Acetate-catalyzed hydroboration of CO2 for the selective formation of methanol-equivalent products

Article abstract of DOI:10.1039/d0cy00118j

The present study details the use of the acetate anion, an inexpensive and robust anion, as a CO₂ hydroboration catalyst for the selective formation, in most cases, of methanol-equivalent borane products. Thus, upon heating (90 °C, PhBr), tetrabutylammonium, sodium and potassium acetate (1, 2 and 3, respectively) effectively catalyze CO₂ hydroboration by pinacolborane (pinB-H) to afford CO₂ reduction products HOCOBpin (A), pinBOCH₂OBpin (B) and methoxyborane (C). In most cases, high selectivity for product C with higher borane loading and longer reaction time with a TON of up to 970 was observed. The reduction catalysis remains efficient at low catalyst loading (down to 0.1 mol%) and may also be performed under solvent-free conditions using salt 1 as a catalyst, reflecting the excellent robustness and stability of the acetate anion. In control experiments, a 1/1 1/pinB-H mixture was found to react fast with CO₂ at room temperature to produce formate species [pinB(O₂CH)(OAc)][N(ⁿBu)₄] (5) through CO₂ insertion into the B-H bond. DFT calculations were also performed to gain insight into the acetate-mediated CO₂ hydroboration catalysis, which further supported the crucial role of acetate as a Lewis base in CO₂ functionalization catalysis by pinB-H. The DFT-estimated mechanism is in line with experimental data and rationalizes the formation of the most thermodynamically stable reduction product C through acetate catalysis.

Full text of DOI:10.1039/d0cy00118j

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Schrekker and S. Dagorne, Catal. Sci. Technol., 2020, DOI: 10.1039/D0CY00118J.
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21 January 2017
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Acetate-catalyzed hydroboration of CO2 for the selective formaDtOioI: n10.1o03f9/D0CY00118J
methanol-equivalent products†
Yuri C. A. Sokolovicza,b, Olalla Nieto Fazac, David Specklina, Béatrice Jacquesa,
Carlos Silva Lópezc, João H. Z. dos Santosb, Henri S. Schrekkerb,*, Samuel Dagornea,*
aInstitute of Chemistry, Université de Strasbourg, CNRS, Strasbourg, France.
bInstitute of Chemistry, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brazil.
c Depto. Química Orgánica, Universidade de Vigo, Facultade de Ciencias
Campus As Lagoas, 32004, Ourense, Spain
†
Electronic supplementary information (ESI) available: NMR data of compounds, catalytic
runs, monitoring experiments. Atom coordinates of all DFT modeled species (xyz files) and
molecular structures of transition states.
* Email : dagorne@unistra.fr, henri.schrekker@ufrgs.br,
1

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Abstract. The present study details the use of the acetate anion, inexpensive and robust anion,
DOI: 10.1039/D0CY00118J
as a CO2 hydroboration catalyst for the selective formation, in most cases, of methanol-
equivalent borane products. Thus, upon heating (90 °C, PhBr), tetrabutylammonium, sodium
and potassium acetate (1, 2 and 3, respectively) effectively catalyze CO2 hydroboration by
pinacolborane (pinB–H) to afford CO2 reduction products HOCOBpin (A), pinBOCH2OBpin
(B) and methoxyborane (C). In most cases, a high selectivity in product C with higher borane
loading and longer reaction time with TONs up to 970. The reduction catalysis remains efficient
at low catalyst loading (down to 0.1 mol%) and may also be performed under solvent-free
conditions using salt 1 as catalyst, reflecting the excellent robustness and stability of the acetate
anion. In control experiments, a 1/1 1/pinB–H mixture was found to react fast with CO2 at room
temperature to produce formate species [pinB(O2CH)(OAc)][N(nBu)4] (5) through CO2
insertion into the B–H bond. DFT calculations were also performed to gain insight on the
acetate-mediated CO2 hydroboration catalysis, which further supported the crucial role of
acetate as a Lewis base for CO2 functionalization catalysis by pinB–H. The DFT-estimated
mechanism is in line with experimental data and rationalizes the formation of the most
thermodynamically stable reduction product C through acetate catalysis.
