Metal-free disproportionation of formic acid mediated by organoboranes

Article abstract of DOI:10.1039/c6sc01410k

In the presence of dialkylboranes, formic acid can be converted to formaldehyde and methanol derivatives without the need for an external reductant. This reactivity, in which formates serve as the sole carbon and hydride sources, represents the first exam

Full text of DOI:10.1039/c6sc01410k

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Metal-free disproportionation of formic acid
mediated by organoboranes†
Cite this: DOI: 10.1039/c6sc01410k
Clement Chauvier, Pierre Thuery and Thibault Cantat*
´
´
In the presence of dialkylboranes, formic acid can be converted to formaldehyde and methanol derivatives
without the need for an external reductant. This reactivity, in which formates serve as the sole carbon and
hydride sources, represents the first example of the disproportionation of formate anions under metal-free
conditions. Capitalizing on both experimental and computational (DFT) mechanistic considerations, the
role of transient borohydride is highlighted in the reduction of formates and this reactivity was further
exemplified in the methylation of TMP (2,2,6,6-tetramethylpiperidine) and in the transfer hydroboration
reactions for the reduction of aldehydes.
Received 30th March 2016
Accepted 19th May 2016
DOI: 10.1039/c6sc01410k
www.rsc.org/chemicalscience
9
production of methanol. This strategy relies on the catalytic
Introduction
2 2
electroreduction of CO to HCO H, a technically and economi-
10
Formic acid is an attractive reductant in organic chemistry cally viable process which is under pilot development. HCO
2
H
because it is a benign and liquid surrogate for molecular can then serve as a C–H bond shuttle to yield methanol,
hydrogen. In fact, the use of HCO
2
H in transfer hydrogenation assuming that efficient systems can promote the dispropor-
1
has been successfully applied to a variety of organic substrates,
tionation of formic acid (Scheme 1). Nevertheless, the dispro-
including carbonyl groups and C]C systems, and the devel- portionation of HCO
2
H is a difficult transformation because the
opment of catalysts for these transformations is still under dehydrogenation of formic acid is favored over the reduction of
2
active study. The reasons behind this success originate in the a formate group. The rst catalysts for the disproportionation of
kinetic and thermodynamic properties of HCO H. Indeed, for- formic acid were only unveiled in 2013 (ref. 11) by Goldberg,
2
0
mic acid presents a redox potential similar to H E (CO /HCO H Miller et al. who showed that iridium(III) complexes could
2
2
2
0
¼
ꢀ0.61 V vs. E (H
2
O/H
2
¼ ꢀ0.41 V at pH ¼ 7 vs. NHE) but the convert formic acid to methanol with a maximum yield of 2.6%.
is in fact the main product during the iridium-catalyzed
H. In 2014, our group reported ruth-
C–H bond has a Bond Dissociation Energy (BDE) which is about
H
2
ꢀ1
3
1
6 kcal mol weaker than the H–H bond and it is thus easier to decomposition of HCO
activate at a metal center. More recently, the high hydrogen enium(II) catalysts for this transformation and the selective
2
9
content of formic acid (4.4%) has been recognized as a means to conversion of HCO
store H2 in liquid form, under ambient conditions. This ruthenium catalysts still represent the state of the art in this
2
H to 50% methanol was achieved. While the
4
concept relies on the selective decomposition of HCO H to H
transformation, Parkin et al. described the rst catalysts
and CO2 and a plethora of molecular catalysts have been without noble metals, using molybdenum(II) complexes. With
2
2
12
5
designed to catalyze this important transformation. While all these systems, H
catalysts based on noble metals are the most efficient systems, least 50% yield.
the state-of-the-art is currently shiing towards earth abundant
is produced as a competing product in at
2
6
7
metal catalysts, based on iron or aluminum complexes. In
015, our group reported the rst organic catalysts for the
2
8
dehydrogenation of formic acid, using dialkyboranes.
