C-H Borylation Catalysis of Heteroaromatics by a Rhenium Boryl Polyhydride

Article abstract of DOI:10.1021/acscatal.1c00869

Transition metal complexes bearing metal-boron bonds are of particular relevance to catalytic C-H borylation reactions, with iridium polyboryl and polyhydrido-boryl complexes the current benchmark catalysts for these transformations. Herein, we demonstrate that polyhydride boryl phosphine rhenium complexes are accessible and catalyze the C-H borylation of heteroaromatic substrates. Reaction of [K(DME)(18-c-6)][ReH4(Bpin)(ν2-HBpin)(κ2-H2Bpin)] 1 with 1,3-bis(diphenylphosphino)propane (dppp) produced [K(18-c-6)][ReH4(ν2-HBpin)(dppp)] 2 through substitution of two equivalents of HBpin, and protonation of 2 formed the neutral complex [ReH6(Bpin)(dppp)] 3. Combined X-ray crystallographic and DFT studies show that 2 is best described as a σ-borane complex, whereas 3 is a boryl complex. Significantly, the boryl complex 3 acted as a catalyst for the C(sp2)-H borylation of a variety of heteroarenes (14 examples including furan, thiophene, pyrrole and indole derivatives) and displayed similar reactivity to the iridium analogues.

Full text of DOI:10.1021/acscatal.1c00869

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Research Article
C−H Borylation Catalysis of Heteroaromatics by a Rhenium Boryl
Polyhydride
Liam J. Donnelly, Teresa Faber, Carole A. Morrison, Gary S. Nichol, Stephen P. Thomas,*
and Jason B. Love*
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ABSTRACT: Transition metal complexes bearing metal−boron
bonds are of particular relevance to catalytic C−H borylation
reactions, with iridium polyboryl and polyhydrido-boryl complexes
the current benchmark catalysts for these transformations. Herein, we
demonstrate that polyhydride boryl phosphine rhenium complexes are
accessible and catalyze the C−H borylation of heteroaromatic
substrates. Reaction of [K(DME)(18-c-6)][ReH4(Bpin)(η2-HBpin)-
(κ2-H2Bpin)] 1 with 1,3-bis(diphenylphosphino)propane (dppp)
produced [K(18-c-6)][ReH4(η2-HBpin)(dppp)] 2 through substitu-
tion of two equivalents of HBpin, and protonation of 2 formed the
neutral complex [ReH6(Bpin)(dppp)] 3. Combined X-ray crystallo-
graphic and DFT studies show that 2 is best described as a σ-borane complex, whereas 3 is a boryl complex. Significantly, the boryl
complex 3 acted as a catalyst for the C(sp2)−H borylation of a variety of heteroarenes (14 examples including furan, thiophene,
pyrrole and indole derivatives) and displayed similar reactivity to the iridium analogues.
KEYWORDS: heteroarene, hydride, borane, phosphine, homogeneous catalysis, crystallography, AIRSS
by the exhaustive deoxygenation of perrhenate, [ReO4]−, with
INTRODUCTION
■
HBpin, and showed that it is a catalyst for the hydroboration of
The synthesis of transition metal complexes containing metal−
N-heteroaromatic substrates.29 Reaction of 1 with H-B-9-BBN
boron bonds is of particular interest due to the relevance of
resulted in reductive borane substitution to form a related
these complexes as intermediates in catalytic C−X (X = H or
dihydroborate complex. These simple methods provided
halogen) borylation to prepare synthetically valuable boronic
straightforward access to reactive rhenium complexes contain-
esters.1−4 The borylation of unactivated C(sp2)−H bonds can
ing boron-centered ligands that can be further functionalized.
be catalyzed by a number of transition metal boryl complexes
Interestingly, 1 also underwent stoichiometric C(sp2)−H
of Ir,5−10 Rh,6,11−13 Co,14−16 Fe,17−20 Ni,21−23 and Pt.24 The
borylation of toluene with high meta-selectivity; unfortunately,
most widely used catalysts are Ir complexes typically of the
attempts to adapt these conditions for catalytic C−H
form [Ir(Bpin)3(L)2], where L is a neutral monodentate or
borylation by addition of stoichiometric boron reagents or
bidentate ligand, and invoke an Ir(III)/Ir(V) redox couple.