2

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DOI: 10.1039/D0CY00118J
Introduction
The reduction/functionalization of carbon dioxide, a cheap and readily available C1 source, is
currently attracting considerable interest to access valuable chemicals, most notably
methanol.1,2 However, the high thermodynamic stability of CO2 requires the development of
effective catalysts for CO2 activation and functionalization catalysis. To this end, numerous
metal-based homogeneous catalysts have been investigated over the past years using hydrogen,3
hydroboranes4 and hydrosilanes5 as reducting agents. While direct CO2 reductive
hydrogenation with H2 is most desired for atom economy, only a few discrete metal catalysts,
mostly based expensive noble metal sources, may efficiently catalyze such a transformation
given the relative inertness of H2.3,3,6 Thus, the use of reductants such as hydroboranes and
hydrosilanes, which are more reactive due to a polar H–E bond (E = B, Si), has led to significant
developments in metal-catalyzed CO2 reductions, including a number of product-selective
catalytic systems (at the formate, ketal, methanol and methane reduction level) with transition
as well as main group metal centers.4,5,7 In contrast, metal-free catalysts for CO2 hydro-
boration/-silylation remain far less studied since the first report by Fontaine using an ambiphilic
phosphino-borane species as a highly active CO2 hydroboration catalyst.8 In the area, additional
ambiphilic as well as Frustrated Lewis pairs (FLP) systems of the type P/B and N/silylium,9
have been reported be effective in CO2 hydroboration catalysis. Also, as first shown by Cantat,10
strong organic Lewis bases may also catalytically hydroborate CO2. The latter approach was
successfully implemented with strong Lewis bases such as guanidines,10a phosphines,9b proton
sponge,11 zwitterionic carbenoids/diazafluorenides,12 abnormal N-heterocyclic carbenes13 and
carbodiimides14. Since most strong bases are air- or/and moisture-sensitive, further progress in
the area may involve the development of highly robust, air-stable and cheap Lewis bases for
CO2 hydroboration. To this end, we have become interested into investigating the acetate anion
for use in CO2 hydroboration catalysis. Though acetate is a weak Lewis base, acetate salts are
3

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particularly attractive for use in catalysis since they are extremely stable, availablDeOoI:n10.a103la9/rDg0eCY00118J
scale, inexpensive, benign and have thus found widespread applications ranging from use as
deicing agents, food additives to catalysts in polymer industry.15
Using simple sodium, potassium and tetrabutylammonium acetate salts, we here report
the acetate-catalyzed reduction of CO2 by hydroboranes for the selective production of the
methanol-equivalent product. In addition to catalysis results, combined experimental and
computational studies were also performed to provide insight into the hydroboration
mechanism, the results of which are also discussed herein.
Results-Discussion
Tetrabutylammonium, sodium and potassium acetate salts (1, 2 and 3, respectively) were tested
as catalysts in the hydroboration of CO2 using pinacol borane (H–Bpin) as a borane source. The
results are compiled in Table 1 and all associated experimental dataare included in the ESI.
Table 1. Acetate-catalyzed reduction of CO2 with pinB–H.[a]
Conv.[b]
(%)
Selectivity (%) [c] in
B
Entry
Cat.
Solvent
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
T
(ºC)
90
t
(h)
2
A
C
TON[d]
12.4
16.4
20
1[e]
2[e]
3
1
62
82
99
40
96
94
97
88
10
0
66
-
7
-
(5 mol %)
1
90
90
26
26
2
27
(5 mol %)
1
0
2
1
1
0
1
0
100
95
(5 mol %)
4
1
90
3
80
(0.5 mol %)
5
1
90
20
46
19
48
< 1
0
98
192
940
970
880
(0.5 mol %)
6
1
(0.1 mol %)
1
90
99
7
Solvent
-free
C6D5Br
90
0
100
99
(0.1 mol %)
8
2
120
0
(0.1 mol%)
4

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DOI: 10.1039/D0CY00118J
9
2
C6D5Br
C6D5Br
C6D5Br
120
120
120
65
65
65
96
85
94
1
1
1
0
0
0
99
99
99
960
(0.1 mol%)
10
11
3
850
(0.1 mol%)
3
940
(0.1 mol%)
[a] Conditions (unless indicated otherwise): [pinB–H]0 = 3 M, P0(CO2) = 1.5 atm, C6D6 or C6D5Br (0.5
1
11
mL). [b] Conv. = borane conversion to A, B and C, as estimated from H and B NMR data (C6Me6
was used as an internal standard). [c] the relative amount of reduction products HOCOBpin (A),
1
11
pinBOCH2OBpin (B) and methoxyborane (C) was estimated from H and B NMR data. [d] TON =
Turnover number (based on borane consumption). [e] Conditions: [pinB–H]0 = 0.3 M, P0(CO2) = 1.5
atm, C6D6 (0.5 mL).
