Investigation of the dinuclear effect of aluminum complexes in the ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1039/c7ra02136d

A series of aluminum (Al) complexes bearing hydrazine-bridging Schiff base and salen ligands were synthesized and investigated as catalysts for the ring-opening polymerization of ε-caprolactone (CL). The introduction of steric bulky groups increases the catalytic activity of the corresponding mononuclear aluminum complex. However, the opposite phenomenon was observed in dinuclear Al complexes bearing salen ligands because the steric repulsion reduced the cooperative activation mechanism in the dinuclear Al system. Among these Al complexes, L^N2Bu-Al₂Me₄ bearing a hydrazine-bridging Schiff base ligand had the highest catalytic activity, approximately 3- to 11-fold higher than that of dinuclear Al complexes bearing salen ligands and mononuclear Al complexes bearing Schiff base ligands. Density functional theory calculations revealed that the mechanism of the coordination of CL to one Al center was initiated by the benzyl alkoxide of another Al center.

Full text of DOI:10.1039/c7ra02136d

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PAPER
Investigation of the dinuclear effect of aluminum
complexes in the ring-opening polymerization of 3-
caprolactone†
Cite this: RSC Adv., 2017, 7, 18851
Chiao-Yin Hsu,a Hsi-Ching Tseng,a Jaya Kishore Vandavasi,c Wei-Yi Lu,a
a
a
Li-Fang Wang,a Michael Y. Chiang,ab Yi-Chun Lai, Hsing-Yin Chen
*
ad
*
and Hsuan-Ying Chen
A series of aluminum (Al) complexes bearing hydrazine-bridging Schiff base and salen ligands were
synthesized and investigated as catalysts for the ring-opening polymerization of 3-caprolactone (CL). The
introduction of steric bulky groups increases the catalytic activity of the corresponding mononuclear
aluminum complex. However, the opposite phenomenon was observed in dinuclear Al complexes
bearing salen ligands because the steric repulsion reduced the cooperative activation mechanism in the
dinuclear Al system. Among these Al complexes, LN2Bu-Al2Me4 bearing a hydrazine-bridging Schiff base
ligand had the highest catalytic activity, approximately 3- to 11-fold higher than that of dinuclear Al
complexes bearing salen ligands and mononuclear Al complexes bearing Schiff base ligands. Density
functional theory calculations revealed that the mechanism of the coordination of CL to one Al center
was initiated by the benzyl alkoxide of another Al center.
Received 21st February 2017
Accepted 22nd March 2017
DOI: 10.1039/c7ra02136d
rsc.li/rsc-advances
shown in Fig. 1, proved that dinuclear Al complexes have higher
catalytic activity than do mononuclear Al complexes.
1. Introduction
A possible coordination–insertion mechanism6a (Fig. 2) was
Biodegradable polymers such as poly-3-caprolactone (PCL) were
designed not only to resolve the pollution problem caused by
petrochemical plastics, but also as biomaterials for various elds
such as tissue engineering,1a–c bone regeneration,1d drug carriers,1e
and biomedical application1f–i because of its biodegradability,
biocompatibility, and permeability. The use metal complexes2 as
catalysts for the ring-opening polymerization (ROP) of 3-capro-
lactone (CL) leads to efficient synthesis of PCL, and aluminum
complexes3,4 are common catalysts for ROP because of their ease
of synthesis, the low cost of their precursor materials, and the
highly catalytic conversion rate of their CL polymerization. The
catalytic activity of aluminum (Al) complexes in cycloester ROP
may be improved by ligand modication because the inuences of
ligands including steric, electronic, and chelating effects, directly
change the catalytic activity of metal complexes.3f Recently, dinu-
clear Al catalysts5,6 were used in cycloesters ROP, and some ROPs,6
reported, in which one Al atom serves as the Lewis acid to
activate the monomer by the monomer coordination. An alkoxy
group on the second Al atom initiates the carbonyl group of the
monomer, and monomer ring opens into another alkoxy group.