Beyond formic acid, methanol can store 12.1 wt% hydrogen
ꢀ
1
and this high energy chemical (4900 vs. 2104 W h L for
HCOOH) represents an excellent energy vector. Formic acid has
been recently proposed as a sustainable intermediate in the
NIMBE, CEA, CNRS, Universit ´e Paris-Saclay, Gif-sur-Yvette, France. E-mail: thibault.
cantat@cea.fr; Fax: +33 1 6908 6640
†
Electronic supplementary information (ESI) available: Experimental and
computational procedures and physical properties of compounds. CCDC
450409–1450413 and 1454182. For ESI and crystallographic data in CIF or
1
Scheme 1 Disproportionation of formic acid to the methanol level
other electronic format see DOI: 10.1039/c6sc01410k
and the competing dehydrogenation.
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Reasoning that on the one hand dialkylboranes are catalysts disproportionation and dehydrogenation of formic acid are
8
for the dehydrogenation of formic acid and that on the other competing in the presence of the dialkylborane, although stoi-
hand hydroboranes can reduce CO to methoxyboranes, in the chiometric amounts of protons are present in the reaction
2
13
+
presence of a catalytic amount of an organic base, we have mixture (in the form of HNEt ) with respect to the formates.
3
+
ꢀ
sought metal-free systems able to mediate the disproportion- Under catalytic conditions (using 5 mol% [HNEt
ation of formic acid. Herein, we demonstrate that stoichio- dehydrogenation of HCO
metric amounts of dialkyborane derivatives can convert together with only a catalytic quantity of CH
3
, 1 ]), the
ꢁ
2
H was observed aer 24 h at 130 C,
3
OBBN (<5 mol%).
, 1 ] was isolated from the reaction between 9-BBN,
rst time. Mechanistic insights derived from the experimental HCOOH and triethylamine and its disproportionation to
results and DFT calculations highlight the role of transient CH OBBN was investigated to gain further insights into this
+
ꢀ
formates to formaldehyde and methanol derivatives, for the
[HNEt
3

3
+
3
ꢀ
borohydride in these transformations.
novel transformation. The thermolysis of [HNEt , 1 ] is ineffi-
cient in THF or benzene and it is best carried out in acetonitrile
ꢁ
at 130 C (entries 1–4 in Table 1). Under these conditions,
+
ꢀ
Results and discussion
[HNEt
3
, 1 ], is completely decomposed to H
2 3
and CH OBBN
aer 10.5 h (entry 4 in Table 1). CH
yield, meaning that 39% of the C–H bonds present in the
formate ligands of 1 are efficiently preserved as C–H bonds in
the methoxide ligand (see ESI†). Increasing the reaction
temperature to 150 C only affects the rate of the thermolysis and
CH OBBN is obtained in 33% yield aer only 4 h. Importantly,
the disproportionation of formates is not specic to the BBN
framework and the dicyclohexylborane derivative [HNEt , 2 ] is
3
3
OBBN is formed in 39%
9-Iodo-9-borabicyclo[3.3.1]nonane (BBN–I) was shown to
promote the catalytic decarboxylation and dehydrogenation of
formic acid and its reaction chemistry was thus explored in the
ꢀ
8
presence of formate anions. Adding 2 molar equivalents of
ꢁ
formic acid and triethylamine to an acetonitrile solution of
BBN–I affords bis(formoxy)borate [HNEt , 1 ] along with
3
+
ꢀ
3
HNEt I, at room temperature (RT) (eqn (1)).
+
ꢀ
3
also prone to generate the corresponding Cy
the latter gave low yields of the methoxyborane at 130 C,
presumably due to the thermal instability of the reaction inter-
2
BOCH
3
. Although
ꢁ
ꢁ
mediates, the reaction proceeded smoothly at 120 C to afford
Cy BOCH in 38% yield aer 7 h (entry 7 in Table 1). In all cases,
2
3
free methanol can be ultimately generated by hydrolysis of
methoxyborane (see ESI†). The nature of the base used to prepare
the starting bis(formoxy)borate also inuences the efficiency of
+
the disproportionation. Indeed, the thermolysis of [iPr
2
EtNH ,
ꢀ
ꢁ
1
] at 130 C leads to CH
3
OBBN in 50% yield within only 4.5 h.
