Heavy-Metal-Free Fischer-Tropsch Type Reaction: Sequential Homologation of Alkylborane Using a Combination of CO and Hydrides as Methylene Source

Article abstract of DOI:10.1021/jacs.0c06580

Carbon homologation reactions occur within the well-known Fischer-Tropsch process, usually mediated by transition metal catalysts at high temperature. Here we report the low-temperature, heavy-metal-free homologation of a carbon chain using CO as a C1-source showing for the first time that transition-metal catalysts are not required for Fischer-Tropsch-type reactivity. Reaction of an alkylborane in the presence of either LiHBEt3 or LiAlH4 resulted in multiple CO insertion/reduction events to afford elongated chains by more than two methylene (-CH2-) units, affording aldehyde products upon oxidative aqueous workup. Theoretical and experimental mechanistic studies indicate that the boron terminus is responsible for CO incorporation as well as sequential hydride delivery leading to reduction of acylborane intermediates to alkylboranes.

Full text of DOI:10.1021/jacs.0c06580

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Heavy-Metal-Free Fischer−Tropsch Type Reaction: Sequential
Homologation of Alkylborane Using a Combination of CO and
Hydrides as Methylene Source
Andreas Phanopoulos,† Shrinwantu Pal,† Takafumi Kawakami,† and Kyoko Nozaki*
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ABSTRACT: Carbon homologation reactions occur within the well-known Fischer−Tropsch process, usually mediated by
transition metal catalysts at high temperature. Here we report the low-temperature, heavy-metal-free homologation of a carbon chain
using CO as a C1-source showing for the first time that transition-metal catalysts are not required for Fischer−Tropsch-type
reactivity. Reaction of an alkylborane in the presence of either LiHBEt3 or LiAlH4 resulted in multiple CO insertion/reduction
events to afford elongated chains by more than two methylene (−CH2−) units, affording aldehyde products upon oxidative aqueous
workup. Theoretical and experimental mechanistic studies indicate that the boron terminus is responsible for CO incorporation as
well as sequential hydride delivery leading to reduction of acylborane intermediates to alkylboranes.
Scheme 1. Formation of Methylene Linkers from CO in the
Absence of Heavy Metals
he Fischer−Tropsch (FT) process is a fundamental
Tindustrial reaction with significant societal and economic
implications that involves the controlled homologation and
hydrogenation of carbon monoxide (CO) to afford higher
hydrocarbons with predominantly straight chains in the range
C4 to C10.1 The process involves the reductive coupling of CO
units into methylene chains and is a practical way of converting
coal to gasoline, diesel oil, wax, and smaller amounts of
oxygenated compounds (e.g., alcohols, acids, and ketones). For
the FT process, iron- and cobalt-based heterogeneous catalysts
have been widely used industrially under two different
operating modes: (i) the high-temperature FT process with
iron catalysts at 573−623 K and (ii) the low-temperature FT
process with either iron or cobalt catalysts at 473−513 K.2
The reductive coupling of CO mediated by heavy metals has
been studied in order to gain insight into possible mechanisms
by which FT processes occur,3−6 but analogous processes in
the absence of heavy metals remain rather unexplored.7−9
Despite a dearth of reports, the nontransition metal mediated
conversion of CO into methylene moieties has been previously
been reported, namely via the reactions of organoboranes with
CO in the presence of hydride species. In 1980, Brown et al.
reported the three-step homologation of a linear alkylborane,
involving the carbonylation of 9-octyl-BBN (BBN = 9-
borabicyclo[2.2.1]nonane) with CO in the presence of
LiHAl(OMe)3, subsequent reduction using LiAlH4, and finally
hydrolysis with CH3SO3H to afford the C1 elongated product,
9-nonyl-BBN (Scheme 1A).10 As the only example of two-
alkylborane. Unfortunately, however, no mechanistic support
was provided in these studies.
