Sustainable Fe-ppm Pd nanoparticle catalysis of Suzuki-Miyaura cross-couplings in water

Article abstract of DOI:10.1126/science.aac6936

Most of today's use of transition metal-catalyzed cross-coupling chemistry relies on expensive quantities of palladium (Pd). Here we report that nanoparticles formed from inexpensive FeCl₃ that naturally contains parts-per-million (ppm) levels of Pd can catalyze Suzuki-Miyaura reactions, including cases that involve highly challenging reaction partners. Nanomicelles are employed to both solubilize and deliver the reaction partners to the Fe-ppm Pd catalyst, resulting in carbon-carbon bond formation. The newly formed catalyst can be isolated and stored at ambient temperatures. Aqueous reaction mixtures containing both the surfactant and the catalyst can be recycled.

Full text of DOI:10.1126/science.aac6936

RESEARCH
| REPORTS
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SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/349/6252/1083/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S8
Table S1
References (34–40)
8 January 2015; accepted 30 July 2015
10.1126/science.aaa6502
For graphene-based materials, heat conduc-
tion is dominated by phonon transport from
lattice vibrations of the covalent sp2 bonding
network, and the electron transport is largely
determined by the delocalized p-bond over the
whole graphene sheet (1, 3, 18, 30). The lattice
vacancies and the residual functional groups on
graphene sheets upon thermal reduction create
substantial numbers of phonon- and electron-
scattering centers, significantly degrading the
thermal and electrical properties (1, 3, 18, 30).
High-temperature annealing heals defects in
the lattice structure and removes oxygen func-
tional groups and significantly increases the
size of the sp2 domains (fig. S6). The crystallite
sizes (Fig. 4F) in parallel and perpendicular di-
rections to the fiber axis have been calculated
from the integrated intensity ratios of the D-
band (1350 cm−1) and the G-band (1581 cm−1)
based on polarized Raman spectra of the opti-
mized graphene fibers annealed at different tem-
peratures (Fig. 4E and fig. S8) (31, 32). At lower
annealing temperatures (e.g., 1800°C), graphene
fibers demonstrate smaller-sized sp2 domains
(40 to 50 nm) with residual defects. The domain
sizes of the optimized graphene fibers in both
longitudinal and transverse directions increase
substantially with the annealing temperature
(Fig. 4F) and approach 783 and 423 nm, respec-
tively, upon annealing at 2850°C. This is further
evidenced by the submicrometer-sized crystal-
line domains along the fiber axis for the high
temperature–treated fibers as observed in the
CATALYSIS
Sustainable Fe–ppm Pd nanoparticle
catalysis of Suzuki-Miyaura
cross-couplings in water
Sachin Handa,1 Ye Wang,1 Fabrice Gallou,2 Bruce H. Lipshutz1*
Most of today’s use of transition metal–catalyzed cross-coupling chemistry relies on
expensive quantities of palladium (Pd). Here we report that nanoparticles formed from
inexpensive FeCl3 that naturally contains parts-per-million (ppm) levels of Pd can catalyze
Suzuki-Miyaura reactions, including cases that involve highly challenging reaction partners.
Nanomicelles are employed to both solubilize and deliver the reaction partners to the
Fe–ppm Pd catalyst, resulting in carbon-carbon bond formation. The newly formed catalyst
can be isolated and stored at ambient temperatures. Aqueous reaction mixtures
containing both the surfactant and the catalyst can be recycled.
recious metal catalysis has been and con-
tinues to be a predominant means of C-C,
C-H, and C-heteroatom bond construc-
tion in organic synthesis. In particular,
palladium-catalyzed Suzuki-Miyaura, Heck,
and Negishi couplings are indispensable, as rec-
ognized by the 2010 Nobel Prize (1, 2). However,
economically accessible supplies of Pd and other
precious metals are dwindling, thus raising con-
cerns about the sustainability of this chemistry (3).
To circumvent this situation, alternative metals
such as nickel (4, 5) and copper (6, 7) have been
studied, especially as applied to the heavily used,
Pd-catalyzed Suzuki-Miyaura reactions (8, 9). De-
spite varying degrees of success, Pd remains, by
P
1Department of Chemistry and Biochemistry, University of
California–Santa Barbara, Santa Barbara, CA 93106, USA.
2Novartis Pharma, Basel, Switzerland.