sacrificial H2 acceptors were unsuccessful. It was, therefore,
The most commonly used catalyst systems use bulky
envisaged that strongly σ-donating, neutral ligands such as
bipyridine or phenanthroline ligands, first developed by
phosphines would help to stabilize the high oxidation state
Hartwig and co-workers.25,26 Smith and co-workers demon-
hydrido-boryl complexes involved in C−H bond activation and
strated the use of the bidentate phosphine complex, [Ir-
subsequent C−B bond formation. Rhenium-catalyzed C−H
(Bpin)3(dppe)], as a catalyst for the selective C−H borylation
borylation has, to the best of our knowledge, only been
of arenes in the presence of haloarenes; the catalyst was formed
reported using low oxidation-state complexes. For example,
in situ from the reaction of [(η5-indene)Ir(COD)] with dppe
Hartwig and co-workers reported the C−H borylation of
and HBpin.7 The related 5-coordinate complexes [Ir-
alkanes and arenes using Cp*Re(CO)3 and B2pin2 under
(Bpin)3(L)] (where L = dtbpe or dippe) have also been
shown to mediate C−H borylation of arenes, albeit stoichio-
metrically.27 Further studies of the stoichiometric reactivity of
Received: February 24, 2021
Revised: May 14, 2021
Published: June 7, 2021
[Ir(Bpin)3(dippe)] reveal a series of borylene, hydrido, and
hydrido boryl complexes that can be formed under catalytic
C−H borylation conditions.28
Recently, we reported the rhenium polyhydride borane
anion [ReH4(Bpin)(η2-HBpin)(κ2-H2Bpin)]− 1, synthesized
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photochemical conditions and an atmosphere of CO (Scheme
Scheme 2. Reaction of 1 with dppp to Produce the Neutral
Phosphine Complex 3 via the Anionic Complex 2
1A).30 Murai and Takai reported the C(sp3)−H borylation of
Scheme 1. Examples of Rhenium-Catalyzed Borylation of
C−H Bonds
The Re−B bond length is 2.188(3) Å, and B1−H1 distance is
1.69(3) Å, both of which are within the range of other high
oxidation-state, elongated σ-borane or hydrido boryl com-
plexes.32,33 The usual caveats of locating hydrides with
confidence from the X-ray structure apply here. As such,
possible hydride positions were identified using ab initio
random structure searching (AIRSS, see Supporting Informa-
tion for details) with the initial positions of the heavier atoms
derived from the crystal structure and geometric constraints on
prospective initial hydride positions to ensure H2 is not
formed.34,35 This approach generates a series of structures with
various combinations of hydride, boryl, σ-borane, and
hydroborate ligands that are then ranked in increasing energy
(Supporting Information). From a data set of 24 optimized
structures obtained, a series of 21 closely related, low-energy
structures (within 5 kJ mol−1) reveal a geometry that matches
the solid-state structure (Figure 1). Given the long B1−H1
tertiary amines at the α-position, facilitated by a pyridyl
directing group under thermal conditions using [ReBr-
(CO)3(THF)]2 as the catalyst and H-B-9-BBN as the boron
source (Scheme 1B).31 However, no examples of C(sp2)−H
borylation of arenes were reported using this catalyst system.
Herein, we report that the reaction of [K(DME)(18-c-
6)][ReH4(Bpin)(η2-HBpin)(κ2-H2Bpin)] 1 with dppp leads to
HBpin ligand substitution to give the Re(V) phosphine
complex [K(18-c-6)][ReH4(η2-HBpin)(dppp)] 2. Complex 2
reacts with 2,6-lutidinium chloride to give the neutral Re(VII)
boryl complex [ReH6(Bpin)(dppp)] 3, which acts as a catalyst
for the C−H borylation of heteroarenes (Scheme 1C). To the
best of our knowledge, this work represents the first example of
a high oxidation-state Re complex acting as a catalyst for this
reaction and draws comparisons to existing reactivity shown by
established iridium examples.
RESULTS AND DISCUSSION
■
An equivalent of bis(diphenylphosphino)propane (dppp) was
added to a solution of 1 in DME at −20 °C, which, on
warming slowly to room temperature and stirring for 72 h,
produces a single phosphine-ligated complex 2 with liberation
of two equivalents of HBpin. Complex 2 was then reacted with
1 equivalent of 2,6-lutidinium chloride, which results in
protonation without H2 evolution to form the neutral complex
1
[ReH6(Bpin)(dppp)] 3 (Scheme 2). The H NMR spectrum
of 3 displays a triplet at −5.57 ppm for 6 hydrides, and the 31
P
NMR spectrum exhibits a broad signal at 6.24 ppm. As with 2,
these resonances are unchanged at −80 °C. A broad signal at
47.3 ppm is seen in the 11B NMR spectrum of 3 and
corresponds to a Re-Bpin environment.
Single-crystal X-ray diffraction analysis of 3 shows the P and
B atoms arranged in a distorted trigonal planar geometry
(Figure 2). The hydrides were located in the difference Fourier
map and display an average Re−H bond length of 1.50(3) Å,
which is comparable to other terminal hydride bond lengths.
Figure 1. X-ray crystal structure (left) and calculated structure (right)
of 3 (toluene solvate and all hydrogen atoms except for the Re
hydrides are omitted; displacement ellipsoids are drawn at 50%
probability). X-ray crystal structure selected distances (Å) and angles
(deg): Re1−B1, 2.188(3); Re1−H, 1.50(3) avg.; B1−H1, 1.69(3);
Re1−P1, 2.4053(9); Re1−P2, 2.4106(9); P2−Re1−P1, 96.22(3);
P1−Re1−B1, 127.9(1); P2−Re1−B1, 135.9(1).
