Alkali metal complexes as efficient catalysts for hydroboration and cyanosilylation of carbonyl compounds

Article abstract of DOI:10.1039/c8dt02032a

We report here reactions between the N-adamantyliminopyrolyl ligand 2-(AdNCH)-C₄H₃NH (L-H) and alkali metal hexamethyldisilazides [MN(SiMe₃)₂] (M = Li, Na and K) to afford the dimeric [{2-(AdNCH)-C₄H<

Full text of DOI:10.1039/c8dt02032a

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Alkali Metal Complexes as Efficient Catalysts for Hydroboration
and Cyanosilylation of Carbonyl Compounds
Adimulam Harinath,a Jayeeta Bhattacharjee,a Hari Pada Nayekb and Tarun K. Panda*a
Received 00th January 20xx,
Accepted 00th January 20xx
We report here reactions between the N-adamantyliminopyrolyl ligand 2-(AdN=CH)-C4H3NH (L–H) and alkali metal
hexamethyldisilazides [MN(SiMe3)2] (M = Li, Na and K) to afford the dimeric [{2-(AdN=CH)-C4H3NLi(THF)}2] (1), [{2-(AdN=CH)-
C4H3N}Na(THF)1.5}2] (2) and polymeric [{2-(AdN=CH)-C4H3NK(THF)}n] (3) complexes at ambient temperature. A one-pot
reaction between L–H, [KN(SiMe3)2] and alkaline earth metal diiodide (AeI2) in a 2:2:1 molar ratio, which resulted in the
formation of a heteroleptic Ae metal complex [2-{2-(AdN=CH)-C4H3N}2Ae(THF)2] [Ae = Mg (4), Ca (5)] is also reported. The
solid-state structures of complexes 1, 3 and 4 were established through single-crystal X-ray diffraction analysis. The alkali
and alkaline earth metal complexes 1–5 were utilised as precatalysts for the catalytic hydroboration of pinacolborane
(HBpin) with aldehydes and ketones and potassium complex 3 was identified as a competent catalyst under mild conditions.
Additionally, cyanosilylation of carbonyl compounds was explored with trimethylsilyl cyanide and aldehyde/ketones, using
the alkali metal precatalyst 3 under mild conditions. In both catalytic processes, the potassium catalyst 3 exhibited a high
tolerance towards a number of functional groups. First-order kinetics with respect to catalyst 3 and each of the substrates
was observed for the hydroboration reaction of carbonyl compounds catalysed by alkali metals.
DOI: 10.1039/x0xx00000x
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polymerisation of lactides. Additionally, 24 Hill et al., R. Muley et al.
and our group also reported active cross-dehydrogenative coupling
of NH/BH, NH/SiH, and OH/SiH using AM complexes.25-30 Although an
extensive array of transition-metal catalysts have been explored in
the hydroboration of carbonyl compounds,31 very limited AM-based
catalysts have been reported till date. Certain selected alkali and
alkaline earth metal catalysts known for hydroboration of carbonyl
compounds are given in Fig 1.32-40
Introduction
In present-day synthetic organic chemistry, the adoption of atom-
economical and eco-friendly chemical processes is one of the most
essential topics in research.1-3 In particular, finding relatively less
toxic and harmless alternatives for metals that have toxic and
detrimental effects on the environment is highly desirable.4-9
Furthermore, the use of abundant metals is encouraged, rather than
rare metals, because of the latter’s potential for becoming depleted
in future. Group 1 metals such as lithium, sodium and potassium are
recognised as being among the most abundant elements in nature,
but their application in synthetic chemistry is limited.10-15 Over the
past few years, several research groups have been keen to explore
the catalytic activity of alkali metals (AMs) in various catalytic
transformations: Gange et al. reported transesterification mediated
by AM oxides,16 the Song group explored the synthesis of alkyl
borates mediated by AM carbonates,17 Zaho et al. reported the
hydroboration of carbonyl compounds using AM hydroxides,18
Grubbs et al. reported C–H bond silylation using AM hydroxides and
AM aldoxides,19-23 while Wu et al. described a highly selective
catalyst (mediated by AM alkoxide) for the ring-opening
a. Department of Chemistry, Indian Institute of Technology Hyderabad
Kandi – 502 285, Sangareddy, Telangana, India.
Figure 1. Selected alkali and alkaline earth metal catalysts reported
E-mail: tpanda@iith.ac.in
in literature.
b. Department of Applied Chemistry, Indian Institute of Technology (ISM) Dhanbad,
826004, Jharkhand, India
In our ongoing research on alkali and alkaline earth metal chemistry,
we have developed a wide range of related catalyst systems for
various organic transformations.41-43 In continuation of that
Electronic Supplementary Information (ESI) available: Text giving X-ray diffraction
analysis, crystallographic table, 1H, 13C{1H}, 11B {1H} and spectra of products obtained
from catalytic reaction are given in Supporting Information.. See
DOI: 10.1039/x0xx00000x
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research, we wanted to introduce a competent AM catalyst for the resonance signals of the pyrrolyl protons of all the above-mentioned
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45
DOI: 10.1039/C8DT02032A
hydroboration of carbonyl compounds, which exhibits greater alkali metal complexes appeared in the expected region. The
efficiency and higher tolerance towards various active functional 13C{1H} NMR spectra also indicated the presence of an imine carbon
groups. In this context, we report here the synthesis and structural atom in each complex, which exhibited resonance signals at δc 151.6
details of N-adamantyl iminopyrolyl alkali metal complexes of (for 1), 151.6 (for 2) and 146.8 ppm (for 3), and the values were
molecular formulae [{2-(AdN=CH)-C4H3NLi(THF)}2] (1), [{2-(AdN=CH)- similar to that of the free ligand (146.4 ppm for 1–H).
