Highly Selective and Solvent-Dependent Reduction of Nitrobenzene to N-Phenylhydroxylamine, Azoxybenzene, and Aniline Catalyzed by Phosphino-Modified Polymer Immobilized Ionic Liquid-Stabilized AuNPs

Article abstract of DOI:10.1021/acscatal.9b00347

Gold nanoparticles stabilized by phosphine-decorated polymer immobilized ionic liquids (AuNP@PPh₂-PIILP) is an extremely efficient multiproduct selective catalyst for the sodium borohydride-mediated reduction of nitrobenzene giving N-phenylhydroxylamine, azoxybenzene, or aniline as the sole product under mild conditions and a very low catalyst loading. The use of a single nanoparticle-based catalyst for the partial and complete reduction of nitroarenes to afford three different products with exceptionally high selectivities is unprecedented. Under optimum conditions, thermodynamically unfavorable N-phenylhydroxylamine can be obtained as the sole product in near quantitative yield in water, whereas a change in reaction solvent to ethanol results in a dramatic switch in selectivity to afford azoxybenzene. The key to obtaining such a high selectivity for N-phenylhydroxylamine is the use of a nitrogen atmosphere at room temperature as reactions conducted under an inert atmosphere occur via the direct pathway and are essentially irreversible, while reactions in air afford significant amounts of azoxy-based products by virtue of competing condensation due to reversible formation of N-phenylhydroxylamine. Ultimately, aniline can also be obtained quantitatively and selectively by adjusting the reaction temperature and time accordingly. Introduction of PEG onto the polyionic liquid resulted in a dramatic improvement in catalyst efficiency such that N-phenylhydroxylamine could be obtained with a turnover number (TON) of 100000 (turnover frequency (TOF) of 73000 h^-1, with >99% selectivity), azoxybenzene with a TON of 55000 (TOF of 37000 h^-1 with 100% selectivity), and aniline with a TON of 500000 (TOF of 62500 h^-1, with 100% selectivity). As the combination of ionic liquid and phosphine is required to achieve high activity and selectivity, further studies are currently underway to explore whether interfacial electronic effects influence adsorption and thereby selectivity and whether channeling of the substrate by the electrostatic potential around the AuNPs is responsible for the high activity. This is the first report of a AuNP-based system that can selectively reduce nitroarenes to either of two synthetically important intermediates as well as aniline and, in this regard, is an exciting discovery that will form the basis to develop a continuous flow process enabling facile scale-up.

Full text of DOI:10.1021/acscatal.9b00347

Subscriber access provided by Bibliothèque de l'Université Paris-Sud
Article
Highly Selective and Solvent-Dependent Reduction of Nitrobenzene
to N-Phenylhydroxylamine, Azoxybenzene and Aniline Catalyzed by
Phosphino-Modified Polymer Immobilized Ionic Liquid-Stabilized AuNPs
Simon Doherty, Julian G. Knight, Tom Backhouse, Ryan John Summers, Einas Abood, william
simpson, William Paget, Richard A. Bourne, Thomas W. Chamberlain, Rebecca Stones, Kevin R.J.
Lovelock, Jake Seymour, Mark Isaacs, Christopher Hardacre, Helen Daly, and Nicholas H. Rees
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00347 • Publication Date (Web): 15 Apr 2019
Downloaded from http://pubs.acs.org on April 15, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
Highly Selective and Solvent-Dependent Reduction
of Nitrobenzene to N-Phenylhydroxylamine,
Azoxybenzene and Aniline Catalyzed by Phosphino-
Modified Polymer Immobilized Ionic Liquid-
Stabilized AuNPs
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
,†
,†
†
†
Simon Doherty,* Julian G. Knight,* Tom Backhouse, Ryan J. Summers, Einas
†
†
†
‡
Abood, William Simpson, William Paget, Richard A. Bourne, Thomas W.
‡,
‡
§
§
Chamberlain, * Rebecca Stones, Kevin R. J. Lovelock, Jake M. Seymour, Mark A.
¥
Ϯ
Ϯ
‖
Isaacs, Christopher Hardacre, Helen Daly, and Nicholas H. Rees
†
NUCAT, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon
Tyne, NE1 7RU, UK
ACS Paragon Plus Environment
1

