Manganese-Catalyzed C(sp2)-H Borylation of Furan and Thiophene Derivatives

Article abstract of DOI:10.1021/acscatal.1c01563

Aryl boronic esters are bench-stable, platform building-blocks that can be accessed through metal-catalyzed aryl C(sp2)-H borylation reactions. C(sp2)-H bond functionalization reactions using rare- and precious-metal catalysts are well established, and while examples utilizing Earth-abundant alternatives have emerged, manganese catalysis remains lacking. The manganese-catalyzed C-H borylation of furan and thiophene derivatives is reported alongside an in situ activation method providing facile access to the active manganese hydride species. Mechanistic investigations showed that blue light irradiation directly affected catalysis by action at the metal center, that C(sp2)-H bond borylation occurs through a C-H metallation pathway, and that the reversible coordination of pinacolborane to the catalyst gave a manganese borohydride complex, which was as an off-cycle resting state.

Full text of DOI:10.1021/acscatal.1c01563

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Research Article
Manganese-Catalyzed C(sp2)−H Borylation of Furan and Thiophene
Derivatives
́
Luke Britton, Maciej Skrodzki, Gary S. Nichol, Andrew P. Dominey, Piotr Pawluc, Jamie H. Docherty,*
and Stephen P. Thomas*
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ABSTRACT: Aryl boronic esters are bench-stable, platform
building-blocks that can be accessed through metal-catalyzed aryl
C(sp2)−H borylation reactions. C(sp2)−H bond functionalization
reactions using rare- and precious-metal catalysts are well
established, and while examples utilizing Earth-abundant alter-
natives have emerged, manganese catalysis remains lacking. The
manganese-catalyzed C−H borylation of furan and thiophene
derivatives is reported alongside an in situ activation method
providing facile access to the active manganese hydride species.
Mechanistic investigations showed that blue light irradiation directly affected catalysis by action at the metal center, that C(sp2)−H
bond borylation occurs through a C−H metallation pathway, and that the reversible coordination of pinacolborane to the catalyst
gave a manganese borohydride complex, which was as an off-cycle resting state.
KEYWORDS: manganese, earth-abundant, C−H borylation, boron, photochemistry
(200−400 nm) to give phenyl-Bcat.11 This was later followed
he development of catalytic protocols based on Earth-
Tabundant metals underpins the sustainable future of by a single catalytic example using a manganese half-sandwich
complex (Cp′Mn(CO)3, where Cp′ = C5H4Me) and the
diboronic ester B2pin2 (B2pin2 = bis(pinacolato)diboron)
(Scheme 1a). Under a pressure of CO (2 atm.) and irradiation
(200−400 nm), phenyl-Bpin was observed in 75% yield after
36 h.10
chemical synthesis by providing a replacement for the
ubiquitous rare and precious metals. A key target for
sustainable catalysis is the selective C(sp2)−H functionaliza-
tion of arenes, in particular, C(sp2)−H borylation.1 Direct
C(sp2)−H borylation provides efficient access to aryl boronic
esters, which are highly versatile synthetic intermediates and a
staple of the pharmaceutical industry.1e,2 Arene C(sp2)−H
borylation is currently dominated by platinum-group metal
catalysts, namely, iridium and rhodium,3 with examples of
C(sp2)−H borylation using first-row transition metals being
generally limited to Fe,4 Co,5 and Ni species.6 Photoinduced
borylations have also been reported.7
The manganese complex, bis[1,2-bis(dimethylphosphino)-
ethane]trihydridomanganese (dmpe2MnH3), has been used to
catalyze hydrogen isotope exchange reactions of arene
C(sp2)−H bonds using C6D6 and D2 as the isotope sources
(Scheme 1b).12 As this was proposed to proceed through a
manganese aryl species, generated by arene C−H metallation,
it was questioned if this manganese aryl species would be long-
lived enough to be intercepted with a suitable borane and
trigger C−H borylation. Further, recent studies on the
analogous iron complex had shown that the metal hydride
could be accessed in situ,4f,h so possibly negating the need to
generate and handle the manganese trihydride (Scheme 1c).
