Dimeric alumatranes as catalysts for trimethylsilylcyanation reaction

Article abstract of DOI:10.1039/c7ra09851k

The solid-state structures of dimeric alumatranes with three five-membered rings chelated by [(OCMe₂CH₂)_nN(CH₂CH₂O)_3-n]^3- (n = 1, L1; n = 2, L2; n = 3, L3), which vary by the num

Full text of DOI:10.1039/c7ra09851k

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Dimeric alumatranes as catalysts for
trimethylsilylcyanation reaction†
Cite this: RSC Adv., 2017, 7, 48151
Yoseph Kim,‡a Kang Mun Lee, ‡b So Han Kim,‡a Jung Hee Moona
a
*
and Youngjo Kim
The solid-state structures of dimeric alumatranes with three five-membered rings chelated by
3ꢀ
[(OCMe2CH2)nN(CH2CH2O)3ꢀn
]
(n ¼ 1, L1; n ¼ 2, L2; n ¼ 3, L3), which vary by the number of CMe2
groups adjacent to the OH functionality [1 (L1H3), 2 (L2H3), and 3 (L3H3)], were determined by single-
crystal X-ray diffraction. The X-ray structures revealed that the aluminum geometries were slightly
distorted trigonal bipyramids. The obtained aluminum complexes are the first structurally characterized
dimeric alumatranes with tricyclic five-membered rings. Quite unexpectedly, the sterically bulky side
arms with dimethyl substituents were always located in the bridging sites, as determined by density
functional theory calculations. Their solution-state structures were analyzed by 1H, 13C, and 27Al NMR
techniques, and their gas-phase structures were determined by mass spectrometry. Unlike
Al(OCH2CH2)3N, complexes 1–3 were all dimeric in the solid state, solution phase, and gas phase. In
addition, they were found to promote the reaction of aryl, heteroaryl, and alkyl aldehydes with
trimethylsilylcyanide to provide the corresponding products in excellent yields under mild conditions of
room temperature, a short reaction time of 1 h, and a very low catalyst loading of 0.5 mol%.
Received 5th September 2017
Accepted 9th October 2017
DOI: 10.1039/c7ra09851k
rsc.li/rsc-advances
interesting point is that they used a dimeric alumatrane as the
starting material. Because ve-coordinate aluminum complexes
Introduction
are generally believed to be important intermediates in
aluminum-catalyzed reactions, ve-coordinate alumatranes could
function as Lewis acid catalysts for the trimethylsilylcyanation
reaction. Although their alumatranes are potentially excellent
Lewis acid catalysts with a loading of only 0.5 mol%,18 the reaction
time of 9–12 h should be shortened to achieve practical
application.
As shown in Chart 1, alumatranes with tricyclic ve-19–33 and
six-membered17,18,34–38 rings are well known. Like other
atranes,39–41 all alumatranes with tricyclic ve- or six-membered
ring systems also have a transannular N / Al interaction from
the bridgehead N atom in the tetradentate ligand to the Al atom.
Although alumatranes with tricyclic six-membered rings (Chart
1(b)) are monomeric or dimeric in solution and in the solid
state, Al(OCH2CH2)3N (Chart 1(a)) has been described as
Cyanohydrin trimethylsilylethers prepared via the trimethylsi-
lylcyanation of aldehydes are versatile and critical intermediates
for a-hydroxy aldehydes and b-amino alcohols.1,2 Various catalysts
for the reaction of trimethylsilylcyanide (TMSCN) with aldehydes
and ketones have been reported.3–5 Among them, aluminum-
based compounds may be one of the most attractive groups of
catalysts because aluminum is the most abundant metal in the
Earth's crust, and its complexes have relatively low toxicity and
high Lewis acidity, rendering them suitable for use as catalysts.
Some examples of aluminum-based catalysts for the efficient
transformation of this reaction have been reported.6–18 However,
high Al catalyst loadings (5–25 mol%) and prolonged reaction
times (3–72 h) are required for the completion of this reaction.6–17
Interestingly, Raders and Verkade have demonstrated that
a 0.5 mol% catalyst loading of alumatranes with three six-
membered rings could complete the trimethylsilylcyanation of
aldehydes at room temperature in only 9–12 h.18 Here, the
aDepartment of Chemistry, BK21+ Program Research Team, Chungbuk National
University, Cheongju, Chungbuk 28644, Republic of Korea. E-mail: ykim@chungbuk.
ac.kr
bDepartment of Chemistry, Institute for Molecular Science and Fusion Technology,
Kangwon National University, Chuncheon, Gangwon 24341, Republic of Korea
† Electronic supplementary information (ESI) available: NMR, IR, EI mass spectra
and computational details. CCDC 1494116–1494118. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c7ra09851k
‡ The rst, second, and third authors contributed equally to this work.
Chart 1 Types of alumatranes with (a) five- and (b) six-membered
rings.
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a dimer28 in the gas phase, a hexamer29 and octamer30 in solu-
tion, and a tetramer31 in the solid state.
Even though some examples of structurally characterized
alumatranes with tricyclic six-membered rings have been re-
ported,17,34–38 few examples of ve-membered alumatranes and
their derivatives20,31–33 have been identied.
Thus, to the best our knowledge, no studies on a structurally
characterized dimeric alumatrane with a tricyclic ve-membered
ring have appeared in the literature. In addition, examples of
ve-membered alumatranes having the same structure in the gas
phase, solution phase, and solid state have, to our knowledge, not
been reported. Herein, we report the logical synthesis, charac-
terization, X-ray structures, theoretical density functional theory
(DFT) studies, and catalytic application of dimeric alumatranes.
