Lewis base activation of borane-dimethylsulfide into strongly reducing ion pairs for the transformation of carbon dioxide to methoxyboranes

Article abstract of DOI:10.1039/c4cc04857a

The hydroboration of carbon dioxide into methoxyboranes by borane-dimethylsulfide using different base catalysts is described. A non-nucleophilic proton sponge is found to be the most active catalyst, with TOF reaching 64 h^-1 at 80°C, and is acting via the activation of BH₃·SMe₂ into a boronium-borohydride ion pair. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04857a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Lewis base activation of borane–dimethylsulfide into
strongly reducing ion pairs for the transformation of
carbon dioxide to methoxyboranes†
Cite this: Chem. Commun., 2014,
50, 11362
Received 25th June 2014,
Accepted 6th August 2014
Marc-Andre Legare, Marc-Andre Courtemanche and Frederic-Georges Fontaine*
´ ´
´
´
´
´
DOI: 10.1039/c4cc04857a
www.rsc.org/chemcomm
The hydroboration of carbon dioxide into methoxyboranes by reactive boranes, 9-borabicyclo[3.3.1]nonane (9-BBN), can be
borane–dimethylsulfide using different base catalysts is described. used as a reductant in the presence of some nitrogen bases to
A non-nucleophilic proton sponge is found to be the most active generate methoxyboranes from CO2.4f It was proposed that the
catalyst, with TOF reaching 64 hÀ1 at 80 8C, and is acting via the role of the catalysts was to activate the borane, either in a
activation of BH3ÁSMe2 into a boronium–borohydride ion pair.
Frustrated Lewis Pair (FLP) fashion or by the formation of ion
pairs. A similar reaction with 9-BBN was also reported using
Due to its availability and its role in climate change, carbon dioxide phosphines as catalysts to reduce carbon dioxide into methoxy-
has attracted considerable interest in recent years. Although several boranes.4g However, the most active metal-free organocatalyst to
sequestration technologies are known,1 chemical storage of CO2 in date is a phosphine–borane complex reported by our group that
the form of valuable hydrogen containing molecules, such as activates borane–dimethylsulfide (BH3ÁSMe2) and CO2 to yield
methanol, is highly desirable since these chemicals could be a methoxyboranes ((MeOBO)n) with a TOF exceeding 900 hÀ1 at
green alternative to fossil fuels, both as a source of organic 70 1C.4b The transformation was proposed to occur through
building blocks and as energy media.1,2
simultaneous nucleophilic activation of the borane and electro-
Even if they are costly and not economically viable for large scale philic fixation of carbon dioxide.8 In such a mechanism, the
processes, hydroboranes are amongst the most efficient reagents for reductant is made more nucleophilic by receiving electron density
the catalytic reduction of carbon dioxide to methanol.3,4 Although from the phosphine moiety. It is important to note that it is one of
the hydroboration of CQO bonds is a well-studied reaction for a the only two CO2 reduction systems reported to use BH3ÁSMe2 as a
large variety of substrates,5 carbon dioxide remains resistant to hydrogen source, the other being a more recent report by Stephan
uncatalyzed hydroboration by neutral hydroboranes, even when of an ambiphilic catalyst for the hydroboration of CO2.4h BH3
using the most reactive BH3 adducts. It has been shown that adducts are by far the most interesting boranes to use, since they
sodium or lithium borohydride can reduce CO2 with low selectivity have the highest hydrogen content of any hydroboranes in addi-
to a mixture of formate and methoxy products without the use of a tion of being less expensive than most of them. Their reactivity,
catalyst, illustrating the superior reducing power of these anionic however, tends to be limited by its strong tendency to form stable
boron hydrides.6 In order to facilitate the reaction of neutral borane adducts with Lewis bases.
