Synergistic effects in ambiphilic phosphino-borane catalysts for the hydroboration of CO2

Article abstract of DOI:10.1039/c6cc02809h

The benefit of combining both a Lewis acid and a Lewis base in a catalytic system has been established for the hydroboration of CO₂, using ferrocene-based phosphine, borane and phosphino-borane derivatives.

Full text of DOI:10.1039/c6cc02809h

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Synergistic Effects in Ambiphilic Phosphino-
Borane Catalysts for the Hydroboration of CO2
Cite this: DOI: 10.1039/x0xx00000x
Anis Tlili,a,b Arnaud Voituriez,b* Angela Marinetti,b* Pierre Thuérya and
Thibault Cantata*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
demonstrates the benefits of the ambiphilic systems, for the first
time.
The benefit of combining both a Lewis acid and a Lewis base
in
a catalytic system has been established for the
hydroboration of CO2, using ferrocene-based phosphine,
borane and phosphino-borane derivatives.
Featuring a carbon atom at the +IV oxidation state, CO2 is a stable
molecule and its conversion to either fuels or chemicals necessitates
the use of a mild reductant and an efficient catalyst, to promote the
cleavage of C–O bonds and the formation of novel C–H and/or C–C
bonds.1 In this respect, hydroboranes (R2BH) have a mild reduction
potential (ca. –0.5 V vs NHE) wellꢀsuited for the reduction of CO2 to
the methanol level.2 In contrast to borohydrides, the reduction of
CO2 with hydroboranes requires a catalyst.3 In fact, the catalytic
hydroboration of CO2 has first been reported in 2010 by Guan and
coworkers who showed that nickel(II) pincer complexes were able to
catalyze the reduction of CO2 to methoxyboranes in the presence of
9ꢀBBN, catBH and pinBH.4 Shortly after, SaboꢀEtienne and
Bontemps described efficient ruthenium catalysts for this
transformation and a variety of inorganic and organometallic
catalysts have been reported since then.5 Notably, the polarized B−H
bond in hydroboranes can also be activated by simple organic
compounds: the organocatalyzed hydroboration of CO2 has been
unveiled in 2013 by Fontaine and coworkers with P/B Frustrated
Lewis Pairs (FLPs), and by our group with guanidines and amidines
bases as well as using an isolated FLPꢀCO2 adduct.6 This collection
of catalysts has been completed with several bases among which
phosphines proved to be potent catalysts for this reduction process.7
In this context, we have also shown recently that
proazaphosphatranes enable the first catalytic methylation of amines
involving CO2 reduction with hydroboranes.7a
Chart 1. Candidates for the catalytic hydroboration of CO2
The ferrocene backbone is a wellꢀsuited platform to assemble Lewis
acids and bases by substituting the cyclopentadienyl ligands on iron.
This approach has been exemplified over the last years by the groups
of Bourissou, Aldridge and Erker to access novel FLPs and
ambiphilic ligands.8 Additionally, rotation around the ferrocene axis
provides some flexibility that can adapt the catalyst to the geometric
constraints associated with the catalytic pathway. In this respect,
ferrocene based FLPs differ from the system used by Fontaine et al.
for the hydroboration of CO2, where a benzene ring maintains a rigid
structure between the P and B centres.6a
In this study, diphenylphosphinoferrocene, 1,9a was isolated by
quenching the lithiated ferrocene9b with chlorodiphenylphosphine
(Scheme 1). Similarly, the corresponding ferrocenylꢀpinacolborane
2-Bpin was obtained in a moderate 25% yield by reacting
bis(pinacolato)diboron with lithiated ferrocene. Crystals of 2-Bpin
were obtained by recrystallization from dichloromethane (see ESI).
When dimesitylboron fluoride was used as an electrophile, 2-BMes2
was obtained in a significantly better yield of 71% (Scheme 1).
Thus, since the hydroboration of CO2 is a reaction of fundamental
importance, it becomes crucial now to better apprehend the role of
the organocatalysts in these reduction processes. In particular, the
possible positive influence of combining a Lewis base with a Lewis
acid in the same catalytic system is still to be established.
