Carbene-9-BBN ring expansions as a route to intramolecular frustrated lewis pairs for CO2 reduction

Article abstract of DOI:10.1002/chem.201304870

Reactions of phosphine-derived carbenes with 9-borabicyclo[3.3.1]nonane (9-BBN) result in ring-expansion reactions to generate novel intramolecular frustrated Lewis pairs (FLPs). These FLPs effect the catalytic reduction of CO₂ in the presence of boranes to give BOB and methoxy-borate species. A BOB well done: Reactions of phosphine-derived carbenes with 9-borabicyclo[3.3.1]nonane (9-BBN) result in ring-expansion reactions to generate novel intramolecular frustrated Lewis pairs (FLPs). These FLPs effect the catalytic reduction of CO₂ in the presence of boranes to give BOB and methoxy-borate species (see scheme).

Full text of DOI:10.1002/chem.201304870

DOI: 10.1002/chem.201304870
Communication
&
Carbenes
Carbene-9-BBN Ring Expansions as a Route to Intramolecular
Frustrated Lewis Pairs for CO2 Reduction**
Tongen Wang and Douglas W. Stephan*[a]
BÀC bond of 9-BBN under mild conditions. Nonetheless, these
Abstract: Reactions of phosphine-derived carbenes with
ring-expanded products behave as intramolecular FLPs, com-
9-borabicyclo[3.3.1]nonane (9-BBN) result in ring-expan-
bining strongly basic sites with a weakly Lewis acidic borane
sion reactions to generate novel intramolecular frustrated
center, to effect the catalytic, metal-free reduction of CO2 in
Lewis pairs (FLPs). These FLPs effect the catalytic reduction
the presence of borane hydrides under mild conditions.
of CO2 in the presence of boranes to give BOB and me-
Only recently have the N-phosphorylated NHCs of the form
thoxy-borate species.
C3H2(NPR2)2 been synthesized.[20–22] Following literature proce-
dures, we prepared the sterically demanding and electron do-
nating analogues C3H2(NPR2)2 (R=tBu 1, NiPr2 2) in straightfor-
ward treatment of imidazolate-phosphine C3H3N(NPR2) with
The first isolation of a stable carbene by Bertrand in 1989[1]
and the subsequent generation of N-heterocyclic carbenes
(NHCs) by Arduengo and co-workers[2] in 1991 are important
milestones in modern chemistry. Since then, NHCs have been
widely utilized in various fields including transition-metal
chemistry,[3–5] catalysis[6–9] and main group chemistry[10–14] be-
cause of their robust two-electron s-donor ability and their
easy modification. NHCs were generally considered to be
robust ligands until the Bertrand group uncovered reactions of
NHCs with pinacolborane (HBpin) resulted in the CÀN bond
cleavage.[15] Subsequently the Hill group discovered related CÀ
N bond activation and ring opening of an NHC on (NHC)BeH2
compound under mild conditions.[16] More recently, Radius and
co-workers reported elegant work on the CÀN bond cleavage
and ring expansion of NHCs using hydrosilanes on heating to
1008C for a few days.[17] Rivard has also reported CÀN bond
cleavage and ring expansion of NHCs with borane hydrides in
the reactions of IPr (IPr=N,N’-bis-[2,6-(diisopropyl)phenyl]imi-
dazol-2-ylidene) with BH2NHDipp (Dipp=2,6-(diisopropyl)-
phenyl) at 1008C.[18]
sodium triflate and the secondary phosphorus chloride, R2PCl.
Deprotonation of these triflate salts with potassium hexa-
methyldisilazane afforded the corresponding carbenes. Com-
pound 1 gives rise to a 31P signal at 98.1 ppm. During the
course of our work an analogous preparation and crystallo-
graphic study of 1 has appeared.[23]
The reaction between 9-BBN and 1 in [D5]bromobenzene at
room temperature was monitored by 31P{1H} NMR spectroscopy
revealing the formation of a new product as evidenced by the
two resonances at d=76.7 and 71.2 ppm. This product also
gave rise to a 11B{1H} NMR resonance at d=50.34 ppm. Subse-
quent workup and extraction into hexane and subsequent
cooling afforded the colorless, crystalline solid 3 in 30% yield.
