Gold-Catalyzed Direct Oxidative Arylation with Boron Coupling Partners

Article abstract of DOI:10.1002/anie.201610457

An efficient synthesis of biaryls through a gold-catalyzed oxidative cross-coupling of arenes with strong electron-deprived aryl boronates is presented herein. Regio- and chemocontrol are achieved by the selective activation of these coupling partners by gold at different oxidation states. Under reaction conditions devoid of basic additives or directing groups, the role of acetato ligand as an internal base has been revealed as a key parameter for expanding the reaction scope in these transformations.

Full text of DOI:10.1002/anie.201610457

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610457
German Edition: DOI: 10.1002/ange.201610457
Cross-Coupling
Gold-Catalyzed Direct Oxidative Arylation with Boron Coupling
Partners
Manuel Hofer, Alexandre Genoux, Roopender Kumar, and Cristina Nevado*
Abstract: An efficient synthesis of biaryls through a gold-
catalyzed oxidative cross-coupling of arenes with strong
electron-deprived aryl boronates is presented herein. Regio-
and chemocontrol are achieved by the selective activation of
these coupling partners by gold at different oxidation states.
Under reaction conditions devoid of basic additives or direct-
ing groups, the role of acetato ligand as an internal base has
been revealed as a key parameter for expanding the reaction
scope in these transformations.
Scheme 1. Gold-catalyzed syntheses of biaryls.
B
iaryl scaffolds are ubiquitous motifs in organic materials,
agrochemicals, and bioactive molecules. Besides classical
transition metal catalyzed cross-coupling reactions between
perfluorinated arenes with highly electronically activated
1
2
[1]
[10]
aryl halides (Ar X) and an organometallic species (Ar M),
arenes (Scheme 1b).
2
efforts towards the direct functionalization of C(sp )ÀH
bonds have gained momentum in recent years. Both, the
direct coupling of two different non-functionalized arenes
Typically, electron-deficient organometallic species are
considered challenging partners in cross-coupling reactions
because of their propensity to undergo fast proto-deactiva-
1
2
[11]
(
(
Ar H, Ar H) as well as the direct arylation combining arenes
Ar H) with an organometallic compound (Ar M) have been
tion, particularly in basic media. We envisaged that the
1
2
development of transmetalation processes under pseudo-
neutral conditions could mitigate such a problem. To this end,
we decided to explore the use of a gold(I) catalyst bearing an
“internal base” to enable the selective activation of strong
electron-deficient aryl boranes. Acetato ligands on gold were
selected to enable the efficient cross-coupling of a wide
variety of arenes with strong electron-deficient boronic esters
thus offering an orthogonal approach to existing Au /Au -
catalyzed arylation methods. Herein, we report the realiza-
tion of this concept together with a study revealing the
mechanism and the selectivity-determining steps underlying
this transformation.
[2]
[3,4]
actively pursued. Despite significant progress,
multiple
challenges, including low reactivity as well as lack of
2
selectivity in the activation of specific C(sp )ÀH bonds,
1
1
coupled with side reactions to give homocoupling (Ar Ar ,
2
2
2
Ar Ar ) or deactivation products (Ar H) are still difficult to
[
5]
master. As a result, a large excess of one of the reactants or
the presence of directing groups are typically needed for
a successful outcome. In this context, gold-catalyzed process-
I
III
I
es are, compared to other metals, underdeveloped even if Au /
III
Au -catalyzed reactions have emerged as competent tools for
[
6]
CÀC bond formation. In a pioneering example, Lloyd-Jones
and Russell reported the oxidative coupling of arenes with
p-Xylene and pentafluorophenylboronic acid pinacol
ester were selected as benchmark reaction partners. Prelimi-
[
7]
weak electron-deficient arylsilanes. An extensive mecha-
nistic study revealed a catalytic cycle in which the selectivity is
[
12]
nary optimization revealed the suitable solvent (1,2-DCE),
attained by the sequential incorporation of both substrates
temperature (1108C), and reactant ratios (2:1) for this
2
onto in situ produced gold(III) species preceding C(sp )À transformation. Different catalysts (Ph PAuX) were used,
3
2
[8]
C(sp ) bond formation (Scheme 1a). While strong electron-
deprived arylsilanes showed low reactivity in this process, the
intrinsic acidity of electron-deficient arenes in combination
with strong bases and/or silver salts has been exploited to
selectively transfer these moieties onto gold(I) complexes in
thus revealing the lack of reactivity of chloride or tosylato
ligands (Table 1, entries 1 and 2), whereas the use of acetato
and pivalato counteranions increased the yields of the desired
biaryl product 1 up to 74% (entries 3 and 4). Experiments
with boronic acids mainly yielded protodeboronation
(entry 5) whereas pinacol and 2,3-pentanediol esters showed
comparable performances (entries 3 and 6–8). Several hyper-
valent iodine reagents were screened as oxidants (entry 9)
[
9]
stoichiometric fashion. Recently, aided by substoichiometric
amounts of silver salts, this process was rendered catalytic as
demonstrated by Larrosa et al. in the direct coupling of
although PhI(OAc) delivered the best outcome (entry 3).
