Mild and Efficient Synthesis of Diverse Organo-AuI-L Complexes in Green Solvents

Article abstract of DOI:10.1002/cssc.201903415

An exceptionally mild and efficient method was developed for the preparation of (hetero)aryl-Au^I-L complexes using ethanol or water as the reaction medium at room temperature and Ar-B(triol)K boronates as the transmetalation partner. The reaction does not need an exogeneous base or other additives, and quantitative yields can be achieved through a simple filtration as the only required purification method, which obviates considerable waste associated with alternative workup methods. A broad reaction scope was demonstrated with respect to both the L and (hetero)aryl ligands on product Au complexes. Despite the polar reaction medium, large polycyclic aromatic hydrocarbon units can be incorporated on the Au complexes in very good to excellent yields. The approach was demonstrated for the chemoselective manipulation of orthogonally protected aryl boronates to afford a new class of N-heterocyclic carbene-Au-aryl complexes. A mechanistic rationale was proposed.

Full text of DOI:10.1002/cssc.201903415

Accepted Article
Title: Mild and Efficient Synthesis of Diverse Organo-Au(I)-L
Complexes in Green Solvents
Authors: Fredric J. L. Ingner, Ann-Cathrin Schmitt, Andreas Orthaber,
Paul J. Gates, and Lukasz T Pilarski
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To be cited as: ChemSusChem 10.1002/cssc.201903415
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10.1002/cssc.201903415
ChemSusChem
FULL PAPER
Mild and Efficient Synthesis of Diverse Organo-Au(I)-L
Complexes in Green Solvents
Fredric J. L. Ingner,[a] Ann-Cathrin Schmitt,[a] Andreas Orthaber,[b] Paul J. Gates,[c] Lukasz T. Pilarski*[a]
[a]
F. J. L. Ingner, A.-C. Schmitt, Dr. L. T. Pilarski
Department of Chemistry - BMC
Uppsala University
BOX 576, 75-123, Uppsala (Sweden)
E-mail: lukasz.pilarski@kemi.uu.se
Dr. A. Orthaber
Department of Chemistry - Ångström
Uppsala University
[b]
[c]
BOX 523, 75-120, Uppsala (Sweden)
Dr. P. J. Gates
School of Chemistry
University of Bristol
Cantock’s Close, Clifton, Bristol, BS8 1TS (UK)
Supporting information for this article is given via a link at the end of the document.
Abstract: An exceptionally mild and efficient method is presented for
the preparation of (hetero)aryl-AuI-L complexes using ethanol or water
as the reaction medium at room temperature and R-B(triol)K
boronates as the transmetalation partner. The reaction returns up to
quantitative yields, needs no exogeneous base or other additives and
a simple filtration is the only required purification method, which
obviates considerable waste associated with alternative workup
methods. A broad reaction scope is demonstrated with respect to both
the L and (hetero)aryl ligands on product Au complexes. Despite the
polar reaction medium, large polycyclic aromatic hydrocarbon units
can be incorporated on the product Au complexes in very good to
excellent yields. We demonstrate the use of our approach for the
chemoselective manipulation of orthogonally protected aryl boronates
to afford a novel new class of NHC-Au-aryl complexes. A mechanistic
rationale is proposed.
For example, complexes 1a[7] (Figure 1) and 1b,[8] respectively,
exhibit anti-malarial and anti-cancer properties. L-AuI-(hetero)aryl
complexes are also of great interest as intermediates in
homogeneous catalysis,[9] as exemplified by 1c.[10] Herein, we
report a uniquely mild, versatile and efficient approach to L-AuI-
(hetero)aryl complexes using green solvents and without any
need for extraction, recrystalisation or chromatography to obtain
analytically pure products.
