Iridium(iii) homo- and heterogeneous catalysed hydrogen borrowing C-N bond formation

Article abstract of DOI:10.1039/c7gc01007a

Pentamethylcyclopentadienyl iridium(iii) complexes of bidentate carbene-triazole ligands were found to be excellent homogeneous catalysts for the hydrogen borrowing mediated coupling of alcohols with amines. The reactions are highly efficient, able to reach completion in under 6 h at 100 °C at low catalyst loadings of 0.5 mol%, and are environmentally benign, the only by-product is water. The Ir(iii) catalysts promoted C-N bond formation across a range of alcohol and amine substrates, including biologically relevant motifs. Covalent attachment to a carbon black surface generated a well-defined "hybrid" heterogeneous catalyst which gave good conversion to products in the coupling reaction of aniline with benzyl alcohol, and could be recycled with no catalyst leeching. This represents the first report of a covalently linked heterogeneous iridium catalyst on carbon used for hydrogen borrowing. Turnover numbers (TON) for the heterogeneous were found to be significantly higher than the corresponding homogeneous reaction. To elucidate the homogeneous reaction mechanism, ¹H NMR studies inconjuction with deuteration experiments allowed a mechanism to be postulated.

Full text of DOI:10.1039/c7gc01007a

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. M. Wong, R. T.
McBurney, S. C. Binding, M. B. Peterson, V. R. Gonçales, J. J. Gooding and B. A. Messerle, Green Chem.,
2017, DOI: 10.1039/C7GC01007A.
This is an Accepted Manuscript, which has been through the
Volume 18 Number
7 7 April 2016 Pages 1821–2242
Royal Society of Chemistry peer review process and has been
accepted for publication.
Green
Chemistry
Accepted Manuscripts are published online shortly after
Cutting-edge research for a greener sustainable future
www.rsc.org/greenchem
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1463-9262
CRITICAL REVIEW
G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
rsc.li/green-chem

Please do not adjust margins
Green Chemistry
Page 1 of 10
View Article Online
DOI: 10.1039/C7GC01007A
Journal Name
ARTICLE
Iridium(III) Homo- and Heterogeneous Catalysed Hydrogen
Borrowing C-N Bond Formation
Chin M. Wong,a,b Roy T. McBurney,b Samantha C. Binding,b Matthew B. Peterson,a,b Vinicius R.
Gonçales,a J. Justin Goodinga and Barbara A. Messerle*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Pentamethylcyclodienyl iridium(III) complexes of bidentate carbene-triazole ligands were found to be excellent
homogeneous catalysts for the hydrogen borrowing mediated coupling of alcohols with amines. The reactions are highly
efficient, able to reach completion in under 6 h at 100 °C at low catalyst loadings of 0.5 mol%, and are environmentally
benign, the only by-product is water. The Ir(III) catalysts promoted C-N bond formation across a range of alcohol and
amine substrates, including biologically relevant motifs. Covalent attachment to a carbon black surface generated a well-
defined “hybrid” heterogeneous catalyst which gave good conversion to products in the coupling reaction of aniline with
benzyl alcohol, and could be recycled with no catalyst leeching. This represents the first report of a covalently linked
heterogeneous iridium catalyst on carbon used for hydrogen borrowing. Turnover numbers (TON) for the heterogeneous
were found to be significantly higher than the corresponding homogeneous reaction. To elucidate the homogeneous
reaction mechanism, 1H NMR studies inconjuction with deuteration experiments allowed a mechanism to be postulated.
www.rsc.org/
mismatched reactivity of alcohol and amine coupling partners.
The carbonyl group, typically an aldehyde, then undergoes
condensation with an amine to form an imine intermediate;
1. Introduction
Alkylated amines play an important role as intermediate
building blocks in many organic synthesis reactions and as final
products employed in pharmaceuticals, agrochemicals,
fragrances, dyes, and other fine chemicals.1-3 Numerous
multistep bench-top reactions are available to synthesise N-
substituted alkylamines, which include reactions of amines
with alkyl halides,4-6 electrophilic amination7-8 and reductive
alkylation routes.9-10 However, these reactions tend to suffer
from additional side reactions, tedious work-up, use of toxic
reagents, and the formation of stoichiometric amounts of
waste.11-12 The alternative synthetic pathway of reacting
alcohols with amines through "hydrogen borrowing" provides
a promising route to create a more sustainable carbon-
nitrogen bond formation reaction.1, 13-20 Inexpensive alcohols
are widely available and as water is the sole by-product of this
reaction, the ability to react amines with alcohols makes it an
atom-economical and environmentally friendly synthetic
method.21
this step yields water as the sole by-product. Hydrogenation of
the imine intermediate by the metal hydride complex ([MH] or
[MH2]) generates the N-alkylamine product and regenerates
the catalyst.
Scheme
1 General mechanistic pathway to react amines with alcohols using the
hydrogen borrowing methodology.
Investigations into homogeneous catalysed C-N bond
transformations using hydrogen borrowing began as early as
1981 with Grigg et al.22 and Watanabe et al.,23 who used
simple coordination complexes of Ru, Ir, and Rh as catalysts.
Since then, a wide range of early and late transition metals
have been explored as catalysts for these reactions including
Hydrogen borrowing facilitates C-N bond formation by
using a catalyst to abstract or “borrow” hydrogen from the
alcohol substrate, thereby oxidising the alcohol to form a more
reactive carbonyl group, see Scheme 1.17 This circumvents the
ruthenium,24-30 iridium,31-40 gold,41 iron42 and cobalt.43-44
A
recent Ru(II) example using an alanine-triazole ligand
overcame a competing transfer hydrogen reaction in order to
efficiently couple ketones with alcohols using hydrogen
borrowing.45 A bisbenzoxazolyl Ir(III) performed exceptionally
well across a range of couplings involving amines, primary and
secondary alcohols, and ketones.46 A related bisbenzothienyl
Ir(III) catalyst was also able to catalyse the cross-coupling of
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 2 of 10
View Article Online
DOI: 10.1039/C7GC01007A
ARTICLE
Journal Name
alcohol,47 and primary alcohols with ketones.48 Despite these containing a Cp* (1,2,3,4,5-pentamethylcyclopentadiene) co-
reports, more active catalysts which work at low catalyst ligand and the weakly coordinating anion BArF
4
loadings (<0.5 mol%), lower temperatures (≤70 °C) and have (B(3,5-(CF3)2C6H3)4) by following
a
modified literature
shorter reaction times (<12 h) are needed.