The ammonium salt 1 was first screened as catalyst due to its better solubility in
common organic solvents compared to inorganic salts 2 and 3. Satisfyingly, in an initial test,
species 1 (5 mol%) was found to catalyze CO2 hydroboration by pinB–H (90 °C, C6D6, 1.5 atm
CO2) to selectively afford the mono-reduction product pinB–CO2H (A) after 2h of reaction,
with a 62% conversion of pinB–H (entry 1, Table 1), thereby showing the ability of the acetate
ion to effectively catalyze CO2 reduction. In such reaction conditions, longer reaction times led
11
1
to higher borane conversion (as estimated from B and H data) along with the formation of
double- and triple-reduction products pinB–O–CH2–O–Bpin (B) and pinB–OMe (C). Thus, a
82% borane conversion to a 66/7/27 A/B/C mixture (along with pinB–O–Bpin as a side-
1
product) was observed after 26h, as deduced from H and 11B NMR monitoring data and
comparison with literature data (entry 2, Table 1).16 In particular, the 1H NMR spectrum of the
crude mixture contains singlet resonances at  8.12, 5.27 and 3.50 ppm assigned to the HCO2-
11
Bpin, pinB–O–CH2–O–Bpin and pinB–OMe moieties, respectively. The B NMR spectrum
features a characteristic broad signal ( 22.9 ppm) assigned to A, B and C. In line with previous
reports on CO2 hydroboration catalysis, the formation of C (from B and H–Bpin) comes along
with the production of side-product pinB–O–Bpin, as evidenced by 11B NMR (br,  = 21.8 ppm;
ESI). Under identical reaction conditions but using a ten-fold increase in borane loading (entry
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3, Table 1), the selective and quantitative formation methoxyborane product C wDaOsI:o10b.1s0e3r9v/De0dCY00118J
after 26 h (90 °C, C6D6).17 This clearly indicates that the lower selectivity in the initial run
(formation of A, B and C) is due to the lower loading in pinB–H, which thus decreases the
reaction rate and limits the extent of the reduction. Remarkably, acetate-catalyzed CO2
reduction is also highly effective at much lower acetate amounts, such as 0.5 and 0.1 mol% in
1 (entries 4-6, table 1) with the nearly quantitative production of methanol-equivalent C after
20 and 46 h, respectively. Again, the large excess in borane vs. CO2 under such low acetate
loadings certainly promote the formation of the triple-reduction CO2 reduced product, thus the
high selectivity in C. It is also noteworthy that CO2 reduction may be performed under solvent-
free conditions (with H–Bpin acting as solvent and reagent) to further boost catalytic activity.
Thus, a 1000/1 H–Bpin/1 mixture heated at 90 °C for 19h under CO2 (1.5 atm) performed best
with the nearly quantitative formation of reduction product C (entry 7, Table 1; TON = 970),
reflecting the excellent stability and robustness catalyst 1 under such reaction conditions.18
Unlike ammonium salt 1, inorganic salts NaOAc (2) and KOAc (3) are inactive CO2
reduction in the presence of H–Bpin at 90 °C in benzene or PhBr due to their poor solubility
under these conditions. However, salt 2 and 3 at 0.1 mol% loading (vs. H–Bpin) both catalyze
CO2 hydroboration at higher temperature (PhBr, 120 °C, 1.5 atm CO2) to selectively afford
methoxyborane C (along with pinB–O–Bpin) in good to nearly quantitative conversion, (entries
7-10, Table 1). The higher reaction temperature (120 ºC for 2 and 3 vs. 90 °C for 1), required
for the solubilization of inorganic salts 2 and 3, is thus not detrimental to catalyst selectivity
and overall effectiveness (TONs: 960 and 940 h-1), though longer reaction times were necessary
for high conversion.
Acetate salts 1-3 were also tested in CO2 reduction catalysis using H3B-SMe2 (BMS), a
cheap hydroborane source since readily available from industrially produced B2H6. The results
are compiled in Table 2.
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DOI: 10.1039/D0CY00118J
Table 2. Acetate-catalyzed reduction of CO2 with BH3-SMe2.[a]
Conv.[b]
Selectivity (%)[c]
Entry
Cat.