To address this question, we report herein the synthesis of simple
phosphine, borane and phosphinoꢀborane compounds based on the
same ferrocene backbone (Chart 1). Comparing the catalytic
performances of the separated Lewis acids and bases and their
combination as intermolecular or intramolecular systems
Scheme 1. Synthesis of 1, 2-Bpin and 2-BMes2
The phosphinoꢀborane derivatives 3-Bpin and 3-BMes2 were
prepared using an established procedure based on a sequential
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DOI: 10.1039/C6CC02809H
halogen/lithium exchange starting from 1,1’ꢀdibromoferrocene is subsequently reduced to the acetal form H2C(OBBN)2, with a
(Scheme 2).8a Thus, 1ꢀbromoꢀ1’ꢀ(diphenylphosphino)ferrocene 4 conversion rate of 90% after just 1 hour. Finally, reduction of the
was obtained in 87% isolated yield. Subsequently, a second acetal provides MeO–BBN. This step is slow and requires 27 hours
bromine/lithium exchange was conducted and the lithiated ferrocene at 25 °C to yield the methoxyborane species quantitatively. The
was quenched with bis(pinacolato)diboron, yielding the desired kinetics of the reaction improved at 70 °C. These conditions revealed
product 3-Bpin
in a low 18 % yield. Other pinacolꢀboron a remarkable catalytic activity of 3-BMes2, allowing the full
electrophiles including 2ꢀisopropoxyꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀ conversion of CO2 to the corresponding methoxyborane with a TON
dioxaborolane, were also tested, without success. Alternative routes of 100, in only 2 hours (Table 1, entry 7). Reducing the loading of
were explored to improve the yields of 3-Bpin. Unfortunately, catalyst 3-BMes2 to 0.1 and 0.05 mol%, afforded an excellent TON
inverting the reaction sequence by quenching 1ꢀbromoꢀ1’ꢀlithioꢀ of 1980, after 20 h at 70 °C, and a TOF of 250 h–1, while the isolated
ferrocene with bis(pinacolato)diboron first proved unproductive. components of the catalytic system, namely 1 and 2-BMes2, are
Also, quenching 4 with trimethyl borate and subsequent reaction unreactive (Table 1, Entries 9ꢀ12).
with pinacol failed.
Table 1. Catalytic reduction of CO2 to methoxyborane CH3OBBN
Scheme 2. Synthesis of 3-Bpin and 3-BMes2
As reported by Bourissou et al., replacing bis(pinacolato)diboron
with dimesitylboron fluoride yielded 3-BMes2 in 82% yield.8a It is
noteworthy that the P/B derivatives 3-BMes2 and 3-Bpin display
31P
NMR
chemical
shifts
similar
to
those
of
(diphenylphosphino)ferrocene (1: δ = ꢀ16.2; 3-Bpin: δ = ꢀ17.4; 3-
BMes2: δ = ꢀ19.3 ppm). This suggests the absence of any interaction
between the two heteroatoms, as noted previously by Bourissou et
al. Moreover, no interaction with CO2 has been observed either by
31P or 1H NMR, when a solution of 3-BMes2 is exposed to an
atmosphere of CO2.10
Having in hand a homogeneous series of Pꢀ, Bꢀ and P/Bꢀbased
catalysts, their reactivity has been investigated in the catalytic
hydroboration of CO2. At room temperature, phosphine 1 (1 mol%)
exhibits a negligible catalytic activity in the conversion of CO2 and
9ꢀborabicyclo[3.3.1]nonane (9ꢀBBN) to methoxyborane MeOꢀBBN
(Table 1, Entry 1). Indeed a Turn Over Number (TON) lower than 5
was observed after 28 h at 25 °C. Increasing the reaction temperature
to 70 °C did not improve the catalytic performances of 1 (Entry 11 in
Table 1). These results are in line with the results of Stephan et al.
on the phosphine catalysed hydroboration of CO2.7b Similarly, the
borane 2-BMes2 is inefficient both at 25 and 70 °C (Entries 2 and 12
in Table 1). In stark contrast, a mixture of 1 and 2-BMes2 provides a
competent catalytic system, enabling the reduction of CO2 to
MeOBBN with a TON of 41 after 28 h at 25 °C (TOF = 1.5 h–1).