C4H3N}Na(THF)1.5}2] (2) and [{2-(AdN=CH)-C4H3NK(THF)}n] (3) and
heteroleptic alkaline earth (Ae) metal complex [2-{2-(AdN=CH)-
C4H3N}2Ae(THF)2] [Ae = Mg (4), Ca (5)]. We also describe the catalytic
efficiency of the potassium complex 3 as a pre-catalyst for the
catalytic hydroboration and cyanosilylation with aldehydes and
ketones.
Results and discussion
Syntheses and structures of alkali metal complexes. The N-
adamantyl iminopyrolyl ligand L–H was prepared by the
condensation reaction between adamantylamine and 2-pyrrole
carboxaldehyde.44 The solid-state structure of the hydrochloride salt
Figure 2. Solid-state structure of lithium complex 1. Selected bond
lengths (Å) and angles (deg) are given. Li1–N1 2.141(3), Li1–N7i
2.106(5), Li1–N1i 2.073(5), Li1–O5 1.940(5), Li1i–N1 2.073(3), N7–Li1i
of the ligand (L–H.HCl) is given in the supporting information. Alkali
2.106(3), N1–Li1-N1i 104.77(14), O5–Li1–N1 107.70(14), N7i–Li1i -N1
120.24(14), N1–Li1–Li1i 53.58(11), O5–Li1–N7i 121.97(15), N1i–Li1–
Li1i 53.58(11), N7i–Li1–Li1i 112.55(18).
metal (Li, Na and K) complexes with molecular formulae [{2-
(AdN=CH)-C4H3NLi(THF)}2] (1), [{2-(AdN=CH)-C4H3N}Na(THF)1.5}2] (2)
and [{2-(AdN=CH)-C4H3NK(THF)}n] (3) were prepared from the
reactions between the protic ligand L–H and [MN(SiMe3)2] (M = Li,
Na, K) in THF through elimination of the volatile
hexamethyldisilazane (Scheme 1). All three complexes were
characterised using spectroscopic and analytical techniques, and
their solid-state structures were confirmed by single-crystal X-ray
diffraction analysis.
Single crystals of complexes 1–3 were obtained from a mixture of
each corresponding complex (with THF) and n-pentane solution. The
molecular structures of 1–3 confirm the attachment of ligand L to the
lithium, sodium and potassium ions respectively. The sodium
complex 2 has higher R factors due to poor X-ray diffraction data set,
and the molecular structure of complex 2 in the solid state is given in
the supporting information (Fig FS2). The lithium complex 1
crystallises in the monoclinic space group P 21/c, with two molecules
in the unit cell. However, the analogous potassium complex 3
crystallises in the triclinic space group P -1, with two molecules in the
unit cell. The details of the structural parameters are given in Table
TS1 in the ESI. The solid-state structures of complexes 1 and 3 are
shown in Figures 2 and 3 respectively.
Molecular structures of the lithium complex 1 and sodium complex 2
(Fig FS 2 in ESI) were found to be quite similar, as well as being
dimeric in nature, when compared to the solid-state structure of the
potassium complex 3, which is polymeric in nature. In both the
dimeric complexes 1 and 2, the coordination polyhedron was formed
by the ligation of the pyrrolylide and imine nitrogen atoms of the
monoanionic bis-iminopyrrolyl ligand L to two alkali metal ions. We
observed that ligand L formed a diamond-shaped M2N2 core in each
case, in which the pyrrolylide nitrogen atom was chelated to the two
metal ions in μ2 coordination mode. In addition, the imine nitrogen
of each ligand moiety coordinated to each metal ion, which also
coordinated to one THF molecule. Thus, each lithium ion in complex
1 was four-fold coordinated and the geometry around it can best be
described as a distorted tetrahedral. In sodium complex 2, one of the
sodium ions coordinated additionally with two THF molecules to
make the two sodium ions non-equivalent. A distorted tetrahedral
Scheme 1. Synthesis of lithium, sodium and potassium complexes 1–
3.
All the three complexes 1–3 show sharp resonance signals at H 8.26
(for 1), 7.96 (for 2) and 8.16 ppm (for 3) in the 1H NMR spectra
measured in C6D6 due to the presence of an imine proton (N=CH) in
the ligand moiety. Two multiplet signals, at H 1.43–1.40 and 3.59–
3.56 ppm, were observed in each complex, which confirmed the
presence of coordinated THF molecules in each compound. The
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geometry was observed around one sodium ion which was four-fold Scheme 2. Synthesis of Ae metal complexes 4–5.
