Metal-free reduction of CO2 with hydroboranes: Two efficient pathways at play for the reduction of CO2 to methanol

Article abstract of DOI:10.1002/chem.201400349

Guanidines and amidines prove to be highly efficient metal-free catalysts for the reduction of CO₂ to methanol with hydroboranes such as 9-borabicyclo[3.3.1]nonane (9-BBN) and catecholborane (catBH). Nitrogen bases, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7- triazabicyclo[4.4.0]dec-5-ene (Me-TBD), and 1,8-diazabicycloundec-7-ene (DBU), are active catalysts for this transformation and Me-TBD can catalyze the reduction of CO₂ to methoxyborane at room temperature with TONs and TOFs of up to 648 and 33 h^-1 (25°C), respectively. Formate HCOOBR₂ and acetal H₂C(OBR₂)₂ derivatives have been identified as reaction intermediates in the reduction of CO₂ with R₂BH, and the first C-H-bond formation is rate determining. Experimental and computational investigations show that TBD and Me-TBD follow distinct mechanisms. The N-H bond of TBD is reactive toward dehydrocoupling with 9-BBN and affords a novel frustrated Lewis pair (FLP) that can activate a CO₂ molecule and form the stable adduct 2, which is the catalytically active species and can facilitate the hydride transfer from the boron to the carbon atoms. In contrast, Me-TBD promotes the reduction of CO₂ through the activation of the hydroborane reagent. Detailed DFT calculations have shown that the computed energy barriers for the two mechanisms are consistent with the experimental findings and account for the reactivity of the different boron reductants.

Full text of DOI:10.1002/chem.201400349

DOI: 10.1002/chem.201400349
Full Paper
&
Organocatalysis
Metal-Free Reduction of CO with Hydroboranes: Two Efficient
2
Pathways at Play for the Reduction of CO to Methanol
2
[
a]
Christophe Das Neves Gomes, Enguerrand Blondiaux, Pierre Thuꢀry, and Thibault Cantat*
Abstract: Guanidines and amidines prove to be highly effi-
Experimental and computational investigations show that
cient metal-free catalysts for the reduction of CO to metha-
TBD and Me-TBD follow distinct mechanisms. The NÀH bond
of TBD is reactive toward dehydrocoupling with 9-BBN and
affords a novel frustrated Lewis pair (FLP) that can activate
2
nol with hydroboranes such as 9-borabicyclo[3.3.1]nonane
(9-BBN) and catecholborane (catBH). Nitrogen bases, such as
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7-
a CO molecule and form the stable adduct 2, which is the
2
triazabicyclo[4.4.0]dec-5-ene (Me-TBD), and 1,8-diazabicy-
cloundec-7-ene (DBU), are active catalysts for this transfor-
catalytically active species and can facilitate the hydride
transfer from the boron to the carbon atoms. In contrast,
mation and Me-TBD can catalyze the reduction of CO to
Me-TBD promotes the reduction of CO through the activa-
2
2
methoxyborane at room temperature with TONs and TOFs
tion of the hydroborane reagent. Detailed DFT calculations
have shown that the computed energy barriers for the two
mechanisms are consistent with the experimental findings
and account for the reactivity of the different boron reduc-
tants.
À1
of up to 648 and 33 h
(258C), respectively. Formate
HCOOBR and acetal H C(OBR ) derivatives have been iden-
2
2
2 2
tified as reaction intermediates in the reduction of CO with
2
R2BH, and the first CÀH-bond formation is rate determining.
Introduction
who used nickel(II) complexes supported by phosphane pincer
[6]
ligands I (Scheme 1). Based on mechanistic investigations,
the authors proposed a catalytic mechanism that involved suc-
cessive s-bond metathesis reactions and the insertion of CO2
into a reactive NiÀH bond. Consecutively, Bontemps and Sabo-
Etienne described ruthenium–polyhydride complexes II able to
Carbon dioxide is an attractive C building block for the pro-
1
duction of chemicals and fuels because it is an economical, re-
[
1]
newable, and nontoxic carbon source. Nonetheless, the kinet-
ic and thermodynamic stability of CO strongly limits the scope
2
[7]
of molecules available from CO2 and efficient catalysts are
needed to promote its reduction. Hydrogen, hydrosilanes, and
hydroboranes possess redox potentials that are well suited for
promote the same catalytic transformation. Importantly, it
was shown that the ruthenium system leads to the formation
of an unexpected C intermediate that results from the CÀO
2
the reduction of CO to formic acid or methanol and recent ef-
2
forts have led to the successful design of metal catalysts able
to accelerate these reactions, most of them being based on
[
2–10]
noble metals.
Nonetheless, in the long term, sustainable
technologies for the reduction of CO must take into account
2
the availability of such metal ions and the cost and/or toxicity.
These limitations have urged the development of metal-free
catalysts for the reduction of CO and, so far, the hydrosilyla-
2
tion of CO (to formic acid derivatives or methoxysilanes) has
2
focused most research efforts, which have led to the successful
[
3,4]
development of organic catalysts.
The reduction of CO with hydroboranes (i.e., 9-borabicyclo-
2
[3.3.1]nonane (9-BBN), pinacolborane (pinBH), and catecholbor-
ane (catBH)) was recently discovered by Guan and co-workers,
[a] C. Das Neves Gomes, E. Blondiaux, Dr. P. Thuꢀry, Dr. T. Cantat
CEA, IRAMIS, NIMBE, CNRS UMR 3299
9
1191 Gif-sur-Yvette (France)
E-mail: thibault.cantat@cea.fr
Homepage: http://iramis.cea.fr/Pisp/thibault.cantat/index.htm
2
Scheme 1. Reported catalysts for the reduction of CO to methanol with hy-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400349.
droboranes. The catalytic performances for II and IV are estimated from the
results described in ref. [7,9].
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coupling of two CO molecules. In 2012, Sgro and Stephan re-
2
Table 1. Catalytic reduction of CO
ed in Equation (1).