*Corresponding author. E-mail: lipshutz@chem.ucsb.edu
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far, the most effective metal for such reactions. In
trace levels, perhaps as impurities in salts of less
expensive metals, Pd could ultimately prove to
be both natural and sustainable for use in
catalysis. Here we disclose such a discovery: a
technique that takes a readily available commer-
cial salt derived from Earth-abundant iron—
which naturally contains parts-per-million (ppm)
levels of Pd—and processes it, in a single step,
into highly active nanoparticles capable of mediat-
ing Suzuki-Miyaura cross-couplings in recyclable
water as the reaction medium.
does the manner in which the salt is reduced.
Attempts to use Fe(acac)3 (acac, acetylacetonate)
and iron pyrophosphate [Fe4(P2O7)3], as well as
several other salts (table S2), led to a far less
reactive catalyst (also formed in situ) than that
derived from FeCl3. Analysis of FeCl3 from a com-
mercially available source by inductively coupled
plasma (ICP) atomic absorption spectrometry
showed that ~300 to 350 ppm Pd was present.
Sources that contained less precious metal (16, 17)
led to incomplete reactions under otherwise iden-
tical conditions. Doping alternative sources of
FeCl3 (≥97% purity) with 350 ppm Pd(OAc)2
(OAc, acetate) yielded nanoparticles of identical
activity, as assessed by using the model reaction
that produced 3 (table S14) (17–20). However,
attempts to use these ppm levels of Pd in the
absence of preformed iron-based nanoparticles
led to virtually no reaction, suggesting that the
release of Pd into the aqueous medium is not
responsible for the catalysis observed. Although
the use of Grignard reagents MeMgX (X = Cl or
Br) and i-PrMgCl yielded material of comparable
activity, both PhMgCl and n-hexyl-MgBr, among
other reductants (e.g., NaBH4, polymethylhydro-
siloxane), led to nanoparticles of inferior quality
(table S4).
Successful couplings require the presence of
both Fe and Pd within these nanocomposites, as
determined by several control reactions (Fig. 3).
Reactions that were attempted using 400 ppm
Pd(OAc)2, with SPhos as a ligand in various ra-
tios, did not lead to product formation. The use
of 5 mol % pure FeCl3 with 400 ppm Pd(OAc)2
and 500 ppm SPhos, without prior treatment
with MeMgCl, afforded none of the biaryl prod-
uct. However, upon reduction of 5 mol % pure
FeCl3 with 10 mol % MeMgCl in the presence of
320 ppm Pd(OAc)2 and 5 mol % SPhos, a highly
active nanocatalyst was generated that mediated
the desired coupling to deliver pure product in a
95% isolated yield.
Doping pure FeCl3 with 500 ppm of other
metals, such as NiCl2, Ni(acac)2, CoCl3, MnCl2,
Cu(OAc)2, or CuBr2, led to catalysts that produced
variable levels of product formation. In all
cases, the yields were ≤38%, as compared with
95% obtained in the presence of added Pd(OAc)2
(table S5).
Solid iron nanoparticles formed from FeCl3
and MeMgCl were collected and analyzed by
transmission electron microscopy (TEM) and
x-ray photoelectron spectroscopy (XPS) (figs.
S8 to S11). As illustrated in fig. S11, XPS anal-
ysis revealed that most of this nanomaterial is
raft-shaped; composed of large amounts of car-
bon (57.4%), oxygen (23.6%), magnesium (6.5%),
and chlorine (9.8%); and characterized by an es-
sentially 1:1 ratio between iron (1.4%) and phos-
phorus (in SPhos; 1.3%). The high levels of carbon
and oxygen are associated with residual solvent
(THF) integrated within these clusters; the C-O
signal appears as a shoulder in the C1s spectrum
(286.5 eV) (figs. S9 to S11). Only 1.4% iron, in the
form of iron oxides (Fe 2p3, 710.86 eV), was pres-
ent in the nanoparticles produced via reduc-
tion of FeCl3 with MeMgCl in THF.