This improved selectivity demonstrates the positive inuence of
ꢁ
Heating this mixture at 130 C leads to the complete bulky tertiary amines, such as iPr
decomposition of the formate ligands in 1 and the concomi- the decreased affinity of the amine for the boron center.
EtN, which might arise from
2
ꢀ
14
1
3
tant formation of CO
2
was observed by GC and C NMR
is also observed in the gas phase.
Notably, a singlet at 3.71 ppm is also observed in the H NMR
spectrum, coupled with a weak resonance at 53.9 ppm that was
attributed to a primary carbon in the C NMR spectrum. To
verify this chemical behavior, the same chemical sequence was
(eqn (2)). As expected, H
2
1
Table 1 Boron mediated disproportionation of formate anions into
a
methoxyboranes
1
3
1
3
reproduced with H CO
2
H. At RT, in acetonitrile, the protons of
1
3
ꢀ
the formate in C-labelled 1 display a doublet (d ¼ 8.43 ppm,
1
13
J
C–H ¼ 202 Hz) coupled to a singlet at 168.1 ppm in the
C
ꢁ
1
NMR spectrum. Aer 3 h at 130 C, the H NMR spectrum of the
crude mixture indicates the formation of H2 gas and
1
3
1
1
13
CH OBBN ( H NMR: d ¼ 3.71 ppm, J
¼ 143 Hz; C NMR:
3
C–H
+
c
ꢁ
b
Entry
R
2
B
BaseH
t [h]
T [ C]
Solvent
Yield [%]
1
3
13
d ¼ 53.9 ppm). Aer 20 h, CO
2 3
and CH OBBN were the only
1
3
+
+
+
+
C-enriched products observed in solution. Aer the removal of
the volatiles under a reduced pressure, the resulting solid was
dissolved in d -toluene to afford a homogeneous solution
comprising methoxyborane (d( B) ¼ 57 ppm) and diboroxane
1
2
3
4
5
6
7
BBN
BBN
BBN
BBN
BBN
Et NH
>48 h
>48 h
4
10.5
4.5
130
130
150
130
130
130
120
d8-THF
<5
<5
33
39
50
<5
38
3
Et
3
Et
3
Et
3
NH
NH
NH
C
6
D
6
CD
CD
CD
CD
3
3
3
3
CN
8
1
1
CN
CN
CN
+
iPr
Et
Et NH
2
EtNH
1
1
(BBN)
2
O (d( B) ¼ 59 ppm) as the sole boron-containing prod-
+
BCy
BCy2
2
3
NH
2.5
+
ucts. These results clearly demonstrate that formate anions can
disproportionate to methoxides in the coordination sphere of
boron, with the associated release of CO
7
3
CD CN
3
a
ꢀ
ꢀ
b
Reaction conditions: 0.125 mmol 1 or 2 , 0.4 mL solvent. Yields
determined by H NMR using mesitylene (10 mL); mean value over at
and diboroxane. The
1
2
c
parallel formation of H
2
and CH
3
OBBN shows that the least 2 runs. Time required to obtain >95% conversion of formate.
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To assess whether H
2
, released by dehydrogenation, plays Although stoichiometric amounts of dialkylboranes are
any role in disproportionation, the corresponding bis(formoxy) required to promote the disproportionation of formates, the
+
ꢀ
ꢀ
ꢀ
borate [Et ND , R B(OCHO) ] was synthesized in situ in formation of methoxyboranes from 1 and 2 represents the
3
2
2
toluene at RT from HCO D. As expected, its thermolysis at rst examples of metal-free mediated disproportionations of
2
ꢁ
1
1
4
30 C in CH
3
CN affords gaseous HD (d
H
¼ 4.54 ppm, t, JHD
¼
formates. The mechanism of this transformation was thus
1
3 Hz), as made evident by H NMR. However, no deuterium- investigated both experimentally and computationally in order
containing reduced product (other than HD) was detected by to track possible reaction intermediates. Monitoring the
1
13
+
ꢀ
either H or C NMR (Fig. S17†) spectroscopy at intermediate or thermal decomposition of [HNEt
3
, 1 ] by NMR reveals the
full conversion of the formate. In addition, the thermolysis of formation of a single transient species 3 (<5% yield), charac-
+
ꢀ
1
aprotic [Na , 1 ] in the presence of the crown ether DB18C6 terized by a singlet at 4.40 ppm in the H NMR spectrum, which
1
3
(
2
dibenzo-18-crown-6) also affords CH OBBN in a 53% yield aer is associated with a C resonance at 80.5 ppm. Reasoning that 3
2 h at 130 C (eqn (4)), thus proving that the disproportion- is an acetal compound, its independent synthesis was carried
ation can proceed without dehydrogenation. In that case, the out by reacting hydroborane 9-BBN with [HNEt
conversion reached a plateau at ca. 80%, induced by the release is obtained as the major reduced product, along with CH
of somewhat unreactive HCO
conditions.
