Magnetically recoverable CuFe2O4 nanoparticles as highly active catalysts for Csp3-Csp and Csp3-Csp 3 oxidative cross-dehydrogenative coupling

Article abstract of DOI:10.1055/s-0033-1339278

This study probes the versatility of [metal] ferrite {[M]Fe ₂O₄} nanoparticles as an effective catalyst platform for oxidative cross-dehydrogenative coupling (CDC) by comparing the reactivity of simple magnetite (Fe₃O

Full text of DOI:10.1055/s-0033-1339278

LETTER
▌1637
lMettear gnetically Recoverable CuFe2O4 Nanoparticles as Highly Active Catalysts
for Csp3–Csp and Csp3–Csp3 Oxidative Cross-Dehydrogenative Coupling
CuFe2O4-Nanoparticle-Catalyzed Cross-Dehydrogenative Coupling
Reuben Hudson, Shingo Ishikawa, Chao-Jun Li,* Audrey Moores*
Department of Chemistry, McGill University, 801 Sherbrooke St. West, Montreal, QC, H3A 0B8, Canada
Fax +1(514)3983797; E-mail: cj.li@mcgill.ca; E-mail: audrey.moores@mcgill.ca
Received: 03.04.2013; Accepted after revision: 24.05.2013
external magnet.3 Most schemes for this type of catalyst
Abstract: This study probes the versatility of [metal] ferrite
recovery take advantage of the magnetic particle (Fe,
{[M]Fe2O4} nanoparticles as an effective catalyst platform for oxi-
Fe3O4, Fe2O3, etc.) only as a support for which to anchor
another catalytically active metal. Catalyst preparation
dative cross-dehydrogenative coupling (CDC) by comparing the re-
activity of simple magnetite (Fe3O4) with that of the copper-
substituted analogue, copper ferrite (CuFe2O4). In either case, the becomes lengthy and complicated when this second
metal4 or organo catalyst5 is anchored via a linker to the
iron within the lattice enables magnetic recovery of the nanoparti-
nanoparticle directly or instead to a protective polymer6 or
cles, simplifying the process of catalyst recycling. Both iron and
copper effectively catalyze the CDC of two sp3 carbons, while cop-
silica7 coating. In an effort to move toward more simple
per provides reactivity that iron cannot: activation of sp-hybridized
schemes, several groups have devolved a series of bare
magnetic nanoparticles as purely heterogeneous catalysts
for organic transformations (Figure 1).
Simple iron8 or iron–iron oxide core-shell nanoparticles
can catalyze olefin hydrogenation,9 dehydrogenation of
ammonia borane10 for release of stored hydrogen, and the
coupling of aryl Grignard reagents with alkyl halides.11
carbons for coupling to sp3 centers.
Key words: ferrite, magnetic nanoparticles, oxygen, cross-
coupling, C–C bond formation
Efforts to both develop more direct chemical syntheses
and to use simple, easily recoverable heterogeneous
catalysts1 to afford the necessary transformations repre-
sent two influential thrusts of the sustainable chemistry
movement.2
With the understanding that iron bestows only a limited
catalytic scope, a bimetalic scheme also emerged.12 Intro-
duction of a metal salt to a dispersion of iron–iron oxide
core shell nanoparticles generates by galvanic reduction
an iron-based nanoparticle decorated with nanoparticles
of a more catalytically relevant metal. Introduction of pal-
Iron-based nanoparticles fit the bill as easily recoverable
heterogeneous catalysts because their magnetic nature en-
ables recovery and recycling by simple application of an
complex, pseudohomogeneous systems
silica or polymer
O
OH
O
NH
[M]
catalytic site
O
O
NH
O
FexOy
FexOy
FexOy
O
S
O
N
ligand
O
H
NH
OH
H2N
metal-binding ligand
bound to protective
coating
organocatalyst
bound directly
to NP
metal-binding
ligand bound
directly to NP
simple, purely heterogeneous systems
[M]
[M]
Fe0
Fe0
MFe2O4
Fe0
Fe3O4
[M]
[M]
Fe NP
Fe-FexOy
core-shell NP
decorated Fe-
ferrite NP
[Metal] ferrite NP
FexOy core-shell
NP
Figure 1 Various types of magnetically recoverable nanoparticle catalysts
SYNLETT 2013, 24, 1637–1642
Advanced online publication: 15.07.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1339278; Art ID: ST-2013-S0303-L
© Georg Thieme Verlag Stuttgart · New York

