Electrophilic Organobismuth Dication Catalyzes Carbonyl Hydrosilylation

Article abstract of DOI:10.1002/chem.202002006

Bismuth compounds are gaining importance as potential alternatives to transition-metal complexes and electron deficient lighter p-block compounds in homogeneous catalysis. Computational analysis on the two-coordinate [(Me₂NC₆H₄)Bi]²⁺ possessing three electrophilic sites is experimentally evidenced by the isolation of [{Me₂NC₆H₄}Bi{OP(NMe₂)₃}₃][B(3,5-C₆H₃Cl₂)₄]₂. These observations led us to generate dicationic organobismuth catalyst, [(Me₂NC₆H₄)Bi(L)₃]²⁺ (L=aldehyde/ketone), evidenced by NMR spectroscopy in solution and by single-crystal X-ray diffraction in the solid state. It efficiently catalyzes hydrosilylation of aldehydes and ketones resulting in silyl ethers as the only products in high yields. Our investigations support a carbonyl activation mechanism at the bismuth center followed by Si?H addition.

Full text of DOI:10.1002/chem.202002006

Accepted Article
Accepted Article
Title: Electrophilic Organobismuth Dication Catalyzes Carbonyl
Hydrosilylation
Authors: Ramkumar Kannan, Selvakumar Balasubramaniam, Sandeep
Kumar, Raju Chambenahalli, Eluvathingal D. Jemmis, and
Ajay Venugopal
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To be cited as: Chem. Eur. J. 10.1002/chem.202002006
Link to VoR: https://doi.org/10.1002/chem.202002006
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Electrophilic Organobismuth Dication Catalyzes Carbonyl
Hydrosilylation
Ramkumar Kannan,[a] Selvakumar Balasubramaniam, [a] Sandeep Kumar, [b] Raju Chambenahalli,[a]
Eluvathingal D. Jemmis,*[b] and Ajay Venugopal*[a]
[a]
[b]
R. Kannan, S. Balasubramaniam, R. Chambenahalli, Dr. A. Venugopal
School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram,
Vithura, Thiruvananthapuram 695551, India
E-mail: venugopal@iisertvm.ac.in
http://faculty.iisertvm.ac.in/venugopal
S. Kumar, Prof. E. D. Jemmis
Department of Inorganic and Physical Chemistry
Indian Institute of Science
Bangalore 560012, India
http://ipc.iisc.ac.in/~edj/
Supporting information for this article is given via a link at the end of the document.
Abstract: Bismuth compounds are gaining importance as potential
alternatives to transition metal complexes and electron deficient
lighter p-block compounds in homogeneous catalysis. Computational
analysis on the two-coordinate [(Me2NC6H4)Bi]2+ possessing three
electrophilic sites is experimentally evidenced by the isolation of
[{Me2NC6H4}Bi{OP(NMe2)3}3][B(3,5-C6H3Cl2)4]2. These observations
led us to generate dicationic organobismuth catalyst,
[(Me2NC6H4)Bi(L)3]2+ (L=aldehyde/ketone), evidenced by NMR
spectroscopy in solution and by single crystal X-ray diffraction in the
solid state. It efficiently catalyzes hydrosilylation of aldehydes and
ketones resulting in silyl ethers as the only products in high yields.
Our investigations support a carbonyl activation mechanism at the
bismuth center followed by Si―H addition.
equivalents of [Me2NC6H4Li] in diethyl ether (Et2O) to obtain
(Me2NC6H4)2BiCl (1). A redistribution reaction between 1 and
BiCl3 resulted in [(Me2NC6H4)BiCl2]2 (2). Compounds 1 and 2
were characterized by NMR spectroscopy, elemental analysis
and single crystal X-ray diffraction studies (See SI, Fig S52 and
Fig S53).
The importance of environmentally benign heavy element
bismuth (Bi) 1 in catalysis 2 has come to the foreground in recent
years with the electrochemical reduction of dinitrogen to
3
4
ammonia and carbon dioxide to formate and homogenous
catalysis involving fluorination of arenes.5 While these reactions
utilize the redox active property of Bi,5,6 electrophilic activation of
CO,7 CO2,8 and sp2
and sp3 C―H bonds
by Bi(III)
9
10
compounds in a stoichiometric manner has motivated chemists
to explore ways to convert these to catalytic processes.