procedure.63 The imidazolium salts 1a-f were complexed to
Heterogeneous catalysis in the field of hydrogen borrowing Ir(III) using [IrCp*Cl2]2 via a silver(I) oxide transmetallation
reactions has also been explored for the reaction of alcohols reaction in acetone. Counter-ion exchange of the crude metal
with amines. Heterogeneous systems have a major advantage complex with NaBArF in dichloromethane yielded the
4
over homogeneous catalysts in that they can simplify the complexes as bright yellow (1a
separation, purification, and recycling following catalytic red-brown (1b) or light brown (1f) crystalline solids. The
processes and hence reduce operational consumption costs. colours of the complexes were clearly result of the
, 1c and 1e), yellow-brown (1d),
a
The majority of heterogeneous catalysts investigated so far for substituents on the aryl group of the ligands, demonstrating
hydrogen borrowing involves metal nanoparticles, oxides or potential for the substituents to tune catalysis reactivity.
hydroxides adsorbed or impregnated on inorganic materials
49-50
All the ligands and Ir(III) complexes were stable in both the
solid state and in solution and were fully characterised by NMR
such as metal oxides, zeolites, silica or alumina.14,
However, key disadvantages of these types of catalysts can spectroscopy, mass spectrometry and elemental analysis, see
1
include metal leaching and uncertain stability and recyclability. ESI. The H NMR spectra of all the complexes showed that the
Comparatively, there have only been a few reports where methylene protons of the CH2 linking the imidazolium and the
isolated complexes immobilised on surfaces have been used as triazole donors as two AB doublets at significantly different
2
a heterogeneous catalyst for catalysed C-N bond formation chemical shifts, with a J of ~16 Hz, illustrating coordination to
reactions via hydrogen borrowing.51-54 A key benefit of using the metal centre, similar to previously reported complexes.65
isolated complexes immobilised on surfaces, so-called “hybrid This difference in chemical shift is due to the diastereotopic
catalysts”, is that the active catalyst sites are well-defined, nature of the protons caused by the chiral coordination around
with easy catalyst optimisation (through ligand modification the metal centre, such that one of the protons is oriented
prior to surface immobilisation), and by being attached to a towards the Cp* ring and hence subjected to a strong ring
surface the catalyst is easily recycled.
current effect, while the other proton is oriented away from
Herein, we report how Ir(III) complexes of easily the Cp* ring. In addition, the observation of pairs of
synthesised bidentate ligands, featuring carbene and triazole resonances attributed to inequivalent ortho-CH3 and meta-CH
1
donors, function as catalysts for hydrogen borrowing. protons of the mesityl ring in the H NMR spectra, as well as
Separately, carbene55-57 and triazole58-60 ligands have proved related exchange cross peaks observed in the 1H-1H NOESY
highly effective as catalysts for
transformations. The combination of carbene and trizaole between the mesityl and carbene group on the NMR time
donors represents great basis from which to tune scale. This hindered rotation was observed for all complexes
a range of organic spectra, indicate restricted rotation around the N-C bond
a
reactivity.61-63 The Ir(III) complexes reported here are 2a
exceptionally efficient catalysts for C-N bond formation
between alcohols and amines, tolerating a wide substrate
scope. In addition, we covalently attached an Ir(III) complex to
a carbon surface and were able to affect the same catalysis
reaction using the new hybrid catalyst with excellent
recyclability. To elucidate the mechanism, the homogeneous
reaction was followed by NMR spectroscopy and deuterium
labelling studies, allowing us to postulate a mechanism.
-f.
2. Results and Discussion
2.1
Homogeneous Catalyst Synthesis
All bidentate carbene-triazole ligands 1a
-f were synthesised Scheme 2 Synthesis of Ir(III) complexes and attachment of carbon surfaces.
using a standard Cu(I) catalysed alkyne-azide cycloaddition
reaction (CuAAC) between 1-propargyl-3-mesitylimidazolium
bromide and a corresponding azide (Scheme 2).64 The CuAAC
reaction is ideal as it installs a second coordination (triazole)
site and provides diversification allowing a series of carbene-
triazole ligands containing various electron-withdrawing or
electron-donating groups located on the aryl ring to be
synthesised.
Single crystals of Ir(III) complexes 2a-f suitable for single
crystal X-ray diffraction analysis were grown by slow diffusion
of hexane into a concentrated dichloromethane solution for
complexes 2a
,
2c and 2f or by slow diffusion of hexane or
2d
cyclohexane into a concentrated toluene solution of 2b
,
and 2e. The solid-state structure of complex 2d is shown in
Figure 1. Structures of all other Ir(III) complexes and a
summary of selected bond lengths and angles of complexes
2a-f are given in the ESI. The solid-state structures of Ir(III)
The bidentate carbene-triazole ligands 1a-f were
subsequently used to synthesise Ir(III) complexes 2a-f
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 3 of 10
View Article Online
DOI: 10.1039/C7GC01007A
Journal Name
ARTICLE
complexes all exhibit
a
piano-stool type octahedral representative of the nitrogen atoms on the ligand
conformation around the metal centre. In all cases, the 6- coordinated to the Ir metal centre appears at ca. 401-402 eV in
membered metallacycle formed upon coordination of the both hybrid catalysts 2-CB and 2-GC. In the XPS spectra of both
bidentate carbene-triazolyl ligands to the metal centre adopts hybrid catalysts, a minor peak was observed within the N1s
a distorted boat conformation. The four torsion angles centred peak at about 399–400 eV, this peak is indicative of an azo
on bend in the boat range from 42° to 46° and average to ~43° bond (N=N). This is likely to originate from incomplete
in Ir(III) complex 2d. The C-Ir-N bite angles are slightly smaller diazonium reaction of the aniline moiety of complex 2f to the
than those reported for analogous Ir(III) complexes containing carbon surface; this has been observed in many previous
bidentate carbene-triazole ligands,65 indicating that upon reports.67 The binding energy of Ir4f representing the Ir(III)
coordination to Ir(III), the bidentate carbene-triazole ligand 1 is metal centre on the carbon surfaces appears at ca. 62-63 eV,
forced into a highly strained conformation. The average Ir-C* in agreement with the binding energy of other reported Ir(III)
distances (C* = centroid of the Cp* ring) and the average Ir- metal complexes.68-69 From the XPS data, the elemental ratio
N(triazole) bond lengths are similar to those reported for of nitrogens to iridiums on the surface was determined to be
analogous Ir(III) complexes with bidentate carbene-triazole65 close to 5:1, which is consistent with the expected molecular
and pyrazole-triazole ligands.66 Although the Ir(III) metal structure, indicating that the Ir(III) complexes were
centres of the various complexes were chelated with bidentate immobilised onto the glassy carbon or carbon black surfaces as
carbene-triazole ligands containing different substituents on intact complexes.
the aryl ring, the selected bond lengths connected to the metal
centre (Ir-C*(centroid of Cp*), Ir-C(carbene), Ir-N(triazole), Ir-
Cl) were generally similar, see ESI Table S6 and Figures S8-S13.