a. Selected (hetero)aryl-AuI complexes and their applications:
Me
Ph3P
N
Au
HN
Me
N
1b: anti-cancer[8]
Fe
OtBu
Cl
N
O
Au
N
R
P
Au
Ph
Ph
1c: putative catalytic intermediates[10]
Introduction
1a: anti-malarial[7]
b. Nucleophilicity parameter (N) for various 2-furanyl boronates:[11]
Synthetic chemistry is under ever greater pressure to provide
environmentally benign routes to functional molecules. Reducing
reliance on hazardous and/or toxic reagents and solvents has
acquired increasing urgency. Several legislative measures across
the world are set to restrict the future use of various toxic,
carcinogenic and/or environmentally damaging solvents.[1] Their
replacement with greener alternatives is a growing concern in
various areas of synthesis,[2] including in organometallics and
homogeneous catalysis[3] and across the pharmaceutical sector.[4]
As part of a wide-ranging project in our group aimed at
investigating new methods to manipulate aryl-transition metal
systems, we sought to develop a reliable and exceptionally mild
approach to the synthesis of diverse complexes of the type L-AuI-
(hetero)aryl. In recent years, organo-AuI complexes have
attracted considerable interest for various forms of biological
activity[5] and for their promise in materials applications.[6]
Me
N
Ar =
Ar
B
Ar BF3K Ar4 Li
B
O
O
Ar-B(mida)
O
Me
O
O
1.84
9.09
7.66
N
0
1
2
3
4
5
6
7
8
9
10 11 12 13
12.55
2.90
6.38
Me
Me
Me
O
B
Li
O
Ar
N
O
Me
Ar
B
Ar
B
O
Me
Me
O
O
O
Ar-B(triol)Li
Ar-B(pin)
ArB(dea)
Figure 1. a) Example organo-AuI complexes and their properties; b) Relative
nucleophilicities of selected arylboronates reported by Mayr and co-workers.[11]
1
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10.1002/cssc.201903415
ChemSusChem
FULL PAPER
Table 1. Selected results from the comparison of different boronates in the
transmetalation to 3a. Ph3PAuCl (0.05 mmol), boronate (1 equiv.), solvent (0.5
mL). Yields determined by 31P NMR spectroscopy.
transition metals.[15f, 15g, 19] For example, Chan-Lam aminations
using [PhB(triol)]K proceed at three times the rate of the
equivalent reactions using PhB(OH)2 or PhBF3K as the arylating
reagent.[20] We envisaged the prospect of an alternative intra-
molecular transmetalation pathway via intermediate alkoxide
adducts (vide infra).
Ph
Ph
Ph
Me
Me
P
Ph
Ph
solvent
Au
Au
P
B
K
+
time, temp.
Ph
Cl
Results and Discussion
2
3a
4a
In initial experiments, we compared a range of 3-tolyl boronates
(2a-d) in the transmetalation to the AuI centre of Ph3PAuCl (3a) in
various solvents (Table 1, entries 1-8). Triolboronate 2a proved
substantially more efficient in EtOH (entry 7) and even H2O (entry
8) than in toluene[12a, 12c, 21] or MeCN,[9s, 22] both of which have
served as solvents of choice in many previously reported
protocols. Moreover, unlike in many previous methods, the
transmetalation occurred efficiently without any exogeneous
heating. Boronate 2a was the only one amongst several (2a-d) to
afford the desired products in H2O, even if elevated temperatures
were used (entries 9-12).
Me
OH
O
Me
Me
O
O
=
B
BF3K
2d
B
B
B
Me
2a
OH
O
Me
O
2b
2c
Entry Boronate Solvent
Temp (°C)
Time (h)
Yield (%)
23
23
24
24
43
60
2a
2a
2a
2a
toluene
CH2Cl2
THF
1
2
3
4
23
24
46
EtOAc
MeCN
23
23
24
24
39
45
Ph
2a
2a
2a
2a
5
6
7
8
Ph
Ph
Ph
P
nPrOH
EtOH
23
23
24
24
88
Ph
Ph
K
2
P
Au
O
EtOH or H2O
rt, 24 h
97
B
Me
+
O
Au
O
R
H2O
23
24
85
Cl
1
:
1
R
3a
4
9
H2O
H2O
H2O
H2O
50
50
50
50
5
5
5
5
0
2b
2c
2d
2a
meta substituents:
ortho substituents:
0
10
11
12
Ph
Ph
Ph
Ph
Ph
Ph
0
Ph
Ph
Ph
P
P
P
87
Au
Au
Au
R
We specifically set out to develop a protocol to prepare L-AuI-Ar
complexes that tolerated a wide functional group and ligand
variety, required only environmentally benign/non-toxic solvent
media and, importantly, worked efficiently without any other
exogeneous additives or heating using well-defined reagents.
Whilst organoboronic acids[12] and their pinacol esters[9m, 12c, 13]
have been conventionally used to generate organo-AuI
complexes, the former are not well defined, as they can contain
varying amounts of boroxine species, and are often used in
excess. Additionally, previously reported methods have often
relied on unsustainable/hazardous solvents, with exogeneous
additives and at elevated temperatures. With our ambition in mind,
we considered boronates of the type [ArB(triol)]M (Figure 1b),
which have never been used in this context, as candidate sources
of the aryl group.[14] Originally developed by Yamamoto and co-
workers as effective reagents in various coupling reactions,[15]
such boronates have also been shown by Mayr and co-workers[11]
to be significantly more nucleophilic than several of their more
commonly used congeners (selected variants are shown in Figure
1b).