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DOI: 10.1039/C8DT02032A
coordinated, whereas a distorted bipyramidal geometry could be
obtained around the second sodium ion which was five-fold
coordinated. Similar coordination for lithium and sodium are quite
well known in literature.46
In the 1H NMR spectra of complexes 4 and 5, measured in C6D6, the
disappearance of signals at H 10.58 ppm, assigned for pyrrole -NH
proton, indicates the formation of monoanionic ligand fragments L
in each complex (Figures FS5–FS12 in ESI). The resonance of the
imine proton was observed as a singlet at H 8.03 ppm (4) and 7.66
ppm (5). In the 13C{1H} NMR spectra, resonance at c 146.8 ppm (for
4) and 145.8 ppm (for 5) can be assigned to the imine carbon (HC=N)
present in the ligand moiety. However, these values are in
accordance to that of the free ligand L–H (146.4 ppm).
Figure 3. Solid-state structure of complex 3. Selected bond lengths
(Å) and angles (deg) are given. K1–N1 2.796(3), K1–N2i 2.863(3), K1–
O1i 2.823(3), K1–O1i 2.823(3), K1–K2 3.6717(6), K2–N1 2.786(4), K2-
N2 2.854(3), K2-O2 2.8133(3), N1–K2-K1 131.06(6), N1–21–O1
91.68(10), O1–K2 -K1 130.55(7), O1–K2–N2 87.92(9), O1–Lk2–C5
102.66(9), O1i–K1–K1i 49.33(6), N1–K2–N2 117.02(10).
The solid-state structure of complex 3 was found to be polymeric in
nature (Fig 3). The molecular structure of complex 3 confirms the 4
coordination of the iminopyrrolide ligand towards two adjacent
potassium ions. In complex 3, both the pyrrolide and imine nitrogen
atoms are chelated to two potassium ions through a bridging
coordination mode. One THF molecule is also coordinated to two
metal ions trough bridging modes, making each potassium ion six-
fold coordinated to adopt a distorted octahedral geometry around it.
In complex 3, the monoanionic iminopyrrolide ligand acts as a
bridging ligand with 4 coordination mode using its two donor
nitrogen atoms. The polymeric structure of complex 3, which
contrasts with the dimeric structure of complexes 1 and 2, can be
attributed to the size of the potassium ion, which is larger than the
lithium and sodium ions. In complexes 1–3, the M–Npyr and M–Nimin
distances are in the range of corresponding metal–nitrogen distances
reported for alkali–metal iminopyrrole complexes in literature.47
Figure 4. Synthesis of alkali metal complex 4. Selected bond lengths
(Å) and angles (deg) are given. Mg1–N1 2.143(2), Mg–N3 2.137(5),
Mg1–N2 2.268(2), Mg1–N4 2.274(2), Mg1–O1 2.172(2), Mg1–O2
2.173(2), N3–Mg1–N1 177.78(9), N3–Mg1–O1 92.74(9), N1–Mg1–O2
94.51(8), O1–Mg1–O2 83.63(8), N2–Mg1–N4 98.47(8), N1–Mg1–N4
100.15(9), O1–Mg1–N2 90.15(8).
Crystals of compounds 4 and 5 were isolated from a mixture of THF
and pentane solution at –35°C. The single-crystal X-ray data obtained
for complex 5 was not complete, and thus a satisfactory molecular
structure could not be obtained. In the molecular structure of
complex 4 in the solid state, the attachment of two iminopyrrolide
ligands to the magnesium ion was confirmed. Complex 4 crystallises
in the monoclinic space group P 21/c, with four molecules in the unit
cell. The details of the structural parameters are given in TS1 in the
supporting information. Figure 4 represents the molecular structure
of complex 4. In complex 4, the magnesium ion is coordinated by two
iminopyrrolyl moieties through 2 NN mode and two THF molecules
to adopt a distorted octahedral geometry around the magnesium
ion. We noted that both the THF molecules are cis to each other. In
complex 4, we observed two sets of Mg–N distances: one short and
one long. The short bond distance of 2.143(2) Å for Mg–Npyr indicates
the presence of larger electron density over pyrrolide nitrogen.
However, the slightly elongated Mg–N distance of 2.274(2) Å
between the imine nitrogen and magnesium ion results from the
lesser electron density over imine nitrogen atom. Nevertheless, both
Mg–N distances are in agreement with reported values.48
Alkaline earth metal complexes. The heteroleptic magnesium and
calcium complexes 4 and 5 were isolated in good yield in a one-pot
reaction of L–H, [KN(SiMe3)2] and the corresponding metal diiodide
(AeI2) in a 2:2:1 molar ratio at room temperature (Scheme 2). The air-
and moisture-sensitive compounds 4 and 5 were characterised by
multinuclear NMR spectroscopic and combustion analysis
techniques. The solid-state structure of complex 4 was established
by single-crystal X-ray diffraction analysis.