This is unsurprising as the aryl substituents are too remote to
influence the conformation around the metal centre.
Figure 1 X-ray crystal structures of Ir(III) complex 2d featuring the p-CF3
substituent, as viewed from (a) above the Ir(III) 6-membered metallocycle and
(b) face-on to the Ir(III) metallocycle. Ellipsoids are shown at the 40% probability
level, hydrogens and BArF counterions have been omitted for clarity. Selected
bond lengths (Å) and angl4es (
°
): C1-Ir 2.058(7), N1-Ir 2.065(5), C*-Ir 1.824, Cl-Ir
2.404(2), C1-Ir-N1 82.9(2). CCDC 1529842.
2.2
Heterogeneous Catalyst Synthesis
The Ir(III) hybrid catalyst containing the bidentate carbene-
triazole ligand was synthesised utilising a direct covalent
attachment of the complex to a carbon surface.63 Complex 2f
containing the para-amino substituent on the aryl ring was
dissolved in a hydrochloric acid solution of nitromethane and
sodium nitrite. Glassy carbon was allowed to stand in this
solution for 24 h, whereas carbon black was stirred into the
solution for the same amount of time. Subsequently, the
glassy carbon was washed thoroughly and dried under a gentle
stream of nitrogen to yield 2-GC. The carbon black reaction
was centrifuged, washed repeatedly with methanol and water,
and then the carbon powder was dried under vacuum
overnight to give 2-CB as a black powder.
Figure 2 a) XPS survey scan of 2-CB with selected narrow scans of N1s and Ir4f
(dashed boxes). b) TGA analysis of 2-CB and unmodified carbon black.
Thermogravimetric analysis was also performed on 2-CB to
analyse the stability of the hybrid complex on carbon black.
From the thermogram of 2-CB, there is one distinct weight loss
process from 100-542 °C (Figure 2b). This process accounts for
26.6% loss in weight due to the whole iridium complex from
the carbon black surface. From this weight loss, the iridium
content on the surface was calculated to be 7.2 wt.%. After
542 °C, the loss in weight of the sample is due to
decomposition of the carbon black material. By ICP-MS, the
quantitative amount of iridium on the carbon black surface in
complex 2-CB was determined to be 0.84 wt.%. The difference
in calculated weight of Ir(III) as determined by TGA and ICP-MS
is likely due to the fact that the total weight loss as observed
by TGA can also be attributed to decomposition of impurities
such as oxidised functional groups on the carbon surfaces.
The Ir(III) hybrid catalysts were characterised using X-ray
photoelectron spectroscopy (XPS), thermogravimetric analysis
(TGA) and inductively coupled plasma mass spectrometry (ICP-
MS). XPS can be used to analyse the elemental composition of
the surfaces, therefore allowing a determination on whether
the chemical structure of the complex is preserved when
immobilised on the surface. The XPS spectra of 2-CB is shown
in Figure 2a and ESI Figure S1; selected binding energies
related to N1s and Ir4f peaks and the atomic ratio of N:Ir are
summarised in the ESI Table S1. The binding energy of N1s
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 4 of 10
View Article Online
DOI: 10.1039/C7GC01007A
ARTICLE
Journal Name
2.3
Homogeneous H2 Borrowing Catalysis for C-N Bond
3
and
4
to 5a was seen to be: p-CF3 (2d) >> m-CF3 (2e) > p-NO2
2c) > p-NH2 2f). Overall, Ir(III)
Formation
(
2b) ~ p-H (2a) > p-CH3
(
(
complexes from ligands containing the more electron-
withdrawing groups (CF3 or NO2) led to better conversions for
the C-N bond formation than those with ligands containing
more electron-donating groups (CH3 or NH2) on the aryl ring.
As a control experiment, the catalytic activity of [IrCp*Cl2]2,
The reaction of benzyl alcohol (3, 1.1 equiv.) and aniline (4, 1.0
equiv.) in air using the unsubstituted complex 2a (0.5 mol%) as
a catalyst, was selected as a model reaction for optimisation
studies. Initially, a solvent screen (toluene-d8, dioxane-d8,
1,1,2,2-tetrachloroethane-d2 and dimethylsulfoxide-d6 at 100
ligand 1d, NaBArF and KOtBu was determined for the reaction
4
°C and tetrahydrofuran-d8 at 60 °C) was conducted using KOH
of
catalyst gave only 50% conversion to 5a, compared against
96% conversion using catalyst 2d. In the absence of ligand 1d
[IrCp*Cl2]2, NaBArF and KOtBu at 100
C gave 70% conversion
to 5a after 15 h. This result is in alignment with the
results reported by Fujita, whereby [IrCp*Cl2]2 in the presence
of NaHCO3 instead of KOtBu gave 44% conversion to 5a 34
There are few examples of hydrogen borrowing at
temperatures lower than 100 C. Complexes 2a and 2d were
found to still catalyse this same reaction at lower
C for 24 hours. Whilst longer than our
C, these reaction times are not longer
3 and 4 after 6 h at 100 °C. Pleasingly, this in situ formed
as the base. From this, toluene-d8 was the found to be the best
solvent for the reaction giving the highest conversion of 60%
after 7 h. In order to improve the conversion of the model
reaction further, the reaction was tested in toluene-d8 using a
series of different bases (KOH, K2CO3, NaHCO3, CsCO3, KOtBu,
DABCO, Et3N, DIPEA). The use of KOtBu as the base provided
increased conversion to product 5a, reaching 80% in 6 h at 100
,
°
4
of
3 and 4
.