1
The H NMR spectrum of 3 is consistent with inequivalent car-
bene backbone CÀH groups as two distinct resonances at d=
3
5.67 (3JH-H =6.3 Hz and JH-P =2.0 Hz) and d=6.01 (3JH-H =6 Hz
3
and JH-P =1 Hz) were observed. An X-ray crystallographic study
revealed 3 as the product is the result of an unusual CÀN
bond cleavage and ring expansion of NHC (Figure 1a). This re-
sults in a six-member ring in which the carbene carbon has in-
serted in to the BÀC bond of 9-BBN affording a BCNC2N ring.
In a similar fashion, the reaction of 9-BBN with 2 afforded the
analogous insertion product 4 in 43% yield. The spectral data
of 4 were analogous to those of 3 and this species was also
characterized by X-ray crystallography (Figure 1b).
Recently, our group has reported the isolation of the simple
NHC adducts of 9-borabicyclo[3.3.1]nonane (9-BBN) en route
to the borenium salt [(IiPr2)(BC8H14)][B(C6F5)4].[19] This species, in
the presence of PtBu3, behaves as a frustrated Lewis pair (FLP)
to activate hydrogen gas and catalyze the hydrogenation of
imines and enamines. Following this work, we were interested
to utilize carbenes of the form C3H2(NPR2)2 to generate an in-
tramolecular borenium-phosphine FLP. However, herein we
report that the reaction of such NHC derivatives with 9-BBN
proceeds through CÀN bond cleavage and insertion into the
The mechanism of these insertions and ring expansions is
thought to proceed by initial coordination of the NHC to the
boron center of 9-BBN. This view is supported by the inhibition
of the reaction by the addition of excess pyridine to the reac-
tion mixture of 1 and 9-BBN. Migration of the H atom from
boron to the electron-deficient carbene carbon atom prompts
coordination of one of the adjacent nitrogen atoms to B, thus
breaking the CÀN bond. The electrophilic carbon center in the
resulting zwitterion prompts subsequent migration of a BÀC
bond to yield the new CÀC bond containing the products 3
and 4 (Scheme 1). Recent computational studies have been
shown to support such a mechanism.[24,25] It is noteworthy that
[a] Dr. T. Wang, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN
[**] 9-BBN=9-borabicyclo[3.3.1]nonane
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304870.
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Communication
Table 1. Catalytic reduction of CO2.[a]
Entry Borane
Cat.
t
Yield TON Product ratio
([mol%]) [h]
[%]
5/6/7
8
1
2
3
4
5
6
7
8
9
HBpin
–
114
114
66
21
66
21
4.5
1.0
0.25 100
11
11
0
95
89
100
100
99
100
100
0
19
89
20
100
99
100
100
100
240
240
0
1
1
1
1
0
0
0
0
–
–
0
0
–
–
–
–
–
–
–
–
–
1
1
HBpin
3(5)
3(1)
4(5)
4(1)
3(1)
3(1)
4(1)
4(1)
0.06 0.01
0.08 2.6
0.02 0.1
0.30 0.3
0
0
0
0
–
–
HBpin[b]
HBpin
HBpin[b]
HBcat[b,c]
HBcat[b]
HBcat[b,c]
HBcat[b]
1
1
1
1
–
–
10
11
BH3(SMe2)[b] 3(1)
BH3(SMe2)[b] 4(1)
80
80
[a] Reactions conducted in J-Young tubes; monitored by NMR spectrosco-
py; C6D5Br (0.8 mL); P(13CO2): CO2 (5 atm), 608C, borane (20 equiv);
[b] borane (100 equiv); no reaction was observed in the absence of FLP
catalyst;[27] [c] 258C.
