Hydrosilylation of Carbonyls Catalyzed by Hydridoborenium Borate Salts: Lewis Acid Activation and Anion Mediated Pathways

Article abstract of DOI:10.1021/acs.inorgchem.0c00646

The electronically unsaturated three-coordinated hydridoborenium cations [LBH]+[HB(C6F5)3]-(1) and [LBH]+[B(C6F5)4]-(2), supported by a bis(phosphinimino)amide ligand, were found to be excellent catalysts for hydrosilylation of a range of aliphatic and ar

Full text of DOI:10.1021/acs.inorgchem.0c00646

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Hydrosilylation of Carbonyls Catalyzed by Hydridoborenium Borate
Salts: Lewis Acid Activation and Anion Mediated Pathways
Sandeep Rawat,† Mamta Bhandari,† Vishal Kumar Porwal, and Sanjay Singh*
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ABSTRACT: The electronically unsaturated three-coordinated hydridoborenium cations [LBH]+[HB(C6F5)3]− (1) and
[LBH]+[B(C6F5)4]− (2), supported by a bis(phosphinimino)amide ligand, were found to be excellent catalysts for hydrosilylation
of a range of aliphatic and aromatic aldehydes and ketones under mild reaction conditions (L = [{(2,4,6-Me3C6H2N)P(Ph2)}2N]).
The key steps of the catalytic cycle for hydrosilylation of PhCHO were monitored via in situ multinuclear NMR measurements for
catalysts 1 and 2. The combined effect of carbonyl activation via the Lewis acidic hydridoborenium cation and the hydridic nature of
the borate counteranion in 1 makes it a more efficient catalyst in comparison to that of carbonyl activation via the predominant
Lewis acid activation pathway operating with catalyst 2. The catalytic cycle of 1 showed hydride transfer from the borate moiety
[HB(C6F5)3]− to PhCHO in the first step, forming [PhCH2−O−B(C6F5)3]−, which subsequently underwent σ-bond metathesis
with Et3SiH to form the product, PhCH2−O−SiEt3. Quantum chemical calculations also support the borate anion mediated
mechanism with 1. In contrast, the reaction catalyzed by 2 proceeds predominantly via the Lewis acid activation of the carbonyl
group involving [LB(H)←OC(H)Ph]+[B(C6F5)4]− as the transition state and [LBOCH2Ph]+[B(C6F5)4]− as the intermediate.
Toward the goal to develop non-transition-metal-based
catalysts, in the late 90s Piers and co-workers used B(C6F5)3 as
a catalyst for hydrosilylation of carbonyl compounds, including
esters and olefins.4,5 This work marked a major step toward the
application of main-group complexes as catalysts. Subse-
quently, the groups of Harder, Manners, Dagorne, Inoue,
Nikonov, Roesky, Singh, Hill, Ingleson, and a few others have
made individual notable contributions in the development of
main-group catalysts such as applications of the complexes of
Mg,6 Ca,7 B,8 Al,9 Si,10 P,11 etc. in various organic
transformations. Additionally, Okuda and co-workers reported
on the chemoselective hydroboration of carbonyl compounds
INTRODUCTION
■
Over the past decades, the hydrosilylation of >CX (O, CR2,
NR) functional groups has become a viable alternative to other
commonly known reduction protocols such as hydrogenation
using molecular hydrogen, use of hydride donor agents, and
transfer hydrogenation.1 For the direct synthesis of silyl ethers
(or protected alcohols), the hydrosilylation of carbonyls
mediated by precious-transition-metal catalysts (Pd, Pt, Rh,
Ir, etc.) is a convenient route.2 The rapidly growing interest in
the catalytic application of inexpensive, eco-friendly, and less
toxic metal complexes such as those of Fe, Co, Mn, Cu, and Ni
in the activation of Si−H bonds has also been witnessed
recently.3 However, there has been limited success in matching
the activity of precious-metal complexes with that of catalysts
based on late transition metals. More recently, complexes from
main-group elements have started to emerge as hydro-
elementation catalysts and as alternatives to both the
precious-metal as well as the late-3d-metal complexes and are
expected to show rapid future growth.4
Received: February 29, 2020
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with lighter alkali-metal hydridoborates, [(L)M][HBPh3] (M
= Li, Na, K; L = tris{2-(dimethylamino)ethyl}amine).12
Interestingly, among these boron remained as one of the
most attractive targets to explore the catalytic role of its
complexes for hydroelementation reactions.8,12,13 Boron
compounds capable of exhibiting strong Lewis acidic behavior,
such as B(C6F5)3, were very promising candidates.13 In this
context, cationic complexes of boron were supposed to fulfill
the requirements of Lewis acidity even better.14,15 It has been
shown that borenium cations offer a perfect balance of stability
and Lewis acidity.16,17
Borenium cations with a hydride functionality have been
considered important reagents with potential applications as
metal-free catalysts in processes such as hydrogenation,18a,b
imine hydrogenation,18c dihydrogen activation, and alkyne 1,2-
carboboration.19 At present, a few well-characterized cationic
mononuclear borenium hydride species are known.20−24
Previously, we have synthesized the borenium hydride cations
[LBH]+[HB(C6F5)3]− (1) and [LBH]+[B(C6F5)4]− (2)
(Figure 1) supported by a bis(phosphinimino)amide ligand
Lewis acid activation mechanism operating with catalyst 2.
