Bismuth-Catalyzed Oxidative Coupling of Arylboronic Acids with Triflate and Nonaflate Salts

Article abstract of DOI:10.1021/jacs.0c05343

Herein we present a Bi-catalyzed cross-coupling of arylboronic acids with perfluoroalkyl sulfonate salts based on a Bi(III)/Bi(V) redox cycle. An electron-deficient sulfone ligand proved to be key for the successful implementation of this protocol, which allows the unusual construction of C(sp2)-O bonds using commercially available NaOTf and KONf as coupling partners. Preliminary mechanistic studies as well as theoretical investigations reveal the intermediacy of a highly electrophilic Bi(V) species, which rapidly eliminates phenyl triflate.

Full text of DOI:10.1021/jacs.0c05343

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Communication
Bismuth-Catalyzed Oxidative Coupling of
Arylboronic Acids with Triflate and Nonaflate Salts
Oriol Planas, Vytautas Peciukenas, and Josep Cornella
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05343 • Publication Date (Web): 14 Jun 2020
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Bismuth-Catalyzed Oxidative Coupling of Arylboronic Acids with
Triflate and Nonaflate Salts
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Oriol Planas, Vytautas Peciukenas, Josep Cornella*
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, 45470, Germany.
Supporting Information Placeholder
ABSTRACT: Herein we present a Bi-catalyzed cross-coupling of
arylboronic acids with perfluoroalkyl sulfonate salts based on a
Bi(III)/Bi(V) redox cycle. An electron-deficient sulfone ligand
proved to be key for the successful implementation of this protocol,
which allows the unusual construction of C(sp2)‒O bonds using
commercially available NaOTf and KONf as coupling partners.
Preliminary mechanistic studies as well as theoretical investiga-
tions reveal the intermediacy of a highly electrophilic Bi(V) spe-
cies, which rapidly eliminates phenyl triflate.
Functional groups such as the trifluoromethanesulfonate (triflate,
OTf) or the nonafluorobutanesulfonate (nonaflate, ONf) are highly
useful moieties when present in organic compounds, especially
when attached to a carbon atom (C‒OTf or C‒ONf).1 Indeed,
C(sp2)‒OTf and C(sp2)‒ONf have been utilized as surrogates of
aryl halides (aka pseudohalides) due to their ability to heavily po-
larize the C‒O bond, facilitating oxidative addition by d-block met-
als. This strategy has been largely exploited with a myriad of com-
binations of both transition metals and coupling partners,2 thus
placing aryl triflates and nonaflates as routine electrophiles in this
large arena.3 From the organometallic standpoint, OTf anions have
also many attractive features. The coordinating properties of triflate
anions have been a matter of intense debate in the recent literature.4
However, it is evident that differently than Ar‒Cl, oxidative addi-
tion complexes of Ar‒OTf would result in a remarkably weaker in-
teraction of the OTf anion and the metal center in solution. Further-
more, in polar and coordinating solvents the OTf anion is generally
relegated to the outer sphere, leaving a vacant coordination site
(Figure 1A), which has been exploited for a variety of organome-
tallic and coordination purposes.5 Yet, the great attributes of OTf
anions ‒ highly electronegative, poor nucleophiles and labile lig-
ands ‒ inherently situates them as one of the foulest anions to un-
dergo C‒O reductive elimination.6
Figure 1. (A) OTf anions as ligands in transition metal chemistry; (B) cat-
alytic Ar‒OTf formation through a Bi(III)/Bi(V) redox system.
Our group has recently started a program to study the catalytic
redox properties of bismuth (Bi) complexes,10 to facilitate transfor-
mations beyond the reactivity of transition metals.11 Hence, based
on the known oxophilicity of Bi complexes12 and their ability to
bind triflate,13 we envisaged that an oxidative protocol based on the
redox couple Bi(III)/Bi(V) could fulfill this synthetic challenge. In-
deed, a decade ago Mukaiyama and co-workers demonstrated that
C‒OTf bonds could be forged from Bi(V) compounds and HOTf,
albeit in low yields.14 Inspired by this early precedent, herein we
report on a catalytic oxidative coupling between arylboronic acids
and triflate salts to furnish Ar‒OTf species (Figure 1B). A ration-
ally designed Bi complex bearing an electron-deficient diaryl-
sulfone ligand unlocks a catalytic redox process and enables the use
of triflate (OTf) and nonaflate (ONf) salts as coupling partners. Pre-
liminary mechanistic investigations and theoretical analysis re-
vealed that the C(sp2)‒O bond formation is extremely fast from
Bi(V), and is suggested to proceed through a 5-membered transi-
tion state.