2
to methoxyborane R
2
BOCH
3
as depict-
ported a ruthenium–tris(aminophosphine) complex III that ex-
hibits a reactivity reminiscent of frustrated-Lewis-pair (FLP) sys-
[
a]
[b]
Entry
Catalyst
[(mol%)]
Borane
(R BH)
T
[8C]
t
TON
TOF
[h ]
tems and catalyzes the hydroboration of CO , with a modest
2
À1 [c]
[
8]
2
[h]
activity. This collection of catalysts was recently enlarged by
[9]
1
2
3
4
5
6
7
8
9
10
–
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
catBH
catBH
catBH
catBH
catBH
9-BBN
9-BBN
9-BBN
9-BBN
9-BBN
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
70
25
70
25
25
70
25
70
>48
no conversion
the contribution from Shintani and Nozaki (i.e., complex IV);
TBD (2.5)
Me-TBD (2.5)
DBU (2.5)
DMAP (2.5)
PS (2.5)
27
7
7
26
24
22
11
0
0.9
3.3
3.2
0.2
0
however, the complexes developed by Guan and co-workers
remain the most efficient catalysts so far in this reaction
(Scheme 1). More recently, Fontaine and co-workers reported
48
>48
>48
23
the first organic catalysts to promote the hydroboration of CO
2
10a]
[
DABCO (2.5)
TBD (1.0)
Me-TBD (1.0)
DBU (1.0)
TBD (0.1)
Me-TBD (0.1)
DBU (0.1)
Me-TBD (1.0)
DBU (1.0)
DBU (1.0)
TBD (1.0)
TBD (1.0)
2 (2.5)
0
0
by means of the ambiphilic phosphine–borane system V,
66
57
63
537
648
351
54
0
0
0
0
26
23
20
24
18
2.9
1.7
3.6
3.6
33
6.9
0.3
0
0
0
0
0.9
0.1
0.1
0.3
0.4
which exhibits a catalytic activity competitive with the metal
33
17
147
20
50
systems I–IV, with a turnover number (TON) of 2950 and a turn-
À1
1
1
over frequency (TOF) of 973 h for the reduction of CO with
2
1
1
1
2
3
4
BH ·SMe as the reductant at 708C. Notably, under the condi-
3
2
tions developed by Guan and co-workers (i.e., with catBH at
168
>200
>200
>200
>200
27
À1
RT), the TON and TOF of V drop to 34 and 17 h , respectively.
15
1
1
1
1
6
7
8
9
An alane version of V was developed recently by Fontaine and
co-workers, which promotes the hydroboration of CO , albeit
2
[
10b]
with lower catalytic activity and decreased stability.
In 2012, our group demonstrated that guanidines and ami-
dines were efficient metal-free catalysts for the reduction of
CO2 to formamide derivatives with hydrosilanes as reduc-
20
4 (2.5)
4 (2.5)
5 (2.5)
5 (2.5)
217
180
84
2
2
2
1
2
3
47
[
4]
tants. Nitrogen bases, therefore, appear as promising candi-
[
a] Reaction time required to achieve a conversion of CO
borane CH OBR in a yield of greater than 90%. [b] The TON is calculated
2
into methoxy-
dates in the organocatalytic hydroboration of CO , which is
2
3
2
supported by recent findings that CO can be reduced to the
2
based on the formation of CÀH bonds in CH
ed for the reaction time t.
3 2
OBR . [c] The TOF is calculat-
methanol level by using a stoichiometric amount of an amine–
[
11]
borane moiety. Herein, we describe the successful utilization
of commercially available nitrogen bases as organocatalysts for
À1
the hydroboration of CO to methanol. Two distinct reaction
33 h . Hydrolysis of CH OBBN by using an excess of water and
2
3
pathways have been unveiled in this 6-electron reduction pro-
cess, based on the isolation of reactive catalytic intermediates
and DFT calculations, that depend on the nature of the cata-
lyst.
distillation of the resulting mixture under reduced pressure af-
fords a solution of methanol in THF in quantitative yield (see
the Experimental section). Monitoring the products distribution
1
13
over time with H and C NMR spectroscopic analysis reveals
that CO is rapidly consumed to yield the boryl formate deriva-
2
tive HCOOBBN (Figure 1). This first reduction step controls the
kinetics of the transformation. Indeed, the formate species is
Results and Discussion
A mixture of 9-BBN and 1,5,7-triazabicyclo[4.4.0]dec-5-ene
(TBD) in a 40:1 ratio (i.e., 2.5 mol% catalyst loading) rapidly
evolved under an atmosphere of CO to produce the methoxy-
2
borane derivative CH OBBN with a TON of 26 after 27 hours at
3
room temperature (Table 1, entry 2). A control experiment con-
firmed that a catalyst is needed to promote this transformation
(
Table 1, entry 1). This reaction is the second example of CO
2
[
10a]
hydroboration with a metal-free catalyst,
thus illustrating
the potential of nitrogen bases in the catalytic reduction of
CO . Under similar reaction conditions, a variety of nitrogen
2
bases were tested as catalysts. Although 4-dimethylaminopyri-
dine (DMAP), 1,4-diazabicyclo[2.2.2]octane (DABCO), and 1,8-
ꢂ
bis(dimethylamino)naphthalene (PS, Proton-sponge ) showed
negligible catalytic activity, the guanidine and amidine moiet-
ies Me-TBD and DBU, respectively, proved to be highly efficient
and led to complete reduction of CO to the methoxy level
2
within 7 hours at room temperature (Table 1, entries 3–7). Im-
portantly, high TONs of 537, 648, and 351 can be achieved
within 33 hours with TBD, Me-TBD, and DBU, respectively
Figure 1. Distribution of the reduction products HCOOBBN, H
CH OBBN (%) over time (min) for the catalytic reduction of CO
presence of (9-BBN)
to the molar quantity of BÀH functionalities).
2
C(OBBN)
(1 bar) in the
(0.60 molL in THF) and TBD (2.5 mol% with respect
2
, and
3
2
À1
2
(Table 1, entries 11–13), which correspond to TOFs of 3.6–
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13
transient and does not accumulate in the reaction mixture be-
cause it is readily reduced to its acetal form H C(OBBN) . The
and C NMR spectroscopic analysis of the crude mixture indi-
cated the formation of a mixture of reduced species, among
which the formate HCOOBBN and acetal H C(OBBN) deriva-
2
2
latter is subsequently converted into methoxyborane. Interest-
2
2
ingly, unlike V, which exhibits an induction period of several
tives were identified [Eq. (2)].
