Aluminum complexes of β-hydroxy-imino ligands: Synthesis, structures and application in the Tishchenko reaction

Article abstract of DOI:10.1016/j.jorganchem.2016.06.008

Four aluminum alkoxide complexes containing β-hydroxy-imino ligands, [(2,6-Me₂C₆H₃)NCPhCH₂CPh₂O]₂AlMe (3a), [(2,6-ⁱPr₂C₆H₃)NCPhCH₂CPh₂O]₂AlMe (3b), [(2,6-Me₂C₆H₃)NCPhCH₂C(C₁₂H₈)OAlMe₂]₂ (3c) and [(2,6-ⁱPr₂C₆H₃)NCPhCH₂C(C₁₂H₈)OAlMe₂]₂ (3d) were synthesized in high yields, and their structural features were provided. The catalytic behavior of those four complexes about the Tishchenko reaction with a range of aromatic aldehydes as substrates were assessed, and it present a synthetically useful protocol to the solvent-free Tishchenko reaction under mild conditions.

Full text of DOI:10.1016/j.jorganchem.2016.06.008

Accepted Manuscript
Aluminum complexes of β-hydroxy-imino ligands: Synthesis, structures and
application in the Tishchenko reaction
Zhiqiang Guo, Song Wang, Xuehong Wei
PII:
S0022-328X(16)30258-3
10.1016/j.jorganchem.2016.06.008
JOM 19525
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 19 January 2016
Revised Date: 7 May 2016
Accepted Date: 5 June 2016
Please cite this article as: Z. Guo, S. Wang, X. Wei, Aluminum complexes of β-hydroxy-imino ligands:
Synthesis, structures and application in the Tishchenko reaction, Journal of Organometallic Chemistry
(2016), doi: 10.1016/j.jorganchem.2016.06.008.
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ACCEPTED MANUSCRIPT
Graphical abstract
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Aluminum Complexes of β-Hydroxy-Imino
Ligands: Synthesis, Structures and Application in
the Tishchenko Reaction
Zhiqiang Guo,a* Song Wang,b Xuehong Wei,a, c*
aScientific Instrument Center, Shanxi University, Taiyuan, 030006, P. R. China
bDepartment of Chemistry, Taiyuan Normal University, Taiyuan 030031, P. R. China
cSchool of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006, P. R.
China
KEYWORDS: Aluminum complex; β-Hydroxy-imine; Structure; Tishchenko reaction
ABSTRACT: Four aluminum alkoxide complexes containing β-hydroxy-imino ligands, [(2,6-
Me2C6H3)NCPhCH2CPh2O]2AlMe (3a), [(2,6-iPr2C6H3)NCPhCH2CPh2O]2AlMe (3b), [(2,6-
Me2C6H3)NCPhCH2C(C12H8)OAlMe2]2 (3c) and [(2,6-iPr2C6H3)NCPhCH2C(C12H8)OAlMe2]2
(3d) were synthesized in high yields, and their structural features were provided. The catalytic
behavior of those four complexes about the Tishchenko reaction with a range of aromatic
*
Corresponding author. Tel.: +86 351 7018390, Fax: +86-351-7011688;
E-mail address: gzq@sxu.edu.cn (Z. Guo), xhwei@sxu.edu.cn (X. Wei).
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aldehydes as substrates were assessed, and it present a synthetically useful protocol to the
solvent-free Tishchenko reaction under mild conditions.
INTRODUCTION
Over the past few years, alkoxide and phenoxide ligands, especially β-hydroxy-imino ligands
(Scheme 1), have attracted increasing attention in stabilizing oxophilic metal centers such as the
early elements of groups 1-4 as well as Al(III) and Zn(II) [1]. Among those complexes, the
phenoxide often yield mononuclear and discrete complexes, however, alkoxides produce highly
aggregated polynuclear species due to the high bridging tendency of the relatively more basic
alkoxide ligands. To overcome this major difficulty, a series of highly stable fluorous tertiary
alkoxide ligands were introduced by carpentier’s group [2], and it proved to be a valuable
strategy. Thereinto, aluminum alkoxide complexes modified by those β-hydroxy-imino ligands as
catalysts/initiators for the ring-opening polymerization (ROP) of cyclicesters have been studied,
and exhibited excellent catalytic activity.
iPr
OH
N
N
N
iPr
R1
tBu
OH
HO
CF3
F3C
tBu
R2
R2
1
2
R1
N
N
N
OH HO
OH
CF3
CF3
F3C
F3C
CF3
CF3
3
4
Scheme 1 Some β-hydroxy-imino ligands.
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As we all know, the nature of the ancillary ligand in the Al coordination sphere play an key
role, which determines the steric and electronic properties of the Al complexes, and in turn the
catalytic performances. Different from carpentier’s strategy, we try to obtain large steric
hindrance group of β-hydroxy-imino ligands access to interesting and useful compounds that are
not available by other means in terms of reactivity and synthetic application.