°C. No reaction was observed when using K2CO3, NaHCO3 or
°
Cs2CO3. The use of organic bases such as DABCO, Et3N or DIPEA
provided less than 10% conversion to the desired product
a
temperature of 60
experiments at 100
°
°
N-benzylaniline
(5a). Certain catalysts reported in the
literature require silver salt additives in order to function
effectively.41, 46 We screened various silver salt additives, such
as AgOTf, AgOAc and AgBF4, however they all had negligible
effect on the percent conversion observed for this reaction.
Clearly our catalysts reported here do not require addition of
silver salts to function effectively, and thus represent a
greener catalysis system compared to other reported
hydrogen borrowing catalysts. The reactions did not require
the addition of molecular sieves to scavenge water.
than most literature reports. Complex 2d gave a near identical
conversion to 5a of 96% after 24 h. Whilst complex 2a
achieved 63% conversion to 5a after 24 h. These results
further demonstrate that complex 2d is a highly active catalyst
for the C-N bond formation between benzyl alcohol and
aniline, therefore it was used as the catalyst for subsequent
substrate scope investigations.
To determine the substrate scope of this Ir(III) catalysed
hydrogen borrowing reaction we separately varied both the
alcohol and amine substrates, see Scheme 3. First, the alcohol
partner was varied in the reaction with aniline (
resulted in over 80% conversion to products 5b-5h after 6h at
100 C. Methyl, methoxy, and bromide substituted benzyl rings
4). All reactions
°
were tolerated as were alkyl and cyclic alcohols. Secondly, the
amine coupling partner was varied in the reaction with benzyl
alcohol (
3). Most reactions resulted in over 90% conversion
after 6 h at 100 °C. Methyl and halogen substituents were well
tolerated as were pyridyl and napthyl rings. The biologically
relevant motifs of dibenzylaniline 5r and piperonyl aniline 5s
gave excellent conversions. N-alkyated products 5c
and 5s could also be obtained at the lower reaction
temperature of 60 C after 24 h; all gave near identical
, 5e, 5i, 5l
°
conversions to those reactions performed at 100
°C (yields in
Figure 3 Catalyst screen for C-N bond formation between benzyl alcohol and
aniline via hydrogen borrowing. Reaction conditions:
3
1.1 equiv.,
4 1.0 equiv.,
square brackets, Scheme 3).
Ir(III) catalyst 2 °C, 6 h.
(0.5 mol%), KOtBu 1.1 equiv., toluene-d8, 100
To further our understanding of the efficiency of the Ir(III)
catalytic reaction, benzyl alcohol ( ) was reacted with different
3
Using the optimised reaction conditions of 0.5 mol%
catalyst loading, KOtBu as a base, toluene-d8 solvent, reaction
aniline substrates functionalised at the para-position in the
presence of 0.5 mol% 2d and KOtBu in toluene-d8 at 100 °C and
the progress of each reaction was monitored at 0.5-1 hour
intervals by 1H NMR spectroscopy. All of the catalytic reactions
showed unprecedented efficiency, reaching at least 80%
conversions in less than 6 hours, see ESI Figure S2 and Table
time of 6 h at 100
were assessed as catalysts for the reaction of benzyl alcohol
) and aniline ( ), see Figure 3. As expected the catalytic
efficiency of each complexes varied depending on the
electronic environment around the metal centre. Complex 2d
°C, the full series of Ir(III) complexes 2a-f
(
3
4
,
S2. The hydrogen borrowing C-N coupling of
2 with
containing an inductively electron-withdrawing p-CF3 on the
p-bromoaniline was the fastest reaction, reaching 96%
aryl ring of the ligand, catalysed the reaction with the highest
conversion in only one hour. Whereas, the reaction of benzyl
conversion of 96% in 6 hours at 100 °C. The observed trend in
catalytic efficiency of the Ir(III) complexes for the conversion of
alcohol 3 with unsubstituted aniline 4 or p-toluidine required 6
h to achieve a good conversion. Thus, a clear improvement in
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 5 of 10
Journal Name
View Article Online
DOI: 10.1039/C7GC01007A
ARTICLE
reaction efficiency was observed when the functionalised cycle gave close to 42% conversion, with only a slight decrease
aniline substrate contained an electron-withdrawing group after the third cycle (37%), see Figure 4a. Despite not reaching
compared to an electron-donating group.
complete conversion after 24 h, the Ir(III) heterogeneous
In comparison to other reported Ir(III) catalysts used for catalyst performs well with high turnover numbers. The
hydrogen borrowing, our catalyst achieves similar conversions excellent recyclability was determined after each recycling
at half the catalyst loading in at least half the time, without step where the filtrate and washings were collected and
addition of silver salts. Our homogeneous catalyst also can analysed by ICP-MS to determine what, if any, loss of iridium
affect complete conversions at 60 °C, in comparison to 100 °C from the carbon surface occurred. It was found that only
which is often reported, confirming that the Ir(III) catalyst 2d 0.33% of total iridium content was lost after the three catalysis
offers a significant improvement over existing Ir(III) catalysts. runs, Figure 4b and Table S3. This is significantly better than
Despite the higher activities resulting in shorter reaction times our findings with a related heterogeneous Rh catalyst which
and reduced reaction temperatures for reactions involving after an initial 12% loss of physisorbed rhodium gave a 3%
primary alcohols, the catalysts reported here could not total loss of rhodium after the second and third cycles.63
catalyse the hydrogen borrowing reaction between the
secondary alcohol, 1-phenly ethanol, and aniline.
There are limited examples of heterogeneous catalysts
used for hydrogen borrowing for the specific reaction between
aniline and benzyl alcohol. A related Ir(III) complex on a silica
support gave higher conversion but had to use a much higher
catalyst loading, hence much lower TONs were obtained.53
A
Pd(II) catalyst on silica was able to produce a higher turnover
as a result of a lower catalyst loading, though this required a
longer reaction time.51
Figure
reaction of benzyl alcohol
conversion after each catalysis cycle and (b) the % iridium remaining on the
4
Heterogeneous recycling experiments using catalyst 2-CB for the
3
and aniline at 100 C for 24 h showing (a) %
4
°
carbon black surface after each cycle. Reagents and conditions:
3 1.1 equiv., 4
1.0 equiv., Ir(III) catalyst 2-CB 0.01 mol%, KOtBu 1.1 equiv., toluene-d8, 100
°
C, 24
h.