R
4e: R = I,
4c: R = Me, EtOH: 62%
4d: R = OMe, EtOH: 78%
H2O: 56%
4b: EtOH: 88%
EtOH: 85%, 97%[a,b]
H2O: 54%
H2O: 56%
4a: R = Me,
EtOH: 97%, 90%[a]
H2O: 85%
para substituents:
4f: R = OMe
EtOH: 88%
H2O: 65%
Ph
Ph
Ph
P
4i: R = Cl
EtOH: 75%, 89%[a,b]
4g: R = I
Au
R
EtOH: 93%, 91%[a,b]
H2O: 52%
H2O: 55%
4h: R = Br
4j: R = CH=CH2, EtOH: 83%
4k: R = CF3, EtOH: 41%
4l: R = NO2, EtOH or H2O: 0%
EtOH: 82%,
H2O: 60%
Ph
Ph
Ph
Ph
Ph
Ph
P
P
Au
Au
S
4n: EtOH: 87%,
96%[a,b]
4m: EtOH: 47%,
96%[a,b]
H2O: 45%
The triol itself, 2-(hydroxymethyl)-2-methylpropane-1,3-diol, has
low toxicity and a significantly lower price compared to pinacol[16]
and is produced on a multi-ton scale annually.[17] We anticipated
that the transmetalation of (hetero)aryl groups from such
boronates to an AuI center should be particularly efficient on
account of their significantly enhanced nucleophilicity[11, 18] and
because they have been shown to benefit transmetalation to other
X-ray crystal
structure of 4n
H2O: 84%
Ph
Au
P
Ph
Ph
Au
Ph
Ph
Fe
O
P
Ph
4p: EtOH: 53%, 88%[a,c]
4o: EtOH: 60%
2
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10.1002/cssc.201903415
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FULL PAPER
27]
and potential applications in molecular recognition.[13b,
Encouraged by the excellent yield of 5e, we turned our attention
to the synthesis of complexes based on several PAH units
(Scheme 3). Products 5l-p were isolated in very good to
quantitative yield: up to 97% yield (5m) using only one equivalent
of the appropriate [aryl-B(triol)]K reagent and in up to 99% yield
(5n) using only 1.2 equivalents. This is a significant improvement
over several previous reports concerning the synthesis of PAH-
Scheme 1. Boronate scope in EtOH and H2O solvents. Ph3PAuCl (0.1 mmol)
boronate (1.0 equiv.). Unless otherwise stated, yield determined by 31P NMR
spectroscopy. [a] Isolated yield; [b] 3.0 equiv. of boronate used. [c] 1.2 equiv. of
boronate used.
Subsequently, we undertook an extensive exploration of the
scope with respect to both five- and six-membered (hetero)aryl
boronates using 3a as the gold precursor (Scheme 1).
containing AuI complexes, for which poor to moderate yields have
The reactions proved very clean: only starting materials and
products could be observed when monitoring reaction progress
using 31P NMR spectroscopy. Very good to excellent conversions
were obtained with almost all substrates in EtOH at room
temperature using only a 1:1 ratio between 2 and 3a. Boronic
acids bearing methyl- (4a,c), methoxy- (4d,f), iodo- (4e,g), bromo-
(4h), chloro- (4i), vinyl- (4j) and trifluoromethyl- (4k) groups
performed well. Gratifyingly, using 1.2–3.0 boronate equivalents
invariably returned >99% conversion and very good to near-
quantitative yields. For example, the vinyl-AuI complex 4m formed
in up to 96% yield, whilst 2-furanyl (4o) and ferrocenyl (4p) units
returned 96% and 88% yield, respectively. The identity of product
complex 4n was confirmed using X-ray crystallography (see SI for
more details); we also observed no difficulties arising from the
previously suggested S···Au coordination in thiophene
substrates.[12c] Generally, electron-rich substrates performed
slightly better and only the very strongly electron-withdrawing
nitro group (4l) prevented product formation. Steric hindrance
from substituents ortho- to the C-Au bond imposed only a modest
reduction in yield (e.g. 4d vs 4f).