Additionally, the intermediate [LBH]+[PhCH2−O−B-
(C6F5)3]− (S12), formed from the reaction between 1 and
PhCHO, was a key intermediate that was successfully isolated
and characterized. This intermediate was further used as a
precatalyst for hydrosilylation of another carbonyl substrate to
experimentally support the hydride migration from the
hydridic borate to the carbonyl as the key step of the
hydrosilylation involving 1.
RESULTS AND DISCUSSION
■
The hydridoborenium borate ion pairs [LBH]+[HB(C6F5)3]−
(1) and [LBH]+[B(C6F5)4]− (2) were generated by hydride
abstraction from LBH2 with B(C6F5)3 and [Ph3C]+[B-
(C6F5)4]−, respectively (LH = [{(2,4,6-Me3C6H2N)P-
(Ph2)}2N]H).24 Compounds 1 and 2 proved to be efficient
catalysts for the hydrosilylation of carbonyl compounds
(aliphatic and aromatic aldehydes and ketones) in CHCl3
medium and showed good catalytic activity at 40 and 70 °C,
respectively. In a control experiment, under similar catalytic
reaction conditions, the use of LBH2 as the catalyst showed no
reaction between benzaldehyde and Et3SiH. This study
justified the need for generation of the cationic complexes 1
and 2 from their common precursor, LBH2. The reaction
conditions for hydrosilylation (with respect to the relative
quantity of reagents and catalyst, reaction temperature, and
duration) were optimized by using benzaldehyde as the
benchmark substrate (see the Supporting Information).
Reducing the catalyst loading from 5 mol % to 2.5 mol %
does not affect the conversion of the desired product; however,
a catalyst loading lower than 2.5 mol % increased the reaction
duration, slowed the conversion, and lowered the purity of the
product. Good catalyst performance was thus obtained on
employing 2.5 mol % of 1 or 2 and 1.0 mmol of benzaldehyde
with 1.2 mmol of Et3SiH in CHCl3. Under these conditions,
quantitative conversion of benzaldehyde to Et3Si−O−CH2Ph
(3a) occurred after 1 h at 40 °C with catalyst 1 and after 7 h at
70 °C with catalyst 2. Usually, solvents play an important role,
possibly by interacting with the transient or stable species
formed during the course of the catalytic reaction.25 Chloro-
form was found to be the most suitable solvent among the
solvents compatible with this reaction (toluene, THF,
dichloromethane, and chloroform) (Tables S1 and S2 in the
Supporting Information). Use of a polar coordinating solvent
such as THF slowed down the reaction, probably due to the
formation of the coordinatively saturated THF adduct of 1 and
2 that delays the substrate binding to the boron center. The
Figure 1. (top) Hydridoborenium cations (1 and 2). (bottom)
Hydrosilylation of carbonyl compounds. Hydrosilylation catalyzed by
1 involves [HB(C6F5)3]− as the hydride source, whereas with 2 the
reaction proceeds with Lewis acid activation of the carbonyl group.
