epoxide and the dihydroxylation product could be induced by the proper choice of the
anion. A rationale for this selectivity might be the influence of the counteranion in the spin state of
iron. The possible role of hidden HOTf in the opening of the epoxide cannot be ruled out when iron
triflate complexes are used (due to the presence of water in the reaction media). The use of acetone
as a solvent and of hydrogen peroxide as terminal oxidant are appealing features when considering
green-chemistry. Studies directed towards a better understanding of the mechanism and a more
efficient ligand design to address the problem of stereocontrol of the C-O bond formed are currently
undergoing in our laboratories.
EXPERIMENTAL SECTION
General experimental details. All of the reactions that involved the use of reagents sensitive to
oxygen or hydrolysis were carried out under an inert atmosphere. The glassware was previously dried
12
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10.1002/cctc.201901045
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in an oven at 110 °C and was set with cycles of vacuum and nitrogen. All chemicals and solvents
were commercially available and were used as given except where specified. The chromatographic
column separations were performed by a flash technique, using silica gel (pore size 60 Å, particle
size 230−400 mesh, Merck grade 9385) or by gravimetric technique using basic Al2O3. For TLC,
silica was used on TLC Alu foils with fluorescent indicator (254 nm) and the detection was performed
by irradiation with UV light (λ = 254 nm or 366 nm). 1H NMR analyses were performed with 300 or
400 MHz spectrometers at room temperature. The coupling constants (J) are expressed in hertz (Hz),
and the chemical shifts (δ) in ppm. 13C NMR analyses were performed with the same instruments at
75.5, and 100 MHz, and attached proton test (APT) sequence was used to distinguish the methine and
methyl carbon signals from those arising from methylene and quaternary carbon atoms. All 13C NMR
spectra were recorded with complete proton decoupling. The 1H NMR signals of the ligand described
in the following have been attributed by correlation spectroscopy (COSY) and nuclear Overhauser
effect spectroscopy (NOESY) techniques. Assignments of the resonance in 13C NMR were made
using the APT pulse sequence and heteronuclear single quantum correlation (HSQC) and
heteronuclear multiple bond correlation (HMBC) techniques. EPR measurements were performed on
powder samples with a Bruker ELEXSYS spectrometer equipped with an ER4102ST standard regular
cavity at X band (9.55 GHz) frequency at room temperature and at 77 K using a cold finger Quartz
Dewar cooled by liquid nitrogen, The derivative dP/dHof power P adsorbed was recorded as a
function of the static magnetic field H. Low resolution MS spectra were recorded with instruments
equipped with electron ionization (EI), ESI/ion trap (using a syringe pump device to directly inject
sample solutions), or fast atom bombardment (FAB) (for Pc-L and metal complexes) sources. The
values are expressed as mass−charge ratio and the relative intensities of the most significant peaks
are shown in brackets. Gaschromatographic analyses were performed with GC-FAST technique using
a Shimadzu GC-2010 equipped with a Supelco SLBTM-5ms capillary column. GC-MS analyses were
performed using a ISQ™ QD Single Quadrupole GC-MS (Thermo Fisher) equipped with a VF-5ms
(30 m x 0.25 mm i.d. x 0.25 µm; Agielent Technology). Elemental analyses and ICP-OES were
recorded in the analytical laboratories of Università degli Studi di Milano. X-ray data collection for
the crystal structure determination was carried out by using Bruker Smart APEX II CCD
diffractometer with the Mo K radiation (λ = 0.71073) at 296 K. Synthesis of the ligands and iron
complexes is described in the experimental section.
General procedure for the synthesis of Fe(III) complexes.
A solution of FeCl3∙6H2O or Fe(OTf)3 (0.36 mmol) in acetonitrile (4.0 mL) was added dropwise to a
solution of ligand 2 or 5 (0.36 mmol) in acetonitrile (4.0 mL). The mixture was left to react for 60
minutes at room temperature showing a change of color from orange to dark brown. The solvent was
13
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10.1002/cctc.201901045
ChemCatChem
then evaporated in vacuum and the crude was treated with Et2O (5 mL) and left under stirring at room
temperature for 30 minutes. The product obtained as dark brown solid was then filtered and dried in
vacuum.