The ubiquitous nature of borylated heterocycles in
pharmaceutical development directed focus to the C−H
borylation of simple heteroarenes with 2-methylfuran used as
Manganese is the third most abundant transition metal and
benefits from low physiological and environmental toxicity.8
Manganese-catalyzed C(sp2)−H functionalization of arenes
has shown promising development with examples including
hydroarylation, alkenylation, and allylation, all of which were
proposed to proceed through manganese aryl intermediates.9
These were typically performed using Mn(0)/(I) carbonyl
species and demonstrated high regioselectivity when using
directing groups.9a,b Despite this, and to the best of our
knowledge, there is currently only a single previous example of
catalytic C(sp2)−H borylation involving a manganese catalyst
(Scheme 1a).10
Received: April 6, 2021
Revised: May 12, 2021
Published: May 27, 2021
Hartwig reported that a manganese-borylcarbonyl complex
[(OC)5MnBcat, where cat = catechol] would mediate the
C(sp2)−H borylation of benzene under UV light irradiation
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%), and in situ activation using NaOtBu (6 mol %) activator
gave equal catalytic activity and demonstrated the effectiveness
of the strategy (entry 2). In situ activation was used to further
expedite the development and negate the need for air- and
moisture-sensitive reagents/(pre-)catalysts.4h,13
Scheme 1. Manganese-Catalyzed C(sp2)−H Borylation
The use of blue light (460 nm) was found to be more
effective than white light and higher-energy ultraviolet light
(<300 nm) (entries 3 and 4). An excess of furan 2a (2.8
equiv), relative to HBpin, was superior to stoichiometric
quantities (entry 5), and substitution of NaOtBu for
carboxylate, metal hydride, or alkyl lithium activators resulted
in reduced yields (see SI, ST2). Alternative boranes and
dioxaborolanes such as 9-borabicyclo[3.3.1]nonane (H-B−9-
BBN), catecholborane (HBcat), and1,8-naphthalenediamina-
toborane (HBdan)) were all unreactive toward C−H
borylation (see SI, ST2). Variation of the diphosphine ligand
(see SI, ST2) or exchange for Mn(I) carbonyl species showed
no reactivity (entries 6 and 7). The requirement for a
manganese precatalyst, an activator, and light irradiation were
confirmed through a series of control experiments (entries 8−
10).
With optimized reaction conditions identified, the reactivity
of the system was assessed by application to a selection of
furan and thiophene derivatives (Table 2). 2-Methylfuran 2a
underwent efficient and regioselective C−H borylation to give
the 5-substituted boronic ester 3a exclusively in high isolated
yield (86%). Substitution of the methyl group for ethyl, octyl,
and benzyl groups all resulted in similar reactivity and
regioselectivity to give the 5-boryl furan derivatives 3b, 3c,
and 3d in 52, 51, and 60% yields, respectively. Borylation of
the parent furan 2e was also successful but gave a 64:36
mixture of mono/disubstituted boryl furans, with borylation
occurring at the 2- and 5-positions only.
Application to thiophene derivatives showed that reactivity
was generally more efficient for 2-substituted thiophenes than
the 3-substituted analogues. This was observed for the methyl-
and phenyl-substituted 5-boryl thiophene derivatives 3i, 3j, 3k,
and 3l. Unsubstituted thiophene 2m showed similar reactivity
to furan 2e, with mono- and diborations observed in a similar
ratio (65:35; cf. 64:36 for furan 2e). For substrates that
showed limited reactivity, borylations were performed using
isolated dmpe2MnH3 4. Furans 2f and 2h and thiophenes 2i,
2j, 2l, 2n, and 2o all achieved a significant increase in reactivity
and yield. Simple carboarenes, aryl halides, and pyrrole
derivatives showed no reactivity, and the presence of nitrile,
alkyne, and carbonyl functionalities gave no observable C−H
borylation.
a
Previous example of manganese-catalyzed C(sp2)−H borylation
b
(Cp′ = C5H4Me). dmpe2MnH3-catalyzed C(sp2)−H deuterium
exchange under thermal and photochemical conditions (dmpe =
c
Me2P(CH2)2PMe2). This workC(sp2)−H borylation using
dmpe2MnBr2 1 as a precatalyst, activated by NaOtBu, under blue
light irradiation.