Examples of catalytic reactions involving highly electrophilic
Bi(III) are rare and such catalysts tend to react even with
conventional solvents.11 Judicious choice of a stable, yet highly
reactive, Bi(III) species is required in solution to perform catalytic
bond activation. Herein, we present the successful generation of
a reactive Bi(III) species in solution and substantiate its
Scheme 1. Synthetic route for the preparation of 1 - 3 and generation of the
active catalyst in CH2Cl2.
electrophilicity by performing carbonyl hydrosilylation,12
a
challenging reaction usually catalyzed by transition metals 13 and
the lighter p-block elements.14,15 Experience with the chelating 2-
Abstraction of the chloride ions from the bismuth centers in
2 should lead to a highly reactive two―coordinate dicationic
species [(Me2NC6H4)Bi]2+, which is expected to possess
[(dimethylamino)]phenyl (Me2NC6H4) ligand in the isolation of
16
electrophilic main group cations
generate in situ
led us to use this ligand to
dicationic Bi(III) species
a
electrophilic sites. Our efforts to access the two coordinate
[(Me2NC6H4)Bi(L)3]2+(L=aldehyde/ketone) for aldehyde and
ketone hydrosilylation under mild conditions.
17
[(Me2NC6H4)Bi]2+ from 2 using Et3Si+
in the presence of the
weakly coordinating anion [B(C6F5)4]- did not succeed. We
examined in detail the acceptor orbitals of this six-valence
electron bismuth species computationally (Figure 1). The lowest
acceptor orbital (LUMO) is primarily a p-orbital based on Bi with
We began the synthetic work by introducing
[(dimethylamino)]phenyl (Me2NC6H4) ligand on to the bismuth
center (Scheme 1). Anhydrous BiCl3 was treated with two
1
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some contribution from the phenyl, (Figure 1, A). The next
highest orbital, LUMO+1, is Bi―N σ* antibonding MO (Figure 1,
The synthesis and structure of 3 substantiates our
hypothesis that a strongly nucleophilic ligand like OP(NMe2)3
stabilizes the dicationic bismuth center in [(Me2NC6H4)Bi]2+
B) largely localized on Bi. Step-wise introduction of
a
nucleophile (L) to [(Me2NC6H4)Bi(L)]2+ is depicted in the
correlation diagram (Figure 1) finally resulting in a twelve
valence electron species [(Me2NC6H4)Bi(L)3]2+ with a nearly
square pyramid geometry. Thus, [(Me2NC6H4)Bi]2+ can be
regarded as possessing three electrophilic sites.
resulting in a stable five-coordinate complex. A weaker
nucleophile like aldehyde or ketone would however result in a
less stable complex amenable to further chemical manipulation
of the carbonyl functionality. The exothermicity of stepwise
addition of aldehydes and ketones, though less than that of
OP(NMe2)3 (See SI, Figure S56, Table S10) prompted us to
generate the aldehyde and ketone adducts of [(Me2NC6H4)Bi]2+,
[(Me2NC6H4)Bi(L)3]2+ in order to further explore their reactivity
with hydridosilanes. Thus, 2 was treated with two equivalents of
20
[(CH2Cl2)Ag][Al{OC(CF3)3}4]
in the presence of three
equivalents of 4-methylbenzaldehyde (4-MePhCHO) in CD2Cl2 ,
which resulted in instantaneous precipitation of two equivalents
of AgCl.21 13C{1H} NMR spectrum of the mother liquor revealed a
de-shielded signal for the carbonyl carbon in 4-MeC6H4CHO at
δ 199.8 ppm (vs δ 192.2 ppm for free 4-MePhCHO in CD2Cl2),
evidencing the formation of [(Me2NC6H4)Bi(4-MePhCHO)3]2+ in
solution (see SI, Figure S43). 1H NMR spectrum of the same
sample exhibited a de-shielded signal for the methyl groups in
Me2NC6H4 at δ 3.48 ppm as compared to the one observed in 2
(δ 2.97 ppm), further confirming the presence of the dicationic
species
in
solution.