[
10a]
hours,
the formation of methoxyborane is observed within
seconds after the reaction mixture is exposed to an atmos-
phere of CO (Table 1, entries 8–23). The nature of the hydro-
2
borane species has only a minor influence on the catalytic ac-
tivity of complex III, which promotes the reduction of CO with
2
pinBH, catBH, or 9-BBN with very similar TONs (i.e., 4–6 after
[
8]
9
6 h at RT). Interestingly, the organocatalytic version of the
reaction [Eq. (1)] depends on the reductant, and longer reac-
tion times are needed to complete the reduction of CO to
2
methoxyborane when catBH is used in place of 9-BBN.
Colorless crystals were deposited within hours
from the reaction mixture. X-ray diffraction studies re-
vealed the formation of a novel CO adduct 2, which
2
formally results from the replacement of the acidic
NÀH proton in 1 with a BBN boryl group (Figure 2).
Although 1 readily decarboxylates under reduced
pressure, the CO molecule is tightly bound to the
2
guanidine–borane backbone in 2 and the adduct is
stable even after 3 hours at 0.1 mbar at 258C. Struc-
turally, the N1ÀC8 bond in 2 is shorter than in
1
(1.410(3) vs 1.480(3) ꢃ, respectively), and the asym-
metry in the CÀO bond lengths is slightly more pro-
nounced in 2 than in 1 (C8ÀO1: 1.299(3), C8ÀO2:
1
.222(2) vs C8ÀO1: 1.257(3), C8ÀO2: 1.229(2) ꢃ, re-
spectively). Compound 2 can, therefore, be described
as an adduct between an intramolecular nitrogen/boron (N/B)
FLP pair and CO . Several examples of FLP–CO adducts have
In fact, Me-TBD can promote the reduction of CO2 to
CH OBcat with a TON of 54 (Table 1, entry 14,). In contrast,
3
2
2
catBH is inert in the presence of CO , when DBU or TBD is
been recently described by the groups of Stephan, Erker, Lam-
2
used as a catalyst, even after 200 hours at 708C (Table 1, en-
tries 15–18). Overall, the organocatalysts tested in Equation (1)
proved to be competitive with the metal and organic catalysts
depicted in Scheme 1, and although the systems developed by
Guan and co-workers remain the most active catalysts, TBD,
Me-TBD, and DBU exhibit a greater activity than the ruthenium
and copper complexes II, III, and IV. Interestingly, TBD, 7-
methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (Me-TBD), and 1,8-di-
azabicycloundec-7-ene (DBU) are unreactive in the presence of
BH ·SMe , whereas this hydroborane species afforded the best
3
2
[
10a]
results with the ambiphilic system V.
Yet, the use of V with
À1
9
-BBN gave a TON of 34 and a TOF of 12 h at 708C, relative
to 648 and 33 h , respectively, with Me-TBD at room tempera-
ture (Table 1, entry 12).
À1
To comprehend the role of the organic catalyst better, the
mechanism at play in the hydroboration of CO was investigat-
2
ed with TBD as the catalyst and 9-BBN as the reductant. We re-
cently demonstrated that guanidine TBD strongly binds to CO
2
[
12]
to yield the stable adduct 1, which could be the starting
step of the catalytic cycle. Reduction of a solution of 1 in THF
Figure 2. Molecular structure of 2 with displacement ellipsoids drawn at the
5
0% probability level. Selected bond lengths [ꢃ] and angles [8]: N1ÀC8:
with one equivalent of the (9-BBN) dimer resulted in the im-
2
1.410(3), C8ÀO1: 1.299(3), C8ÀO2: 1.222(2), O1ÀB1: 1.537(3); O1-C8-O2:
1
mediate reduction of the activated CO molecule at 258C. H
123.9(2).
2
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mertsma, and Tamm, and the reaction chemistry of these ad-
ducts was investigated in the presence of stoichiometric
[
13]
amounts of amine–boranes.
Noticeably, these systems in-
volve mostly phosphanes and N-heterocyclic carbenes (NHCs)
as Lewis bases and only one nitrogen FLP has been reported
[
13a]
for the activation of CO2.
This latter system resembles 2,
with a CO2 molecule bound to an intramolecular amidine–
borane FLP (N–C: 1.402(5) and B–O: 1.493(5) vs N–C: 1.410(3)
and B–O: 1.537(3) ꢃ, respectively, in 2).
The reaction of TBD with 9-BBN was explored to rationalize
the formation of the BÀN bond in 2. The addition of 9-BBN to
a solution of TBD in THF affords adduct 3, which was identified
1
13
in situ by H and C NMR spectroscopic analysis. Yet, 3 is un-
stable at room temperature and evolves into 4, with the con-
1
comitant loss of H (identified by H NMR spectroscopy). By
2
Figure 4. Molecular structure of 5 with displacement ellipsoids drawn at the
using this synthetic route, 4 was obtained in 75% yield of the
isolated product after 1 hour at 708C [Eq. (3)] and was charac-
terized by elemental and X-ray analyses (Figure 3). Compound
5
1
0% probability level. Selected bond lengths [ꢃ] and angles [8]: N1ÀB1:
.555(2), N2ÀB2: 1.550(2), C1ÀN1: 1.3434(19), C1ÀN2: 1.337(2), C1ÀN3:
1.3545(19); N1-C1-N2: 116.31(12).
Figure 3. Molecular structure of 4 with displacement ellipsoids drawn at the
50% probability level. Selected bond lengths [ꢃ] and angles [8]: N1ÀB1:
1.601(2), N2_2ÀB1: 1.598(2), N1ÀC1: 1.3376(19), N2ÀC1: 1.3409(18), N3ÀC1:
1.3704(18); N1-C1-N2: 123.47(13).
Scheme 2. Computed pathway for the conversion of TBD into 2, 4, and 5 at
the M05-2X/6-31+G* level of theory. The enthalpy and free energy associat-
ed with the dimerization of TBD-BBN to 4 was computed by replacing BBN
2
with BMe boryl groups (in 4 and TBD-BBN) on the energy surface.
4
directly results from the dimerization of the FLP responsible
for the activation of CO in 2; therefore, the reaction chemistry
2
of 4 with 9-BBN and CO was investigated. Compound 4 reacts
level of theory indeed confirm that the formation of 4 from
2
À1
with 9-BBN at 1008C to yield the borane adduct 5 in which the
BÀH bond of the incoming borane is activated by the N/B FLP
system [Eq. (4) and Figure 4]. Similarly, exposing a solution of 4
TBD and 9-BBN is exergonic (DG=À21.2 kcalmol ; Scheme 2).