1638
R. Hudson et al.
LETTER
ladium into such a scheme enables Suzuki–Miyaura cou- ward Csp–Csp3 coupling and marginal yield increase for
pling,12a while copper facilitates the Huigsen 1,3-dipolar CuFe2O4-nanoparticle-catalyzed Csp–Csp coupling vali-
cycloaddition.12b
dates our approach of comprehensively comparing these
two catalyst systems.
Catalysts that rely on a reduced iron core will always be
susceptible to oxidative degradation over time. To over- In order to establish the reusability of the catalyst and its
come this limitation, the exploration of oxidized ferrite operation under a heterogeneous mechanism, we per-
nanoparticles13 and their metal-substituted analogues14 formed several additional tests on the model reaction of
(again to overcome the limited catalytic scope of iron copper ferrite catalyzed coupling of 2-phenyl-1,2,3,4-tet-
alone) have been explored to facilitate numerous organic rahydroisoquinoline with nitromethane. After being sub-
transformations. For example, Fe3O4 can catalyze the ox- jected to catalytic conditions, the nanoparticles were
idation of olefins and alcohols,13b as well as A3 coupling13c magnetically recovered, the supernatant decanted and fil-
and similar reactions.13a Introduction of another catalyti- tered through Celite to remove any remaining particulate
cally active metal into the ferrite lattice can expand the matter. This process was carried out immediately after re-
scope of accessible reactions.14c,e,g Cobalt ferrite nanopar- moving the reaction vessel from the heat source in order
ticles can catalyze the Knoevenagel reaction,14h while to disfavor any equilibrium shifts that may occur if the so-
copper ferrite can catalyze C–O,14l,o C–C,15 and C–N14g lution were allowed to cool to room temperature. This su-
coupling reactions as well as the Biginelli condensa- pernatant was then used for a second round of catalysis,
tion,14m and azide–alkyne ‘click’ reaction.14n This study affording only a 40% yield. This value is in accordance
examines for the first time a comparative analysis of a with the uncatalyzed yield (41%), suggesting that no
simple (Fe3O4) vs. a substituted ferrite (MFe2O4) for the leached species are leaving the nanoparticle to conduct
catalysis of one class of reactions – in this case the oxida- homogeneous catalysis. To further corroborate this claim,
tive CDC of Csp3 carbons with other Csp3 or Csp carbons ICP analysis was performed on the supernatant, indicating
(Scheme 1).
that less than 0.39 ppm of dissolved copper was present in
solution. These results strongly suggest a heterogeneous
mechanism. Furthermore, the nanoparticles could be eas-
ily recovered (Figure 2), washed with ethyl acetate, and
recycled up to ten times with little appreciable decrease in
yield (Figure 3). Additionally, TEM images of the
nanoparticles indicate no discernable change in size,
shape, or morphology from before the reaction to after ten
Csp3–H + Csp3–H
or
Csp3–H + Csp–H
Csp3–Csp3
or
[M]ferrite nanoparticles
[O]
+ H2O
Csp3–Csp
Scheme 1 Oxidative CDC with Fe3O4 and CuFe2O4 nanoparticles
The oxidative CDC represents a unique synthetic chal- cycles of catalysis (see the Supporting Information). Be-
lenge whereby two C–H bonds are coupled to form a new fore catalysis the average particle size was 34 ± 11.6 nm,
C–C bond.16 Most schemes for C–H bond formation re- while after ten cycles they measured at 31 ± 12.6 nm.
quire prefunctionalization. By circumventing functional-
ization steps, CDC reactions effectively shorten synthetic
routes. Such transformations have been carried out with
various catalysts including copper16d or iron17 and a range
of oxidants from hydrogen peroxide,18 O2,19 tert-butylhy-
droperoxide (TBHP),20 2,3-dichloro-5,6-dicyanobenzo-
quinone21 (DDQ) or even in the absence22 of an oxidant.
Since both iron and copper represent effective catalysts
for CDC reactions, we sought to compare the established
Figure 2 (A) Photograph of reaction mixture with active stirring;
(B) photograph of CuFe2O4 nanoparticles adsorbed to the stir bar and
attracted to an external magnet.
15
reactivity of Fe3O4 with that of the copper-substituted
analogue (CuFe2O4) in hopes that the latter would both in-
crease yields and open new catalytic avenues, specifically
Csp–Csp3 coupling. The direct and exhaustive compari-
son of the activity of magnetite (Fe3O4) vs. copper ferrite
nanoparticles for this reaction is unique to this study.
To compare the catalytic efficiency of Fe3O4 and CuFe2O4
nanoparticles for CDC reactivity,23 N-arylated analogues
of tetrahydroisoquinoline (a common natural product sub-
structure) were coupled with various aromatic alkynes or
nitroalkanes (Table 1). Both displayed excellent yields
coupling to nitroalkanes, but only copper ferrite afforded
the alkynylated product. This second result is not surpris-
ing given the ability of copper to activate alkyne species.
Figure 3 Recycling of CuFe2O4 nanoparticles (0.02 mmol) for the
coupling of nitromethane (0.5 mL) with 2-phenyl-1,2,3,4-tetra-
This disparity in yield between CuFe2O4 and Fe3O4 to- hydroisoquinoline (0.2 mmol) with O2 (1 atm) at 100 °C for 24 h
Synlett 2013, 24, 1637–1642
© Georg Thieme Verlag Stuttgart · New York