Cryogenic TEM (cryo-TEM) analysis revealed
the aggregation of rafts of metal nanoparticles,
either inside or around the nanomicellar sur-
face (Fig. 4, A to C). Scanning electron micros-
copy (SEM), together with energy-dispersive
x-ray (EDX) analyses, showed the presence of
hybrid nanoparticles containing both iron and
ligand (Fig. 4, D and E, and figs. S12 to S14). Fur-
ther analyses by atomic force microscopy (AFM)
At the heart of this advance lies the conflu-
ence of several reaction variables: the origin
and source of the iron salt, the presence of ppm
levels of Pd, the manner through which these
are converted to nanoparticles, and the use of
aqueous micellar conditions for catalysis. The
catalyst preparation calls for the use of inex-
pensive (97% purity) FeCl3 containing ppm lev-
els of Pd, admixed with a ligand and dissolved
specifically in tetrahydrofuran (THF). Treatment
of this solution at room temperature with two
equivalents of a Grignard reagent, also in THF,
quickly affords nanoparticles that, after solvent
removal in vacuo, can be used directly in Suzuki-
Miyaura reactions. The in situ generation of
5 mole percent (mol %) of these Fe–ppm Pd
nanoparticles was found empirically to be suf-
ficient. Next, an aqueous solution containing 2
weight percent (wt %) of our commercially avail-
able designer surfactant TPGS-750-M (10) and
a base (K3PO4⋅H2O, 1.5 to 2.0 equivalents) is
added to the nanoparticles. Reaction partners
1 and 2 as model substrates are then intro-
duced, leading to biaryl product 3 (Fig. 1). The
choice of ligand is crucial (Fig. 1), with SPhos
(2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl)
and XPhos (2-dicyclohexylphosphino-2′,4′,6′-
triisopropylbiphenyl) affording the best results.
Vigorous stirring at temperatures between am-
bient and 45°C, depending upon the extent of crys-
tallinity of the reaction partners, is sufficient to
drive couplings to completion, typically in the
12- to 24-hour time frame (at a global concentra-
tion of 0.5 M).
We examined the scope of this technology
and found that many representative cases afford
good-to-excellent isolated yields (Fig. 2). A broad
variety of aromatic and heteroaromatic arrays,
with either partner being the aryl halide or boron
derivative, can be tolerated. Functional groups
including CF3, amines, acetals, amides, aldehydes,
esters, ketones, phosphate esters, nitro groups,
polyaromatics, sulfonamides, and carbamates
are represented among these examples. Several
types of heteroaromatic arrays are also ame-
nable, including nitrogen-containing moieties
that might present complications as competing
ligands for Pd. Both bromides and iodides are
excellent educts, and the boron species involved
can be any of those commonly employed: boronic
acids, Bpin (boronic acid pinacol ester) (11) or
N-methyliminodiacetic acid (MIDA) boronates
(12, 13), and BF3K salts (14, 15).
Fig. 1. Ligand optimization for Fe–ppm Pd catalysis of Suzuki-Miyaura cross-couplings. Conditions
were as follows: 4-bromoanisole (0.5 mmol), naphthalene-1-boronic acid (0.75 mmol), 5 mol % Fe–ppm
Pd nanoparticles, K3PO4•H2O (0.75 mmol), 2 wt % TPGS-750-M (1 ml). Asterisks indicate yields based
on gas chromatography–mass spectrometry (GC-MS).
The composition of the iron salt plays a role
in the activity of the resulting nanoparticles, as
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Fig. 2. Couplings between aryl halides (Ar-X) and aryl (Ar') or alkenyl boron derivatives. Unless otherwise noted, conditions for these couplings were as
follows: Ar-X (0.5 mmol), Ar'-BRn (0.6 mmol), FeCl3 (5 mol %), SPhos (5 mol %), MeMgCl (10 mol %), K3PO4•H2O (0.75 mmol),TPGS-750-M (2 wt %, 1 ml), 45°C.
Room temperature, rt. Asterisks indicate the use of Ar-B(OH)2 or Ar-B(MIDA) (1.2 mmol) and K3PO4•H2O (1.5 mmol). Reported yields are for isolated,
chromatographically purified materials. 320 ppm Pd is required (the general procedure is described in detail in the supplementary materials).
Fig. 3. Control reactions documenting the importance of
both Fe and Pd in catalyst formation. Details are provided
in the supplementary materials.
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(Fig. 4F) revealed an atypical arrangement of
iron atoms intermixed mainly with Mg. These
particles have a long shelf life (≥1 month at
room temperature) and are virtually identical
in catalytic activity to those prepared and used
in situ.
Thermogravimetric analysis (TGA) of nano-
material revealed about a 40% total drop in
weight between 60° and 145°C, corresponding
to the loss of THF bound within the catalyst.