3. Mechanistic Investigations
Although a precise mechanism for the C-N bond formation
reaction of alcohols with amines via hydrogen borrowing using
iridium catalysts has not yet been clarified, several theoretical
studies using density functional theory (DFT) calculations have
been reported recently.37, 67, 70-71 From these DFT studies, the
catalysed reaction of alcohols with amines has been
demonstrated to be composed of three main multistep
reactions: (1) metal catalysed oxidation of the alcohol, (2)
nucleophilic addition of the amine to the aldehyde, and (3)
metal catalysed reduction of the imine to the amine product.
We conducted NMR spectroscopy based investigations in an
attempt to probe the reaction mechanism. The results
obtained allowed us to postulate a plausible mechanism.
Scheme 3. Substrate scope for the homogeneous Ir(III) catalysed hydrogen
borrowing catalysed coupling of amines and alcohols. %Conversions were
determined by 1H NMR spectroscopy. Isolated yields are given in parentheses ().
%Conversions after 24 h at 60 °C are given in square brackets [].
2.4 Heterogeneous H2 Borrowing Catalysis for C-N Bond Formation
Ir(III) heterogeneous catalyst 2-CB was tested as a catalyst
3.1
NMR Hydride Investigations
for the reaction of benzyl alcohol
3 with aniline 4. At a catalyst
loading of 0.01 mol%, 2-CB gave 42% conversion to benzyl
aniline after 24 h at 100 °C which equates to a turnover
number (TON) of 4362. We recycled the heterogeneous
catalyst over three reaction cycles (24 h at 100 °C), after each
cycle the conversion was measured. It was found that each
The first key step in the hydrogen borrowing reaction is the
oxidation of the alcohol substrate to an aldehyde. This is
thought to proceed via -hydride elimination from the alcohol
β
upon coordination to the metal centre with the metal centre
capturing the hydride. Therefore, we examined what
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 6 of 10
View Article Online
DOI: 10.1039/C7GC01007A
ARTICLE
Journal Name
substrates were necessary to generate an Ir(III) hydride
From the hydride experiments, Ir(III) hydride complex II
complex and if a base was required. Iridium complex 2d was was proven to be an active catalytic intermediate, with the
reacted with ca. 5 equivalents of benzyl alcohol ( ) or aniline hydride most likely originating from the alcohol substrate. To
) in toluene-d8 in separate NMR tubes and monitored by determine that the hydride on complex II does not originate
NMR spectroscopy. In both cases, no reaction was observed. from the amine substrate, complex 2d was used to catalyse
Subsequently, ca. 5 equivalents of KOtBu was added to both the reaction of benzyl alcohol (
) with deuterated aniline
NMR tubes. No reaction was observed for the reaction mixture substrates 4-d5 or 4-d7 in non-deuterated toluene for 6 hours
containing aniline (
). Whereas, upon addition of KOtBu to the at 100 °C (Scheme 4a). The products were then isolated and
NMR tube containing Ir(III) complex 2d and benzyl alcohol (
), analysed by 1H NMR in toluene-d8, and 2H NMR in toluene
5 was formed;
3
(
4
3
4
3
a hydride species (broad resonance, δ -13.3 ppm) was which showed that only deuterated product 5a-d
observed by 1H NMR spectroscopy. A 1H-13C HMBC experiment no deuteration of the NH or CH2 moieties was observed.
revealed a correlation between the hydride resonance and a
carbon signal at δ 155.3 ppm which was attributed to the
carbene carbon coordinated to the iridium metal centre (see
Scheme 5 inset). This confirms that a hydride was bound to the
Ir metal centre. Additionally, a sharp singlet resonance appears
over time at δ 9.52 ppm, indicative of an aldehyde, possibly
coordinated to the Ir(III) centre, complex I. Whilst the initial
Ir(III) hydride complex forms quickly, the catalysed conversion
of benzyl alcohol to benzaldehyde proceeds slowly.
To further confirm that an Ir(III) hydride complex was part
of the catalytic cycle, an iridium hydride complex II was
prepared and tested as a catalyst for this reaction. Complex 2d
was reacted with sodium borohydride in methanol and Ir(III)
hydride complex II was isolated as a pale yellow solid.
Complex II was characterised by 1D and 2D NMR and mass
spectrometry, see ESI. A 1H-13C HMBC NMR experiment
revealed a correlation between the hydride resonance at δ -
13.3 ppm and a 13C resonance at δ 155.1 ppm, similar to that
observed previously in the HMBC NMR experiment containing
iridium complex 2d, benzyl alcohol (3
) and KOtBu in toluene-d8.
The 13C resonance at δ 155.1 ppm was identified as the
carbene carbon resonance coordinated to the metal centre.
Complex II was also used as a catalyst in place of complex 2d
Scheme 4 Deuteration NMR studies of the C-N bond formation reaction of benzyl
alcohol ( ) with deuterated aniline substrates ( ) catalysed by complex 2d
3
4
.
Complex 2d was next used as catalyst for the reaction of
deuterated benzyl alcohols 3-Cd2 and 3-Od1 with aniline ( ) in
toluene for 6 hours at 100 °C (Scheme 4b). The products were
in the reaction of benzyl alcohol (3) and aniline (4) under the
4
optimised conditions identified previously. This reaction
reached a conversion to N-benzylaniline (5a) of 77%, which
was lower compared to the 96% conversion observed when
using 2d as catalyst. Despite the lower conversion, this result
proves that Ir(III) hydride complex II is an intermediate during
this reaction and that Cl- is required to achieve complete
conversion, possibly by sequestering K+ from the base.
1
2
isolated and analysed by H NMR in toluene-d8 and H NMR in
toluene. The reaction of benzyl alcohol-d2 (3-Cd2) and aniline
(
4
) gave a 1:1 mixture of N-benzyl aniline (5a), and the
deuterated N-benzylaniline product (5a Cd1) containing only
one deuterium on the benzyl position. On the other hand, the
reaction between benzyl alcohol-OD (3-Od1) and aniline (
gave 5a as the major product (>90%) with a minor deuterated
-
4
)
Iridium hydride complex II was proposed to activate the
imine intermediate leading to hydride transfer to the imine.
Diphenylmethanimine, was synthesised then reacted in an
NMR tube with hydride complex II in toluene-d8. 1H NMR
analysis of this reaction did not present any evidence of the
N-benzylaniline product (5a-Cd1) (for the relevant spectra, see
ESI Figures S3 – S4).
