Mechanistic and asymmetric investigations of the Au-catalysed cross-coupling between aryldiazonium salts and arylboronic acids using (P,N) gold complexes

Article abstract of DOI:10.1039/c8cc07530a

In order to explore the different mechanisms possibly occurring in the Au-catalysed cross-coupling of ArN₂BF₄ and ArB(OH)₂ in the presence of CsF, various stoichiometric experiments were performed on gold complexes with (P,N) ligands. Employing 2-pyridylphenyl-diphenylphosphine allowed us to suggest three different mechanistic pathways, starting either with a transmetallation step, via two consecutive single electron transfers, or by implying a transmetallation between Au(i) and Au(iii) species. Moreover, when using commercially available chiral (P,N) ligands, the asymmetric formation of atropoisomeric biaryls from suitable aryldiazonium salts and arylboronic acids could be achieved with e.e. up to 26%.

Full text of DOI:10.1039/c8cc07530a

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Mechanistic and asymmetric investigations
of the Au-catalysed cross-coupling between
aryldiazonium salts and arylboronic acids
using (P,N) gold complexes†
Cite this: Chem. Commun., 2018,
54, 12867
Received 17th September 2018,
Accepted 19th October 2018
Alexis Tabey, Murielle Berlande, Philippe Hermange * and Eric Fouquet*
DOI: 10.1039/c8cc07530a
rsc.li/chemcomm
In order to explore the different mechanisms possibly occurring in ensure the access to the Au(III) intermediates,7 or designing
the Au-catalysed cross-coupling of ArN2BF4 and ArB(OH)2 in the specific (P,P), (P,N) and (N,N) ligands to enable the oxidative
presence of CsF, various stoichiometric experiments were performed addition onto aryl halide.8 On the other hand, employing
on gold complexes with (P,N) ligands. Employing 2-pyridylphenyl- aryldiazonium salts as electrophiles9 also led to extremely
diphenylphosphine allowed us to suggest three different mechanistic fruitful results in gold-catalysed arylation reactions. Indeed,
pathways, starting either with a transmetallation step, via two con- upon thermal, light-induced, base-induced or photoredox-
secutive single electron transfers, or by implying a transmetallation induced10 activation, the aryl radical generated in situ from the
between Au(I) and Au(III) species. Moreover, when using commercially diazonium salt can produce the desired Au(III)-aryl species after
available chiral (P,N) ligands, the asymmetric formation of atropoiso- two single electron transfers (SET).11 These complexes, obtained
meric biaryls from suitable aryldiazonium salts and arylboronic acids under extremely mild conditions from simple Au(I) precursors, are
could be achieved with e.e. up to 26%.
able to engage in a wide variety of transformations,12,13 including
the synthesis of biaryl compounds from the cross-coupling of
The synthesis of biaryl compounds by transition metal cata- aryldiazonium salts (or derivatives) and arylboronic acids. Inter-
lysed reactions has been the subject of numerous research estingly, this reaction could be catalysed by either coordinatively
studies these last few decades, and various elements in the saturated or cationic Au species, and various conditions were
d-block were explored.1 Among them, Pd-based catalysts were reported (Scheme 1).14
undoubtedly the most successful species to enable the coupling
However, contrary to the well-established catalytic cycle for
of suitable aryl electrophiles and aryl nucleophiles with high palladium catalysed cross-couplings involving successive oxidative
efficiency and wide functional-group tolerance. Indeed, the addition, transmetallation and reductive elimination steps,1,2
palladium atom is easily able to undergo changes of its oxida- mechanistic pathways leading to intermediate A, which undergoes
tion state, and its catalytic activity can be finely tuned by the reductive elimination,15 are still unclear in these gold-catalysed
coordination to specifically designed monodentate/bidentate aryl–aryl cross-couplings. Based on stoichiometric 1H and 31P NMR
ligands.2 In contrast, the use of gold catalysts for the synthesis studies, transmetallation was proposed to be the first step in
of aryl–aryl cross-coupling products has remained extremely ref. 14a and b (catalytic cycle I). In contrast, ref. 14d and e
challenging for a long time.3,4 Indeed, their reluctance to suggested that the double SET occurred first (catalytic cycle II),
engage in Au(I)/Au(III) catalytic cycles has been seen as a barrier taking into account the modelling experiments on related gold-
for such reactions,5 and most of the research studies in homo- catalysed transformations involving aryldiazonium salts16 and
geneous gold catalysis were focused on employing its carbo- literature reports on the stoichiometric synthesis of aryl–Au(III)