Hydroboration of carbonyl compounds. After the successful
synthesis of alkali and alkaline earth metal complexes 1–5, we
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employed them as catalysts for the hydroboration of un-activated converted to 2-(cinnamyloxy)-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
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DOI: 10.1039/C8DT02032A
carbonyl compounds. The present study also allows us to compare lane with a yield of 94%, indicating that the catalyst selectively
the catalytic activity of a number of transition metal complexes and reduced the carbonyl moiety over the double bond (C=C) present in
main group metal complexes already known to effect the reduction the cinnamaldehyde (Table 2, entry 8, FS 36–38 in ESI). Aldehydes
of carbonyl compounds under benign conditions. To investigate the with heterocycles reacted rapidly with HBpin to give the desired
catalytic efficiency of complexes 1–5, we screened the elementary borate ester product in high yield within one hour (Table 2, entries
reaction using p-methoxy benzaldehyde and pinacolborane (HBpin) 12–15). To assess the functional group tolerance with respect to
with 3 mol% of the catalyst being loaded in neat condition at ambient catalyst 3, we chose the aldehydes with OH, NH2, NH and CN as
temperature. The hydroboration reaction proceeded rapidly to substrates. To our delight, 2-hydroxy benzaldehyde or 4-hydroxy
produce p-OMeC6H4CH2OBpin within 30 minutes when complexes 1 benzaldehyde could be selectively converted to the corresponding
and 2 were used as catalysts (Table 1, entries 1–2). When complex borate esters by reducing carbonyl functionality, leaving behind only
3 was used as the catalyst (Table 1, entry 3, FS 18–20 in ESI), the dehydroocupling of hydroxyl with HBpin (Table 2, entries 10–11,
complete conversion was achieved within 20 minutes. In contrast, FS 42–47 in ESI).50 Similarly, when pyrrol 2-carboxaldehyde was
the use of Ae metal complexes
4 and 5 exhibited sluggish reacted with HBpin in the presence of catalyst 3, a chemoselective
activity towards catalytic hydroboration of p-methoxy product PyCH2OBpin was obtained, leaving the NH moiety
benzaldehyde. Additionally, the reaction proceeded smoothly in unaffected (Table 2, entries 14, FS 54–56 in ESI). Analogously,
organic solvents such as THF, toluene, benzene and hexane (Table 1, corresponding chemoselective carbonyl reduced products were
entries 6–9).
isolated by the reaction of indole 3 carboxaldehyde, 2,5 pyrrole
dicarboxaldehyde with HBpin, using complex 3 as the catalyst (Table
2, entry 15–16, FS 57–62 in ESI). The reaction of
ferrocenecarboxaldehyde (as aldehyde source) with HBpin also
yielded the corresponding borate ester without affecting the
ferrocene ring (Table 2, entry 17, FS 63–65 in ESI).
Table 2. Substrate scope for the hydroboration of carbonyl
compounds catalysed by catalyst 3.
Scheme 3. Hydroboration of carbonyl compounds catalysed by
complexes 1–5.
S.
Substrate
Product
Time Yield
No.
1
(h)
<0.5
(%)
99
Table 1. Screening of the complexes 1–5 as catalysts for
hydroboration of carbonyl compounds.
2
<0.5
95
Entry
Substrate
Cat
(3 mol%)
Time
(Min)
Solvent
Conv (%)
3
1
1
92
90
1
2
3
4
5
6
7
p-OMePhCHO
p-OMePhCHO
p-OMePhCHO
p-OMePhCHO
p-OMePhCHO
p-OMePhCHO
p-OMePhCHO
1
2
3
4
5
3
3
30
30
20
30
30
30
30
Neat
Neat
Neat
Neat
Neat
THF
99
99
99
50
50
96
98
4 b
5
6
1
1
1
95
95
93
7 b
Toluen
e
8
1
94
8
9
p-OMePhCHO
p-OMePhCHO
3
3
30
30
Hexane
90
98
Benzen
e
9
1
1
94
85
Reaction conditions: catalysts 1–5 (0.03 mmol), p-methoxy benzaldehyde (1
mmol) with pinacolborane (1 mmol).
10 b
To explore the scope of the reaction, a range of aryl aldehydes were
subjected to hydroboration with pinacolborane using potassium
complex 3 (3 mol %). The results are outlined in Table 2. Aromatic
aldehydes with one or more electron-releasing groups required one
hour to yield 90–99% conversion at room temperature (Table 2,
entries 2–4, FS 18–26 in ESI). Substrates with electron-withdrawing
functional groups (F, Br, NO2) underwent smooth hydroboration
reactions to afford the corresponding borate ester within one hour
(Table 2, entries 5–7, FS 27–35 in ESI). Cinnamaldehyde could also be
11 b
1
90
12 b
13
1
1
93
95
14 b
15 b
1
1
93
92
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16b
1
90
Selective hydroboration of aldehydes over ketonVeieswaAnrtdicleoOtnhliener
DOI: 10.1039/C8DT02032A
functional groups present in the substrate is an important
17a
1
4
95
91
transformation in organic synthesis.49 To investigate the
catalyst selectivity between aldehydes and ketones, we
subjected stoichiometric amounts of benzaldehyde and
acetophenone to one equivalent of HBpin using catalyst loading
18 a,b
19a
3
95
of
3 mol% and observed 95% selectivity in aldehyde
hydroboration (Scheme 5, entry 1, FS 94–96 in ESI). Analogous
results were obtained when 4-fluoro benzaldehyde and 4-
chlorobenzophenone were simultaneously reacted with HBpin
in 1:1:1 molar ratio (Scheme 5, entry 2, FS 97–99 in ESI). The
selectivity of the aldehyde over ketone could also be achieved
through the intramolecular hydroboration reaction of 4-acetyl
benzaldehyde with one equivalent HBpin in the presence of
20 a
3
3
96
93
21 a,b
22 a,b
3
95
23 a
3
3
95
95
catalyst
3 to give 4-CH3COC6H4CH2OBpin exclusively (Scheme 5,
entry 3, FS105-107 in ESI). The reaction of N-(4-formylphenyl)
acetamide and 4-formylphenyl acetate with HBpin yielded the
corresponding mono-borate ester by selective reduction of the
aldehyde functional group over the amido and ester groups
(Scheme 5, entries 5–6, FS108–113 in ESI). The methodology
was successfully utilised for the selective reduction of aldehyde
over a number of potential reducing groups present in steroid
molecule 16-dehydropregnolone acetate (Scheme 5, entries 7,
FS 114–116 in ESI).
