Iron-catalyzed C-H borylation of arenes

Article abstract of DOI:10.1021/jacs.5b00895

Well-defined iron bis(diphosphine) complexes are active catalysts for the dehydrogenative C-H borylation of aromatic and heteroaromatic derivatives with pinacolborane. The corresponding borylated compounds were isolated in moderate to good yields (25-73%) with a 5 mol% catalyst loading under UV irradiation (350 nm) at room temperature. Stoichiometric reactivity studies and isolation of an original trans-hydrido(boryl)iron complex, Fe(H)(Bpin)(dmpe)₂, allowed us to propose a mechanism showing the role of some key catalytic species.

Full text of DOI:10.1021/jacs.5b00895

Communication
pubs.acs.org/JACS
Iron-Catalyzed C−H Borylation of Arenes
Thomas Dombray,† C. Gunnar Werncke,‡,§ Shi Jiang,† Mary Grellier,‡,§ Laure Vendier,‡,§
Sebastien Bontemps,*,‡,§ Jean-Baptiste Sortais,*,† Sylviane Sabo-Etienne,*,‡,§ and Christophe Darcel*,†
́
†Team Organometallics: Materials and Catalysis, Centre for Catalysis and Green Chemistry, Institut des Sciences Chimiques de
Rennes, UMR 6226 CNRSUniversite
́ ́ ́
de Rennes 1, Campus de Beaulieu, 263 av. du General Leclerc, 35042 Rennes Cedex, France
‡Laboratoire de Chimie de Coordination (LCC), CNRS, 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France
§Universite
́
de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
S
* Supporting Information
analogous ruthenium complexes can notably activate hydro-
boranes.14 Complexes of the type Fe(X)2(L)2 (X = H, Me; L =
dppe, dmpe; complexes 1−3) have been tested as catalyst (5
mol%) for the borylation of ethylbenzene with HBpin under
UV irradiation at 350 nm at room temperature for 72 h in neat
conditions (Scheme 1). While complex 1, featuring diaryl-
ABSTRACT: Well-defined iron bis(diphosphine) com-
plexes are active catalysts for the dehydrogenative C−H
borylation of aromatic and heteroaromatic derivatives with
pinacolborane. The corresponding borylated compounds
were isolated in moderate to good yields (25−73%) with a
5 mol% catalyst loading under UV irradiation (350 nm) at
room temperature. Stoichiometric reactivity studies and
isolation of an original trans-hydrido(boryl)iron complex,
Fe(H)(Bpin)(dmpe)2, allowed us to propose a mechanism
showing the role of some key catalytic species.
a
Scheme 1. Iron-Catalyzed Borylation of Ethylbenzene
elective carbon−hydrogen bond activation and functional-
Sization is highly attractive to provide more complex
molecules by building C−C and C−heteroatom bonds in an
eco-friendly manner.1 Of particular interest is the borylation of
arenes and heteroarenes, which represents one of the most
elegant ways to obtain (hetero)-arylboronate esters, versatile
synthons for Suzuki−Miyaura reactions.2 Very efficient catalytic
borylation processes have been developed using noble-metal-
based catalysts such as iridium and rhodium complexes.2 With
earth-abundant transition metals as catalysts, borylation has
been performed with electrophilic halide derivatives,3 and
recently Chirik et al. achieved efficient catalyzed arene
borylations via C−H activation using a pincer cobalt complex.4
In the case of iron, which has become a valuable alternative to
precious transition metals in homogeneous catalysis,5 achieving
such a transformation remains challenging. Despite early
stoichiometric studies by Hartwig et al. on the ability of iron
carbonyl complexes such as CpFe(Bcat)(CO)2 to mediate
borylation of arenes under photochemical conditions,2e,6 the
main difficulty appears to be the regeneration of the active
species after B−C coupling. Two catalytic iron systems,
involving stoichiometric amounts either of hydrogen acceptor7
or base and oxidant,8 were reported before the recent disclosure
by Mankad et al. of the activity of bimetallic complexes (Cu−Fe
and Zn−Fe) under photochemical conditions (450-W Hg
lamp).9 However, dehydrogenative C−H borylation with a
mononuclear iron catalyst, without any additives, has never
been reported.
a
Experimental conditions: EtPh (1.5 mmol). Reported values are %
1
isolated yields. Product ratios were determined by H NMR. dppe =
bis(diphenylphosphinoethane); dmpe = bis(dimethylphosphino-
ethane).
phosphine moieties, failed to catalyze the reaction,15 the
dihydride complex 2, bearing more basic alkyl substituents,
afforded the borylarene compound in 52% isolated yield of a
68:32 mixture of meta and para derivatives, respectively.