ACS Catalysis
Page 2 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
‡
Institute of Process Research & Development, School of Chemistry and School of
Chemical and Process Engineering, University of Leeds, Woodhouse Lane, Leeds, LS2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
9
JT, UK
§
School of Chemistry, Food and Pharmacy, University of Reading, Reading, RG6 6AT,
UK
¥
EPSRC National Facility for XPS (HarwellXPS), Room G.63, Research Complex at Harwell
(RCaH), Rutherford Appleton Laboratory, Harwell, Oxford, Didcot, OX11 0FA, UK
Ϯ
School of Chemical Engineering and Analytical Science, The Mill, Sackville Street Campus, The
University of Manchester, Manchester M13 9PL, UK
‖
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1
3QR
ACS Paragon Plus Environment
2

Page 3 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
Table of Contents and Abstract Graphics
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment
3

ACS Catalysis
Page 4 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ABSTRACT: Gold nanoparticles stabilized by phosphine-decorated polymer immobilized
ionic liquids (AuNP@PPh -PIILP) is an extremely efficient multi-product selective catalyst
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
for the sodium borohydride-mediated reduction of nitrobenzene giving either N-
phenylhydroxylamine, azoxybenzene or aniline as the sole product under mild conditions
and a very low catalyst loading. The use of a single nanoparticle-based catalyst for the
partial and complete reduction of nitroarenes to afford three different products with
exceptionally high selectivities is unprecedented. Under optimum conditions,
thermodynamically unfavorable N-phenylhydroxylamine can be obtained as the sole
product in near quantitative yield in water whereas a change in reaction solvent to ethanol
results in a dramatic switch in selectivity to afford azoxybenzene. The key to obtaining
such a high selectivity for N-phenylhydroxylamine is the use of a nitrogen atmosphere at
room temperature as reactions conducted under an inert atmosphere occur via the direct
pathway and are essentially irreversible while reactions in air afford significant amounts
of azoxy-based products by virtue of competing condensation due to reversible formation
of N-phenylhydroxylamine. Ultimately, aniline can also be obtained quantitatively and
selectively by adjusting the reaction temperature and time accordingly. Introduction of
ACS Paragon Plus Environment
4

Page 5 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
PEG onto to the polyionic liquid resulted in a dramatic improvement in catalyst efficiency
such that N-phenylhydroxylamine could be obtained with a TON of 100,000 (TOF of
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
7
3,000 h , with >99% selectivity), azoxybenzene with a TON of 55,000 (TOF of 37,000
-1
-1
h with 100% selectivity) and aniline with a TON of 500,000 (TOF of 62,500 h , with
100% selectivity). As the combination of ionic liquid and phosphine are required to
achieve high activity and selectivity further studies are currently underway to explore
whether interfacial electronic effects influence adsorption and thereby selectivity and
whether channeling of the substrate by the electrostatic potential around the AuNPs is
responsible for the high activity. This is the first report of a AuNP-based system that can
selectively reduce nitroarenes to either of two synthetically important intermediates as
well as aniline and, in this regard, is an exciting discovery that will form the basis to
develop a continuous flow process enabling facile scale-up.
KEYWORDS: Nanoparticle catalysis, gold, selective partial reduction, nitroarenes, N-
phenylhydroxylamine, azoxybenzene, aniline, switchable selectivity
ACS Paragon Plus Environment
5

ACS Catalysis
Page 6 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment
6

Page 7 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
INTRODUCTION
Selectivity in catalysis is immensely important for the industrial scale production of
commodity chemicals as well as fine chemicals and pharmaceuticals as it limits the
production of waste, streamlines the process by avoiding or simplifying purification
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
procedures, reduces costs and improves green credentials. While selectivity in
homogeneous catalysis is well-understood and can often be optimized by tuning steric
2
and electronic parameters, tuning selectivity of nanoparticles or heterogeneous catalysts
3
is much more challenging and less-well developed. To this end, until recently, the vast
majority of studies have employed organic modifiers which improve or enhance selectivity
through steric effects resulting from specific noncovalent molecular interactions.4
However, there is now a growing body of evidence that activity and selectivity of
nanoparticle-based catalysts can be modified/optimized by modulating their surface
electronic structure. For example, PVP-stabilized rhodium nanoparticles modified with
phosphine catalyze the chemoselective hydrogenation of functionalized aromatic
5
compounds with >92% selectivity compared with 70% for the unmodified catalyst while
ACS Paragon Plus Environment
7