24 a,b
25 a,b
26 a,b
3
4
85
90
27 a,b
4
93
a
Reaction conditions: catalyst 3 (3 mol%), 5 mol% catalyst loading,
b
reaction carried out at 30C. 0.5 ml toluene used as a solvent. Yields
are determined by using 1H NMR (400 MHz) integration of
characteristic product signal present in the reaction mixtures.
a
We wanted to extend this protocol for hydroboration of ketones, and
during our initial comparison between acetophenone and HBpin, we
observed that while 60% conversion of substrate occurred in one
hour, three hours were required for the near-quantitative conversion
(93%) of acetophenone, indicating that ketone hydroboration was
relatively slower than that of aldehydes (Table 2, entry 19). However,
aromatic ketones could be converted to the corresponding borate
ester through a longer reaction time (three–four hours). The
aromatic ketones with electron-releasing as well as electron-
withdrawing functional groups underwent smooth reaction with
HBpin to yield the corresponding reduced products within four hours
at ambient temperature (Table 2, entries 20–21). To test the
tolerance of catalyst 3 (5 mol%), 2-amino-, 3-amino- and 4-
aminoacetophenone or 2-amino benzophenone were reacted with
HBpin to yield the corresponding chemoselective borate ester
product. In each case, no formation of cross-dehydrocoupling
product50 between amine and borane substrates (Table 2, entries
22–25) was observed. However, the reaction with di-keto
compounds, such as benzil or acenaphthoquinone, with two
equivalents of HBpin was slightly sluggish, and required four hours in
room temperature to afford the doubly hydroborated product in
good yield (Table 2, entry 26–27, FS 90–95 in ESI). There was no
evidence of the formation of the singly hydroborated product in
either reaction.
Scheme 5. Selective hydroboration of aldehydes over ketones
catalysed by complex 3.
Plausible mechanism. The traditional mechanism involves the
formation of an active metal hydride which acts as the active catalyst
for the hydroboration reaction for carbonyl compounds.51–63 In
contrast to this, recently, Sen et al. and Ma et al. reported the use of
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calcium iodide and ytterbium iodide complexes respectively as
catalysts for the hydroboration of aldehydes as well as ketones, using
the Lewis-acidic nature of metal towards the carbonyl compound,
rather than formation of discrete metal hydride species. Thus, it is
useful to elucidate on the mechanistic pathway of our process of
hydroboration of aldehydes and ketones, as mediated by catalyst 3.
Based on the kinetic study, a plausible mechanism for the
hydroboration of aldehyde and ketones is proposed in Scheme
4. In the first step of the proposed mechanism, the alkali metal
View Article Online
In literature a wide range of catalystDhOaI:v1e0.1b0e39e/nC8rDeTp0o20rt3e2dA
including alkali metal fluorides for cyano sillylation of carbonyl
compounds. 65-69 After screening the efficiency of catalyst
3 for
the hydroboration reaction, we were interested in probing our
catalyst further by addition of trimethylsilyl cyanide to the
carbonyl compounds to produce corresponding cyanohydrins.
The initial reaction was performed using one equivalent of
benzaldehyde with one equivalent of TMSCN in the presence of
3 mol% of catalyst 3. We observed that quantitative conversion
ion of complex
an active catalytic species
3
polarises the carbonyl oxygen moiety to give
, which is well known in literature.64
was accomplished within 30 minutes (Table 3, entry 1, FS 120–
121 in ESI). Additionally, the use of various solvents did not
impede this reaction. Moreover, to explore the substrate scope,
we conducted the same reaction using a number of carbonyl
compounds with electron-donating and electron-withdrawing
I
In the next step, a nucleophilic attack occurs on the electron-
deficient carbonyl carbon atom by the incoming hydride,
leading to the formation of the four-membered transition state
(
II), which undergoes  bond metathesis to give another
functional groups. Catalyst
3 exhibited excellent tolerance
intermediate III. In the final step, III reacts further with another
molecule of ketone to give the hydroborate product
towards these functionalities and yielded 80–95% of the
product (Table 3, entries 2–7, FS 122–133 in ESI) under optimum
conditions. Not surprisingly, the cyanosilylation reaction of
ketones took slightly longer to yield near-quantitative
conversion (1–1.5 hours, Table 3, entries 10–12, FS 138–143 in
ESI). Moreover, the reaction of 2-pyrrole carboxaldehyde,
indole 3-carboxaldehyde, and 2,5 pyrrol dicarboxaldehyde with
TMSCN led to the chemoselective cyanosilylation of carbonyl
moiety, leaving behind a free amine NH group in each case
(Table 3, entries 8, 9 and 13, FS 134–137 and143–144 in ESI).
