Tandem frustrated lewis pair/tris(pentafluorophenyl)borane-catalyzed deoxygenative hydrosilylation of carbon dioxide

Article abstract of DOI:10.1021/ja105320c

The frustrated Lewis pair system consisting of 2 equiv of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluorophenyl)borane [B(C ₆F₅)₃] activates carbon dioxide to form a boratocarbamate-TMPH ion pair. In the presence of triethylsilane, this species is converted to a silyl carbamate and the known ion pair [TMPH] ⁺[HB(C₆F₅)₃]^-, which recently was shown to react with CO₂ via transfer of the hydride from the hydridoborate to form the formatoborate [TMPH]⁺[HC(O)OB(C ₆F₅)₃]^-. In the presence of extra B(C₆F₅)₃ (0.1-1.0 equiv) and excess triethylsilane, the formatoborate is rapidly hydrosilated to form a formatosilane and regenerate [TMPH]⁺[HB(C₆F ₅)₃]^-. The formatosilane in turn is rapidly hydrosilated by the B(C₆F₅)₃/Et₃SiH system to CH₄, with (Et₃Si)₂O as the byproduct. At low [Et₃SiH], intermediate CO₂ reduction products are observed; addition of more CO₂/Et₃SiH results in resumed hydrosilylation, indicating that this is a robust, living tandem catalytic system for the deoxygenative reduction of CO₂ to CH₄.