Material heated beyond 145°C was stable up to
380°C. However, when the catalyst was preheated
at 80°C under a vacuum for 12 hours (fig. S6), a
loss of catalytic activity was observed, indicating
the importance of THF in maintaining the nano-
cage structure.
Upon completion of a Suzuki-Miyaura coupl-
ing, in-flask extraction with a single organic sol-
vent (e.g., ethyl acetate or methyl tert-butyl ether)
at ambient temperatures produces crude mate-
rial that can be further purified by standard means
(fig. S2). The remaining aqueous mixture con-
taining both nanomicelles and nanoparticles of
iron can then be recycled, with a modest aug-
mentation of Pd [i.e., 160 ppm Pd(OAc)2] at every
other recycling to compensate for losses during
extraction. Although the external addition of this
Pd salt extends the catalyst activity, the manner
in which it is reduced to active Pd(0) and how it
is incorporated into the Fe nanoparticles remain
unclear. Either the same or different educts can
be used in these couplings, indicating the robust-
ness of the process. Alternatively, with solid
products, dilution with water could be followed by
simple filtration to produce the targeted material
Fig. 4. Catalyst characterization. (A to C) Cryo-TEM images of Fe-Pd nanorods in aqueous TPGS-750-M.
(D and E) SEM images of the solid nanomaterial. (F) AFM image of the solid nanomaterial.
Fig. 5. Further applications of Fe–ppm Pd–catalyzed couplings. (A) Sequential reactions, including a Suzuki-Miyaura coupling using Fe–ppm Pd
nanoparticles as the catalyst (TMSI, TMS iodide; DIPEA, diisopropylethylamine; cBRIDP, di-t-butyl(2,2-diphenyl-1-methyl-1-cyclopropyl)phosphine; KO-t-Bu,
potassium t-butoxide; TIPSOH, triisopropylsilyl alcohol). (B) A representative example suggestive of the extension of this approach to Sonogashira couplings
(OTBS, t-butyldimethylsilyloxy).
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directly. The diluted filtrate could be augmented
with TPGS-750-M to the original level (2 wt %) and
reused, thereby creating little to no wastewater
stream. The environmental factor (E factor) (21),
a metric of “greenness” that has previously been
applied to micellar catalysis (22), is very low (E
factor = 3).
18. C. L. Chun, D. R. Baer, D. W. Matson, J. E. Amonette, R. L. Penn,
Environ. Sci. Technol. 44, 5079–5085 (2010).
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made use of the University of California–Santa Barbara (UCSB)
Materials Research Laboratory Central Facilities, supported by
NSF’s MRSEC program under award no. DMR-1121053. ICP-MS
analyses were provided by J. Reilly (Novartis, Cambridge, MA). We
also acknowledge support from NIH in the form of a Shared
Instrument Grant to UCSB(1S10OD012077-01A1). A preliminary
patent covering this chemistry has been filed by the University of
California–Santa Barbara. The experimental data reported in this
paper are available in the supplementary materials.
We used ICP to analyze the palladium content
(<10 ppm) of a product formed via the technology
presented here, and we compared the result with
that quantified following a traditional Suzuki-
Miyaura coupling in organic solvent (fig. S4). Re-
sidual palladium in the product derived from a
standard coupling in dioxane was far higher than
that observed using our nanoparticle approach.
Prospects for incorporating this water-based
nanomicelle-nanometal technology into a one-
pot sequence of reactions are shown in Fig. 5A.
