Dual gold and photoredox catalysed C-H activation of arenes for aryl-aryl cross couplings

Article abstract of DOI:10.1039/C6SC05469B

A mild and fully catalytic aryl-aryl cross coupling via gold-catalysed C-H activation has been achieved by merging gold and photoredox catalysis. The procedure is free of stoichiometric oxidants and additives, which were previously required in gold-catalysed C-H activation reactions. Exploiting dual gold and photoredox catalysis confers regioselectivity via the crucial gold-catalysed C-H activation step, which is not present in the unselective photocatalysis-only counterpart.

Full text of DOI:10.1039/C6SC05469B

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Dual Gold and Photoredox Catalysed C‐H Activation of Arenes for
Aryl‐Aryl Cross Couplings
V. Gauchot,a D. R. Sutherlanda and A.‐L. Lee*a
Received 00th January 20xx,
Accepted 00th January 20xx
A mild and fully catalytic aryl‐aryl cross coupling via gold‐catalysed C‐H activation has been achieved by merging gold and
photoredox catalysis. The procedure is free of stoichiometric oxidants and additives, which were previously required in
gold‐catalysed C‐H activation reactions. Exploiting dual gold and photoredox catalysis confers regioselectivity via the
crucial gold‐catalysed C‐H activation step, which is not present in the unselective photocatalysis‐only counterpart.
DOI: 10.1039/x0xx00000x
www.rsc.org/
and Toste.9,10,11 Its use in cross‐couplings has only very recently
been reported: Sonogashira‐type couplings12 and Suzuki‐type
couplings were revealed this year, the latter independently by
our group13 and Fouquet.14,15 To the best of our knowledge,
however, aryl‐aryl couplings via C(sp2)‐H activation using dual
gold and photoredox catalysis has yet to be achieved, although
it was recently attempted by Maestri and Malacria.16 Under
their conditions, they instead discovered that the coupling
between unactivated arenes and diazonium salts could occur
under photocatalysis‐only conditions (no gold) through
Introduction
The increased drive to develop more sustainable methods for
synthesis has led to a surge in research on C‐H functionalisations.1
Within this context, direct aryl C‐H functionalisations using gold
catalysis2 is a relatively young and overlooked field compared to the
more developed palladium, ruthenium and rhodium counterparts.
Nevertheless, the mild conditions under which gold‐catalysis can
activate C‐H bonds, as well as the regioselectivity observed in the
absence of directing groups,2 provides many golden opportunities
for this developing field. In the specific area of aryl‐aryl cross‐
couplings via C‐H activation, Lloyd‐Jones and Russell elegantly
showcased that gold catalysis can be used to site selectively arylate
arylsilanes (Scheme 1a).3 More recently, Larrosa disclosed his
seminal work on oxidative cross‐couplings via double C‐H activation
to couple electron‐poor with electron‐rich arenes.4 Despite these
advances, there remain several limitations, one of which is the
often limited arene substrate scope.2a The other major limitation is
the requirement for a stoichiometric oxidant to access the
Au(I)/Au(III) cycle required for cross‐couplings:2,5 the benefit of
employing C‐H activation to avoid arene prefunctionalisation is thus
somewhat offset by the generation of stoichiometric organic waste
from the oxidant, and the use of the latter can also limit functional
group tolerance. There is therefore a clear need to develop
couplings that do not require stoichiometric oxidants.2a Within this
context, we herein disclose the first dual gold and photoredox
catalysed aryl‐aryl cross coupling via C‐H activation (Scheme 1b),
which also constitutes the first gold‐catalysed C(sp2)‐H activation
reaction which does not require stoichiometric oxidants.
mechanistically
distinct
formal
homolytic
aromatic
substitutions, which does not involve C‐H activation. However,
poor regioselectivities (mixtures of ortho, meta and para
coupling) were observed and 40 equivalents of arene were
generally required for this radical reaction.16 Therefore, aryl‐
aryl couplings via C(sp2)‐H activation involving dual gold and
photoredox catalysis is clearly desirable, as it will not only
prove for the first time that catalytic oxidants can be utilised in
the general field of gold‐catalysed C‐H activations, but it
should also significantly improve the regioselectivities and
arene equivalents in the aryl‐aryl couplings, compared to the
mechanistically distinct photocatalysis‐only reaction.