Results and discussion
Synthesis of new alumatrane complexes 1–3
Scheme 1 Synthetic routes for dimeric alumatranes 1–3.
As shown in Chart 2, the R1 groups of the tetradentate tris(2-oxy-
3,5-dialkylbenzyl)amine ligand in alumatranes with tricyclic six-
member rings could act as “picket fences” to prevent oligomer
formation. A similar approach was applied to make dimeric alu-
matranes with tricyclic ve-membered rings. Thus, the introduc-
tion of steric congestion in the vicinity of the hydroxyl groups of
the triethanolamine ligands could inhibit oligomer formation.
Using this idea, we have successfully prepared monomeric bora-
tranes,42 monomeric germatranes,43 and monomeric or dimeric
titanatranes.44–46 This idea further prompted us to prepare alu-
matranes with tricyclic ve-membered rings, which may be
dimeric in the gas phase, solution phase, and solid state.
The alcoholysis of AlMe3 has proven to be a useful synthetic
route for alumatranes.17–38 As shown in Scheme 1, the addition of
AlMe3 to a solution of (HOCMe2CH2)nN(CH2CH2OH)3ꢀn (n ¼ 1,
L1H3; n ¼ 2, L2H3; n ¼ 3, L3H3) in toluene gave novel alumatranes
1–3 as colorless crystals aer workup. These reactions proceeded
readily at ambient temperature, resulting in good isolated yields of
62–79%. The crude compounds were puried by washing with
n-hexane and were recrystallized in toluene. Importantly, in
contrast to aluminum complexes with Al–Me bonds,47 complexes
1–3 are very stable at room temperature for more than 1 week, even
in chloroform-d1 and benzene-d6 solutions. They are soluble in
various solvents, including toluene, chloroform, methanol, and
acetone.
modes of the ligands. X-ray-quality single crystals were obtained
from toluene solutions maintained at ꢀ20 ꢁC in a refrigerator for
a few days. The molecular structures and their selected bond
lengths and angles are shown in Fig. 1 and Table 1, respectively.
In the solid state, complexes 1–3 exist as dimers with pseudo-Ci
symmetry. Each is composed of two alumatrane units with a four-
membered Al2O2 ring linked by two Al–O bonds. To our knowl-
edge, compounds 1–3 represent the only structurally characterized
examples of an alumatrane dimer with all ve-membered rings
reported thus far. The aluminum atoms in 1–3 adopt a slightly
distorted trigonal bipyramid geometry with Oax–Al–Nax angles
(:O10–Al–N in Table 1) of 161.66(12)ꢁ in 1, 160.53(5)ꢁ in 2, and
161.25(5)ꢁ in 3. Trigonal bipyramidal and square pyramidal are
two possible coordination geometries around the metal center in
ve-coordinate systems. They could also be determined by the
trigonality parameter s (s ¼ [a ꢀ b]/60, where a and b are the
largest and next-largest interligand bond angles, respectively).48,49
The largest and next-largest interligand bond angles are :O10–Al–
N [161.66(12)ꢁ in 1, 160.53(5)ꢁ in 2, and 161.25(5)ꢁ in 3] and :O2–
Al–O3 [119.88(15)ꢁ in 1, 117.56(6)ꢁ in 2, and 120.75(6)ꢁ in 3],
respectively. Thus, the s values of 0.71 for 1, 0.72 for 2, and 0.68 for
3 means that complexes 1–3 have distorted trigonal bipyramidal
structures; the trigonality parameter s for regular trigonal bipyra-
midal complexes is 1.0, and s for perfect square pyramidal
complexes is zero.
Solid-state structures of alumatranes 1–3
All Al–O bond distances of dimeric compounds 1–3 were
˚
observed as ca. 1.74–1.86 A, which are similar to those found in
Complexes 1–3 were subjected to X-ray diffraction analysis to
determine the geometry around the central Al atoms and binding
typical pentacoordinate aluminum complexes.50,51 Moreover, two
equatorial bonds, Al–O2 and Al–O3 [1.753(3) and 1.755(3) A in 1,
˚
˚
˚
1.7604(11) and 1.7539(11) A in 2, and 1.7436(11) and 1.7601(12) A
in 3], are substantially shorter than those between the aluminum
and bridging oxygen atoms in the other equatorial Al–O1 bond
0
˚
and in one axial Al–O1 bond [1.841(2) and 1.848(2) A in 1,
˚
˚
1.8540(10) and 1.8565(10) A in 2, and 1.8593(11) and 1.8515(11) A
in 3]. The longer bridging Al–O1 and Al–O10 distances are also
observed in dimeric alumatranes with tricyclic six-membered
rings.17,33
Chart 2 Logical design of dimeric alumatranes with tricyclic five-
membered rings.
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Fig. 1 X-ray crystal structures of 1 (left), 2 (center), and 3 (right) (50% thermal ellipsoids). All H atoms and toluene molecules (for 1 and 2) are
omitted for clarity.
Table 1 Selected bond lengths (A˚) and bond angles (deg)
showed dramatic dimer-to-monomer structural changes
induced by an increase of steric hindrance in the side arm of the
1
2
3
tetradentate ligands.45 However, alumatranes are insensitive to
the effects of ligand change. Unexpectedly, the sterically bulky
side arm with dimethyl substituents in dimeric 1–3 is always
located in the bridging sites; however, the less hindered side
arms with no substituents in the dimeric titanatranes45 are
placed in the same positions.