The polymer chain alternates between two Al atoms, making it
an extremely efficient polymerization process. Carpentier6e re-
˚
ported that the Al–Al distance in his study is 2.8–3.0 A, which
could open up a cooperatively activation mechanism.
Dinuclear Ti complexes7 bearing hydrazine-bridging Schiff base
ligands shown in Fig. 3, have been used as catalysts in L-lactide
polymerization, where they had higher catalytic activity than did
mononuclear Ti complexes bearing Schiff base ligands. The Ti–Ti
˚
distance in that study was 3.244 A with two bridging isoprop-
oxides. Hydrazine-bridging Schiff base ligands appear to be the
perfect ligands for use in a dinuclear Al system in CL polymeri-
zation. Herein, a series of hydrazine-bridging Schiff base ligands,
salen ligands, Schiff base ligands, and associated Al complexes
were synthesized. The catalytic comparison in CL polymerization
between di- and mononuclear Al complexes and the possible
mechanism using density functional theory (DFT) were studied.
aDepartment of Medicinal and Applied Chemistry, Kaohsiung Medical University,
Kaohsiung 80708, Taiwan, Republic of China. E-mail: hchen@kmu.edu.tw; Fax:
+886-7-3125339; Tel: +886-7-3121101-2585
bDepartment of Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan,
Republic of China
cDepartment of Chemistry, University of Ottawa, Ontario, Canada
2. Results and discussion
2.1. Synthesis and characterization of Al complexes
dDepartment of Medical Research, Kaohsiung Medical University Hospital, Kaohsiung
80708, Taiwan, Republic of China
† Electronic supplementary information (ESI) available: Polymer characterization
data, and details of the kinetic study. CCDC 1482938. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c7ra02136d
Symmetrical and asymmetrical hydrazine-bridging Schiff base
ligands,7 and salen ligands8 were synthesized using acid-
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Fig. 1 Structures and their catalytic activity of Al complexes bearing Schiff base ligands.
The X-ray structure of LClBu-AlMe2 (5) (Fig. 4, CCDC 1482938)
illustrates the distorted tetrahedral geometry of the Al complex
with two methyl groups. The angles of the N(1)–Al(1)–O(1) and
C(23)–Al(1)–C(24) planes are 93.32(6)ꢁ and 119.59(8)ꢁ, respec-
tively. The distances of Al(1)–O(1), Al(1)–N(1), Al(1)–C(23),
and Al(1)–C(24) are 1.7624(12), 1.9923(15), 1.9563(19), and
˚
Fig. 2 Possible coordination–insertion mechanism of cyclic ester
ROP by using dinuclear Al complexes as catalysts.
1.9587(19) A, respectively.
2.2. Polymerization of 3-caprolactone
catalyzed condensation following the literature procedures. The
synthesis of Al complexes was described in Experimental
section, and Al complexes were obtained as evidenced by the
appearance of methyl groups showed a singlet peak in the
upeld region (ꢀ0.5 to ꢀ0.75 ppm) and an 6 : 1 integral ratio
(two methyl groups : one imine proton) in 1H NMR spectra. The
crystal of LClBu-AlMe2 (5) was observed in CDCl3 in the nuclear
CL polymerization in toluene using Al complexes as catalysts
with BnOH as the initiator was investigated in a nitrogen
atmosphere at room temperature (Table 1 and Fig. 5). As
shown in entries 1–8 of Table 1, all Al complexes were active in
CL polymerization, and dinuclear Al complexes, LN–NH-Al2Me4
(2) and LN–NBu-Al2Me4 (3), and mononuclear Al complex, LClH
-
AlMe2 (2), exhibited a low controllability of larger MnGPC and
a broad polydispersity index (PDI, 1.70–2.18) compared with
ꢁ
magnetic resonance (NMR) tube at ꢀ20 C aer 1 month.
Fig. 3 Comparison of L-LA polymerization between di- and mononuclear Ti complexes.