The use of dual6 gold and photoredox catalysis7 to access
Au(I)/Au(III) catalytic cycles was recently pioneered by Glorius8
a. Institute of Chemical Sciences
Heriot‐Watt University
Edinburgh EH14 4AS, Scotland, United Kingdom.
E‐mail: A.Lee@hw.ac.uk
Electronic Supplementary Information (ESI) available: Experimental procedures,
full optimisation tables, control reaction, mechanistic studies, characterisation
data and copies of NMR spectra of new compounds. See DOI: 10.1039/x0xx00000x
Scheme 1 Gold‐Catalysed Aryl‐Aryl Couplings via C‐H Activation
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Table 2 Aryldiazonium Scope
Results and discussion
View Article Online
DOI: 10.1039/C6SC05469B
Since electrophilic Au(III) is known to C‐H activate electron rich
arenes,17,3g,18,19 and using insights gained from our previous
studies,13 we surmised that a combination of an aryldiazonium
salt20 with PPh3AuNTf2 and a photoredox catalyst should furnish an
electrophilic arylAu(III) species (III, Scheme 3) capable of C‐H
activating a suitable arene in order to form our cross‐coupled
product (see later for mechanism). We thus initiated our studies
using mesitylene 1a as the arene with Ru(bpy)3(PF6)2 as the
photoredox catalyst (Table 1). To our delight, the coupling product
3aa was observed in a promising 31% yield (Entry 1).
Crucially, control experiments in the absence of gold catalyst,21
Ru catalyst or light resulted in little or no conversion (see ESI),
confirming that it is a dual gold/photoredox coupling reaction under
these conditions (see Scheme
2 for further confirmation).
Optimisation studies showed that a small excess of arene 1a is
beneficial (Entry 3) but a large excess hampers the reaction in this
case (Entry 4). Employment of organic dyes22 eosin Y and
fluorescein instead of Ru(bpy)3(PF6)2 proved to be a potentially
greener alternative (Entries 5‐6), although we opted to continue our
studies using the better performing Ru catalyst. Finally, a good 3aa
yield of 81% was achieved by increasing the gold catalyst loading
(Entry 7).23
Table 1 Selected Optimisation Reactions
Entrya
x
1
1
3
10
3
y
2
1
1
1
1
1
1
z
5
5
5
5
5
5
10
Modification
Yield (%)b
As for the arene scope, Au(III)‐mediated C‐H activation is
known to proceed via electrophilic aromatic substitution onto
Au(III) (see later), thereby rendering electron‐poor arenes
unsuitable candidates for these conditions. With this in mind,
suitable electron neutral and electron rich arenes were
evaluated as shown in Table 3. While steric hindrance in the
form of double ortho substitution is tolerated in mesitylene
3aa (73%), the yield begins to drop off with increasingly
1
2
3
4
5
6
7
‐
‐
‐
‐
31
51
67
54
62
58
81
Eosin Y instead of [Ru]
Fluorescein instead of [Ru]
‐
3
3
aDegassed MeCN. bDetermined by 1H NMR analysis using dimethylsulfone as internal
standard.
hindered ortho‐substituents (3ba, 3ca). Para‐ and meta‐xylene
also couples with high yields (82%), as does toluene (3da) and
tbutylbenzene (3ea). Predictably, 3da and 3ea are formed as
o‐/p‐isomers, although the major para‐3da can be isolated in a
good 56% yield. The p‐/o‐ ratio is a good 5.7:1 for the more
With these optimised conditions in hand, an aryldiazonium
substrate scope was carried out (Table 2). Ester‐ (3aa) and amide‐
substituted (3ak) substrates, as well as halogenated substrates 3ab‐
3ae react smoothly (50‐80%), as do meta‐ and para‐substituted
nitro substrates (3af‐3ag). The ortho‐substituted 3ah, however, is
furnished in a modest 37% yield, presumably due to steric effects.
Yields of 3ai and 3aj were moderate under standard conditions, but
the yield of 3ai was successfully improved to 60% under more
forcing conditions (10 equiv. 1a and 50 °C). Predictably,8a,12b,13
electron‐rich aryldiazoniums react more sluggishly, with decreasing
yields observed with more electron rich aryls (3am‐3an 48%, while
3ao <26%).
hindered 3ga
.