Interestingly, when the dimethyliron analogue Fe(Me)2-
(dmpe)2 (3)16 was used as a catalyst precursor, the isolated
yield increased to 73%, with the same meta:para ratio. Control
experiments showed the necessity of UV irradiation, as no
coupling product was detected when the reaction was
performed in toluene up to 100 °C, under visible light
conditions or in the dark.17,18
Given the better yield obtained with complex 3, and its
higher stability compared to the dihydride analogue 2, we
probed the scope of the reaction with 3 as catalyst precursor
(Scheme 2). Isolated yields ranging from 34 to 73% were
obtained for the alkyl-substituted arenes, whereas evaluation of
the crude reaction mixture by NMR spectroscopy showed
higher yields.18a Only in the case of toluene, trace amounts of
tol(Bpin)2 were detected by GC-MS analysis. We also probed
the tolerance toward ethers and amino groups. Anisole can be
To tackle this domain,10 we considered bis(diphosphine)
iron dihydride complexes as promising catalyst precursors.11,12
Indeed, in-depth studies by Field and Perutz et al. have shown
the ability of such complexes to activate C−H bonds,13 whereas
Received: January 27, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b00895
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Journal of the American Chemical Society
Communication
with HBpin, under irradiation, to afford the new hydrido-
(boryl)iron complex Fe(H)(Bpin)(dmpe)2 (4), as evidenced
by spectroscopic and X-ray diffraction studies. In solution, two
isomers in a 1:1 ratio, 4-trans and 4-cis, were identified by NMR
spectroscopy (Scheme 4). Complex 4-trans is characterized by
Scheme 2. Functionalized Arene Borylation Catalyzed by
Fe(Me)2(dmpe)2 (3)
a
Scheme 4. Synthesis of the Hydrido(boryl)iron Complex 4
1
a hydride signal in H NMR at −12.75 ppm (q, JHP = 42 Hz)
and a sharp doublet in 31P NMR (77.7 ppm, JHP = 40 Hz).
1
Variable-temperature H and 31P NMR analyses indicate that
complex 4-cis is highly fluxional, showing at 213 K a multiplet at
−13.6 ppm in 1H NMR and four signals at 77.3, 76.2, 59.9, and
58.4 ppm in 31P{1H} NMR, interpreted as an AA′MN second-
order system by NMR data simulation.18a These features are
similar to those of the ruthenium analogues cis- and trans-
Ru(H)(Bpin)(dmpe)2, which were also obtained under UV
irradiation.14a The solution IR spectrum displays two bands at
1684 and 1797 cm−1, assigned to the Fe−H stretching
frequencies of 4-trans and 4-cis, respectively, by comparison
with DFT calculations conducted on both isomers (νcalc,Fe−Htrans
= 1718 cm−1; νcalc,Fe−Hcis = 1856 cm−1).18a When the 4-trans
and 4-cis solution was evaporated to dryness, the resulting solid
a
Reported values are % isolated yields. Product ratios were determined
1
by H NMR.
borylated in a modest isolated yield (25%), whereas dimethyl-
amino-containing arenes, namely N,N-dimethylaniline and
N,N-dimethylbenzylamine, can be efficiently borylated in 57%
isolated yield (75% in the crude mixture for N,N-dimethyl-
aniline). Overall, the observed regioselectivities (meta > para ≫
ortho) correspond to the trend commonly observed in
transition metal C−H borylation.2c
was identified as solely 4-trans (νFe−H = 1671 cm−1; 1H and 31
P
solid-state NMR resonances at −13 and 78 ppm, respectively).
When complex 4-trans was redissolved, signals for 4-cis and 4-
trans were observed in an identical equimolar ratio as a result of
fast isomerization in solution.
We were able to obtain crystals of 4-trans suitable for X-ray
diffraction analysis (Figure 1, left). The iron center adopts an
The catalytic performance of 3 was then evaluated with
heterocyclic derivatives (Scheme 3). Starting from 2-methyl-
Scheme 3. Catalytic Borylation of Heterocycles Using
Fe(Me)2(dmpe)2 (3)
a
Figure 1. X-ray diffraction structure of 4-trans (left), and DFT/
B3PW91-D3BJ calculated geometry (right). Hydrogen atoms, except
the hydride, were omitted for clarity.
a
Reported values are % isolated yields. Product ratios were determined
1
by H NMR.
furan, the corresponding 4-Bpin-2-methylfuran and 5-Bpin-2-
methylfuran were obtained in an overall 67% isolated yield in a
ratio of 18:82. 1-Benzofurane led to the corresponding 2-Bpin-
1-benzofurane in 35% yield. When starting from 3-methyl-
thiophene, only 8% of a borylated compound was obtained,
while no reaction took place with 2-methylthiophene. By
contrast with iridium- and cobalt-catalyzed reactions,2,4 no
coupling product was detected with pyridine derivatives.