hydroboranes with CO2, transition metal catalysts have been used
In an effort to broaden the scope of BH3ÁSMe2 as a reducing
to activate the B–H bonds and reduce CO2 via an hydrometallation agent for the functionalization of CO2, the effect of various
reaction.3,7 The most notable example of this type of catalysis is a commercially available bases on the catalytic activity was studied.
nickel pincer complex developed by Guan et al. that catalyzes the We thus decided to monitor by 1H NMR spectroscopy the reaction
hydroboration of CO2 to methoxyboranes with a TOF of 495 hÀ1 between the bases and 25 equiv. of BH3ÁSMe2 under one atm of
using HBcat.7b Recently, Cantat has shown that one of the most CO2 at 80 1C in a J Young NMR tube. The yields were calculated
using the integration of the (MeOBO)n generated in comparison to
the internal standard (hexamethylbenzene) present in the solution,
as shown in Fig. S1 (ESI†). Contrary to the reactivity observed using
9-BBN as a reductant,4g phosphines (PPh3, PtBu3 and PCy3) proved
ineffective for the CO2 reduction, as they are known to generate
stable BH3ÁPR3 adducts.9 Inorganic bases such as CsF, KCN and
CsCO3 also did not have any effect on the reduction of CO2.
However, sodium alkoxides (MeONa, EtONa, and tBuONa) proved
´
Departement de Chimie and Centre de Recherche sur la Catalyse et la Chimie Verte
´
´
´
(C3V), Universite Laval, 1045 Avenue de la Medecine, Quebec, Canada.
E-mail: frederic.fontaine@chm.ulaval.ca; Fax: +1-418-656-7916;
Tel: +1-418-656-5140
† Electronic supplementary information (ESI) available: Synthetic procedures,
solution NMR data and crystallographic information. CCDC 1009675. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc04857a
11362 | Chem. Commun., 2014, 50, 11362--11365
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
and the TOF of the reaction (64 hÀ1), presumably by increasing the
solubility of the reaction intermediates (vide infra). Letting the
reaction run for a longer time period allowed for complete conver-
sion of the BH3ÁSMe2 (Table 1, entry 11).
Table 1 Catalytic activity of different bases for the hydroboration of CO2
by BH3ÁSMe2
Entry
Base (mol%)
t (h)
T (1C)
Yield (%) (TON)
TOF
In order to shed light on the mode of activation of BH3 by
bidentate ligands, the reaction was studied using 1 since it proved to
be the most active catalyst. This exceptionally basic amine is thus
called because of its almost exclusive affinity for protons, a result of
the unique steric environment around its nitrogen atoms which
prevents them to coordinate larger electrophiles. The notable
exceptions include the fixation of a handful of boron cations,12 as
1
2
3
4
5
6
7
MeONa (4)
EtONa (4)
tBuONa (4)
DBU (4)
TBD (4)
Terpyridine (4)
1 (4)
1 (4)
1 (4)
1 (4)
1 (4)
1
1
1
1
1
1
1
1
24
1
21
24
0.16
3
24
1
80
80
80
80
80
80
80
25
25
80
80
80
25b
25b
25
80
80
1.3 (1)
2.6 (2)
4.0 (3)
23 (17)
25 (19)
47 (35)
68 (51)
4.0 (3)
39 (29)
85 (64)
499 (75)
7.6 (23)
24 (18)
21 (62)
28 (84)
21 (64)
25 (75)
1
2
3
17
19
35
51
3
8a
9a
10a
11a
12a
13
14a
15a
16a
17a
well as two transition metal complexes and group XIII species.13
1
1
64
4
was reported to react rapidly with B2H6 or boron trifluoride gas
to afford species identified as [PS(BX2)]+[BX4]À (X = H, F)12b or
[PS(BH2)]+[B2H7]À,12c but were not structurally characterized. These
species are reminiscent of the ‘diammonia of diborane’ that was
proposed as a key intermediate in the dehydrogenation of
ammonia–borane.12d–f Interestingly, Vedejs and coworkers were
able to prepare a 9-BBN derived PS stabilized boronium by a
more aggressive method.12g The cation thus formed was found
to be a highly reactive electrophile, contrary to classical boronium
cations. This property was proposed to arise from the steric
demands of the proton sponge framework. It is important to note
that no catalytic activity was ever reported for these species.
