A cationic aluminium complex: An efficient mononuclear main-group catalyst for the cyanosilylation of carbonyl compounds

Article abstract of DOI:10.1039/c7dt01760j

A structurally characterized cationic aluminium complex [(AT)Al(DMAP)]⁺[OTf]^- (3) stabilized through a relatively nonbulky aminotroponate (AT) ligand is reported (DMAP = 4-(dimethylamino)pyridine). This compound was found to work as

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A Cationic Aluminium Complex: An Efficient Mononuclear Main-
Group Catalyst for the Cyanosilylation of Carbonyl Compounds
Mahendra Kumar Sharma,a Soumen Sinhababu,a Goutam Mukherjee,a Gopalan Rajaramanb and
Selvarajan Nagendrana*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: A structurally characterized cationic aluminium complex
[(AT)Al(DMAP)]+[OTf]− (3) stabilized through less bulky
aminotroponate (AT) ligand is reported (DMAP
=
4-
(dimethylamino)pyridine). It works as an excellent mononuclear
main-group catalyst (1 to 2 mol%) for the cyanosilylation of a
variety of aldehydes and ketones by consuming just 5 to 30 min at
room temperature.
In recent years, catalysis using compounds with earth-
abundant main-group elements is attracting the attention due
to their benign environmental nature, low cost, and so forth.1-2
In this regard, organoaluminium compounds are also studied
actively as catalysts for various organic transformations.3
Hydroboration of alkynes4 and carbonyl compounds;5-7
dehydrocoupling of boranes with amines, thiols, and phenols;4
hydrosilylation of olefins,8 carbonyl compounds,9-10 and
imines;9 ethylene polymerization;11 and cyanosilylation of
carbonyl compounds12 were achieved using various aluminium
compounds. With respect to cyanosilylation of carbonyl
compounds, [LAlH(OTf)]
catalyst known till now,13 and it takes 1-5 h with 0.1-2 mol%
catalyst loadings. Catalysts (LAl[(μ-S)(m-pyrimidine)(CH2)2]2
LAl[(μ-O)(o-C6H4)CN(C5NH4)]2 LAlH[(μ-O)(o-C4H4)CN(2,6-
iPr2C6H3)]
and LAl[(μ-NH)(o-C8SH8)(COOC2H5)]2 (L
B is the best mononuclear main-group
C
,
D
,
Chart 1. Aluminium catalysts for the cyanosilylation of carbonyl compounds
E
F
=
HC{C(Me)N(Ar)}2, Ar = 2,6-(i-Pr)2C6H3; Tf = SO2CF3) take 3-6 h
with 1-2 mol% catalyst loadings to afford corresponding
cyanohydrin trimethylsilyethers.14 Germanium(II) cyanide [t-
binding of substrates to the cationic aluminium atom. Though
this vision is attractive, it is a challenge as the less bulky ligands
may not offer enough stability to the cationic aluminium atom.
Nevertheless, we were able to synthesize such a cationic
Bu2(ATI)GeCN]
H (ATI = aminotroponiminato ligand) works as a
aluminium compound [(AT)Al(DMAP)]+[OTf]−
(3) through the
catalyst (1 mol%), and cyanosilylates a few aliphatic aldehydes
within 45-135 minutes.15 To further improve the efficiency of
cyanosilylation reactions of carbonyl compounds using main-
group catalysts, we planned to employ a cationic aluminium
catalyst stabilized by a less bulky ligand to facilitate the facile
stabilization offered by a semi-bulky aminotroponate (AT)
ligand that binds through nitrogen and oxygen atoms. With
this successful isolation, here we demonstrate compound
3 as
the most efficient mononuclear main-group catalyst for the
cyanosilylation of carbonyl compounds. Using 1-2 mol% of
compound
cyanosilylated in 5-30 min at room temperature.
The cationic aluminium compound was isolated according to
3, a variety of aldehydes and ketones are
3
the following three-step synthetic route. In-situ lithiation of 2-
n
ibutylaminotropone using one equivalent of BuLi, followed by
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the addition of 0.5 equivalent of AlCl3 afforded and
Journal Name
3
, singlet resonances at
as a respectively.