Single
crystals
of
[(Me2NC6H4)Bi(4-MePhCHO)3][Al{OC(CF3)3}4] (4) were obtained
from a concentrated CH2Cl2 solution and X-Ray diffraction
experiment confirmed the structure of the aldehyde adduct in the
solid state (see Figure 3 and SI, Figure S55). The Bi―O
distances are found in the range of 2.343 to 2.391 Å with an
average value of 2.367 Å, the shortest Bi―carbonyl distance
documented in the literature.22
Figure 1. A schematic correlation diagram (not drawn to scale) of the acceptor
orbitals in [(Me2NC6H4)Bi]2+ (A, B), [(Me2NC6H4)LBi]2+ (C, D), and two
[(Me2NC6H4)L2Bi]2+ (E), where L=PhCHO. The exothermicity (ΔE kcalmol-1) of
the stepwise addition of PhCHO is provided in the bottom and for L= Cl-,
OP(NMe2)3 and Ph2CO is provided in SI, Table S10.
To test the hypothesis on the electronic structure of
[(Me2NC6H4)Bi]2+, we performed a salt metathesis reaction
18
between 1 and [(MeCN)2Ag][A] (A=B(3,5-C6H3Cl2)4)
in the
presence of the nucleophilic hexamethylphosphoramide
(OP(NMe2)3). It resulted in a stable dicationic bismuth compound,
[{Me2NC6H4}Bi{OP(NMe2)3}3][B(3,5-C6H3Cl2)4]2 (3) in 90% yield
(Scheme 1, Figure 2). NMR spectroscopy shows that 3 is stable
Figure 3. Solid state structure of the cationic part of 4. Hydrogen atoms are
omitted for clarity. (See SI, Figure S55, Table S9 for more details).
in
solution
unlike
the
related
compound
[PhBi{OP(NMe2)3}4][PF6]2 undergoing re-distribution in solution
to Ph3Bi and [Ph2Bi{OP(NMe2)3}2][PF6].19
In a similar way, [(Me2NC6H4)Bi(Ph2CO)3][Al{OC(CF3)3}4]
(5) was generated in CD2Cl2. It exhibited a signal for the
carbonyl carbon at δ 205.3 ppm in the 13C{1H} NMR spectrum,
which is de-shielded by 9 ppm from the one observed in free
Ph2CO (see SI, Figure S46). We also found that the addition of
further equivalents of the respective carbonyls to the solution
containing [(Me2NC6H4)Bi(L)3]2+ resulted in the upfield shift of the
carbonyl carbon signal in the 13C{1H} NMR spectrum, pointing
out the dynamic nature of the Bi―carbonyl interaction in
[(Me2NC6H4)Bi(L)3]2+ (see SI, Figure S48-S51).
After evidencing [(Me2NC6H4)Bi(L)3]2+ in solution, we
began exploring its potential in hydrosilylation. (Me2NC6H4)BiCl2
(2 mol%) was treated with [(MeCN)2Ag][A] (4 mol%) in CD2Cl2 in
the presence of PhCHO (249 µmol) at 25 °C (Scheme 2). AgCl
precipitation was observed in quantitative amount and separated
Figure 2. Solid state structure of the cationic part of 3. Hydrogen atoms are
omitted for clarity. (See SI, Figure S54, Table S8 for more details).
2
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from the solution by filtration. Et3SiH (298 µmol) was added to
this filtered solution. The progress of PhCHO hydrosilylation was
monitored by H NMR spectroscopy and the reaction completed
boron center.15e,f Hence, we speculate
hydrosilylation pathway.15a, 16a
a C=O activation
1
within an hour. PhCH2OSiEt3 was observed as the only product
with a yield of 98% with respect to PhCHO (Scheme 2)
corresponding to a TOF of 49 h-1, ten times more than the
activity observed in [1,2-(Ph2MeSb)2C6H4][BF4]2 (TOF = 4.2 h-
1).15a Using the same synthetic procedure, we successfully
performed hydrosilylation of
ortho-phthalaldehyde and
terephthalaldehyde and observed the corresponding silyl ethers
in high yields (Scheme 2). We were also successful in probing
the activity of the bismuth catalyst with a simple aliphatic
aldehyde, isobutyraldehyde, as well as an α,β-unsaturated
aldehyde, cinnamaldehyde without observing any undesired
products (Scheme 2). The overall catalytic efficiency of
[(Me2NC6H4)Bi(L)3][A]2 for aldehyde hydrosilylation in terms of
turn over numbers and frequencies is close to that of B(C6F5)3
15f,g
and better than other reported main group Lewis acids.15a-e,
23a,b
Scheme 3. Catalytic hydrosilylation of ketones. Yield (with respect to ketone),
reaction time, turn-over number and frequency are provided below each
product. See SI, section 2 for more details.