Having established that 2 and 3 serve as catalyst precursors,
we sought mechanistic information. We first examined the
reactivity of the iron complexes with pinacolborane. To our
delight, both the dihydride and dimethyl complexes reacted
octahedral geometry, with the four phosphorus atoms located
in the equatorial plane. The Fe−B distance of 2.024(4) Å is in
the range of iron−boryl bonds,19 slightly shorter than the Fe−
Bpin distance (2.065(3) Å) recently reported for the iron(I)
boryl complex Fe(Bpin)(dpbz)2,3e and slightly longer than in
the series of CpFe(Bpin)(PR3)2 complexes (1.996(3)−
2.002(10) Å).20 The hydride has been localized by Fourier
difference map analysis with a Fe−H distance of 1.63(8) Å, and
DFT calculation ascertained the hydrogen location, with
d
calc(Fe−H) = 1.561 Å (Figure 1, right).
B
DOI: 10.1021/jacs.5b00895
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Journal of the American Chemical Society
Communication
We then conducted a series of experiments to gain more
information on the role of 2, 3, and 4 using multinuclear H,
with the arene to afford the aryl hydride intermediate 5, which
can react with HBpin to produce 4. The hydrido(boryl)
complex 4 can then react with an incoming arene to afford
ultimately B−C coupling and restore the dihydride complex 2.
At this stage, direct formation of Ar−Bpin by the reaction of
the hydrido(aryl) complex 5 with HBpin under irradiation
cannot be ruled out.22 When the dimethyl complex 3 was
used,17 its reaction with HBpin under irradiation afforded the
hydrido(boryl) complex 4, which can lead to the coupling
product or to the transient Fe(0) species.
In summary, we report the first dehydrogenative borylation
of (hetero)arenes catalyzed by iron without the need for any
additives, although this method led to lower yields in
comparison to previously described Ir-, Rh-, and Co-catalyzed
borylations. Our system operates at ambient temperature under
UV irradiation (350 nm) with an electron-rich iron catalyst. We
could isolate and characterize the first hydrido(boryl)iron
complex and prove its involvement in the catalytic process.
Studies of the scope of these findings are currently underway in
our laboratories.
1
2H, 31P, and 11B NMR spectroscopy: (i) Monitoring the
catalytic borylation reaction using 2 or 3 as catalyst precursors,
in C7D8 or C6D6, respectively, showed the formation of the
corresponding borylated product and the coexistence of 2, 4
(major species), and unknown organometallic species together
with evolution of H2 and HD (Figures S17−S19). (ii) When
HBpin was added to a C6D6 solution of 3, monitoring the
reaction without irradiation at room temperature for 3 days led
to the detection of the hydrido(boryl) complex 4 in 15%
conversion. In the absence of any arene, addition of neat HBpin
to 3 and irradiation resulted in the formation of complex 4
together with Me-Bpin, as detected by the 11B NMR signal at
+33.9 ppm.21 (iii) We checked that isolated complex 4 catalyzes
the borylation of ethylbenzene in the standard conditions
(isolated yield of EtC6H4Bpin, 45% with a m/p ratio of 68/32).
(iv) At the stoichiometric level, complex 4, which is stable in
C6D6 without irradiation, undergoes B−C coupling upon
irradiation, and the deuterated complexes FeH(D)(dmpe)2
(2D) and FeD(C6D5)(dmpe)2 (5D) as well as evolution of
HD and H2 were detected (Figures S20, S21). When 4 was
dissolved in 30 equiv of DBpin, upon irradiation, scrambling
resulted in the formation of FeD(Bpin)(dmpe)2 (4D) and
HBpin (Figures S22, S23). (v) Finally, the hydrido(aryl)iron
complex FeH(C6H5)(dmpe)2 (5), in situ generated by the
reaction of 2 with benzene at room temperature under
irradiation,13a reacted with HBpin in the absence of irradiation,
and a mixture of 2 and 4 was characterized (Figures S24−S27; a
small amount of the B−C product was detected by GC). This is
in agreement with the reported reversibility of C−H arene
activation at [Fe(dmpe)2].13
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, NMR and IR data, DFT calculations, and
crystallographic data for 4 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*sebastien.bontemps@lcc-toulouse.fr
*jean-baptiste.sortais@univ-rennes1.fr
*sylviane.sabo@lcc-toulouse.fr
*christophe.darcel@univ-rennes1.fr
■
On the basis of all these data, we propose a mechanism for
the iron-catalyzed borylation of arenes under irradiation, which
is illustrated in Scheme 5. The dihydride complex 2 affords the
related Fe(0) compound under irradiation.13c This highly
reactive species can react with HBpin to generate the
hydrido(boryl) complex 4 (the observed H/D scrambling
experiments indicate that oxidative addition of HBpin to the
iron(0) species Fe(dmpe)2 is reversible under irradiation) or
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the CNRS and the ANR agency (program blanc 12-
BS07-0011-01 “Ironhyc”). C.D. and J.-B.S. thank the University
of Rennes 1. Dr. Christian Bijani (LCC) is warmly acknowl-
edged for NMR analyses. Computer time was given by the
HPC resources of CALMIP (Toulouse, France) under the
allocation 2014-[P0909].
Scheme 5. Proposed Mechanism for the Iron-Catalyzed
Borylation of Arenes under UV Irradiation
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DOI: 10.1021/jacs.5b00895
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