Solvent
C6D6
C6D6
C6D6
C6D6
C6D6
T (ºC) t (h)
(%)
100
100
68
D
30
0
E
TON[d]
1[e]
2
1
RT
RT
RT
60
17
17
48
20
20
70
10
10
(10 mol %)
1
100
(10 mol %)
3
1
(1 mol %)
1
0
100
100
100
68
4
96
0
96
(1 mol %)
5
1
60
75
0
750
(0.1 mol %)
[a] Conditions (unless indicated otherwise): [Me2S-BH3]0 = 3.3 M, P0(CO2) = 1.5 atm, C6D6 or C6D5Br
1
(0.5 mL). [b] Conv. = borane conversion, as estimated from H and 11B NMR data. [c] the relative
amount of reduction products D and E was estimated from 1H and 11B NMR data. [d] TON = Turnover
number, based on borane consumption (C6Me6 was used as an internal standard). [e] Conditions: [Me2S-
BH3]0 = 0.33 M, P0(CO2) = 1.5 atm, C6D6 (0.5 mL).
Satisfyingly, ammonium salt 1 (10 mol%) catalyzes CO2 hydroboration at room
temperature in the presence of BMS to quantitatively afford after 17h a 70/30 mixture of
trimethoxyboroxine (E) and B(OMe)3 (D), as estimated from 1H and 11B data and comparison
1
with literature data (entry 1, Table 2).8a,19 In particular, the H spectrum of the crude mixture
contains characteristic singlet resonances at  3.35 and 3.44 ppm (in a 66/34 ratio), attributed
to the OMe moieties of products E and D, respectively (see ESI). 11B NMR data clearly
indicated the complete consumption of BMS along the formation D and E ( 18.7 and 19.1
ppm, respectively; ESI).19 Using a ten-fold increase in the BMS feed (while keeping a 10%
catalyst loading; entry 2, Table 1) led to the quantitative formation of reduction product E after
17 h at RT, indicating that BMS was a rate-limiting reagent in the initial run. With a lower
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catalyst loading (1 mol% of catalyst 1), the selective formation of E was also obserDvOeId: 10a.1t0r3o9/oDm0CY00118J
temperature, yet with only 68% BMS consumption to E after 48 h (entry 3, Table 2).
Quantitative CO2 reduction to boroxine E was however achieved at 60 °C using 1 mol % of
catalyst 1 (entry 4, Table 2). CO2 reduction catalysis also proceeds using 0.1 mol% of 1 (vs.
BMS) to produce E (75% BMS consumption after 20h; entry 5, Table 2), further establishing
1 as an effective CO2 reduction catalyst by BMS to methanol-equivalent products (TON up to
750). In contrast, no CO2 reduction (by BMS) activity was observed using inorganic salts 2 and
3 at room temperature due to the poor solubility of those salts, while higher reaction
temperatures (from 40 to 80 °C) only led to unidentified decomposition products. Overall, the
above results establish the acetate ion, a weak Lewis base, as a robust and effective CO2
hydroboration catalyst using either pinB–H or BMS as a hydroborane source.
Stoichiometric control experiments on the 1/pinB–H/CO2 system. To better
understand the acetate-catalyzed CO2 hydroboration and the role of the acetate ion, NMR-scale
control reactions with the 1/pinB–H/CO2 system were carried out and monitored by 1H and 11B
NMR (Figure 1). 1H and 11B NMR data recorded after the addition of 1 to 1 equiv of pinB–H
(RT, C6D6, 15min) suggests the labile coordination of the acetate ion to pinB–H, with a fast
acetate coordination/decoordination on the NMR timescale under the studied conditions.20
However, an adduct of the type [pinB(H)(OAc)]- could not be completely characterized.21
While no reaction was observed between pinB–H and CO2 (1.5 atm) after 24 h at RT in C6D6,
the 1/1 1/pinB–H mixture reacts fast with CO2 at room temperature (1.5 atm CO2, C6D6, 15min)
to quantitatively afford the formate insertion product pinB(CO2H)(OAc)- (4, Figure 1), which
results from CO2 insertion into the B–H bond. This clearly reflects a more nucleophilic pinB–
H hydride, further suggesting acetate coordination to pinB–H. The identity of species 4 is
proposed on the basis of 1H, 13C, 11B NMR and 2D (HSQC and HMBC) NMR data, which all
support the presence of a formate group connected to a four-coordinate B(III) center. For
8

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instance, the 1H and 13C NMR data of the reaction of 4 with CO2 contains characteristic HCO2
View Article Online
DOI: 10.1039/D0CY00118J
11
( 9.18 ppm) and HCO2 ( 66.0 ppm) resonances for a formate moiety, while the B NMR
spectrum, with a singlet signal at  = 8.35 ppm, agrees with a four-coordinated B(III) center.