This is in stark contrast to the photocatalysis‐only reaction
(Scheme 2). In the absence of gold, yield (30%) and selectivity
(0.88:1:0.15 of 3ae:3ae’:3ae’’) are both very poor (Scheme 2b)
compared to the fully selective dual catalytic reaction (Scheme
2a). Adopting the literature photocatalysis‐only conditions16
also result in a similarly unselective reaction, although the
conversion is improved (58% combined yield of inseparable
isomers, Scheme 2c). These controls show the significant
benefit of utilising the regioselectivity conferred by the gold
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C‐H activation step in the dual gold and photoredox reaction
ARTICLE
Next,
mesitylenes
bearing
electronV‐wiewitAhrdticrleawOnilninge
(Scheme 2a) and is further proof that the reaction described substituents successfully couple (TableDO3,I: 10.1039/C6SC05469B
3ia‐3ja, 52‐57%),
here is not a photocatalysis‐only reaction.
although the yield drops to 37% in the presence of the more
withdrawing CN group (3ka). Biphenyl pleasingly reacts
exclusively at the para‐position to yield triaryl 3la in 56% yield.
Finally, very electron rich arenes and heteroarenes such as
1,3,5‐trimethoxybenzene and N‐methyl indole do not currently
cross‐couple well under these conditions, due to competing
azo coupling (see ESI).24,25
Table 3 Arene Scope
Pleasingly, however, more electron rich arenes are viable
arenes for intramolecular C‐H couplings, as exemplified by the
formation of 5 in 83% yield (Scheme 3). While these cyclisations
have been attempted under photocatalysis‐only conditions,
reported yields were very low (0‐25%) due to competing
deazotisation.26 Carbazole27 7 can also be accessed from 6 in 82%
yield, which is of note as traditional Pschorr cyclisations28 do not
typically work well for carbazoles. Indeed, 5 and 7 are only formed
in 36% and 41% (NMR yields) respectively in the absence of gold.
Moreover, the readily oxidisable sulfide (4) and benzyl (6) are
tolerated under these conditions, showcasing the potential of dual
catalysis to significantly improve the C‐H activation cross coupling
under mild conditions compared to previously required
stoichiometric oxidant conditions.
Dual gold and photoredox catalysed:
ArN2BF4 2a
PPh3AuNTf2 (10 mol%)
Ru(bpy)3(PF6)2 (2.5 mol%)
(1)
Scheme 3 Intramolecular Aryl‐Aryl Coupling via C‐H Activation
Ar
MeCN, 50 oC, 16 h
(10 equiv.)
3ae, 82%
1e
high conversion, regioselective
Ar'
N2BF4
2+*
-N2
Ru(bpy)3
Photoredox
As above but without gold:
ArN2BF
Ar
Ar'
4 2a
catalysis
Ru(bpy)3
3+
Ru(bpy)3(PF6)2
(2.5 mol%)
2+
+
(2)
II
Ru(bpy)3
+
Ar
L
SET
Ar
3ae'
X
Au(II) Ar'
MeCN, 50 oC, 16 h
III
3ae''
(10 equiv.)
3ae
L
Radical
Ar'
X
Au(III) Ar'
1e
30%, unselective (0.88 : 1 : 0.15),
chain
Ar'
inseparable isomers
I
L
Ph3P Ar'
-N2
Au(I)
X
Ar'N2BF4
Previous photocatalysis only conditions:16
VI
Ar Ar'
3
Gold
Catalysis
Ar
Reductive
elimination
ArN2BF4 2a
Ru(bpy)3(PF6)2
(0.5 mol%)
Ar
H
L
L
Au(III)
Au(III) Ar'
X
Ar'
X
(3)
+
+
Ar
-H+
IV
V
Ar
MeCN, r.t., 16 h
(40 equiv.)
Ar
3ae'
L=PPh3
3ae''
3ae
1e
58%, unselective (0.84 : 1 : 0.13),
Scheme 4 Plausible Mechanism
inseparable isomers
Based on a combination of various literature reports,8a,9a,12b,29,13
a plausible mechanism for the cross‐coupling is shown in Scheme 4.