Received: June 18, 2020
̈
carbon homologation, in 1992, Yalpani and Koster reported
the reaction of (Et2BH)2 with CO at 80 °C to detect butanol
after alkaline hydroperoxide oxidation (Scheme 1B).11 In this
report, the authors proposed that CO inserted into the ethyl−
boron bond to form a three-membered cyclic intermediate,
and then further addition of hydride, gives a homologue of
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More recently, in 2013, Stephan et al. reported that the
reaction of B(C6F5)3 with CO in the presence of t-Bu3P and
H2 resulted in the formal methylene insertion into a C−B bond
(Scheme 1C).12 In this study, formation of a formylborate
species is proposed as the intermediate, which undergoes C6F5
migration from the boron atom to the formyl carbon. Also in
2013, Braunschweig et al. reported the reactivity of CO across
a stabilized B−B triple bond in which each of the boron atoms
was stabilized by IDip (IDip = 1,3-bis-(2,6-diisopropylphenyl)-
imidazol-2-ylidene).13 In addition to carbon monoxide, other
divalent carbon species or their analogs are known to undergo
insertion to a C−B bond to provide homologated alkylborane
products, such as the well-known Matteson and Aggarwal
homologations.14−17
were conducted with LiHBEt3 or LiAlH4; a series of aldehydes
varying in their chain length (2-1, 2-2, ... 2-8) were detected by
GC analysis (Figure 1). The yield of aldehydes 2-1, 2-2, and 2-
With these foundations, here we report our unprecedented
finding of heavy-metal-free, low-temperature FT-like multiple
carbon homologation. Treatment of the alkyl-BBN produced
via hydroboration of olefin with CO and LiHBEt3 and
subsequent oxidation led to the formation of aldehydes with
multiple methylene group enchainment (Scheme 1E), adding
to the recent exciting progress boron species have made in
showing transition-metal-like behavior.18−20 The experimental
and computational analyses elucidated the reaction mecha-
nism.
Figure 1. GC trace showing formation of Cn elongated aldehydes (2-
n) corresponding to Table 1 (entry 3). Tetradecane was used as an
internal standard.
≥3 were quantified to be 28%, 30%, and 4% with LiHBEt3
(Table 1, entry 3) and 21%, 21%, and 10% with LiAlH4 (Table
1, entry 4). 2-Phenylethanol and cyclooctanone were observed
as byproducts of oxidative workup from the residual 2-
phenethylboron and 9-BBN fragments. Addition of other
hydride reagents such as iso-Bu2AlH (DIBAL-H), LiH, or KH
(Table 1, entries 5−7) were not effective for CO
incorporation, thus providing 2-phenethyl alcohol (1) as the
major product after oxidative workup.
The multiple carbon insertion into a C−B bond was
observed as follows. A trialkylborane, 9-(2-phenethyl)-BBN,
prepared by hydroboration of styrene with 9-H-BBN,21 was
allowed to react with CO in the presence and absence of
hydride reagents. After subsequent oxidative workup with
hydrogen peroxide in a pH 7 buffer, the product mixture was
1
analyzed by GC, GC-MS, and H NMR spectroscopy. The
representative results are summarized in Table 1.
In order to elucidate the mechanistic details of this novel
reactivity, in situ analyses of the reaction mixtures and DFT
calculations were undertaken.25−27 The results of our DFT
studies are summarized in Scheme 2. Initial coordination of
CO to the B-center of phenethyl-BBN (A) forms B, and the
subsequent CO insertion into the C−B bond of alkyl-BBN (via
TSBC) affords acylborane C.28 Attempts for the direct
spectroscopic observation of this acylborane species were
unsuccessful (see Supporting Information).29 This agrees with
our calculations where not only CO insertion from A to C is
endergonic but also the barrier for the reverse reaction (C to B,
ΔG‡ = 17.3 kcal/mol) is accessible. Thus, no detectable
reaction would take place in the absence of hydride reagents.
Coordination of another CO molecule to the B-center of C to
give D is also accessible if C is generated. In the presence of
hydride donors (such as LiHBEt3), the B-center of the BBN
fragment of C accepts a hydride to form borate E, which is
exergonic by 3.8 kcal/mol (relative to C). Once E forms,
reduction of the CO-fragment affords three-membered ring
alkoxide F22−24 via TSEF associated with a low-barrier (ΔG‡ =
8.9 kcal/mol) hydride transfer. The partial negative charge that
is built up on the oxygen of F is stabilized through interaction
with Lewis acids present, e.g., the newly formed BEt3. Since F
marks the lowest point in the energy profile, oxidative workup
at this stage leads to the C1 elongated aldehyde 2-1, as was
indeed observed (Table 1, entries 2−5). The existence of F in
the reaction mixture was also unambiguously confirmed by
ESI-MS analysis (vide infra).