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Fig. 4 Molecular structure of LClBu-AlMe2 (5) as 30% probability ellipsoids (all of the hydrogen atoms were omitted for clarity).
other Al complexes, and it may be ascribed to the trans- ligand in the dinuclear system. In addition, mononuclear Al
esterication9e or produce inactive impurity aer the reaction complexes bearing hydrazine-bridging Schiff base ligands,
of BnOH and Al complexes. In dinuclear Al complexes, the LClH-AlMe2 (4) and LClBu-AlMe2 (5), revealed the greater cata-
catalytic activity of LN2Bu-Al2Me4 (1) bearing a hydrazine- lytic activity than those, LHBu-AlMe2 (6), LBu-AlMe2 (7), and LBn
-
bridging Schiff base ligand was higher (approximately 3- to AlMe2 (8), bearing normal Schiff base ligands. This may be
11-fold) than that of Al complexes bearing salen and Schiff attributed to hydrazine group, as an electron withdrawing
base ligands. This reveals that the dinuclear Al complex group, increasing the Lewis acidity of the Al center. On the
bearing a hydrazine-bridging Schiff base ligand has greater basis of the linear relationship between MnGPC and ([CL]0
cooperation than those bearing salen ligands. LN–NBu-Al2Me4 conv.)/[BnOH] exhibited in Fig. 6, polymerizing CL by using
(1) exhibited a lower polymerization rate than did LN–NH LN2Bu-Al2Me4 (1) as the catalyst demonstrated high controlla-
Al2Me4 (2), revealing that a steric bulky ligand decreased the bility with a narrow PDI.
ꢂ
-
1
catalytic activity of Al complexes. However, in mononuclear Al
The H NMR spectrum of PCL (Fig. 7, PCL from entry 11,
complexes, the opposite phenomenon was observed, similar to Table 1) shows that peaks at 7.36 ppm (C6H5CH2–, peak a),
in the literature.4f This may be because the cooperation 5.12 ppm (C6H5CH2–, peak b), and 3.66 ppm (CH2CH2OH, peak
0
between the two Al centers was blocked by the steric bulky c) with an integral ratio of 5 : 2 : 2 between Ha, Hb, and Hc ,
Table 1 3-Caprolactone polymerization by using each of the Al complexes as catalysts at room temperatured
Conv.a (%)
MnCal
MnGPC
MnNMR
PDIc
kobs (error)/min
b
c
b
Entry
Catalyst
Time (min)
1d
LN2Bu-Al2Me4 (1)
LN–NH-Al2Me4 (2)
LN–NBu-Al2Me4 (3)
LClH-AlMe2 (4)
60/90
60/1395
60/1330
60/720
60/230
60/1260
60/360
60/2560
1000
30
20
450
1500
80/88
46/95
9/100
22/99
28/95
18/94
17/90
0/96
96
97
99
90
95
5100
5500
5800
5800
5500
5500
5300
5600
11 100
2900
1500
11 500
16 400
22 000
7200
5900
11 200
5800
11 000
3700
6700
6900
4200
11 500
4700
2300
1.20
2.18
1.70
1.73
1.16
1.15
1.16
1.16
1.39
1.06
1.06
1.31
1.30
1.32
0.0261 (22)
0.0085 (4)
0.0023 (8)
0.0065 (7)
0.0098 (4)
0.0021 (1)
0.0065 (3)
0.0014 (5)
0.0035 (2)
0.1413 (72)
0.2541 (127)
2d
11 800
11 600
16 100
5000
5900
6900
7700
29 700
3100
3d
4e
5e
LClBu-AlMe2 (5)
LHBu-AlMe2 (6)4r
LBu-AlMe2 (7)
6e
7e
8e
LBn-AlMe2 (8)
9f
LN2Bu-Al2Me4 (1)
LN2Bu-Al2Me4 (1)
LN2Bu-Al2Me4 (1)
LN2Bu-Al2Me4 (1)
LN2Bu-Al2Me4 (1)
LN2Bu-Al2Me4 (1)
10g
11h
12i
13j
14k
1600
l
22 300
47 400
57 400
11 500
17 100
20 300
—
l
—
l
1500
96
—
a
Obtained from 1H NMR analysis. b Calculated from the molecular weight of monomer ꢂ [monomer]0/[BnOH]0 ꢂ conversion yield + Mw(BnOH).