Though species 4 could not be isolated in a pure form, its in situ formation rationalizes the
observed acetate-promoted CO2 functionalization. Thus, as supported by DFT studies (vide
infra), species 4 is presumably a key initial intermediate permitting CO2 hydroboration catalysis
with pinB–H.
H
1) 1 (1 equiv), RT, 15 min
2) CO2 (1.5 atm), RT, 15min
O
O
O
O
O
O
B
O
B H
O
N(nBu)4
4
Figure 1. Reaction scheme for the formation of borate species 4 and the corresponding 1H NMR
spectra (C6D6) of HBpin, 1, the 1/1 HBpin/1 mixture and species 4.
DFT-estimated mechanism. To gain additional insight on and further rationalize the
acetate-catalyzed CO2 hydroboration by pinB–H, mechanistic pathways were computed by
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DFT at the wB97xD/def2tzvpp//B3LYP/6-31G* theory level. The DFT-derived proposed
DOI: 10.1039/D0CY00118J
mechanism is depicted in Figure 2 and the associated molecular structures are included in the
ESI. First, combining pinB–H with the acetate ion affords the corresponding borohydride Lewis
adduct OAc-Ad, through an exergonic reaction (G = -5.6 kcal.mol-1), in line with
experimental indications. The reaction of borohydride OAc-Ad with CO2 is then rather facile
(energy barrier, G = 16.5 kcal.mol-1) to yield the thermodynamically favored formate
insertion product IV (G = -21.3 kcal.mol-1). The reaction proceeds through an early transition
state TS-1 (G = 10.9 kcal.mol-1) featuring an elongated B–H bond (1.323 vs. 1.229 Å in pinB–
H) and a short B–H/CO2 distance (1.551 Å). Under catalytic reaction conditions (i.e. excess of
pinB–H), IV may also act as a Lewis base and coordinate a pinB–H unit to provide, through a
low barrier process (G = 10.4 kcal.mol-1), dinuclear Lewis adduct V (G = -12.0 kcal.mol-
1), in which the two boron centers are linked by a formate bridge.22 The formation of model V
is an endergonic process (G = 9.3 kcal.mol-1) from IV and pinB-H) and V appears to be a
central intermediate in the present catalysis. It thus may evolve through two pathways,
altogether rationalizating the formation of reduction products A, B and C. The first pathway
involves the exergonic reduction of a second CO2 molecule by the B–H moiety in V leading to
diboron diformate VI (G = -24.7 kcal.mol-1) through a moderate barrier (TS-3, G‡ = 11.5
kcal.mol-1; G = 23.5 kcal.mol-1). VI may then dissociate via a low barrier (TS-4, G‡ = -
18.6 kcal.mol-1) to mono-reduction product pinBO2CH (A’, model of experimentally observed
A) and re-generate species IV. From that point, it should be noted that further reduction of A’
by reaction with pinB–H to produce B’ (model of species B) was DFT-estimated to proceed via
a high energy barrier (42.6 kcal/mol), which is not compatible with the reaction conditions. The
second pathway from intermediate V initially proceeds with an intramolecular reduction of the
B-OCHO-B bridging formate by the terminal B–H moiety to yield the corresponding ketal
product VII (G = -24.1 kcal.mol-1). The reaction occurs through four-center transition state
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TS-5 (G‡ = 14.2 kcal.mol-1), an exergonic reaction with an energy barrier (26.2 kcal.mol-1)
DOI: 10.1039/D0CY00118J
compatible with reaction conditions. Acetate decoordination from VII may produce the second-
reduction product B’ given the similar stability of VII and B’ (G = -1.1 kcal.mol-1). For the
formation of methanol-equivalent C’, direct reaction of B’ with pinB–H was first DFT-
estimated, yet leading to C’ through a prohibitively high energy barrier (63.2 kcal/mol). Rather,
both B’ and acetate adduct VII react similarly to undergo an intramolecular reaction through a
concerted asynchronous four-center transition state (TS-6 and TS-7, respectively) producing
formaldehyde and boroxide pinB–O–Bpin VIII (G = -26.9 kcal.mol-1, experimentally
observed) from B’, and formaldehyde and acetate adduct IX (G = -31.9 kcal.mol-1) from VII.