Table 1. Reaction of 9-(2-Phenethyl)-BBN with CO in the
Presence and Absence of Hydrides
a
2-2
a
a
a
Hydride
reagent
CO
1
2-1
n = 1
(%)
2-≥3
Entry
(MPa) (%) n = 0 (%)
n ≥ 2 (%)
1
2
3
4
5
6
7
8
None
2.0
2.0
2.0
2.0
5.0
2.0
2.0
2.0
62
0
6
1
4
17
11
26
0
64
28
21
48
0
0
0
30
21
0
0
0
0
0
0
4
10
0
0
LiHB(s-Bu)3
LiHBEt3
LiAlH4
LiHBEt3
i-Bu2AlH
LiH
0
0
0
0
KH
a
GC yields determined using tetradecane as internal standard.
First, in the absence of any hydride reagent, no CO uptake
into the product was observed and thus 2-phenylethanol (1)
was obtained as the major product (entry 1) after the oxidative
cleavage of the C−B bond of unreacted phenethyl-BBN. In the
presence of LiHB(sec-Bu)3 (L-Selectride, entry 2), the C1
elongated aldehyde 2-1 was obtained in a moderate yield
(64%), which is consistent with the previous reports with
LiHAl(OMe)3,22 LiHAl(OtBu)3,23 or KHB(OiPr)3.24 In
contrast, multiple CO uptake was detected when the reactions
The key for multiple methylene elongation is the
regeneration of alkyl borane species A′ from intermediate F.
Here, the B-center plays a critical role: once the B-center in F
accepts a hydride from LiHBEt3, the anionic borate species G
B
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Scheme 2. Mechanism for the Boron-Mediated Homologation of Alkenes Using CO and LiBHEt3 as the Methylene Source
undergoes a subsequent barrier-less C−O bond cleavage
(deoxygenation) to regenerate the alkylborane A′. This species
can then be involved in further homologation (in a manner
identical to that described for A) to form F′, which is
homologous with F.
Chart 1. Comparison of Masses of Isotopically Labelled
a
Intermediates Observed in ESI-MS Analyses of Reactions
of A with Either LiHBEt3 or LiDBEt3 and Either 12CO or
13CO
Oxidative workup of F’ leads to the formation of aldehyde 2-
2 (Table 1, entries 3−4). The highly exergonic nature (driving
force) for the overall reaction from A to F′ is attributed to the
formation of strong B−O (or Al−O) and C−C bonds. As a
side path from intermediate F, a competitive CO-coordination
to form H is also feasible. The existence of H would retard the
reaction from F to G, especially under high CO pressure. This
is consistent with the observation that a higher CO pressure of
5.0 MPa completely shut down the multiple CO insertion
leading to the isolation of 2-1 in 48% yield as the sole aldehyde
product (entry 5 in Table 1).
To support the mechanism proposed above, several high-
pressure in situ NMR experiments were performed including
isotopic labeling studies. Mixing 9-(2-phenethyl)-BBN(A) with
1 equiv of LiHBEt3 and either 12CO or 13CO demonstrated
that CO uptake into the product is reversible with a rate
comparable to the NMR time scale by a series of broad peaks
within δC = 70−90 ppm (Figure S1). When lithium
triethylborodeuteride (LiDBEt3)30 was used as a deuteride
source, a low-field shift in the 2H NMR signal from δD = −1.79
ppm (free LiDBEt3 δD = −1.74 ppm) to a series of peaks
within δD = 2.11−3.84 was observed upon the addition of CO,
indicating that the deuteride is indeed acting as a reducing
agent to form −CD2− or −CD(OH)− moieties (Figure S2).
Additionally, when a combination of LiDBEt3 and 13CO was
a
See Figures S3, S4 in the Supporting Information for mass spectra
(I-d1; expected mass = 280.2165, ΔMr = 8.2 ppm), while using
13CO instead of 12CO, a mass peak of 280.2151 was observed
(I*, expected mass = 280.2136, ΔMr = 5.6 ppm). Finally, using
both LiDBEt3 and 13CO gave a mass peak of 281.1804 (I*-d1,
expected mass 281.2199, ΔMr = 140 ppm). This further
supports the reduction of the carbonyl moiety by the
borohydride/deuteride reagents. Moreover, other peak clusters
follow a similar pattern and can be analogously assigned (see
Charts S1 and S2).