Many examples with high-valent transition metals have been re-
ported to accommodate OTf anions in the primary coordination
sphere.7 However, reductive elimination primarily occurred at
other anionic ligand sites and the M‒OTf bond remained unal-
tered.8 During the synthesis of tri-substituted olefins, Gaunt and co-
workers suggested that C(sp2)‒OTf bonds could be formed through
an unusual reductive elimination from a Cu(III) center,9 although
further evidence was not provided. Indeed, examples of well-de-
fined transition metal complexes that forge C(sp2)‒OTf bonds still
remain elusive. Notwithstanding, the development of a catalytic
protocol which enables the formation of Ar‒OTf from the corre-
sponding organometallic reagent (Ar‒M) and a commercially
available triflate salt (MOTf) would be highly desirable both from
the synthetic and fundamental point of view.
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We started our investigations by optimizing the coupling of phe-
boronic acids bearing unsaturated moieties boded well in this meth-
odology, as exemplified by the presence of alkynyl (2p) and vinyl
(2q) groups. In spite of the large variety of arylboronic acids ame-
nable for this transformation, modest yields were obtained in the
presence of certain functionalities. Due to the high reactivity to-
wards oxidation with [Cl2pyrF]BF4, fluorene derivative 2r was ob-
tained in 38% yield.15 Substrates bearing strong electron-withdraw-
ing groups such as CF3 (2s) and reactive carbonyl functionalities at
the para-position (2t and 2u) also struggled to undergo C‒O bond
formation, demonstrating some limitations to the scope of this re-
action.
nylboronic acid (1a) with NaOTf to generate phenyltriflate (2a)
(Table 1). Based on our previous studies on Bi-catalyzed fluorina-
tion, bismines featuring a diarylsulfone backbone (4a-c) were se-
lected as catalysts,15 together with N-fluoro-2,6-dichloro-
pyiridinium tetrafluoroborate ([Cl2pyrF]BF4) as oxidant. To pro-
mote transmetalation, we selected K2CO3 as it has been recently
demonstrated to be an excellent base for this purpose.16 In our ini-
tial attempts, the unsubstituted bismine catalyst (4a) provided no
reactivity towards 2a (entry 1). However, when a CF3 group was
introduced in meta-position to the Bi (4b), an encouraging 11% of
2a was obtained; interestingly, the formation of protodeboronation
byproduct 3 was largely suppressed (entry 2). In line with these re-
sults, when two CF3 are introduced in the backbone of the sulfone
(4c) the reactivity towards 2a increased to 32%, while the for-
mation of 3 was still largely reduced (entry 3). When K2CO3 is re-
placed by NaF, a reversed trend in the product distribution is ob-
served, substantially favoring undesired 3 (entry 4). Surprisingly,
addition of 4Å molecular sieves (MS) boosted the formation of 2a
to 54% yield, while formation of 3 was still minimized (entry 5).
Remarkably, when K2CO3 was replaced by the weaker Na3PO4,
nearly quantitative formation of 2a was achieved (entry 6). The use
of 5Å MS proved crucial to completely suppress the formation of
3, thus obtaining the desired 2a in >95% yield (90% isolated) (entry
7). Unfortunately, lower catalyst loadings resulted in poor yields
(entry 8).
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Table 2. Scope of the Bi-catalyzed oxidative coupling of arylboronic acids
and sodium triflate.a
Table 1. Optimization of the reaction conditions.a
aReaction conditions: 1 (0.3 mmol), NaOTf (0.33 mmol), 4c (0.03 mmol),
[Cl2pyrF]BF4 (0.33 mmol), Na3PO4 (0.6 mmol) and 5Å MS (120 mg) in
aReactions performed at 0.025 mmol of 1a. Yields determined by 19F NMR
using 1-fluoro-4-nitrobenzene as internal standard. b Isolated yield of pure
material of a reaction performed at 0.3 mmols of 1a.
b
CHCl3 at 60 °C for 16 h. Yields of isolated pure material. Reaction per-
formed at 90 °C with 2.0 equiv. of NaF as base. cYields determined by 19
F
d
NMR using 1-fluoro-4-nitrobenzene as internal standard. Reactions per-
e
formed at 0.025 mmol of the corresponding arylboronic acids. Reaction
performed at 90 °C with 4.0 equiv. of Na3PO4 as base.