ACS Catalysis
Page 8 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
addition of tricyclohexylphosphine to silica-supported CuNPs results in a significant
improvement for the semi-hydrogenation of 1-phenyl-1-propyne to cis-β-methylstyrene.6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Other relevant studies include gold nanoparticles ligated by secondary phosphine oxides
which catalyze the hydrogenation of -unsaturated aldehydes with 100% selectivity for
7
reduction of the carbonyl and a pronounced ligand effect for RhNP and RuNP catalyzed
8
9
hydrogenation of aromatic ketones and substituted arenes. Most recently, platinum
nanoparticles stabilized on triphenylphosphine-modified silica showed markedly higher
selectivity for the chemoselective hydrogenation of acetophenone and phenylacetylene
than its unmodified counterpart; this too has been attributed to an increase in surface
10
electron density by the electron donating phosphine. In addition, amine-based modifiers
have also been shown to alter the surface electronic structure of platinum nanowires and
particles and thereby optimize reaction selectivity for the partial hydrogenation of
11
nitroarenes to N-arylhydroxylamines. The high activity of amino-modified Ru/-Al O for
2
3
the aqueous phase hydrogenation of levulinic acid to -valerolactone was attributed to
12
highly dispersed ruthenium centers with an electron-rich state and the efficacy of RuNP
stabilized in amine-modified porous organic polymers for the catalytic transfer
ACS Paragon Plus Environment
8

Page 9 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
hydrogenation of nitroarenes was attributed to a combination of nano-confinement and
electron donation13 while the efficacy of PdNPs supported on amine-rich silica hollow
nanospheres for the hydrogenation of quinoline has been associated with an ultra-small
particle size and high surface electron density.14 Amine modified supports have been
shown to improve activity and selectivity for PtNP, Pt/Co-NP and PdNP-catalyzed semi-
hydrogenation of alkynes15a-c and selectivity for PtNP-catalyzed reduction of the C=O
double bond in cinnamaldehyde15d while functionalization of platinum nanoparticles with
L-proline enhances the activity and selectivity for the hydrogenation of acetophenone.16
Aspartic acid also improves the activity and selectivity of platinum nanoparticle catalysts
for the selective hydrogenation of -unsaturated aldehydes to unsaturated alcohols
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
17
through cooperation of steric and electronic effects. Finally, a series of elegant studies
by the Glorious group have also demonstrated that the performance of supported
heterogeneous catalysts can be tuned by surface modification with N-heterocyclic
1
8
9
carbenes which activate unreactive Pd/Al O for the Buchwald-Hartwig amination,
2
3
1
improve the selectivity of Ru/K-Al O for the hydrogenation of aryl ketones and alkynes
2
3
ACS Paragon Plus Environment
9

ACS Catalysis
Page 10 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
and influence the performance and stability of ruthenium and palladium nanoparticles as
catalysts for oxidations and reductions.20
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
We have recently been exploring the concept of HeteroAtom Donor-modified Polymer-
Immobilized Ionic Liquid Stabilized NPs (NP@HAD-PIILS) reasoning that covalent
attachment of an ionic liquid to a polymer would combine the favorable characteristics of
ILs, such as their tunable physicochemical properties, ease of modification and
enhancement in reaction rates and selectivity with the advantages associated with
attachment to a solid support, to limit loss of ionic liquid, facilitate product separation,
catalyst recovery and recycle and reduce the volume of IL as the catalyst is retained in a
small amount of PIIL.21 While heteroatom donors were initially incorporated into ionic
22
liquids to stabilize NPs with respect to agglomeration under conditions of catalysis, it is
now clear that these donors could also enable the surface electronic structure to be
modified and/or NP size and morphology to be controlled and, in this regard, HAD-PIILs
may well prove to be tunable multifunctional supports for developing more efficient
catalysts and processes.14c,7a,15a,15d,22p Gratifyingly, our initial foray in this area has proven
extremely promising as PEG-modified phosphine-decorated polymer-immobilized ionic
ACS Paragon Plus Environment
10