We then successfully performed hydrosilylation of other
aromatic and aliphatic ketones (Scheme 3). In all the cases,
hydrosilylation proceeded efficiently under mild reaction
conditions and high yields of the corresponding silyl ethers were
observed. As a general observation, hydrosilylation of aromatic
ketones was faster than the aliphatic ketones. This can be
rationalized based on the fact that C=O is more polarized in
aromatic ketones thus facilitating the effective coordination of
the oxygen atom to the Bi center in the catalyst. Our
experiments on Ph2CO hydrosilylation infer high turn-over
number and frequency of 100 and 200 h-1 respectively. Neither
presence of excess of silane nor increase in reaction
temperature (up to 55 °C) lead to the over-reduction of Ph2CO,
often observed in the case of B(C6F5)3 and other p-block Lewis
acids,23 thereby highlighting the selectivity of the bismuth
catalyst towards silyl ethers over de-oxygenated products. We
performed a preliminary test for chemoselectivity between
PhCHO and Ph2CO towards hydrosilylation (Scheme 4).25 It was
observed that the more basic PhCHO selectively transforms to
PhCH2OSiEt3.in the presence of Ph2CO (Scheme 4).
Scheme 2. Catalytic hydrosilylation of aldehydes. Yield (with respect to
aldehyde), reaction time, turn-over number and frequency are provided below
each product. See SI, section 2 for more details.
Having witnessed facile aldehyde hydrosilylation, we set
out to study the more challenging ketone hydrosilylation. We
tested the in-situ generated [{Me2NC6H4}Bi(PhMeCO)3][A]2 (L =
PhMeCO)
Hydrosilylation of acetophenone proceeded in a much slower
pace compared to benzaldehye with
TOF of 2.5 h-1.
Acetophenone being less basic than benzaldehyde and the fact
that hydrosilylation of the former is faster than the latter points
out that these reactions proceed via the coordination of carbonyl
oxygen to the bismuth center. Our observations on
benzaldehyde and acetophenone hydrosilylation with the
dicationic bismuth catalyst contrasts the findings with B(C6F5)3,24
which is proposed to proceed via Si―H coordination to the
for
ketone
hydrosilylation
using
Et3SiH.
a
Scheme 4. Chemoselective hydrosilylation of benzaldehyde over
benzophenone. See SI section 4 for more details.
To establish the active catalytic species, control
experiments on Ph2CO hydrosilylation were performed (Scheme
5). We observed that both anhydrous BiCl3, 1 and 2 did not
catalyze Ph2CO hydrosilylation. [(MeCN)2Ag][A] exhibited low
catalytic activity, but resulted in a mixture of products - silyl ether,
3
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Ph2C(H)OSiEt3 (A) and the deoxygenated Ph2CH2 (B), inferring
that it lacks efficiency as well as control in a single product
formation.26 However, [{Me2NC6H4}Bi(L)3][A]2 was found to be
highly efficient and selective towards the formation of
hydrosilylated product, confirming that [{Me2NC6H4}Bi(L)3][A]2 is
the active catalytic species involved in the catalytic
hydrosilylation of aldehydes and ketones.
Mr. Alex P. Andrews are thanked for help in refining single
crystal X-ray data.
Keywords: Main group compounds • Lewis acids • reactive
cations • bismuth • hydrosilylation
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[{Me2NC6H4}Bi(HSiEt3)3]2+ in the presence of Et3SiH, we resorted
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L=benzaldehyde. Energetics indicates that Si-H activation at the
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hydrosilylation of aldehydes and ketones where a mixture of silyl
ether and de-oxygenated product results15f our catalyst is highly
selective towards silyl ether. Further computational and
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mechanistic steps.
While cationic bismuth compounds were mainly limited in
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investigations have provided a new approach for catalytic
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organobismuth center. Our efforts to expand this concept to a
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Acknowledgements
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We thank IISER Thiruvananthapuram for generous funding and
research facilities. SERB is thanked for supporting research
through Year of Science Chair Professorship to E.D.J., and
SERC, IISc, for computational facilities. Dr. Babu Varghese and
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experiments to prove the identity of [(Me2NC6H4)Bi(L)3]2+
.
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Table of Contents
We present the first bismuth catalyzed carbonyl hydrosilylation. The catalytically active electrophilic
dicationic bismuth species [(Me2NC6H4)Bi(L)3]2+ (L=aldehyde/ketone) is characterized both in solution
and in the solid state. Control experiments and computation studies support a carbonyl activation
mechanism at bismuth followed by silane addition.
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