c
Obtained from GPC analysis and calibration based on the polystyrene standard. Values of MnGPC are the values obtained from GPC times 0.56.
d
e
Reaction condition: toluene (5 mL), [CL] ¼ 2.0 M, [CL] : [Cat] : [BnOH] ¼ 100 : 0.5 : 2. Reaction condition: toluene (5 mL), [CL] ¼ 2.0 M,
f
g
[CL] : [Cat] : [BnOH] ¼ 100 : 1 : 2. Reaction condition: toluene (5 mL), [CL] ¼ 2.0 M, [CL] : [Cat] : [BnOH] ¼ 100 : 0.25 : 1.0. Reaction
h
condition: toluene (5 mL), [CL]
¼
2.0 M, [CL] : [Cat] : [BnOH]
¼
100 : 1 : 4. Reaction condition: toluene (5 mL), [CL]
¼
2.0 M,
i
j
[CL] : [Cat] : [BnOH] ¼ 100 : 2 : 8. Reaction condition: toluene (5 mL), [CL] ¼ 4.0 M, [CL] : [Cat] : [BnOH] ¼ 200 : 0.5 : 2. Reaction condition:
k
toluene (5 mL), [CL] ¼ 6.0 M, [CL] : [Cat] : [BnOH] ¼ 300 : 0.5 : 2. Reaction condition: toluene (5 mL), [CL] ¼ 8.0 M, [CL] : [Cat] : [BnOH] ¼
l
400 : 0.5 : 2. Not available.
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Fig. 5 Comparison of the catalytic activity of di- and mononuclear Al complexes.
2.4. Polymerization mechanism of LN2Bu-Al2Me4 using DFT
calculation
To shed some light on how dinuclear Al catalysts cooperatively
polymerizate the CL, DFT calculations were performed for the
LN2Bu-Al2Me4 (1) system. To reduce the computational cost, the
tBu groups on the hydrazine-bridging Schiff base ligand were
replaced by hydrogen atoms and the methoxide was used
instead of benzyl alkoxide.
The structure of the active catalyst from the reaction of LN2Bu
-
Al2Me4 (1) with 4 equivalent of BnOH could not be isolated in the
1
experiments. However, the H NMR spectrum (Fig. S17†) of the
mixture of benzyl alcohol and LN2Bu-Al2Me4 (1) revealed that
methyl groups associated with Al center disappear, and new
methylene groups of benzyl alkoxide grow up. Based on the
acceptable assumption that alcohol is able to replace methyl
groups3b–e,j–n,4q,r and referring to the crystal structure of dinuclear
Ti catalyst previously reported,7 our DFT calculations successfully
located the optimized geometry of the dinuclear Al catalyst (Fig. 9).
In this C2 symmetric structure, two Al metals are bridged by two
Fig. 6 Linear plot of MnGPC versus [CL]0 ꢂ conv./[BnOH]0 with
blue dots indicating polydispersity indices (Table 1, entries 1 and
9–14).
˚
alkoxide groups with an Al–Al distance of 2.83 A. The coordination
indicating that the polymer chain should be capped with one
benzyl ester and one hydroxy end and suggesting that the
polymerization occurs through the insertion of a benzyl alkoxy
group into CL.
geometry of the Al metals is characterized by a distorted square
pyramid in which four oxygen atoms form the base and a nitrogen
atom is located at the apex. The vacant site opposite to the
nitrogen can be used to accommodate the CL monomer.
