Beyond Hydrofunctionalisation: A Well-Defined Calcium Compound Catalysed Mild and Efficient Carbonyl Cyanosilylation

Article abstract of DOI:10.1002/chem.201705795

Organocalcium compounds have been reported as efficient catalysts for various transformations, for cases in which one of the substrates contained an E?H (E=B, N, Si, P) bond. Here, we look at the possibility of employing an organocalcium compound for a tr

Full text of DOI:10.1002/chem.201705795

A Journal of
Accepted Article
Title: Beyond Hydrofunctionalization: A Well-Defined Calcium
Compound Catalyzed Mild and Efficient Carbonyl Cyanosilylation
Authors: Sandeep Yadav, Ruchi Dixit, Kumar Vanka, and Sakya S.
Sen
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10.1002/chem.201705795
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Beyond Hydrofunctionalization: A Well-Defined Calcium
Compound Catalyzed Mild and Efficient Carbonyl Cyanosilylation
Sandeep Yadav,[a] Ruchi Dixit,[b] Kumar Vanka,[b] and Sakya S. Sen*[a]
Dedicated to Professor Dietmar Stalke on the occasion of his 60th birthday
Abstract: Organocalcium compounds have been reported as
efficient catalysts for various transformations, for cases where one of
the substrates contained a E–H (E=B, N, Si, P) bond. Here, we look
at the possibility of employing an organocalcium compound for a
transformation in which none of the precursors has a polar E–H
bond. This study demonstrates the utilization of a well-defined
amidinatocalcium iodide, [PhC(NiPr)2CaI] (1) for cyanosilylation of a
variety of aldehydes and ketones with Me3SiCN under ambient
conditions without the need of any co-catalyst. The reaction
mechanism involves a weak adduct formation between 1 and
Me3SiCN leading to the activation of the Si–C bond, which
subsequently undergoes σ-bond metathesis with a C=O moiety.
Such a mechanistic pathway is unprecedented in alkaline earth
metal chemistry. Experimental and computational studies support
the mechanism.
reactions where neither of the substrates possess an E–H bond.
Scheme 1. The tentative catalytic cycle for organocalcium catalyzed
hydrofunctionalization of carbonyl compounds [Y=H, alkyl]. What happens if
there is no E-H bond present in the substrates?
To expand the catalytic regime of hydrocarbon soluble
well-defined calcium compounds, we have turned our attention
towards the cyanosilylation of carbonyl compounds with
Me3SiCN,[21] where none of the substrates possessed an E–H
bond. Unlike carbonyl hydroboration by heavier main group
compounds, which is increasingly being reported in the
literature,[22] the catalytic carbonyl cyanosilylation by compounds
with heavier main group elements has seen only limited success
with p-block elements from the groups of Roesky,[23-25]
Nagendran,[26,27] and others.[28,29] Furthermore, cyanosilylation of
carbonyl compounds has not been achieved thus far with any
alkaline earth metal complex. Notable advances to this end have
been made with lanthanide based polyoxometallates.[30] Our
initial entrance into the calcium chemistry was through the
preparation of soluble and easily accessible [PhC(NiPr)2CaI]
There is an increasing demand to explore efficient, sustainable
catalysts based on earth-abundant elements that can rival
precious metal catalysts in high-value transformations.[1-4]
Among various earth-abundant elements, organocalcium
compounds have recently garnered considerable interest due to
their high terrestrial abundance, low-cost, non-toxicity regardless
of concentration, and biocompatibility. A range of catalytic
processes such as hydroamination, hydrophosphination,
hydroboration, hydrosilylation, and the hydrogenation of C=C,
C=O or C=N bonds involving hydrocarbon soluble calcium
compounds has appeared in recent years through the studies
from a number of research groups of whom Harder, Hill, Roesky,
Westerhausen, Sarazin, and Ward are especially prominent.[5-20]
A common feature of all aforementioned reactions is that one of
the precursors contains an E–H bond that undergoes σ-bond
metathesis with the organocalcium compound to generate the
active catalyst. The latter undergoes an insertion reaction with
(1).[31] After discovering that
1 efficiently catalyzes the
hydroboration of aldehydes, ketones, and imines,[32] we sought
to look into the viability of cyanosilylation of carbonyl compounds
with the former to enhance the utility of 1 as a catalyst.