Yields 3a·2H2O
MS (FAB)
94% [iron source: FeCl3∙6H2O]
m/z (%) 422 (100), 424 (63) [M-Cl]
Elem. An.
C18H28Cl3FeN4O2 Calcd: C 43.71; H 5.71; N 11.33; Found: C 43.81; H 5.49; N 11.47
3b
95% [iron source: Fe(OTf)3 anhydrous]
MS (FAB)
Elem. An.
m/z (%) 501 (100), 503 (27) [M-2OTf]
C21H24F9FeN4O9S3 Calcd: C 31.55; H 3.03; N 7.01; Found: C 32.01; H 3.24; N 7.49
6a·2H2O
97% [iron source: FeCl3∙6H2O]
MS (FAB)
Elem. An.
m/z (%) 567 (100), 569 (62) [M-2Cl]
C32H40Cl3FeN4O2 Calcd: C 56.95; H 5.37; N 8.30; Found: C 56.83; H 5.73; N 8.08
6b
93% [iron source: Fe(OTf)3 anhydrous]
MS (FAB)
Elem. An.
m/z (%) 830 (100), 831 (37) [M-OTf]
C35H36F9FeN4O9S3 Calcd: C 42.91; H 3.70; N 5.72; Found: C 42.34; H 4.04; N 6.08
General catalytic procedure
The catalyst (0.025 mmol), the substrate (0.5 mmol) and dodecane (60 µL, used as GC internal
standard) were dissolved in acetone (10.0 mL). H2O2 30% (1.5 mmol) was added and the mixture
was heated for 10 hours at 60 °C. Then H2O2 30% (1.5 mmol) was added and the mixture was heated
for further 12-14 hours. After this period the catalyst was removed by filtration on celite pad and a
sample of the so-obtained crude (c = 0.1 mg/mL) was used for GC and GC-MS analyses.
ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures, text, figures and tables reporting full NMR spectra for all ligands
and characterization of the iron complexes. EPR data for complexes 6a·2H2O and 6b. General
catalytic procedures and procedures employed for the isolation of selected reaction products,
including figure and tables reporting full NMR characterization. GC methods used and GC-MS
1
spectra for all the catalytic run reported, and H NMR of selected reaction crudes. X-Ray data for
complex 6a’. Tables containing the results of the cost evaluation studies. This material is available
free of charge via the Internet
ACKNOWLEDGEMENT
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10.1002/cctc.201901045
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Financial support from the Università degli Studi di Milano “Unimi – piano di sostegno alla ricerca 2015-17
– Linea 2 Azione B " and from Ministerio de Economia y Competividad (MINECO), Agencia Estatal de
Investigacion (AEI) and Fondo Europeo de Desarroillo Regional (FEDER) (CTQ2016-76840-R) is gratefully
acknowledged. A. C. and G. T. are grateful to Nicola Panza for precious help in the synthesis of ligand 4, and
to Dr. Matteo Compagnoni for his advices and help in performing a cost evaluation study. M.S. gratefully
thanks Prof. Cesare Oliva for useful discussions.
KEYWORDS
Dihydroxylation; Epoxidation; Macrocycles; Nonheme iron complexes; Pyridine containing macrocyclic
ligands.
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GRAPHICAL ABSTRACT
Controllable iron(III)-catalysed alkene epoxidation or dihydroxylation reactions are performed by judicious
choice of anion in [Fe(III)(Pc-L)] catalysts (Pc-L = pyridine-based 12-membered tetraaza-macrocyclic ligands).
The catalytic system proved its robustness by performing several catalytic cycles, without observing catalyst
deactivation. The use of acetone as a solvent and hydrogen peroxide as terminal oxidant renders this catalytic
system appealing.
18
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