Structures of compounds
crystal ray diffraction studies (See the Supporting
-
77.95 and -78.04 ppm were seen,
aminotroponate stabilized aluminium(III) monochloride
yellow solid in 73% yield (Scheme 1).
1
1
and 3 were characterized by single
X
-
Information for details); they crystallized in the orthorhombic
and triclinic space groups Pnna and P1, respectively. In
compound
pyramidal [
1
, the geometry around aluminium atom is square
τ
= 0.007; τ = 1 (trigonal bipyramidal), = 0 (square
τ
pyramidal)]16 with two nitrogen, two oxygen, and a chlorine
Scheme 1. Synthesis of aminotroponatealuminium chloride 1
atoms (Figure S1; see SI). In compound
separated from the aluminium atom with a closest cation-
1 with AgOTf in anion distance of 5.426 Å between Al(1)-O(4). The lengths of
3, triflate is completely
An equimolar reaction of compound
dichloromethane at 0 C gave aluminium triflate
as brown solid (Scheme 2). Addition of 4-
(dimethylamino)pyridine to a toluene solution of compound
at room temperature afforded the cationic aluminium(III)
complex in a quantitative yield as a yellow solid (Scheme 3).
Al(1)-N(3)DMAP and average Al-O bonds are 1.963(4) and
1.830(3) Å, respectively. The average Al-Nligand bond (1.915(4)
°
2 in 97% yield
a
Å) is slightly shorter than that in compound
1 (1.940(2) Å). This
2
is due to the cationic nature of aluminium atom in compound
3.
3
Scheme 2. Synthesis of aminotroponatealuminium triflate 2
Compounds
as benzene, toluene, diethyl ether, tetrahydrofuran,
dichloromethane, and chloroform. Compounds are stable
1-3 are soluble in common organic solvents such
1-3
at room temperature under an inert atmosphere of dry
nitrogen or argon.
Figure 1. Molecular structure of compound 3. All hydrogen atoms are omitted for
clarity and thermal ellipsoids are drawn at the 40% probability level. Selected
bond lengths (Å) and angles (°): Al1-O1 1.826(3), Al1-O2 1.834(3), Al1-N1
1.907(4), Al1-N2 1.923(3), Al1-N3 1.963(4); O1-Al1-N1 83.44(2), O1-Al1-N3
94.70(1), O2-Al1-N1 93.56(1), O1-Al1-O2 171.68(2). Data collection temperature:
100 K.
Scheme 3. Synthesis of cationic aluminium complex 3
Compounds 1-3 are characterized in solution by NMR
1
spectroscopic studies. In the H NMR spectrum of compound
1
, the methyl, methine, and methylene protons of the iso-butyl
With the successful synthesis and characterization of
compound , a reaction of benzaldehyde with a slight excess
of trimethylsilylcyanide (TMSCN) and 1 mol% of catalyst in
substituents resonate as a broad singlet (0.96 ppm), a
multiplet (2.15-2.24 ppm), and a broad singlet (3.65 ppm),
respectively. Its seven-membered ring protons resonate as two
triplets (6.77 ppm and 7.13 ppm) and a multiplet (7.22-7.36
3
3
neat conditions was tried. Interestingly, quantitative
conversion of benzaldehyde to the corresponding cyanohydrin
trimethylsilylether (PhCH(CN)OSiMe3) occurred within 10 min
at room temperature (time was monitored by 1H NMR
spectroscopic analysis) (entry 1, Table 1). The same reaction
took 60 minutes when the catalyst was changed from cationic
ppm). In compound 2, iso-butyl protons resonate as a doublet
(0.91 ppm), a multiplet (2.00-2.09 ppm), and a doublet (3.64
ppm). Its seven membered ring protons appear as a triplet
(6.92 ppm) and two multiplets (7.18-7.48 ppm). Compound
3
shows three broad singlet resonances (0.84, 1.95 and 3.54
ppm) for its methyl, methine, and methylene protons,
respectively. The methyl protons of 4-(dimethylamino)pyridine
donor attached to the aluminum atom were seen as a singlet
at 3.10 ppm. Its seven-membered ring and pyridyl protons
resonate between 6.60 to 7.98 ppm. In the 13C NMR spectra of
aluminium compound
(entry 2, Table 1). This shows the efficiency of the cationic
aluminium catalyst . When the catalyst loading was reduced
to 0.5 mol% in the compound catalyzed reaction between
3 to neutral aluminium compound 2
3
3
benzaldehyde and TMSCN, 25 min was required for complete
conversion (entry 3, Table 1); Further, no reactions were
observed without the catalysts under the same reaction
conditions (entry 4, Table 1).
compounds 1, 2, and 3, ten, twelve, and fifteen signals were
observed, respectively. In the 19F NMR spectra of compounds
2
2 | J. Name., 2012, 00, 1-3
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under the same catalyst loading of 2 mol% (Table 2, entries 9-
12).