a model substrate. The potential for C−H borylation was
tested using isolated dmpe2MnH3 4 (3 mol %) as a precatalyst
(see Supporting Information (SI) for stoichiometric generation
of dmpe2MnH3 4) and pinacolborane (HBpin) (1 equiv) in n-
C6H14 (1 M) under blue light irradiation. This gave the
borylation of 2-methylfuran 2a with high yield (93%) and
exclusive regioselectivity for the 5-borylated regioisomer 3a
(Table 1, entry 1). Exchange of the dmpe2MnH3 4 precatalyst
for the manganese(II) halide precursor, dmpe2MnBr2 1 (3 mol
a
Table 1. Deviations from Optimized Conditions
To gain mechanistic insight into the precatalyst activation
and borylation reaction, a series of single-turnover experiments
were carried out. Due to the poor solubility of dmpe2MnBr2 1
in n-C6H14, activation studies were performed in THF.
Reaction of the activator, NaOtBu, with HBpin and the
dmpe2MnBr2 precatalyst 1 in the absence of light irradiation
showed no observable formation of dmpe2MnH3 4 at room
temperature or 60 °C. Instead, a new manganese hydride
species was produced and observed to increase in concen-
entry
deviation
yield (%)
1
2
3
4
5
6
7
8
9
10
precatalyst = dmpe2MnH3 4 No NaOtBu
93
86
30
13
60
0
0
0
0
0
none
white light
UV light (CFL)
furan/HBpin (1:1)
precatalyst = (CO)5MnBr
precatalyst = Cp′Mn(CO)3
no precatalyst
1
tration over time, as determined by H NMR spectroscopy
no NaOtBu
1
(Scheme 2a). In combination with 11B, H−11B HMQC, and
no light (60 and 100 °C)
1
variable-temperature H NMR spectroscopies, the structure
a
was suggested to be a hydride-bridged manganese borohydride
complex dmpe2Mn(μ-H)2Bpin 6.4d,14 Under both thermal and
photochemical conditions, dmpe2MnH3 4 is reported to
readily dissociate dihydrogen to give a Mn(I) hydride species,
2-Methylfuran 2a (2.8 equiv), HBpin (1 equiv), dmpe2MnBr2 1 (3
mol %), NaOtBu (6 mol %), n-C6H14 (1 M), 60 °C, 72 h. Yields
determined by H NMR spectroscopy of the crude reaction mixtures
using product:substrate ratio. Blue light (460 nm). Cp′ = C5H4Me.
1
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a
Table 2. Scope of Manganese-Catalyzed C−H Borylation of Furan and Thiophene Derivatives
a
Reaction conditions 1: arene 2 (2.8 equiv), HBpin (1 equiv), dmpe2MnBr2 1 (3 mol %), NaOtBu (6 mol %), n-C6H14 (1 M), 60 °C, 72 h. Isolated
b
yields reported. Yields determined by 1H NMR spectroscopy of the crude reaction mixture using product:substrate ratio (reaction conditions 1).
c
Reaction conditions 2: arene 2 (2.8 equiv), HBpin (1 equiv), dmpe2MnH3 4 (3 mol %), n-C6H14 (1 M), 60 °C, 24 h. Isolated yields reported.
d
1
Yield determined by H NMR spectroscopy of the crude reaction mixture using product:substrate ratio (reaction conditions 2). Mono: 2-boryl
product. Di: 2,5-diboryl product. Mono:Di = molar ratio of products.
which was trapped here by HBpin to give dmpe2Mn(μ-
H)2Bpin 6.12 Blue light irradiation accelerated the formation of
the manganese borohydride adduct dmpe2Mn(μ-H)2Bpin 6 as
for DBpin led to the deuterated borohydride adduct,
dmpe2Mn(μ-H/D)2Bpin d-6, as well as the observation of
HD in situ and the incorporation of deuterium to the dmpe
ligands of the complex (Scheme 2b, II).12
1
observed by H NMR spectroscopy, presumably by increasing
Blue light irradiation of dmpe2Mn(μ-H)2Bpin 6 with 2-
methylfuran 2a resulted in the formation of the 5-boryl
product 3a and regeneration of dmpe2MnH3 4 (Scheme 2b,
III). Use of the deuterated complex, dmpe2Mn(μ-H/D)2Bpin
d-6, led to deuterium incorporation observed in both the furan
substrate (d-2a) and 5-boryl product d-3a (Scheme 2b, IV),
indicating that a reversible C−H metallation process was
occurring.