Page 11 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
liquid-stabilized palladium nanoparticles were shown to be remarkably active and
selective catalysts for hydrogenation of -unsaturated aldehydes, ketones and nitriles,23
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
24
hydrogenation and transfer hydrogenation of nitroarenes in water and Suzuki-Miyaura
cross-couplings in aqueous media.25
With the aim of further exploring the concept of PIIL stabilized NPs we have recently
extended our study to include gold nanoparticle-based systems in order to undertake a
comparative study of their efficacy as catalysts for the reduction of nitroarenes. Herein,
we report that AuNP@PPh -PIILP and its PEGylated counterpart AuNP@PPh -
2
2
PEGPIILP are highly efficient and remarkably selective catalysts for the sodium
borohydride-mediated reduction of nitroarenes to either N-phenylhydroxylamine,
azoxybenzene or aniline (Scheme 1). Although AuNP catalyzed reduction of nitroarenes
26
with sodium borohydride have been widely reported in the literature, the overwhelming
majority of systems are highly selective for aniline and this is the first example of a AuNP-
based catalyst that is completely selective for the aqueous phase sodium borohydride-
mediated reduction of a nitroarene to either the corresponding N-arylhydroxylamine,
azoxyarene or aniline. To this end, there are very few reports of a single catalyst that can
ACS Paragon Plus Environment
11

ACS Catalysis
Page 12 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
selectively reduce a nitroarene to more than one intermediate with high efficacy. For
example, RuNPs catalyze the transfer hydrogenation of nitroarenes to the corresponding
azoxyarene, azoarene or aniline, albeit with moderate selectivities and at elevated
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
temperatures with a high catalyst loading.27a More recently, Au/meso-CeO has been
2
shown to be a highly selective and switchable catalyst for the transfer hydrogenation of
nitroarenes to the corresponding azoxyarene, azoarene or aniline and high yields of each
product could be obtained by modifying the reaction conditions.27b
O-
N
[H2]
nsation
O
N
conde
III
N
H O
2
NO2
[H2]
-
H O
2
I
N
(b)
N
IV
(a)
AuNP
H
OH
[
H ]
2
N
AuNP@PPh -PIILP
2
NH
[H2]
HN
II
V
[
H ]
2
H N
2
H N
2
Scheme 1 General mechanisms (Haber) for the reduction of nitroarenes (a) direct
pathway (b) condensation pathway, highlighting the selective partial reduction to N-
phenylhydroxylamine (green), azoxybenzene (blue) and complete reduction to aniline
(
red) achieved with AuNP@PPh -PIILP (3a) and AuNP@PPh -PEGPIILP (3a.PEG).
2
2
ACS Paragon Plus Environment
12

Page 13 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
Selective partial hydrogenation of nitroarenes is of considerable interest as N-
arylhydroxylamines are synthetically important intermediates to high value products
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
28
29
including biologically active motifs, polymerization inhibitors and reagents for use in
organic synthesis as they undergo a range of transformations including cyclizations,30a
the Bamberger rearrangement30b and gold-catalyzed additions to multiple bonds.30c
Although there are numerous procedures for the synthesis of N-arylhydroxylamines
31
including stoichiometric reductions with zinc or tin as well as catalytic reductions using
Rh/C,32a,b Pt/SiO2,32c RuNP/carbon nanotubes,32d RuNP/polystyrene,32e and
PtNP/Amberlite IRA32f all with hydrazine, AgNP/TiO with ammonia-borane32g and ultra-
2
thin PtNW/amine,11a PtNP/amine,11b Pt/SiO -amine, Pt/C-DMSO33b and platinum metal
33a
2
catalysts33c with hydrogen, most suffer severe limitations including low isolated yields,
poor selectivity, slow rates, a lack of scalability and/or recyclability as well as poor
environmental and economical credentials. Similarly, azoxyarenes are an important class
of compound which find use as dyes, reducing agents and chemical stabilizers.34
35
34,36
Azoxyarenes have been prepared by stoichiometric or catalytic
oxidative coupling
ACS Paragon Plus Environment
13

ACS Catalysis
Page 14 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
of arylamines as well as selective reduction of nitroarenes,27,37 however, these
approaches also suffer the same drawbacks as those detailed above for the selective
synthesis of N-arylhydroxylamines. As such, the discovery of a single catalyst that is
highly selective for the reduction of nitroarenes to N-arylhydroxylamines, azoxyarenes
and the corresponding aniline at low catalyst loadings in either water or ethanol under
mild conditions is exceptional and will provide a platform to engineer new catalyst
technology for efficient cost-effective production of these target intermediates.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
EXPERIMENTAL SECTION
Synthesis of Catalyst Precursor [AuCl ]Cl@PPh -PIILP (2a). A 100 mL round bottom
4
2
flask was charged with the appropriate PPh -PIILP (1.79 g, 2.0 mmol) support and water
2
(15 mL). To this was added a solution of potassium tetrachloroaurate (0.755 g, 2.0 mmol)
in water (4-5 mL) and the resulting mixture stirred vigorously for 6 hours. The yellow
precipitate was collected after centrifugation by filtration through a frit and the solid was
washed with water (10 mL), ethanol (2 x 10 mL) and diethyl ether (3 x 10 mL) to afford
ACS Paragon Plus Environment
14