When phenylsilane (PhSiH3) was further added to the
reaction of alkyl-BBN, CO, and LiHBEt3, homologated alkenes
and alkanes were generated, although the yields were very low
(see Figure S5). In contrast, when silane was the sole hydride
source (in the absence of LiHBEt3), only 2-phenylethanol was
detected after oxidative workup, implying no CO insertion
took place. Coexistence of both PhSiH3 and LiHBEt3 under
CO pressure was essential for the alkene and alkane
generation; addition of PhSiH3 after release of the CO
pressure afforded only aldehydes in the same way as without
PhSiH3 (see Supporting Information for details).
2
used, the H signals change from broad singlets to give more
complex splitting patterns, indicating that the deuterium is
bound to the labeled carbonyl moiety.
High-resolution mass spectrometry analysis further revealed
that the elongated carbon chain originates from the hydride in
LiHBEt3 and the CO in the isotope labeling experiments
(Figure S3). The major mass envelope for the reaction using
LiHBEt3 and 12CO corresponded to a molecular ion peak of
279.2127, which we propose to be the structure I depicted in
Chart 1 (Mr = 279.2102 for 11B). Similar insertion of the O
atom into the B−C bond of the 9-BBN fragment31 has been
reported.32 By isotope labeling of the reagents, the peak
accordingly changes its mass number with enough precision to
differentiate between the exchange of 12C to 13C and 1H to 2H.
When LiDBEt3 was used, the mass peak increased to 280.2188
In conclusion, we found multiple methylene elongations
mediated by boron are accessible, representing the first heavy-
C
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Monoxide by Organometallic Iron Complexes. Nat. Commun. 2018, 9
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improvement in terms of yield and selectivity, we believe our
mechanistic studies shed light on the new direction of
organoboron chemistry as a possible substitute for organo-
transition metal chemistry.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c06580.
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Experimental details and additional NMR and GC
charts, mass spectra (PDF)
(10) Brown, H. C.; Ford, T. M.; Hubbard, J. L. Homologation of
Organoboranes via Carbonylation−Reduction. J. Org. Chem. 1980, 45,
4067−4068.
AUTHOR INFORMATION
Corresponding Author
Kyoko Nozaki − Department of Chemistry and Biotechnology,
Graduate School of Engineering, The University of Tokyo,
Bunkyo-ku, Tokyo 113-8656, Japan; orcid.org/0000-0002-
0321-5299; Email: nozaki@chembio.t.u-tokyo.ac.jp
■
̈
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C.; Hammond, K.; Jimenez-Halla, J. O. C.; Kramer, T.;
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Authors
Andreas Phanopoulos − Department of Chemistry and
Biotechnology, Graduate School of Engineering, The University
of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan; orcid.org/
0000-0002-1572-5560
Shrinwantu Pal − Department of Chemistry and Biotechnology,
Graduate School of Engineering, The University of Tokyo,
Bunkyo-ku, Tokyo 113-8656, Japan; orcid.org/0000-0001-
6260-1803
Takafumi Kawakami − Department of Chemistry and
Biotechnology, Graduate School of Engineering, The University
of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
Complete contact information is available at:
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(18) Power, P. P. Main-Group Elements as Transition Metals.
Nature 2010, 463 (7278), 171−177.
Author Contributions
́
́
(19) Legare, M.-A.; Pranckevicius, C.; Braunschweig, H. Metal-
lomimetic Chemistry of Boron. Chem. Rev. 2019, 119 (14), 8231−
8261.
†A.P., S.P., and T.K. contributed equally.
Notes
(20) Melen, R. L. Frontiers in molecular p-block chemistry: From
structure to reactivity. Science 2019, 363 (6426), 479−484.
(21) Tomizawa, T.; Orimoto, K.; Niwa, T.; Nakada, M. Preparation
of Imides via the Palladium-Catalyzed Coupling Reaction of
Organoborons with Methyl N-[Methoxy(methylthio)methylene]-
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI JP18H05259.
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