Phosphine catalyzed reduction of CO2 with boranes

Article abstract of DOI:10.1039/c4cc02103g

Phosphine is shown to catalyse the reaction of CO₂ with 9-BBN to give mixtures of HCO₂(B(C₈H₁₄)) 3, H ₂C(OB(C₈H₁₄))₂4 and MeOB(C ₈H₁₄) 5; at 0.02 mol% of tBu₃P 5 is obtained in 98% yield at 60 °C with TON of almost 5500 and a TOF of 170. Under stoichiometric conditions the species (R₃PCH₂O)(HC(O)O) B(C₈H₁₄) (R = tBu 1,4-MeC₆H₄2) were isolated and characterized. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02103g

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Phosphine catalyzed reduction of CO2 with
boranes†
Tongen Wanga and Douglas W. Stephan*ab
Cite this: Chem. Commun., 2014,
50, 7007
Received 20th March 2014,
Accepted 13th May 2014
DOI: 10.1039/c4cc02103g
www.rsc.org/chemcomm
Phosphine is shown to catalyse the reaction of CO2 with 9-BBN to give to reduce CO2 to CO.14 The Berke group used Re–H and B(C6F5)3 as
mixtures of HCO2(B(C8H14)) 3, H2C(OB(C8H14))2 4 and MeOB(C8H14) 5; FLPs in the hydrogenation of CO2 into formate.15 In recent efforts,
at 0.02 mol% of tBu3P 5 is obtained in 98% yield at 60 8C with TON of Fontaine and coworkers16 have demonstrated the catalytic activity of
almost 5500 and a TOF of 170. Under stoichiometric conditions the R2PC6H4B(O2C6H4) in the hydroboration of CO2 to methoxyborane,
species (R3PCH2O)(HC(O)O)B(C8H14) (R = tBu 1,4-MeC6H4 2) were ultimately generating methanol on hydrolysis. Recently, we reported
isolated and characterized.
the N,N0-diphosphanyl-imidazol-2-ylidene (NHC-P) ring expansion
with 9-BBN to intramolecular FLPs which catalyze the hydroboration
Prompted by concerns over global warming, climate changes and the of CO2 to methoxyborane.17 In the present report, we demonstrate
need for renewable fuels, numerous research groups have undertaken that an intermolecular FLP approach is also effective as catalytic
efforts seeking to develop highly efficient methodologies to utilize CO2 amounts of phosphines in the presence of 9-BBN effect the reduction
as a C1 source for organic synthesis.1 To this end, a number of of CO2, in one case achieving TON of over 5500.
transition metal catalysts have been developed which effect the hydro-
The combination of tBu3P and 9-BBN in bromobenzene-d5
genation of CO2 to formic acid2 and methanol,3 the hydrosilylation of under 13CO2 (4 atm) at 25 1C for 6 hours, resulted in the
CO2 to formic acid,4 methanol1k,5 and methane,6 and the reduction of formation of new products as evidenced by H NMR spectroscopy.
1
CO2 with borane reagents into CO7 and methoxide.8 Sabo-Etienne and Two doublets appeared at 8.74 ppm (1JC–H = 208 Hz), 5.46 ppm
co-workers9 provided an interesting example of the reduction of CO2 by (1JC–H = 165 Hz) and one doublet of doublets appeared at 4.32 ppm
[RuH2(H2)2(PCy3)2] and pinacolborane (HBpin) generating the formic (1JC–H = 145 Hz, 2JH–P = 1 Hz). The first two signals were attributed to
acid ester, CH2(OBpin2)2 and the methoxide MeOBpin. More recently, H13COOB and CH2(OB)2 fragments, respectively by analogy to the
our group demonstrated a related CO2 reduction with HBpin, known species HCOOBpin and CH2(OBpin)2.9 These assignments
catecholborane (HBcat) or 9-borabicyclo[3.3.1]nonane (9-BBN) to were further confirmed by the corresponding 13C NMR signals at
1
1
generate methoxyboranes using a Ru-tris(aminophosphine) catalyst.10 169.1 ppm (d, JC–H = 208 Hz) and 85.44 ppm (t, JC–H = 165 Hz).