The DFT-calculated reaction pathway and the related free
energy changes for the ROP of CL by the dinuclear Al catalyst are
illustrated in Fig. 10. The rst step is the association of CL and
2.3. Kinetic studies of CL polymerization catalyzed by LN2Bu
Al2Me4
-
Kinetic studies were performed at room temperature with the catalyst. The calculations predict that CL can coordinate
respect to the ratio of [CL]0/[LN2Bu-Al2Me4 (1) + 4BnOH] ([CL] ¼ with the Al center through the carbonyl oxygen (Ocarbonyl) to
2.0 M in 5 mL of toluene) as shown in Table S2 and Fig. S2.† The form a stable complex a, which is 2.6 kcal molꢀ1 lower than the
preliminary results indicated a rst-order dependency on [CL] separated state. We also considered the Al–CL complex bound
(Fig. S2†). By plotting kobs against [LN2Bu-Al2Me4 (1) + 4BnOH], through the ester oxygen (Oester) of CL; nonetheless, the binding
a kprop values of 7.37 (Mꢀ1 minꢀ1) was obtained (Fig. 8). Poly- free energy of the Oester-bound complex is 1.1 kcal molꢀ1
merizing CL by using LN2Bu-Al2Me4 (1) at room temperature indicating that it is not a stable complex. That CL prefers
,
demonstrated the following rate law:
coordination to the Al center via Ocarbonyl rather than Oester has
been reported previously.4s
d[CL]/dt ¼ 7.37 ꢂ [CL][LN2Bu-Al2Me4 (1) + 4BnOH]
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Fig. 7 1H NMR spectrum of PCL in CDCl3 catalyzed by LN2Bu-Al2Me4 (1) with BnOH (entry 11, Table 1).
Upon association with the CL monomer, one of the
bridging alkoxides leaves the CL-coordinated Al center (a / b)
and then inserts into the activated carbonyl carbon (b / c).
Notice that during the alkoxide insertion, the ester oxygen of
CL concurrently coordinates to the other Al center but the ring
structure of CL remains closed. The CL in c immediately
undergoes ring opening to achieve a more thermodynamically
more stable state d. It is assumed that the free CL monomers
could subsequently attack the Ocarbonyl-coordinated Al center
(right Al site) of d and dissociate the carbonyl group of the
open-ring CL to generate a new propagating alkoxide; that is,
return to state b to accomplish the catalytic cycle. According to
the present DFT calculations, the rate of propagation is
determined by the step of alkoxide insertion b / c with an
activation free energy of 19.2 kcal molꢀ1. This theoretical value
is in good agreement with the experimental measurement of
the propagating rate constant kprop ¼ 7.37 Mꢀ1 minꢀ1, which
equates to an activation free energy of 18.7 kcal molꢀ1 using
the Eyring equation.
Fig. 8 Linear plot of kobs against [LN2Bu-Al2Me4 (1) + 4BnOH] for CL
polymerization with [CL] ¼ 2.0 M in toluene (5 mL).
3. Conclusions
A comparison of CL polymerization between mononuclear Al
complexes bearing Schiff base ligands, and dinuclear Al
complexes bearing hydrazine-bridging Schiff base and salen
ligands was investigated. Mononuclear Al complexes with steric
bulky groups increased the catalytic activity, whereas the
opposite phenomenon was observed in dinuclear Al complexes
bearing salen ligands. Steric repulsion may reduce the cooper-
atively activation mechanism in a dinuclear Al system. LN2Bu
-
Al2Me4 (1) bearing a hydrazine-bridging Schiff base ligand had
the highest catalytic activity (approximately 3- to 11-fold), higher
than that of dinuclear Al complexes bearing salen ligands and
mononuclear Al complexes bearing Schiff base ligands. A
Fig. 9 DFT-optimized structure of an active dinuclear Al catalyst
(distances in angstroms).
