Enantioselective aryl-aryl coupling facilitated by chiral binuclear gold complexes

Article abstract of DOI:10.1039/c9cc07175j

Herein, we report stoichiometric investigations embodying the first highly enantioselective aryl-aryl coupling facilitated by a gold complex. With up to 91% ee, this is the first demonstration of a transmetalation and C(sp²)-C(sp²) r

Full text of DOI:10.1039/c9cc07175j

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Enantioselective aryl–aryl coupling facilitated
by chiral binuclear gold complexes†
Cite this: Chem. Commun., 2019,
55, 12988
Jonas Himmelstrup,a Mikkel B. Buendia,a Xing-Wen Sun
and Søren Kramer
*
b
a
Received 13th September 2019,
Accepted 5th October 2019
DOI: 10.1039/c9cc07175j
rsc.li/chemcomm
Herein, we report stoichiometric investigations embodying the first cross-coupling between arylboronic esters and electron-rich
highly enantioselective aryl–aryl coupling facilitated by a gold complex. arenes.14 The same year, Bourissou et al. showed that cross-
With up to 91% ee, this is the first demonstration of a transmetalation coupling with aryl iodides is possible by using a P,N-ligand.15
and C(sp2)–C(sp2) reductive elimination sequence with high enantios- Finally, Xie et al. have reported that cross-coupling of two
electivity using a gold complex. The results offer a basis for development organometallic nucleophiles can be catalyzed by a binuclear
of enantioselective gold-catalyzed aryl–aryl coupling reactions.
gold complex bearing a PNP-type ligand.16 The ease of C–H
functionalization, mild reaction conditions, and especially the
Atropisomers can be found in natural products and they play an orthogonality to palladium-catalyzed cross-coupling reactions,
increasingly important role in medicinal chemistry.1 In addition, in terms of functional group tolerance (e.g. aryl halides) and
atropisomeric C2-symmetric ligands, such as the BINOL, BINAP, and regioselectivity, makes gold catalysis an appealing complementary
SEGPHOS ligand-families, have revolutionized asymmetric metal method for aryl–aryl coupling reactions.
catalysis.2 Other applications of atropisomers include BINOL-based
In spite of the intensive research efforts in gold-catalyzed
Brønsted acid catalysts for enantioselective Brønsted acid catalysis,3 aryl–aryl coupling reactions, there are still no examples of enantio-
and atropisomeric phosphines for enantioselective nucleophilic selective synthesis of atropisomers using oxidative gold catalysis.17
phosphine catalysis.4 Due to the high importance of atropisomers, In 2018, Hermange and Fouquet et al. reported the only example
their enantioselective synthesis has received considerable attention of a stoichiometric study toward this goal.18 By using stoichio-
in recent years.5,6
metric amounts of mononuclear gold complexes bearing chiral
Since the seminal report on gold-catalyzed homocoupling of P,N-ligands, they obtained the atropisomeric product in very
arenes by C–H functionalization by Tse et al. in 2008,7 the field modest 26% ee (44% yield).
of gold-catalyzed aryl–aryl coupling reactions has advanced
Herein, we report that by using binuclear gold complexes
tremendously (Scheme 1). In 2012, Lloyd-Jones and Russell bearing C2-symmetric chiral ligands, the atropisomeric biaryl
et al. reported the selective cross-coupling of arylsilanes with product can be obtained in up to 91% ee (70% yield). These
arenes in the presence of an external oxidant.8 In 2015, Larrosa stoichiometric investigations embody the first demonstration of
et al. developed a selective cross-coupling reaction taking place a highly enantioselective transmetalation and aryl–aryl reductive
through double C–H functionalization.9 The same year, You elimination sequence for a gold complex. The realization of high
et al. showed that by using a directing group, cross-coupling enantioselectivity for these elementary steps offers a basis for
between arylboronic acids and arenes is also possible.10 Based successful development of enantioselective gold-catalyzed aryl–
on a seminal publication from Glorius et al.,11 various gold- aryl cross-coupling reactions and the results can serve as a guide
catalyzed cross-coupling reactions using aryldiazonium salts for ligand choice for such reactions.19
under external oxidant-free conditions have been developed for
As part of our interest in asymmetric catalysis and catalysis
aryl–aryl bond formation.12,13 In 2017, Nevado et al. reported with group 11 metals, we set out to investigate the possibility for
enantioselective aryl–aryl coupling reactions with gold complexes.20
Inspired by a report from 2014 by Toste and coworkers,21 we
hypothesized that enantioselectivity could be achieved for an
a Department of Chemistry, Technical University of Denmark, 2800 Kgs. Lyngby,
Denmark
aryl–aryl coupling reaction by the introduction of appropriate
b Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433,
chiral bisphosphine ligands on gold.22,23
China. E-mail: sokr@kemi.dtu.dk
We initiated the investigation by subjecting (S)-BINAP(AuCl)2
to 2-methoxynaphthaleneboronic acid (1) and CsF in CH2Cl2 at
† Electronic supplementary information (ESI) available: Experimental procedures,
NMR data and spectra, and HPLC traces. See DOI: 10.1039/c9cc07175j
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Table 1 Initial optimization of reaction conditions for enantioselective
aryl–aryl coupling mediated by a chiral gold complex
Entry
CsF (equiv.)