3
ꢁ
+
ꢀ
3
, 1 ] (eqn (5)). 3
OBBN
2
Na under the applied reaction (10 : 1.3 ratio), and the formulation of an acetal compound was
3
conrmed by X-ray diffraction (Fig. 1A). 3 is a zwitterionic
adduct of formaldehyde in which the carbon atom is bound to
NEt and the oxygen atom coordinates a formoxyborane group,
3
namely BBNOCHO. The latter formate ligand coordinates
15
a second equivalent of the Lewis acid BBNOCHO.
Overall, the disproportionation of formate anions mediated
by dialkylboranes proceeds with yields and selectivities (up to
9,11,12
53%), comparable to state-of-the-art metal catalysts.
The nding that nucleophilic bases such as NEt can stabi-
3
lize an intermediate formaldehyde adduct suggests the occur-
rence of a novel and unique transformation, namely the
selective disproportionation of formates to CO and formalde-
2
hyde. We have thus sought a Lewis base able to direct the
selectivity towards the formation of formaldehyde surrogates
from the disproportionation of boron formates. A phospho-
+
ꢀ
nium salt [Cy
3
PH , 1 ] was synthesized and its thermolysis
ꢁ
leads, aer 5.5 h at 130 C, to the formation of H , CO , free
2
2
PCy3 and the formaldehyde adduct 4 as the only reduction
product (eqn (6)).
The X-ray analysis of crystals obtained by slowly cooling the
reaction mixture reveals that 4 features a formaldehyde unit
C-coordinated to the phosphorus Lewis base and O-coordinated
to a BBN–OCHO unit (Fig. 1B). It thus resembles the related
(
tBu
coworkers, from the hydroboration of CO
exhibits an enhanced stability towards reduction in comparison
3
PCH
2
O)(HC(O)O)B(C
8
H
14
)
isolated by Stephan and
16
2
. As expected, 4
to 3. Its conversion to CH
3
OBBN is slow and requires 10 h at
Fig. 1 ORTEP views of 3 (A) and 4 (B). The hydrogen atoms of the BBN,
ethyl and cyclohexyl groups are omitted for clarity.
ꢁ
17
130 C to proceed (30% yield).
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1
ꢀ
two k -HCOO ligands. Hydride transfer from a monodentate
ꢀ
formate ligand thus provides intermediate 6 , which further
dissociates into CH (OBBN) and a free HCO2 anion. This
ꢀ
2
2
pathway represents a boron variant of the MPV mechanism.
Nevertheless, its occurrence is highly unlikely because the
ꢀ
ꢀ
transition state (TS5ꢀ6) connecting 5 and 6 is high in energy
ꢀ
1
and it involves an overall barrier of 43.9 kcal mol , incom-
patible with the experimental conditions.