Scheme 2 Dual Gold and Photoredox Catalysis Confers Regioselectivity
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Initial oxidation of the Au(I) catalyst I via addition of an aryl radical for stoichiometric oxidants and its corresponding waste. As is the
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2
DOI: 10.1039/C6SC05469B
(I→II), is followed by a subsequent SET to form Au(III) intermediate case with current gold‐catalysed C‐H activation reactions, the
III,30 regenerating the photocatalyst. Alternatively, quantum yield arene substrate scope for the intermolecular coupling still has its
calculations carried out on related dual triphenylphosphine limitations (although the intramolecular version shows great
gold/visible‐light catalysed systems revealed that species II can also promise) and addressing this issue remains a future challenge for
undergo SET with another equivalent of diazonium salt, to the field. Nevertheless, we envisage that the development of the
simultaneously yield the Au(III) species III along with an aryl first fully catalytic system constitutes significant progress for the
radical.12b The arene partner then undergoes electrophilic auration field of gold‐catalysed C‐H activation and functionalisation of
with the Lewis acidic species III to give intermediate IV, which arenes. In addition, control experiments show that exploiting dual
explains the regioselectivities observed.17,3g,18,19 The corresponding gold and photoredox catalysis confers regioselectivity via the crucial
intermediate V then reductively eliminates to form the cross‐ gold‐catalysed C‐H activation step, which is not present in the
coupled product 3, while regenerating Au(I) catalyst I.
unselective photocatalysis‐only counterpart.
In order to lend support to this mechanism, two reactions were
set up using equimolar amounts of PPh3AuNTf2, 1a and 2d, and 2.5
mol% of Ru(bpy)3(PF6)2 in the presence and absence of light
respectively. 31P NMR monitoring reveals that a new signal at  23.1
ppm appears for the irradiated reaction (Figure 1), but is absent
from the dark reaction. The transient species III is highly unstable31
and cannot be isolated, however, the signal at  23.1 ppm
corresponds to species VI32 which is formed by reductive
elimination of III.9a The detection of VI therefore implies that III is
present in the reaction.9a,13
Acknowledgements
We gratefully acknowledge the Leverhulme Trust (RPG‐2014‐
345) for funding and Heriot‐Watt University for a James Watt
Scholarship (DRS). Mass spectrometry data was acquired at
the EPSRC UK National Mass Spectrometry Facility at Swansea
University.
Notes and references
1
Selected review: (a) H. M. L. Davies and D. Morton, J. Org.
Chem., 2016, 81, 343; (b) T. Gensch, M. N. Hopkinson, F. Glorius
and J. Wencel‐Delord, Chem. Soc. Rev., 2016, 45, 2900; (c) J. F.
Hartwig and M. A. Larsen, ACS Central Science, 2016, 2, 281.
Reviews: (a) S. Kramer, Chem. Eur. J., 2016, 22, 15584; (b) T. C.
Boorman and I. Larrosa, Chem. Soc. Rev., 2011, 40, 1910; (c) T.
deꢀHaro and C. Nevado, Synthesis, 2011, 2530.
(a) L. T. Ball, G. C. Lloyd‐Jones and C. A. Russell, Science, 2012,
337, 1644; (b) L. T. Ball, G. C. Lloyd‐Jones and C. A. Russell, J.
Am. Chem. Soc., 2014, 136, 254; (c) A. J. Cresswell and G. C.
Lloyd‐Jones, Chem. Eur. J., 2016, 22, 12641; see also: (d) K. Hata,
H. Ito, Y. Segawa and K. Itami, Beilstein J. Org. Chem., 2015, 11,
2737; intramolecular: (e) T. J. A. Corrie, L. T. Ball, C. A. Russell
and G. C. Lloyd‐Jones, J. Am. Chem. Soc., 2017, 139, 245; using
arylboronic acids: (f) M. Hofer, A. Genoux, R. Kumar and C.
Nevado, Angew. Chem. Int. Ed., 2017, 56, 1021; (g) Q. Wu, C.