Incentivized by the seminal works on electrostatic activation of
multiple bonds by an early main group element by Clark and
others,[33] we hypothesized that the electrostatic interaction
between the Ca atom of 1 and Me3SiCN will activate the Si-C
bond to such a level that it would permit further nucleophilic
attack from the carbonyl moiety. Here, we describe our initial
efforts to define a catalytic, molecular cyanosilylation process
based upon a calcium complex and provide a mechanistic
appraisal based upon stoichiometric reactivity and DFT based
calculations.
the unsaturated C=X bond to generate
a species that
subsequently reacts with another molecule of E-H to form the
product and regenerate the catalyst (Scheme 1). Therefore, the
catalytic landscape of organocalcium compounds is primarily
restricted to those processes where one of the substrates
contains a polar E–H bond and is generally not known for those
[a]
S. Yadav and Dr. S. S. Sen
Inorganic Chemistry and Catalysis Division
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pashan, Pune 411008 (India)
E-mail: ss.sen@ncl.res.in
Homepage: http://academic.ncl.res.in/ss.sen
R. Dixit and Dr. K. Vanka
Physical and Material Chemistry Division
CSIR-National Chemical Laboratory
Dr. Homi Bhabha Road, Pashan, Pune 411008 (India)
Calcium compound 1 was examined as a catalyst for
cyanosilylation of a variety of aldehydes and ketones with
Me3SiCN, as illustrated in Scheme 2. A brief screening of
solvents showed that that most of the organic solvents were
suitable for the reaction. However, for aldehydes, toluene and for
ketones, THF afforded the best results. Conversion of aldehydes
was efficient at catalyst loadings of 2 mol % within 30 minutes
[b]
Supporting information for this article is given via a link at the end of
the document.
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1
(entries 2a-2p). As the experiments were monitored by H NMR
spectroscopy, most of the reactions were over before the
reaction mixture could be analyzed. In fact, a visual evaluation of
all such reactions qualitatively indicated that they were complete
in less than 10 min. Electron donating as well as withdrawing
groups were well tolerated. For the analogous reactions with
substrates including naphthalene and heterocycles, the desired
products were obtained in good yields (2k: 96%, 2l: 97%, 2m:
79%). Consistent with recent studies, 1,2-addition of Me3SiCN to
cinnamaldehyde was observed as a result of the highly
electrophilic character of the carbonyl moiety (2j). Aldehydes
were selectively and exclusively cyanosilylated in the presence
of amide (2n), acid (2o) and ester (2p).
Aromatic ketones were identified as equally suitable
substrates for cyanosilylation, demonstrated by the conversion
of acetophenone although marginally higher catalyst loadings (3
mol %) and extended reaction time (2 h) were necessary to
achieve productive conversion. Even the sterically demanding
benzophenone was converted to the corresponding
cyanosilylated product (3g) under the same conditions in 82%
yield, which had not been achieved with any other main group
catalyst thus far. The cyanosilylation of a wide variety of
aromatic ketones bearing various functional groups to the
respective cyanohydrin trimethylsilyl ethers was successful
under standard reaction conditions. The cyanosilylation of
acetophenone derivatives with electron withdrawing substituents
(3c, 3d) took places smoothly. In contrast, acetophenone
derivatives with electron donating substituents (3b, 3e, 3h) were
not found to be very suitable for productive catalysis. Among the
main group catalysts, the catalytic efficiency of 1 was found to
be higher than our previously reported silane catalyst
[PhC(NtBu)2Si(H)(Me)Cl] and comparable with other known
aluminum catalysts reported by Zhi, Nagendran, Roesky and
others.[23-27]
Scheme 2. The scope of cyanosilylation with aldehyde and ketone substrates.
