Iron/N-doped graphene nano-structured catalysts for general cyclopropanation of olefins

Article abstract of DOI:10.1039/d0sc01650k

The first examples of heterogeneous Fe-catalysed cyclopropanation reactions are presented. Pyrolysis of in situ-generated iron/phenanthroline complexes in the presence of a carbonaceous material leads to specific supported nanosized iron particles, which are effective catalysts for carbene transfer reactions. Using olefins as substrates, cyclopropanes are obtained in high yields and moderate diastereoselectivities. The developed protocol is scalable and the activity of the recycled catalyst after deactivation can be effectively restored using an oxidative reactivation protocol under mild conditions. This journal is

Full text of DOI:10.1039/d0sc01650k

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Iron/N-doped graphene nano-structured catalysts
for general cyclopropanation of olefins†
Cite this: DOI: 10.1039/d0sc01650k
b
Abhijnan Sarkar,a Dario Formenti,b Francesco Ferretti, a Carsten Kreyenschulte,
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
b
b
b
a
*
*
Stephan Bartling,
Kathrin Junge,
Matthias Beller
and Fabio Ragaini
The first examples of heterogeneous Fe-catalysed cyclopropanation reactions are presented. Pyrolysis of in
situ-generated iron/phenanthroline complexes in the presence of a carbonaceous material leads to specific
supported nanosized iron particles, which are effective catalysts for carbene transfer reactions. Using olefins
as substrates, cyclopropanes are obtained in high yields and moderate diastereoselectivities. The developed
protocol is scalable and the activity of the recycled catalyst after deactivation can be effectively restored
using an oxidative reactivation protocol under mild conditions.
Received 20th March 2020
Accepted 1st June 2020
DOI: 10.1039/d0sc01650k
rsc.li/chemical-science
Inspired by the availability of different Fe oxidation states in
such materials,20,21 we considered them also to be promising
Introduction
candidates for processes known to be catalysed by iron species
in higher oxidation states. To prove this idea, the cyclo-
propanation of olens using diazo compounds as carbene
sources was chosen as a model reaction. Indeed, this trans-
formation is efficiently catalysed by homogeneous Fe(II/III)
complexes based on nitrogen-containing ligands such as
porphyrins, phthalocyanines or other multidentate N- and N,O-
ligands.22
The use of available, environmentally compatible and inexpen-
sive 3d-metals in catalysis is an hot topic in modern chemistry.1–4
Among the various transition metals, particularly Fe is a privi-
leged element due to its abundance, very low price and negligible
toxicity.5–7 In the past decade, a remarkable number of reports
were disclosed concerning the use of specic well-dened iron
homogeneous complexes, supported or unsupported Fe-based
nanoparticles, or other kinds of aggregates (e.g. nanoclusters) in
various catalytic transformations.6,8–15 In 2013, inspired by elec-
trocatalysis, some of us reported the development of an iron-
based heterogeneous catalyst prepared by the pyrolysis of
Fe(OAc)2/Phen (Phen ¼ 1,10-phenanthroline) impregnated onto
Vulcan carbon.16 According to the most recent and accurate
characterization, the active material Fe/Phen@C-800 is
composed of Fe-based nanoparticles (NPs), small Fe clusters,
and even single Fe atoms embedded in a nitrogen doped carbon
matrix (see below for a more detailed description). Based on this
work, a whole family of catalytically interesting materials has
been prepared using different approaches.17,18 So far, they have
been mainly used in reduction/oxidation reactions and no
application in other advanced organic transformations has been
reported.19 Clearly, the discovery of new reactivity for iron-based
heterogeneous catalysts will give new impetus for 3d-metal
catalysis in general, and probably facilitate their actual imple-
mentation in industry (Scheme 1).
Cyclopropanes constitute an important scaffold in natural
products and many biologically active molecules including
several pharmaceuticals and agrochemicals.23 Furthermore,
a
`
Dipartimento di Chimica – Universita degli Studi di Milano, Via C. Golgi 19, 20133
Milano, Italy. E-mail: fabio.ragaini@unimi.it
bLeibniz-Institut fur Katalyse e.V., Albert-Einstein-Straße 29a, 18059 Rostock,
¨
Germany. E-mail: matthias.beller@catalysis.de
† Electronic supplementary information (ESI) available: Experimental details for
catalyst preparation and catalytic reactions, additional catalytic results, catalyst
Scheme 1 (A) Preparation of Fe/Phen@C-800, (B) selected known
catalytic applications and (C) the present work: cyclopropanation
reactions using Fe/Phen@C-800 as the catalyst.
characterisation,
and
cyclopropane
characterisation.