Al–O1
1.841(2)
1.848(2)
1.753(3)
1.755(3)
1.8540(10)
1.8565(10)
1.7604(11)
1.7539(11)
2.1055(12)
120.52(6)
120.08(5)
117.56(6)
77.85(4)
102.72(5)
102.94(5)
82.68(5)
86.90(5)
86.99(5)
1.8593(11)
1.8515(11)
1.7436(11)
1.7601(12)
2.1158(14)
116.65(6)
120.85(5)
120.75(6)
78.54(5)
101.46(5)
102.45(5)
82.71(5)
87.30(5)
86.75(5)
Al–O10
Al–O2
Al–O3
Al–N
2.074(3)
O1–Al–O2
O1–Al–O3
O2–Al–O3
O1–Al–O10
O2–Al–O10
O3–Al–O10
O1–Al–N
O2–Al–N
O3–Al–N
O10–Al–N
Al–O1–Al0
121.41(14)
117.23(14)
119.88(15)
78.18(11)
101.37(12)
102.11(13)
83.53(11)
86.95(12)
87.39(13)
161.66(12)
101.81(11)
Theoretical calculations for the structures of alumatranes 1–3
Unlike compound 3, compounds 1 and 2 may have ve addi-
tional geometric isomers, i.e., 1a–1e and 2a–2e, which can be
classied by how many dimethyl-substituted bridging arms
exist (see Chart 3). Thus, dimeric alumatranes chelated by L1
could have six isomers such as 1 (with two dimethyl substitu-
ents at the bridging side arm), 1a and 1d (with one dimethyl
substituent at the bridge), and 1b, 1c, and 1e (with no dimethyl
substituents at the bridge). The same trends for 2 could be
applied. Among these structures, 1a, 1b, 2a, and 2b is the
enantiomer of 1d, 1e, 2d, and 2e, respectively. Thus, enantio-
meric pairs of 1a/1d, 1b/1e, 2a/2d, and 2b/2e have the nonsu-
perimposable mirror image.
160.53(5)
102.14(4)
161.25(5)
101.46(5)
˚
The transannular Al–Nax interaction distances of 2.074(3) A
˚
˚
in 1, 2.1055(12) A in 2, and 2.1158(14) A in 3 are slightly longer
than the sum of the ionic radii of Al3+ and N3ꢀ (2.00 A) and
52
˚
that of the covalent radii of Al and N (2.05 A).53 This means that
˚
The presence of dimethyl-substituted bridging arms in alu-
matrane isomers could play a signicant role in determining
their thermodynamic stability. To obtain the thermodynamic
stabilities for each isomers shown in Chart 3, the relative free
energy (DG/kcal molꢀ1) of the ground-state optimized structures
in the gas phase was calculated using the B3LYP functional and
6-31G(d) basis set. The structural geometries used for the
calculations were optimized on the basis of the X-ray structures
of 1 and 2, and the energy states of the isomers were given
relative to 1 or 2 because 1 and 2 were assigned to zero DG (kcal
molꢀ1). Computed free energy diagram for 1, 2, and their
possible isomers is shown in Fig. 2. Since enantiomers exhibit
identical thermodynamic stabilities, data for only one enan-
tiomer was given.
all dative N / Al coordinating bonds in 1–3 have a substantial
degree of single bond character. All Al–N bond distances in 1–3
are among the longest of those observed for other structurally
characterized alumatranes with tricyclic ve-membered (2.003–
20,31–33
17,34–38
˚
˚
2.094 A)
or six-membered rings (2.026–2.083 A).
The sum of the Oeq–Al–Oeq angles (:O1–Al–O2 + :O1–Al–
O3 + :O2–Al–O3) are 358.52ꢁ in 1, 358.16ꢁ in 2, and 358.25ꢁ in
3. Interestingly, the obtuse Oeq–Al–Oax angle [av ¼ 93.89ꢁ for 1,
94.50ꢁ for 2, and 94.15ꢁ for 3; (:O1–Al–O10 + :O2–Al–O10 +
:O3–Al–O10)/3] and acute Oeq–Al–Nax angle [av ¼ 85.96ꢁ for 1,
85.52ꢁ for 2, and 85.59ꢁ for 3; (:O1–Al–N + :O2–Al–N + :O3–
Al–N)/3] reect a displacement of the aluminum atoms toward
the bridging oxygen atoms. No direct Al–Al0 interactions occur
in 1–3.
Even though boron and aluminum, which are in the same
group of the periodic table, have similar chemical properties,
boratranes42 and alumatranes chelated by L1–L3 are mono-
meric and dimeric, respectively. In addition, titanatranes
According to Fig. 2, the DG values of 1a–1c were 1.09, 3.68 and
3.85 kcal molꢀ1, respectively, higher than that of 1 (Fig. 2, le),
distinctly indicating that the structure of 1 is the most thermo-
dynamically stable isomer among six possible ones. The thermo-
dynamic stability was proportional to the number of dimethyl-
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1a. Whereas the DG values of 2 and 2a, bearing two dimethyl-
substituted bridging arms each, differ slightly (by
0.46 kcal molꢀ1), those of 2b and 2c, which having one and no
dimethyl-substituted bridging arms, respectively, linearly
increase.