Figure 1. 29Si NMR from the reaction the
reaction mixture of 1 and Me3SiCN (left). The
above NMR was taken after 30 min, when
both Me3SiCN and Int_2 were present; after
2 h all Me3SiCN was consumed and only
Int_2 was left (below).The IR spectrum of the
Int_2 is on the right.
aReaction conditions: 2 mol% catalyst, 30 min reaction at room temperature in
toluene. Yields were determined by 1H NMR spectroscopy using 1,3,5-
trimethyl benzene as an internal standard.
Efforts to gain some mechanistic insights were undertaken.
Two possible pathways may be considered for the mechanism.
By analogy to the mechanism proposed for organolanthanide
catalyzed cyanosilylation of ketones,34 the Ca-catalyst can
undergo σ-bond metathesis with Me3SiCN affording the "Ca-CN
complex" (Int_2) that acts as the catalyst for the cycle.
Alternatively, 1 can form an adduct with Me3SiCN (Int_1) and
activates the Si-C bond. As Me3SiCN is more basic than THF,
replacement of one of the THF molecules by Me3SiCN is viable.
The cycloaddition of the Si–C σ bond of the Me3SiCN fragment
to the O=C bond of the carbonyl moiety could result in the
formation of cyanohydrin. In order to understand which
mechanism is operational, a few stoichiometric reactions were
aReaction conditions: 3 mol% catalyst, 2 h reaction at room temperature in
THF. Yields were determined by 1H NMR spectroscopy using 1,3,5-trimethyl
benzene as an internal standard.
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1
undertaken. The H NMR spectrum of the 1:1 reaction of 1 and
would proceed through the second pathway, where the transition
state TS_1 would lead to the formation of the product (Pdt)
along with the regeneration of the catalyst 1. The free energy
profile with the intermediate and transition state structures are
shown in Figure S1 in the SI file.
Me3SiCN at ambient temperature shows the development of a
new SiMe3 peak at δ 0.06 ppm along with the free Me3SiCN
peak at δ 0.23 ppm. In the 29Si NMR, a new peak developed at δ
7.39 ppm with the free Me3SiCN peak at δ -11.6 ppm (Figure 1).
No peak at δ 0.38 ppm in the H NMR and δ 10.5 ppm in the
29Si NMR negate the possibility of the metathesis reaction
1
between 1 and Me3SiCN and the formation of Me3SiI during the
reaction. Monitoring the NMR after
2
h
showed the
disappearance of the Me3SiCN resonance and the presence of
only one resonance at δ 7.39 ppm in the 29Si NMR. New
resonances appeared at the 13C NMR spectrum at δ 1.59 and
127.43 ppm, which are different from those in Me3SiCN (δ -1.95
and 127.62 ppm). The new resonances are indicative of the
formation of a weakly bound adduct. The IR spectrum of the
intermediate showed a CN stretching band at ν 2067.7 cm-1 (free
Me3SiCN at ν 2192 cm-1). The decrease in stretching frequency
indicates the decrease of triple bond character, which is
anticipated due to electrostatic interaction between the C≡N
bond and the calcium atom (Figure 1, right). Taken together,
these data surmise that Me3SiI was not formed during the
catalytic cycle and suggests the possible formation of a weakly
bound adduct, Int_1. The role of the calcium complex is to pre-
orient the substrate as well as activate the C–Si bond in
Me3SiCN. By monitoring the reaction of Int_1 with 1 equivalent
1
Scheme 3. The catalytic cycle and reaction mechanism for the
cinnamaldehyde cyanosilylation reaction by catalyst 1, calculated at the
PBE/QZVP level of theory with DFT. ΔG and ΔG# represent the Gibbs free
energy of reaction and the Gibbs free energy of activation respectively. All
values are in kcal/mol.
of benzaldehyde by H NMR, we observed the formation of the
characteristic C-H resonance of the Pdt at δ 4.69 ppm
Full quantum chemical calculations have also been done
using density functional theory (DFT) at the PBE/QZVP level of
theory {for further details, please see the Supporting Information
(SI) file}. Cinnamaldehyde was chosen as the substrate for the
calculations because the obtained product yield was observed to
be highest for this case (see Scheme 2, 2j). The mechanism
obtained for this is shown in Scheme 3.