Here again, acetate coordination favors the reaction as reflected by the substantially lower
energy barrier for formaldehyde formation from acetate adduct VII vs. B’ (G = 25.1 and
37.0 kcal.mol-1, respectively) due to the lower energy of TS-7 vs. TS-6. The acetate pathway
is thus clearly favored. Comparing the molecular structures of TS-7 and TS-6 shows that acetate
coordination significantly lengthens the adjacent B–OCH2 bond, in line with a more
nucleophilic B-OCH2 oxygen upon Lewis adduct formation, which in turn allows B–O–B bond
formation at a lower energy cost along with formaldehyde formation. The comparable stability
of VIII and adduct IX suggests that both species are accessible through acetate de-
coordination/coordination under the reaction conditions. Then, formaldehyde may
subsequently be reduced in the presence of pinB–H and borohydride OAc-Ad to produce final
reduction product pinB–OMe C’ (model of C, experimentally observed) and re-generating
borohydride OAc-Ad through a highly exergonic reaction (G = -62.6 kcal.mol-1). The reaction
proceeds with a reasonable activation barrier (G = 21.1 kcal.mol-1) though the associated
transition state TS-8 (G‡ = -10.6 kcal.mol-1) is surprisingly low for such an entropically
disfavored process. The structure of TS-8 indicates that pinB–H and OAc-Ad reduce
formaldehyde in cooperative manner, where pinB–H may act as a Lewis acid for H2C=O
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DOI: 10.1039/D0CY00118J
activation and OAc-Ad as a hydride source.
Altogether, the proposed DFT mechanism for CO2 hydroboration is consistent with all
experimental and reaction conditions as well as product formation/selectivity (in A, B’ and C’).
In particular, it may highlighted that: i) the formation of the most thermodynamically stable
final model product pinB–OMe (C’) certainly drives the reaction to completion explaining the
experimentally observed high selectivity in C with longer reaction time and higher borane
loading; ii) the experimental energy barrier of the overall reaction (35.5 kcal.mol-1, estimated
from the difference between the highest energy entity, i.e. TS-5, and the most stable and
observable intermediate, i.e. OAc-Ad) is in line with the rather harsch reaction conditions; iii)
the rate limiting step of the overall reaction (through TS-5) is thus the formation of the second
reduction product B’ then readily reduced to C’, consistent with experimental data (at most,
trace amount of reduction product B are observed at the end of the reaction).
Summary – Conclusion
The present study shows that the acetate ion, a weak Lewis base, may effectively
catalyze CO2 hydroboration to methanol-equivalent products using pinB–H and BH3-SMe2 as
hydroboranes (TON up to 970). Taking advantage of the robustness and stability of such a
common and robust anion, the catalysis may be efficiently performed with a low catalyst
loading (down to 0.1 mol%) and, in some cases, under solvent-free conditions without any loss
in catalytic activity. Combined experimental and DFT calculations data provided mechanistic
insights on the CO2 hydroboration catalysis by pinB–H, showing the key role of acetate
interaction with pinB–H for CO2 functionalization and subsequent catalysis to occur.
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O
O
DOI: 10.1039/D0CY00118J
H
O
TS-1
H
O
B
O
O
O
O
CO2
O
O
O
O
B H
B
O
-5.6
OAc-Ad
10.9 (16.5)
O
B H
1
O
O
0.0
-21.3
IV
-18.6 (7.7)
TS-4
O
O
O
B
TS-2
O
H
-10.9 (10.4)
O
-5.0
A’
H
O
H
O
O
O B
O B
O
O
O
O
O
O
O
O
B
O
H
B
H
O
V
TS-3
O
VI
O
-12.0
-24.7
CO2
TS-5
11.5 (23.5)
14.2 (26.2)
O
O
O
O
O
O
O
O
O
B
O
O
B
O
B’
B
VII
B
H
O
H
-25.2
O
-24.1
O
H
O
H
O
-
O
H
O
H
O
O
H O
O
H
B
O
O
O
B
B
O
O
B
O
O
TS-7
1.0 (25.1)
TS-6
11.8 (37.0)
O
O
O
O
O
O
O
O
O
+
B O B
VIII
+
O
B O B
O
IX
H
H
O
H
H
O
O
-31.9
-26.2
H
H
O
H
O
O
O
O
H
pinB–H + OAc-Ad
B
B
O
O
OAc-Ad
TS-8
-10.6 (21.1)
O
O
B O
H
C’
C
H
H
-62.5
Figure 2. DFT-estimated mechanism (wB97xD/def2tzvpp//B3LYP/6-31G*, benzene) of
acetate-catalyzed CO2 hydroboration and the associated Gibbs free energies. The values in
parenthesis correspond to the energy barriers.