A key factor of the association between the structural
features and thermodynamic stabilities of the isomers was
found from the angle (4N) between the two unbridged arms of
each structure optimized by theoretical calculations (Table S1 in
ESI†). The 4N is dened as the angle of CH2–N–CH2 (CH2 from
the unbridged arm, see the inset gure of Table S1†), indicating
the angle strain between the two unbridged arms centered at
the N atom. These dimeric structures have two 4N values (4N1
and 4N2), and each value is dictated by the number of CH3 pairs
substituted onto the unbridged arms. Whereas the 4N between
the two unbridged arms without CH3 substituents was observed
to be ꢂ114ꢁ (4N1 and 4N2 for 1 and 4N1 for 1a, Table S1†), this
angle between the unbridged arm with CH3 substituents and
the unbridged arm without CH3 substituents was increased to
ꢂ116ꢁ (4N2 for 1a and 4N1 ꢀ 4N2 for 1b–c, Table S1†). These 4N
values are the same for 2 and 2a. Furthermore, the 4N between
both unbridged arms with CH3 substituents (in 2b or 2c) was
observed as >118ꢁ. These results distinctly indicate that the CH3
substituents on the unbridged arms increase the 4N values and
that CH3 substituents evoke angle strain centered at the N atom
in the structure of each isomer. Consequently, we found that 1
and 2 are the most thermodynamically stable structures in
comparison to the other isomers because these structures have
the smallest angle strain among the isomer structures.
Chart 3 All possible structural isomers for 1 and 2.
Unlike dimeric alumatranes 1 and 2, we recently found that
the corresponding titanatranes chelated by L1 and L2 always
had the less hindered side arms with no substituents in the
bridging sites.45 Even though titanatranes have different site
preference, such the correlation between the angle strains and
thermodynamic stabilities were exhibited once again from the
calculation results of dimeric titanatrane complexes45 (see Ti1–
Ti3 in Table S2 in ESI†). The more increasing the substituted
methyl groups on unbridged arms, the larger 4N values are (Ti1:
ꢂ106ꢁ, Ti2: ꢂ105ꢁ and Ti3: ꢂ104.6ꢁ in Table S2†) and simul-
taneously, these complexes become thermodynamically
unstable (DG of Ti1: 0.48 kcal molꢀ1 and DG of Ti2:
0.22 kcal molꢀ1 in comparison of DG of Ti3, Table S2†). These
results distinctly indicate that the angle strain between
unbridged arms in dimeric complexes can evoke those ther-
modynamical unstability.
Fig. 2 Computed free energy diagram for 1, 2, and their theoretical
isomers.
Solution- and gas-phase structures of alumatranes 1–3
Structurally characterized compounds 1–3 were also investi-
gated by 1H, 13C, and 27Al NMR spectroscopies, elemental
analysis, and electron ionization mass spectrometry (EI-MS) to
determine their solution- and gas-phase structures. All chem-
ical shis of the protons and carbons for 1–3 were within their
expected ranges. Compounds 1 and 2 may exist as isomers, as
shown in Chart 1; however, their H NMR spectra display well-
dened, sharp resonances with expected integrations. In
compound 1, the 1H NMR spectrum shows two triplets and one
substituted bridging arms in the order of 1 > a pair of enantiomers
1a/1d > a pair of enantiomers 1b/1e > 1c. The DG values of 2a–2c
were calculated to be 0.46, 1.63 and 3.46 kcal molꢀ1 higher than
that of 2. Like 1 and its isomers, the similar stability order of 2 >
a pair of enantiomers 2a/2d > a pair of enantiomers 2b/2e > 2c was
also observed. In particular, the energy states for 1b and 1c
(>3.6 kcal molꢀ1), which have no dimethyl-substituted bridging
arms, are conspicuously enhanced compared to those for 1 and
1
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Table 2 Optimization studies for the trimethylsilylcyanation reaction
singlet for the methylene protons in an integration ratio of
2 : 2 : 1 (Fig. S1 and S4 in ESI†). In addition, the H NMR for
of benzaldehyde at room temperaturea
1
compound 2 shows four singlets for the methyl protons with the
integration ratio of 1 : 1 : 1 : 1 (Fig. S10 and S13 in ESI†). These
data support only structures 1 and 2 among all possible struc-
1
tural isomers shown in Chart 3. The H and 13C NMR spectra
support dimeric structures in solution; NMR separations for the
bridging and terminal side arms were observed.
Entry
Catalyst
Mol%
Solvent
t (h)
Yieldb,c (%)
The coordination number and geometry around aluminum
correlate well with the 27Al NMR chemical shi. The 27Al NMR
spectra of 1–3 were collected with the samples dissolved in
CDCl3, and two broad peaks at approximately 6 ppm and
65 ppm were observed (Fig. S7, S16 and S28 in ESI†).54 Low-eld
signals in their 27Al NMR spectra could be denitely assigned to
the aluminosilicate peak of the NMR tube. The both penta-
coordinate alumatranes N(C6H4O)3Al–NH2CH2Ph19 and
Al(OCH2CH2)3N21 showed similar signals at 66 ppm in their 27Al
NMR spectra. Although the 27Al NMR signal for the penta-
coordinate six-membered system34 was shied downeld to
37.2 ppm, which is within the expected region from 33 to
61 ppm for monomeric ve-coordinate aluminum alkoxides,55
the 27Al NMR signal at approximately 6 ppm for 1–3 is very
similar to those for other reported pentacoordinate aluma-
tranes with tricyclic ve-membered rings.19,21 Thus, 1H, 13C, and
27Al NMR data support that alumatranes 1–3 in the solution
phase exist as pentacoordinate dimeric structures. Especially,
2D NMR (COSY, HSQC, and HMBC) spectra for compounds 2
and 3 made the proper assignment of NMR peaks (see ESI†).
The electron impact mass spectra (70 eV) of 1–3 show that
the molecular peaks of compounds 1–3 appeared at 402, 458,
and 514 m/z, respectively (Fig. S8, S20 and S32 in ESI†). In
addition, the absence of other peaks between 500 and 1000 m/z
excludes the existence of oligomeric species other than dimers.