Figure 1. POV-ray drawing of: a) 3 and b) 4. All hydrogen atoms are omitted
for clarity.
sumption of 96% of the borane and affording a product mix-
ture of HCO2Bpin (5a), CH2(OBpin)2 (6a), and MeOBpin (7a), in
a ratio of 1:0.06:0.01 (entry 2, Table 1) together with the by-
product O(Bpin)2. Under comparable conditions compound 4
produces a similar product ratio of 5a/6a/7a of 1.0:0.02:0.11
in 21 h (entry 4, Table 1). Increasing the concentration of HBpin
to 100 equivalents in the presence of 3 or 4 had very different
effects. In the former, the major product becomes the me-
thoxy-borane 7a (entry 3, Table 1) whereas when using 4 only
a small impact on the product distribution is observed (en-
tries 4 and 5, Table 1). In contrast, changing the borane to
HBcat and using either 3 or 4 as the catalyst resulted in much
faster and complete conversions to the fully reduced product
MeOBcat (7b), with no evidence of HCO2Bcat (5b), CH2(OBcat)2
(6b). Increasing the temperature from 25 to 608C reduced the
time to complete reduction of CO2 from 21 to 4.5 h (entries 6
and 7, Table 1) when 3 was used as the catalyst. On the other
hand, when performing the catalysis using 4, the correspond-
ing reaction times were 1 h and 15 min (entries 8 and 9,
Table 1). The higher reactivity of 3 in these latter cases, is
thought to be the result of the stronger Lewis basicity of
P(NiPr2)2 over PtBu2, which accelerates CO2 binding and enhan-
ces activation for subsequent reduction. It is interesting that
use of BH3(SMe2) resulted in similar reactivity of 3 and 4 (en-
tries 10 and 11, Table 1) leading to 80% consumption of the
100 equivalents of borane in 11 h affording exclusively
(MeOBO)3 (8) as the observed product. These observations
confirm a marked dependence of the rate of reduction on
both the nature of the FLP catalyst and the reducing agent
(borane).
Scheme 1. Proposed mechanism for the formation of 3 and 4.
the ring expansion observed in the formation of 3 and 4 is
reminiscent of the work of Soderquist’s group who exploited
the insertion into the BÀC bonds of 9-BBN to generate oxaza-
borolidines and chiral 9-BBD derivatives.[26]
Compounds 3 or 4 were exposed to an atmosphere of 13CO2
at room temperature. Although compound 3 showed no ap-
parent reaction with CO2, compound 4 exhibits a 31P resonance
at 20.2 ppm and a 13C{1H} signal at 166.8 ppm with a PÀC cou-
pling constant of 184.5 Hz. This is consistent with the more
basic nature of the P centers in 4. Despite this evidence for
a CO2 adduct of 4 efforts to isolate this species were unsuc-
cessful, suggesting an equilibrium binding. Nonetheless, such
equilibria are well suited to catalytic reductions. To this end,
catalytic amounts (1–5 mol%) of compounds 3 or 4 were ex-
posed to CO2 in the presence of one of the reducing boranes
HBpin, HBcat, or BH3(SMe2) at 608C. Using 5 mol% catalyst and
5 atm CO2 at 608C, in the presence of 20 equiv of borane, 3
catalyzed reduction of CO2 with HBpin resulting in the con-
Mechanistically, these reactions are thought to exploit the
intramolecular FLP nature of the products 3 and 4. These com-
pounds are thought to activate CO2 in a transient fashion al-
lowing reaction with borane to generate the initial boron-for-
mate species 5. It should be noted that efforts to either ob-
serve or isolate these intramolecule FLP–CO2 complexes were
unsuccessful. Nonetheless, subsequent reaction with additional
borane is thought to afford the methylene CH2(OBR2)2 (6) and
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Communication
[1] G. Bertrand, A. Igau, A. Baceiredo, G. Trinquier, Angew. Chem. 1989, 101,
617; Angew. Chem. Int. Ed. Engl. 1989, 28, 621.
[2] A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113, 361–
363.
[3] S. Dꢁez-Gonzꢂlez, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109, 3612–
3676.
[4] M. Poyatos, J. A. Mata, E. Peris, Chem. Rev. 2009, 109, 3677–3707.
[5] O. Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev. 2009,
109, 3445–3478.
methoxy-boranes MeOBR2 (7) products and R2BOBR2. The role
of 3 and 4 in the catalysis was further supported by experi-
ments in which the phosphines PtBu3 and PhP(NiPr2)2 were
combined with HBcat and CO2 at room temperature for 21 h
and at 608C for 8 h, respectively. In these cases, no MeOBcat
was formed. These data suggest that it is the intramolecular
binding of CO2 that initiates these reductions (Scheme 2).
