Catalytic borylation of methane

Article abstract of DOI:10.1126/science.aad9730

Despite steady progress in catalytic methods for the borylation of hydrocarbons, methane has not yet been subject to this transformation. Here we report the iridium-catalyzed borylation of methane using bis(pinacolborane) in cyclohexane solvent. Initially, trace amounts of borylated products were detected with phenanthroline-coordinated Ir complexes. A combination of experimental high-pressure and high-throughput screening, and computational mechanism discovery techniques helped to rationalize the foundation of the catalysis and identify improved phosphine-coordinated catalytic complexes. Optimized conditions of 150°C and 3500-kilopascal pressure led to yields as high as ～52%, turnover numbers of 100, and improved chemoselectivity for monoborylated versus diborylated methane.

Full text of DOI:10.1126/science.aad9730

RESEARCH
| REPORTS
results further highlight the impact of catalyst
on both reactivity and selectivity in the C–H
borylation of light alkanes.
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Overall, we have demonstrated that catalyst
structure has a major impact on reaction rates
and selectivities in the C–H borylation of meth-
ane. Over-functionalization of the initial product,
CH3Bpin, can be limited through the appropriate
selection of catalyst. These results open up ex-
citing possibilities for catalyst design (to further
modulate reactivity and selectivity in methane
C–H borylation) as well as the application of the
concepts delineated here for other light alkane
C–H functionalization reactions.
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SUPPLEMENTARY MATERIALS
ACKNOWLEDGMENTS
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C–H BOND ACTIVATION
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Despite steady progress in catalytic methods for the borylation of hydrocarbons, methane has
not yet been subject to this transformation. Here we report the iridium-catalyzed borylation of
methane using bis(pinacolborane) in cyclohexane solvent. Initially, trace amounts of borylated
products were detected with phenanthroline-coordinated Ir complexes. A combination of
experimental high-pressure and high-throughput screening, and computational mechanism
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ctivation of methane is challenging because
it is nonpolar, has strong sp3 C–H bonds, is
sparingly soluble in both polar and non-
polar solvents, and has very high ioniza-
tion energies and very low triple, boiling,
tions with methane have not been thoroughly
explored, despite this reaction being known for
more than a decade. Fundamentally important is
that the methyl-derived product is arguably a
form of a mildly nucleophilic methyl transfer re-
agent, which complements the chemistry observed
in electrophilic activation reactions in Shilov-type
chemistry (9). Theory predicts that borylation of
hydrocarbons with a borane (Eq. 1) is thermo-
neutral, whereas the weaker B–B bond in diboron
reagents provides an enthalpic driving force of
at least 12 kcal/mol, as shown in Eq. 2 (18). These
considerations led us to pursue the catalytic
borylation of methane using diboron reagents
such as B2pin2 (pin = pinacolate).
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and flashing points (1–8). Homogeneous catalysts
that convert methane to products that could be
used as liquid fuels are known, but these sys-
tems often require strong electrophiles and, in
some cases, superacids and/or powerful oxidants
(1, 2, 9–17). Chemoselectivity is another limita-
tion in methane activation and functionalization.
For instance, H3C-R (R = functional group) pro-
ducts resulting from methane activation and func-
tionalization have more reactive C–H bonds than
methane itself, hence often resulting in poor se-
lectivity, overfunctionalization, and overoxidation.
The pioneering work by Hartwig, Marder, and
Smith on C–H bond borylation inspired our in-
vestigation into the catalytic functionalization of
methane using a similar approach (18). Whereas
stoichiometric and catalytic borylations of al-
kanes show marked selectivity for monoborylation
of terminal methyl groups (18), analogous reac-
2
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Chem. Soc. 126
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Iridium systems are particularly promising for
C-H activation of methane (1, 2), and some of the
most active borylation catalysts use this transition
metal (18). Therefore, we focused our attention
on the commercially available iridium reagents
[Ir(COD)(m-Cl)]2, [Ir(COD)(m-OMe)]2 (COD = 1,5-
cyclooctadiene), and (MesH)Ir(Bpin)3 (MesH =
mesitylene) (19), modifying them with a range of
1
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*Corresponding author. E-mail: smithmil@msu.edu (M.R.S.);
mbaik2805@kaist.ac.kr (M.-H.B.); mindiola@sas.upenn.edu (D.J.M.)
4
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.
1424 25 MARCH 2016 • VOL 351 ISSUE 6280
sciencemag.org SCIENCE