Similar data have been reported in the literature,28 and the mass
spectra data indicate that 1–3 exist as dimeric structures in the
gas phase.
1
2
3
4
5
6
7
8
1
2
3
0.5
0.5
0.5
0.5
—
0.1
0.1
0.1
0.1
0.05
0.05
0.05
0.05
0.1
0.1
0.1
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Toluene
Toluene
Toluene
1
1
1
1
1
6
6
6
6
6
6
6
6
6
6
6
95
90
84
15
0
93
87
83
13
39
31
26
5
d
[AlL]2
—
1
2
3
d
9
[AlL]2
10
11
12
13
14
15
16
1
2
3
d
[AlL]2
1
2
3
67
59
54
a
Reaction conditions: 2 mmol benzaldehyde, 3.5 mmol TMSCN, and
b
5 mL solvent. Isolated yields aer silica-gel column chromatography
c
d
based on benzaldehyde. Average of two runs. [AlL]2 ¼ [Al(OC6H2-
2,4-Me2-6-CH2)3N]2.
temperature (entries 10–13). Polar solvent such as MeCN is better
than non-polar solvent of toluene under the same reaction
condition (entries 6–8 and 14–16).
Such an order of the catalytic activities for 1–3 is signicantly
correlated with the dissociation free energy barrier of each bond
between Al and bridged O atom. In order to exhibit efficiently
catalytic behaviors of these alumatrane complexes, the disso-
ciation of Al–O bond have to be especially well occurred for the
insertion of substrates to Al center. These energy barriers could
be calculated as the thermal stabilities between before and aer
dissociation of Al–O bonds in gas phase. The values of 1–3 were
estimated as 1740 kcal molꢀ1 for 1, 1766 kcal molꢀ1 for 2 and
1780 kcal molꢀ1 for 3, indicating that the more decreasing the
dissociation energy barrier of Al–O bond are, the more
increasing the catalytic activities show. Consequently, the
reason why the high catalytic activity in 1 could be shown
compared to 2 or 3 is the weakest bond strength between Al and
bridged O atom in 1 among those of alumatrane complexes. For
the trimethylsilylcyanation reaction, a polar solvent such as
acetonitrile is normally used to attain a high yield. As expected,
changing the solvent from polar acetonitrile to nonpolar
toluene caused a decrease of catalytic activity (entries 14–16).
Since our systems 1–3 have higher solubility in CH3CN and
toluene than [AlL]2,18 they showed higher catalytic activity than
[AlL]2. However, the maintained homogeneity aer completion
of the reaction prevents the recycling of catalysts 1–3 for this
reaction.
Catalytic activities
As shown in Table 2, to optimize the conditions for the trime-
thylsilylcyanation reaction, we used TMSCN and benzaldehyde as
model substrates in four catalytic systems: compounds 1–3 and
the previously reported alumatrane with a tricyclic six-membered
ring,18 which was used as a trimethylsilylcyanation catalyst to
synthesize 2-phenyl-2-trimethylsilyloxyacetonitrile in 92% iso-
lated yield at room temperature in 9 h. Initially, we reduced the
reaction time from 9 h to 1 h for the comparison of catalytic
activity. Under the same reaction conditions, catalyst 1 showed
the highest catalytic activity (entries 1–4). As expected, no catalytic
activity was observed when no catalyst was used (entry 5). As the
amount of catalyst was decreased from 0.5 mol% to 0.1 mol%, the
reaction time required to achieve similar catalytic activity for all
catalysts increased from 1 h to 6 h (entries 6–9). The order of the
catalytic activity did not change between catalysts; thus, catalyst 1
was determined to be the best catalytic system among the four.
Even at a low catalyst loading of 0.05 mol%, catalyst 1 showed an
isolated yield of 39% for a reaction time of 6 h at room
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Table 4 Trimethylsilylcyanation of heteroaryl and alkyl aldehydes
using catalyst 1a
With the optimized conditions of 0.5 mol% 1, 5 mL CH3CN,
rt and 1 h in hand, we examined the effect of various aryl
aldehydes (instead of benzaldehyde) on the trimethylsilylcya-
nation reaction (Table 3). The coupling of benzaldehyde (entry
1) with TMSCN efficiently generated the desired product in 95%
isolated yield. Electron-donating aldehydes such as p-tolualde-
hyde (entry 2) and p-anisaldehyde (entry 3), electron-
withdrawing aldehydes such as a,a,a-triuorotolualdehyde
(entry 4), 4-nitrobenzaldehyde (entry 5), 4-cyanobenzaldehyde
(entry 6), methyl 4-formylbenzoate (entry 7), and 4-chlor-
obenzaldehyde (entry 8) and electron-neutral aldehydes such as
1-naphthaldehyde (entry 9) and 2-naphthaldehde (entry 10)
were also effective in the trimethylsilylcyanation reaction. When
0.5 mol% 1 was employed in this reaction, three aryl alde-
Entry
1
RC(]O)H
Product
Yieldb,c (%)
91
2
3
4
5
86
93
90
83
hydes—the
highly
electron-withdrawing
a,a,a-tri-
uorotolualdehyde (entry 4) and 4-chlorobenzaldehyde (entry 8)
and the sterically hindered electron-neutral 1-naphthaldehyde
(entry 9)—resulted in only 83–85% isolated yields. Other aryl
aldehydes showed activities similar to that of benzaldehyde.