Further structural confirmation of the manganese−HBpin
borohydride complex was gained through the synthesis,
isolation, and characterization of the analogous HBdan and
HBcat complexes, dmpe2Mn(μ-H)2Bdan 7 and dmpe2Mn(μ-
the rate of H2 dissociation.
To confirm the identity of any intermediate manganese
hydride species, further mechanistic studies were performed
using isolated dmpe2MnH3 4. The reaction of dmpe2MnH3 4
with HBpin under blue light irradiation again showed the
1
formation of dmpe2Mn(μ-H)2Bpin 6, as observed by H and
11B NMR spectroscopy (Scheme 2b, I). Single crystals suitable
for X-ray analysis were obtained from a concentrated n-C6H14
solution at −40 °C, unambiguously confirming the structure of
the hydride-bridged manganese borohydride complex,
dmpe2Mn(μ-H)2Bpin 6 (Scheme 2c). Substitution of HBpin
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Scheme 2. Mechanistic Investigations 1
c
X-ray crystal structure of dmpe2Mn(μ-H)2Bpin 6; ellipsoids are set at 50% probability. X-ray crystal structure of dmpe2Mn(μ-H)2Bdan 7;
ellipsoids are set to 30% probability. C-bound H atoms are omitted for clarity.
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Scheme 3. Mechanistic Investigations 2
a
1
2
Reaction conditions 1: dmpe2MnH3 4 (1 equiv), 5-d-2c (xs.), n-C6H14 (1 M), 60 °C, 48 h. Deuterium incorporation determined by H and D
b
NMR spectroscopy. I) Standard reaction conditions, arene: 2-methylfuran 2a. II) Standard reaction conditions, arene: 2-octylfuran 2c or 5-d-
octylfuran 5-d-2c.
H)2Bcat 8. These complexes displayed comparative 1H and 11
B
for arene−borane precoordination through the neighboring
heteroatom to achieve C−B formation and could explain the
lack of reactivity with carboarenes.
1
NMR resonances as well as the characteristic H−1B cross
peak (HMQC). Single crystals suitable for X-ray analysis of
dmpe2Mn(μ-H)2Bdan 7 were obtained from a concentrated n-
C6H14/C6D6 solution and again showed the bridging
borohydride bonding arrangement (Scheme 2c).
Irradiation of dmpe2MnH3 4 in the presence of excess
thiophene 2m again led to no evidence of an isolatable C−H
metallated manganese aryl complex. Instead, small quantities of
a thiophene-coordinated Mn(I) hydride complex, dmpe2MnH-
The relative stability of the manganese borohydride adducts
6−8 was assessed through a series of exchange/competition
reactions that showed a relationship corresponding to the
relative Lewis acidity of each boron reagent (Scheme 2c).
Addition of HBpin to dmpe2Mn(μ-H)2Bdan 7 gave dmpe2Mn-
(μ-H)2Bpin 6 through displacement of HBdan, as observed by
1H and 11B NMR spectroscopy. This reaction was found to be
irreversible by the addition of HBdan to dmpe2Mn(μ-H)2Bpin
6 giving no observable formation of dmpe2Mn(μ-H)2Bdan 7.
The addition of, the more Lewis acidic, HBcat to dmpe2Mn(μ-
H)2Bpin 6 gave dmpe2Mn(μ-H)2Bcat 8.