The hydroboration of CO2 using BH3ÁSMe2 was very slow when
1 (1)
1
2-BH4 (4)
2-BH4 (1)
2-BH4 (1)
2-BH4 (1)
2-BH4 (1)
108
62
4
64
3
24
Conditions: 0.01 mmol of base catalyst in benzene-d6 (0.4 mL), the
number of equivalents of BH3ÁSMe2 was calculated to obtain the desired
catalytic loading, 1 atm CO2. The yield was measured by NMR spectro-
scopy using hexamethylbenzene as an internal standard. TONs and TOFs
a
are given in regard to the number of hydrides transferred. Solvent =
b
dichloromethane-d2. Although the tube was kept at room temperature,
the exothermic reaction caused internal heating of the system.
to convert some of the CO2 to (MeOBO)n, notably getting up to 3 catalyzed by 1 at room temperature in benzene-d6. However, a
turn-overs in the first hour of reaction with tBuONa (Table 1, solution of BH3ÁSMe2 and 1 heated to 70 1C for an hour in the
entries 1–3). It is known that the addition of alkoxides to BH3 leads absence of CO2 was found to react rapidly and exothermically
to the generation of alkoxyborohydrides.10 These species possess upon exposure to CO2 at room temperature. These observations
increased nucleophilicity when compared to neutral BH3 adducts suggest that a reaction between 1 and BH3ÁSMe2 is the limiting
and can disproportionate to give sodium borohydride.11 The low step in the catalytic transformation. It was possible to isolate
activity of sodium alkoxides can either be attributed to their low single crystals from a reaction mixture between 1, 50 equiv. of
solubility or to the low rate of the disproportion reaction in BH3ÁSMe2 and CO2. Although the limited solubility prevented
benzene-d6. On the other hand, anionic amides (lithium diisopro- NMR characterization, the solid state structure consisted of the
pylamide and lithium dimethylamide) were found to be inactive in boronium salt [PS(BH2)]2[B12H12] (2-B12H12) (Fig. 1). the resolved
the reduction of CO2.
solid state structure of the boronium cation is similar to a previously
Several nitrogen Lewis bases also proved to be inefficient under reported structure of [PS(BH2)][B19H22].12h Since the B12H122À dianion
the conditions tested, including triethylamine, diisopropylethylamine, is generally isolated as a product of the disproportionation of [B3H8]À,
2,2,6,6-tetramethylpiperidine, pyridine, 4-dimethylaminopyridine, which is prepared in oxidative conditions from BH4À 14
it could be
,
2,6-lutidine, 4-pyrrolopyridine and 2,6-di-tert-butylpyridine. How- suggested that the initial step in the catalytic transformation involves
ever, as shown in Table 1, some species proved to exhibit signifi- the generation of the ion pair [PS(BH2)][BH4] (2-BH4).
cant activity in the generation of methoxyboranes.