For further confirmation that the hydride bound to the
iridium metal in II derives from the benzylic CH2 on the
alcohol, complex 2d was reacted in a series of NMR tube
experiments with ca. 5 equivalents of deuterated benzyl
alcohols 3-Cd2 or 3-Od1 and KOtBu in toluene or toluene-d8
activation of the C=N double bond by the hydride complex II
.
Subsequently, addition of benzyl alcohol (3) to the reaction
mixture containing complex II and diphenylmethanimine,
showed no reaction. Upon addition of approximately
stoichiometric amount of KOtBu to the same reaction mixture
and heating at 100 °C, the N-benzylaniline 5a was observed.
These results imply that hydride transfer from iridium hydride
complex II to the imine is a slow process that requires base
and alcohol in order to progress C-N bond formation.
1
(Scheme 4d, e). The H NMR spectrum of benzyl alcohol-d2 (3-
Cd2) and complex 2d displayed no hydride or aldehyde
resonances (see SI, Figure S5). The 2D NMR experiments for
this reaction in non-deuterated toluene showed that the
deuterated hydrogen resonances appear at δ 9.52 ppm,
corresponding to the aldehyde proton of benzaldehyde. Whilst
3.2
NMR Deuteration Investigations
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 7 of 10
Journal Name
View Article Online
DOI: 10.1039/C7GC01007A
ARTICLE
hydride complex II was expected to be formed during this alkylated amine product in step H. The catalyst is then
NMR experiment, no deuterated hydride resonance was regenerated and the catalytic cycle continues. From our NMR
observed either in the 2H NMR spectrum, likely because the studies, we postulated that steps F and G are reversible and
resonance was too broad. The 1H NMR spectrum from the highly favour the reverse reaction. It is only step H that is
reaction of benzyl alcohol-OD (3-OD) and Ir(III) complex 2d in irreversible, hence driving the overall cycle toward N-alkylated
toluene-d8 with KOtBu (Scheme 5, inset) showed immediate product.
formation of a hydride resonance at δ -13.3 ppm as well as the
distinctive aldehyde proton resonance at
δ 9.57 ppm
attributed to benzaldehyde. In the 2D NMR spectra, no
deuterated resonances were observed either in the hydride or
aldehyde regions.
The combination of all the deuteration NMR studies
provides clear experimental evidence that the hydride on
iridium complex II must come from one of the benzylic CH2
protons during the oxidation of benzyl alcohol
(3) to
benzaldehyde and not from the OH of the benzyl alcohol or
from the amine substrate. These results also demonstrate the
lack of the reaction between alcohol and alcohol, or alcohol
and aldehyde under a catalysed hydrogen borrowing reaction
in the absence of amine substrate.
3.3
Base Requirement
We attempted to use catalytic quantities of KOtBu (0.5
mol% or 10 mol%) in these reactions. However, this resulted in
very poor yields of N-alkylated product. Using no base, the
reaction did not proceed at all. It appears that a stoichiometric
quantity of base is required to deprotonate all alcohol
substrate present in order to drive the reaction forward. This
finding is reinforced from our other experimental findings that
most steps do not proceed without base present. This is
potentially due to our catalyst only being able to
accommodate one hydride. Literature reports usually describe
metal catalysts with the ability to coordinate two hydrides or a
metal complex that can deprotonate the alcohol substrate.25, 39
Scheme 5
Proposed mechanism of Ir(III) catalysed C-N bond formation
reaction of alcohols with amines via hydrogen borrowing. Inset, hydride-carbene
coupling observed in 1H-13C HMBC NMR.
Considering the lack of reactivity observed with secondary
alcohols, one possible explanation is the inability of the
catalyst to dehydrogenate the secondary alcohol to the
corresponding ketone. This is could be due to the sterics
around the active catalytic site which is contained within a
3.4
Proposed Mechanism
Based on our NMR studies and DFT studies reported in the pocket defined by a bulky carbene-triazole ligand in addition to
literature,37, 67, 70-71 a mechanism was proposed using our Ir(III) the large Cp* co-ligand.
complex 2d to catalyse C-N bond formation between alcohols
and amines via hydrogen borrowing (Scheme 5). In step A of
4. Conclusions
the proposed mechanism, the Ir(III) complex exchanges a
chloride for an alkoxide to produce an iridium alkoxide
Six new Ir(III) complexes 2a-f were synthesised from a set
of ligands 1a-f based on a bidentate carbene-triazole motif.
From this series of Ir(III) complexes, complex 2d, featuring a p-
CF3 substituent on the ligand, emerged as the most efficient
catalyst for C-N bond formation reactions of alcohols with
amines using hydrogen borrowing. Ir(III) complex 2d was found
to be an excellent catalyst, catalysing reactions of a range of
alcohol and amine substrates, and tolerating a range of
functional group substituents on those substrates, including
biologically relevant molecules. Catalyst 2d is selective for
primary alcohols over secondary alcohols which did not react
under these conditions. Reactions proceeded at a low catalyst
loading of 0.5 mol% and reached completion between 1-6
hours at 100 °C, showing unprecedented efficiency to date
compared to other similar Ir(III) complexes. Furthermore, Ir(III)
catalyst 2d was just as efficient at catalysing the couplings of
complex. Subsequent
from the benzylic CH2 of the alkoxide in steps B and C yields an
aldehyde and a hydride bound to the Ir(III) metal centre
β-hydride elimination of a hydrogen
I.
These steps are facilitated by partial de-coordination of the
bidentate ligand, most likely via dissociation of the triazole
donor from the metal centre. In the presence of an amine
substrate, the release of the aldehyde in step D allows imine
formation away from the metal centre in step E, and also
releases a water molecule as the only by-product of this
reaction. Imine intermediate III enters the catalytic cycle to
form a complex with Ir(III) hydride complex II at step F, aided
by partial decoordination of the bidentate ligand. The proximal
orientation of the activated imine and hydride promotes the
hydride transfer from the Ir metal centre to the imine in step
G. An alkoxide then exchanges with the deprotonated amine
t
which then picks up a proton from BuOH to generate the N-
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 8 of 10
View Article Online
DOI: 10.1039/C7GC01007A
ARTICLE
Journal Name
5
6
7
8
S. Shekhar, P. Ryberg, J. F. Hartwig, J. S. Mathew, D. G.
Blackmond, E. R. Strieter and S. L. Buchwald, J. Am. Chem.
Soc., 2006, 128, 3584.