See
DOI:
10.1039/d0sc01650k
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their highly strained structure provides a surprising reactivity
making them versatile reagents in synthetic organic chem-
istry.24 Thus, many homogeneous complexes based on Fe, Ru,
Co, and Cu have been developed, which successfully catalyse
this transformation.25,26 Along with homogeneous catalysts,
immobilized metal complexes (mainly based on Cu and, to
a less extent, on Ru and Rh) are also known to be active in
cyclopropanations. However to date, very few reports describe
the use of truly heterogeneous catalysts. Aer the pioneering
work of Mayoral in 1997,27 Cu NPs on TiO2–Al2O3,28 Al2O3 (ref.
29) and core–shell Fe–FexOy (ref. 30) were reported as catalysts.
More recently, the groups of Coleman and Mack described the
use of silver foil as a recyclable catalyst under mechanochemical
conditions.31,32
Here, we present for the rst time a heterogeneous iron
catalyst for such reactions, which can be conveniently used and
professionally recycled.
Results and discussion
Optimization and scope of olen cyclopropanation using the
Fe/Phen@C-800 catalyst
Initially, we explored the activity of Fe/Phen@C-800 in the
reaction of a-methylstyrene 1a with ethyl diazoacetate (EDA) as
a model transformation (Scheme 2). To our delight, the Fe/
Phen@C-800 catalyst is able to efficiently catalyse this reaction
affording the cyclopropane 2a in very high to excellent yields
with a moderate diastereoselectivity in favour of the trans
isomer. Control experiments using materials prepared by the
same procedure employed for Fe/Phen@C-800, but omitting
either Fe(OAc)2 or Phen, or by using Fe(OAc)2 and Phen as such
resulted in no detectable formation of the desired product
(Table S2, entries 6–8†).
Notably, the nature of the solvent has a minimal inuence
both on the reaction yield and diastereoselectivity making this
reaction versatile in terms of the media prole (Table S2†).
Applying 1,2-dimethoxyethane (DME) as solvent, 60 ꢀC was
identied as the optimal reaction temperature (Table S3†). Even
though in most optimization experiments an excess of the
olen (5-fold amount) with respect to the diazo compound has
been used, it is possible to decrease the amount of 1a (1.5 eq.)
and still a very good yield of the cyclopropanes can be achieved
(Table S4, entry 3†). This demonstrates the applicability of the
procedure even to more expensive olens. Under the latter
conditions, diethyl fumarate and diethyl maleate derived from
EDA homocoupling were also observed as side products.
Advantageously, the catalyst is water tolerant and only a slightly
Scheme 3 Substrate scope with respect to olefins. Reaction condi-
tions: 0.50 mmol of EDA, 2.50 mmol of alkene, 18.5 mg of Fe/
Phen@C-800 (corresponding to 2 mol% Fe), and 3 mL of DME at 60 ꢀC
for 4 h. Yields are based on the starting EDA and refer to the isolated
compounds (sum of the trans and cis diastereoisomers). Diastereo-
meric trans/cis ratios (dr) are based on separated diastereomers.
aIsolated yield of the major isomer in parentheses, dr measured by 1H
NMR. bOnly major isomer isolated, dr measured by 1H NMR. c1.5 mmol
of alkene was employed. dReaction time: 8 h. Isolated as an isomeric
mixture. Where possible, dr are measured by 1H NMR.
decreased yield was obtained using a “wet” solvent (Table S4,
entry 4†).
Given the good results obtained in the benchmark reaction
and the apparent robustness of the catalyst, the substrate scope
was investigated (Scheme 3).
Styrene and related derivatives (1b–1e) gave the corre-
sponding cyclopropanes in high yields regardless of the posi-
tion of the substituent on the arene ring. Similarly, substrates
with electron-donating groups (1f–1g) afforded the corre-
sponding cyclopropanes in very high yields. Also halogen-con-
taining substrates (1h–1j, 1m, 1p, 1q) were well tolerated,
maintaining intact the C–X bond, which can allow further
functionalization. To our surprise, pentauoro styrene (1l) as an
example of an electron-poor substrate afforded the corre-
sponding product in a good yield. Notably, more challenging
aliphatic substrates such as 1-octene (1t) and b-pinene (1v) gave
the products in satisfactory yields. Using vinyl cyclohexene (1u)
the cyclopropanation reaction occurred regioselectively on the
Scheme 2 Cyclopropanation of 1a using Fe/Phen@C-800 as the
catalyst.