[6] N-Heterocyclic Carbenes in Synthesis (Ed.: S. Nolan), Wiley-VCH, Wein-
heim, 2006.
[7] N-Heterocyclic Carbenes in Transition Metal Catalysis, Vol. 21 (Ed.: F. Glo-
rius), Springer, Berlin, 2007.
[8] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 2007, 107, 5606–5655.
[9] N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic
Tools (Ed.: S. Dꢁez-Gonzꢂlez), RSC, Cambridge, 2011.
[10] Y. Z. Wang, G. H. Robinson, Dalton Trans. 2012, 41, 337–345.
[11] Y. Z. Wang, G. H. Robinson, Inorg. Chem. 2011, 50, 12326–12337.
[12] Y. Z. Wang, G. H. Robinson, Chem. Commun. 2009, 5201–5213.
[13] D. Martin, M. Soleilhavoup, G. Bertrand, Chem. Sci. 2011, 2, 389–399.
[14] U. Siemeling, Aust. J. Chem. 2011, 64, 1109–1112.
[15] G. D. Frey, J. D. Masuda, B. Donnadieu, G. Bertrand, Angew. Chem. 2010,
122, 9634–9637; Angew. Chem. Int. Ed. 2010, 49, 9444–9447.
[16] M. Arrowsmith, M. S. Hill, G. Kociok-Kohn, D. J. MacDougall, M. F.
Mahon, Angew. Chem. 2012, 124, 2140–2142; Angew. Chem. Int. Ed.
2012, 51, 2098–2100.
Scheme 2. Proposed mechanism for the reduction of CO2 by 3 or 4.
[17] D. Schmidt, J. H. J. Berthel, S. Pietsch, U. Radius, Angew. Chem. 2012,
124, 9011–9015; Angew. Chem. Int. Ed. 2012, 51, 8881–8885.
[18] S. M. I. Al-Rafia, R. McDonald, M. J. Ferguson, E. Rivard, Chem. Eur. J.
2012, 18, 13810–13820.
[19] J. M. Farrell, J. A. Hatnean, D. W. Stephan, J. Am. Chem. Soc. 2012, 134,
15728–15731.
[20] A. P. Marchenko, H. N. Koidan, I. I. Pervak, A. N. Huryeva, E. V. Zarudnit-
skii, A. A. Tolmachev, A. N. Kostyuk, Tetrahedron Lett. 2012, 53, 494–496.
[21] A. P. Marchenko, H. N. Koidan, A. N. Huryeva, E. V. Zarudnitskii, A. A.
Yurchenko, A. N. Kostyuk, J. Org. Chem. 2010, 75, 7141–7145.
[22] P. Nꢃgele, U. Herrlich, F. Rominger, P. Hofmann, Organometallics 2013,
32, 181–192.
[23] P. Ai, A. A. Danopoulos, P. Braunstein, K. Y. Monakhovc, Chem. Commun.
2013, 50, 103–105.
[24] M. R. Momeni, E. Rivard, A. Brown, Organometallics 2013, 32, 6201.
[25] K. J. Iversen, D. J. D. Wilson, J. L. Dutton, Organometallics 2013, 32, 6209.
[26] C. H. Burgos, E. Canales, K. Matos, J. A. Soderquist, J. Am. Chem. Soc.
2005, 127, 8044–8049.
[27] M. A. Courtemanche, M. A. Legare, L. Maron, F. G. Fontaine, J. Am.
Chem. Soc. 2013, 135, 9326–9329.
[28] G. Mꢄnard, D. W. Stephan, Dalton Trans. 2013, 42, 5447–5453.
[29] G. Mꢄnard, T. M. Gilbert, J. A. Hatnean, A. Kraft, I. Krossing, D. W. Ste-
phan, Organometallics 2013, 32, 4416–4422.