J. F. Hartwig, in Palladium-Catalyzed Amination of Aryl
Halides and Sulfonates, ed. Wiley-VCH Verlag GmbH,
Germany, 2000, pp. 195-262.
E. Fernandez and J. M. Brown, in Electrophilic Amination
Routes from Alkenes, ed. A. Ricci, Wiley-VCH Verlag
GmbH, Weinheim, 2007, pp. 37-63.
J.-P. Genet, C. Greck and D. Lavergne, in Stereoselective
Electrophilic Amination with Sulfonyloxycarbamates and
Azodicarboxylates, ed. A. Ricci, Wiley-VCH Verlag GmbH,
Weinheim, 2007, pp. 65-102.
alcohols with amines at a lower temperature (60 °C) in 24
hours.
Heterogeneous Ir(III) carbene-triazole hybrid complexes
covalently attached to carbon black and glassy carbon were
synthesised and fully characterised by XPS, TGA and ICP-MS.
For C-N bond formation between benzyl alcohol and aniline
using hydrogen borrowing, the heterogeneous Ir(III) hybrid
catalyst 2-CB gave a moderate conversion to benzyl alcohol of
42% with a turnover number of 4363. Whilst the conversions
were slightly lower compared to the homogenous catalysis,
the turnover number achieved using the hybrid catalyst was
significantly higher than that of the homogeneous catalysed
reactions, which suggests that the hybrid complex was a more
efficient catalyst. The excellent recyclability was confirmed by
ICP-MS which showed that the total loss of iridium over three
cycles was 0.33%, this is an order of magnitude better than our
previously reported heterogeneous Rh catalyst.63
9
10
G. W. Gribble, Chem. Soc. Rev., 1998, 27, 395.
C. A. Olsen, H. Franzyk and J. W. Jaroszewski, Synthesis,
2005, 2005, 2631.
11
12
13
14
A. Nandakumar, S. P. Midya, V. G. Landge and E.
Balaraman, Angew. Chem. Int. Ed., 2015, 54, 11022.
A. F. Abdel-Magid, K. G. Carson, B. D. Harris, C. A.
Maryanoff and R. D. Shah, J. Org. Chem., 1996, 61, 3849.
S. Bähn, S. Imm, L. Neubert, M. Zhang, H. Neumann and
M. Beller, ChemCatChem, 2011, 3, 1853.
Q. Yang, Q. Wang and Z. Yu, Chem. Soc. Rev., 2015, 44,
2305.
The mechanism of the C-N bond formation using hydrogen
borrowing with catalyst 2d was studied using deuterium
labelling and NMR spectroscopy. To corroborate our proposed
mechanism an intermediate hydride Ir(III) complex was
observed and the source of the hydride was clearly identified
15
16
R. H. Crabtree, Organometallics, 2011, 30, 17.
T. D. Nixon, M. K. Whittlesey and J. M. J. Williams, Dalton
Trans., 2009, 2009, 753.
A. J. A. Watson and J. M. J. Williams, Science, 2010, 329,
635.
G. Guillena, D. J. Ramón and M. Yus, Chem. Rev., 2010,
110, 1611.
F. Huang, Z. Liu and Z. Yu, Angew. Chem. Int. Ed., 2016, 55,
862.
G. Chelucci, Coord. Chem. Rev., 2017, 331, 1.
A. C. Marr, Catal. Sci. Tech., 2012, 2, 279.
R. Grigg, T. R. B. Mitchell, S. Sutthivaiyakit and N.
Tongpenyai, J. Chem. Soc., Chem. Commun., 1981, 1981,
611.
as the benzyl hydrogens from the alcohol substrate.
A
stoichiometric quantity of base is necessary for the reaction to
reach complete conversion.
17
18
19
Acknowledgements
Financial support from the University of New South Wales,
Macquarie University and the Australian Research Council are
gratefully acknowledged. This research was supported under
the Australian Research Council's Discovery Projects funding
scheme (project numbers DP130101838 and DP150103065).
We are grateful to the NMR facilities at the Mark Wainwright
Analytical Centre, UNSW, and Macquarie University. We are
grateful to the Cabot Corporation for the donation of the
carbon black (Vulcan XC-72R). C.M.W. thanks the Australian
Government for the award of an International Postgraduate
Research Scholarship (IPRS). M.B.P thanks the Australian
Government for the Australian Government Research Training
Program Scholarship. J.J.G. acknowledges the ARC for an ARC
Australian Laureate Fellowship (FL150100060).
20
21
22
23
24
25
Y. Watanabe, Y. Tsuji and Y. Ohsugi, Tetrahedron Lett.,
1981, 22, 2667.
D. Hollmann, S. Bähn, A. Tillack, R. Parton, R. Altink and
M. Beller, Tetrahedron Lett., 2008, 49, 5742.
M. H. S. A. Hamid, C. L. Allen, G. W. Lamb, A. C. Maxwell,
H. C. Maytum, A. J. A. Watson and J. M. J. Williams, J. Am.
Chem. Soc., 2009, 131, 1766.
A. Eka Putra, Y. Oe and T. Ohta, Eur. J. Org. Chem., 2013,
2013, 6146.
A. B. Enyong and B. Moasser, J. Org. Chem., 2014, 79,
7553.
26
27
28
29
Notes and references
T. T. Dang, B. Ramalingam and A. M. Seayad, ACS Catal.,
2015, 5, 4082.
1
J. Leonard, A. J. Blacker, S. P. Marsden, M. F. Jones, K. R.
Mulholland and R. Newton, Org. Proc. Res. Dev., 2015, 19,
1400.
V. R. Jumde, L. Gonsalvi, A. Guerriero, M. Peruzzini and M.
Taddei, Eur. J. Org. Chem., 2015, 2015, 1829.
K. O. Marichev and J. M. Takacs, ACS Catal., 2016, 6, 2205.
K.-i. Fujita, K. Yamamoto and R. Yamaguchi, Org. Lett.,
2002, 4, 2691.
B. Blank, S. Michlik and R. Kempe, Chem. Eur. J., 2009, 15,
3790.
S. Michlik and R. Kempe, Chem. Eur. J., 2010, 16, 13193.
R. Kawahara, K.-i. Fujita and R. Yamaguchi, Adv. Syn.
Catal., 2011, 353, 1161.
2
T. Akizawa, N. Kurita, M. Mizobuchi, M. Fukagawa, Y.
Onishi, T. Yamaguchi, A. R. Ellis, S. Fukuma, M. Alan
Brookhart, T. Hasegawa, K. Kurokawa and S. Fukuhara,
Sci. Rep., 2016, 6, 19612.