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Scheme
4 Substrate scope with respect to diazo compounds.
Reaction conditions: 0.50 mmol of 3a–d, 2.50 mmol of alkene, 18.5
mg of Fe/Phen@C-800 (2 mol% Fe), and 3 mL of DME, at 60 ꢀC for 4 h.
Isolated yields are reported, based on the starting EDA; dr were
measured by 1H NMR. aReaction conditions: 100 ꢀC in toluene for 8 h.
terminal olenic bond maintaining intact the internal one. The
selectivity for the terminal double bond in 1u is explained by the
lack of activity of the catalyst in the case of internal olens
(Fig. S1†), most likely due to a hindered approach of the
substrate to the carbene formed on the surface of the catalyst.
Under the optimized conditions, the scope of the diazo
compound was also examined (Scheme 4). Mono-substituted
diazo compounds (ester or ketone) gave the corresponding
cyclopropanes 2aa–2ab in excellent yields. Interestingly, the use
of more sterically demanding diazo compounds has a dramatic
positive effect on the diasteroselection, furnishing the trans
isomer with >99 : 1 dr. The use of di-substituted diazomethanes
proved to be more challenging. Nevertheless, diphenyldiazo-
methane yielded the product 2ac in a moderate yield (100 ꢀC, 8
h in toluene).
Fig. 1 Recycling and reactivation steps. For experimental details
regarding the recycling runs and the reactivation procedures see the
ESI.†
occurred even by treating the parent material only with
styrene under the reaction conditions. Also in this case, the
activity was restored by oxidative treatment (see the ESI† for
experimental details).
Note that most oxidative reactivation processes are based on
the use of air at high temperatures, but such a treatment would
result in the complete burning of the carbon support of our
catalyst. Hydrogen peroxide has been reported as a milder
alternative in a few cases.35–37 Moreover, the Fen+/H2O2 system
(known as Fenton's reagent) is also effective in these processes
thanks to the generated HOc radicals.38 Thus, it appears that in
our case both the direct action of H2O2 and related Fenton-like
reactions are responsible for the removal of the oligomers/
polymers from the spent catalyst.
Catalyst reactivation and recycling
In order to demonstrate the practical utility of this protocol, the
scalability and recyclability of the system were studied. The
model reaction was successfully scaled-up to 15-fold without
signicant variations of yield and diasteroisomeric ratio
compared to the small-scale run. Regarding the recyclability,
a signicant decrease of the activity was observed aer the 4th
run (D.1 in Fig. 1, cycles 1–5). ICP analysis of the solution
Characterisation of fresh, spent and reactivated catalysts
showed a negligible (ca. 0.1%) loss of iron from the catalyst, so To further elucidate the reason for the declined activity, the
this cannot be the reason for deactivation. fresh, spent and regenerated catalysts were characterized by X-
In order to make the whole process both efficient and ray photoelectron spectroscopy (XPS) and scanning trans-
effective, two routes of re-activation of the spent catalyst were mission electron microscopy (STEM). XPS N 1s analysis showed
explored. The material obtained aer 5 runs (Fe/Phen@C- common peaks at around 399 eV, 400 eV, and 401 eV for all the
800_S) was thermally treated under an inert atmosphere (R.1 materials (Fig. S2–S6†). These results are in agreement with
in Fig. 1). However, despite a partial reactivation was gained, three different nitrogen bonding situations, namely N bonded
still complete conversion of EDA could not be achieved. In in residual organic matrices and/or Fe–Nx centers, pyrrolic-N,
contrast, by treating Fe/Phen@C-800_S with an aqueous and graphitic-N, respectively.16,39,40 A minor peak at 398.1 eV
solution of 3 v/v% H2O2, the initial catalyst activity was ascribed to pyridinic-N has been detected in Fe/Phen@C-800_S.