We also screened various heterocyclic and alkyl aldehydes
using the optimized trimethylsilylcyanation conditions of
0.5 mol% 1, 5 mL CH3CN, rt and 1 h (Table 4). With 2- and 3-
pyridine-carboxaldehyde
and
6-methyl-2-pyridine-
carboxaldehyde, good product yields of 91%, 86%, and 93%,
respectively, were obtained (entries 1–3). As shown in Table 4
(entries 4–6), 6,2- and 3-thiopenecarboxaldehyde and 4-methyl-
2-thiazolecarboxaldehyde also worked well in this reaction,
giving 90%, 83%, and 91% yields of the corresponding prod-
ucts, respectively. Straight-chain alkyl aldehydes (entries 7 and
8), a conjugated aldehyde (entry 9), and an aldehyde with
a sterically hindered cyclic ring (entry 10) also provided the
corresponding products in reasonably good yields of 80–87%.
A possible mechanism for alumatrane catalyzed trime-
thylsilylcyanation reaction is proposed in Scheme 2. The
6
7
8
91
80
87
9
81
81
Table 3 Trimethylsilylcyanation of various aryl aldehydes using cata-
lyst 1a
10
a
Reaction conditions: 2 mmol RC(]O)H, 3.5 mmol TMSCN, 5 mL
b
CH3CN, and rt.
chromatography based on aryl aldehyde. Average of two runs.
Isolated yields aer silica-gel column
c
Entry
ArC(]O)H
Yieldb,c (%)
catalytic cycle starts with the coordination of aldehyde to the
Lewis acidic Al center in alumatrane 1, generating neutral
monomeric alumatrane 10. Generally, dimeric alumatranes in
the presence of aldehyde could be easily converted to mono-
meric specie, which was supported by structurally characterized
monomeric alumatrane adduct with benzaldehyde obtained
from the reaction between dimeric alumatrane and benzalde-
hyde.17 In addition, we also calculated the dissociation free
energy barrier of Al–O bond as 1740 kcal molꢀ1 for 1,
1766 kcal molꢀ1 for 2 and 1780 kcal molꢀ1 for 3, indicating that
monomeric alumatrane containing aldehyde could be easily
generated. Then, a cyanide anion as a nucleophile attacks the
1
2
3
4
5
6
7
8
9
10
Benzaldehyde
p-Tolualdehyde
p-Anisaldehyde
a,a,a-Triuoro-p-tolualdehyde
4-Nitrobenzaldehyde
4-Cyanobenzaldehyde
Methyl 4-formylbenzoate
4-Chlorobenzaldehyde
1-Naphthaldehyde
2-Naphthaldehde
95
90
94
85
91
92
90
83
85
91
a
Reaction conditions: 2 mmol ArC(]O)H, 3.5 mmol TMSCN, 5 mL
b
CH3CN, 0.5 mol% 1, rt, and 1 h. Isolated yields aer silica-gel
column chromatography based on aryl aldehyde. c Average of two runs.
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and used as supplied unless otherwise indicated. Toluene, THF,
diethylether, and n-hexane were dried with sodium diphe-
˚
nylketyl and stored over activated 3 A molecular sieves. All
deuterated solvents such as CDCl3 and C6D6 (Cambridge
Isotope Laboratories) were used aer drying over activated
˚
molecular sieves (5 A).
Measurements
1H and 13C NMR spectra were recorded at ambient temperature
on a 400 MHz NMR spectrometer using standard parameters.
All chemical shis are reported in d units with reference to the
1
peaks of residual CDCl3 (d 7.24, H NMR; d 77.0, 13C NMR) or
1
C6D6 (d 7.16, H NMR; d 128.0, 13C NMR). 27Al NMR spectros-
copy was carried out at the Korea Basic Science Institute. EI-MS
was performed on a VG Auto Spec. Elemental analyses were
performed using an EA 1110-FISONS analyzer (CE Instruments).
Scheme 2 Plausible mechanism for trimethylsilylcyanation of alde-
hydes using catalyst 1.
Synthesis
(HOCH2CH2)2N(CH2CMe2OH) (L1H3), (HOCH2CH2)N(CH2-
CMe2OH)2 (L2H3), and N(CH2CMe2OH)3 (L3H3) were synthe-
sized by literature procedures.44
Synthesis of 1. To a stirred, colorless solution of L1H3 (0.35 g,
2.0 mmol) in 20 mL of THF was added AlMe3 (1.0 mL of 2 M
solution in toluene, 2.0 mmol) at 0 ꢁC. The reaction mixture was
allowed to warm to room temperature and was stirred over-
night. The residue obtained by removing the solvent under
vacuum was recrystallized in toluene. The desired product 1 was
electron-decient carbonyl carbon on the activated aldehyde to
make new ionic species 100. Finally, the production of the
desired product facilitates the regeneration of the catalyst 1.