With the optimal conditions in hand, the scope of the Bi-cata-
lyzed C(sp2)‒OTf bond reaction was investigated using a variety of
arylboronic acid derivatives (Table 2). The methodology boded
well with Me groups in both para- (2b) and ortho-positions (2c).
Remarkably, when the steric encumbrance at the ortho-position
was further increased, excellent yields of the corresponding triflate
were obtained (2d and 2e). Furthermore, the presence of alkyl moi-
eties in other positions of the aryl ring did not affect the reactivity
(2f and 2g). The protocol accommodates various functional groups,
including ethers (2h and 2i) and halogens (2j, 2k, and 2l), albeit in
moderate yields. Arylboronic acids substituted with a trimethylsilyl
group (TMS), Ph or an ester at the para-position afforded good to
excellent yields of the corresponding aryl triflates (2m - 2o). Aryl-
Having established a protocol for the successful coupling of
NaOTf, we turned our attention to the use of less nucleophilic no-
naflate salts as coupling partners. A brief re-examination of the re-
action parameters revealed bismine nonaflate 4d as the catalyst of
choice to couple arylboronic acids with commercially available
KONf.15 With the optimized conditions shown in Table 3, Ph‒ONf
(5a) was isolated in a satisfactory 97% yield. The various aryl-
boronic acids scrutinized in the nonaflate synthesis revealed com-
parable reactivity to NaOTf. Aryl nonaflates containing ortho-sub-
stituents such as Br (5b) and Me (5c) were obtained in excellent
yields. Furthermore, a TMS moiety can also be accommodated to
the protocol (5d) as well as unsaturated alkynyl functionalities (5e).
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Journal of the American Chemical Society
has been previously observed in high-valent Cu cross-couplings,24
suggesting a concerted reductive elimination through the metal.
Table 3. Scope of the Bi-catalyzed oxidative coupling of arylboronic acids
and potassium nonaflate.a
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aReaction conditions: 1 (0.3 mmol), KONf (0.33 mmol), 4d (0.03 mmol),
[Cl2pyrF]BF4 (0.33 mmol), Na3PO4 (1.2 mmol) and 5Å MS (120 mg) in
CHCl3 at 60 °C for 16 h. Yields of isolated pure material.
The unprecedented catalytic C‒OTf and C‒ONf bond forming re-
action using 4c and 4d led us to explore the operative mechanism
governing this transformation. First, we interrogated the transmeta-
lation step between 1a and 4c (Figure 2A). When the reaction was
performed in the presence of Na3PO4 and 5Å MS, transmetalation
occurred efficiently and 6 was obtained in 86% yield (entry 1). In
the absence of base, 6 was also obtained in slightly lower yields
(62%, entry 2). In sharp contrast, when the reaction was performed
without MS (entry 3), formation of 6 was dramatically reduced
(21%). In absence of both MS and Na3PO4 6 was not detected.
These results demonstrate the importance of molecular sieves in
this transformation, not only as dehydrating agent,17 but also as po-
tential heterogeneous Brønsted base,18 promoting transmetalation
to the Bi(III) center. At this point, the oxidation-reductive elimina-
tion sequence from phenylbismine 6 was studied utilizing different
oxidants and triflate sources (Figure 2B, top). After oxidizing 6
15
with XeF2 to the high-valent Bi(V) difluoride species,10b,
TMSOTf was added, resulting in a rapid color change from pale to
dark yellow. Analysis of the reaction crude revealed quantitative
formation of 2a. This result points to the formation of a highly elec-
trophilic Bi(V) intermediate (7a) bearing a OTf moiety, as a conse-
quence of fluoride abstraction by TMSOTf. Indeed, when TMSOTf
was added at –41 °C, intermediate 7a could be detected by HRMS
(Figure 2B). Furthermore, using [Cl2pyrF]BF4 as oxidant together
with NaOTf similar yields for 2a were obtained (92%, Figure 2B,
bottom). It is important to mention that only trace amounts of
fluorobenzene were detected, which shows the preferential for-
mation of C‒OTf over C‒F bonds (vide infra).19 Related interme-
diates have been previously postulated by Mukaiyama, in the
C(sp2)‒OTs coupling from Bi(V) intermediates.14 Based on these
experimental results, preliminary theoretical studies were per-
formed to investigate a putative reductive elimination from 7, bear-
ing both a F (7a) or a OTf (7b) as counterions.15 As shown in Figure
2C, two possible scenarios were postulated. On one hand, reductive
elimination can occur through a 3-membered transition state (Fig-
ure 2C, pathway a), reminiscent of concerted reductive elimina-
tions performed by d-block elements. Alternatively, reductive elim-
ination could also occur via a 5-membered transition state (Figure
2C, pathway b), where two oxygens of the OTf are involved. This
latter hypothesis has been previously invoked to explain the selec-
tivity of Bi-mediated couplings such as -arylation of phenols20
and N-arylation of pyridones,21 among other transformations.22 In
accordance with these previous reports, our theoretical analysis
predicts that the 5-membered TS2 is slightly favored over TS1,
pointing towards TS2 as the preferable pathway for the C‒O bond
forming event. NBO analysis on the Bi center also provided addi-
tional information about this process.23 In the case of 7a, the NBO
charge on the Bi decreases from 2.17 to 1.84 in TS1 and 1.88 in
TS2; and is further reduced to 1.50 in 8. The same trend is observed
from 7b.15 This progressive change in charge at the metal center
Figure 2. (A) Study of the transmetalation step: influence of the molecular
sieves and the base; (B) Stoichiometric sequence of oxidative addition –
reductive elimination; (C) Theoretical analysis of the C‒O bond forming
step. aYields determined by 1H NMR using 1,3,5-trimethoxybenzene as in-
ternal standard. bYields determined by 19F NMR using 1-fluoro-4-nitroben-
zen as internal standard.