philic Lewis acid properties for pi-activation and cascade
reactions.6 However, new strategies appeared recently to over-
come this issue, for example using a strong external oxidant to
´
Univ. Bordeaux, Institut des Sciences Moleculaires, UMR-CNRS 5255,
´
351 Cours de la Liberation, 33405 Talence Cedex, France.
E-mail: philippe.hermange@u-bordeaux.fr, eric.fouquet@u-bordeaux.fr
† Electronic supplementary information (ESI) available: General experimental
procedures and analysis/spectroscopic data (1H NMR, 13C NMR, 31P NMR and
HPLC chromatograms). CCDC 1843202. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c8cc07530a
Scheme 1 Reported gold-catalysed cross-coupling between aryldiazonium
salts (or arylazosulfones) with arylboronic acids. For the exact conditions,
see ref. 14.
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complexes.11 An in-between situation was described by Lee et al.,
whose NMR studies were in favour of either transmetallation first
with PPh3AuNTf2 or oxidation first with PPh3AuCl, thus showing
the complexity of mechanism determination in this reaction.
Finally, despite the evident analogy with the well-known [Pd]- or
[Ni]-catalysed syntheses of atropoisomeric biaryls,17 possible asym-
metric induction with a gold-based system has not been described
yet to our knowledge, but could open a new area of research in
this field.
In view of these observations, we decided to perform
stoichiometric experiments to investigate mechanistic and
asymmetric possibilities of our conditions of gold-catalysed
cross-coupling between aryldiazonium salts and arylboronic
acids.14b Some of our preliminary experiments with PPh3AuCl
as a precursor were unsuccessful,14b and all the isolated
aryl–Au(III) species reported in the literature contained a
supplementary ligand, i.e. a chloride ion when starting from
aryldiazonium chloride,11b,e or pyridine moieties present either
on the aryldiazonium tetrafluoroborate11d or on the phosphine
ligand.11a,c Thus, 2-pyridylphenyl-diphenylphosphine L1 was
chosen as the preferential ligand for stoichiometric studies,
as the catalytic ability of L1-AuCl in the cross-coupling reaction
could be confirmed (39% yield of product 3a, Scheme 2(A)).
Then, transmetallation of this model gold chloride complex
with phenylboronic acid 2a in the presence of two equivalents
of CsF was explored following pathway I, and pleasingly it led to
the desired L1AuPh (B1) with an excellent isolated yield of
94%.18 Interestingly, adding one equivalent of diazonium 1a to
an acetonitrile solution of B1 under photoredox conditions
produced the cross-coupling biaryl compound 3a in 30% yield
(Scheme 2(B), pathway I), i.e. in a comparable yield to the one
obtained employing catalytic amounts of the gold precursor
(39%, Scheme 2(A)). Thus, as previously demonstrated with
Ph3AuCl,14b this would suggest that Cycle I could operate in the
catalytic version of this reaction. Moreover, when L1AuCl was
reacted directly with 4-methoxybenzenediazonium salt 1a
Scheme 2 Catalytic and stoichiometric experiments of the cross-coupling
between 4-methoxybenzenediazonium tetrafluoroborate 1a and phenyl-
boronic acid 2a using L1AuCl as the gold precursor, and possible catalytic
cycles.
under blue light irradiation with Ru(bpy)3(PF6)2 as the photo- but unfortunately, it led only to unusable complex mixtures
catalyst, the Au(III)-complex E1 was also isolated with an excel- and/or degradation.
lent yield of 94% (Scheme 2(B), pathway II). Interestingly, the
In a second part, the use of chiral catalysts was investigated
31P NMR signal at 30.0 ppm was in accordance to a trans in combination with more hindered starting materials to
configuration between the chloro ligand and the phosphorous potentially produce enantioenriched atropoisomeric biaryl
atom as reported previously in the literature.11b Subsequently, compounds.