been common until now.[13b,
For all our examples,
22c, 28]
analytically pure products could be obtained via a simple filtration,
obviating the need for additional extraction, recrystalisation or
chromatography. This is especially pleasing, as purification has
relied in some previous reports on the use of chlorinated solvents,
which are increasingly the subject of evermore stringent
legislative regulation.[1]
Moreover, no recrystallization or chromatographic separation
were required: although each reaction mixture remained
heterogeneous throughout, analytically pure products could be
obtained in simply via a sole filtration at the end of the reaction
time. This is a marked advantage over previously reported
procedures for the preparation of L-AuI-(hetero)aryl complexes,
which have often relied on the use of halogenated solvents during
the work-up procedure. Even water proved to be a viable, albeit
slightly less effective solvent (Scheme 1, yields from reactions
performed in water shown in blue).[2b, 3]
We next sought to test the scope of the reaction with respect a
variety of L-type ligands on the AuI center. Trialkylphosphine
ligands with both large (5a) and small (5b) cone angles[23]
performed well. Weaker σ-donor ligands, e.g. tris(4-
trifluoromethylphenyl)phosphine (5c) and triethylphosphite (5d)
also gave very good yields.
Our reaction conditions returned the trimetallic ferrocene-based
complex 5e in 97% isolated yield. The efficiency of this reaction is
particularly notable, given the presence of multiple planar π-
extended and apolar groups. Dinuclear AuI complexes have
attracted attention in various contexts, including as potential anti-
cancer therapies[24] and in catalysis.[25] Additionally, NHC-Au-aryl
complexes bearing
a variety of functional groups formed
efficiently (5f-k). The only exception was the 4-methoxyphenyl
variant (5i) presumably due to protodeauration, which is
accelerated by polar protic solvents for NHC-AuI complexes with
electron-rich arene ligands[26] (cf. 4f, Scheme 1). We also noted
that the reaction scaled well: Compound 5g was prepared on a
0.4 mmol scale, which is 40 times the scale used in our initial
optimisation.
Various organometallic complexes incorporating polycyclic
aromatic hydrocarbons (PAHs), including those of AuI, have
attracted attention for their interesting photophysical properties
Scheme 2. Scope with respect to the L ligand. Isolated yields unless otherwise
specified. 0.1 mmol scale of LAuCl [a] 1.0 equiv. of boronate used.; [b] 1.5 equiv.
boronate; [c] Yield determined by 31P NMR spectroscopy; [d] Ratio of reagents
used: [(dppf)(AuCl)2] 1:2 boronate; [e] 1.2 equiv boronate [f] Yield determined
by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as standard. [g]
Reaction performed on 0.4 mmol scale.
3
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10.1002/cssc.201903415
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N-Aryl carbazole units feature in a range of organic electronic
applications as easily tunable charge transporters.[29] The
auration under our conditions of N-Phenyl carbazole (product 5p)
occurred with 86% conversion using only 1 equivalent of the
corresponding triol boronate and in 94% isolated yield using 3
equivalents.
In a more general context, the manipulation of PAHs can often
suffer from their poor solubility in many solvents. In contrast to
their -B(pin) and -BF3K congeners, each of the starting material
[aryl-B(triol)]K substrates dissolved easily in EtOH at room
temperature (Figure 2). This should open new opportunities for
the manipulation of PAHs.[30]
the differentiated reactivity of 2a, 2b and/or 2c to generate a new
class of decorated NHC-AuI-aryl complexes that retained versatile
boronate functionality for further manipulation. In a competition
experiment, boronate 2e gave significantly faster transmetalation
to PPh3AuCl (3a) than did 2b or 2c in EtOH (Scheme 4a). As
shown in Scheme 4b, we found that Ir-catalyzed borylation[33] of
IPr-Au-Cl gave diborylated complex 6 in quantitative yield. To the
best of our knowledge, this marks the first reported use of C-H
functionalization methodology to derivatize coordinated NHC
ligands.[34]
Subsequently, several aryl groups could be introduced in good to
excellent yields (products 7a-c) to the AuI center using
[ArB(triol)]K boronates under our mild conditions without any
evidence of C-B bond cleavage on the boryl NHC moiety in 6. By
Ph
Ph
Ph
iPr
iPr
Ph
Ph
contrast, attempts to generate complexes
7
from the
P
Ph
N
N
corresponding (hetero)arylboronic acids invariably led to
intractable mixtures.
P
Au
iPr
iPr
Au
Au
a. Boronate competition in EtOH
Me
5m: quant. (97%)[a]
5n: 99%[c]
5l: 67% (91%)[a,b]
PPh3AuCl (3a)
O
B
B
O
O
(1 equiv.)
4e
+
+
4a
K
Ph
EtOH
rt, 24 h
Ph
Ph
with 2b: 100%
with 2c: 79%
0%
21%
P
Ph
Me
Ph
I
P
Au
2b: B = B(OH)2
2c: B = B(pin)
Au
Ph
2e
b. Synthesis of borylated NHC-Au-aryl complexes
N
iPr
iPr
IPr
Au
[Ir(OMe)COD]2
dtbpy
cat.