RR’CHOBpin under immediate regeneration of active species .
I
Table 3. Substrate scope for the cyanosilylation of carbonyl
compounds catalysed by catalyst 3.
S.
No.
1
Substrate
Product
Time
Yield
30
30
99
99
2
3
30
45
99
99
4 a
Scheme 4. Proposed catalytic cycle for hydroboration of
carbonyl compounds catalysed by alkali metal complexes
5
6
30
40
45
99
99
99
1–3.
For greater insight into the mechanism of the reaction, we
carried out two control reactions of excess 2-methoxy-
7 a
benzaldehyde and pinacolborane with catalyst
and observed no significant changes under similar reaction
conditions (FS 117–119), indicating that catalyst is quite stable
in the presence of the two substrates. To eliminate the
possibility of the formation of KH as the active species by the
3 separately,
8
50
60
99
99
3
9 a
reaction of alkali metal complex 3 and HBpin, we performed the
10
11
60
70
99
99
catalytic hydroboration reaction between 2-hydroxy-
benzaldehyde and HBpin by using KH or KN(SiMe3)2 under
similar reaction conditions. In both cases, we observed the
formation of cross dehydrocoupled (CDC) product C6H4CHO-2-
OBpin as the sole product (FS 165 in ESI), indicating no presence
of KH or KN(SiMe3)2 in the catalytic hydroboration of carbonyl
compounds as proposed in Scheme 4.
12 a
13a
90
80
99
99
Cyanosilylation of carbonyl compounds.
6 | J. Name., 2012, 00, 1-3
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a
Reaction conditions: Catalyst 3 (3 mol%), reaction carried out at 30C 0.5
ml toluene used as a solvent and yields were determined by using 1H NMR
spectroscopy.
catalytic hydroboration of a wide range of aldehydes and ketones,
View Article Online
DOI: 10.1039/C8DT02032A
thus manifesting its versatility as a pre-catalyst for this reaction.
Complex 3 has also exhibited unusual tolerance towards active
functional groups such as -NH, -OH, -S, -CN, -OMe, -F, -Cl, etc. at
ambient temperature. Further, complex 3 is an active catalyst for the
cyanosilylation reaction of carbonyl compounds. The proposed
mechanism, based on controlled reactions, suggests the possible
formation of a Lewis acid-base adduct by the metal ion and carbonyl
compound as the transition state, excluding the formation of metal
hydride. The kinetic studies suggest that the hydroboration reactions
displayed first-order kinetics with respect to the catalyst 3.
Scheme 6. Selective cyanosilylation of aldehydes over ketones
catalysed by complex
3.
Experimental
General considerations.
To investigate the selectivity of catalyst
3, we performed a
reaction with 4-acetyl benzaldehyde with one equivalent of All manipulations of air-sensitive materials were performed under
inert atmosphere in flame-dried Schlenk-type glassware, either on a
dual manifold Schlenk line interfaced with a high vacuum (10−4 Torr)
or in an argon-filled M. Braun glovebox. 1H NMR (400 MHz), 11B NMR
(128.4 MHz) and 13C{1H} (100 MHz) spectra were recorded on a
BRUKER AVANCE III-400 spectrometer. Pyrrole-2-carboxaldehyde,
adamantylamine, aldehydes and ketones and trimethylsilyl cyanide,
KN(SiMe3)2, NaN(SiMe3)2, LiN(SiMe3)2, MgI2 were purchased from
Sigma Aldrich. Amines were distilled over CaH2 prior to use. NMR
solvent (CDCl3 and C6D6) was purchased from Sigma Aldrich and C6D6
was dried over Na/K alloy.
TMSCN in the presence of 3 mol% of catalyst and we obtained
a 95% yield of 4-acetylphenyl-2-((trimethylsilyl)oxy acetonitrile,
indicating that the aldehyde group was selected over the ketone
group for cyanosilylation (Scheme 6, entry 1, FS 146–147 in ESI).
Analogous results were observed when 4-formyl benzoate, N-
(4-formyl-phenyl)acetamide and 4-formyl phenylacetate was
reacted with TMSCN to yield the corresponding cyanohydrin by
selective reduction of the aldehyde functional group over the
amido and ester groups (Scheme 6, entries 2-3, FS 148–151 in
ESI).