Alternatively, a mechanism relying on the transient forma-
tion of a borohydride species is favored. The dissociation of one
ꢀ
ꢀ1
formate ligand from 1 is slightly endergonic (+11.1 kcal mol
)
but it provides an unsaturated formatoborane (7), able to
ꢀ
promote the decarboxylation of HCO
2
with a low energy barrier
ꢀ
1
of 15.5 kcal mol by hydride abstraction. This process affords
a reductant, BBN(H)(OCOH) (8 ). The displacement of
a formate ligand in 1 with 8 provides dimer 9 (+18.0
kcal mol ) which features a bridging HCOO ligand, similar to
and 3. Hydride transfer from boron to the bidentate formate
ligand in 9 yields 6 , with a low energy barrier of 16.1
kcal mol . From 6 , the acetal CH (OBBN) (+10.1 kcal mol
can be released or trapped as adduct 3 (ꢀ8 kcal mol ) in the
presence of NEt . Overall, this mechanism only involves an
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ1
ꢀ
ꢀ
5
ꢀ
ꢀ
ꢀ
Scheme 2 Computed mechanism for the disproportionation of 1 to
the formaldehyde level. The full energy surface is provided in Scheme S1.†
ꢀ
1
ꢀ
ꢀ1
)
2
2
ꢀ1
3
ꢀ1
ꢀ
energy barrier of 34.1 kcal mol , corresponding to the energy
The disproportionation of 1 to CH
2
(OBBN)
2
and CH
3
OBBN
span between the starting materials and TS9ꢀ6, and it is
was investigated using DFT calculations (M06-2X/6-311+G(d,p)
level of theory, in acetonitrile (PCM)) so as to determine how
a C–H bond from a formate anion can be transferred to a second
formate ligand in the boron coordination sphere. The thermo-
compatible with thermolysis occurring within several hours at
ꢁ
1
30 C.
ꢀ
dynamic balance for the disproportionation of 1 is given in
ꢀ
Scheme 2. From 2 equiv. of 1 the formation of CH
2
(OBBN)
2
is
ꢀ1
slightly endergonic with DG ¼ 10.1 kcal mol and 2 equiv. of
ꢀ
HCO
1
1
2
and 1 equiv. of CO
2
are released. Further addition of
ꢀ
equiv. of 1 enables the formation of CH
3
OBBN together with
ꢀ
ꢀ1
equiv. of HCO
2
and CO
2
(DG ¼ ꢀ3.9 kcal mol ). It is note-
ꢁ
ꢀ
worthy that, under the applied reaction conditions (130 C), 1
Experimentally, strong evidence for the involvement of a B–H
ꢀ
also catalyzes the decarboxylation of the free HCO2 anions, bond was further gained by carrying out the thermolysis of
+
12
ꢀ
13
thereby shiing the equilibria towards the formation of the [HNEt
products (see ESI†).
3
, (H C(O)O)
2
BBN ] under CO
2
atmosphere (1 atm)
ꢁ
12
(eqn (7)). Aer 3 h at 130 C, a mixture of H C(O)O[B] and
Experimental observations show that the disproportionation H C(O)O[B] along with CH
of 1 to the formaldehyde redox state is rate determining, as 9 h, the additional formation of CH
1
3
12
OBBN was observed and, aer
3
ꢀ
13
3
OBBN became evident
CH
mechanism of this step was computed. Two routes can be between CO
proposed. First, transfer hydrogenation with aluminum(III) for an MPV pathway, it rather points to the involvement of an
2
(OBBN)
2
and 3 do not accumulate, and thus only the (Fig. S14†). This result stresses the presence of an equilibrium
ꢀ
2
and HCOO and, while such a process is unlikely
ꢀ
ꢀ
isopropoxide salts is well documented and a mechanism intermediate borohydride species BBN(H) (OCHO) (8 ). This
ꢀ
4ꢀ
similar to the Meerwein–Pondorf–Verley (MPV) reaction repre- proposal is also consistent with the observation that HB(C
6
F
5
)
3
18
sents a rst plausible pathway. In a second option, the (obtained by H
decarboxylation of a formate ligand could afford a boron are able to reduce CO
splitting with a Frustrated Lewis Pair) and BH
2
to the methanol and formate levels
2
19
hydride, whose B–H functionality reduces a second formate respectively.
anion. DFT calculations were performed to distinguish the two
The thermal decarboxylation of a formate anion in 1
ꢀ
ꢀ
ꢀ
pathways and the results are summarized in Scheme 2, with the provides a reductant (8 ) and we have computed that 8 has
ꢀ
ꢀ1
full energy surface being provided in Scheme S1 (ESI†). Starting a hydride donor ability similar to BBNH
from 2 equiv. of 1 , the exclusion of one formate ligand yields respectively).
