A silver-free system for the direct C-H auration of arenes and heteroarenes from gold chloride complexes

Article abstract of DOI:10.1039/c3cy00240c

A new methodology for the direct C-H auration of electron-deficient arenes and heteroarenes with simple bases and readily available [Au(PR₃)Cl] complexes is described. This system allows the preparation of a wide scope of aryl-Au(i) compounds without the need for using Ag(i) additives or preparing and isolating basic Au(i) hydroxide complexes.

Full text of DOI:10.1039/c3cy00240c

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A silver-free system for the direct C–H auration of
arenes and heteroarenes from gold chloride
complexes†
Cite this: DOI: 10.1039/c3cy00240c
Received 12th April 2013,
Accepted 13th June 2013
Nanna Ahlsten, Gregory J. P. Perry, Xacobe C. Cambeiro, Tanya C. Boorman and
Igor Larrosa*
DOI: 10.1039/c3cy00240c
www.rsc.org/catalysis
A new methodology for the direct C–H auration of electron-
deficient arenes and heteroarenes with simple bases and readily
available [Au(PR3)Cl] complexes is described. This system allows
the preparation of a wide scope of aryl–Au(I) compounds without
the need for using Ag(I) additives or preparing and isolating basic
Au(I) hydroxide complexes.
Introduction
Scheme 1 Transformations of aryl–Au(I) compounds.
Direct functionalisation of arenes by metal-catalysed C–H acti-
vation is among the most active topics in organic synthesis.
Although dominated by Pd, other metals such as Cu, Rh, Ru, Ir
or Co have been shown in recent years to mediate this type of
reaction.1 Au(III) salts have also been used in C–H activation of
electron-rich arenes under mild conditions.2,3 In contrast to
Au(III), Au(I)-promoted C–H activation to afford (hetero)aryl–Au(I)
compounds has been demonstrated only recently and is specific
for electron-poor arenes.4
Studies on the chemistry of (hetero)aryl–Au(I) compounds
(Scheme 1) have shown them to be suitable organometallic
partners for Pd- or Ni-catalysed cross-coupling reactions,5
although systems catalytic in Au are scarce.6 These species have
also been shown to react with appropriate electrophiles such
as CO2,7 electrophilic halogen sources8 or strong acids.6a,8b
Finally, oxidation with I(III) reagents has allowed their use
in the direct C–H arylation of electron-rich arenes exploiting
the orthogonal selectivity of both Au oxidation states towards
C–H activation, which opens a path for the development
of Au(I/III)-catalysed double C–H activation cross-coupling
reactions.9
Aryl–Au(I) compounds are remarkably stable towards
common decomposition pathways that affect related organo-
metallic compounds.10,11 Thus, in addition to being air and
moisture-stable, they do not participate in oxidative addition
reactions,12 undergo protodeauration only in the presence
of relatively strong acids6a,8b,13 and, although one-electron
reduction to Au(0) is possible with mild reductants such as
Pd(0), this reaction is sufficiently slow to be outcompeted by
other processes.6,14
This combination of rich reactivity and stability towards
undesired decomposition pathways makes aryl–Au(I) com-
pounds extremely interesting intermediates in the design of
metal-catalysed transformations.
Two C–H auration systems have been reported to date by
Nolan’s group4b,c (eqn (1)) and our group4a (eqn (2)) based on
the use of a [Au(IPr)(OH)] or a [Au(PR3)Cl]–Ag2O–K2CO3–PivOH
reagent system, respectively.
School of Biological and Chemical Sciences, Queen Mary University of London,
Joseph Priestley Building, Mile End Road, E1 4NS, London, UK.