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Fig. 10 Possible reaction path for the ring-opening polymerization of 3-caprolactone catalyzed by a dinuclear Al catalyst; the relative free
energies are shown in kcal molꢀ1 and the geometries of TS1st and TS3rd respectively resemble those of b and c and are therefore not shown.
mechanism study using DFT calculations revealed that the 31G* level combined with the D3 version of Grimme's disper-
cooperation that coordinated the CL with one Al center was sion correction. Calculations were performed by Gaussian 09
initiated by the benzyl alkoxide of another Al center.
program.
4.1. Synthesis of LClBu-H
4. Experimental section
4-Chlorobenzaldehyde (7.03 g, 50 mmol) was added drop-wise
into an ethanol solution (100 mL) of the hydrazine mono-
hydrate (5.0 g, 0.10 mol) at 0 ꢁC for 6 h. Volatile materials
were removed under vacuum to give a white powder. 3.685 g of
the white powder was reacted with 3,5-di-tert-butyl-2-
hydroxybenzaldehyde (5.85 g, 25 mmol) in ethanol (100 mL)
and reuxed for one day. The product precipitated as a yellow
Standard Schlenk techniques and
a N2-lled glovebox
were used all over the isolation and treatment of all the
compounds. Solvents, 3-caprolactone, and deuterated solvents
were puried prior to use. Salicylaldehyde, 3,5-di-tert-butyl-2-
hydroxybenzaldehyde, 4-chlorobenzaldehyde, hydrazine mon-
ohydrate, ethane-1,2-diamine, benzyl amine, and benzyl alcohol
were purchased from Aldrich. 1H and 13C NMR spectra were
1
solid which was ltered. Yield: 6.05 g (65%). H NMR (CDCl3,
1
recorded on a Varian Gemini2000-200 (200 MHz for H and 50
200 MHz): d 8.77, 8.61 (2H, s, CH]N), 7.80, 7.43 (4H, d, J ¼ 8 Hz,
ArH), 7.46, 7.17 (2H, s, ArH), 7.05–6.92 (2H, m, ArH). 13C NMR
(CDCl3, 50 MHz): d 166.68, 165.24 (C]N), 160.23, 157.03,
141.10, 136.80, 132.26, 132.33, 129.69, 128.12, 126.97, 116.79
(Ar), 35.10, 34.15 (C(CH3)3), 31.45, 29.44 (C(CH3)3). Mp ¼ 155 ꢁC.
MHz for 13C) spectrometer with chemical shis given in ppm
from the internal TMS or center line of CDCl3. Microanalyses
were performed using a Heraeus CHN-O-RAPID instrument.
The gel permeation chromatography (GPC) measurements were
performed on a Waters 1515 Isotratic HPLC pump system
equipped with a differential Waters 2414 refractive index
detector using THF (HPLC grade) as the eluent. The chro-
4.2. Synthesis of LN2Bu-Al2Me4
matographic column was a Water Styragel Column (HR4E), and A mixture of LN2Bu-H (2.32 g, 5 mmol) and AlMe3 (6 mL, 2.0 M in
the calibration curve was made by polystyrene standards to toluene, 12 mmol) in toluene (25 mL), was stirred at 0 ꢁC for one
calculate Mn(GPC). Ligands of LN2Bu-H,7 LN–NH-H,9a LN–NBu-H,9b day. Volatile materials were removed under vacuum to give red
LClH-H,7a LHBu-H,9c LBu-H,9d LBn-H,11 and Al complexes of LHBu
-
mud and then it was washed with hexane (50 mL). The product
AlMe2 (6) (ref. 3d and 4r) and LBu-AlMe2 (7) (ref. 10) were precipitated as a deep yellow powder which was ltered. Yield:
prepared following literature procedures. All the symmetrical 2.45 g (80%). 1H NMR (CDCl3, 200 MHz): d 8.57 (2H, s, CH]N),
ligands were reacted with two equivalent of AlMe3 in toluene to 7.62 (2H, s, Ar-H), 6.95 (2H, s, Ar-H), 1.42, 1.31 (36H, s, C(CH3)3),
obtain a moderate yield of Al compounds (Scheme 1(A) and (B)). ꢀ0.60 (12H, s, Al(CH3)2). 13C NMR (CDCl3, 50 MHz): d 167.52
The synthesis of LHBu-AlMe2 (6), LBu-AlMe2 (7), LHBn-AlMe2 (8), (C]N), 162.30, 140.85, 140.45, 134.26, 128.81, 116.18 (Ar),
LClH-AlMe2 (4), and LClBu-AlMe2 (5) was similar to the afore- 35.30, 34.11 (C(CH3)3), 31.18, 29.28 (C(CH3)3), ꢀ8.79 (Al(CH3)2).