Taken these results together, the reaction is proposed to follow
the catalytic cycle depicted in Figure 3. Initially, bismine A under-
goes transmetalation (TM) with the corresponding arylboronic
acid, thus forming aryl bismine B. Subsequently, B undergoes for-
mal oxidative addition (OA) with [Cl2pyrF]BF4, furnishing the pro-
posed high-valent Bi(V) intermediate C. Reductive elimination
(RE) from C delivers the desired aryl triflate with concomitant re-
generation of A. Due to the structural similarities between OTf and
ONf, we believe that a similar mechanism is operating for the cou-
pling of the latter.
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Figure 3. Postulated mechanism for the Bi-catalyzed oxidative coupling of
arylboronic acids and triflate salts.
In summary, an unprecedented oxidative coupling of arylboronic
acids with triflate and nonaflate salts has been developed exploiting
the reactivity of the Bi(III)/Bi(V) redox couple. A highly electron
withdrawing diarylsulfone ligand unlocked a catalytic process
which proceeds under mild conditions and accommodates various
functional groups. The results presented in this study unveil bis-
muth redox catalysis as a promising tool to perform transformations
beyond the scope of transition metals, while mimicking their fun-
damental organometallic steps.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, analytical data (1H, 19F, 11B and 13C
NMR, HRMS) for all new compounds, computational results, in-
cluding Tables (S1-S10) and Figures (S1-S20). This material is
available free of charge via the internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*cornella@kofo.mpg.de
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
Financial support for this work was provided by Max-Planck-Ge-
sellschaft, Max-Planck-Institut für Kohlenforschung, Fonds der
Chemischen Industrie (FCI-VCI). This project has received fund-
ing from European Union’s Horizon 2020 research and innovation
programme under the agreements No. 850496 (ERC Starting Grant,
J. C.) and No. 833361 (Marie Skłodowska Curie Fellowship, O. P.).
We thank Prof. Dr. A. Fürstner for insightful discussions and gen-
erous support. We also thank Dr. Kalishankar Bhattacharyya and
Dr. Dimitrios Pantazis for insightful suggestions and support in the
computational studies, as well as the analytical department at the
MPI-Kohlenforschung for support in the characterization of com-
pounds.
(8) (a) Bour, J. R.; Camasso, N. M.; Sanford, M. S. Oxidation of Ni(II)
to Ni(IV) with Aryl Electrophiles Enables Ni-Mediated Aryl–CF3 Cou-
pling. J. Am. Chem. Soc. 2015, 137, 8034‒8037. (b) Camasso, N. M.; San-
ford, M. S. Design, synthesis, and carbon-heteroatom coupling reactions of
organometallic nickel(IV) complexes. Science 2015, 347, 1218‒1220. (c)
Canty, A. J.; Ariafard, A.; Camasso, N. M.; Higgs, A. T.; Yates, B. F.; San-
ford, M. S. Computational study of C(sp3)‒O bond formation at a PdIV cen-
tre. Dalton Trans. 2017, 46, 3742‒3748. (d) Nebra, N. High-Valent NiIII and
NiIV Species Relevant to C–C and C–Heteroatom Cross-Coupling Reac-
tions: State of the Art. Molecules, 2020, 25, 1141.
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