PhICl2 (equiv.)
Yielda (%)
eeb (%)
1
2
3
3
6
6
3
3
2
59
71
73
50
50
50
a
b
Based on 1H NMR relative to an internal standard. Determined by
chiral HPLC after purification.
Scheme 1 Milestones in development of gold-catalyzed aryl–aryl coupling.
Fig. 1 Ligands utilized for investigation of the aryl–aryl coupling reaction.
30 1C in order to facilitate double transmetalation to the gold
complex (Table 1). Subsequently, the reaction mixture was
cooled to À78 1C and PhICl2 added to oxidize the intermediate Introducing tolyl groups on the BINAP scaffold gave a significant
complex, (S)-BINAP(AuAr)2. Upon heating to room temperature, increase in enantioselectivity affording the product in 78% ee
the biaryl product forms by reductive elimination.21 By varying (entry 3). However, the use of a more sterically hindered BINAP
the amount of CsF and PhICl2, clean double transmetalation was ligand containing xylyl groups led to diminished reactivity and
observed by 31P NMR after 48 hours, and 73% yield obtained of enantioselectivity (entry 4). Other variations to the BINAP motif,
the aryl–aryl coupling product (Table 1, entry 3). Importantly, the (R)-H8-BINAP and (R)-SYNPHOS, gave 58 and 63% ee, respectively
biaryl coupling product was formed with 50% ee, which is (entries 5 and 6). Next, the MeO-BIPHEP ligand family was examined.
already the best enantioselectivity reported for aryl–aryl coupling The use of (R)-MeO-BIPHEP(AuCl)2 led to a faster transmetalation
with a gold complex.
than for any of the other complexes tested. Furthermore, the biaryl
In order to further optimize the enantioselectivity, a series of product 2 was obtained in 77% yield and 76% ee (entry 7). Increasing
gold complexes bearing chiral bisphosphine ligands were the sterical bulk led to markedly slower transmetalation and poor
synthesized (Fig. 1) and their potential for enantioselective yields, as observed for (R)-Tol-MeO-BIPHEP(AuCl)2 and (R)-Xyl-MeO-
aryl–aryl coupling evaluated (Table 2). First, it was confirmed BIPHEP(AuCl)2 (entries 8 and 9). (R)-C3-TUNEPHOS, which structu-
that the ligand controls the major enantiomer formed. The use rally resembles the MeO-BIPHEP ligand family, was also tested and
of (R)-BINAP(AuCl)2 leads to the opposite enantiomer of the afforded the product with 66% ee (entry 10). Finally, the SEGPHOS
biaryl product 2 compared to (S)-BINAP(AuCl)2 (entries 1 and 2).24 ligand class was examined. By using (R)-SEGPHOS(AuCl)2 as the gold
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Table 2 Influence on yield and enantioselectivity from different chiral
ligands
Entry
Ligand
Yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
(S)-BINAP
(R)-BINAP
(R)-Tol-BINAP
(S)-Xyl-BINAP
(R)-H8-BINAP
(R)-SYNPHOS
(R)-MeO-BIPHEP
(R)-Tol-MeO-BIPHEP
(R)-Xyl-MeO-BIPHEP
(R)-C3-TUNEPHOS
(R)-SEGPHOS
(R)-DM-SEGPHOS
(R)-DTBM-SEGPHOS
(R)-DIFLUORPHOS
73
64
50
À51
À78
49
À58
À63
À76
ND
À51
À66
À91
À71
À11
À76
59
40c
59
44
Fig. 2 31P NMR of the starting complex (top), the reaction mixture after
transmetalation (middle), and the reaction mixture after reductive elimination
(bottom).
77d
13c
15c
58e
70
9
10
11
12
13
14
39c
43
starting complex is cleanly reformed suggesting the potential
for development of a catalytic cycle (Fig. 2).