À1
Nonetheless, the dissociation of 4 requires only 5.4 kcalmol
and adducts 2 and 5 lie close in energy to 4 (DG=À24.2 and
À1
in THF to an atmosphere of CO leads to the formation of 2 in
À20.7 kcalmol , respectively; Scheme 2). Experimentally, both
2
quantitative yield at 1008C [Eq. (5)].
2 and 5 promote the reduction of CO and the treatment of 5
2
with an excess of CO leads to the formation of 2 and a 2:3
2
mixture of H C(OBBN) and CH OBBN after 21 hours at 708C
2
2
3
[
Eq. (6)].
The chemical behavior depicted in Equations (4) and (5)
clearly demonstrates that the dissociation of 4 is accessible to
generate adducts. DFT calculations at the M05–2X/6-31+G*
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The reduction of the activated CO molecule in 2 is some-
carbon atom. The corresponding transition state TS
27.7 kcalmol above the starting materials, thus demonstrat-
ing the positive influence of the catalyst on the reaction rates.
lies
6-7
2
À1
what faster. In fact, the addition of 0.5 equivalents of (9-BBN)2
to a solution of 2 in THF leads to the formation of 5 and
H C(OBBN) in 23% yield after 30 minutes at 708C, and the
Importantly, 5 does not form a stable adduct with CO , and
2
2
2
conversion of 2 into 5 and CH OBBN is complete within
a TS for the hydride transfer from 5 to CO could not be locat-
3
2
8
hours at 708C in the presence of 2 equivalents of (9-BBN)
ed on the energy surface. This finding is in agreement with the
lower reactivity of 5, for which the Lewis acidity of the boron
center and the hydridicity of the borane are masked by coordi-
nation to the N/B FLP. Overall, the computed mechanism high-
lights the important role of the FLP system in increasing the
nucleophilicity of the O centers in CO2 while maintaining
a high electrophilicity at the carbon atom. Indeed, the comput-
ed natural charge for the C center in 6 is +1.06, a value similar
to that in free CO (q = +1.06), whereas the charge at the O
2
0
ÀII
[Eqs (7)and (8)]. The formation of the C and C products from
the stoichiometric reactions depicted in Equations (6)–(8)
strongly suggests that the guanidine–borane FLP system is
active in the catalytic reduction of CO . Indeed, 2 exhibits a cat-
2
alytic activity identical to TBD for the reduction of CO2 to
methanol with 9-BBN (Table 1, entries 2 and 19). Compound 5
presents a somewhat lower catalytic activity (Table 1, entries 22
and 23) and requires about 84 hours at 258C to achieve com-
2
C
plete reduction of CO . Interestingly, 5 was identified as the
centers decreases from À0.53 in free CO to À0.67 in 2 (À0.61
2
2
catalyst resting state at the end of the reaction performed
with TBD or 2 (Table 1, entries 2, 8, 11, 19, and 20). Overall,
these observations reveal that the guanidine–borane FLP
forms under the applied reaction conditions and participates
in the catalytic cycle.
in 6). The ability of 2 to act as a Lewis base to activate the hy-
droborane reagent was confirmed experimentally by treating 2
with 0.5 equivalents of 9-BBNI [Eq. (9)].
The catalytic reduction of CO to methoxyborane is kinetical-
2
ly controlled by the first hydroboration step, which yields the
formoxyborane intermediate (Figure 1). The potential-energy
surface of the catalytic hydroboration of CO to HCOOBBN was
2
computed at the DFT level to rationalize the experimental find-
ings and identify the rate-determining states in the mechanism
(the results are summarized in Scheme 3). In the absence of
The iodide anion is readily displaced by 2 to afford an un-
precedented adduct 8, in which the CO molecule is activated
2
by the guanidine function, namely, one borane and one boro-
nium center (Figure 5). As such, the elongation of the CÀO
bond is more pronounced than in 2 (C8ÀO1: 1.268(2) and C8À
O2: 1.257(2) ꢃ) and the structural parameters are similar to
those of the previously reported Mes P-CO -(AlX ) (X=Cl,
3
2
3 2
[13i]
Br).
Notably, 8 is also a unique example of a boronium
cation stabilized by two activated CO2 molecules, and the
Scheme 3. Computed pathway for the catalytic reduction of CO
2
with
0
.5 equivalents of (9-BBN) with catalyst 2 (at the M05–2X/6-31+G* level of
2
theory). The labels in the boxes denote the compounds characterized exper-
imentally (see the Experimental Section).
catalysts, the direct reduction of CO to the boryl–formate de-
2
À1
rivative is exergonic (DG=À10.8 kcalmol ) and requires an
À1
activation energy of 30.2 kcalmol . In contrast, the FLP-acti-
Figure 5. Molecular structure of the cation in 8 with displacement ellipsoids
drawn at the 50% probability level. Selected bond lengths [ꢃ] and angles
vated CO molecule in 2 acts as a Lewis base to coordinate the
2
incoming hydroborane species, and the resulting adduct 6
readily undergoes a hydride migration from the boron to the
[
8]: N1ÀC8: 1.386(3), C8ÀO1: 1.268(2), C8ÀO2: 1.257(2), O1ÀB1: 1.566(3), O2À
B2: 1.589(3); O1-C8-O2: 124.64(18).
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1
1
B NMR spectrum exhibits two resonances, in agreement with
the presence of two aminoborane groups (Dd=4.2 ppm) and
a carbon charge of q = +0.99 relative to +1.06 in both CO
C
2
and 6. The subsequent hydride migration from the boron to
the carbon atoms, therefore, requires a high activation energy
[
14]
a boronium center (Dd=58.7 vs 3.9 ppm in 2).