Full text of DOI:10.1021/ja105320c

Published on Web 07/15/2010
Tandem Frustrated Lewis Pair/Tris(pentafluorophenyl)borane-Catalyzed
Deoxygenative Hydrosilylation of Carbon Dioxide
Andreas Berkefeld, Warren E. Piers,* and Masood Parvez
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary, Alberta, Canada T2N 1N4
Received June 17, 2010; E-mail: wpiers@ucalgary.ca
Scheme 1
Abstract: The frustrated Lewis pair system consisting of 2
equiv of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluo-
rophenyl)borane [B(C6F5)3] activates carbon dioxide to form a
boratocarbamate-TMPH ion pair. In the presence of triethyl-
silane, this species is converted to a silyl carbamate and the
known ion pair [TMPH]+[HB(C6F5)3]-, which recently was
shown to react with CO2 via transfer of the hydride from the
hydridoborate to form the formatoborate [TMPH]+[HC(O)-
OB(C6F5)3]-. In the presence of extra B(C6F5)3 (0.1-1.0 equiv)
and excess triethylsilane, the formatoborate is rapidly
hydrosilated to form
a
formatosilane and regenerate
[TMPH]+[HB(C6F5)3]-. The formatosilane in turn is rapidly
hydrosilated by the B(C6F5)3/Et3SiH system to CH4, with
(Et3Si)2O as the byproduct. At low [Et3SiH], intermediate CO2
reduction products are observed; addition of more CO2/Et3SiH
results in resumed hydrosilylation, indicating that this is a
robust, living tandem catalytic system for the deoxygenative
reduction of CO2 to CH4.
The utilization of carbon dioxide as a sustainable and nontoxic C1
feedstock for the production of value-added chemical products such
as carboxylic acids or fuels such as methanol and methane is of current
interest.1 The high thermodynamic stability of CO2 necessitates its
catalytic activation and coupling to a thermodynamic driver for efficient
conversion. Transition-metal-based catalysts have played a dominant
role in CO2 conversion, but recently, an increasing number of
organocatalytic CO2 reduction schemes have emerged.2 For example,
N-heterocyclic carbenes (NHCs) reversibly form zwitterionic adducts
NHC ·CO2 that are considered key intermediates in the reductive
deoxygenation of CO2 using diphenylsilane as a sacrificial reducing
agent, affording CH3OH upon workup.3
In this context, activation of CO2 by transition-metal-free
“frustrated Lewis pairs” (FLPs)4 has led to the development of
stoichiometric reductions of CO2 to CH3OH. Here, the FLPs form
bridging carboxylate species5 that can accept hydrogen from
ammonia borane6 or via a thermally driven, multistep self-reduction
in which the key step is a reversible B-H bond addition of
hydridotris(pentafluorophenyl)borate to one CdO double bond of
CO2, affording the formatoborate anion [HC(O)OB(C6F5)3]-.7
Ultimately, hydrolysis of CH3O-LA (LA ) BX3 or AlX3) is
required in order to obtain methanol.
The ammonium hydridoborate ion pair 1 formed by treatment of
the FLP B(C6F5)3/2,2,6,6-tetramethylpiperidine (TMP) and hydrogen10
(32 mM, C6D5Br) reacted with CO2 (2-4 atm) in the presence of
Et3SiH (18 equiv) at 56 °C to afford the previously reported7
formatoborate 2 exclusively (see Scheme 1).11 The reaction was
monitored by 1H and 19F NMR spectroscopy, and integration versus
an internal standard (C6H5CF3, 9 mM) revealed that no Et3SiH was
consumed. Thus, although the formation of 2 is reversible,7 there does
not appear to be sufficient free B(C6F5)3 present under these conditions
to activate silane for further reduction of 2.
Accordingly, we carried out a reaction under identical conditions
with an additional 1.0 equiv of B(C6F5)3 (relative to 1) present.
This resulted in the immediate and complete conversion of 2 back
into 1 at room temperature and the appearance of the products of
CO2 hydrosilylation. Further monitoring of the reaction by 1H and
19F NMR spectroscopy at 56 °C showed that silane was gradually
consumed and that CH4 along with 2 equiv of (Et3Si)2O were
formed as the ultimate reaction products. Minor amounts of
bis(triethylsilyl)acetal, (Et3SiO)2CH2, (∼10%) were also present.
Interestingly, 1 and B(C6F5)3 were the only boron-containing
compounds detectable during the reaction but diminished in favor
of a new species, 3, upon complete silane consumption. At the same
time, the characteristic signals of HCO2SiEt3, {Et3SiO}2CH2, and
Et3SiOCH3 in C6D5Br became evident in the 1H NMR spectra. Upon
addition of further silane equivalents and pressurization with fresh
CO2, these partially reduced intermediates were depleted and
methane formation resumed, indicating a “living” catalytic system.
Boron-hydrogen bond addition to CO2 mediated by phospho-
nium or ammonium borate ion pairs formed via FLP hydrogen
splitting thus offers a potential entry point into catalytic CO2 fixation
in the presence of a suitable reducing agent (oxygen acceptor). We
have shown that perfluoroarylboranes are excellent catalysts for
the reductive hydrosilylation of carbonyl functions8 and C-O
bonds,9 a potentially useful reaction for subsequent steps in the
reductive deoxygenation of CO2 to CH4.
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10660 J. AM. CHEM. SOC. 2010, 132, 10660–10661
10.1021/ja105320c  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Collectively, these observations suggest that the chemistry
depicted in Scheme 1 is operative. Analysis of the 19F NMR spectra
indicated that the coordinated B(C6F5)3 in compound 3 is labile in
solution.12 Thus, the equilibrium between 3 and 2/B(C6F5)3 is rapid
and provides a source of free borane to activate the Et3SiH present
via 4, as previously reported.8 The fact that neither 3 nor 2 was
detected in the reaction mixture in the presence of silane implies
that B-H bond addition of 1 to CO2 is the rate-limiting step.
Consequently, the overall rate of silane consumption showed a
zeroth-order concentration dependence over four half-lives (∼240
min) when the reaction was monitored at 56 °C by 1H NMR
spectroscopy (see Figure S2 in the Supporting Information). As
the silane concentration decreased, the concentration of CH4
increased at a rate one-quarter that of silane consumption while
[1] remained constant, as expected. Once 2 was generated, the
reaction with B(C6F5)3-activated silane 4 was rapid and eventually
produced CH4 and (Et3Si)2O (as shown in Scheme 1) via well-
documented B(C6F5)3-mediated transformations.13
In accord with this postulate, the reaction of a 2:1 mixture of 2
and B(C6F5)3 (in equilibrium with 3) with Et3SiH (1.2 equiv vs 2)
at room temperature instantly afforded (Et3SiO)2CH2 (57%) along
with starting material (42%) and trace amounts of Et3SiOCH3 and
CH4, suggesting that the triethylsilylformate intermediate is highly
reactive toward 4. Therefore, its production from 2 is critical. Here,
the involvement of a silylium cation to react with anionic 2 greatly
enhances its conversion rate in comparison with that for the
reduction of 2 with further equivalents of anionic 1.7 In other words,
silylium ion transfer to the formate moiety of 2 from 4 is
Coulombically favored over hydride transfer from 1 and occurs
under much milder conditions.7 The reaction of 4 with 2 gives
highly reactive HCO2SiEt3 but also regenerates 1 for rate-limiting
activation of CO2.
B(C6F5)3 with CO2 in the presence of silane (Scheme 2). In the
absence of Et3SiH, ion pair 5 was generated and could be
characterized by X-ray crystallography.11 As with other complexes
of this type,5,6 CO2 activation is reversible, and small quantities of
free borane are accessible to added silane, which rapidly converts
5 into the triethylsilyl carbamate and 1, which in turn becomes
available for catalytic reduction of CO2 to CH4 as in Scheme 1.
In summary, ammonium borate 1 and B(C6F5)3 act in tandem to
catalytically convert CO2 to CH4 using triethylsilane as the reductant.
The rate-limiting step involves transfer of hydride from 1 to CO2,
suggesting that a more nucleophilic hydridoborate might improve the
rate of conversion. However, use of the less Lewis acidic borane B(4-
C6F4H)314 resulted in a kinetic profile essentially identical to that
obtained for B(C6F5)3 (Figure S3). Ongoing work will explore a range
of boranes, amines, and sacrificial reductants aimed at increasing the
turnover frequencies on the basis of the mechanistic details uncovered
through these detailed spectroscopic studies.
Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council (NSERC) of Canada.
A.B. thanks the German Research Foundation (DFG) for a
postdoctoral fellowship.
Compound 3 was generated separately by treatment of solutions
of 2 with B(C6F5)3 and was isolated by slow hexane diffusion into
the reaction mixture at -30 °C. Its solid-state structure was
elucidated by single-crystal X-ray diffraction, and an ORTEP
diagram is shown in Figure 1 along with selected metrical data
implying that the negative charge is delocalized over the bridging
formate and flanking borane fragments. This compound was
proposed by Ashley et al.7 as an intermediate in the conversion of
2 to [H3COB(C6F5)3]-[TMPH]+ that accepts hydride from 1; here
it serves as a reservoir of borane catalyst for hydrosilylation of 2
and subsequent intermediates.
Supporting Information Available: Crystallographic data for 3 and
5 (CIF), additional experimental and spectroscopic details, and complete
ref 1a. This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 1. ORTEP plot for 3 (50% thermal displacement ellipsoids).
Selected bond lengths (Å) and angles (deg): C1-O1, 1.256(3); C1-O2,
1.268(3); O1-B1, 1.587(3); O2-B2, 1.584(3); O1-C1-O2, 120.2(2); O1/
2-C1-H, 119.9. The largest deviation from the least-squares plane through
atoms B1, O1, C1, O2, and B2 is 0.015 Å for O1.
The above experiments were performed with isolated 1, but this
can be bypassed simply through the use of a 2:1 mixture of TMP/
JA105320C
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