N
N
(pin)B
B(pin)
5p: 86% (94%)[a,b]
B2(pin)2
5o: 81%, (91)[a,b]
iPr
iPr
Au
Cl
THF, 50 °C, 18 h
Cl
Scheme 3. Synthesis of PAH-containing complexes. General conditions as in
Scheme 2. PPh3AuCl complex (0.1 mmol), boronate (1 equiv.). Yields
determined by 31P NMR spectroscopy unless otherwise stated; [a] Isolated yield.
[b] 3.0 equiv. boronate used. [c] 1.2 equiv. boronate used.
6: quant.
iPr
iPr
N
N
[ArB(triol)]K
EtOH, rt
(pin)B
B(pin)
iPr
iPr
Au
Ar
7
Ar =
7c:
90%
S
7a: 91%
7b: 76%
c. Proposed transmetalation pathway
L
L
Figure 2. Photograph of pyrenyl boronates (2 mmol) in EtOH (2 mL).
Au
Au
O
R
R
O
2
EtOH
To address a related environmental aspect, and to mitigate
concerns that the overall method is reliant on unrenewable
solvents, we synthesized a representative range of the parent
(het)
O
Me
3
(het)
Me
O
B
O
B
O
9
OEt
OEt
ArB(triol)K reagents in 2-methyltetrahydrofuran,
a solvent
8
obtained from renewable resources.[31] These syntheses returned
comparable or higher yields than the more commonly used THF
or toluene solvents (see SI for further details).
Scheme 4. a. Competition study: Ph3PAuCl (0.1 mmol) and boronates (1 equiv.
each) were used. b. Use of differentiated boronate reactivity to generate novel
organogold complexes. c. Tentative intramolecular transmetalation pathway
based on preliminary mechanistic data (see SI for further information).
Species bearing multiple, differentiated boronate groups can
impart various unique advantages in synthesis.[32] As part of a
wider program of studies into AuI complexes, we sought to exploit
4
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10.1002/cssc.201903415
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FULL PAPER
J. D. Hayler, D. K. Leahy, E. M. Simmons, Organometallics
2019, 38, 36-46.
We propose that, in addition to their significant nucleophilicity,
boronates 2 benefit from the lability of an alkoxide residue, leading
to the formation of complexes 8. Thereafter, fast intramolecular
transmetalation (transition states 9, Scheme 4c) presumably
occurs; hydroxide and alkoxide organo-AuI complexes are known
to participate in transmetalation processes.[12c, 35]. Experiments in
support of this proposed pathway are presented in the Supporting
Information.
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In summary, we have described that triol-based boronates afford
a mild, convenient, high-yielding and versatile pathway to L-AuI-
(hetero)aryl complexes. Our method works efficiently in green
solvents at room temperature, often without an excess of the
boronate
reagent. No
extraction, recrystallisation
or
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chromatography methods are required: product isolation relies
solely on filtration. Uniquely, our scope includes phosphine,
phosphite and NHC L-type ligands, as well as diversely
substituted (hetero)aryl and PAH-based aryl groups transferred to
the AuI center. The significantly greater efficiency of
transmetalation from triol-based boronates compared to their
congeners permits the diversification of C-H borylated NHC-Au
complexes. We envisage that this can be leveraged to construct
new AuI complexes with valuable bioactivity profiles and
properties useful in organic electronics applications. Work on this
continues in our laboratory.
Experimental Section
Representative procedure: To a 12 mL microwave vial equipped with a
magnetic stirrer bar were added Ph3PAuCl (0.1 mmol, 1 equiv.) and the
desired triol-based aryl boronate (0.3 mmol, 3 equiv.). EtOH (1.0 mL) was
added and the resulting heterogeneous mixture was stirred at room
temperature for the indicated time. After stirring, the precipitate was
washed with EtOH (2 x 4 mL) and dried under reduced pressure to afford
the analytically pure corresponding aryl Au(I) complex.
Acknowledgements
We thank the Swedish Research Council (Vetenskapsrådet) for
funding, as well as Dr Johanna Larsson, and Dr Christine Dyrager
for proof-reading the manuscript.
Keywords: gold • green solvents • boronates • NHCs •
synthesis
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Entry for the Table of Contents
Triols and triturations: The high-yielding syntheses of diverse aryl-Au(I) complexes is achieved under mild conditions in green
solvents (ethanol or water) by harnessing the unique properties of aryl-B(triol)K boronates. The method permits transmetalation from
aryl-B(triol)K boronates whilst preserving aryl-B(pin) boronates on the substrate. Pure products are obtained through a simple
filtration.
7
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