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distances in both the solid-state and calculated structures and
that the crystals of 3 are colorless, it is evident that 3 is best
described as a boryl hexahydride complex (i.e., ReVII).
difference to the X-ray structure is a closer K1−H4 contact
(2.956 Å compared with 3.19(5) Å). The calculated structure
contains an elongated B1−H1 distance compared with the
solid-state structure (1.522 Å), although this bond distance
remains in the range reported for σ-borane complexes and is
within one esd range of the X-ray structure (Figure 2).32,37 The
hydride positions are consistent with those from the X-ray
structure, indicating a high level of confidence for their
assignment.
The time course of the reaction between 1 and dppp was
monitored by NMR spectroscopy in d8-THF and revealed full
conversion of 1 to a monodentate complex (major) and a
bidentate complex (minor) after 2 h. The monodentate
1
complex corresponds to a doublet at −8.05 ppm in the H
NMR spectrum and a broad signal at 52.9 ppm in the 11B
NMR spectrum. This monodentate complex, then, slowly
converts to the bidentate complex, which is characterized by a
The ready synthesis of the well-defined Re(VII) boryl
complex 3 prompted its exploration as a catalyst for the
C(sp2)−H borylation of arenes (Scheme 3). Optimized
1
triplet at −8.16 ppm in the H NMR spectrum and a broad
signal at 53.8 ppm in the 11B NMR spectrum. Correlations
between the hydride signals and two signals at 24.5 and 11.6
Scheme 3. Substrate Scope for the Catalytic
Dehydrogenative C−H Borylation of Heteroarenes
1
ppm in the 31P NMR spectrum are observed in the H−31P
a
HMBC NMR spectrum. This suggests that the second
equivalent of HBpin is significantly less labile than the first.
A screen of phosphine ligands showed that the range of ligands
that can successfully and selectively coordinate to 1 is narrow
(Supporting Information).
When dimethoxyethane (DME) is used as the reaction
solvent, [K(18-c-6)][ReH4(η2-HBpin)(dppp)] 2 is isolated by
crystallization from the reaction mixture as large yellow blocks.
However, 2 is unstable once isolated from the reaction mixture
and decomposes rapidly upon drying under vacuum or at
ambient temperature/pressure to give multiple phosphine-
coordinated rhenium hydride complexes. This could be due to
dissociation of HBpin or H2 to form an unstable 16-electron
complex, which reacts unselectively with itself or with the
solvent. The X-ray crystal structure of 2 (Figure 2) reveals a
a
Reaction conditions: heteroarene 4 (0.50 mmol), HBpin (0.75
mmol), catalyst 3 (0.0125 mmol), THF (2.5 mL), 80 °C for 16 h.
NMR yields determined by 1H NMR spectroscopy by integration
against mesitylene (10 μL, 0.0719 mmol), isolated yields in
Figure 2. X-ray crystal structure (left) and calculated structure (right)
of 2 (all hydrogen atoms except for the Re hydrides are omitted;
displacement ellipsoids are drawn at 50% probability). X-ray crystal
structure selected distances (Å) and angles (deg): Re1−B1, 2.149(4);
Re1−H, 1.54(4) avg.; B1−H1, 1.48(4); H5−K1, 2.74(5); H4−K1,
3.19(5); Re1−P1, 2.3571(9).; Re1−P2, 2.3465(9); P2−Re1−P1,
93.28(3); P2−Re1−B1, 141.5(1); B1−Re1−P1, 91.36(9).
b
parentheses. Three equivalents of HBpin (1.50 mmol).
conditions were established for the C−H borylation of 2-
methylfuran 4a using 2 mol % of 3 and 1.5 equivalent of
HBpin in THF at 80 °C for 16 h (see Supporting Information
for full details); this gives the borylated furan 5a with high
regioselectivity for the C5-Bpin isomer over the C4-Bpin
isomer (>95% conversion, 95:5). No product is seen using
B2pin2 as the boron source for this reaction. Increasing the
catalyst loading results in an enhanced rate of reaction and
lowering the loading of HBpin from 1.5 to 1.0 equivalent
decreases conversion. In contrast, catalytic borylation reactivity
is not observed using complexes 1 or 2 showing that the
neutral, phosphine-coordinated complex 3 displays significant
differences in reactivity.
rhenium pentahydride anion in which the P and B atoms are
arranged in a distorted T-shape geometry with an elongated
Re−B bond (Re1−B1 = 2.149(4) Å), which could reflect
contributions from σ-borane structures because of the close
B1−H1 distance (1.48(4) Å). The hydrides were located in
the difference Fourier map and refined freely. The structure
also exhibits a close Re−H−K contact (H5−K1 = 2.74(5) Å),
which is similar to the those seen in other potassium salts of
rhenium polyhydride anions.36 Once again, possible hydride
positions were identified using AIRSS where, of the data set of
26 optimized structures generated, the same lowest energy
geometry is obtained 18 times, in which the only significant
Mono- and disubstituted furans 4a−4c are tolerated under
these reaction conditions with good selectivity for the C5-Bpin
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regiosiomer. Furan 4b reacts with 3 equiv of HBpin to give the
doubly borylated products exclusively. Thiophene substrates
are also converted to borylated products although the yields
are lower than seen for furans. Thiophene 4d gives a mixture of
2-boryl and 2,5-diboryl thiophenes. Monosubstituted thio-
phenes 4e−4i are converted with high selectivity for the 5-
boryl products. 3,4-Ethylenedioxythiophene (EDOT) 4g and
2,2′-bithiophene 4i give a mixture of monosubstituted (major)
and disubstituted (minor) boronic ester products. Pyrroles and
indoles 4j−4n are tolerated and give high yields and
regioselectivities. Notably, substrates containing carbon halide
bonds (C−Cl/Br) 4m and 4n favor C−H borylation over
Miyaura borylation of the C−X bond. N-Methyl and N-Boc
protected pyrroles and indoles only show trace conversion.