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DOI: 10.1039/D0CY00118J
Experimental Section
General considerations
Pinacolborane (HBpin), C6D6 and C6D6Br were purchased from Aldrich and stored over
molecular sieves (4 Å) for at least 20 h. Borane dimethyl sulfide was purchased from Strem
Chemicals and stored over molecular sieves (4 Å) for at least 20 h. Sodium acetate (2) and
potassium acetate (3) were purchased from Aldrich and dried at 100 ºC under high vacuum for
24 h prior to use. Methanol was distilled over KOH and stored over activated molecular sieves
(4 Å) for at least 48 h. 1D and 2D NMR spectra were recorded before and after the CO2
injection, using Bruker AC 300 MHz, 400 MHz, 500 MHz and 600 MHz NMR spectrometers.
NMR chemical shift values were determined relative to the residual protons in C6D6 and
1
C6D5Br as internal reference for H (δ of the most downfield signal = 7.16, 7.30, 5.32, 7.26
ppm) and 13C{1H} (δ of the most downfield signal = 128.39, 130.89, 53.84, 77.23 ppm).
All DFT calculations have been performed with the Gaussian 09 suite of programs, at
the wB97xD/def2tzvpp//B3LYP/6-31G* level for the optimization and characterization of all
stationary points. The effect of solvent has been included both in geometry optimizations and
energy refinement using the integral equation formalism variant of PCM (IEFPCM) with the
solvent parameters corresponding to benzene. Harmonic analysis has been carried out at all
stationary points, in order to characterize them either as minima or transition states in the
potential energy surface, and the stability of the wavefunction has also been checked, to ensure
that it is a minimum in the corresponding functional space. For the sake of simplicity, we have
not included in our calculations the cationic counterion in the anionic structures.
[N(nBu)4][OAc] (1). Tetrabutylammonium acetate [N(nBu)4][OAc] (1) was synthesized on a
gram-scale via a salt metathesis reaction between a stoichiometric amount of potassium acetate
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and tetrabutylammonium chloride in dry methanol as solvent. Salt 1 was isolated inDqOuI:a1n0.t1i0t3a9t/iDv0eCY00118J
yield as a colorless solid. 1H NMR (400 MHz, C6D6): δ 3.40 (m, 8H, nBu), 2.39 (s, OAc), 1.55
(m, 8H, nBu), 1.39-1.34 (m, 8H, nBu), 0.97 (t, 8H, nBu). 13C NMR (100 MHz, C6D6): δ 174.1
(C=O), 58.7 (Me-Ac), 24.7 (nBu), 24.6 (nBu), 20.3 (nBu), 14.1 (nBu).
General procedure for the catalytic reduction of CO2 by hydroboration with acetate salts
1-3. The catalytic hydroboration of CO2 was divided in two steps. First, in a dry box, a NMR J.
Young tube was charged with the acetate catalyst (1-3). In the case of 5 and 10% mol. catalyst
loadings, pure catalyst 1-3 was directly added to the NMR tube. In the case of 0.1, 0.5 and 1%
mol. catalyst loadings, the appropriate volume of a pre-prepared 1 M solution of catalyst 1-3
(in THF) was added to the NMR tube (THF was then removed in vacuo for a couple of hours).
C6D6 (0.5 mL) and the appropriate amount of borane were subsequently added to the tube along
with C6Me6 (internal standard). Outside the dry box, the tube was cooled with liquid N2, the
tube was freezed-pumped-thawed, then pressurized with 1.5 bar of CO2 and allowed to warm
to room temperature. The tube was then placed into an oil bath, pre-heated at the desired
reaction temperature. The reaction was monitored by 1H and 11B NMR and products assignment
was based on literature data.