Shintani and Nozaki have reported the copper-catalyzed hydro- A peak at 52.45 ppm (dt, JC–P = 56 Hz, JC–H = 145 Hz) in the 13C
1
1
boration of CO2 with HBpin yielding formic acid ester.11
NMR spectrum also gave rise to the corresponding doublet at
While these strategies for CO2 reduction exploit transition metal 43.3 ppm in the 31P{1H} NMR spectrum were assigned to an
based catalysts, non-metal catalysts have also been developed. For [tBu3P13CH2OBR2] fragment.
example, N-heterocyclic carbenes1k,5 have also been exploited for
The analogous stoichiometric reactions of other phosphines with
reduction of CO2. In addition, frustrated Lewis pairs (FLPs) have CO2 and 9-BBN afforded similar products (see ESI†) as evidenced by
been utilized to activate and reduce CO2.12 Piers and co-workers used 1H, 31P and 11B and 13C NMR spectra. In the case of Ph3P, the
B(C6F5)3 and amines to transform CO2 to methane with silanes.13 We product proved difficult to isolate. In the case of the corresponding
have reported use of Zn(II) and in situ generated carbodiphosphines mixture of Cy3P, 9-BBN and 13CO2, the phosphine–borane
adduct (Cy3P)9-BBN was observed as the predominant species
in solution as evidenced by the 31P and 11B NMR spectra,
although trace amount of [Cy3P13CH2OBR2] species and
HCOOBR2 were observed (see ESI†). This is consistent with
the lesser steric demands and stronger basicity. The corresponding
reaction of (4-MeC6H4)3P with 9-BBN and 13CO2 afforded isolation
of ((4-MeC6H4)3PCH2O)(HC(O)O)B(C8H14) 2 (Scheme 1, Fig. 1(b)),
which was spectroscopically similar to 1.
a Department of Chemistry, University of Toronto, 80 St. George Street, M5S 3H6,
Toronto, Ontario, Canada. E-mail: dstephan@chem.utoronto.ca;
Web: http://www.chem.utoronto.ca/staff/DSTEPHAN
b Chemistry Department-Faculty of Science, King Abdulaziz University,
Jeddah 21589, Saudi Arabia
† Electronic supplementary information (ESI) available: Synthetic details and
spectral data are deposited. CCDC 992442 and 992443. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4cc02103g
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Table 1 Product ratios for reduction of CO2 with 9-BBN catalyzed by
phosphinesa
Entry Phosphines
Conc. (mol%) t/T (h 1Cꢀ1
)
3
4
5
1
2
3
4
5
6
7
8
9
10
No catalyst
tBu3P
Ph3P
—
4
4
4
4
1
1
2
0.2
0.02
19/25
16/25
19/25
19/25
15/25
30/25
20/25
13/25
16/25
32/60
0
0
0
36 15 49
60 36
28 34 38
28 39 33
7
8
19 24 57
3
0
4
(4-MeC6H4)3P
(3,5-Me2C6H3)3P
(4-MeC6H4)3Pb
Ph3Pb
Scheme 1 Isolation of 1 and 2.
30 63
41 51
tBu3Pb
tBu3Pb
29 68
98
tBu3Pb
2
a
Bromobenzene; 13CO2: 5 atm; all 9-BBN dimer (0.094 mmol) was
consumed; ratios based on integration of resonances in 1H NMR
spectra using the protonated solvent as the internal standard. 9-BBN
b
dimer (0.410 mmol) was consumed.