Similarly, di-ortho-substituted heteroaromatics show signifi-
cantly reduced conversion to the boronic ester products. No
C(sp3)−H, N−H, or dppp borylation is seen in these
reactions.
Scheme 4. Mechanistic investigation of catalytic C−H
borylation
The time course of a catalytic borylation of 4a was
1
monitored by H NMR spectroscopy in d8-THF (Figure 3).
borylation catalysis and inert to hydride substitution by HBpin,
reinforcing its role in a deactivation pathway.
Complex 7 is presumably formed during standard catalytic
conditions from boryl complex 3 by coordination of H2,
liberated by C−H borylation of the substrate, to a transiently
formed ReH5(dppp). When heptahydride 7 was heated in the
presence of an excess of HBpin a small amount of boryl
1
complex 3 is seen in the H NMR spectrum, along with
dinuclear 8 and an unknown complex 9, which has a hydride
resonance at −8.52 ppm (Scheme 4B). To assess the effect of
H2 evolution and its role within catalysis, the borylation of 3-
hexylthiophene 4e was conducted under standard conditions
in a Teflon-tapped NMR tube and compared with a reaction
under static vacuum to enable the H2 formed during the
reaction to escape into the headspace of the NMR tube. The
progress of both reactions and Re hydride speciation was
monitored by in situ 1H NMR spectroscopy (Supporting
Information). By comparing the yield of borylated product 5e
in each reaction over time, it is clear that both reactions follow
the same profile until ∼240 min, upon which they diverge with
the rate of reaction for the standard reaction slower than that
of the reaction under partial static vacuum. At 1000 min the
final yield of 5e is 37% for the standard reaction and 44% for
the vacuum reaction, indicating the deleterious effect of H2 on
catalysis. Quantification of the Re hydride species present by
Figure 3. Analysis by 1H NMR spectroscopy of the Re hydride
species during the catalytic C−H borylation of 2-methylfuran 4a.
Reaction conditions: heteroarene 4 (0.20 mmol), HBpin (0.3 mmol),
catalyst 3 (0.004 mmol), d8-THF (0.5 mL), 80 °C for 16 h.
1
integration of the relevant hydride signals in the H NMR
spectra show a much higher proportion of heptahydride 7 in
the standard reaction; this is as expected, given the higher
concentration of H2 in this reaction compared with the
reaction under static vacuum. Given the decreased catalytic
activity of heptahydride 7 compared with boryl 3, the increased
concentration of 7 supports the observed decrease in the rate
of reaction. The standard reaction also appears to have a higher
relative concentration of dimer 8 compared with the reaction
under static vacuum, which again supports the lower rate of
reaction by loss of active Re hydride species through this
deactivation pathway. Significantly, the changes in speciation
correlate with the observed evolution of H2 (4.55 ppm in the
1H NMR spectra) during the course of the reaction, with a
consistently lower concentration of H2 seen in solution in the
reaction under static vacuum compared with the standard
reaction.