Page 15 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
the corresponding afford the desired precatalyst 2a as a pale-yellow powder in 93% yield
2.2 g). ICP-OES data: 0.73 wt% gold and a gold loading of 0.037 mmol g-1.
(
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Synthesis of [AuCl ]Br@PPh -PEGPIILP (2a.PEG). Tetrachloroaurate loaded polymer
4
2
2a.PEG was prepared according to the procedure described above for 2a and isolated
as a pale-yellow powder in 87% yield. ICP-OES data: 3.1 wt% gold and a gold loading of
-1
0
.16 mmol g .
General procedure for the selective reduction of nitroarenes to arylhydroxylamines. A
Schlenk flask equipped with a magnetic stirrer was charged with precatalyst (0.5 mol, 0.005 mol
-1
%
gold based on the gold loading determined as mmol Au g polymer using ICP-OES data) and
evacuated and backfilled with nitrogen. NaBH (0.096 g, 2.5 mmol) was added followed
4
immediately by water (2.5 mL) and the resulting mixture stirred vigorously for 5 min at room
temperature. After this time, nitroarene (1 mmol) was added and the reaction stirred for
the appropriate time. The reaction was quenched by addition of ethyl acetate (20 mL)
followed by water (5 mL), the organic phase separated after extractions and the solvent
1
removed under reduced pressure. The residue was analyzed by H NMR spectroscopy
ACS Paragon Plus Environment
15

ACS Catalysis
Page 16 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
using 1,4-dioxane as internal standard to quantify the composition of starting material and
products and determine the selectivity.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
General procedure for the selective reduction of nitrobenzene to azoxybenzene. A
Schlenk flash was charged with precatalyst (0.5 mol, 0.005 mol % gold based on the gold
-1
loading determined as mmol Au g polymer using ICP-OES data) under a nitrogen atmosphere
and NaBH (0.096 g, 2.5 mmol) and ethanol (2 mL) added. The solution was stirred for 5
4
mins at room temperature after which time nitrobenzene (102 µL, 1 mmol) was added
and the resulting mixture stirred for the appropriate time. After this, stirring was stopped,
1,4 dioxane (85 μL, 1 mmol) was added as the internal reference and an aliquot (ca. 0.05
1
mL) removed and diluted with CDCl (0.35 mL). The reaction was analyzed using H NMR
3
spectroscopy to quantify the composition and selectivity.
General procedure for the reduction of nitroarenes to arylamines. A Schlenk flask was
charged with precatalyst (0.5 mol, 0.005 mol % gold based on the gold loading determined as
-1
mmol Au g polymer using ICP-OES data) under a nitrogen atmosphere and NaBH (0.192 g, 5.0
4
mmol) and water (2 mL) added. The solution was then stirred for 5 mins at room temperature
after which nitrobenzene (102 µL, 1 mmol) was added dropwise and the mixture heated
ACS Paragon Plus Environment
16

Page 17 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
to 60°C with stirring for the appropriate time. After this time, the reaction was cooled to
room temperature then quenched by addition of ethyl acetate (40 mL) and water (5 mL),
the aqueous phase separated after extraction and the solvent removed under reduced
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
pressure. The residue was analyzed by H NMR spectroscopy using 1,4-dioxane as
internal standard to quantify the composition and selectivity.
RESULTS AND DISCUSSION
Synthesis and characterization of phosphine-modified gold precatalysts and
nanoparticles
The polymers, their tetrachloroaurate loaded precursors, and the corresponding
polymer immobilized ionic liquid stabilized AuNPs developed in this project are shown in
Figure 1. These polymers have been designed to form a protective cage or shell over the
AuNP surface that provides stabilization through electrostatic interactions with the
imidazolium fragment as well as Au-P interactions with the phosphine-decorated
38
polymer. Polymer immobilized ionic liquids 1a and 1b were prepared by AIBN-initiated
ACS Paragon Plus Environment
17