2
Finally, control experiments in the absence of either gold or
oxidant showed no formation of the desired product
[
*] M. Hofer, A. Genoux, R. Kumar, Prof. Dr. C. Nevado
Department of Chemistry, University of Zꢀrich
Winterthurerstrasse 190, 8057 Zꢀrich (Switzerland)
E-mail: cristina.nevado@chem.uzh.ch
(entries 10 and 11).
The optimized reaction conditions (conditions A: Table 1
entry 3; conditions B: entry 7) were successfully applied to
the direct coupling of a series of simple arenes lacking strong
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201610457.
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Table 1: Optimization of the reaction conditions.
method compared to classical cross-coupling reactions. X-
Ray diffraction analysis of 9 confirmed the bond formation at
[
13]
the para position of the electron-rich arene.
Esters and
acetates (12–15), which are susceptible to basic hydrolysis,
were nicely accommodated under the neutral reaction con-
ditions that proved to be compatible with functionalized
heteroarenes (16) as well. No reactivity was found for
substrates which potentially interact as coordinating ligands,
such as pyridine (17) and for electron-poor fluorinated arenes
1
[a]
Entry
R
Oxidant
X
Yield [%]
[
b]
1
2
3
4
5
6
7
8
9
Pin
Pin
Pin
Pin
(OH)
Gly
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
Cl
8
10
OTs
OAc
OPiv
OAc
OAc
OAc
OAc
OAc
no Au
OAc
2
73 (A)
74
(18, 19). The scope with respect to the boronic esters (Ar -
PhI(OAc)2
BX ) was investigated next (Scheme 2b). Tetra-fluorosubsti-
2
[
c]
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
12
11
2
tuted boronic esters gave the corresponding biaryls in good
yields (20–22). Furthermore, di- and trifluorophenylboronic
esters with different fluorination patterns (23–32) were also
efficiently converted, thus covering a wide range of fluori-
nated compounds as the electron-deficient coupling partner.
Bromo and iodo substituents were also tolerated (20, 25–26,
31), thus offering the possibility to use the corresponding
biaryl products in subsequent cross-coupling reactions. With
few exceptions, these reactions displayed excellent levels of
regioselectivity (> 92%) and chemoselectivity (> 93%) for
[
d]
[
e]
Pent
77 (B)
0
0–46
0
0
[
f]
Cat
Pin
Pin
Pin
PhI(OAc)
2
[
g]
[Ox]
1
1
0
1
PhI(OAc)2
no
1
9
[
a] Determined by F NMR spectroscopy. [b] Pinacol ester. [c] Penta-
fluorobenzene was observed. [d] Ethyleneglycol ester. [e] 2,4-Pentanediol
ester. [f] Catechol ester. [g] [Ox]=PhI(OTFA), PhI(OPiv) , HBX (1-
hydroxy-1l ,2-benziodoxol-3(1H)-one), PBX (1-pivaloyloxy-1l ,2-benz-
iodoxol-3(1H)-one). 1,2-DCE=1,2-dichloroethane, TFA=trifluoroace-
tate, Piv=pivaloyl.