Over 16 h, the hydride signal at −6.08 ppm corresponding to
complex 3 diminishes, and two new signals at −6.27 (t) and
−6.34 (p) ppm are observed. Both signals correspond to
catalyst deactivation and are identified as arising from the
heptahydride complex [ReH7(dppp)] 7 and the dinuclear
octahydride complex [ReH4(dppp)]2 8, respectively (Figure
3). Heptahydride 7 has been previously reported and was
synthesized independently to confirm this assignment.38 The
complex was originally reported to be inert to H2 loss when
heated under reflux in THF in the presence of PPh3, pyridine,
and hydrosilanes. Interestingly, we find that 7 acts as a catalyst
for C−H borylation, albeit at a significantly reduced rate
compared with 3, achieving only 48% NMR yield after 16 h
1
(Scheme 4A). In contrast, in situ H/11B/31P NMR spectros-
copy shows that dinuclear complex 8 is inactive in C−H
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1
The dinuclear complex 8 is the sole product formed on
heating 3 or 7 at 80 °C in toluene and crystallizes directly from
the reaction mixture (Scheme 4C). The 1H NMR spectrum of
isolated 8 matches that corresponding to the hydride
resonance at −6.34 (p), observed under standard catalytic
conditions. It should be noted that the formation of 8 by
heating 3 at 80 °C for 16 h in toluene results in stoichiometric
borylation of toluene to give a 74:26 regioisomeric mixture of
the meta and para borylated products in 48% NMR yield
against an internal standard. The borylation of carboarenes is
limited to stoichiometric reactivity using 3. The X-ray crystal
structure of 8, which has two chemically equivalent but
crystallographically independent molecules in the asymmetric
unit (Figure 4), confirms that it is a dimeric complex with four
in the H NMR spectrum corresponding to a phosphine-
coordinated Re hydride complex 9 (Scheme 4B and D), as well
as several minor signals between −6.00 and −7.50 ppm; H2
evolution is also observed (singlet at 4.55 ppm). The
formation of 9 does not coincide with the observation of an
obvious Re−B resonance in the 11B NMR spectrum. A similar,
but shorter-lived complex is seen if the reaction is undertaken
in d8-THF, but it is not present under standard catalysis
conditions. It was not possible to isolate complex 9 as it is
unstable under vacuum and its formulation is difficult to
discern as integration of the 1H NMR spectrum is inconclusive
due to some decomposition. It is likely, however, that 9 is a
higher-order boryl complex, for example Re-
H4(Bpin)3(dppp).28 Complex 9 is found to be active in the
catalytic borylation of 3-hexylthiophene 4e: complex 9 was
generated in situ by the reaction between 3 and HBpin for 3 h
in d12-cyclohexane, after which addition of 4e and further
heating for 16 h results in the borylated product 5e in 36%
yield. Complex 9 does not react with 4e at room temperature
but, after 20 min heating in the presence of thiophene 4e, it is
almost fully consumed with the only hydride species seen
being 3. From this point, the reaction follows a similar profile
to standard catalytic reaction conditions using 3. Complex 9 is
only observed in significant quantities in the presence of a large
excess of HBpin and in the absence of substrate, therefore it
may only be formed transiently under standard catalytic
conditions. These results suggest that 9 is likely a higher-order
boryl complex such as ReH4(BPin)3(dppp), which may act
similarly to the known iridium tris(boryl) complexes in
hydroborylation catalysis.43−45
On the basis of these results a catalytic cycle is proposed
with complex 9 as an off-cycle intermediate present in very low
concentrations and in equilibrium with 3 (Scheme 5A): (1)
Elimination of an equivalent of H2 from precatalyst 3 forms a
coordinatively unsaturated boryl complex Int-A; (2) oxidative
addition of the arene substrate gives a Re aryl intermediate Int-
B; (3) reductive elimination of the boronic ester product and
formation of a coordinatively unsaturated pentahydride
complex Int-C; and (4) addition of an equivalent of HBpin
and elimination of H2 reforms Int-A. The reaction of
precatalyst 3 with an excess of HBpin will lead to the
formation of the higher-order boryl 9 and the reaction of 3
with H2 will form heptahydride 7. Catalyst deactivation can
occur through the dimerization of 3 or 7 to form 8. We cannot
discount a mechanism involving 9 as an active species for C−H
borylation, where it is present as an on-cycle intermediate. The
formation of 9 under high HBpin concentrations may also limit
the formation of the off-cycle heptahydride 7. Computational
studies on the reaction intermediates and potential reaction
pathways may provide more definitive insight into the reaction
mechanism.
Figure 4. X-ray crystal structure of 8 (one of the two molecules in the
asymmetric unit is shown, all hydrogen atoms except for the Re
hydrides are omitted; displacement ellipsoids are drawn at 50%
probability). Selected distances (Å) and angles (deg): Re1−Ht,
1.47(4) avg.; Re1−Hb, 1.89(4) avg.; Re1−Re1′, 2.5383(5); Re1−P1,
2.327(1); Re1−P2, 2.3265(8); P1−Re1−P2, 96.62(3); P1−Re1−
Re1′, 133.81(3); P2−Re1−Re1′, 129.57(3); P1−Re1−Re1′−P1′,
180.00(5).
bridging hydrides between the Re atoms and two terminal
hydrides on each Re atom; the hydride atoms were located in
the difference Fourier map and refined freely. Similar dinuclear
complexes have been synthesized bearing monodentate or
tridentate phosphines;39−41 however, to the best of our
knowledge, complex 8 is the first example with a bidentate
phosphine.
The reactivity of Re polyhydrides is largely determined by
their ability to generate vacant coordination sites by liberation
of H2, either thermally or photochemically, and they have been
shown to participate in C−H bond activation chemistry.42 It is
likely that complex 3 participates in a similar manner to
activate the C−H bonds of the heteroarenes. However, under
catalytic conditions, there is potential for the boryl complex 3
to sequentially lose equivalents of H2 and add equivalents of
HBpin to form higher-order boryl complexes, which may be
catalytically relevant. To investigate this, a solution of the boryl
complex 3 and HBpin (20 equiv) in d12-cyclohexane was
heated at 80 °C and monitored by NMR spectroscopy
(Scheme 4D). After 3 h, a broad singlet is seen at −8.50 ppm
CONCLUSIONS
■
Two new Re boron-polyhydride complexes 2 and 3 have been
synthesized and characterized from the previously reported 1.