Experimental
General considerations
All reactions of air- and moisture-sensitive materials were isolated as colorless crystals aer the solution remained at
1
ꢁ
carried out under dinitrogen using standard Schlenk-type ꢀ20 C in a refrigerator for a few days (62%, 0.25 g). H NMR
glassware on a dual manifold Schlenk line in a glove box.56 (CDCl3, ppm): d 3.75 (t, 8H, J ¼ 6.0 Hz, OCH2CH2N), 2.83 (t, 8H, J
Dinitrogen was deoxygenated using an activated Cu catalyst and ¼ 6.0 Hz, OCH2CH2N), 2.74 (s, 4H, OCMe2CH2N), 1.24 (s, 6H,
dried with drierite.57 All chemicals were purchased from Aldrich OCMe2), 1.19 (s, 6H, OCMe2). 13C NMR (CDCl3, ppm): d 68.3
Table 5 Crystallographic data for 1–3
1
2
3
Chemical formula
Formula weight
Crystal system
Space group
C
15H24AlNO3
C68H112Al4N4O12
1285.53
Monoclinic
P21/n
13.6533(6)
14.1939(6)
18.8530(8)
90.00
98.997(2)
90.00
3608.6(3)
2
C12H24AlNO3
257.30
Monoclinic
P21/n
9.4843(5)
10.5629(6)
14.1175(7)
90.00
91.340(3)
90.00
293.33
Monoclinic
P21/c
10.3390(5)
9.2969(4)
17.3082(8)
90.00
107.191(3)
90.00
˚
a (A)
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
3
˚
V (A )
1589.35(13)
4
1.226
1413.93(13)
4
1.209
Z
dcalcd (g cmꢀ3
F(000)
)
1.183
1392
632
560
Reections collected
# of independent reections
# of parameters
19 264
79 871
24 117
3343 [R(int) ¼ 0.0340]
13 281 [R(int) ¼ 0.0668]
4377 [R(int) ¼ 0.0571]
184
407
160
R1 (I > 2s(I))a
0.0903
0.2667
0.0579
0.1526
0.0513
0.1350
wR2 (I > 2s(I))b
GOF (I > 2s(I))
1.082
1.017
1.060
P
P
P
P
a
b
2
R1 ¼ ||Fo| ꢀ |Fc||/ |Fo|. wR2 ¼ { [w(Fo ꢀ Fc2)2]/ [w(Fo2)2]}1/2
.
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(OCMe2CH2N), 65.2 (OCMe2), 64.8 (OCH2), 58.0 (OCH2CH2N), glove box, and 5 mL of freshly distilled acetonitrile was added
57.8 (OCH2CH2N), 32.49 (OCMe2), 32.45 (OCMe2). 1H NMR via a syringe, resulting in a clear solution. The corresponding
(C6D6, ppm): d 3.59 (t, 8H, J ¼ 5.8 Hz, OCH2), 2.36 (s, 4H, aldehyde (2 mmol) and TMSCN (3.5 mmol) were added
OCMe2CH2N), 2.23 (t, 8H, J ¼ 5.8 Hz, OCH2CH2N), 1.34 (s, 6H, sequentially under nitrogen, and the reaction mixture was
OCMe2), 1.26 (s, 6H, OCMe2). 13C NMR (C6D6, ppm): d 69.0 stirred at room temperature. Aer 1 h, the solvent and excess
(OCMe2CH2N), 65.4 (OCMe2), 65.0 (OCMe2CH2N), 58.3 (OCH2- TMSCN were evaporated at reduced pressure on a Schlenk line
CH2N), 58.0 (OCH2CH2N), 32.8 (OCMe2), 31.9 (OCMe2). 27Al at 70 C, and then 10 mL of hexanes was added. The precipi-
ꢁ
NMR (CDCl3, ppm): d 5.08 (Dn1/2 ¼ 1978.9 Hz). EI-MS (% tated catalyst was ltered, and the crude product was puried by
intensity): m/z 402 (4.0%, M+), 387 (42%, M+ ꢀ Me), 372 (3.0%, column chromatography (5% ethyl acetate in hexane).
M+ ꢀ 2Me), 358 (20%, M+ ꢀ 3Me), 344 (100% M+ ꢀ 4Me). Anal.
calc. for C16H32Al2N2O6: C, 47.76; H, 8.02; N, 6.96. Found: C,
47.59; H, 8.21; N, 7.02.
X-ray structural determination for 1–3
The crystallographic measurements were performed at 296(2) K
for all complexes 1–3 using a Bruker APEX II diffractometer with
Synthesis of 2. In a manner analogous to that used in the
synthesis of 1, the desired product 2 was prepared as colorless
crystals from a solution of AlMe3 (1.0 mL of 2 M solution in
toluene, 2.0 mmol) and L2H3 (0.41 g, 2.0 mmol) in THF in
˚
Mo Ka (l ¼ 0.71073 A) radiation. Specimens of suitable quality
and size were selected, mounted, and centered in the X-ray
beam using a video camera. The structures were solved by
direct methods and rened by full-matrix least-squares
methods using the SHELXTL58 program package with aniso-
tropic thermal parameters for all non-hydrogen atoms, result-
ing in the X-ray crystallographic data of 1–3 in CIF formats
(CCDC 1494116–1494118). Final renement based on the
reections (I > 2s(I)) converged at R1 ¼ 0.0903, wR2 ¼ 0.2667,
and GOF ¼ 1.082 for 1, at R1 ¼ 0.0579, wR2 ¼ 0.1526, and GOF ¼
1.017 for 2, and at R1 ¼ 0.0513, wR2 ¼ 0.1350, and GOF ¼ 1.060
for 3. Further details are listed in Table 5.