and demonstrated their Lewis acid behavior as well.22,24 In a
continuation of our efforts to exploit the Lewis acidity of these
ionic complexes and to discover new catalysts from main-group
compounds, herein we employed 1 and 2 for the hydro-
silylation of aldehydes and ketones (Figure 1). Currently, only
two reports are available that utilize [9-BBN·(2,6-lutidi-
1
formation of 3a was confirmed by its H NMR spectrum,
which showed a singlet corresponding to the benzylic −CH2
moiety at 4.73 ppm. A quartet and triplet due to the −OSiEt3
group appeared at 0.64 and 0.97 ppm, respectively. In the 13C
NMR spectrum, signals for −OSiEt3 (4.5 and 6.8 ppm) and
−CH2OSiEt3 (64.7 ppm) as a result of hydrosilylation also
revealed the formation of 3a. Subsequently, various aldehydes
and ketones were smoothly converted to their corresponding
silyl ethers 3a−x (Table 1) and 4a−l (Table 2), respectively, in
very good yields under the optimized reaction conditions.
Halogenated benzaldehydes could be easily converted to
their corresponding triethylsilyl ethers (3b−e,t,u), and no σ-
bond metathesis between the halogen moiety and Et3SiH was
observed. In our attempts to explore the substrate scope, we
have also been able to demonstrate good chemo- and
regioselectivity of our catalysts toward carbonyls in the
ne)]+NTf2 and {Al[OC(CF3)3]4]− salts of a ferrocene-based
−
planar-chiral borenium cation to catalyze the hydrosilylation of
ketones.14a,d Mechanistic investigations of these catalysts
suggest the borenium ion activation of Et3SiH as the key
step, similar to that outlined for B(C6F5)3-catalyzed hydro-
silylation of carbonyls by Piers and co-workers.4c In contrast to
these, herein we have observed that the hydride functionality
plays an important role in catalysts 1 and 2 and that these
catalysts adopt reaction pathways different from those for
previously reported borenium ion catalyzed carbonyl hydro-
silylations. The combination of the Lewis acidic hydridobore-
nium cation and the hydridic borate in 1 assist the catalysis
reaction more efficiently in comparison to the predominant
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Table 1. Substrate Scope of Hydrosilylation of Aldehydes
Catalyzed by Hydridoborenium Borate Salts 1 and 2
Table 2. Substrate Scope of Hydrosilylation of Ketones
Catalyzed by Hydridoborenium Borate Salts 1 and 2
a
Values in parentheses indicate the yields of isolated products that
were characterized by 1H and 13C NMR. Reactions were performed at
40 °C (for cat. 1) and 70 °C (for cat. 2).
groups in conjugation with the aromatic rings such as
isophthalaldehyde, terephthalaldehyde, and 2,6-diacetylpyri-
dine furnished the reduction of both carbonyl moieties,
yielding 3p,q and 4d, respectively.
Mechanistic Investigations of Hydrosilylation of
Carbonyl Compounds with [LBH]+[HB(C6F5)3]− (1) as
the Catalyst. To elucidate the mechanism of reaction by the
present catalysts, we first performed the stoichiometric reaction
of [LBH]+[HB(C6F5)3]− (1) with PhCHO and monitored the
progress of the reaction using hetronuclear NMR spectroscopy.
These investigations clearly identified the intermediate
[LBH]+[PhCH2−O−B(C6F5)3]− (S12), formed as a result of
hydride migration from [HB(C6F5)3]− to PhCHO (Figure 2).
Intermediate S12 was stable enough to be isolated and
characterized by using multinuclear NMR (1H, 13C{1H}, 11B,
31P{1H}, and 19F) and IR spectroscopy. The disappearance of
a
Values in parentheses indicate the yields of isolated products that
were characterized by 1H and 13C NMR. Reactions were performed at
40 °C (for cat. 1) and 70 °C (for cat. 2). Due to volatile nature of
1
b
the aldehydic PhCHO peak in the H NMR spectrum and
the product the yield was calculated by NMR using hexamethylben-
zene as an internal standard.
absence of a carbonyl >CO stretching frequency in the IR
spectrum confirmed the formation of S12. The formation of
S12 was facile at room temperature, and subsequent addition of
an stoichiometric amount of Et3SiH gave the product 3a. The
formation of 3a was slow at room temperature but can be
expedited by heating the reaction mixture to 40 °C.