Synthesis of [2-(AdN=CH)C4H3NH] (L–H). To an ethanol solution
Sequential
cyanosilylation-hydroboration
of
4
acetyl
(15 mL) of pyrrole-2-carboxaldehyde (2.0 g, 21.0 mmol), ethanol
solution of adamantylamine (3.2 g, 21.0 mmol), was added
while stirring. The reaction was stirred for another eight hours
at room temperature. The solution was evaporated under
reduced pressure to obtain a colourless solid. The compound
was re-crystallised in hot ethanol to give colourless crystals.
benzaldehyde. Sequential cyanosilylation-hydroboration of
dicarbonyls was examined, using one equivalent of 4-acetyl
benzaldehyde with 1 equivalent of TMSCN in the presence of 3 mol%
of catalyst 3 to yield the corresponding cyanohydrin product by
selective reaction of the aldehyde group at room temperature.
Moreover, the addition of one equivalent of HBpin and 5 mol% of
catalyst 3 to the cyanohydrin caused the reaction of the ketone with
HBpin at 30C and conversion to cyanohydrin borate ester (FS 152–
154 in ESI). However, a mixture of products was obtained when other
dialdehydes and diketones were used as substrates under similar
conditions.
1
Yield: 4.5 g (95%). H NMR (400 MHz, CDCl3): δH 10.50 (b, 1H,
NH), 8.17 (s, 1H, N=C-H), 6.82 (s, 1H, Py-H), 6.51 - 6.49 (m, 1H,
Py-H), 6.24 - 6.23 (m, 1H, Py-H), 2.04 (m, 3H, Ad-H), 1.81 -
1.69(m, 12H, Ad-H) ppm. 13C-{1H} NMR (100 MHz, C6D6): δC
146.4 (PyC=N), 131.2 (Py-C), 121.3 (Py-C), 113.5 (Py-C), 109.3
(PyC), 56.9 (Ad-C), 43.3 (Ad-C), 36.6 (Ad-C), 29.6 (Ad-C) ppm.
(C15H20N2) (228.3). Calc. C 78.90, H 8.83, N 12.27; found C 78.27
H 8.56, N 11.79.
Synthesis of [{2-(AdN=CH)C4H3NLi(THF)}2] (1). In a 10 mL
sample vial, 50 mg (0.219 mmol) of
L–H and 37 mg of
[LiN(SiMe3)2] (0.219 mmol) were mixed together with 5 mL of
THF. After six hours of stirring at room temperature, a small
amount of THF (2 mL) and n-pentane (2 mL) were added to it
Scheme 7. Sequential cyanosilylation/hydroboration of 4-acetyl
benzaldehyde catalysed by complex 3.
and kept at −35
C. Colourless crystals of complex 2 were
obtained after two days.
1
Yield: 60 mg (89%). H NMR (400 MHz, C6D6): δH 8.26 (s, 2H,
N=C-H), 7.57 (b, 2H, Py-H), 7.05-7.04 (m, 2H, Py-H), 6.83 (b, 2H,
Py-H), 3.44 (m, THF), 2.02 (m, 7H, Ad-H), 1.73 - 1.60(m, 10H, Ad-
H), 1.43 - 1.40 (m, THF), 1.29 (m, 12, Ad-H) ppm. 13C-{1H} NMR
(100 MHz, C6D6): δC 151.6 (PyC=N), 138.4 (Py-C), 131.8 (Py-C),
115.0 (Py-C), 108.1 (PyC), 65.3 (THF), 52.6 (Ad-C), 41.4 (Ad-C),
34.3 (Ad-C), 27.5 (Ad-C) 22.8(THF) ppm. (C38H54Li2N4O2) (612.7).
Conclusions
We have successfully synthesised alkali (Li, Na, K) and alkaline earth
metal (Mg and Ca) complexes supported by N-adamantyl-
iminipyrrolyl ligand and established their molecular structures.
Potassium complex 3 has proved its competence as a catalyst for the
Calc. C 74.49, H 8.88, N 9.14; found C 73.85 H 8.55, N 8.69
.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal Name
Hydroboration of aldehydes or ketones was carried VoieuwtAurtsicilnegOntlhinee
following standard protocol. In the glove boxD, tOhIe: 1c0h.1o0s3e9n/Cp8reD-Tc0a2t0a3ly2sAt
(0.03–0.05 mmol) was loaded into a Schlenk tube, and subsequently,
HBpin (pinacolborane) (1 mmol) followed by aldehyde or ketone (1
mmol) was added. The reaction was stirred in an oil bath at the
desired temperature (30°C). Substrate conversion was monitored by
examination of the 1H NMR, which indicated the formation of a new
CH2 peak for aldehydes and CH peak for ketones.
Synthesis of [{2-(AdN=CH)-C4H3N}Na(THF)2}2] (2). In a 10 mL
sample vial, 50 mg (0.219 mmol) of L–H and 40.7 mg (0.219
mmol) of [NaN(SiMe3)2] were mixed together with 5 mL of THF.
After six hours of stirring, a small amount of THF (2 mL) and n-
pentane (2 mL) were added to it and kept at −35
C. Colourless
crystals of complex were obtained after two days.