E-mail: i.larrosa@qmul.ac.uk; Web: http://webspace.qmul.ac.uk/ilarrosa/index.html
† Electronic supplementary information (ESI) available: Experimental details,
characterisation data and spectra of new compounds. See DOI: 10.1039/
c3cy00240c
(1)
c
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However, shifting to the stronger, but still simple, bases KOH
(entry 2) or KOtBu (entry 3) allowed us to obtain product 3aa in
47 and 80% yields, respectively. Interestingly, use of the more
soluble LiOtBu (entry 4) resulted in a decreased yield, while the
less soluble NaOtBu (entry 5) gave a result comparable to that
obtained with KOtBu. Consistently, NaOH (entry 6) afforded a
somewhat higher yield than KOH, although still lower than the
tert-butoxide salts. The use of 4 equiv. of 2a was necessary for
optimal performance of the reaction, lower yields being
obtained with smaller amounts of 2a (entry 7).17 Less polar
solvents, shown to be effective in direct auration with
[Au(IPr)(OH)], turned out to be detrimental in this case (entries
8 and 9), while dioxane (entries 10 and 11) afforded a yield
very similar to that obtained in DMF. Finally, increasing the
temperature to 75 1C in DMF with NaOtBu (entry 12) allowed us
to obtain the aurated product in 82% yield after purification
(90% NMR yield).
(2)
It is worth noting that the second of these systems, despite
working in an overall less basic medium, shows a higher activity,
as exemplified by the direct auration of 1,3,5-trifluorobenzene
(pKa = 31.5)15 which is not possible under the standard condi-
tions reported for the [Au(IPr)(OH)] system. The exact origin of
this difference in activity has been a matter of discussion,4c,16
although no thorough mechanistic studies have been published
to date.
With these optimised conditions, we set out to explore the
scope of the reaction, aiming to determine the reactivity of this
system towards electron-poor arenes (Scheme 2). A variety of
polyfluorinated benzenes 2b–e, as well as 1-fluoro-3-nitrobenzene
2f, reacted smoothly to afford the corresponding aryl–Au(I)
We considered that the development of a system using a
readily available [Au(PR3)Cl] starting complex and a simple
base, without other additives, would not only be synthetically
useful but also help us determine which are the real differences
between the reactivity of phosphine- and NHC-ligated gold
complexes towards C–H activation of electron-deficient arenes.
Results and discussion
We took as a starting point for this investigation our previously
reported conditions for the direct auration of electron-deficient
arenes ([Au(PtBu3)Cl], PivOH, Ag2O and K2CO3, eqn (2)) and studied
the possibility of performing the reaction with only the base, in the
absence of PivOH or Ag+ salts. Unsurprisingly, K2CO3 alone was not
able to promote the reaction by itself when applied to [Au(PtBu3)Cl]
(1a) and 1,3-dinitrobenzene (2a) in DMF (Table 1, entry 1).
Table 1 Optimisation of the reaction conditionsa
Entry
Base
Solvent
Yield (%)
1
2
3
4
K2CO3
KOH
DMF
DMF
DMF
DMF
DMF
DMF
DMF
THF
PhCH3
1,4-Dioxane
1,4-Dioxane
DMF
0
47
80
57
81
64
31
45
KOtBu
LiOtBu
NaOtBu
NaOH
5
6
7b
8
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOH
9
30
80
10
11
12c
74
NaOtBu
90 (82)d
Scheme 2 Scope of the direct C–H auration of arenes. Reactions carried out
between the 0.06 and 0.1 mmol scale with 1a as the limiting reagent, 4 equiv. of
2 and 4 equiv. of NaOtBu in DMF (0.2 M) at 50 1C for 15 h, unless stated other-
wise. Yields are of the pure isolated product. a Regioisomer ratios determined
using 1H NMR of the crude mixture.
a
Reactions carried out on the 0.03 mmol scale with 1a as the limiting
reagent, 4 equiv. of 2a, 4 equiv. of base and 0.15 mL of solvent (0.2 M),
at 50 1C for 18 h. Yields determined using 1H NMR with an internal
b
c
standard. Reaction performed with 2 equiv. of 2a. Reaction at the
0.1 mmol scale, at 75 1C for 5 h. Isolated yield.
d
c
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compounds 3ab–3af in excellent yields with only slight variations
in the general procedure in some cases (use of NaOH as the base
in 1,4-dioxane). Remarkably, even 2d reacted at 50 1C without
any additive. This contrasts with previous observations where
[Au(IPr)(OH)] was not able to activate this substrate under the
standard conditions (eqn (1)), requiring the use of a silver salt as
an additive.16 A higher basicity of the active phosphine–Au(I)
complex compared to that of [Au(IPr)(OH)] could explain this
difference (vide infra). Benzene derivatives bearing two F atoms
and another halogen such as Br or I (2g-h) reacted preferentially
with the C–H bond ortho to the two F atoms, with virtually
complete regioselectivity at nearly room temperature. These sub-
strates are particularly interesting since, after functionalisation of
the C–Au bond, they would give place to products bearing a useful
functional group for further reactions via other metal-catalysed
coupling processes.