Blue light irradiation of dmpe2MnH3 4 in the presence of 2-
methylfuran 2a led to no observable manganese aryl complex,
arising from C−H bond metallation. However, irradiation in
the presence of 5-d-2-octylfuran, 5-d-2c, led to HD-observed in
situ, deuterium incorporation into the precatalyst, dmpe2Mn-
(H/D)3 d-4, and H/D scrambling of the furan, thus indicating
a reversible C−H metallation process (Scheme 3a). Deuterium
incorporation at the 3- and 4-positions of the furan was
observed, suggesting that the catalyst was capable of insertion
into all of the C−H bonds but only underwent C−B formation
in the 5-position. These observations may suggest a necessity
1
(thiophene) 9, were observed by H NMR spectroscopy (see
SI, Part 8). The 1H−31P coupling within the associated hydride
signal was akin to that of the trans-adducted Mn(I) hydride
complexes prepared by Jones and co-workers,12 although the
exact configuration of the thiophene coordination was
undetermined. When reacted with HBpin, immediate and
quantitative conversion to the borohydride complex,
dmpe2Mn(μ-H)2Bpin 6, was observed.
It was noted that during the mechanistic investigations, two
further hydride signals were consistently observed despite
changes in the substrate and solvent. Attempts to isolate these
species, alongside recent studies by Emslie and co-workers,14d
confirmed these to be the phosphine complexes,
(dmpe2MnH)2(μ-dmpe) and (dmpe2MnH)2(κ1-dmpe) (see
SI, Part 8). These complexes are presumably the product of
thermal or irradiative degradation of dmpe2MnH3 4.
Finally, the rate-limiting step of the C−H borylation
reaction was investigated. A light/dark experiment was used
to confirm that the C−H borylation reaction could be
accurately followed by NMR spectroscopy. The borylation of
2-methylfuran 2a under alternating blue light irradiation and
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darkness confirmed that C−H borylation only occurred under
irradiation and no persistent reaction was observed in the dark
(Scheme 3b, I). A kinetic isotope effect (KIE) of 1.6 was found
for the borylation of 2-octylfuran 2c versus 5-d-2-octylfuran 5-
d-2c, with the initial rates of reaction being determined by
monitoring the formation of borylated furan product 3c using
1H NMR spectroscopy (Scheme 3b, II). The reversibility of
the C−H metallation step (Scheme 3a) presumably accounts
for the smaller than average value of a typical primary KIE.15
Use of higher power light sources (450 and 365 nm) increased
the rate of reaction (see SI, Part 10).
By monitoring reaction progression, it was possible to
determine the relative ratio of manganese species present
throughout the borylation reaction (see SI, Part 10). In the
early stages of the reaction, when the concentration of HBpin
was high, the majority of manganese species were the
borohydride adduct dmpe2Mn(μ-H)2Bpin 6. The concen-
tration of this species was then observed to decrease as the
reaction proceeded. The growth and subsequent depletion of
dmpe2Mn(μ-H)2Bpin 6 suggested that this complex was an
off-cycle resting state. In support of this, use of dmpe2Mn(μ-
H)2Bpin 6 as the precatalyst, in place of dmpe2MnH3 4, gave
equal activity in the borylation of 2-methylfuran 2a (93%
yield) under standard conditions.
adducted by HBpin to give dmpe2Mn(μ-H)2Bpin 6, an off-
cycle, resting state and catalyst reservoir.
Alternatively, dmpe2MnH 5 undergoes C(sp2)−H bond
metallation, potentially by initial heteroatom coordination,
producing the manganese aryl complex dmpe2MnH2(2-
methylfuryl) 7. Further dissociation of dihydrogen results in
a highly reactive coordinately unsaturated manganese aryl
complex, dmpe2Mn(2-methylfuryl). Finally, in the presence of
HBpin, dmpe2Mn(2-methylfuryl) can either undergo oxidative
addition followed by a subsequent reductive elimination, or a
single-step σ-bond metathesis reaction to give the aryl boronic
ester product 3a while regenerating the catalyst, dmpe2MnH 5.
The addition of NaOtBu and NaBr to a standard catalysis
reaction using dmpe2MnH3 4 showed no inhibition of catalytic
activity; thus, the reduced activity observed when performing
reactions using dmpe2MnBr2 1 and in situ activation can be
attributed to incomplete activation of the dmpe2MnBr2
precatalyst.