presence of different functional groups such as conjugated or
isolated olefins (3o,v), where only 1,2-addition took place and
the double bond of the substrate remained intact. In the case of
hydrosilylation of heteroaromatic ketones such as 2-acetylpyr-
idine, 2,6-diacetylpyridine, and 2-acetylthiophene a good
tolerance toward the dearomatization of the heteroaromatic
rings6b,7c,11d was observed and smooth reduction of the ketone
group took place to afford their corresponding silyl ethers
4d,e,l (Table 2). Further, substrates containing two carbonyl
1
In the H NMR spectrum of the reaction mixture, the
appearance of a signal at 4.46 ppm was assigned to the benzylic
−CH2 moiety of [PhCH2−O−B(C6F5)3]− (S12). Upon
addition of Et3SiH a downfield shift for −CH2 protons (4.73
ppm) was observed, which indicates the formation of Et3Si−
O−CH2Ph (3a) (Figure 3). The concomitant regeneration of
1 was also clear from the disappearence of the 11B NMR signal
C
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Figure 2. Proposed catalytic cycle for the hydrosilylation of PhCHO using Et3SiH and the catalyst [LBH]+[HB(C6F5)3]− (1).
at −2.50 ppm (for [PhCH2−O−B(C6F5)3]−) and reappear-
ance of the signal at −25.1 ppm (for [HB(C6F5)3]−). We
propose that Et3SiH reacts with S12 to form the four-
membered cyclic transition state S13, which easily undergoes a
σ-bond metathesis reaction at 40 °C to convert into 3a and
regenerate the catalyst 1. Due to fast conversion of S13 into the
product at 40 °C, we could not observe experimental evidence
for S13, but computational studies suggest its involvement (see
below Figure 4).
cently, in 2018, Mosch-Zanetti et al. showed the insertion of
PhCHO into the B−H bond of the anion in [Mo(OSiR3)-
(NtBu)(2,4-tBu2-6-{(N-Ph)CH}C6H2)]+[HB(C6F5)3]−, re-
sulting in the formation of [Mo(OSiR3)(NtBu)(2,4-tBu2-6-
{(N-Ph)CH}C6H2)]+[PhCH2−O−B(C6F5)3]− (R = Et, Ph).31
Given the fact that hydrosilylation with 1 involved the hydride
transfer from the anion, we could not ascertain from our
experiments the role of the cation in 1; however, it is likely that
the cation assists in activating the carbonyl in this process and
facilitates the hydride transfer from the anion [HB(C6F5)3]−.
Lewis acids such as B(C6F5)3 are known to form weak labile
adducts with carbonyl oxygen. Piers et al. found hydride
abstraction from R3SiH by B(C6F5)3 via [Et3Si- - -H−
B(C6F5)3] as the key activating step in B(C6F5)3-catalyzed
hydrosilylation of carbonyls.4a,c In view of this, we were
prompted to see the involvement of such a species in the
present case; therefore, when equimolar quantities of 1 and
Et3SiH were mixed, no evidence for the formation of LBH2
and [Et3Si- - -H−B(C6F5)3] or even [L(H)B−H- - -
SiEt3]+[HB(C6F5)3]− was observed from 1H, 11B, and
31P{1H} NMR spectra, which only showed signals for 1 and
Et3SiH. Thus, the alternative pathway involving borenium ion
activation of Et3SiH was not found to be convincing in this
case (see Figure S19 in the Supporting Information).
̈
Recently, Vanka et al. performed computational work that
supports the idea of B(C6F5)3 to function more as an initiator
rather than a pure catalyst in many metal-free transformations
where the anion [HB(C6F5)3]− is generated in the process and
in further steps remains a spectator.26 However, in the present
work the anion [HB(C6F5)3]− is not just a spectator but
actually cooperatively participates in the reaction. Previously,
Piers et al. also found hydride delivery from [HB(C6F5)3]− in
carbonyl hydrosilylation catalyzed by B(C6F5)3.4c Similarly,
insertion of CO2 in the borohydride bond of [HB(C6F5)3]−
was observed by Ashley et al. when CO2 was reacted with the
salt [Me4C5NH2]+[HB(C6F5)3]−, leading to the formation of
the structurally characterized formatoborate complex
[Me4C5NH]+[HCO2B(C6F5)3]−.27 Similarly, Parkin et al.