2
1
Yield: 75 mg (92%). H NMR (400 MHz, C6D6): δH 7.95 (s, 2H,
N=C-H), 6.46-6.45 (m, 2H, Py-H), 6.33 - 6.32 (m, 2H, Py-H), 6.24
- 6.22 (m, 2H, Py-H), 3.59 - 3.55 (m, THF), 2.03 (m, 6H, Ad-H),
1.76 - 1.76 (m, 12H, Ad-H), 1.66 - 1.61 (m, 12H, Ad-H), 1.43 -
1.40 (m, THF) ppm. 13C-{1H} NMR (100 MHz, C6D6): δC 151.6
(PyC=N), 138.4 (Py-C), 131.8 (Py-C), 115.0 (Py-C), 108.1 (PyC),
65.3 (THF), 52.6 (Ad-C), 41.4 (Ad-C), 34.3 (Ad-C), 27.5 (Ad-C)
22.8 (THF) ppm. (C42H62Na2N4O3) (716.9). Calc. C 70.36, H 8.72,
N 7.81; found C 69.68 H 8.31, N 7.41.
Typical procedure for TMSCN addition to carbonyl compounds:
TMSCN addition of carbonyl compounds was carried out using the
following standard protocol. In the glove box, the chosen pre-catalyst
(0.03 mmol) was loaded into a Schlenk tube, and subsequently,
TMSCN (1 mmol) followed by aldehyde or ketone (1 mmol) was
added. The reaction was stirred in an oil bath at the desired
temperature (30°C).
Synthesis of [{2-(AdN=CH)C4H3NK(THF)}n] (3). In a 10 mL
sample vial, one equivalent (50 mg, 0.219 mmol) of L1 and one
equivalent of [KN(SiMe3)2] (43.8 mg, 0.219 mmol) were mixed
together with 5 mL of THF. After six hours, a small amount of
THF (2 mL) and n-pentane (2 mL) were added to it and kept at
Conflicts of interest
There are no conflicts to declare.
−35
days.

C. Colourless crystals of complex
3
were obtained after two
Acknowledgements
1
Yield: 70 mg (91%). H NMR (400 MHz, C6D6): δH 8.16 (s, 1H,
N=C-H), 7.49 (m, 1H, Py-H), 6.96-6.95 (m, 1H, Py-H), 6.75 (m, 1H,
Financial support from the JICA FRIENDSHIP Project under the
Py-H), 3.34-3.31 (m, THF), 1.92 (m, 3H, Ad-H), 1.63-1.62(m, 6H, Collaboration Kick-starter Programme (CKP) and Instrumental
Ad-H), 1.43-1.40 (m, THF), 1.20 - 1.17 (m, 6H, Ad-H) ppm. 13C-
support from the Indian Institute of Technology Hyderabad is
{1H} NMR (100 MHz, C6D6): δC 146.1 (PyC=N), 132.4 (Py-C), 114.4
gratefully acknowledged. A.H. and J.B. thank CSIR and UGC India
(Py-C), 110.3 (PyC), 68.4 (THF), 57.5 (Ad-C), 44.2 (Ad-C), 37.4
respectively for their PhD fellowships. We thank Prof. Kazushi
(Ad-C), 30.6 (Ad-C), 26.3 (THF) ppm. (C19H27KN2O) (338.5). Calc.
Mashima and Dr. Hayato Tsurugi, Osaka University, Japan, for their
C 67.41, H 8.04, N 8.28; found C 66.59 H 7.71, N 7.65.
generous support, and Dr. A. K. Mishra, IIT Hyderabad, for fruitful
discussion.
Synthesis of [
Ca (5)]. Inside the glove box, in a 25 mL pre-dried Schlenk flask,
compound (50 mg, 0.219 mmol) was mixed with 44 mg of

2-{2-(AdN=CH)-C4H3N}2Ae(THF)2] [Ae = Mg (4),
L–H
Notes and references
[KN(SiMe3)2] and 31 mg MgI2 (0.1095 mmol) or 32 mg CaI2
(0.1095 mmol) in 10 mL THF solvent at ambient temperature
and the reaction mixture was stirred for eight hours. The white
precipitate of KI was filtered, and the filtrate was evaporated
under reduced pressure. The resulting white residue was
purified further by washing with pentane and was re-
1
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4
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: Yield: 130 mg (92%). 1H NMR (400 MHz, C6D6): δH 8.03 (s, 2H,
N=C-H), 6.49 (m, 4H, Py-H), 6.26-6.24 (m, 2H, Py-H), 3.59-3.55
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: Yield: 135 mg (93%). 1H NMR (400 MHz, C6D6): δH 7.98 (s, 2H,
N=C-H), 6.47 (m, 2H, Py-H), 6.35 (m, 2H, Py-H), 6.25-6.24 (m,2H,
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DOI: 10.1039/C8DT02032A
Alkali Metal Complexes as Efficient Catalysts for Hydroboration and
Cyanosilylation of Carbonyl Compounds
Adimulam Hrinath,a Jayeeta Bhattacharjee,a Hari Pada Nayek,b and Tarun K. Panda*a
Graphical Abstract
Catalytic hydroboration of aldehydes and ketones with pinacolborane (HBpin) and catalytic
cyanosilylation of carbonyl compounds with trimethylsilyl cyanide using alkali metal (Li, Na, K)
complexes as precatalyst under mild conditions are reported.