Table 1. Cyanosilylation of benzaldehyde using TMSCN with compounds 2
and 3 as catalyst.
Table 2. Cyanosilylation of ketones catalyzed by compound 3.
Entry
Catalyst
Catalyst
(mol%)
Time
Yield (%)
1
2
3
4
3
1
10 min
60 min
25 min
12 h
99
99
99
0
2
1
Catalyst
(mol%)
Time
(min)
Yield
(%)
Entry
1
Substrate
3
0.5
0
2/3
1
1
5
5
99
99
Conditions: benzaldehyde (1 mmol), TMSCN (1.2 mmol), room
temperature. Yields were obtained by 1H NMR spectroscopy.
2
3
In view of these data, further catalytic reactions were
1
1
5
5
99
99
performed using compound
and various aliphatic/aromatic aldehydes/ketones were
converted to the corresponding cyanohydrin
3 with at least 1 mol% loadings,
4
5
trimethylsilylethers in excellent yields at room temperature
(Scheme 4, Table S2, and Table 2). The reaction of aliphatic
aldehydes
iso-butyraldehyde,
iso-valeraldehyde,
1
1
5
5
99
99
propionaldehyde, n-butyraldehyde, heptaldehyde, and 2-
phenyl propionaldehyde with a slight excess of TMSCN in the
presence of catalyst
3 (1 mol%) took only 5 min at ambient
6
7
temperature to give the corresponding cyanosilylated products
in quantitative yields (Table S2, entries 1-6, respectively).
Heterocyclic
aldehydes,
furfural
and
3-
thiophenecarboaldehyde produced the corresponding
cyanohydrin trimethylsilylether (99% yield) in 10 min with 1
1
2
5
99
98
mol% of compound
3 (Table S2, entries 7 and 8, respectively).
Aromatic aldehydes with different electron withdrawing and
electron donating substituents required higher catalyst
loadings to have reaction times closer to aliphatic aldehydes.
Thus, they are converted to corresponding cyanohydrin
trimethylsilylethers (93-99% yields) in 10-20 min with 2 mol%
8
15
of catalyst
3 (Table S2, entries 10-22). With 1 mol% of catalyst
9
2
2
2
30
30
30
95
98
97
3, ferrocene carboxaldehyde took 25 min to produce the
desired cyanosilylated product in 99% yield (Table S2, entry
23).
10
11
Scheme 4. Cyanosilylation of aldehydes catalyzed by 3
12
2
25
97
Similarly, reactions of ketones with TMSCN were investigated
using catalyst
acetone,
3
. Like aliphatic aldehydes, aliphatic ketones,
2-pentanone, 2-octanone, and
Conditions: ketones (1 mmol), TMSCN (1.2 mmol), room temperature.
Yields were obtained by 1H NMR spectroscopy.
methylisopropylketone gave corresponding cyanosilylated
products quantitatively in 5 minutes with 1 mol% of catalyst
3
The higher catalytic efficiency of cationic aluminium
(Table 2, entries 1-4). This trend was also seen with cyclic
ketones, cyclopentanone, cyclohexanone, and cycloheptanone
(Table 2, entries 5-8). Unlike aromatic aldehydes, aromatic
ketones took relatively more reaction times (25 to 30 min)
compound
compounds than the most efficient aluminium catalyst
known till date may be due to the higher Lewis acidity/charge
on the aluminium atom than that in compound . To prove
3 in the cyanosilylation reactions of carbonyl
B
B
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Main-group Catalysis: A cationic aluminium complex supported by a semi-bulky aminotroponate
ligand is synthesized. It functions as an excellent catalyst for the cyanosilylation of carbonyl
compounds using trimethylsilyl cyanide (TMSCN).