Polychlorinated benzenes 2i–k were also reactive, although
in this case at least three chlorine atoms were necessary for
good yields. 1,3-Di(trifluoromethyl)benzene 2l failed to afford
any aurated product, as well as benzenes without a C–H bond
ortho to two electron-withdrawing groups (2m-n).
A wide scope was also observed for electron-deficient hetero-
arenes (Scheme 3), with a clear dependence of reactivity on the
acidity of the C–H bond to be activated. A brief re-examination
of the reaction conditions showed that NaOH in 1,4-dioxane
consistently afforded higher yields in the case of heteroarenes.
Under these conditions, oxazole (4a), thiazole (4b), benzoxazole
(4c) and benzothiazole (4d) all reacted to afford the corre-
sponding auration products 5aa–5ad in good yields. In the case
of thiazole, displaying two roughly equally acidic protons (pKa =
29.5 for the C2 position and 29.6 for C4),15 selective C2-auration
(>99 : 1) to give 5ab could be achieved at 35 1C. Additionally,
blocking the C2 position with a Br substituent allowed prepara-
tion of the C4-aurated product 5ae, keeping the Br intact and
available for further functionalisation. The less acidic thio-
phene (4f, pKa = 32.5)15 was found not to be active in the
reaction, thus determining an upper limit for the activity of our
system. However, the presence of just one halogen substituent
lowered the pKa sufficiently for the corresponding aurated
products 5ag–5ai to be obtained in excellent yields and regio-
selectivities, with auration occurring selectively a to the sulfur.
Similarly, just one strong electron-withdrawing group allowed
C2-auration of indole-type substrates (pKa of N-methylindole =
37.3)15 to obtain 5aj, and two electron-withdrawing groups
allowed preparation of pyridyl–Au(I) compounds 5ak–5al.
Finally, a stable, electron-deficient imidazole-type substrate
such as caffeine (4m) could be used, affording product
5am in good yield and using only 1.1 equiv. of the parent
heteroarene.
Scheme 3 Scope of the direct C–H auration of heteroarenes. Reactions carried
out between the 0.06 and 0.1 mmol scale with 1a as the limiting reagent,
4 equiv. of the corresponding heteroarene 4 and 4 equiv. of NaOH in 1,4-dioxane
(0.2 M) at 50 1C for 15 h, unless stated otherwise. Yields are of the pure isolated
product. a Regioisomer ratios determined using 1H NMR of the crude product.
b Reaction carried out with 1.1 equiv. of caffeine.
Phosphine ligands other than PtBu3 were also explored
(Scheme 4). For the highly electron-deficient tetrachlorobenzene,
both alkyl and aryl phosphines (3ai–3ci) afforded high yields, and
only with the very bulky XPhos (3di) was the yield significantly
reduced. Reaction of 1,3,5-trichloro- and 1,3-difluorobenzene
with [Au(PEt3)Cl] also led to excellent yields, comparable to those
Scheme 4 Application of different phosphines. Reactions carried out between
the 0.06 and 0.1 mmol scale with 1a as the limiting reagent, 4 equiv. of 2 and
4 equiv. of NaOtBu in DMF (0.2 M) at 50 1C for 15 h, unless stated otherwise.
obtained with the PtBu3 ligand (compare 3bj, 3be and 3bd in Yields are of the pure isolated product.
c
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Scheme 7 Previously reported products obtained from basic treatment of
[Au(L)(NCMe)]BF4 complex 10. L = XPhos or JohnPhos.23
Scheme 5 Direct auration with [Au(IPr)Cl].
Scheme 4 with 3aj, 3ae and 3ad in Scheme 2). More interestingly,
in some cases where [Au(PtBu3)Cl] had offered limited or no
success, the use of a PEt3 ligand allowed efficient preparation of
the corresponding aryl–Au(I) compounds. This allowed efficient
auration of the previously unreactive 1,3-dichlorobenzene (3bk),
and even satisfactory reaction with 1,3-dibromobenzene (3bo).