ACS Catalysis
Page 18 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
radical polymerization of the constituent imidazolium-modified monomer or its PEGylated
counterpart with cation appropriate cross-linker and 4-diphenylphosphinostyrene in the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
desired ratio.23-25 Both polymers were impregnated by exchanging half of the halide
-
anions with [AuCl ] to afford the corresponding tetrachloroaurate-loaded precursors 2a
4
(
[AuCl ]Cl@PPh -PIILP) and 2a.PEG ([AuCl ]Br@PPh -PEGPIILP) which have a gold to
4
2
4
2
phosphine stoichiometry of one; full characterization details are provided in the ESI. As
we have previously demonstrated that palladium nanoparticle-based catalysts generated
in situ by reduction of the corresponding tetrachloropalladate-loaded precursor
immediately prior to use are as efficient as their ex situ prepared counterparts,
precursors,24-25 2a and 2a.PEG were used for catalyst evaluation and optimization while
samples of 3a and 3a.PEG were generated under conditions of catalysis to obtain TEM
and XPS characterization data on the active species. The generation of catalyst in situ
immediately prior to addition of substrate offers several potential practical advantages as
it eliminates the need to prepare, isolate and store nanoparticle catalysts, and as such
streamlines the protocol; it also improves versatility by enabling different reducing agents
ACS Paragon Plus Environment
18

Page 19 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
and/or conditions to be examined and thereby facilitates rapid catalyst and reaction
screening.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-
Figure 1 Composition of polymers 1a and 1a.PEG, synthesis of [AuCl ] impregnated
4
polymers 2a and 2a.PEG and PIILP-stabilized gold nanoparticles 3a (red) and 3a.PEG
(green) and composition of 3b (orange) and 3b.PEG (blue).
The gold loadings of precatalysts 2a and 2a.PEG were determined to be 0.037 mmol
-1
-1
31
g and 0.16 mmol g , respectively, using ICP-OES. The solid state P NMR spectra of
2a and 2a.PEG confirm the presence of a Au----P interaction which is clearly evident from
ACS Paragon Plus Environment
19

ACS Catalysis
Page 20 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
1
the low field chemical shifts of  27.1 and 24.5 ppm, respectively; for comparison the P
NMR signals for polymers 1a and 1a.PEG appear at  -5 and -10.4 ppm, respectively
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
39
(
see ESI file for full details); these complexation shifts are similar to previously reported
40
13
values. The C CP/MAS NMR spectra of 1a and 1a.PEG and 2a and 2a.PEG contain
a set of characteristic signals between  124-144 ppm associated with the imidazolium
ring and the aromatic carbon atoms as well as signals in the range  11-50 ppm which
correspond to the methyl groups attached to the imidazolium ring and the aliphatic carbon
atoms of the polystyrene backbone (see ESI file for full details). ATR-IR spectra of
catalysts 2a and 2b, treated in-situ with NaBH /ethanol solution to form AuNP, showed
4
no adsorption of CO due to weak adsorption of CO on metallic Au. Surface
characterization of the tetrachloroaurate-loaded precursors (2a, 2b, 2a.PEG and 2b.PEG)
and equivalent reduced catalysts (3a, 3b, 3a.PEG and 3b.PEG) was undertaken by X-ray
photoelectron spectroscopy (XPS) and a shift in the Au 4f and Au 4f doublets to lower
5/2
7/2
binding energies upon reduction was observed (see ESI file for details). For example, Au
4
f7/2 binding energies of 87.4 eV for 2b and 87.6 eV for 2b.PEG are entirely consistent
3+
41a,b
with the Au ion
whilst 83.8 eV for 3b and 83.8 eV for 3b.PEG are assigned to a lower
ACS Paragon Plus Environment
20

Page 21 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
0
41b,c
oxidation state of the Au species (Au ) (Figure 2a-b).
For example, the Au 4f7/2 binding
energies are similar to those found for AuNPs supported on IL-hybrid -Al O materials in
2
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
which the ionic liquid moiety forms a thin layer that acts as a cage for the AuNPs.41d The
presence of additional Au 4f doublets in the precursor spectra at higher binding energies
0
1
than the Au doublets of the chemically reduced catalysts are assigned as Au species
resulting from decomposition caused by exposure to the X-ray source during
acquisition,41e this is particularly evident in the spectra for 2a in which the sample has
undergone near complete reduction. This is verified by considering the N 1s region, which
along with the expected signal for the imidazolium environment in each sample, shows
additional signals at lower binding energies corresponding to uncharged, and in some
cases anionic, nitrogen species which are assigned to damage.41f There is no evidence
from the XPS of any Au---P interactions caused by electron donation from the phosphino
group, identified by solid state NMR spectroscopy as the binding energies for the Au 4f5/2
and Au 4f doublets for both 2a and 2a.PEG are very similar to those of 2b and 2b.PEG
7/2
(
Figure 2b and ESI); moreover, no shift in the binding energies of the P 2p3/2 and P 2p1/2
doublets was observed upon reduction of the gold salt (Figure 2c).
ACS Paragon Plus Environment
21