HCO2(B(C8H14)) 3, the diolate-linked compound H2C(OB(C8H14))2 4
and methoxide product MeOB(C8H14) 5 fragments in the ratio of
3:4:5 of 36 : 15 : 49 (Table 1, entry 2). The formate product 3 was
evident from the 1H NMR doublet at 8.91 ppm (1JC–H = 199.7 Hz) and
the 13C NMR doublet at 166.9 ppm (1JC–H = 200 Hz). The diolate
derivative spectrum and a triplet at 85.44 ppm (1JC–H = 165 Hz) in 13
C
NMR spectrum. The boron-methoxide exhibited a doublet at
1
3.57 ppm (1JC–H = 143 Hz) in H NMR spectrum and a quartet at
52.69 ppm (1JC–H = 143 Hz) in 13C NMR spectrum consistent with the
formulation as 5. The electron deficient or sterically encumbered
phosphines (C6F5)3P, (2,4,6-Me3C6H2)3P and (2-MeC6H4)3P showed
no reactivity even at 60 1C. Employing 4 mol% Ph3P, (4-MeC6H4)3P
and (3,5-Me2C6H3)3P were also effective in reduction of CO2 produ-
cing mixtures of 3–5 (Table 1, entries 3–5). Lowering the concen-
tration of catalysts to 1–2 mol% increases the proportion of 5 in the
products (Table 1, entries 6–8). In the case of tBu3P lowering the
concentration of tBu3P to 0.2 and 0.02 mol% results in even higher
production of 5 (Table 1, entry 9 and 10); in the latter case 98% of the
borane was converted to 5 with a minor amount of 4 (2%) after
31.5 h at 60 1C. This corresponds to a TON of 5556 and a TOF of 176.
The consumption of borane in these reactions was monitored
in the reactions using 1.0 mol% tBu3P, 1.0 mol% Ph3P and
1.0 mol% ( p-MeC6H4)3P as the catalyst. These reactions were
complete in 16, 20 and 30 h, respectively (Fig. 2).
Fig. 1 POV-ray depiction of (a) 1, (b) 2; C, black; O, red; B, green; P, orange;
H, grey.
Crystals of a product 1 and 2 were obtained from stoichiometric
In the latter two cases, an induction period was observed. In these
reaction of phosphine and 9-BBN (HBC8H14) under 13CO2 in bromo- cases, doubling the phosphine concentration halved the observed
benzene-d5 layered with hexanes. X-ray study of these crystals induction period. This is consistent with the slow formation of a
confirmed the formulations as (R3PCH2O)(HC(O)O)B(C8H14) (R = phosphine derivative which is subsequently and sequentially reduced
tBu 1, 4-MeC6H4 2) (Fig. 1), in which CO2 is incorporated both as a to give 3, 4 and 5. In addition it suggests that as the concentration of
formate and in the phosphonium methoxy-fragment. These zwitter- the reduction product increase the reaction accelerates. This view is
ions incorporate phosphonium fragments linked via a CH2O linker consistent with the observation of only traces of 3 during these
to the borate center. The four coordinate B atoms in each case are induction periods. These data support a mechanism in which initial
also coordinated to a formate anion. The B–O bond distances FLP-like binding of CO2 takes place and is subsequently reduced
ranging from 1.499(2) to 1.568(3) Å. The remaining metric para- (Scheme 2). The slow binding of CO2 by the arylphosphines is
meters are unexceptional.
consistent with the reduced steric demands prompting an equili-
As the formation of these products illustrates the stoichiometric brium involving the classical Lewis P–B adduct. In the case of tBu3P,
reduction of CO2, we probed the reactions under catalytic conditions. steric demands preclude such adduct formation. In addition, the
Thus, 4 mol% of tBu3P and 9-BBN (0.094 mmol) were combined weaker basicity of the aryl-phosphines presumable slows nucleophilic
with excess 13CO2 at room temperature (Table 1, entry 2). After 16 h, attack of CO2. The species 1 or 2 are generated in the reaction under
the CO2-reduction products include boron-bound formate species, stoichiometric conditions inferring that further reaction with borane
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Fig. 2 Consumption of borane in by phosphine catalysed CO2 reduction
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´
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=
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species (Scheme 2).
In conclusion, herein we have described a facile approach
to the metal-free catalytic reductions of CO2 in the presence of
9-BBN. These reactions proceed via an FLP-type CO2 activation
intermediate. Subsequent reaction with borane effects sequential
reduction ultimately yielding the boron-methoxide. In the best
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systems for CO2 reductions are the subject of continuing study
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The authors gratefully acknowledge the financial support of
the NSERC of Canada and DWS is grateful for the award of a
Canada Research Chair.
¨
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