À1
À1
The mechanism for the reduction of CO with 9-BBN cata-
of 40.6 kcalmol , that is 10.4 kcalmol greater than the
energy necessary for the uncatalyzed pathway (Scheme 4). As
a result, an alternative pathway was computed to account for
the catalytic activity of Me-TBD. In agreement with Equa-
2
lyzed by Me-TBD was also investigated to determine the
impact of replacing the NÀH and NÀB functionalities in TBD
and 2 by an inert NÀCH linkage. Experimentally, Me-TBD does
3
[
12]
not react with CO . Yet, the addition of 0.5 equivalents of (9-
tion (10), the addition of (9-BBN) to Me-TBD leads to the for-
2
2
BBN) to a solution of Me-TBD in THF affords adduct 9, which
was characterized in solution by H and C NMR spectroscopic
mation of the stable adduct 9. Coordination of the lone pair of
electrons on the nitrogen atom to the vacant orbital in the
boron atom results in an increase of the BÀH-bond polarity
2
1
13
analysis. Compound 9 promotes the reduction of CO to the
2
formate adduct 11 within minutes at 258C, and the formation
and a decrease in the q value from +0.16 in free 9-BBN to
H
1
13
of the CÀH bond is supported by H and C NMR spectroscop-
À0.05 in 9 upon coordination (in parallel, the q value increas-
B
ic analysis [Eq. (10)].
es slightly from +0.35 to +0.36). This activation of the hydro-
borane species is efficient and leads to the reduction of CO
2
À1
with
a
low energy barrier of 23.8 kcalmol
via TS9-10
(
Scheme 5). The resulting ion pair 10 readily converts into the
more stable 11 after nucleophilic attack of the formate anion
onto the electrophilic boron center, as expected from Equa-
tion (10). Me-TBD is then released by de-coordination of
HCO BBN to close the catalytic cycle.
2
A mechanism similar to 2, which relies on the activation of
CO2 by the guanidine catalyst, was first computed through
DFT calculations (Scheme 4). The activation of CO in this path-
2
way requires the coordination of both Me-TBD and 9-BBN to
À1
yield adduct 12 (DG= +17.6 kcalmol ). Importantly, the CO2
molecule in 12 is less electrophilic than in free CO , with
2
Scheme 5. Computed pathway (M05-2X/6-31+G*) for the catalytic reduc-
tion of CO to the formate derivative HCOOBBN, with Me-TBD, through the
2
activation of 9-BBN by using the guanidine catalyst. The labels in the boxes
denote the compounds characterized experimentally (see the Experimental
Section).
Overall, TBD and Me-TBD follow two distinct mechanisms
that are both efficient in the catalytic hydroboration of CO2
(
Scheme 6). Under the applied reaction conditions, TBD is con-
verted into an N/B FLP system that can promote the activation
of CO by increasing the nucleophilicity of the O atom while
2
maintaining the partial positive charge on the C center (6).
This mechanism is similar to the reaction pathway proposed
by Fontaine and co-workers for the hydroboration of CO pro-
2
Scheme 4. Computed pathway (M05-2X/6-31+G*) for the catalytic reduc-
moted by V, although the authors did not compute any transi-
tion of CO
2
to the formate derivative HCOOBBN, with Me-TBD, through the
by using the guanidine catalyst.
[10a]
activation of CO
2
tion state.
In contrast, Me-TBD preferentially activates the
&
&
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6
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Scheme 6. Schematic pathways for the catalytic hydroboration of CO
2
to formoxyborane HCOOBR
2
with TBD and 6 (left) or Me-TBD (middle) as the catalysts.
The activation energies (in THF) are summarized (right) for the reduction of CO
for the different mechanisms.
2 2
to the formate derivative HCOOBR with 9-BBN and catBH as the reductants
hydroborane species by increasing its hydridic character. Be- Conclusion
cause DBU lacks any NÀH functionality, it is likely that this ami-
dine catalyst follows a reaction path similar to Me-TBD. The
rate-determining state of highest energy is the transition state
that results in the formation of the CÀH bond in both mecha-
nisms (Scheme 6). Because these transition states lead to the
formation of zwitterions or ion pairs, their energy is better de-
scribed by taking into account the influence of the solvent.
The energy demand for each pathway was, therefore, comput-
ed by means of a polarizable continuum model (PCM) for the
THF solvation (Scheme 6, right-hand side). The influence of the
polar solvent has only a minor impact on the hydroboration of
We have described herein the first nitrogen bases able to pro-
mote the catalytic hydroboration of CO . Reactive hydrobor-
2
anes include 9-BBN and catBH and enable the reduction of
CO to the methoxide level. Guanidine and amidine derivatives,
2
such as TBD, Me-TBD, and DBU, proved to be active catalysts
for this transformation and Me-TBD catalyzes the reduction of
CO to methoxyborane at room temperature with TONs and
2
À1
TOFs of up to 648 and 33 h (258C), respectively. Formate
HCOOBR and acetal H C(OBR ) derivatives were identified as
2
2
2 2
reaction intermediates in the reduction of CO with R BH and
2
2
CO catalyzed by 6 because the two rate-determining states
the first CÀH-bond formation is rate determining. Experimental
and computational investigations showed that TBD and Me-
TBD follow different mechanisms. Although Me-TBD promotes
2
#
9-BBN
have a similar polarity. The activation energy for 6 (DG
=
1
À1
3
0.1 kcalmol ) is only slightly favored relative to the uncata-
À1
lyzed reaction (DG=30.8 kcalmol ). Notably, the transition
the reduction of CO through the activation of the hydrobor-
2
9ÀBBN
state TS2
that corresponds to the reduction of CO pro-
ane reagent and formation of ion pairs, TBD is converted into
2
moted by Me-TBD is strongly facilitated by the polar solvent
because it connects the neutral starting materials with an ion
pair (i.e., 10). As such, Me-TBD lowers the activation energy for
an N/B frustrated Lewis pair that activates CO and facilitates
2
the hydride transfer from the boron to the carbon atom. Cur-
rent efforts are devoted to translating these conclusions into
the design of new efficient catalysts that combine supplemen-
tary coordination sites for the activation of the hydroborane
moiety and CO2.
#
9-BBN
the conversion of CO into formoxyborane to DG 2
=
2
À1
10.6 kcalmol (Scheme 6). These results also explain the differ-
ence in reactivity between TBD and Me-TBD when catBH is
used as the reductant. Indeed, catBH has a lower acidity than
9
-BBN and its utilization is more efficient in a pathway that in-
Experimental Section
volves the direct activation of the reductant by Me-TBD. Com-
#
catBH
À1
putationally, the activation energy of DG 1
=33.4 kcalmol
General considerations
required for the reduction of CO with catBH and TBD is in-
2
All the reactions and manipulations were performed at 208C in a re-
circulating mBraun LabMaster DP inert-atmosphere (Ar) drybox
and with vacuum Schlenk lines. Glassware was dried overnight at
compatible with catalytic activity. Nonetheless, Me-TBD pro-
motes the hydroboration of CO with an energy demand of
2
À1
1
13
1
6.5 kcalmol , in line with the catalytic results (Table 1,
608C before use. H and C NMR spectra were obtained by using
11
entry 14).