2
(34 vs. 33 kcal mol
ꢀ
20
ꢀ
ꢀ
8 (and 1 ) is thus a potent reductant and its
ꢀ
dimer
5
in
a
slightly endergonic process (DG
¼
reductive properties were further investigated, in the presence
ꢀ
1
ꢀ
+
4.4 kcal mol ). In dimer 5 , one formate ligand is bridging of various oxidants (eqn (9) and (10)). Building on the dispro-
ꢀ
two boron centers and this activation increases the electrophi- portionation of formates in 1 to CH
licity of the carbon atom. In fact, the NBO charge of the carbon a secondary amine was attempted. The tetramethylpiper-
OBBN, the methylation of
3
2
0
ꢀ
+
ꢀ
atom in the m (O,O )–HCOO ligand is 0.77 vs. 0.71 in the other idinium salt [TMPH , 1 ] was rst prepared and decomposed
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ꢁ
quantitatively within 17 h at 130 C in acetonitrile. Interestingly, Research Council (ERC Starting Grant Agreement no. 336467).
CH OBBN does not form under these conditions and N-methyl- T. C. thanks the Foundation Louis D. – Institut de France for its
3
tetramethylpiperidine was obtained instead in 23% yield, along support.
with H , CO , BBN O and free TMP (eqn (8)). In this reaction,
2
2
2
ꢀ
the formate ligands in 1 serve both as a H and C source for the
21
formation of the N–CH
Interestingly, when the thermolysis of [HNEt
out in the presence of 1 molar equiv. of benzaldehyde,
3
linkage.
Notes and references
+
ꢀ
3
, 1 ] is carried
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PhCH
2
OBBN (10a) is obtained in a 67% yield, further conrm-
ꢀ
ing that bis(formoxy)borate 1 is able to deliver a useful
reducing agent (eqn (9)). The parallel formation of CH OBBN
3
(12% yield) shows that the disproportionation of formates is
ꢀ
a surprisingly facile process from 1 . This result represents the

rst example of reduction of a carbonyl substrate by transfer
hydrogenation from formic acid, under metal-free conditions. It
can also be described as a transfer hydroboration. In fact, the
22
reduction of various aldehydes can be efficiently carried out
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+
ꢀ
with 2 molar equivalents of [HNEt , 1 ]. Under these condi-
3
tions, benzaldehyde is reduced to 10a in 99% yield and bor-
ylethers 10b and 10c were obtained respectively in 99 and 80%
yield from the corresponding aldehydes (eqn (10)).
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c) B. Loges, A. Boddien, F. G ¨a rtner, H. Junge and
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8
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9
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In conclusion, we have shown that formic acid can undergo 11 A. J. Miller, D. M. Heinekey, J. M. Mayer and K. I. Goldberg,
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disproportionation of formates to formaldehyde and methanol
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conditions, for the rst time. Current work in our laboratory is 14 H. C. Brown and S. U. Kulkarni, Inorg. Chem., 1977, 16, 3090–
devoted to facilitating the generation of borohydrides from
3094.
formic acid and increasing the hydride donor ability of the 15 In contrast, the combination of [iPr
+
ꢀ
2
EtNH , 1 ] with 9-BBN
and CH OBBN, without
resulting B–H group.
exclusively leads to H
2
C(OBBN)
2
3
involvement of any zwitterionic formaldehyde adduct.
1
6 T. Wang and D. W. Stephan, Chem. Commun., 2014, 50,
Acknowledgements
7007–7010.
For nancial support of this work, we acknowledge CEA, CNRS, 17 Analogously, the thermolysis of the adduct 3 eventually
the CHARMMMAT Laboratory of Excellence and the European
3
affords CH OBBN albeit much faster than 4.
This journal is © The Royal Society of Chemistry 2016
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1
1
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2
1818–1827.
Chem. Sci.
This journal is © The Royal Society of Chemistry 2016