reported the hydrosilylation of CO2 by R3SiH catalyzed with
a combination of {[TismiPrBenz]M}[HB(C6F5)3] (M = Zn, Mg)
and B(C6F5)3. The CO2 insertion into [HB(C6F5)3]− was a
crucial step of the reaction that formed the formatoborate-
bridged complex [TismiPrBenz]MOC(H)OB(C6F5)3, which was
structurally characterized.28 The zwitterionic phosphonium
borate salt (C6H2Me3)2P(H)CH2CH2B(H)(C6F5)2 reduces
benzaldehyde stoichiometrically to give the benzyl alcohol
derivative (C6H2Me3)2P(H)CH2CH2B(OCH2Ph)(C6F5)2.29
Repo et al. demonstrated reduction of PhCHO using a
stoichiometric amount of the borate [Me4C5NH]+[HB-
(C6F5)3]− and formation of [Me4C5NH2]+[PhCH2−O−B-
(C6F5)3]− as a result of PhCHO insertion into the B−H bond
(where Me4C5NH = 2,2,6,6-tetramethylpiperidine).30 Re-
To further verify our proposed mechanism, the isolated
benzyloxyborate intermediate [LBH]+[PhCH2−O−B-
(C6F5)3]− (S12) was used in a separate reaction in a catalytic
amount (5 mol %) for the hydrosilylation of isophthalalde-
hyde, which led to the formation of the product 3p in
quantitative yield. We anticipate that S12 first converted into
equal amounts of 3a and 1 and the re-formed catalyst 1
catalyzed the hydrosilylation of isophthalaldehyde (See Figures
S8 and S9 in the Supporting Information). This reaction
strengthens our claim of proposing [LBH]+[PhCH2−O−
B(C6F5)3]− (S12) as the key intermediate.
Quantum Chemical Computational Results on the
Catalytic Hydrosilylation of Benzaldehyde via an Anion
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1
Figure 3. Stacked H NMR (top) and 11B NMR (bottom) spectra of the stoichiometric reaction of [LBH]+[HB(C6F5)3]− (1) with PhCHO in
CDCl3 to generate S12 and the subsequent reaction of S12 with Et3SiH. The spectra in red are those of the catalyst [LBH]+[HB(C6F5)3]− (1). The
spectra in green show the characteristic signals of S12 (prepared in situ in a room-temperature reaction between 1 and PhCHO); note the
appearance of the −OCH2Ph signal and disappearance of the signal for the anion [HB(C6F5)3]−. The spectra in blue show the formation of the
product Et3Si−O−CH2Ph (3a) upon addition of Et3SiH at 40 °C and regeneration of 1. The 11B signal for the cationic B moiety was too broad to
be observed due to low symmetry around the cationic B center in comparison to the anionic B center, which gives a sharp signal due to its higher
symmetry and higher coordination number.
Mediated Pathway. The steps involved in the hydro-
silylation of benzaldehyde with catalyst 1 were studied using
computational methods. We have optimized the geometry and
examined the changes in the electrostatic potential (ESP) of
the possible intermediate steps and the final product and
calculated their free energies in the solvent phase in ground
states. The geometry optimizations, frequency calculations, and
NBO analyses of all these structures have been carried out at
the (U)M06-2X/CC-PVDZ level with CPCM as the solvent
model and CHCl3 as the solvent using the Gaussian09 suit of
program.32 The transition states were also investigated and
confirmed by the IRC calculations employed in Gaussian09.
The visualization of ESPs and MOs was done using
Chemcraft33 and Molden.34
computations reveal that S12′ (ΔG = −16.7 kcal/mol),
corresponding to the anion of the experimentally observed
intermediate [LBH]+[PhCH2−O−B(C6F5)3]− (S12), is ther-
modynamically stable and its formation takes place in spite of a
large activation barrier (ΔG⧧ = 54.7 kcal/mol) via the
transition state S11′. The formation of S12′ takes place via
hydride transfer through a four-membered cyclic transition
state with an imaginary frequency of −256.93 cm−1. The
transition state S11′ involves a weak interaction between
PhCHO and [HB(C6F5)3]−, and the driving force for its
conversion to S12′ could be due to the favorable interactions
between an electrophilic carbonyl and a nucleophilic borate
moiety (also favored by the electronegativity difference of B
and O, 2.04 and 3.44, respectively, on the Pauling scale) as also
depicted in the ESP plot of the complex between PhCHO and
[HB(C6F5)3]− (Figure S17 in the Supporting Information).