ACS Catalysis
Page 22 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2 XPS scan showing a) full scan of 2a.PEG; b) Au 4f core level of 2b, 2b.PEG and
3b and 3b.PEG; and (c) P 2p core level of 2a, 2a.PEG and 3a and 3a.PEG. All referenced
to the C 1s alkyl peak at 284.8 eV.
ACS Paragon Plus Environment
22

Page 23 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
TEM micrographs of in situ generated 3a and 3a.PEG revealed that the gold
nanoparticles were near monodisperse with average diameters of 3.4 ± 0.9 and 2.5 ± 0.6
nm, respectively, representative micrographs and associated distribution histograms
based on > 100 particles are shown in Figure 3 (see ESI file for TEM micrographs of 3b
and 3b.PEG). For comparison, AuNPs supported on IL-hybrid -Al O have slightly larger
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
3
mean diameters of 6.5-6.7 nm, however, these systems were prepared by sputtering
deposition using ultrapure gold foil whereas 3a and 3a.PEG were prepared by reduction
of a gold(III) salt.41d Interestingly, we have recently reported that palladium nanoparticles
stabilized by PPh -PIILP and PPh -PEGPIILP have vastly disparate sizes and those
2
2
stabilized by polymer immobilized ionic liquid containing PEG and PPh were also smaller
2
than those stabilized by PPh -modifed PIIL; thus, the number and type of heteroatom
2
donor may well influence nucleation and growth of NPs. The strong surface plasmon
resonance (SPR) adsorption for AuNPs is dependent on the size and shape as well as
the dielectric constant of the local medium which for this study will be associated with the
nature of the polymeric shell.38,42,43 The strong SPR bands at 519 nm in the UV-vis spectra
of AuNP@PPh -PIILP (3a) and AuNP@PPh -PEGPIILP (3a.PEG) and at 517 nm for
2
2
ACS Paragon Plus Environment
23

ACS Catalysis
Page 24 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
AuNP@PEG-PIILP (3b.PEG) are consistent with formation of spherical particles while
max for AuNP@PIILP (3b) is red shifted to 525 nm, which may well reflect the different
nature of the ligand-surface interactions stabilizing the NPs as the ionic microenvironment
in each is approximately the same but 3b is the only system that does not contain
heteroatom donor atoms. As a comparison, gold nanoparticles stabilized by neutral PEG-
modified heteroatom donor mono- bis- and tris-1,2,3-triazoles as well as nona-PEG-
branched triazole dendrimers with diameters close to 3 nm also exhibit SPR bands
around 515 nm.26d-f
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment
24

Page 25 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3 HRTEM images of (a-b) 3a and (d-e) 3a-PEG, with the observed atomic spacing
(parallel white lines) confirming the metallic nature of the AuNPs, and (c and f)
corresponding particle size distributions determined by counting >100 particles. Mean
nanoparticle diameters are of 3.4 ± 0.9 nm and 2.5 ± 0.6 nm for 3a and 3a-PEG,
respectively. Black and white scale bars are 25 and 1 nm, respectively.
Selective Reduction of Nitrobenzene to N-Phenylhydroxylamine
ACS Paragon Plus Environment
25

ACS Catalysis
Page 26 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The reduction of nitroarenes was initially targeted on the basis that there have been two
recent reports that nitrogen donor-modified platinum nanoparticles selectively catalyze
the highly selective partial hydrogenation of nitrobenzene to N-phenylhydroxylamine.11
As the high selectivity of these systems was attributed to electron donation from the amine
to the surface platinum atoms, we became interested in exploring and comparing the
efficacy of phosphine-decorated PIIL-stabilized AuNPs as catalysts for the reduction of
nitroarenes to assess the influence of the heteroatom donor on selectivity; moreover,
embedding the selectivity modifying heteroatom donor in to the support will eliminate the
need to separate or recover the additive which will simplify work-up and facilitate
recycling. To this end, we have recently demonstrated that PdNP@PIILP are highly
efficient catalysts for the complete reduction of nitroarenes to the corresponding aniline
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
in water under extremely mild conditions and the TOF of 17,125 h is the highest to be
reported for the aqueous phase transfer hydrogenation of a nitroarene catalyzed by a
PdNP-based system.24
ACS Paragon Plus Environment
26