a Bruker DPX 200 MHz spectrometer. B NMR spectra were ob-
Chem. Eur. J. 2014, 20, 1 – 10
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7
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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&
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1
3
1
1
tained by using a Bruker Avance 400 MHz spectrometer. Chemical
and C{ H} NMR spectra in [D ]THF. H NMR ([D ]THF, 298 K): d=
8 8
1
13
1
shifts for H and C{ H} NMR spectra were referenced to solvent
6.29 (brs, 1H; NH TBD), 3.36 (pseudo-t, 2H; CH TBD), 3.28–3.15
(m, 6H; CH ), 1.96–1.40 (m, 17H), 0.80 ppm (brs, 2H); C NMR
([D ]THF, 298 K): d=152.2 (NCN ), 48.1 (CH ), 43.1 (CH ), 39.3 (CH ),
2
11
13
impurities. Chemical shifts for B NMR spectra were referenced by
using Et O·BF as an external standard. Elemental analyses were
2
2
3
8
2
2
2
2
performed at the CNRS facility in Gif-Sur-Yvette (France). Unless
otherwise noted, reagents were purchased from commercial sup-
pliers and dried over molecular sieves (4 ꢃ) prior to use. The mo-
lecular sieves (4 ꢃ; Aldrich) were dried under a dynamic vacuum at
37.0 (CH ), 31.4, 26.1, 25.9, 23.1, 22.6 ppm (CH ).
2 2
4
: A 20 mL round-bottom flask equipped with a stirring bar and
a J. Young valve was charged with TBD (163.1 mg, 1.17 mmol,
equiv), (9-BBN)2 (143.0 mg, 0.590 mmol, 0.5 equiv), and THF
3.5 mL). The flask was sealed, and the reaction mixture was stirred
1
(
2
508C for 48 h prior to use. THF, [D ]THF, toluene, pentane, and
8
[
D ]benzene were dried over a sodium(0)/benzophenone mixture
6
for 1 h at 708C, thus leading to the formation of a white solid. The
reaction mixture was cooled to room temperature, and the precipi-
tate was recovered by filtration. The resulting solid is washed with
and distilled before use. CD CN and CD Cl were dried over CaH
2
3
2
2
and distilled before use. Carbon dioxide was purchased from
Messer in a 5.5 purity gas bottle.
Et O (3ꢄ5 mL) and dried under reduced pressure to afford 4
2
(226.0 mg, 0.436 mmol, 75%). X-ray-quality samples of 4 were ob-
Computational details
tained in an NMR tube by cooling a solution of 4 in THF from 100
to 258C. Note: the insolubility of 4 at room temperature in THF,
Et O, pentane, benzene, acetonitrile, CH Cl , and pyridine preclud-
The M05–2X functional was employed to optimize the equilibrium
molecular structure of the model compounds. This functional was
specifically developed to describe organic systems with nonbond-
ing interactions and proved to be efficient and reliable for investi-
gating reaction mechanisms. The 6–31+G* sets were used for the
carbon, hydrogen, nitrogen, oxygen, and silicon atoms, except for
2
2
2
1
13
11
ed the recording of meaningful H, C, or B NMR spectroscopic
data. Elemental analysis (%) calcd for C H B N (518.40): C 69.51,
30
52
2
6
H 10.11, N 16.21; found: C 69.43, H 10.22, N 16.33.
5: A 20 mL round-bottom flask equipped with a stirring bar and
a J. Young valve was charged with 4 (100.0 mg, 0.190 mmol,
the carbon atom of the CO pattern and the hydride moiety of the
2
borane pattern, which were described by using 6–311+ +G** as
the basis. All the geometries were fully optimized without any
symmetry or geometry constrains. Harmonic vibrational analyses
were performed to confirm and characterize the structures as
minima or transition states. The vibrational data were used to relax
the geometry of each transition state toward the reactants and
products to confirm its nature. Free energies were calculated
within the harmonic approximation for vibrational frequencies. The
effect of the THF solvent on the energy demand for each pathway
was evaluated through single-point calculations with the polariza-
ble-continuum model (PCM). All the calculations were carried out
1 equiv), (9-BBN) (51.0 mg, 0.210 mmol, 1.1 equiv), and THF (5 mL).
2
The flask was sealed, and the reaction mixture was stirred for
150 min at 1008C. The mixture was cooled to room temperature
and concentrated down to 2 mL. A white solid formed during the
evaporation of the solvent. The resulting solid was washed with
Et O (3ꢄ5 mL) and dried under reduced pressure to afford 5
2
(110.5 mg, 0.290 mmol, 76%). X-ray-quality samples of 5 were ob-
tained in an NMR tube by cooling a solution of 5 in THF from 100
1
to 258C. H NMR ([D
]THF, 298 K): d=3.41 (pseudo-t, 4H; CH
TBD), 2.11–1.36 (m, 29H; CH ), 1.02 ppm
(brs, 4H; CH BBN); C NMR ([D ]THF, 298 K): d=157.3 (NCN ), 48.4
(CH ), 44.5 (CH ), 33.1 (CH ), 25.9 (CH ), 25.6 (CH), 24.0 ppm (CH );
B NMR ([D ]THF, 298 K): d=12.8 ppm (brs); elemental analysis (%)
TBD),
8
2
3.34–3.12 (m, 4H; CH
2
2
1
3
8
2
[15]
by using the Gaussian09 suite of codes.
2
2
2
2
2
11
8
calcd for C H B N (381.21): C 72.46, H 10.84, N 11.02; found: C
23
41
2
3
Synthesis
7
1.85, H 10.88, N 10.69.
: A 20 mL round bottom-flask equipped with a stirring bar and
a J. Young valve was charged with 2 (30.0 mg, 0.099 mmol,
2
: A 20 mL round-bottom flask equipped with a stirring bar and
8
a J. Young valve was charged with 4 (71.0 mg, 0.14 mmol) and THF
(
4 mL). The reaction mixture was exposed to a CO2 atmosphere
1
0
equiv), B-iodo-9-BBN solution in hexanes (50 mL, 1m, 0.050 mmol,
.51 equiv), and toluene (1.5 mL). The flask was sealed, and the re-
(
1 bar), and the flask was sealed and heated to 1008C for 75 min.