mentioned method except that one equivalent of AlMe3 was Anal. calcd (found) for C34H54N2O2Al2: C, 70.80 (71.14); H, 9.44
used. DFT geometry optimizations were carried out at M06/6- (9.74); N, 4.86 (4.76)%. Mp ¼ 224 ꢁC.
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Scheme 1 Synthesis of hydrazine-bridging Schiff base ligands, salen ligands, and associated Al complexes.
4.3. Synthesis of LN–NH-Al2Me4
ꢀ9.18 (Al(CH3)2). Anal. calcd (found) for C36H58N2O2Al2: C,
71.49 (71.82); H, 9.67 (9.56); N, 4.63 (4.99)%. Mp ¼ 180 ꢁC.
Using a method similar to that for LN2Bu-Al2Me4 expect LN–NH-H
was used in place of LN2Bu-H. Yield: 0.99 g (52%). 1H NMR
(CDCl3, 200 MHz): d 8.01 (2H, s, CH]N), 7.42 (2H, t, J ¼ 8.0 Hz,
Ar-H), 7.04 (2H, d, J ¼ 8.0 Hz, Ar-H), 6.89 (2H, d, J ¼ 8.0 Hz, Ar-
H), 6.71 (2H, t, J ¼ 8.0 Hz, Ar-H), 3.93 (4H, s, NCH2CH2N), ꢀ0.67
(12H, s, Al(CH3)2). 13C NMR (CDCl3, 50 MHz): d 173.25 (C]N),
168.47, 164.08, 137.55, 134.84, 121.59, 117.82 (Ar), 56.21
(NCH2CH2N), ꢀ9.31 (Al(CH3)2). Anal. calcd (found) for
4.5. Synthesis of LClH-AlMe2
A mixture of LCl–H-H (1.29 g, 5 mmol) and AlMe3 (3 mL, 2.0 M in
toluene, 6 mmol) in toluene (25 mL), was stirred at 0 ꢁC for one
day. Volatile materials were removed under vacuum to give red
viscous mud and then it was washed with hexane (50 mL). The
product precipitated as a yellow powder which was ltered.
Yield: 0.13 g (8%). 1H NMR (CDCl3, 200 MHz): d 8.72, 8.68 (2H, s,
CH]N), 7.74, 7.49 (2H, d, J ¼ 8.0 Hz, Ar-H), 7.29, 7.28 (2H, m,
Ar-H), 6.94, 6.84 (2H, m, Ar-H), ꢀ0.62 (6H, s, Al(CH3)2). 13C NMR
(CDCl3, 50 MHz): d 169.45, 164.49 (C]N), 158.36, 138.49,
137.91, 135.14, 131.09, 129.79, 129.38, 121.61, 118.20, 117.19
(Ar), ꢀ9.98 (Al(CH3)2). Anal. calcd (found) for C16H16N2O2ClAl2:
C, 61.06 (60.55); H, 5.12 (5.44); N, 8.90 (8.45)%. Mp ¼ 120 ꢁC.
C
20H26N2O2Al2: C, 63.15 (63.75); H, 6.89 (6.71); N, 7.36 (7.55)%.
ꢁ
Mp ¼ 153 C.