In order to better understand the reaction between proton
Some reduction products were observed when BH3ÁSMe2 was added sponge and BH3ÁSMe2, the products formed were characterized
to strong Lewis bases, such as DBU (1,8-diazabicycloundec-7-ene) using 11B NMR spectroscopy while controlling the stoichiometry
and TBD (triazabicyclo[4.4.0]dec-5-ene), giving respective TOF of of the reagents. Over the course of the reaction, three different
17 and 19 hÀ1 (Table 1, entries 4 and 5). However, multidentate boronium salts were observed, depending on the reaction condi-
amines, such as terpyridine and proton sponge (PS; 1) gave the best tions. In all cases, the formation of the dihydroboronium cation is
activity, with observed TOF of 35 and 51 hÀ1 when the reaction was supported by 1H NMR by a significant downfield shift for the proton
carried out at 80 1C in benzene-d6 (Table 1, entries 6 and 7). When sponge resonances to a doublet of doublets at 7.78 ppm ( J = 8 Hz)
the reaction was carried out in the presence of 1 as catalyst at and two doublets at 8.23 and 8.08 ppm ( J = 8 Hz) in dichloro-
ambient temperature a lower activity was observed. The TON and methane-d2, with a slight variation on the chemical shift depending
TOF dropped to reach in the best conditions, 29 (Table 1, entry 9) on the counterion (see ESI†). Although the resonances of the [BH2]+
and 3 hÀ1, respectively (Table 1, entry 8). It is good to note that were not observed by 1H NMR spectroscopy, the 11B NMR spectra of
under these conditions 1 did not present any activity in presence of derivatives of 2 did show a broad resonance at ca. 1.5 ppm, which is
9-BBN, as reported by Cantat et al.4f Running the reaction in similar to that of previously reported [PS(BH2)][B2H7].12c Stirring 1 in
dichloromethane-d2 (Table 1, entry 10) did increase the TON (64) diethyl ether with 2 equiv. of BH3ÁSMe2 for 16 hours allowed us to
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 11362--11365 | 11363

View Article Online
ChemComm
Communication
Fig. 1 The ORTEP plot of [PS(BH2)]2[B12H12] with thermal ellipsoids set at
the 40% probability level. Selected bond lengths (Å) and angles (1): N1–B1:
1.601(2), N1–C1: 1.511(2), N2–B1: 1.600(2), N1–B1–N2: 111.62(13), C1–C10–C9:
126.11(15), C1–N1–B1: 111.07(12), C9–N2–B1: 111.07(13).
Fig. 2 Proposed mechanism for the reduction of CO2 to methoxyboranes
catalyzed by 1.
obtain colourless crystals. 11B NMR spectroscopy for this compound
showed, in addition to the resonance at 1.57 ppm for the borenium
species [PS(BH2)]+, a sharp quintuplet at À38.4 ppm due to the (Table 1, entry 13). The heat produced by the exothermic hydro-
[BH4]À anion. The latter was confirmed by observation of a sharp boration of carbon dioxide likely contributed to the increase in
4-line 1H NMR resonance at d À0.06. These spectroscopic findings catalytic activity, obtaining a TOF of 108 hÀ1. However, when
strongly suggest the presence of the [BH4]À anion and the formation looking at its reactivity at 80 1C or for a longer time period, the
of salt 2-BH4 ([PS(BH2)][BH4]). However, using 10 equiv. of BH3ÁSMe2 catalytic activity 2-BH4 was found to be similar to that of 1
in presence of 1 in toluene afforded [PS(BH2)][B3H8] (2-B3H8) as the (Table 1, entries 14–17).
main product (Scheme 1) after heating. The [B3H8]À anion was
From these observations, it is possible to propose a catalytic
cycle, as shown in Fig. 2. In a first step that is rate-limiting, the
bidentate ligand activates borane–dimethylsulfide to generate
characterized by the presence of a nonuplet at À30.0 ppm in the 11
B
NMR spectrum.15
Although the [B12H12]À anion could not be observed by NMR a highly reactive boronium–borohydride ion pair. Similar
spectroscopy, the crystallization of all samples of [PS(BH2)][B3H8] boronium species has been synthesized with TBD,16 but under
consistently yielded 2-B12H12. It is rational to propose that initial catalytic conditions, thus explaining the need for higher reac-
activation of BH3ÁSMe2 by proton sponge affords the ion pair tion temperature in order to achieve the catalytic reduction of
2-BH4, which can degrade to 2-B3H8 by reaction with BH3ÁSMe2 CO2 since no activity was observed between DBU and TBD in
and to 2-B12H12. The activity of these compounds was studied presence of BH3ÁSMe2 at 25 1C.4f The [BH4]À generated can
by treating them with an atmosphere of carbon dioxide. In this in turn react with carbon dioxide to give formate derivatives.