Figure 4 depicts the energy profile diagram for the optimized
intermediate species and the transition states involved. These
E
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Figure 4. Free energy diagram for the hydrosilylation of benzaldehyde with Et3SiH leading to the formation of PhCH2−O−SiEt3 via the anion
[HB(C6F5)3]− mediated pathway. The horizontal axis represents the arbitrary reaction coordinates.
31P{1H}). Owing to the Lewis acidic nature of the cationic
The next step of the reaction involves the interaction of Et3SiH
with S12’ resulting in the formation of the product 3a (−22.5
kcal/mol). We envisaged that formation of 3a occurs via a
cyclic transition state similar to that for S13 (Figure 2).
However, our efforts to locate such a transition state were not
successful. Instead, computationally we found that the
transformation is favorable in two steps, which also seems to
be consistent with our experimental observations. In this we
allowed S12′ to interact with Et3SiH, which afforded S13a′
(−11.7 kcal/mol) containing a weak Si- - -O interaction (3.73
Å) and elongated B−O bond (1.476 Å). Then S13a′ was
allowed to rearrange into the slightly more stable species S13b′
(−17.7 kcal/mol). Species S13b′ basically consists of the
complex [PhCH2−O−Et3Si- - -H- - -B(C6F5)3]−, which is even
lower in energy than S13a′, with a weak interaction of hydride
between boron and silicon. This complex easily releases the
final product PhCH2OEt3Si (3a) (ΔG = −22.5 kcal/mol) and
regenerates the borate anion [HB(C6F5)3]−. The combined
outcome of S13a′ and S13b′ is reminiscent of the anticipated
four-membered cyclic transition state S13′ involving σ-bond
metathesis. Figure S17 in the Supporting Information shows
the ESP plots of S12′, Et3SiH, S13a′, and S13b′ supporting the
interactions mentioned and the proposed transformations
discussed above. Overall, this is consistent with the
experimental observations that the anion plays the major role
in catalyzing hydrosilylation of carbonyls with catalyst 1.
Mechanism of Hydrosilylation of Carbonyl Com-
pounds with [LBH]+[B(C6F5)4]− (2) as the Catalyst. With
clear spectroscopic evidence of involvement of the anion of
[LBH]+[HB(C6F5)3]− (1) (Figure 3; vide supra) as the
hydride source in the hydrosilylation of carbonyls with the
cation possibly assisting in the Lewis acid activation of the
>CO group, it was worth investigating the same reaction
using a similar hydridoborenium cation but with the
counteranion devoid of any hydride. Therefore, the complex
[LBH]+[B(C6F5)4]− (2) was an ideal candidate for such
investigations.24 The stoichiometric reaction of [LBH]+[B-
(C6F5)4]− (2) with PhCHO at 70 °C was followed by
heteronuclear NMR spectroscopy (1H, 13C{1H}, 11B, 19F, and
boron center of 2, it can catalyze the reaction either via an
aldehyde activation pathway or a silane activation4c,14d
pathway. The NMR investigations of a mixture of 2 and
PhCHO revealed a sluggish formation of the benzyloxy
intermediate [LB−O−CH2Ph]+[B(C6F5)4]− (S22) formed
due to hydride transfer from [LBH]+ to PhCHO along with
unreacted 2 and PhCHO (Figure 5) probably forming an
Figure 5. Proposed catalytic cycle for the hydrosilylation of PhCHO
using Et3SiH and the catalyst [LBH]+[B(C6F5)4]− (2) via a Lewis
acid mediated reaction pathway.