The reaction mixture was cooled to room temperature, and the
volatiles were removed under reduced pressure to afford 2 as
a white solid (83.0 mg, 0.14 mmol, 100%). X-ray-quality samples of
action mixture was stirred for 30 min at room temperature, thus
leading to the formation of a white precipitate. The solid was re-
covered by filtration, washed with pentane (3ꢄ5 mL), and dried
2
were obtained in an NMR tube by diffusion of pentane into a so-
1
under reduced pressure to afford 8 (85%). H NMR (CD Cl , 298 K):
1
2
2
lution of 2 in THF. H NMR ([D ]THF, 298 K): d=3.69 (pseudo-t, 2H;
CH2 TBD), 3.53 (pseudo-t, 2H; CH2 TBD), 3.39–3.25 (m, 4H; CH
TBD), 2.13–1.29 (m, 16H), 0.69 ppm (brs, 2H); C NMR ([D ]THF,
8
d=3.58 (pseudo t, 4H; CH2 TBD), 3.66–3.44 (m, 12H; CH TBD),
2
2
13
2
.14–1.45 (m, 44H), 1.14 (brs, 2H), 0.71 ppm (brs, 4H); C NMR
1
3
8
(
CD Cl , 298 K): d=157.7 (OCO), 148.8 (NCN ), 50.0 (CH ), 44.6
2
2
2
2
2
98 K): d=152.4 (OCO), 151.1 (NCN ), 49.3 (CH ), 48.4 (CH ), 44.3
2 2 2
(
2
CH ), 44.1 (CH ), 33.7, 33.5, 32.2, 31.9, 31.5, 24.5, 24.2, 23.9, 23.5,
2.6 (br), 21.2, 20.8 ppm; B NMR (CD Cl , 298 K): d=58.7 (brs, 1B),
2 2
2 2
(
CH ), 41.7 (CH ), 33.1 (CH ), 32.7 (CH ), 25.4 (CH), 22.3 (CH ),
11
2
2
2
2
2
11
2
2.2 ppm (CH2); B NMR ([D ]THF, 298 K): d=3.3 ppm (brs, 1B);
8
4
.2 ppm (brs, 2B).
1
H NMR (CD Cl , 298 K): d=3.73 (pseudo-t, 2H; CH TBD), 3.54
2
2
2
Formation in situ and characterization of 9 and 11: A 2.5 mL
NMR tube equipped with a J. Young valve was charged with Me-
(
pseudo-t, 2H; CH TBD), 3.30–3.23 (m, 4H; CH TBD), 2.03–1.65 (m,
2 2
1
3
1
2
2H), 1.56–1.41 (m, 4H), 0.69 ppm (brs, 2H); C NMR (CD Cl ,
2 2
98 K): d=153.1 (OCO), 150.2 (NCN ), 49.0 (CH ), 48.2 (CH ), 43.7
TBD (16.9 mg, 0.11 mmol, 1 equiv), (9-BBN) (13.5 mg, 0.055 mmol,
2
2
2
2
0
.5 equiv), and [D ]THF (0.5 mL). Compound 9 was formed quanti-
8
(
(
(
CH ), 41.5 (CH ), 32.5, 32.0, 24.7, 24.6, 21.7(1) (CH ), 21.6(8) ppm
CH2); B NMR (CD Cl , 298 K): d=3.9 ppm (brs); elemental analysis
%) calcd for C H BN O (303.21): C 63.38, H 8.64, N 13.86; found:
2
2
2
1
13
1
11
tatively and identified by its H and C{ H} NMR spectra in [D ]THF.
8
2
2
The reaction mixture was exposed to an atmosphere of CO (1 bar)
2
1
6
26
3
2
1
13
1
to yield 11, which was identified by its H and C{ H} NMR spectra
C 63.20, H 8.78, N 13.89.
in [D ]THF.
8
Formation in situ and characterization of 3: A 2.5 mL NMR tube
equipped with a J. Young valve was charged with TBD (16.7 mg,
1
9: H NMR ([D ]THF, 298 K): d=3.33–3.13 (m, 6H; CH MTBD), 3.08
8
2
0
.12 mmol, 1 equiv), (9-BBN) (14.6 mg, 0.06 mmol, 0.5 equiv), and
(pseudo-t, 2H; CH MTBD), 3.01 (s, 3H; CH ), 2.03–1.31 (m, 17H),
2 3
0.80 ppm (brs, 2H); C NMR ([D ]THF, 298 K): d=159.4 (NCN ), 48.7
8 2
2
1
13
[
D ]THF (0.5 mL) to form 3 (quant.), which was identified by its H
8
&
&
Chem. Eur. J. 2014, 20, 1 – 10
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Full Paper
(
3
CH ), 48.3 (CH ), 48.2 (CH ), 43.2 (CH ), 40.2 (CH ), 36.6 (CH BBN),
1.2 (1C), 26.8, 26.1, 25.8, 23.0 ppm.
em 2013, 5, 117; c) O. Jacquet, C. D. Gomes, M. Ephritikhine, T. Cantat, J.
Am. Chem. Soc. 2012, 134, 2934.
[5] a) O. Jacquet, X. Frogneux, C. D. Gomes, T. Cantat, Chem. Sci. 2013, 4,
2
2
2
2
3
2
1
1
8
(
4
2
1: H NMR ([D ]THF, 298 K): d=8.33 (s, 1H; HCOO), 3.52–3.12 (m,
8
2
7
127; b) M. Khandelwal, R. J. Wehmschulte, Angew. Chem. 2012, 124,
435; Angew. Chem. Int. Ed. 2012, 51, 7323; c) T. Matsuo, H. Kawaguchi,
H; CH MTBD), 3.06 (s, 3H; CH ), 2.14–1.32 (m, 16H), 1.06 ppm
2
3
1
3
bsr, 2H); C NMR ([D ]THF, 298 K): d=163.6 (HCOO), 161.5 (NCN ),
8 2
J. Am. Chem. Soc. 2006, 128, 12362; d) S. Park, D. Bꢀzier, M. Brookhart, J.
Am. Chem. Soc. 2012, 134, 11404.
8.7 (CH ), 48.3 (CH ), 47.8 (CH ), 42.3 (CH ), 42.1 (CH ), 32.6 (CH ),
2 2 2 3 2 2
6.8, 25.1, 22.4 ppm.
[6] a) S. Chakraborty, J. Zhang, J. A. Krause, H. R. Guan, J. Am. Chem. Soc.