LETTER
CuFe2O4-Nanoparticle-Catalyzed Cross-Dehydrogenative Coupling
1639
Table 1 Comparison of Csp3–Csp3 and Csp3–Csp CDC Reaction Catalyzed by Fe3O4 and CuFe2O4 Nanoparticlesa
Ar1
N
Ar2
R1
Ar2
R2
[M]ferrite nanoparticles
(10 mol%)
Ar1
N
H
or
or
H
R1
100 °C, [O], 24 h
Ar1 R3
N
R3
H
R2
R1
NO2
NO2
R2
Entry
1c
Substrate
Product
Yield of Fe3O4 (%)b Yield of CuFe2O4 (%)b
N
N
trace
68
N
N
N
N
2c
3c
trace
trace
71
61
OMe
N
OMe
N
4c
5c
trace
trace
69
53
OMe
OMe
N
N
N
OMe
N
N
N
6d
90e
69e
92
76
NO2
N
7d
NO2
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1637–1642

1640
R. Hudson et al.
LETTER
Table 1 Comparison of Csp3–Csp3 and Csp3–Csp CDC Reaction Catalyzed by Fe3O4 and CuFe2O4 Nanoparticlesa (continued)
Ar1
N
Ar2
R1
Ar2
R2
[M]ferrite nanoparticles
(10 mol%)
Ar1
N
H
or
or
H
R1
100 °C, [O], 24 h
Ar1 R3
N
R3
H
R2
R1
NO2
NO2
R2
Entry
8d
Substrate
Product
Yield of Fe3O4 (%)b Yield of CuFe2O4 (%)b
N
N
59e
73
NO2
N
N
N
9d
10d
11d
72e
91e
93e
87
91
92
NO2
N
NO2
OMe
OMe
OMe
OMe
N
N
N
NO2
OMe
OMe
N
12d
13d
79
32
88
41
NO2
N
N
O2N
a Tertiary amine (0.2 mmol), [M]ferrite nanoparticles (10 mol%), 100 °C, 24 h.
b Isolated yield.
c Aromatic alkyne (0.22 mmol), decane (0.5 ml), [O] = DDQ (0.2 mmol).
d Nitroalkane (0.5 mL), [O] = O2 (1 atm).
e Csp3–Csp3 for Fe3O4-nanoparticle-catalyzed CDC reactions were previously reported15 by our group, verified in this study and reproduced
herein for comparison.
A mechanism has already been proposed for the Fe3O4- Both iron and copper are effective catalysts for the CDC
nanoparticle-catalyzed CDC reaction of nitroalkanes to of two Csp3–H bonds. The ability of both CuFe2O4 and
tertiary amines.15 Herein we propose a similar mechanism Fe3O4 to catalyze these reactions is therefore not surpris-
for CuFe2O4-nanoparticle-catalyzed coupling of aromatic ing. Copper, however, offers the unique benefit of alkyne
alkynes to tertiary amines (Scheme 2). The notable differ- activation, which means CuFe2O4 nanoparticles expand
ence is that we propose that copper must activate the al- the scope of [metal] ferrite nanoparticles beyond that of
kyne while the iminium cation generated from the tertiary simple unsubstituted Fe3O4 nanoparticles. This finding
amine can coordinate to a neighboring iron or copper then implies that the ferrite lattice is a versatile catalyst
atom within the ferrite lattice, before the two are ultimate- platform into which various metals can be substituted to
ly coupled. The proposed route of coupling through an afford different reactivities. No matter the metal substitu-