Du, Y. Huang, X. Liu, Z. Long, F. Song and J. You, Chem. Sci.,
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2
3
Figure 1 31P NMR Studies in CD3CN
Furthermore, control experiments using various gold(I) species
fail to form the coupling product 3 (see ESI), lending further support
to the hypothesis that intermediate III is the key species in the
crucial electrophilic auration step. In addition, control experiments
shown in Scheme 2 confirm that the mechanism is distinct from the
formal homolytic aromatic substitutions observed in the
photocatalysis‐only reactions, since the regioselectivity observed
supports the electrophilic auration step shown in Scheme 4 rather
than the former, which is unselective. Additionally, the involvement
of an aryl cation intermediate from the aryldiazonium salt 233 can
also be discounted by the fact that electron withdrawing
aryldiazoniums react more readily than their electron rich
counterparts (Table 2).
4
(a) X. C. Cambeiro, N. Ahlsten and I. Larrosa, J. Am. Chem. Soc.,
2015, 137, 15636; stoichiometric version: (b) X. C. Cambeiro, T.
C. Boorman, P. Lu and I. Larrosa, Angew. Chem. Int. Ed., 2013,
52, 1781.
Review on Au(I)/Au(III) catalysis: M. N. Hopkinson, A. D. Gee
and V. Gouverneur, Chem. Eur. J., 2011, 17, 8248.
For reviews on cooperative photoredox catalysis, see: (a) X.
Lang, J. Zhao and X. Chen, Chem. Soc. Rev., 2016, 45, 3026; (b)
M. N. Hopkinson, B. Sahoo, J.‐L. Li and F. Glorius, Chem. Eur. J,
2014, 20, 3874.
5
6
7
8
Reviews on photoredox gold catalysis: (a) M. N. Hopkinson, A.
Tlahuext‐Aca and F. Glorius, Acc. Chem. Res., 2016, 49, 2261; (b)
T. McCallum, S. Rohe and L. Barriault, Synlett, DOI: 10.1055/s‐
0036‐1588644.
(a) B. Sahoo, M. N. Hopkinson and F. Glorius, J. Am. Chem. Soc.,
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Glorius, Adv. Synth. Catal., 2014, 356, 2794; (c) A. Tlahuext‐Aca,
Conclusions
In conclusion, we have developed the first dual
gold/photoredox method for aryl‐aryl cross coupling via direct C‐H
activation of arenes under mild conditions. The use of dual catalysis
has allowed us to address and overcome a major limitation
encountered with gold‐catalysed C‐H activations: the requirement
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M. N. Hopkinson, R. A. Garza‐Sanchez and F. Glorius, Chem. Eur. 30 A. Tlahuext‐Aca, M. N. Hopkinson, C. G. Daniliuc and F. Glorius,
View Article Online
J, 2016, 22, 5909.
Chem. Eur. J., 2016, 22, 11587.
DOI: 10.1039/C6SC05469B
9
(a) X.‐Z. Shu, M. Zhang, Y. He, H. Frei and F. D. Toste, J. Am. 31 Reductive elimination steps involving Au(III) can be extremely
Chem. Soc., 2014, 136, 5844; see also: (b) Y. He, H. Wu and F. D.
Toste, Chem. Sci., 2015, 6, 1194.
fast, see: W. J. Wolf, M. S. Winston and F. D. Toste, Nature
Chem., 2014, 6, 159.
10 For other examples of dual gold/photoredox catalysed 32 D. Marcoux and A. B. Charette, J. Org. Chem., 2008, 73, 590.
reactions, see ref. 12‐14 and: (a) S. Cai, K. Yang and D. Z. Wang, 33 C. P. S. J., M. Howard, M. Katharine and S. Brian, Bull. Chem.
Org. Lett., 2014, 16, 2606; (b) D. V. Patil, H. Yun and S. Shin, Adv.
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11 For an elegant alternative, examples of photoredox using only
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Hashmi, Chem. Commun., 2016, 52, 6435; (b) L. Huang, M.
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18 Selected reactions involving Au(III)‐promoted C‐H activation,
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Zhang and J. Wang, Org. Lett., 2011, 13, 4988; (b) A. Pradal, P. Y.
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22 D. A. Nicewicz and T. M. Nguyen, ACS Catalysis, 2013, 4, 355.
23 The same reaction carried out in an open flask with non‐
degassed solvents yielded 51% of 3aa.
24 L. M. Anderson, A. R. Butler and A. S. McIntosh, J. Chem. Soc.,
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