The boryl complex 3 is an effective catalyst for the C−H
borylation of heteroarenes proceeding with good yields and
regioselectivities. An investigation of the mechanism of this
reaction identifies potential reactive intermediates and
deactivation pathways. This work represents a new application
of high-oxidation-state Re complexes in C−H functionalization
catalysis and may have applications to related transformations,
for example C−H silylation and germylation. Further work to
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orcid.org/0000-0001-8614-2947;
Email: Stephen.Thomas@ed.ac.uk
Scheme 5. Proposed Catalytic Cycle for the C−H
Borylation of Heteroarenes
Authors
Liam J. Donnelly − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, U.K.
Teresa Faber − EaStCHEM School of Chemistry, University of
Edinburgh, Edinburgh EH9 3FJ, U.K.; Present
Address: T.F.: Organisch-Chemisches Institut,
̈
̈
̈
Westfalische Willhelms-Universitat Munster,
Corrensstrasse 40, 48149 Munster, Germany.
̈
Carole A. Morrison − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, U.K.
Gary S. Nichol − EaStCHEM School of Chemistry, University
of Edinburgh, Edinburgh EH9 3FJ, U.K.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c00869
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the University of Edinburgh, the EPSRC (UK), and
the EPSRC CRITICAT Centre for Doctoral Training (PhD
studentship to L.J.D.; Grant EP/L016419/1) for financial
support. S.P.T. thanks The Royal Society for a University
Research Fellowship. T.F. thanks the DAAD for a RISE
Worldwide scholarship. We thank the University of Edin-
burgh’s ECDF and the EaStCHEM Research computing
facility for hardware and software provision.
optimize the ligand structure to inhibit the formation of
heptahydride 7 and dimer 8 could develop a class of catalysts
that can compete with established Ir examples. Exploration of
ligand scaffolds other than phosphines may also enable the
catalytic C−H borylation of more challenging C(sp2)−H
bonds of carboarenes and pyridines, and C(sp3)−H bonds of
alkanes and is subject to ongoing investigations in our
laboratory.
REFERENCES
■
(1) Mkhalid, I. A. I.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.;
Hartwig, J. F. C−H Activation for the Construction of C−B Bonds.
Chem. Rev. 2010, 110, 890−931.
(2) Fyfe, J. W. B.; Watson, A. J. B. Recent Developments in
Organoboron Chemistry: Old Dogs, New Tricks. Chem. 2017, 3, 31−
55.
(3) Xu, L.; Wang, G.; Zhang, S.; Wang, H.; Wang, L.; Liu, L.; Jiao, J.;
Li, P. Recent advances in catalytic C−H borylation reactions.
Tetrahedron 2017, 73, 7123−7157.
(4) Hartwig, J. F. Borylation and Silylation of C−H Bonds: A
Platform for Diverse C−H Bond Functionalizations. Acc. Chem. Res.
2012, 45, 864−873.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c00869.
(5) Press, L. P.; Kosanovich, A. J.; McCulloch, B. J.; Ozerov, O. V.
High-Turnover Aromatic C−H Borylation Catalyzed by POCOP-
Type Pincer Complexes of Iridium. J. Am. Chem. Soc. 2016, 138,
9487−9497.
Syntheses and characterization, NMR spectra, crystallo-
graphic details, AIRSS/CASTEP details for 2, 3 and 8,
synthetic and spectroscopic details (PDF)
Crystallographic data for 2 (CCDC 2060777) (CIF)
Crystallographic data for 3 (CCDC 2060776) (CIF)
Crystallographic data for 8 (CCDC 2060778) (CIF)
AIRSS/CASTEP data for 2 and 3 (ZIP)
(6) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Thermal,
Catalytic, Regiospecific Functionalization of Alkanes. Science 2000,
287, 1995.
(7) Cho, J.-Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E.; Smith, M. R.
Remarkably Selective Iridium Catalysts for the Elaboration of
Aromatic C-H Bonds. Science 2002, 295, 305.
(8) Saito, Y.; Segawa, Y.; Itami, K. para-C−H Borylation of Benzene
Derivatives by a Bulky Iridium Catalyst. J. Am. Chem. Soc. 2015, 137,
5193−5198.
(9) Davis, H. J.; Mihai, M. T.; Phipps, R. J. Ion Pair-Directed
Regiocontrol in Transition-Metal Catalysis: A Meta-Selective C−H
Borylation of Aromatic Quaternary Ammonium Salts. J. Am. Chem.
Soc. 2016, 138, 12759−12762.
AUTHOR INFORMATION
Corresponding Authors
Jason B. Love − EaStCHEM School of Chemistry, University
of Edinburgh, Edinburgh EH9 3FJ, U.K.; orcid.org/0000-
0002-2956-258X; Email: Jason.Love@ed.ac.uk
Stephen P. Thomas − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, U.K.;
■
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ACS Catal. 2021, 11, 7394−7400

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
(10) Oeschger, R.; Su, B.; Yu, I.; Ehinger, C.; Romero, E.; He, S.;
Hartwig, J. Diverse functionalization of strong alkyl C−H bonds by
undirected borylation. Science 2020, 368, 736.