RESEARCH
| REPORTS
nitrogen-based ligands, some of which are summa-
rized in Table 1. Suitable catalyst and reaction
conditions were identified systematically by means
of a high-pressure, high-throughput reactor (see
fig. S1 for details). Both [Ir(COD)(m-OMe)]2 and
(MesH)Ir(Bpin)3 complexes gave some conver-
sion to borylated methane products in cyclohexane
(CyH) or tetrahydrofuran (THF) at pressures
as low as 2068 kPa. Product yields were determined
by gas chromatography–mass spectrometry (GC-
MS) techniques with mesitylene as an internal
standard.
(supplementary text). Likewise, increasing the
temperature to 150°C did not improve the
reaction (table S6).
The mechanism of methane borylation was mod-
eled with density functional theory calculations
on the Ir-phenanthroline system, and the pro-
posed catalytic cycle is summarized in Fig. 1. Be-
fore catalysis can take place, the [Ir(COD)(m-X)]2
(X− = OMe or Cl) undergoes a series of ligand
substitutions to ultimately yield (phen)Ir(Bpin)3
Table 1. 1,10-phenanthroline ligands used in the borylation of methane. The ligands were added
in a 2:1 ratio relative to dimeric Ir reagent and in a 1:1 ratio relative to independently prepared (MesH)
Ir(Bpin)3. Solvent was either tetrahydrofuran (THF) or cyclohexane (CyH). Results with other ligands
are shown in table S4.
Ir(I) precatalysts with supporting ligand com-
binations were exposed for 16 hours at 120°C
to 2068 kPa of methane and B2pin2. Our re-
sults indicate that L3 (3,4,7,8-tetramethyl-1,10-
phenanthroline) is the best nitrogen ligand. Among
the products detected in the reaction mixture
were H3CBpin (1), H2C[Bpin]2 (2), HBpin (3),
and H3COBpin. We also observed the produc-
tion of O[Bpin]2. Because hydrolysis of 1 and 2
is very slow on the basis of control experiments,
we propose O[Bpin]2 to derive from a combina-
tion of hydrolysis of 3 during aerobic workup and
analysis by GC-MS, as well as decomposition of
B2pin2 or 3. The decomposition of B2pin2 may be
metal-catalyzed, as ring-opening of pinacolborane
with Ir catalysts has been documented recently
(20). We did not observe any tri- or tetraborylated
methyl products, H4-xC[Bpin]x (x = 3 or 4), where-
as borylation of the solvent is barely detected
under our conditions. Increasing CH4 pressures
in small increments to 8274 kPa did not improve
the mono- or diborylation reaction appreciably.
Although gem-diborylation of alkanes is unknown,
the gem-diborylation of benzylic groups has
been documented (21, 22). Because three boryl
moieties become incorporated into the active cat-
alyst, as illustrated in Fig. 1, the diboron additive
has an immediate impact on the reactivity. For
instance, no reaction takes place when B2cat2 (cat:
catechol) is used instead of B2pin2, which is con-
sistent with previous experimental and computa-
tional studies showing that borylation is favored
for more electron-rich catalysts (23, 24).
Entry
Ir reagent
Loading (mol %)
Ligand
Solvent
Percent yield 1
Ratio
1:2
.1..................................................................................2.....5..............................................................................................................................
[Ir(COD)(m-OMe)]2
L3
THF
2.6
n/a
.2..................................................................................5.....0..............................................................................................................................
[Ir(COD)(m-OMe)]2
L1
L2
L3
THF
THF
1.5
1.5
n/a
n/a
.3..................................................................................5.....0..............................................................................................................................
[Ir(COD)(m-OMe)]2
.4..................................................................................5.....0..............................................................................................................................
[Ir(COD)(m-OMe)]2
THF
CyH
2.7
2.0
n/a
3.9:1
2.5:1
.5..................................................................................2.....5..............................................................................................................................
[Ir(COD)(m-OMe)]2
[Ir(COD)(m-OMe)]2
(MesH)Ir(Bpin)3
L3
L3
L3
.6..................................................................................5.....0..............................................................................................................................
CyH
THF
THF
2.3
4.1
2.4
7
8
9
...................................................................................2.....5..............................................................................................................................
n/a
(MesH)Ir(Bpin)3
...................................................................................5.....0..............................................................................................................................
L5
n/a
(MesH)Ir(Bpin)3
...................................................................................2.....5..............................................................................................................................
L5
CyH
1.7
2.9:1
Fig. 1. Proposed cycle
for the monoboryla-
tion of methane with
1,10-phenanthroline
as a supporting
ligand.
The observed lower yield of 3 compared to
1 and 2 may be due to a second, slower borylation
cycle that consumes 3 (Eq. 1). Consistent with
our findings, we have observed that 3 can be
used as a reactant replacing B2(pin)2, but this
reaction is much slower at 120°C (table S7).
Other diboron reagents, such as B2(OH)4 or
B2(NMe2)4, produced complex mixtures of prod-
ucts with intractable precipitates.
Table 1 summarizes some of our screening re-
sults with the most promising chelating polypyridyl
ligands. The use of ligands L1 and L2 gave detect-
able amounts of H3CBpin, whereas the best re-
sults were obtained with L3 (3,4,7,8-tetramethyl-1,
10-phenanthroline), which showed yields as
high as 4.1% and chemoselectivity ratios of
mono- versus diborylated products 1:2 as high
as 4:1. Surprisingly, even [Ir(COD)(m-OMe)]2 with-
out exogenous ligand resulted in some boryla-
tion (<1%) but overall (25), the results listed in
Table 1 suggest these systems to be stoichio-
metric with respect to methane borylation
SCIENCE sciencemag.org
25 MARCH 2016 • VOL 351 ISSUE 6280 1425

