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was then evacuated. The entire bomb wDaOs It:h1e0n.10im39m/Ce5rCseCd04in33a7A–
70 °C dry ice-isopropanol bath to keep the solution frozen and
cool the headspace. The bomb was then opened to 1 atm of
CO2 for 10 minutes to allow the temperature to equilibrate.
Subsequently the bomb was sealed and allowed to warm to 25
°C to achieve ~1.5 atm pressure. Safety Warning: if CO2 gas was
introduced below -78 °C, dry ice would condense in the reaction
vessel and the final pressure becomes time-dependent and can
no longer be calculated easily. Using our protocol with a low-
melting solvent (i.e., the solvent is not frozen at -70 °C), the
final pressure is again time-dependent, because of the
dramatically increased solubility of CO2 at -70 °C. In both
scenarios prolonged CO2 exposure could cause serious
explosions due to uncontrolled high pressures and make the
results incomparable to others due to unknown CO2 pressure.
The slightly slower conversion of the second batch of HBcat was
likely due to the inefficient mixing of the reactants caused by
the large amount of solid produced in the reaction.
reduction products HCOOBpin, CH2(OBpin)2 and CH3OBpin (Table 1,
entry 6). Using 100 eq. of BH3·SMe2 (with respect to catalyst 1) as
the reductant under 1.5 atm of CO2 the reaction achieved a TON of
294 with BH3 within 44 h at 25 °C to yield (CH3OBO)3 (Table 1, entry
7). Increasing the reaction temperature from 25 °C to 70 °C only
improved the reaction rate by a factor of ~2 (Table 1, entry 8).
Next, we tested the catalytic activity of 2. When a CDCl3
solution of 2 and 100 eq. of 9-BBN was exposed to 1.5 atm of CO2 at
25 °C, CH2(OBBN)2 and CH3OBBN were produced with an overall
TON of 61 within 19 h (Table 1, entry 9); the reaction is much
slower than that catalyzed by 1. When the same reaction was
carried out at 70 °C, however, the reaction rate is comparable to
that catalyzed by 1 at 70 °C, i.e., the reaction reached 66 TON
within 2 h (Table 1, entry 10). Compared to 1, 2 showed a higher
activity when HBpin was used as the reductant, i.e., the reaction
catalyzed by 2 gave CH3OBpin as the dominant CO2 reduction
product with a TON of 75 in 46 h at 90 °C (Table 1, entry 11). In
contrast, when HBcat was used as the reductant, catalyst 2 showed
lower activity than 1 (Table 1, entry 12). We speculate that the
difference in catalytic activity between 1 and 2 may originate partly
from the preferred interactions between the catalyst and borane:
the larger system in 1 interacts with the aromatic backbone of
HBcat more strongly, while the longer aliphatic propyl chain and
smaller system in 2 favor the aliphatic backbone of HBpin.
Interestingly, 2 showed much higher catalytic activity than 1 when
BH3·SMe2 was used as the reductant, i.e., complete conversion to
(CH3OBO)3 was observed in 7 h at 25 °C with a TON of 298 and
average TOF of 42.6 h-1 (Table 1, entry 13). This reaction also has a
short induction period at 25°C (Fig. 2). A TOF of 56 h-1 at the fast
catalysis stage was extracted from the plot of TON vs time. Such
‖
1
2
3
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TOFs make
2 one of the best organocatalysts for this
transformation.
4
In summary, we have demonstrated compounds 1 and 2 not
only bind CO2 reversibly via the formal insertion of CO2 into a C–H
bond of the C5 ring, but also catalyze the hydroboration of CO2 to
methylborylethers which upon hydrolysis can produce methanol.
These air- and moisture-stable compounds that consist of only
carbon, hydrogen, and nitrogen are the first catalysts with carbon-
centered activity for the reduction of CO2 to methylborylethers.
These catalysts feature broad borane scope and their catalytic
activities are comparable to the best organocatalysts with
heteroatom-based activity. The mechanism of the catalytic
reactions are currently being investigated via experimental and
computational methods in our laboratory.
5
6
We acknowledge Natural Science and Engineering
Research Council of Canada (NSERC) for funding. Y. Y. greatly
thanks the government of Ontario for an Ontario Trillium
Scholarship. M. X. gratefully thanks University of Toronto for
the University of Toronto Excellence Award and Charlie Kivi for
X-ray crystallography.
7
Notes and references
8
9
a) M. D. Anker, M. Arrowsmith, P. Bellham, M. S. Hill, G. Kociok-
Kohn, D. J. Liptrot, M. F. Mahon, and C. Weetman, Chem. Sci.,
2014, 5, 2826-2830; b) J. A. B. Abdalla, I. M. Riddlestone,
R.Tirfoin, and S. Aldridge, Angew. Chem. Int. Ed., 2015, 54,
ASAP, DOI: 10.1002/anie.201500570.
a) M.-A. Courtemanche, M.-A. Le´gare´, L. Maron, and F.-G.
Fontaine, J. Am. Chem. Soc., 2013, 135, 9326-9329 ; b) M-A.
§
Compound 2 is an orange oil and can be synthesized using a
modified literature procedure. Compared to 1, whose solution
is stable in air for several hours, the solution of 2 can be stored
at –15 °C in air for weeks without significant change. Compound
2 is soluble in all common organic solvents. For the synthetic
protocol of 2 and CO2 binding experiments, see ESI.
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Chem. Commun., 2015, 00, 1-4 | 3
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