sp2-hybridized intermediate suggests that this coupling tion, the iron within the lattice imparts a magnetic nature,
could also be referred to as pseudo sp2.
which offers an easy and environmentally benign means
for catalyst recovery and recycling.
Synlett 2013, 24, 1637–1642
© Georg Thieme Verlag Stuttgart · New York

LETTER
CuFe2O4-Nanoparticle-Catalyzed Cross-Dehydrogenative Coupling
1641
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Scheme 2 Proposed mechanism for CuFe2O4 nanoparticle catalyzed
Csp3–Csp CDC
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Acknowledgment
We thank the Natural Science and Engineering Council of Canada
(NSERC), the Canada Foundation for Innovation (CFI), the Canada
Research Chairs (CRC), the Fonds de Recherche sur la Nature et les
Technologies (FQRNT), the Center for Green Chemistry and Cata-
lysis (CCGC), and the Green Chemistry – NSERC Collaborative
Research and Training Experience (CREATE) program, for their fi-
nancial support. We also thank Professor Kevin J. Wilkinson from
Université de Montréal for ICP analysis.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
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(23) Experimental Procedure
stir bar were added to a reaction vessel to which a refluxing
condenser was connected and a balloon of O2 sealed the top
and reacted at 100 °C for 24 h. For coupling with aromatic
alkynes, CuFe2O3 nanoaprticles (0.02 mmol), aromatic
alkyne (0.22 mmol), 2-aryl-1,2,3,4-tetrahydroisoquinolines
(0.2 mmol), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(0.2 mmol), decane (0.5 mL), and a magnetic stir bar were
added to a reaction vessel, sealed, and reacted at 100 °C for
24 h. The nanoparticles were magnetically recovered,
washed with EtOAc, air-dried, and reused without further
modification (only for the recycling tests). The reaction
supernatant was filtered through Celite, and any volatile
compounds were removed under vacuum. The residue was
purified by flash column chromatography on silica gel
(eluent: hexane–EtOAc, 5:1).
CuFe2O3 (<50 nm particle size), Fe2O4 (<50 nm particle
size) and other reagents were purchased from Sigma-Aldrich
and used as received. 2-Aryl-1,2,3,4-
tetrahydroisoquinolines were prepared by a previously
reported method. For coupling with nitroalkanes, CuFe2O3
nanoparticles (0.02 mmol), nitroalkane (0.5 mL), 2-aryl-
1,2,3,4-tetrahydroisoquinolines (0.2 mmol), and a magnetic
Synlett 2013, 24, 1637–1642
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