RESEARCH
| REPORTS
Fig. 2. Computed structures of catalytic cycle states in Fig. 1. Nonessential hydrogens are omitted for clarity.
ligand (Me2PCH2CH2PMe2) improved the reac-
Table 2. Variations of catalyst loading and time with the ligand dmpe for the borylation of
methane. The ligand dmpe (Me2PCH2CH2PMe2) was used in a 2:1 ratio relative to the Ir precatalyst
[Ir(COD)(m-Cl)]2 in cyclohexane (CyH) under 3447 kPa of CH4.
tion substantially. Table 2 summarizes the best
results from our screening. Varying catalyst load-
ings from 0.5 to 25 mole percent (mol %) led to
conversion yields as high as 52% and catalytic
turnover numbers (TONs) up to 104 with selectiv-
ity of 3:1 for monoborylated product 1 versus 2.
Increasing the mol % of catalyst resulted in lower
conversion, though the selectivity for mono- versus
diborylation (1:2 ratio) of methane increased to
as high as 9:1 (entry 1). Pressures below 1379 kPa
afforded lower conversions, whereas pressures
above 3447 kPa did not greatly improve the over-
all yield of products. Reactions required 16 hours
for completion, and control experiments using
similar amounts of dmpe/[Ir(COD)(m-Cl)]2 and
1 as a reagent with 40 equivalents of B2(pin)2
(with or without methane) did yield the diboryl-
ated product 2. This result implies that the yield
of monoborylation product is always greater than
for diborylation with the dmpe scaffold.
Entry
Loading
(mol %)
Time
Percent yield 1
1:2
TON
(hours)
1
25
16
9.4
9:1
<1
.....................................................................................................................................................................................................................
2
10
16
16
5:1
~1
.....................................................................................................................................................................................................................
3
5.0
16
16
16
...................................................................................................................................2...3...............................................................................
5:1
4
.4.........................................1....0........................................................................................................................................................................
25 5:1 25
5
0.5
5
2
3:1
104
.
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6
25
6
17
..............................................................................................................................................................................7...:.1............................<...1....
7
10
6
...................................................................................................................................2...5...............................................................................
7:1
~2
An inverse relationship between precatalyst con-
centration and borylation conversion has previous-
ly been observed in borylations with [Ir(COD)(m-Cl)]2
precatalysts and N-chelating ligands, but no ex-
planation was provided for this behavior (29).
Recently, Finke and co-workers have analyzed
similar counterintuitive behavior in hydrogenations
with Ziegler-type nanoparticle catalysts prepared
from Ir precatalysts (30). Likewise, benzene boryl-
ation has been described with Ir nanoparticles at
80°C with activities that are considerably lower
than those for homogeneous catalysts (31). Both
of these Ir nanoparticle–catalyzed reactions are
poisoned by Hg. In our case, Hg addition to the re-
actions listed in Table 2 did not suppress catalysis. In
addition, borylations with dmpe and phenanthroline-
based ligands at 150°C with identical precatalyst
loadings and concentrations give very different
conversions (table S6). These observations are con-
sistent with a homogeneous process in which the
nature of the ligand affects catalysis. Lastly, meth-
ane activation over Ir/ZrO2 has been described,
but high temperatures (~600°C) are typically re-
quired for these processes (32).
8
5.0
6
27
6:1
5
.....................................................................................................................................................................................................................
9
..........................................1....0........................................................................................................................................................................