4.4. Synthesis of LN–NBu-Al2Me4
Using a method similar to that for LN2Bu-Al2Me4 expect LN–NBu-H
was used in place of LN2Bu-H. Yield: 0.37 g (12%). 1H NMR
(CDCl3, 200 MHz): d 8.01 (2H, s, CH]N), 7.50, 6.86 (2H, s, Ar-H),
3.87 (4H, s, NCH2CH2N), 1.39, 1.22 (36H, s, C(CH3)3), ꢀ0.69
(12H, s, Al(CH3)2). 13C NMR (CDCl3, 50 MHz): d 174.23 (C]N),
4.6. Synthesis of LClBu-AlMe2
161.73, 140.32, 139.20, 132.54, 128.83, 117.86 (Ar), 56.61 Using a method similar to that for LClH-AlMe2 expect LClBu-H
(NCH2CH2N), 35.22, 33.93 (C(CH3)3), 31.24, 29.27 (C(CH3)3), was used in place of LClH-H. Yield: 1.59 g (74%). 1H NMR
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(CDCl3, 200 MHz): d 8.69 (2H, s, CH]N), 7.72, 7.47 (4H, d, J ¼ Instead, we performed the calculation of potential energy
8.0 Hz, Ar-H), 7.59, 7.10 (2H, s, Ar-H), 1.42, 1.31 (18H, s, surface scan from c to d with increasing the Ccarbonyl–Oester bond
C(CH3)3), ꢀ0.63 (6H, s, Al(CH3)2). 13C NMR (CDCl3, 50 MHz): length and approximately identied the maximum point along
d 170.57, 162.13 (C]N), 157.24, 140.34, 139.74, 138.12, 133.10, the scan path as the TS3rd. Vibrational frequency analysis of this
131.38, 129.62, 128.99, 116.76 (Ar), 35.25, 34.07 (C(CH3)3), 31.27, approximate TS3rd indeed shows an imaginary vibration corre-
29.30 (C(CH3)3), ꢀ8.83 (Al(CH3)2). Anal. calcd (found) for C24- sponding to the Ccarbonyl–Oester bond stretching.
H32N2OClAl: C, 67.51 (67.75); H, 7.55 (7.47); N, 6.56 (6.66)%. Mp
ꢁ
¼ 150 C.
Acknowledgements
4.7. Synthesis of LBn-AlMe2
This study is supported by Kaohsiung Medical University “Aim
for the top 500 universities grant” under Grant No. KMU-
DT105009, NSYSU-KMU JOINT RESEARCH PROJECT, (#NSY-
SUKMU 105-I004), and the Ministry of Science and Technology
(Grant MOST 105-2113-M-037-007). We thank Center for
Research Resources and Development at Kaohsiung Medical
University for the instrumentation and equipment support.
Using a method similar to that for LClH-AlMe2 expect LBn-H was
used in place of LClH-H. Yield: 1.09 g (82%). 1H NMR (CDCl3, 200
MHz): d 7.88 (1H, s, CH]N), 7.35 (5H, br, Bn-H) 7.17–6.95, 6.66–
6.57 (4H, m, Ar-H), 5.15, 4.80 (2H, d, J ¼ 14 Hz, CH2Ph), ꢀ0.85
(6H, s, Al(CH3)2). 13C NMR (CDCl3, 50 MHz): d 167.36 (C]N),
163.89, 137.56, 134.28, 133.08, 129.26, 128.67, 128.15, 119.80,
116.65, 114.61 (Ar), 58.65 (CH2Ph), ꢀ7.74 (Al(CH3)2). Anal. calcd
(found) for C16H18NOAl: C, 71.89 (71.48); H, 6.79 (7.01); N, 5.24 References
ꢁ
(5.33)%. Mp ¼ 125 C.
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~
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4.9. Computational methods
The B3LYP functional12 was adopted in the present DFT calcu-
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calculations were accomplished by the Gaussian 09 program.15
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transition state TS3rd for the ring-opening step (c / d) failed.
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