In contrast to other main-group intramolecular FLP systems,
3 and 4 combine strongly donating basic centers with weakly
acidic boron centers. The weaker Lewis acidity is thought to
offer lability of the reduced CO2 fragments, prompting catalytic
turnover in the reduction cycle. This stands in contrast to FLPs
derived from strong Lewis acids in which only stoichiometric
reductions are observed.[28–46] This situation is analogous to the
recently reported work from Fontaine and co-workers, in
which they exploited intramolecular FLPs for similar CO2 reduc-
tions.[27]
In summary, herein we have reported the insertion of CÀN
bonds of an N-heterocyclic carbene into 9-BBN. The resulting
intramolecular phosphine-borane acts as a catalyst for the acti-
vation of CO2 in the presence of several borane reducing
agents. Although use of HBpin was shown to provide a product
mixture dominated by the boron-formate, use of HBcat and
BH3(SMe2) resulted in the complete reduction of CO2 to the
corresponding methoxy-borane. These findings point out that
the use of weak Lewis acids in the FLP activation and reduc-
tion of CO2 facilitates catalytic turn-over. Efforts to develop re-
lated catalytic processes that exploit oxygen atom acceptors
such as olefins to generate highly desirable by-products are
on-going.
[30] Y. Jiang, O. Blacque, T. Fox, H. Berke, J. Am. Chem. Soc. 2013, 135, 7751–
7760.
[31] R. Dobrovetsky, D. W. Stephan, Angew. Chem. 2013, 125, 2576–2579;
Angew. Chem. Int. Ed. 2013, 52, 2516–2519.
[32] M. J. Sgro, J. Dçmer, D. W. Stephan, Chem. Commun. 2012, 48, 7253–
7255.
[33] R. C. Neu, G. Mꢄnard, D. W. Stephan, Dalton Trans. 2012, 41, 9016–
9018.
[34] G. Mꢄnard, D. W. Stephan, J. Am. Chem. Soc. 2010, 132, 1796–1797.
[35] C. M. Mçmming, E. Otten, G. Kehr, R. Frçhlich, S. Grimme, D. W. Stephan,
G. Erker, Angew. Chem. 2009, 121, 6770–6773; Angew. Chem. Int. Ed.
2009, 48, 6643–6646.
Acknowledgement
D.W.S. gratefully acknowledges the financial support of NSERC
of Canada and the award of a Canada Research Chair.
[36] A. E. Ashley, A. L. Thompson, D. O’Hare, Angew. Chem. 2009, 121,
10023–10027; Angew. Chem. Int. Ed. 2009, 48, 9839–9843.
[37] L. J. Hounjet, C. B. Caputo, D. W. Stephan, Angew. Chem. 2012, 124,
4792–4795; Angew. Chem. Int. Ed. 2012, 51, 4714–4717.
[38] A. M. Chapman, M. F. Haddow, D. F. Wass, J. Am. Chem. Soc. 2011, 133,
8826–8829.
Keywords: borane hydrides · boranes · carbenes · carbon
dioxide · frustrated Lewis pairs
[39] M. A. Dureen, D. W. Stephan, J. Am. Chem. Soc. 2010, 132, 13559–
13568.
[40] M. J. Sgro, D. W. Stephan, Chem. Commun. 2013, 49, 2610–2612.
Chem. Eur. J. 2014, 20, 3036 – 3039
www.chemeurj.org
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Communication
[41] F. Bertini, V. Lyaskoyskyy, B. Timmer, F. de Kanter, M. Lutz, A. Ehlers, J.
Slootweg, K. Lammertsma, J. Am. Chem. Soc. 2012, 134, 201–204.
[42] C. F. Jiang, D. W. Stephan, Dalton Trans. 2013, 42, 3318–3325.
[43] A. L. Travis, S. C. Binding, H. Zaher, T. A. Q. Arnold, J. C. Buffet, D. O’Hare,
Dalton Trans. 2013, 42, 2431–2437.
[46] C. Appelt, H. Westenberg, F. Bertini, A. W. Ehlers, J. C. Slootweg, K. Lam-
mertsma, W. Uhl, Angew. Chem. 2011, 123, 4011–4014; Angew. Chem.
Int. Ed. 2011, 50, 3925–3928.
[44] E. Theuergarten, J. Schlosser, D. Schluns, M. Freytag, C. G. Daniliuc, P. G.
Jones, M. Tamm, Dalton Trans. 2012, 41, 9101–9110.
Received: December 13, 2013
[45] M. Harhausen, R. Frohlich, G. Kehr, G. Erker, Organometallics 2012, 31,
2801–2809
Published online on February 13, 2014
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