Hydroboration of CO2 by pinB–H catalysed by 1 (0.5 mol%) with [pinBH]0 = 3 M. Catalyst
1 (7.46 µmol, 1.0 equiv), HBpin (212 mg, 1.49 mmol, 200 equiv), and hexamethylbenzene (4
mg, 24.6 µmol, 1.5 equiv) as an internal standard in C6D6 (0.5 mL) were transferred to a NMR
J Young tube. After CO2 pressurization under 1.5 atm, the tube was heated at 90 ºC and the
reaction monitored by 1H and 11B NMR. The 1H and 11B NMR spectra showed the consumption
of 96 % of HBpin to form CH3OBpin after 20 h. The Turnover number (TON), i.e. borane
conversion to A, B and C, was calculated by the integration of the 1H NMR signals of product
A (HOCOBpin,  = 8.23 ppm) and methoxyborane C (MeO-Bpin,  = 3.44 ppm), using the
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hexamethylbenzene signal ( = 2.06 ppm) as reference. After 20 h, the reaction achieved more
DOI: 10.1039/D0CY00118J
than 98% of selectivity in C. The formation of A and C was further evidenced and quantified
by 11B NMR spectroscopy in accordance with literature data.
Generation of [pinB(O2CH)(OAc)][NnBu4] (4). Under a nitrogen atmosphere, a C6D6 solution
(0.5 mL) containing salt 1 (15 mg, 49.75 µmol) and pinB–H (6.37 mg, 49.75 µmol) was
1
11
transferred to a NMR J. Young tube. H and B NMR spectra were immediately recorded
suggesting a labile interaction between the OAc- anion and pinB–H. The NMR tube was then
charged with 1.5 bar of CO2 at room temperature. The resulting mixture was immediately
1
analyzed by H, 13C, HSQC and HMBC spectroscopy showing showing the complete
conversion latter which also showed being unstable when isolated. 1H NMR (300 MHz, C6D6)
δ 9.18 (s, HCOO, 1H), 3.27 – 2.88 (m, CH2, 2H), 2.06 (s, CH3, 3H), 1.52 – 1.40 (m, CH2, 2H),
1.40 – 1.34 (m, CH2, 2H), 1.32 (s, CH3, 3H), 0.95 (t, CH3, 3H). 11B NMR (128 MHz, C6D6, δ):
8.35 (s, B). 13C NMR (101 MHz, C6D6, δ): 171.40 (OCOCH3), 165.97 (OCOH), 79.64 (Ctert),
58.53 (CH2), 25.97 (CH3), 24.61 (CH3COO), 24.21 (CH2), 20.1 (CH2), 13.95 (CH3).
Conflicts of interest
There are no conflicts to declare
Acknowledgments. The CNRS (Paris, France) and the University of Strasbourg are
acknowledged for financial support. The Brazilian agency Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq; Science without Borders Special Visiting
Scientist project 405784/2013-9) is acknowledged for financial support. J. H. Z. S., and H. S.
S. are grateful to CNPq for the PQ fellowships. Jordan Parmentier (University of Strasbourg)
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is acknowledged for low temperature 1D and 2D NMR experiments and additional blank
DOI: 10.1039/D0CY00118J
catalytic runs.
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17. A blank test run under such conditions (with only pinB–H under 1.5 atm of CO2) showed
no reduction reaction after 26 h (90 °C, C6D6).
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DOI: 10.1039/D0CY00118J
18. The reaction mixture was re-pressurized with 1.5 atm of CO2 after 8 h of reaction.
19. (a) B. Wrackmeyer and O. L. Tok, Zeitschrift für Naturforschung B, 2006, 61, 949–955.
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20. The 11B spectrum of a 1/1 1/pinB–H mixture (C6D6, RT) features a broad singlet signal at
 = 11.4 ppm significantly upfield shifted compared to that of three-coordinate pinB–H ( 27.9
1
ppm), in line with a four-coordinate boron species and thus adduct formation. The H NMR
data are also consistent with the adduct formation, with, in particular, a upfield shift of the
MeCO2- 1H NMR resonance ( 2.18 ppm) versus that in 1 ( 2.39 ppm). The NMR data are all
consistent with an effective Cs symmetry at RT, consistent with
a
fast
coordination/decoordination of OAc- on the NMR time scale. Such a fast exchange also
rationalizes the non-observation of the B–H 1H NMR resonance for the presumed adduct.
21. Low temperature NMR analysis of a 1/1 1/pinB–H mixture did not provide additional
evidence on adduct formation. The putative [pinB(H)(OAc)][TBA] salt could not be isolated,
perhaps due to the labile interaction between OAc- to pinB–H.
22. The formation of a dinuclear boron species with a bridging acetate (VI’, ESI) is also likely
and was also computed to yield slightly higher energy barriers, but could also account for the
formation of formyl product A’ (see ESI).
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Table of Contents only
The acetate anion is a highly robust and effective catalyst for the selective hydroboration of
CO2 to methanol-equivalent products.