In summary, organocalcium compounds are thus far
known
to
catalyze
hydroboration,
hydroamination,
hydrophosphination, and hydrosilylation of multiple bonds. We
have introduced a well-defined organocalcium catalyst for
cyanosilylation of carbonyl compounds, where none of the
precursors contains a polar E-H bond. This is also the first report
on an alkaline earth metal catalyzed cyaosilylation reaction. The
catalyst is found to be very effective for the cyanosilyltion of a
wide range of aldehydes and ketones under ambient conditions.
As neither the precursor or nor the catalyst contains any E-H
bond, the mechanism of the reaction is different from that
proposed previously for organocalcium catalyzed reactions. Our
combined experimental and computational studies (with DFT
based calculations)lead us to conclude that the calcium complex
forms a weak adduct with trimethylsilyl cyanide, leading to
polarization of the Si-C bond and facilitates the carbonyl attack.
The observation of cyanosilylation of carbonyl compounds with 1
will not only expand the catalytic regime of calcium complexes
but reduce the gap between the chemistry of alkaline earth
metals and transition metals/lanthanides in homogeneous
catalysis.
1 can react with trimethylsilyl cyanide (Me3SiCN) to give
Int_1, where the nitrogen of the cyanide group shows a weak
interaction with the calcium of catalyst 1.This intermediate
complex, Int_1, is seen to being thermodynamically stable (ΔG =
-8.0 kcal/mol), (see Scheme 1 below). Subsequent to this, there
are two possible pathways that can be followed as the reaction
proceeds. In the first possible pathway, Int_1 can transform into
Int_2 (see FigureS1 in the SI file) via a four membered
transitionstate,TS_2, with a free energy barrier (ΔG#) of 33.1
kcal/mol and a reaction free energy (ΔG) of 2.5 kcal/mol. In the
alternative pathway, nucleophilic attack by the carbonyl oxygen
of the cinnamaldehyde can occur at the silicon centre of
Me3SiCN in Int_1. This will lead to the cyanide being transferred
from the silicon centre to the electrophilic carbonyl carbon of the
cinnamaldehyde, via a C-C bond formation reaction (see
Scheme 1). This occurs through a four membered transition
state (TS_1) and has a barrier of 28.1 kcal/mol. Therefore, a
comparison of the two competing pathways shows that the
second pathway is thermodynamically (by 3.5 kcal/mol) and
kinetically (by 5.0 kcal/mol) more favourable than the first.
Indeed, applying the Arrhenius equation to compare the rates, it
is seen that the second pathway would be approximately 4000
times faster than first. Hence, it becomes clear that the reaction
Experimental Section
Experimental Details, details of DFT calculations, and representative
NMR spectra are given in the supporting information.
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Acknowledgements
This work was supported by Ramanujan research grant
(SB/S2/RJN-073/2014). S. Y and R. D. thank CSIR, India for the
research fellowships. We thank the reviewers for their critical
suggestions to improve the manuscript.
Keywords: Ca-catalysis • cyanosilylation • DFT• transition metal
free catalysis • carbonyls
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Entry for the Table of Contents
COMMUNICATION
Moving over from catalyzing only
hydroelemenation reactions, this
work describes the first use of a well-
defined calcium compound for
cyanosilylation of a range of
aldehydes and ketones.
Spectroscopic and DFT studies
propose an unprecedented
mechanism that involves a weak
adduct formation between the
catalyst and Me3SiCN, followed by σ-
bond metathesis with the C=O group.
Sandeep Yadav, Ruchi Dixit, Kumar
Vanka, Sakya S. Sen*
Page No. – Page No.
Beyond Hydrofunctionalization: A
Well-defined Calcium Compound
Catalyzed Mild and Efficient
Carbonyl Cyanosilylation**
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