2
3
i
3
[12]
the heterocoupling (versus homo) product.
Control experiments to establish the reaction mechanism
were designed next. First, Ph PAuOAc was exposed to
3
activating and/or directing groups, including different xylenes,
benzene, and toluene (1–5, Scheme 2a). Anisoles (6, 7),
including halogen-containing ones (8–11), underwent the
reaction with complete site selectivity, also highlighting the
functional-group compatibility and orthogonality of this
(C F )Bpin in 1,2-DCE at high temperature and produced
6
5
the pentafluorophenylgold(I) complex 33 (see Scheme 3) in
85% yield (Table 2, entry 1). A similar result was obtained in
the presence of two equivalents of p-xylene, which does not
I
seem to interfere with the B/Au transmetalation process. No
reaction was detected with
Ph PAuCl, thus signalling the
3
low reactivity of chloride in
I
this B/Au exchange (entry 2).
Other electron-deficient boron
partners were also easily trans-
ferred to gold(I) (entry 3)
whereas neither phenyl nor p-
methoxyphenyl boron deriva-
tives produced the arylgold(I)
complex (see the Supporting
[
12]
Information). These series of
experiments uncovered the
selective reactivity of electron-
I
deficient boronates in the B/Au
transmetalation, and the role of
the acetato ligand on gold, as an
internal base, to attain effi-
[
14]
ciency.
Notably, in contrast to well-
established
Suzuki–Miyaura
reactions in which the base is
crucial to enable the transmeta-
[
15]
lation step, no addition of an
external base was needed here.
Moreover, unlike the previously
reported silver-promoted trans-
fer of electron-deficient arenes
Scheme 2. Scope with respect to the arene and boron reagents in the gold-catalyzed direct arylation.
[a] Reaction conditions A: Table 1, entry 3 (using Bpin) and Reaction conditions B: Table 1, entry 7 (using
Bpent). [b] Site selectivity 79%. [c] In benzene as solvent, PhI(OAc) (0.70 mmol); chemoselectivity 71%.
2
[10,16]
to gold,
no other additives
[
d] Ph PAuOAc (10 mol%). [e] Toluene (1.5 mmol). [f] Regioisomeric p/o/m ratio ca. 3:2:1 [g] Site
3
selectivity 66%. [h] Ph PAuOAc (7.5 mol%). [i] Site selectivity 78%. EWG=electron-withdrawing group.
were required for a successful
3
2
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I
1
Table 2: B/Au Transmetalation.
moiety. Reactions were also monitored by both H and
1
9
F NMR spectroscopy to identify potential reaction inter-
mediates. The reaction of (C F )Bpin in the presence of
6
5
PhI(OAc)2 with Ph PAuOAc, both in stoichiometric and
3
1
Entry
Conditions
R
Yield [%]
catalytic fashion with benzene as solvent, clearly showed 33
and 35 in the reaction media (see section 5 in the Supporting
[
a,b]
1
2
3
X=OAc, 908C, 8 h
X=Cl, 908C, 12 h
X=OAc, 908C, 14 h
C6F5
C6F5
2,4,6-C H F
2 3
33, 85
33, 0
[
12]
Information).
[
a]
34, 89
6
Based on the data presented in the previous sections,
a proposal for this gold-catalyzed direct arylation is shown in
[
a] Yield after column chromatography. [b] 94% when the reaction was
run in the presence of p-xylene.
Scheme 4. The catalytic cycle can be initiated by a selective B/
I
Au transmetalation of Ph PAuOAc (A) and the electron-
3
deficient boronic ester to form an arylgold(I) complex (B).
outcome. The ability of PhI(OAc) to oxidize the electron-
2
[17]
deficient arylgold(I) species was investigated next.