1
In summary, the manganese-catalyzed C(sp2)−H borylation
of furan and thiophene derivatives has been developed using
the in situ activation of a Mn(II) precatalyst. The combination
of NaOtBu and HBpin provided an efficient means of accessing
the photoactive species, dmpe2MnH3 4. Mechanistic inves-
tigations resulted in the identification of key reaction
intermediates that suggest the boronic ester products are
obtained exclusively through a C−H metallation pathway.
Kinetic studies confirmed that C−H metallation was also the
rate-limiting step. Alternatively, coordination with pinacolbor-
ane results in the manganese borohydride adduct, dmpe2Mn-
(μ-H)2Bpin 6, which acts as a catalyst resting state and
reservoir.
Based on the accumulated mechanistic studies, a catalytic
cycle was proposed (Scheme 4). Precatalyst, dmpe2MnH3 4,
could either be generated in situ from the corresponding
dibromide precursor, dmpe2MnBr2 1, or used directly.
Thermal or photochemical loss of dihydrogen gives a highly
reactive Mn(I) hydride species, dmpe2MnH 5, which is rapidly
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01563.
■
Scheme 4. Proposed Mechanism for Manganese-Catalyzed
C−H Borylation of Furan and Thiophene Derivatives.
sı
General experimental, mechanistic and kinetic studies,
characterization data, and crystallographic data (PDF)
(CIF)
(CIF)
AUTHOR INFORMATION
Corresponding Authors
■
Jamie H. Docherty − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom; Email: jamie.docherty@ed.ac.uk
Stephen P. Thomas − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom; orcid.org/0000-0001-8614-2947;
Email: stephen.thomas@ed.ac.uk
Authors
Luke Britton − EaStCHEM School of Chemistry, University of
Edinburgh, Edinburgh EH9 3FJ, United Kingdom
Maciej Skrodzki − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom; Faculty of Chemistry, Adam Mickiewicz
́
University, 61-614 Poznan, Poland
Gary S. Nichol − EaStCHEM School of Chemistry, University
of Edinburgh, Edinburgh EH9 3FJ, United Kingdom
Andrew P. Dominey − GSK Medicines Research Centre,
Stevenage SG1 2NY, United Kingdom
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R.; Dorcet, V.; Vives, T.; Duhayon, C.; Mauduit, M.; Baslé, O. Visible
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for Directed C(sp2)−H and C(sp3)−H Borylation of Diverse Classes
of Substrates. J. Am. Chem. Soc. 2021, 143, 5022−5037.
́
Piotr Pawluc − Faculty of Chemistry, Adam Mickiewicz
́
University, 61-614 Poznan, Poland
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c01563
Author Contributions
L.B., M.S., and J.H.D. performed the practical work. G.S.N.
carried out X-ray crystallography. J.H.D. and S.P.T. conceived
the reaction. S.P.T., P.P., and A.P.D. advised investigations. All
authors contributed to the manuscript.
Notes
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with Earth-Abundant 3d Transition Metals. Angew. Chem., Int. Ed.
2019, 58, 12803−12818. (c) Yan, G.; Jiang, Y.; Kuang, C.; Wang, S.;
Liu, H.; Zhang, Y.; Wang, J. Nano-Fe2O3-Catalyzed Direct Borylation
of Arenes. Chem. Commun. 2010, 46, 3170−3172. (d) Hatanaka, T.;
Ohki, Y.; Tatsumi, K. C-H Bond Activation/Borylation of Furans and
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tivity. J. Am. Chem. Soc. 2013, 135, 17258−17261. (f) Dombray, T.;
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Borylation of Arenes. J. Am. Chem. Soc. 2015, 137, 4062−4065.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.P.T. thanks The Royal Society for a University Research
Fellowship (RF191015). J.H.D. and S.P.T. acknowledge GSK
and EPSRC (PIII0002) and The Royal Society (RF191015)
for postdoctoral funding. L.B. acknowledges The Royal Society
and The University of Edinburgh for a Ph.D. studentship
(RF191015). M.S. acknowledges a Ph.D. studentship grant
(POWR.03.02.00-00-I026/16) cofinanced by the European
Union through the European Social Fund under the Opera-
tional Program Knowledge Education Development.
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