+
manner, [PS(BH2)][B3H8] was found to be unreactive towards CO2. The formate generated can abstract the BH2 fragment gener-
Unsurprisingly, a dicholoromethane-d2 solution of 2-BH4 in absence ating reduced species such as HCOOBH2. In turn, BH3ÁSMe2
of additional boranes reacted instantaneously at room temperature can very efficiently reduce formatoboranes into (MeOBO)n, thus
with carbon dioxide. Within five minutes, two different signals explaining the highest TOF obtained using BH3ÁSMe2 instead of
appeared in the 1H NMR spectrum at 8.27 and 8.33 ppm, assigned other organic hydroboranes.
to the formation of formate products, along with the complete
In summary, the use of BH3ÁSMe2 as a reducing agent for CO2
À
disappearance of the BH4 resonances. No products of higher hydroboration is possible in presence of strong Lewis bases.
reduction level were observed in this manner, suggesting that the Although limited activity is observed with monodentate species
catalyst is only active in the reduction of carbon dioxide. Similarly, a such as DBU and TMP, bidentate amines such as proton sponge
solution of 2-BH4 and 25 equiv. of BH3ÁSMe2 reacted within minutes proved to be the active catalysts in this reaction. It was possible
with carbon dioxide to give methoxyboranes at room temperature to isolate single crystals that demonstrated the presence of the
boronium species [PS(BH2)]+. 11B NMR studies demonstrate that
the most likely active species for the CO2 reduction into formates
was the borate ion, BH4À, whereas the BH3ÁSMe2 in solution can
reduce the formatoborane derivatives into methoxyboranes.
Optimization of this system is currently underway.
The authors would like to acknowledge National Sciences
and Engineering Research Council of Canada (NSERC, Canada)
and the Centre de Catalyse et Chimie Verte (Quebec) for
Scheme 1 Reaction of PS with BH3ÁSMe2 to give boronium salts.
11364 | Chem. Commun., 2014, 50, 11362--11365
financial support. M.-A. C. and M.-A. L. would like to thank
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
´
´
8 M.-A. Courtemanche, M.-A. Legare, L. Maron and F.-G. Fontaine,
J. Am. Chem. Soc., 2014, 136, 10708–10717.
NSERC and FQRNT for scholarships. We acknowledge W. Bi for
the resolution of the crystal structure.
9 H. Schmidbaur, T. Wimmer, G. Reber and G. Mu¨ller, Angew. Chem.,
Int. Ed. Engl., 1988, 27, 1071–1074.
10 E. C. Ashby, F. R. Dobbs and H. P. Hopkins Jr., J. Am. Chem. Soc.,
1975, 97, 3158–3162.
Notes and references
1 D. M. D’Alessandro, B. Smit and J. R. Long, Angew. Chem., Int. Ed., 11 J. L. Hubbard, Sodium Trimethoxyborohydride, e-EROS Encyclopedia
2010, 49, 6058–6082. of Reagents for Organic Synthesis, 2001.
2 G. A. Olah, A. Goeppert and G. K. S. Prakash, Beyond Oil and Gas: The 12 (a) D. D. Axtell, A. C. Campbell, P. C. Keller and J. V. Rund, J. Coord.
Methanol Economy, Wiley-VCH, Weinheim, 2006.
Chem., 1976, 5, 129–134; (b) T. Onak, H. Rosendo, G. Siwapinyoyos,
R. Kubo and L. Liauw, Inorg. Chem., 1979, 18, 2943–2945; (c) P. C.
Keller and J. V. Rund, Inorg. Chem., 1979, 18, 3197–3199; (d) D. R.