2
010, 132, 8872; b) S. Chakraborty, Y. J. Patel, J. A. Krause, H. R. Guan,
Typical procedure for the catalytic hydroboration of CO2 to
methanol: The typical procedure is detailed for the conversion of
CO into methanol with TBD as the catalyst and (9-BBN) as the re-
ductant (Table 1, entry 2). A 2.5 mL NMR tube equipped with a J.
Young valve was charged with TBD (1.7 mg, 0.012 mmol,
Polyhedron 2012, 32, 30; c) S. Chakraborty, J. Zhang, Y. J. Patel, J. A.
Krause, H. R. Guan, Inorg. Chem. 2013, 52, 37.
7] a) S. Bontemps, L. Vendier, S. Sabo-Etienne, Angew. Chem. 2012, 124,
2
2
[
1703; Angew. Chem. Int. Ed. 2012, 51, 1671; b) S. Bontemps, S. Sabo-Eti-
enne, Angew. Chem. 2013, 125, 10443; Angew. Chem. Int. Ed. 2013, 52,
0
.050 equiv), (9-BBN)2 (58.6 mg, 0.240 mmol, 1.00 equiv), and
10253.
[
D ]THF (0.40 mL). The reaction mixture was exposed to an atmos-
[8] M. J. Sgro, D. W. Stephan, Angew. Chem. 2012, 124, 11505; Angew.
Chem. Int. Ed. 2012, 51, 11343.
8
phere of CO (1 bar), and the flask was sealed. The formation of
CH OBBN was followed by H NMR spectroscopic analysis in
2
1
[
9] R. Shintani, K. Nozaki, Organometallics 2013, 32, 2459.
3
1
[10] a) M. A. Courtemanche, M. A. Legare, L. Maron, F. G. Fontaine, J. Am.
Chem. Soc. 2013, 135, 9326; b) M. A. Courtemanche, J. Larouche, M. A.
Legare, W. H. Bi, L. Maron, F. G. Fontaine, Organometallics 2013, 32,
[
D ]THF with Ph CH as an internal standard. Selected H and
8
2
2
1
3
C NMR spectroscopic data ([D ]THF) for: HCOOBBN (d=8.24
8
(HCOO) and 163.2 ppm (HCOO)), H C(OBBN) (d=5.54 (H C(OBBN) )
2 2 2 2
6804.
and 86.5 ppm (H C(OBBN) )), and CH OBBN (d=3.71 (CH OBBN)
and 53.4 ppm (CH OBBN)). When full conversion of CO was ach-
2
2
3
3
[
11] A. E. Ashley, A. L. Thompson, D. O’Hare, Angew. Chem. 2009, 121, 10023;
Angew. Chem. Int. Ed. 2009, 48, 9839.
3
2
ieved (CH OBBN; 0.114 mmol, 1.00 equiv), H O (20 mL, 1.1 mmol,
[12] C. Villiers, J. P. Dognon, R. Pollet, P. Thuery, M. Ephritikhine, Angew.
Chem. 2010, 122, 3543; Angew. Chem. Int. Ed. 2010, 49, 3465.
3
2
1
0 equiv) was added to the reaction mixture. The reaction mixture
is distilled after 1 h under reduced pressure to afford a solution of
methanol in THF (0.105 mmol, 0.921 equiv, 92%). The formation of
methanol was determined by using H NMR spectroscopy with
[13] a) M. A. Dureen, D. W. Stephan, J. Am. Chem. Soc. 2010, 132, 13559;
b) M. J. Sgro, J. Domer, D. W. Stephan, Chem. Commun. 2012, 48, 7253;
c) X. X. Zhao, D. W. Stephan, Chem. Commun. 2011, 47, 1833; d) I.
Peuser, R. C. Neu, X. X. Zhao, M. Ulrich, B. Schirmer, J. A. Tannert, G.
Kehr, R. Frohlich, S. Grimme, G. Erker, D. W. Stephan, Chem. Eur. J. 2011,
1
[
D ]THF as the solvent and Ph CH as an internal standard.
8 2 2
1
7, 9640; e) C. M. Mçmming, E. Otten, G. Kehr, R. Frçhlich, S. Grimme,
D. W. Stephan, G. Erker, Angew. Chem. 2009, 121, 6770; Angew. Chem.
Int. Ed. 2009, 48, 6643; f) F. Bertini, V. Lyaskoyskyy, B. J. J. Timmer, F. J. J.
de Kanter, M. Lutz, A. W. Ehlers, J. C. Slootweg, K. Lammertsma, J. Am.
Chem. Soc. 2012, 134, 201; g) C. Appelt, H. Westenberg, F. Bertini, A. W.
Ehlers, J. C. Slootweg, K. Lammertsma, W. Uhl, Angew. Chem. 2011, 123,
Acknowledgements
For financial support of this work, we acknowledge CEA, CNRS,
ANR (Research Grant to T.C.), ADEME, and the CHARMMMAT
Laboratory of Excellence. T.C. thanks the Fondation Louis D.: In-
stitut de France for support. We thank CINES for the allowance
of computer time (Project No. c2013086494).
4
011; Angew. Chem. Int. Ed. 2011, 50, 3925; h) E. L. Kolychev, T. Bannen-
berg, M. Freytag, C. G. Daniliuc, P. G. Jones, M. Tamm, Chem. Eur. J.
012, 18, 16938; i) G. Menard, D. W. Stephan, J. Am. Chem. Soc. 2010,
2
132, 1796.
[
14] W. E. Piers, S. C. Bourke, K. D. Conroy, Angew. Chem. 2005, 117, 5142;
Angew. Chem. Int. Ed. 2005, 44, 5016.
Keywords: boranes · carbon dioxide · density functional
calculations · guanidines · organocatalysis · reduction
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Received: January 28, 2014
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Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
9
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Highly efficient metal-free catalysis for
&
Organocatalysis
the reduction of CO to methanol with
2
C. Das Neves Gomes, E. Blondiaux,
P. Thuꢀry, T. Cantat*
hydroboranes as the reductant and gua-
nidines and amidines as the catalyst
(
see picture). Experimental and DFT re-
&& – &&
sults show that the reaction proceeds
through two different pathways de-
pending on the nature of the organoca-
talyst.
Metal-Free Reduction of CO with
Hydroboranes: Two Efficient Pathways
2
at Play for the Reduction of CO to
2
Methanol
&
&
Chem. Eur. J. 2014, 20, 1 – 10
www.chemeurj.org
10
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!