30
31
32
3
4
M. Daubresse and G. C. Alexander, Int. J. Obe., 2015, 39,
377.
O. Navarro, N. Marion, J. Mei and S. P. Nolan, Chem. Eur.
J., 2006, 12, 5142.
33
34
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Green Chemistry
Page 9 of 10
Journal Name
View Article Online
DOI: 10.1039/C7GC01007A
ARTICLE
35
36
37
38
39
40
S. Michlik, T. Hille and R. Kempe, Adv. Synth. Catal., 2012, 66
354, 847.
C. M. Wong, K. Q. Vuong, M. R. D. Gatus, C. Hua, M.
Bhadbhade and B. A. Messerle, Organometallics, 2012,
31, 7500.
G. Zeng, S. Sakaki, K.-i. Fujita, H. Sano and R. Yamaguchi,
ACS Catal., 2014, 4, 1010.
I. Cumpstey, S. Agrawal, K. E. Borbas and B. Martin-
Matute, Chem. Commun., 2011, 47, 7827.
G.-M. Zhao, H.-l. Liu, X.-r. Huang, D.-d. Zhang and X. Yang,
RSC Adv., 2015, 5, 22996.
S. Ruch, T. Irrgang and R. Kempe, Chem. Eur. J., 2014, 20,
13279.
O. Saidi, A. J. Blacker, M. M. Farah, S. P. Marsden and J.
M. J. Williams, Angew. Chem. Int. Ed., 2009, 48, 7375.
O. Saidi and J. M. J. Williams, in Iridium-Catalyzed
67
68
69
70
S. Mukhopadhyay, R. S. Singh, A. Biswas and D. S. Pandey,
Chem. Commun., 2016, 52, 3840.
G. Gunniya Hariyanandam, D. Hyun, P. Natarajan, K.-D.
Jung and S. Yoon, Catal. Today, 2016, 265, 52.
D. Balcells, A. Nova, E. Clot, D. Gnanamgari, R. H. Crabtree
and O. Eisenstein, Organometallics, 2008, 27, 2529.
A. Bartoszewicz, G. González Miera, R. o. Marcos, P.-O.
Hydrogen Transfer Reactions, ed. P. G. Andersson, 71
Springer, Berlin, Heidelberg, 2011, pp. 77-106.
H. Yang, R. Mao, C. Luo, C. Lu and G. Cheng, Tetrahedron,
2014, 70, 8829.
Norrby and B. Martıń -Matute, ACS Catal., 2015, 5, 3704.
41
42
43
T. Yan, B. L. Feringa and K. Barta, ACS Catal., 2016, 6, 381.
S. Rösler, M. Ertl, T. Irrgang and R. Kempe, Angew. Chem.
Int. Ed., 2015, 54, 15046.
44
45
G. Zhang, Z. Yin and S. Zheng, Org. Lett., 2016, 18, 300.
Y. Yang, A. Qin, K. Zhao, D. Wang and X. Shi, Adv. Syn.
Catal., 2016, 358, 1433.
46
47
48
D. Wang, K. Zhao, C. Xu, H. Miao and Y. Ding, ACS Catal.,
2014, 4, 3910.
D. Wang, K. Zhao, X. Yu, H. Miao and Y. Ding, RSC Adv.,
2014, 4, 42924.
D. Wang, K. Zhao, P. Ma, C. Xu and Y. Ding, Tetrahedron
Lett., 2014, 55, 7233.
49
50
K.-i. Shimizu, Catal. Sci. Tech., 2015, 5, 1412.
K.-i. Shimizu, N. Imaiida, K. Kon, S. M. A. Hakim Siddiki and
A. Satsuma, ACS Catal., 2013, 3, 998.
51
52
53
T. T. Dang, S. P. Shan, B. Ramalingam and A. M. Seayad,
RSC Adv., 2015, 5, 42399.
S. Pei Shan, T. T. Dang, A. M. Seayad and B. Ramalingam,
ChemCatChem, 2014, 6, 808.
D. Wang, X.-Q. Guo, C.-X. Wang, Y.-N. Wang, R. Zhong, X.-
H. Zhu, L.-H. Cai, Z.-W. Gao and X.-F. Hou, Adv. Synth.
Catal., 2013, 355, 1117.
54
55
A. M. Rasero-Almansa, A. Corma, M. Iglesias and F.
Sánchez, ChemCatChem, 2014, 6, 1794.
W. A. Herrmann, M. Elison, J. Fischer, C. Köcher and G. R.
J. Artus, Angew. Chem. Int. Ed., 1995, 34, 2371.
W. A. Herrmann, Angew. Chem. Int. Ed., 2002, 41, 1290.
S. Díez-González, N. Marion and S. P. Nolan, Chem. Rev.,
2009, 109, 3612.
56
57
58
D. Wang, R. Cai, S. Sharma, J. Jirak, S. K. Thummanapelli,
N. G. Akhmedov, H. Zhang, X. Liu, J. L. Petersen and X. Shi,
J. Am. Chem. Soc., 2012, 134, 9012.
59
60
61
62
63
64
65
H. Duan, S. Sengupta, J. L. Petersen, N. G. Akhmedov and
X. Shi, J. Am. Chem. Soc., 2009, 131, 12100.
Y. Yang, W. Hu, X. Ye, D. Wang and X. Shi, Adv. Syn. Catal.,
2016, 358, 2583.
S. Gu, H. Xu, N. Zhang and W. Chen, Chemistry – An Asian
Journal, 2010, 5, 1677.
S. Warsink, R. M. Drost, M. Lutz, A. L. Spek and C. J.
Elsevier, Organometallics, 2010, 29, 3109.
C. M. Wong, D. B. Walker, A. H. Soeriyadi, J. J. Gooding
and B. A. Messerle, Chem. Sci., 2016, 7, 1996.
H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem.
Int. Ed., 2001, 40, 2004.
K. Q. Vuong, M. G. Timerbulatova, M. B. Peterson, M.
Bhadbhade and B. A. Messerle, Dalton Trans., 2013, 42,
14298.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Green Chemistry
Page 10 of 10
View Article Online
DOI: 10.1039/C7GC01007A
TOC Text
Ir(III) complexes were found to be highly active catalysts for the hydrogen borrowing
coupling of amines and alcohols.