Heteroaryl iodide 4, containing carbamate and
trimethylsilyl (TMS) protecting groups, was gen-
erated in situ and then subjected to cross-coupling
with alkenyl tetrafluoroborate salt 5, using the
Fe–ppm Pd nanoparticle protocol. The coupling
product 6 was then exposed to aqueous base to
23. P. N. Dube et al., Chem. Biol. Drug Des. 84, 409–419 (2014).
SUPPLEMENTARY MATERIALS
ACKNOWLEDGMENTS
www.sciencemag.org/content/349/6252/1087/suppl/DC1
Materials and Methods
Figs. S1 to S18
Tables S1 to S15
References (24–34)
We thank Novartis for financial support; J. Feng for technical
assistance; M. Cornish for acquisition of the AFM images;
S. Kraemer for obtaining the cryo-TEM, SEM, and EDX data; and
J. Matthey for providing Pd salts. Parts of this work were carried
out in the Characterization Facility, University of Minnesota, which
receives partial support from NSF through the Materials Research
Science and Engineering Center (MRSEC) program. This work also
13 June 2015; accepted 30 July 2015
10.1126/science.aac6936
NATURAL HAZARDS
Slip pulse and resonance of the
remove the TMS groups and effect elimination to Kathmandu basin during the 2015
7, followed by butoxycarbonyl (Boc) deprotection
to 8. Final aryl amination with bromobenzene to
Gorkha earthquake, Nepal
9 provided entry to the bioactive class of 2,4,5-
substituted pyrazol-3-one compounds in a one-
J. Galetzka,1* D. Melgar,2 J. F. Genrich,1 J. Geng,3 S. Owen,4 E. O. Lindsey,3 X. Xu,3
pot sequence with an overall isolated yield of
Y. Bock,3 J.-P. Avouac,5,1† L. B. Adhikari,6 B. N. Upreti,7 B. Pratt-Sitaula,8
68% (23).
T. N. Bhattarai,9 B. P. Sitaula,9 A. Moore,4 K. W. Hudnut,10 W. Szeliga,11
In addition, testing the potential for this mixed-
metal catalyst system to effect other important
Pd-catalyzed reactions, such as Sonogashira cou-
plings, was carried out (in the absence of added
J. Normandeau,12 M. Fend,12 M. Flouzat,13 L. Bollinger,13 P. Shrestha,6 B. Koirala,6
U. Gautam,6 M. Bhatterai,6 R. Gupta,6 T. Kandel,6 C. Timsina,6 S. N. Sapkota,6
S. Rajaure,6 N. Maharjan6
copper), following the example illustrated in
Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake
physics and associated ground shaking.The 25 April 2015 moment magnitude 7.8 earthquake in
Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a
high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric
synthetic aperture radar data to model the earthquake rupture as a slip pulse ~20 kilometers in
width, ~6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which
propagated toward the Kathmandu basin at ~3.3 kilometers per second over ~140 kilometers.
The smooth slip onset, indicating a large (~5-meter) slip-weakening distance, caused moderate
ground shaking at high frequencies (>1 hertz; peak ground acceleration, ~16% of Earth’s
gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of
4 to 5 seconds caused the collapse of tall structures, including cultural artifacts.
Fig. 5B. The prognosis for a similar outcome is
good.
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long segment of the fault (Fig. 1A), with a hypo-
central depth of ~15 km and a dip angle of 7°
to 12° (5, 6). The MHT accommodates most of
the convergence between India and southern
Tibet, with a convergence rate between 17 and
21 mm/year (7). For the 2015 event, which re-
sulted in over 8000 deaths (mostly in Kathmandu
and adjacent districts), modified Mercalli inten-
sities (MMIs) reported by the National Society
for Earthquake Technology–Nepal (NSET) (8)
reached up to IX (violent shaking) and exceeded
VI (strong shaking) over an area 170 km by 40 km.
Kathmandu has been struck by repeated earth-
quakes in the past, with major destruction [MMI >
X (extreme shaking)] in the years 1255, 1344, 1408,
1681, 1833, and 1934 (9–11). These earthquakes
all occurred close to Kathmandu and have been
T
tent of the STF, and hence the intensity of the
near-field ground motion (1), whereas the tail,
which discriminates pulse-like and crack-like rup-
tures (2), has a low-frequency signature. Therefore,
resolving the STF with band-limited strong-motion
records is difficult. Combining high-rate Global
Positioning System (GPS) waveforms (3, 4), which
capture both dynamic and permanent deforma-
tion, overcomes this limitation.
The 25 April 2015 moment magnitude (Mw)
7.8 earthquake in Gorkha, Nepal resulted from
the unzipping of the lower edge of the locked
portion of the Main Himalayan Thrust (MHT)
fault, along which the Himalayan wedge is thrust
13. S. J. Lee, T. M. Anderson, M. D. Burke, Angew. Chem. Int. Ed.
49, 8860–8863 (2010).
14. G. A. Molander, Y. Yokoyama, J. Org. Chem. 71, 2493–2498
(2006).
15. S. Darses, J.-P. Genet, Chem. Rev. 108, 288–325 (2008).
16. T. H. Bointon et al., Nano Lett. 14, 1751–1755 (2014).
17. S. L. Buchwald, C. Bolm, Angew. Chem. Int. Ed. 48, 5586–5587
(2009).
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