(11) Thongpaen, J.; Manguin, R.; Dorcet, V.; Vives, T.; Duhayon,
C.; Mauduit, M.; Baslé, O. Visible Light Induced Rhodium(I)-
Catalyzed C−H Borylation. Angew. Chem., Int. Ed. 2019, 58, 15244−
15248.
complexes of boron−hydride ligands and their application in catalysis.
Chem. Sci. 2020, 11, 9994−9999.
(30) Chen, H.; Hartwig, J. F. Catalytic, Regiospecific End-
Functionalization of Alkanes: Rhenium-Catalyzed Borylation under
Photochemical Conditions. Angew. Chem., Int. Ed. 1999, 38, 3391−
3393.
(31) Murai, M.; Omura, T.; Kuninobu, Y.; Takai, K. Rhenium-
catalysed dehydrogenative borylation of primary and secondary
C(sp3)−H bonds adjacent to a nitrogen atom. Chem. Commun.
2015, 51, 4583−4586.
(32) Esteruelas, M. A.; Fernández, I.; García-Yebra, C.; Martín, J.;
̃
Onate, E. Elongated σ-Borane versus σ-Borane in Pincer−POP−
Osmium Complexes. Organometallics 2017, 36, 2298−2307.
(33) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D.; Fan, Y.;
Webster, C. E.; Hall, M. B. Rhodium Boryl Complexes in the
Catalytic, Terminal Functionalization of Alkanes. J. Am. Chem. Soc.
2005, 127, 2538−2552.
(34) Pickard, C. J.; Needs, R. J. Ab initio random structure
searching. J. Phys.: Condens. Matter 2011, 23, No. 053201.
(35) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert,
M. J.; Refson, K.; Payne, M. C. First principles methods using
CASTEP. Z. Kristallogr. - Cryst. Mater. 2005, 220, 567−570.
(36) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Intra- and inter-
ion-pair protonic-hydridic bonding in polyhydridobis(phosphine)-
rhenates. Can. J. Chem. 2001, 79, 964−976.
(37) Hebden, T. J.; Denney, M. C.; Pons, V.; Piccoli, P. M. B.;
Koetzle, T. F.; Schultz, A. J.; Kaminsky, W.; Goldberg, K. I.;
Heinekey, D. M. σ-Borane Complexes of Iridium: Synthesis and
Structural Characterization. J. Am. Chem. Soc. 2008, 130, 10812−
10820.
(38) Luo, X. L.; Crabtree, R. H. Synthesis and structural studies of
some new rhenium phosphine heptahydride complexes. Evidence for
classical structures in solution. J. Am. Chem. Soc. 1990, 112, 4813−
4821.
(39) Kosanovich, A. J.; Reibenspies, J. H.; Ozerov, O. V. Complexes
of High-Valent Rhenium Supported by the PCP Pincer. Organo-
metallics 2016, 35, 513−519.
(40) Cotton, F. A.; Luck, R. L. Solid-state geometric isomers of
octahydridotetrakis(triphenylphosphine)dirhenium. Inorg. Chem.
1989, 28, 4522−4527.
(12) Wen, J.; Wang, D.; Qian, J.; Wang, D.; Zhu, C.; Zhao, Y.; Shi,
Z. Rhodium-Catalyzed PIII-Directed ortho-C−H Borylation of
Arylphosphines. Angew. Chem., Int. Ed. 2019, 58, 2078−2082.
(13) Esteruelas, M. A.; Oliván, M.; Vélez, A. POP−Rhodium-
Promoted C−H and B−H Bond Activation and C−B Bond
Formation. Organometallics 2015, 34, 1911−1924.
(14) Obligacion, J. V.; Semproni, S. P.; Chirik, P. J. Cobalt-Catalyzed
C−H Borylation. J. Am. Chem. Soc. 2014, 136, 4133−4136.
́
(15) Pabst, T. P.; Obligacion, J. V.; Rochette, E.; Pappas, I.; Chirik,
P. J. Cobalt-Catalyzed Borylation of Fluorinated Arenes: Thermody-
namic Control of C(sp2)-H Oxidative Addition Results in ortho-to-
Fluorine Selectivity. J. Am. Chem. Soc. 2019, 141, 15378−15389.
(16) Agahi, R.; Challinor, A. J.; Dunne, J.; Docherty, J. H.; Carter, N.
B.; Thomas, S. P. Regiodivergent hydrosilylation, hydrogenation, [2π
+ 2π]-cycloaddition and C−H borylation using counterion activated
earth-abundant metal catalysis. Chem. Sci. 2019, 10, 5079−5084.
(17) Dombray, T.; Werncke, C. G.; Jiang, S.; Grellier, M.; Vendier,
L.; Bontemps, S.; Sortais, J.-B.; Sabo-Etienne, S.; Darcel, C. Iron-
Catalyzed C−H Borylation of Arenes. J. Am. Chem. Soc. 2015, 137,
4062−4065.