47
a
b
Based on 1H NMR relative to an internal standard. Determined by
After the oxidation at À78 1C, the reductive elimination is
facilitated by removing the dry ice-bath and allowing the reaction
to quickly heat to room temperature. The same yield and ee was
obtained for (R)-SEGPHOS(AuCl)2 when letting the reaction
mixture heat to room temperature very slowly (over 7 h). This
result suggests that there is little influence of the heating profile
c
chiral HPLC after purification. Additional ArB(OH2) (3 equiv.) and CsF
(6 equiv.) added after 48 h, and the transmetalation stirred for another
d
e
24 h. Transmetalation stirred for 24 h. Additional ArB(OH2) (1 equiv.)
and CsF (2 equiv.) added after 48 h, and the transmetalation stirred for
another 24 h. ND = not determined.
complex, the atropisomeric product 2 was formed in 91% ee and on the enantiodetermining step. In contrast, the enantioselec-
70% yield (entry 11).25 Similar to the trend observed for the BINAP tivity is sensitive to the solvent used as a reaction in a toluene :
and MeO-BIPHEP ligand families, the introduction of more sterical CH2Cl2 (7: 3) solvent mixture afforded the product with 76% ee,
bulk on the parent SEGPHOS ligand led to decreased yields and and a reaction in DCE (À30 1C to RT) led to 74% ee.
enantioselectivities as observed for both (R)-DM-SEGPHOS(AuCl)2
We hypothesize that the reactions follow the mechanism
and (R)-DTBM-SEGPHOS(AuCl)2 (entries 12 and 13). As the only elucidated by Toste et al. for the achiral dppp(AuCl)2 complex.21
one of the complexes examined, (R)-DTBM-SEGPHOS(AuCl)2 led to a After transmetalation with the arylboronic acid, the gold
very unclean transmetalation. The use of (R)-DIFLUORPHOS(AuCl)2, complex is oxidized by PhICl2 to a mixed-valence Au(I)–Au(III)
a fluoride-containing SEGPHOS derivative, led to 76% ee (entry 14). complex,30 which can undergo fast intramolecular transmetala-
Interestingly, the use of binuclear complexes appears crucial under tion placing both aryl moieties on one gold atom. Finally,
our reaction conditions as PPh3AuCl led to clean transmetalation reductive elimination forms the enantioenriched biaryl product
(6 h), but only 5% yield of biaryl 2 after the second step. Beneficial and regenerates (R)-SEGPHOS(AuCl)2. Currently, it is unclear
effects of binuclear complexes were also reported by Xie et al.16
whether the intramolecular transmetalation or aryl–aryl reduc-
The general trends for the 13 examined chiral gold com- tive elimination is enantiodetermining; however, none of these
plexes are that a significant increase of the steric bulk within a elementary steps have been reported as enantiodetermining
ligand class leads to slower, and in rare cases unclean, trans- before with a gold complex.31
metalation. The enantioselectivity does not benefit from the
In summary, we have demonstrated the first highly enantio-
increase in steric bulk. The best enantioselectivity was achieved selective aryl–aryl coupling mediated by a gold complex.
with the complex bearing the relatively cheap (R)-SEGPHOS By synthesizing and evaluating 13 binuclear gold complexes
ligand.26 The obtained 91% ee is even more notable, when bearing different C2-symmetric chiral ligands, we found that
keeping in mind that asymmetric gold catalysis is a very difficult the atropisomeric biaryl product (2) can be obtained in 91% ee
task with enantioselectivities reported for gold(III) catalysis in the and 70% yield by using the (R)-SEGPHOS(AuCl)2 complex.
range of 41–90% ee23,27 and for gold(I) catalysis, in many cases, At the end of the reaction, (R)-SEGPHOS(AuCl)2 is reformed.
around 80–95% ee.22,28
The results reported here indicate the potential for development
When monitoring the reaction with (R)-SEGPHOS(AuCl)2 by of highly enantioselective aryl–aryl coupling reactions catalyzed by
31P NMR, one major peak is observed at the end of the chiral gold complexes.
transmetalation (Fig. 2). This observation indicates that trans-
The authors are deeply appreciative of generous financial support
metalation with the arylboronic acid 1 proceeds with high from the Lundbeck Foundation (Grant No. R250-2017-1292) and the
diastereoselectivity.29 After the reductive elimination of 2, the Technical University of Denmark.
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14 M. Hofer, A. Genoux, R. Kumar and C. Nevado, Angew. Chem., Int.
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Conflicts of interest
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and D. Bourissou, Nat. Commun., 2017, 8, 565; (b) J. Rodriguez,
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stoichiometric study, see: (c) M. J. Harper, C. J. Arthur, J. Crosby, E. J.
Emmett, R. L. Falconer, A. J. Fensham-Smith, P. J. Gates, T. Leman,
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There are no conflicts to declare.
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29 High diastereoselectivities were also observed for other complexes
which ultimately led to lower enantioselectivity, e.g. (S)-BINAP(AuCl)2,
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