.1.0........................................0.....5.....................................................................................2...8........................................6...:.1............................5..6.....
.1.1.........................................1..0......................................................................................2...2........................................3...:.1............................2..2.....
.1.2........................................5.....0......................................................................................2..7........................................4...:.1...................................
.1.3........................................1....0......................................................................................2...4........................................4...:.1............................2..4.....
.1.4........................................0.....5......................................................................................1..6........................................2...:.1............................3..2.....
6
21
8:1
21
6
2
2
5
2
2
(phen = 1,10-phenanthroline). This complex is the
most plausible resting state of the catalyst and
consists of an Ir(III)-d6 center in a pseudo–
square-pyramidal coordination geometry labeled
as a (see Fig. 2). The catalytic cycle commences
with weak binding of methane at the empty co-
ordination site to give the intermediate complex
b, followed by oxidative addition traversing the
likely rate-determining transition state b-TS at
25.9 kcal/mol (26). The iridium center in this inter-
mediate c adopts a rare, but not unprecedented,
seven-coordinate geometry (27). Next, the hydride
and borane ligands swap position to give access to
c-iso that can undergo reductive elimination of
the boryl-methane product 1 to afford the Ir(III)-
complex d, which reacts with another equivalent
of the diboron source to regenerate the catalyst
resting state a. We considered several alternative
mechanisms, most notably a s-bond metathesis
pathway (28), but found that the mechanism shown
in Fig. 1 is energetically most favorable. A de-
tailed analysis of the computational results sug-
gested a potential optimization strategy: As the
H–CH3 bond is cleaved at the transition state
b-TS, the Ir-center must undergo formally an
oxidation from Ir(III) to Ir(V). Therefore, the hard
N-based Lewis base ligands may not provide the
ideal supporting ligand framework, as these lig-
ands tend to decrease the polarizability of the
valence electrons of the metal. Softer Lewis bases,
such as the phosphine analogs of the N ligands,
seemed likely to prove beneficial by increasing
the polarizability of the metal.
We tested the simple qualitative rationale
from our computer model by exploring whether
phosphine ligands offered improved reactivity
toward C–B bond formation. Initial screens
showed that phosphine ligands do not result in
any notable borylation at 120°C with 2068 to
3447 kPa of methane, but at 150°C the dmpe
Because dmpe/[Ir(COD)(m-Cl)]2 afforded the
cleanest yield of monoborylated product 1, we
conducted isotopic labeling studies using 13CH4-
enriched methane (99% atom enriched, 1379 kPa)
to unambiguously establish that methane gas is
the source of methyl in 1. As anticipated, GC-MS
results conclusively established the formation of
1426 25 MARCH 2016 • VOL 351 ISSUE 6280
sciencemag.org SCIENCE

RESEARCH
| REPORTS
1-13C as the only product derived from 13CH4
borylation (fig. S7), excluding the possibility of
pinacol or solvent degradation as possible sources
of CH3. Mechanistically, we expect the phosphine
system to follow the same route outlined above
for the polypyridyl systems.
MAGNETOHYDRODYNAMICS
Large-scale magnetic fields
at high Reynolds numbers in
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ACKNOWLEDGMENTS
A
U
p
n
o
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S
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p
r
o
v
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s
i
i
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i
B
s
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n
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t
w
a
h
a
s
b
e
e
n
f
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d
f
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h
i
s
w
y
o
t
t
t
r
h
k
.
F
n
o
k
u
n
n
i
n
a
v
S
t
n
c
i
a
l
s
u
p
o
r
y
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*Corresponding author. E-mail: hotta@chiba-u.jp
SCIENCE sciencemag.org
25 MARCH 2016 • VOL 351 ISSUE 6280 1427

Catalytic borylation of methane
Kyle T. Smith et al.
Science 351, 1424 (2016);
DOI: 10.1126/science.aad9730
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