Page 27 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
A series of catalytic reactions were initially conducted with nitrobenzene as the
benchmark substrate using a protocol recently developed for the PdNP-catalyzed
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
24
reduction of nitroarenes as a lead. A preliminary reaction conducted in water under air
for 40 min at 25 °C using a 0.005 mol% loading of AuNP@PPh -PIILP (3a), generated in
2
situ by reduction of tetrachloroaurate loaded 2a, with a substrate:NaBH ratio of 2.5 gave
4
35% conversion and 62% selectivity for N-phenylhydroxylamine (Table 1 entry 1). Under
these conditions, azoxybenzene was identified as the other major species (10%) together
with trace amounts of azobenzene (2%) and aniline (2%). An increase in the reaction time
to 6 h resulted in complete conversion affording a complex mixture containing
azoxybenzene (41%), N-phenylhydroxylamine (36%) and azobenzene (20%) as the
major components together with a minor amount of diphenylhydrazine (<1%) and aniline
(2%); this product distribution is consistent with reduction via the condensation pathway
(Table 1 entry 2). Speculating that reversible reduction of rapidly formed nitrosobenzene
to N-phenylhydroxylamine in air would facilitate access to the condensation pathway to
afford azoxy-based reduction products, the same reaction was conducted under a
nitrogen atmosphere. Remarkably, the same reaction conducted under a nitrogen
ACS Paragon Plus Environment
27

ACS Catalysis
Page 28 of 83
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
atmosphere for 40 min resulted in a dramatic improvement in selectivity from 62% to 97%
as well as an increase in conversion to 45%; moreover, high selectivity for N-
phenylhydroxylamine was retained when the reaction time was extended to 6 h (Table 1
entries 3 and 4), which is in stark contrast to the corresponding reaction conducted in air
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Table 1 entries 1 and 2). Thus, it appears that irreversible formation of N-
phenylhydroxylamine under nitrogen suppresses the condensation pathway as only trace
quantities of azoxybenzene (1%) and aniline (<1%) were detected by NMR spectroscopy
and GLC analyses. Indeed, we are confident that azoxybenzene only forms after
exposure of the sample to oxygen during work-up, as monitoring of the NMR sample
showed a significant increase in azoxybenzene with time at the expense of N-
phenylhydroxylamine; as such all subsequent studies were conducted with careful and
meticulous exclusion of air. A survey of the performance of 3a in selected solvents
revealed that the highest conversions and selectivities were obtained in water whereas
markedly lower yields and/or selectivities were obtained under the same conditions in
ethanol, a 1:1 mixture of ethanol and water, 2-methyltetrahydrofuran, a 1:1 mixture of
water and 2-methyltetrahydrofuran and toluene (Table 1 entries 3-9). Most interestingly,
ACS Paragon Plus Environment
28

Page 29 of 83
ACS Catalysis
1
2
3
4
5
6
7
8
9
this study revealed that the use of ethanol as solvent gave a complete switch in selectivity
to afford azoxybenzene as the sole product albeit with a conversion of only 19% (Table 1
entry 5); full details are discussed in the following section on selective reduction of
nitrobenzene to azoxybenzene and aniline. To this end, AuNPs supported on meso-CeO2
catalyze the sodium borohydride-mediated transfer hydrogenation of nitroarenes with
solvent-dependent switchable selectivity to afford azoxyarene as the sole product in 2-
propanol and the azoarene in a mixture of 2-propanol and water.27b
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Thus, water was identified as the solvent of choice based on the high conversion and
superior and unprecedented selectivity for N-phenylhydroxylamine as well as its
environmentally benign properties, practical advantages and the potential benefit of the
hydrophobic effect.
Table 1 Selective reduction of nitrobenzene to N-phenylhydroxylamine by NaBH4
as a function of catalyst, solvent and temperature.a
NO2
NHOH
catalyst
solvent, NaBH4
ACS Paragon Plus Environment
29