Schultz and R. W. Parry, J. Am. Chem. Soc., 1958, 80, 4–8; (e) M. Bowden,
D. J. Heldebrant, A. Karkamkar, T. Proffen, G. K. Schenter and T. Autrey,
Chem. Commun., 2010, 46, 8564–8566; ( f ) C. W. Hamilton, R. T. Baker,
A. Staubitz and I. Manners, Chem. Soc. Rev., 2009, 38, 279–293;
(g) A. Prokofjevs, J. W. Kampf and E. Vedejs, Angew. Chem., Int. Ed.,
3 (a) K. Huang, C.-L. Sun and Z.-J. Shi, Chem. Soc. Rev., 2011, 40, 2435–2452;
(b) T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365–2387;
(c) M. Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975–2992.
4 (a) F.-G. Fontaine, M.-A. Courtemanche and M.-A. Legare, Chem. –
Eur. J., 2014, 20, 2990–2996; (b) M.-A. Courtemanche, M.-A. Legare,
L. Maron and F.-G. Fontaine, J. Am. Chem. Soc., 2013, 135, 9326–9329;
´
´
´
´
´
´
(c) M.-A. Courtemanche, J. Larouche, M.-A. Legare, B. Wenhua, L. Maron
and F.-G. Fontaine, Organometallics, 2013, 32, 6804–6811; (d) S. N. Riduan,
Y. Zhang and J. Y. Ying, Angew. Chem., Int. Ed., 2009, 48, 3322–3325;
(e) A. Berkefeld, W. E. Piers and M. Parvez, J. Am. Chem. Soc., 2010, 132,
ˇ
2011, 50, 2098–2101; (h) M. G. S. Londesborough, J. Bould, T. Base,
´
ˇ ´
D. Hnyk, M. Bakardjiev, J. Holub, I. Cısarova and J. D. Kennedy,
Inorg. Chem., 2010, 49, 4092–4098.
´
10660–10661; ( f ) C. Das Neves Gomes, E. Blondiaux, P. Thuery and 13 (a) J. P. Collman, M. Zhong, C. Zhang and S. Constanzo, J. Org.
T. Cantat, Chem. – Eur. J., 2014, 23, 7098–7106; (g) T. Wang and
D. W. Stephan, Chem. Commun., 2014, 50, 7007–7010; (h) T. Wang and
D. W. Stephan, Chem. – Eur. J., 2014, 20, 3036–3039.
Chem., 2001, 66, 7892–7897; (b) T. Yamasaki, N. Ozaki, Y. Saika,
K. Ohta, K. Goboh, F. Nakamura, M. Hashimoto and S. Okeya,
¨
Chem. Lett., 2004, 33, 928–929; (c) J. F. Kogel, X. Xie, E. Baal,
5 E. Mincione, J. Org. Chem., 1978, 43, 1829–1830.
D. Gesevicius, B. Oelkers, B. Kovacevic and J. Sundermeyer,
6 (a) T. J. Wartik and R. K. Pearson, J. Inorg. Nucl. Chem., 1958, 7, 404–411;
Chem. – Eur. J., 2014, 20, 7670–7685.
(b) H. E. Heller, W. G. Brown and J. G. Hurr Jr., J. Am. Chem. Soc., 1950, 14 V. Geis, K. Guttsche, C. Knapp, H. Scherer and R. Uzun, Dalton
72, 2560. Trans., 2009, 2687–2694.
7 (a) R. Shintani and K. Nozaki, Organometallics, 2013, 32, 2459–2462; 15 J. D. Woollins, Inorganic Experiments, Wiley-VCH, Weinheim, 3rd
(b) S. Chakraborty, J. Zhang, J. A. Krause and H. J. Guan, J. Am. Chem. Soc., revised edn, 2010, pp. 405–408.
2010, 132, 8872–8873; (c) S. Bontemps, L. Vendier and S. Sabo-Etienne, 16 O. Ciobanu, E. Kaifer, M. Enders and H.-J. Himmel, Angew. Chem.,
Angew. Chem., Int. Ed., 2012, 51, 1671–1674.
Int. Ed., 2009, 48, 5538–5541.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 11362--11365 | 11365