The
reaction of the electron-deficient arylgold(I) complexes 33
and 34 with PhI(OAc)2 in benzene cleanly delivered the
bis(acetate)gold(III) species 35 (83%) and 36 (70%), respec-
tively (Scheme 3, Equation 1). X-Ray diffraction analysis
confirmed the trans disposition of the two acetato ligands in
these rare examples of non-cyclometallated bis(acetate)gold-
[6g,13]
(
III) complexes (Scheme 3).
Interestingly, 35 reacted with
p-xylene to give 1 in 59% yield whereas Ph PAuC F (33) and
3
6 5
p-xylene reacted in the presence of PhI(OAc) in 1,2-DCE at
2
9
08C to furnish 1 with similar efficiency (Scheme 3, Equa-
tions 2 and 3).
Scheme 4. Mechanistic proposal.
This intermediate is oxidized by PhI(OAc) in the second step
2
to form a trans-bis(acetate)arylgold(III) complex (C). Acti-
vation of the arene by an S Ar process to form a bis(aryl)gold-
E
[
18]
[19]
(
III) species (D) and subsequent reductive elimination
produces the heterocoupling product and regenerates the
gold(I) catalyst A. Selectivity in the reaction is thus achieved
by careful manipulation of the electronic properties of the
coupling partners and their specific reactivity with the gold
species in different oxidation states present in the reaction
media.
Finally, the complementarity of this method to those
[
7,10]
previously reported was also evaluated.
The competitive
arylation of 1,3-dimethoxy-benzene with two different elec-
tron-deficient aryl silanes and boronates under the standard
reaction conditions of Table 1 produced 88% yield of the
heterocoupling product 27, stemming from the boronic ester
while no reactivity with the arylsilane was observed
Scheme 3. Oxidation of arylgold(I) species to bis(acetato)gold(III)
complexes and stoichiometric experiments. The solid-state molecular
structures of 35 and 36 are shown with atoms drawn using 50%
probability ellipsoids.
(Scheme 5). For this and additional control experiments, see
section 4 in the Supporting Information.
In summary, an efficient cross-coupling of arenes with
strong electron-deprived aryl boranes has been developed.
Nonsymmetric biaryls can be synthesized with high levels of
regio- and chemoselectivity under reaction conditions devoid
of basic additives or directing groups. This methodology
shows orthogonal reactivity and complementary scope with
respect to already existing methods and does not require the
use of a large excess of any of the reaction partners.
In the latter case, no reactivity was observed in the
absence of PhI(OAc) , and is in line with the observation
2
made for either catalytic (Table 1, entry 11) or stoichiometric
reactions (Table 2, entry 1). These results are in line with
previous experiments, thus confirming the activation of
[
17]
electron-rich arenes by gold(III) species.
Furthermore,
I
III
the observed selectivities and reaction scope are in agreement
Mechanistic investigations uncovered a Au /Au catalytic
I
with an S Ar mechanism for the incorporation of the arene
process initiated by a B/Au transmetalation to transfer the
E
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Acknowledgments
We thank the European Research Council (ERC Starting
grant agreement no. 307948) and the Swiss National Science
Foundation (200020_146853) for financial support. We thank
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Cai, S. Sharma, J. Jirak, S. K. Thummanapelli, N. G. Akhmedov,
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134, 9012; c) D. Whitaker, J. Burꢁs, I. Larrosa, J. Am. Chem. Soc.
Manuscript received: October 25, 2016
2016, 138, 8384.
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Cross-Coupling
M. Hofer, A. Genoux, R. Kumar,
C. Nevado*
&&&& — &&&&
Gold-Catalyzed Direct Oxidative Arylation
with Boron Coupling Partners
Gold couple: A gold-catalyzed synthesis
of biaryls combining arenes and electron-
poor aryl boranes is presented here.
Under reaction conditions devoid of basic
additives or directing groups, this meth-
odology shows orthogonal reactivity and
complementary scope with respect to
existing methods. The high regio- and
chemoselectivity stems from the specific
reactivity of the reaction partners with
gold at different oxidation states and the
use of an acetato ligand as an internal
base.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!