1
a yield of 79% (0.36 g). H NMR (CDCl3, ppm): d 3.73 (m, 4H,
CH2N), 2.88 (m, 4H, OCH2), 2.79 (m, 10H, OCMe2CH2N), 1.46 (s,
6H, OCMe2), 1.35 (s, 6H, OCMe2), 1.17 (s, 6H, OCMe2), 1.12 (s,
6H, OCMe2). 13C NMR (CDCl3, ppm): d 71.7 (OCMe2CH2N), 68.9
(OCMe2), 68.0 (OCMe2), 67.6 (OCH2CH2N), 62.2 (OCH2CH2N),
58.4 (OCMe2CH2N), 32.2 (OCMe2), 31.8 (OCMe2) 29.9 (OCMe2),
29.5 (OCMe2). 1H NMR (C6D6, ppm): d 3.80 (m, 4H, CH2N), 2.47
(m, 4H, OCH2), 2.35 (m, 10H, OCMe2CH2N), 1.58 (s, 6H, OCMe2),
1.51 (s, 6H, OCMe2), 1.32 (d, J ¼ 4.4 Hz, 12H, OCMe2). 13C NMR
(C6D6, ppm): d 71.6 (OCMe2CH2N), 68.5 (OCMe2), 68.3 (OCMe2),
67.0 (OCH2CH2N), 62.3 (OCH2CH2N), 58.9 (OCMe2CH2N), 32.9
(OCMe2), 32.3 (OCMe2) 30.2 (OCMe2), 30.1 (OCMe2). 27Al NMR
(CDCl3, ppm): d 7.90 (Dn1/2 ¼ 2962.6 Hz). EI-MS (% intensity): m/
z 458 (3.0%, M+), 443 (50%, M+ ꢀ Me), 413 (0.99%, M+ ꢀ 2Me),
400 (100%, M+ ꢀ 3Me), 309 (20%, M+ ꢀ 4Me), 385 (19% M+ ꢀ
5Me), 370 (3.0% M+ ꢀ 6Me). Anal. calc. for C20H40Al2N2O6: C,
52.39; H, 8.79; N, 6.11. Found: C, 52.48; H, 8.91; N, 5.98.
Computational details for 1, 2 and their isomers
The geometry optimization for the ground-state (S0) structures
of 1, 2 and their isomers based on the X-ray structures of 1 and 2
were performed at the B3LYP/6-31+G(d) level of theory. Imagi-
nary frequencies for the optimized structures were not
observed. All the calculations were performed for gas-phase
molecules and were carried out using the Gaussian 09 so-
ware package.59 The dissociation energy barriers between Al and
bridged O atom of 1–3 could be calculated as the thermal
stabilities between before and aer dissociation of Al–O bonds
in gas phase.
Synthesis of 3. In a manner analogous to that used in the
procedure for 1, the desired product 3 as colorless crystals was
prepared from a solution of AlMe3 (1.0 mL of 2 M solution in
toluene, 2.0 mmol) and L3H3 (0.46 g, 2.0 mmol) in THF in
1
a yield of 67% (0.34 g). H NMR (CDCl3, ppm): d 2.79 (s, 4H,
CH2N), 2.78 (s, 4H, CH2N), 2.78 (s, 4H, CH2N), 1.41 (s, 12H,
OCMe2), 1.18 (s, 12H, OCMe2), 1.16 (s, 12H, OCMe2). 13C NMR
Conclusions
(CDCl3, ppm): d 72.8 (CH2N), 71.5 (OCMe2), 71.0 (CH2N), 68.4
1
(OCMe2), 32.2 (OCMe2), 31.9 (OCMe2), 29.1 (OCMe2). H NMR We designed and explored novel dimeric alumatranes with
(C6D6, ppm): d 2.484 (s, 4H, CH2N), 2.482 (s, 4H, CH2N), 2.37 (s, tricyclic ve-membered rings. The obtained alumatranes were
4H, CH2N), 1.54 (s, 12H, OCMe2), 1.38 (s, 12H, OCMe2), 1.34 (s, all dimeric in the solid state, solution phase, and the gas phase.
12H, OCMe2). 13C NMR (C6D6, ppm): d 72.7 (CH2N), 71.5 According to single-crystal X-ray analysis, the rst structurally
(OCMe2), 70.7 (CH2N), 68.7 (OCMe2), 32.6 (OCMe2), 32.3 characterized dimeric alumatranes abnormally had their steri-
(OCMe2), 29.5 (OCMe2). 27Al NMR (CDCl3, ppm): d 8.05 (Dn1/2
¼
cally bulky side arms with dimethyl substituents in the bridging
1874.7 Hz). EI-MS (% intensity): m/z 514 (6.00%, M+), 499 sites of the tetradentate ligand, which was also determined by
(63.00%, M+ ꢀ Me), 456 (100.0%, M+ ꢀ 4Me), 441 (44.00%, M+ ꢀ DFT calculations. The new alumatranes were used as catalysts
5Me), 426 (3.00% M+ ꢀ 6Me). Anal. calc. for C24H48Al2N2O6: C, for the trimethylsilylcyanation reaction of aldehydes under
56.01; H, 9.40; N, 5.44. Found: C, 56.22; H, 9.31; N, 5.50%.
extremely mild conditions of room temperature, less than
0.5 mol% catalyst loading, and a short reaction time of 1 h. The
new catalytic systems showed high catalytic activities regardless
of the aldehyde type, which included electron-rich, neutral, and
decient aryl aldehydes, heterocyclic aldehydes, and alkyl
Representative procedures for the trimethylsilylcyanation
reaction
In a glove box, a 10 mL vial was charged with 1–3 (0.01 mmol, aldehydes. Further explorations of the synthesis and applica-
0.5 mol% relative to aldehyde); the vial was removed from the tion of chiral alumatranes are in progress.
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24 M.-A. Munoz-Hernandez, P. Wei, S. Liu and D. A. Atwood,
Coord. Chem. Rev., 2000, 210, 1–10.
Conflicts of interest
25 Y. Opornsawad, B. Ksapabutr, S. Wongkasemjit and
R. M. Laine, Eur. Polym. J., 2001, 37, 1877–1885.
26 A. Singh and R. C. Mehrotra, Coord. Chem. Rev., 2004, 248,
101–118.
There are no conicts to declare.
Acknowledgements
27 L. Fernandez, P. Viruela-Martin, J. Latorre, C. Guillem,
This work was supported by the National Research Foundation
of Korea (NRF), the Korean Ministry of Education (MOE)
through Basic Science Research Program (grant number
2015R1D1A1A01061043).
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