(18) Britton, L.; Docherty, J. H.; Dominey, A. P.; Thomas, S. P.
Iron-Catalysed C(sp2)-H Borylation Enabled by Carboxylate
Activation. Molecules 2020, 25, 905.
(19) Kato, T.; Kuriyama, S.; Nakajima, K.; Nishibayashi, Y. Catalytic
C−H Borylation Using Iron Complexes Bearing 4,5,6,7-Tetrahy-
droisoindol-2-ide-Based PNP-Type Pincer Ligand. Chem. - Asian J.
2019, 14, 2097−2101.
(20) Mazzacano, T. J.; Mankad, N. P. Base Metal Catalysts for
Photochemical C−H Borylation That Utilize Metal−Metal Cooper-
ativity. J. Am. Chem. Soc. 2013, 135, 17258−17261.
(21) Tian, Y.-M.; Guo, X.-N.; Wu, Z.; Friedrich, A.; Westcott, S. A.;
Braunschweig, H.; Radius, U.; Marder, T. B. Ni-Catalyzed Traceless,
Directed C3-Selective C−H Borylation of Indoles. J. Am. Chem. Soc.
2020, 142, 13136−13144.
(22) Zhang, H.; Hagihara, S.; Itami, K. Aromatic C−H Borylation by
Nickel Catalysis. Chem. Lett. 2015, 44, 779−781.
(23) Furukawa, T.; Tobisu, M.; Chatani, N. Nickel-catalyzed
borylation of arenes and indoles via C−H bond cleavage. Chem.
Commun. 2015, 51, 6508−6511.
(24) Furukawa, T.; Tobisu, M.; Chatani, N. C−H Functionalization
at Sterically Congested Positions by the Platinum-Catalyzed
Borylation of Arenes. J. Am. Chem. Soc. 2015, 137, 12211−12214.
(25) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.;
Hartwig, J. F. Mild Iridium-Catalyzed Borylation of Arenes. High
Turnover Numbers, Room Temperature Reactions, and Isolation of a
Potential Intermediate. J. Am. Chem. Soc. 2002, 124, 390−391.
(26) Liskey, C. W.; Hartwig, J. F. Iridium-Catalyzed Borylation of
Secondary C−H Bonds in Cyclic Ethers. J. Am. Chem. Soc. 2012, 134,
12422−12425.
(27) Chotana, G. A.; Vanchura, B. A., II; Tse, M. K.; Staples, R. J.;
Maleczka, E., Jr.; Smith, M. R., III Getting the sterics just right: a five-
coordinate iridium trisboryl complex that reacts with C−H bonds at
room temperature. Chem. Commun. 2009, 5731−5733.
(41) Bau, R.; Carroll, W. E.; Teller, R. G.; Koetzle, T. F. A quadruply
hydrogen-bridged metal-metal bond. The neutron diffraction analysis
of octahydridotetrakis(diethylphenylphosphine)dirhenium(IV). J. Am.
Chem. Soc. 1977, 99, 3872−3874.
(42) Zeiher, E. H. K.; DeWit, D. G.; Caulton, K. G. Mechanistic
features of carbon-hydrogen bond activation by the rhenium complex
ReH7[P(C6H11)3]2. J. Am. Chem. Soc. 1984, 106, 7006−7011.
(43) Tamura, H.; Yamazaki, H.; Sato, H.; Sakaki, S. Iridium-
Catalyzed Borylation of Benzene with Diboron. Theoretical
Elucidation of Catalytic Cycle Including Unusual Iridium(V)
Intermediate. J. Am. Chem. Soc. 2003, 125, 16114−16126.
(44) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama, T.; Miyaura,
N.; Hartwig, J. F. Mechanism of the Mild Functionalization of Arenes
by Diboron Reagents Catalyzed by Iridium Complexes. Intermediacy
and Chemistry of Bipyridine-Ligated Iridium Trisboryl Complexes. J.
Am. Chem. Soc. 2005, 127, 14263−14278.
(45) Vanchura, B. A., II; Preshlock, S. M.; Roosen, P. C.; Kallepalli,
V. A.; Staples, R. J.; Maleczka, E., Jr.; Singleton, D. A.; Smith, M. R.,
III Electronic effects in iridium C−H borylations: insights from
unencumbered substrates and variation of boryl ligand substituents.
Chem. Commun. 2010, 46, 7724−7726.
(28) Ghaffari, B.; Vanchura, B. A.; Chotana, G. A.; Staples, R. J.;
Holmes, D.; Maleczka, R. E.; Smith, M. R. Reversible Borylene
Formation from Ring Opening of Pinacolborane and Other
Intermediates Generated from Five-Coordinate Tris-Boryl Com-
plexes: Implications for Catalytic C−H Borylation. Organometallics
2015, 34, 4732−4740.
(29) Donnelly, L. J.; Parsons, S.; Morrison, C. A.; Thomas, S. P.;
Love, J. B. Synthesis and structures of anionic rhenium polyhydride
7400
https://doi.org/10.1021/acscatal.1c00869
ACS Catal. 2021, 11, 7394−7400