E1 was reacted with phenylboronic acid and cesium fluoride
The commercially available ligand (R)-(Sp)-(À)-PFNMe L2
under the exact transmetallation conditions of pathway I, and was firstly chosen for its established ability to produce good
3a could be isolated in 35% yield in this case,19 therefore enantioselectivities in Pd-catalysed atropoisomeric biaryl
suggesting that Cycle II could also be a viable mechanism. couplings.20 Pathway I was considered, and transmetallation
Finally, we hypothesised that Au(I)–Au(III) ligand exchange of 2b with L2AuCl was achieved smoothly. Indeed, the Au(I)
might occur during the course of the reaction, and the reaction complex B2 was isolated in good crude yields (70–96%, see the
of L1AuPh (B1) with E1 was investigated. Interestingly, along ESI†) under the conditions previously developed, i.e. with two
with the homo-coupling product 4a resulting from the decom- equivalents of cesium fluoride at 50 1C in acetonitrile for 16 h.
position of E1 under the reaction conditions, the desired cross- Then, the reaction of this compound with one equivalent of the
coupling product 3a was obtained in 15% yield, confirming the diazonium salt 1b under photoredox conditions provided the
possibility of a still undescribed third catalytic cycle (Cycle III). desired biaryl product 3b with isolated yields ranging from 28%
To gain further insight into the precise intermediates involved to 41%, corresponding to a mean yield of 33% over the four
catalytically, we attempted to prepare the corresponding experiments. Unfortunately, a very low asymmetric induction
cationic and fluoride gold species for these three pathways, was measured for 3b using chiral HPLC (mean enantiomeric
12868 | Chem. Commun., 2018, 54, 12867--12870
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Table 1 Screening of ligands and substrates in the asymmetric gold-
catalytic cycle III and decomposition pathways of E2, we
decided to focus solely on the fully reproducible pathway I for
the other experiments. (P,N) Ligands with chiral oxazoline
moieties were investigated and L3AuCl was prepared,21 which
produced single crystals suitable for X-ray diffraction analysis
(Table 1).§ Interestingly, the crystal structure confirmed the
expected absence of nitrogen coordination in this Au(I) complex.
Then, the stoichiometric biaryl couplings were realised starting
from this compound, and the yield of 3b obtained was similar to
the one with L2AuCl for pathway I (42%, entry 3), but led to a
slightly higher enantiomeric excess of 12%. Ligand L3 was selected
to further investigate the substrate effects, which quickly appeared
to be the determinant. Indeed, changing only the aryldiazonium
salt to 2-methoxybenzenediazonium tetrafluoroborate 1c resulted
in an important decrease in both the yield and enantioselectivity.
For instance, the desired biaryl 3c was isolated in only 23% yield,
as an almost racemic mixture (entry 4). The synthesis of 3d
mediated by L3AuCl was also explored, starting either from
1d/2d or 1e/2e reagent couples (Table 1, entry 5 and entry 6,
respectively). Unfortunately, all isolated yields and measured e.e. of
3d were relatively low (inferior to 10%). Finally, the best results
were obtained when employing 1b, 2b and AuCl coordinated by
(Sp)-1-(diphenylphosphino)-2-[(4S)-4-isopropyl-2-oxazolyl]-ferrocene
L322 as starting materials, the desired biaryl product 3b being
isolated in a good 44% yield over two steps, and with an improved
enantiomeric excess of +26% in this case (Table 1, entry 7).
To conclude, investigations on the gold-catalysed cross-
coupling of aryldiazonium tetrafluoroborate salts and arylboro-
nic acids in the presence of cesium fluoride were realised using
(P,N) gold complexes. In a first part, stoichiometric experi-
ments employing 2-pyridylphenyl-diphenylphosphine L1 as a
ligand for AuCl allowed us to propose three catalytic cycles
from the same precatalyst, which demonstrated a mechanistic
peculiarity of gold when compared to Pd-catalysed aryl–aryl
cross couplings. In a second part, preliminary stoichiometric
experiments proved that Au(I) catalysts could induce the
asymmetric formation of atropoisomeric biaryl compounds
from suitable aryldiazonium salts and arylboronic acids with
e.e. up to 26% when coordinated to chiral (P,N) structures.