restored (R.2 in Fig. 1)! The reactivated catalyst, Fe/Phen@C- C 1s core levels displayed an analogous pattern for all the
800_R, could be further recycled as the fresh one (Fig. 1, cycle materials (Fig. S2†). Indeed, signals corresponding to C]C/C–
7). Such an oxidative regeneration is typical for catalysts that H (284.8 eV), C]N or C–O (approximately at 285 eV), and C–N or
suffer from physicochemical deactivation (e.g. fouling or C]O (285–291 eV) functionalities can be detected.41 Deconvo-
poisoning).33,34 Apparently, the olenic substrates and in lution of the Fe 2p3/2 region showed four pairs of peaks which
particular styrenes underwent to a small extent oligo- and/or can be attributed to various Fe states. Peaks at around 708 eV,
polymerization, which deactivates the iron catalyst. Indeed, 711 eV, and 714 eV in the region correspond to Fe(0), Fe(II) and
we veried that complete deactivation of the catalyst Fe(III), respectively.42 Additionally, the peak at 710 eV suggests
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the presence of Fe–Nx bonds which is in agreement with the protecting role of graphene towards oxidation. The nitrogen-
previous results from N 1s spectra.43
doped amorphous carbonaceous matrix attached to the
Finally, the interpretation of the O 1s region is not trivial support showed the presence of dispersed Fe clusters. Their
because of the large amount of oxygen functionalities both identity is also conrmed by electron energy loss spectra (see
derived from oxygen groups present in the carbonaceous Fig. S10B†). The complex pattern revealed by Fe 2p XPS
matrix44 and iron oxides.45 For this reason, unambiguous spectra reects the variety of iron functionalities (metallic
assignments are not possible here. In general, only small Fe, FeOx, and FeNx centres) in the material. The spent cata-
changes in the XPS data can be observed when comparing lyst (Fe/Phen@C-800_S) showed similar structures: still
the fresh, spent, and regenerated catalysts. STEM images dened Fe-based NPs (mainly metallic and to a less extent
and analytical data of the three materials were then oxidic) enveloped in carbon shells and dispersed iron clus-
acquired. As depicted in Fig. 2, the annular dark eld (ADF) ters can be clearly observed (Fig. 2B, S8 and S10C†). Finally,
STEM images and related elemental maps show the presence the reactivated catalyst Fe/Phen@C-800_R generally showed
of Fe, N, and O on the carbon support. Nitrogen is mainly more extended oxidic particles, while maintaining intact
distributed in a phase also containing C, O and Fe on the small iron clusters (Fig. 2C, S9 and S10D†).
surface of the support alongside Fe-based particles. In
The general morphology and distribution of Fe, N, O and C
addition to the elemental maps, the different contrast visible remained similar between the three different states of the
in both the annular bright eld (ABF) and high angle annular catalyst as shown by the STEM data. Combined with the small
dark eld (HAADF) images (Fig. S7†) conrmed the changes in the electronic structure revealed by XPS, the data are
predominant presence of probably metallic Fe NPs in addi- consistent with the deactivation of the catalyst being due to
tion to Fe oxide NPs. Whereas layers of graphene covered the fouling, which is removed by the H2O2 treatment, rather than by
former, the latter are not covered thus indicating a possible a structural change of the catalyst itself.
Conclusions
In conclusion, here we report the rst iron-based heterogeneous
catalyst for cyclopropanation reactions. While in the past, this
class of non-noble metal catalysts was generally employed for
reduction or oxidation reactions, they are indeed effective
catalysts also for carbene transfer reactions. The developed
protocol allows obtaining several cyclopropanes from aromatic
and aliphatic olens and different diazo compounds in a prac-
tical and efficient manner. The heterogeneous nature of the
catalyst and its robustness make it also an ideal candidate for
ow chemistry applications. The deactivation of the catalyst has
also been studied and an oxidative regeneration protocol was
effectively developed, which may be of more general use even
for other reactions.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
`
This work has been supported by the Ministero dell'Universita e
della Ricerca (MIUR) (PRIN 20154X9ATP). Financial support of
the BMBF is also gratefully acknowledged. F. F. thanks the
`
Dipartimento di Chimica, Universita degli Studi di Milano, for
nancial support (Piano di Sostegno alla Ricerca 2019). D. F.
thanks LIKAT for a Post-Doctoral Fellowship. Dr Pavel Ryab-
chuk and Dr Annette-Enrica Surkus (both LIKAT, Germany) are
gratefully acknowledged for fruitful discussions. We thank Prof.
¨
¨
Dr Hansjorg Grutzmacher (ETH, Switzerland) for suggesting us
Fig. 2 ADF-STEM images and EELS elemental maps of Fe/Phen@C-
800 (A), Fe/Phen@C-800_S (B) and Fe/Phen@C-800_R (C).
to test the use of H2O2 for the reactivation of the catalyst.
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26 H. Lebel, J.-F. Marcoux, C. Molinaro and A. B. Charette,
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