In order to determine the origin of this system’s ability
to perform the auration of less acidic substrates, such as
1,3,5-trifluorobenzene, in contrast to the [Au(IPr)(OH)] complex,
we tested the auration in our conditions with [Au(IPr)Cl]
(6, Scheme 5). Treatment of 6 with 2b and NaOH or NaOtBu
cleanly provided the aryl–Au(I) complex 7b in 95 and 93%
yields, respectively, showing that the auration process with
NHC ligands does not require isolation of the gold hydroxide
complex.18 Reaction with 2d under the same conditions, how-
ever, did not afford the corresponding aryl–Au(I) compound.
This shows that, at least under these reaction conditions,
phosphine-ligated Au(I) complexes are intrinsically more active
in the auration reaction than the corresponding NHC-ligated
ones.19
The exact nature of the species effecting the C–H activation
proved elusive. Titration of 1a with NaOtBu or NaOH, moni-
tored by in situ 31P NMR, showed complete disappearance of
the signal corresponding to the starting material (90.7 ppm),
giving place to a new peak at 83.2 ppm with 1.0 equiv. of
NaOtBu or at 83.5 ppm with 1.5 equiv. of NaOH. Further
addition of base did not give place to any other observable
product. The product of reaction between 1a and NaOtBu,
unfortunately, resisted all attempts to isolate it, but it has been
reported previously that treatment of [Au(PR3)Cl] complexes
with NaOtBu in THF gives place to the corresponding
[Au(PtBu3)OtBu] complexes20 (Scheme 6). Higher order com-
plexes of the type {[Au(PR3)]2OR}+ have also been described,21
but their preparation usually involves the use of a ‘‘cationic’’
gold complex (such as [Au(PR3)(BF4)]), which makes it unlikely
in our case. Thus, we hypothesise that the product of 1a +
NaOtBu could be 8. Regarding the reaction between 1a and
NaOH, the NMR signals of the product matched those pre-
viously described for {[Au(PtBu3)]3O}BF4 (90).22 Addition of
NaBF4 into the mixture of 1a + NaOH allowed isolation of 90
as a white solid in 92% yield, leading us to conclude that the
product obtained is {[Au(PtBu3)]3O}Cl (9).
However, the reaction of 1a, 2a and either 1.0 equiv. of
NaOtBu or 1.5 equiv. of NaOH afforded the auration product
3aa in only 45 and 15% yield, respectively. These results seem
to indicate different behaviour of compounds 8 and 9: in the
case of 8 the yield obtained was moderate, still lower than that
obtained under the standard conditions but indicative of some
activity. On the other hand, 9 was less reactive and required the
presence of a large excess of base (only 15% yield was obtained,
despite the presence of 1.5 equiv. of base, more than theoreti-
cally needed for formation of 9).
Trinuclear m3-oxo Au(I) species with biphenyl(dialkyl)-
phosphines (12, Scheme 7) have recently been shown to be
related to 10, 11 and 13 by an equilibrium which depends on
the stoichiometry with respect to the base, as well as on the
steric properties of the phosphine ligand.23 The analogous
compound 1d (PR3 = XPhos) was effective in the auration
process under the conditions reported here (product 3di in
Scheme 4), leading us to hypothesise that a qualitatively similar
situation could take place also with the other phosphines.
Thus, in situ formation of a Au(I) hydroxide species, even as
a minor component in the equilibrium mixture, can be hypo-
thesised. Alternatively, direct deprotonation of the arene by the
base followed by attack of the resulting aryl anion on an
electrophilic gold(I) complex cannot be discarded either.24
Conclusions
In summary, we have developed a simplified methodology for
efficiently performing direct C–H auration of electron-deficient
arenes and heteroarenes, combining readily available [Au(PR3)Cl]
complexes and simple bases. This system shows excellent activity
for a wide range of substrates without requiring the use of silver
salts as additives or the preparation and isolation of Au(I)
hydroxide complexes.
Acknowledgements
We gratefully acknowledge the Engineering and Physical Sciences
Research Council for generous funding, the European Research
Council for a Starting Research Grant (to I.L.), the Marie Curie
Foundation for an Intra-European Fellowship (to X.C.C.) and the
EPSRC National Mass Spectrometry Service (Swansea).
Scheme 6 Reactions of [Au(PtBu3)Cl] (1a) with NaOtBu and NaOH.
c
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c
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c
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