equilibrium. S22 was easily characterized by the characteristic
PhCH2O− signal at 4.44 ppm in the 1H NMR spectrum of this
mixture. It is proposed that the formation of S22 proceeds via
the Lewis acid activation of the carbonyl group by the cationic
boron hydride moiety (shown as S21) followed by hydride
migration, a similar hydride migration to PhCHO was
observed with a cationic aluminum hydride catalyst.9a It is
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possible that S22 is labile with respect to its dissociation back
into 2 and PhCHO (Figure 5). Addition of Et3SiH to this
mixture leads to a slow consumption of S22, leading to the
formation of the product 3a. The possible pathway from S22 to
3a conversion could resemble a four-membered cyclic
structure shown as S23 that may involve σ-bond metathesis
of Si−H and B−O bonds. The structure for S23 is proposed in
view of the absence of the involvement of silylium cation (see
below) and also in view of a computed analogous transition
state of our previous report on the hydroboration of carbonyls
with a similar aluminum cation.9a The alternative silane
activation pathway (see Figure S19 in the Supporting
Information) was not supported by our observations from
multinuclear NMR measurements, as the reaction between 2
and Et3SiH did not provide evidence for the interaction of
silane with the cationic boron center of 2, thus eliminating the
possibility of formation of species such as LBH2 and silylium
species [Et3Si]+[B(C6F5)4]− or [L(H)B−H- - -SiEt3]+[B-
(C6F5)4]−. Additionally, the complete absence of deoxygenated
product rules out the involvement of the silylium cation
[Et3Si]+[B(C6F5)4]− in the reactions catalyzed by 1 or 2. This
aspect has been thoroughly established by Piers and co-
workers in their endeavor to establish the [Et3Si]+[B(C6F5)4]−-
catalyzed mechanism of ketone hydrosilylation, where
PhCH2CH3 and Et3SiOSiEt3 were the only products in
[Et3Si]+[B(C6F5)4]−-catalyzed hydrosilylation of acetophe-
none using Et3SiH as the hydride source.4c
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00646.
■
sı
General information on synthetic methods, catalytic
reactions, and characterization and computational
analyses and tables (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Sanjay Singh − Department of Chemical Sciences, Indian
Institute of Science Education and Research Mohali Knowledge
City, Mohali 140306, Punjab, India; orcid.org/0000-0003-
0806-3105; Email: sanjaysingh@iisermohali.ac.in
Authors
Sandeep Rawat − Department of Chemical Sciences, Indian
Institute of Science Education and Research Mohali Knowledge
City, Mohali 140306, Punjab, India; orcid.org/0000-0002-
1882-9285
Mamta Bhandari − Department of Chemical Sciences, Indian
Institute of Science Education and Research Mohali Knowledge
City, Mohali 140306, Punjab, India; orcid.org/0000-0001-
6510-1249
Vishal Kumar Porwal − Department of Chemical Sciences,
Indian Institute of Science Education and Research Mohali
Knowledge City, Mohali 140306, Punjab, India; orcid.org/
0000-0003-3671-1368
CONCLUSIONS
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00646
■
In conclusion, the hydridoborenium salts [LBH]+[HB-
(C6F5)3]− (1) and [LBH]+[B(C6F5)4]− (2) were found to
be good catalysts for the hydrosilylation of a variety of
aldehydes and ketones in the presence of Et3SiH. Catalysts 1
and 2 were easily obtained by hydride abstraction from LBH2
using B(C6F5)3 and [Ph3C]+[B(C6F5)4]−, respectively (L =
[{(2,4,6-Me3C6H2N)P(Ph2)}2N]). On the basis of the
observations from multinuclear NMR measurements it has
been concluded that catalyst 1 involved the activation of the
carbonyl group by hydride transfer from the borate anion
[HB(C6F5)3]− of 1 to the carbonyl carbon of benzaldehyde
that formed a stable and isolable intermediate
[LBH]+[PhCH2−O−B(C6F5)3]− (S12). The intermediate
S12 subsequently reacted with Et3SiH via σ-bond metathesis
of Si−H and B−O bonds to form the product PhCH2OSiEt3
and regeneration of the catalyst 1. In contrast to 1, the reaction
mechanism for catalyst 2 was observed to proceed through the
formation of a labile Lewis adduct between the hydridobore-
nium cation [LBH]+ of 2 and the benzaldehyde oxygen. This
adduct was thought to be responsible for the activation of
benzaldehyde leading to the intermediate [PhCH2−O−
BL]+[B(C6F5)4]− (S22), which is displaced into the product
upon reaction with Et3SiH at 70 °C. The reaction mechanism
for catalyst 1 was also studied by a computational approach
that supports our experimental observations for an anion-
mediated pathway. Our experimental observations and out-
come of the in situ NMR investigations revealed that the anion
mediated pathway of catalyst 1 is more efficient than the Lewis
acid mediated process of catalyst 2. Currently, we are in the
process of finding applications of these catalyst for other
hydrofunctionalization reactions and carrying out a detailed
investigation of their mechanisms.
Author Contributions
†S.R. and M.B. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge IISER Mohali for the NMR and HRMS
central facility. The HPC facility of IISER Mohali for providing
resources for computational studies and all other infra-
structural support are also gratefully acknowledged. The
authors thank Mandeep Kaur for useful discussions.
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