Due to the high substrate dependence of the outcome of the
reaction, thorough research studies will be probably needed to
achieve a widely applicable method. However, we believe that
this work could open the way for further applications of chiral
mediated cross-coupling of aryldiazonium salts and arylboronic acidsa
Entry Path
L
1
2
3
Yielda (%)
e.e.b (%)
1
2
3
4
5
6
7
I
II
I
I
I
I
I
L2 1b 2b 3b 33 Æ 6c (n = 4) +5 Æ 2c (n = 4)
L2 1b 2b 3b 9 Æ 13c (n = 6) À8 Æ 0c (n = 2d)
L3 1b 2b 3b 42
L3 1c 2b 3c 23
L3 1d 2d 3d o10e
+12
À3
ndf
+3
L3 1e 2e 3d
5
L4 1b 2b 3b 44
+26
Conditions A: arylboronic acid (1–1.5 equiv.), CsF (2 equiv.), CH3CN,
50 1C, 2–72 h; conditions B: aryldiazonium tetrafluoroborate (1 equiv.),
Ru(bpy)3PF6 (2 mol%), blue LED irradiation, 30 1C, 2–3 h. After step 1,
the mixture was evaporated, filtered on celite, evaporated under
reduced pressure and triturated in Et2O or pentane to give the crude
product, which was engaged without further purification in step 2 after
NMR analyses. For the exact conditions, see the ESI. Mercury drawing of
one of the structure of L3AuCl obtained by X-ray diffraction analysis
(50% thermal ellipsoids).a Yield of product 3 over the two steps after
b
isolation via preparative TLC on silica gel. Determined by chiral HPLC
(measurement error of Æ2%). The plus sign was arbitrarily attributed if
the major enantiomer of 3 corresponded to the HLPC pic with the lower
c
retention time, and the minus sign in the opposite case. Mean Æ
d
standard deviation of n experiments. Calculated for the two success-
e
ful reactions out of six performed. Product 3d was isolated in a
mixture with multiple side-products, and a second purification by
f
preparative TLC did not improved the purity of the sample. nd: not
determined.
excess of +5 Æ 2%, n = 4, Table 1, entry 1). On the other hand, (P,N) ligands in gold-catalysed reactions implying Au(III) inter-
directly adding 1b to L2AuCl in acetonitrile in the presence of mediates in their catalytic cycles.7,8,11–13,23 Such possibilities
2+
2 mol% of the Ru(bpy)3 as a photocatalyst under blue LED are currently being investigated by our team and will be
irradiation leads to the major formation of the gold complex reported in due time.
E2, characterised by its signal at 20.6 ppm in 31P NMR.‡
This study was supported by a public grant from the French
However, a lack of reproducibility was observed for step 2 in Agence Nationale de la Recherche (AAP JCJC 2015), referenced
this case, as the desired product 3b could only be isolated for ANR-15-CE18-0015-01 and named SuCarB. Analyses were per-
two of the six experiments, despite a full conversion of the formed at the CESAMO (UMR 5255).
starting complex observed in all cases (Table 1, entry 2). A mean
yield of only 9% was calculated (n = 6), and a low e.e. was again
measured (À8 Æ 0%, n = 2), which precluded compelling
Conflicts of interest
mechanistic conclusions. As this poor reproducibility might be
due in these conditions to the competition between pathway I, There are no conflicts to declare.
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‡ A second minor peak was detected at 13.2 ppm by 31P NMR (o10%).
The exact structure of this side-product could not be clearly deter-
mined. See the ESI† for details.
§ Vapor diffusion method employing dichloromethane and diethyl
ether as solvents. Crystallographic data for this structure have been
deposited with the Cambridge Crystallographic Data Centre as supple-
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12870 | Chem. Commun., 2018, 54, 12867--12870
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