This result demonstrates for the first time the synergistic and positive
influence of combining a Lewis base and a Lewis acid in the
reduction of CO2 with hydroboranes. Replacing 2-BMes2 with the
less acidic 2-Bpin borane somewhat deactivates the catalytic system
and a TON of 36 (TOF = 1.3 h–1) is measured under the same
conditions. To further establish the benefit of an ambiphilic
structure, the catalytic potential of the intramolecular Lewis pairs 3-
BMes2 and 3-Bpin has been determined. 3-BMes2 presents a better
activity than the mixture of 1 and 2-BMes2 and a TON of 100 was
reached after 28 hours at 25 °C (TOF = 3.6 h–1).
[a] reactions performed in JꢀYoung tubes; monitored by NMR spectroscopy
with 0.0048 mmol of catalyst, 0.48 mmol of 9ꢀBBN in d8ꢀTHF (0.40 mL) and
exposed to an atmosphere of CO2 (1 bar), [b] The TON is obtained by 1H
NMR monitoring (CH3OꢀBBN formation), with mesitylene as internal
standard.
Dialkylboranes, other than 9ꢀBBN, were also tested as potential
reductants. As expected, the hydroboration of CO2 with
catecholborane is slower than with 9ꢀBBN because of the lower
Lewis acidity of the borane. In fact, under the optimized reaction
conditions (48 h at 70 °C), 3-BMes2 promotes the reduction of CO2
to MeOBcat with a modest TOF of 23. Replacing catecholborane
.
with BH3 SMe2 did not lead to any significant product formation.
Again, replacing the BMes2 unit with a Bpin fragment somewhat
slows down the catalytic reaction and a TOF of 3.0 h–1 is measured
with 3-Bpin, at 25 °C (vs 3.6 h–1 for BMes2). The difference of
reactivity between 3-BMes2 and 3-Bpin is more pronounced at
70 °C, with TOFs values of 50 and 29 h–1, respectively (Entries 7ꢀ8
It is worth noting here, that the reaction course follows a profile
similar to the one observed previously for guanidines and N/B
FLPs.6f Indeed, monitoring the product distribution over time, using
1H and 13C NMR spectroscopy, reveals that CO2 is first reduced to
the boryl formate HCOO–BBN which is present in only a very low
concentration and does not accumulate (Fig. S1). This intermediate
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in Table 1). This behaviour directly follows from the Arrhenius rate
law.
†
Footnotes should appear here. These might include comments
relevant to but not central to the matter under discussion, limited
experimental and spectral data, and crystallographic data.
Previous studies by the groups of Stephan and Cantat have
established that basic guanidines, Nꢀheterocyclic carbenes and
phosphines can promote the hydroboration of CO2.6f, 11 Mechanistic
and computational investigations have shown that basic catalysts
proceed through the activation of the B–H bond, by coordination of
the Lewis base to the hydroborane.6f Therefore we propose here that
the benefit of combining a trivalent phosphorus function with a
remote Lewis acid, as in 3-BMes2, results from a concomitant Hꢀ
transfer / boraneꢀformate adduct formation, in the first step of the
CO2 reduction process (Scheme 3). In this approach, linking together
the Lewis acid and the Lewis base functions should reduce the
volume of the nonꢀproductive region of the conformational space
between the catalysts and the substrates and thus increase further the
reaction rates.12 In fact, experimentally, catalysts 3-BMes2 and 3-
Bpin are about twice as reactive as the corresponding bimolecular
systems at 25 °C. This difference of reactivity corresponds roughly
to a decrease of ca. 1 kcal/mol in activation energy, consistent with
an entropic effect.
Electronic Supplementary Information (ESI) available: Fig. S1 and
experimental procedures. See DOI: 10.1039/c000000x/
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Scheme 3. Proposed synergistic effect of phosphino-borane catalysts in the
.
hydroboration of CO2
5
Overall, we have demonstrated for the first time that the combination
of phosphines and boranes offers a synergistic effect in the catalytic
reduction of CO2 with hydroboranes. While the phosphine and the
borane fragments, taken separately, do not catalyse the reduction of
CO2, the P/B derivatives 3, combining the two functions, display
high catalytic activity.
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Acknowledgements
We thank the CHARMMMAT Laboratory of Excellence, CEA, CNRS,
Institut de Chimie des Substances Naturelles, and the ANR for financial
support. The European Research Council is acknowledged for an ERC
Starting Grant to T.C (Grant Agreement no. 336467).
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