Recently, we report a series of amidinatoaluminum complexes [3], asymmetric guanidinato
aluminum complexes [4] and bidentate N, O-aluminum complexes [5] as pre-catalysts for the
Tishchenko reaction or the Meerwein-Ponndorf-Verley (MPV) reaction. As our continuous
interest in developing aluminum alkoxide complexes as catalysts/initiators, here we report the
synthesis and structural features of four aluminum alkoxide complexes containing β-hydroxy-
imino ligands with different bonding modes, and their catalytic activity for the Tishchenko
reaction with a range of aromatic aldehydes.
RESULTS AND DISCUSSION
Synthesis and structure of β-hydroxy-imino ligands and their complex.
Four
β-hydroxy-imino
ligands, (2,6-Me2C6H3)NC(Ph)CH2C(Ph)2OH (2a),
(2,6-
iPr2C6H3)NC(Ph) -CH2C(Ph)2OH (2b), (2,6-Me2C6H3)NCPhCH2C(C12H8)OH (2c) and (2,6-
iPr2C6H3)NCPhCH2C (C12H8)OH (2d) were prepared in good yield by lithiation of imines (1a or
1b) with lithium diisopropylamide (LDA) and the lithiated species were then treated with the
corresponding ketone in Et2O, and quenching the ether solution with water (Scheme 2). In
addition to characterisation by NMR spectroscopy, the solid-state structure of 2a (Fig. 1) was
determined by single crystal X-ray diffraction.
Reactions of the β-hydroxy-imino ligand 2a-2d with AlMe3 proceeded cleanly in Et2O at room
tempretrure, via methane elimination, to afford the corresponding aluminum alkoxide complexes.
Among them, whichever the different amounts of AlMe3 or the addition order of the reagents,
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only the bis-ligand complexes [(2,6-Me2C6H3)NCPhCH2CPh2O]2AlMe (3a) and [(2, 6-
iPr2C6H3)NCPhCH2CPh2O]2AlMe (3b) could be isolated. While 1 equiv of 2c and 2d vs AlMe3
were used, complexes [(2,6-Me2C6H3)NCPhCH2C(C12H8)OAlMe2]2 (3c) and [(2,6-
iPr2C6H3)NCPhCH2C(C12H8)OAlMe2]2 (3d) were isolated in high yield. All complexes have
been characterized by NMR spectroscopy, elemental analyses, and single crystal X-ray
diffraction.
Scheme 2 Synthetic routes to the corresponding aluminum complexes.
X-ray single crystal structures of 2a, 3a-3d.
Crystals of ligand 2a were obtained from a saturated hexane solution. As shown in Figure 1,
which revealed there is a strong intramolecular N-H-O hydrogen bond.
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Fig. 1. ORTEP diagram of compound 2a. Thermal ellipsoids are drawn at the 20% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg):
N1-C9 1.276(3), N1-C1 1.429(3), O1-C17 1.428(2), C9-C16 1.513(3), C16-C17 1.529(3), C9-
C10 1.494(3), C17-C18 1.532(3), C17-C24 1.526(3); C9-N1-C1 123.01(19), N1-C9-C16
118.1(2), C9-C16-C17 116.03(18), O1-C17-C16 108.97(17), C10-C9-C16 115.99(19).
Single crystals of 3a suitable for X-ray diffraction studies were obtained by recrystallization in
hexane at -20 oC. The molecular structure of 3a is shown in Figure 2 where the aluminum center
is coordinated to methyl and two chelating β-hydroxy-imino ligands. The ligands are coordinated
to the aluminum center through both of their oxygen and nitrogen atoms. Thus, the aluminum
center has a distorted trigonal bipyramidal geometry with the N and N’ atoms occupying the axial
positions with N-Al-N’ = 174.67(13)° and the O and O’ atoms occupying the equatorial
positions with O-Al-O’ = 115.13(14)°. The Al-O bond distances are 1.7504(19) Å. It is shorter
than those of tetradentate fluorinated dialkoxy-diimino aluminum complexes, {MeONEtNO}AlMe
( 1.778(1) Å, 1.807(5) Å) and rac-{ONCyNO}AlMe ( 1.767(1) Å, 1.834(1) Å) [6]. The observed
donor acceptor Al-N (2.254(2) Å) distances is substantially longer than those of formally
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covalent Al-N bonds (1.89-2.20 Å) of the above mentioned amide compounds. The longer Al-N
bond distances are in good agreement with axial coordination positions and the formation of
relatively weak neutral donor metal bonding interactions.
Fig. 2. ORTEP diagram of compound 3a. Thermal ellipsoids are drawn at the 20% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg):
N-C9 1.284(3), N-C1 1.452(3), O-C17 1.403(3), Al-O 1.7504(19), Al-C24 1.942(5), Al-N
2.254(2), C9-C16 1.504(4), C16-C17 1.550(4); O-Al-C24 122.43(7), O-Al-N 87.85(8), C24-Al-
N 92.66(7), C17-O-Al 132.46(16), C9-N-Al 117.75(18), C9-C16-C17 113.7(2), C17-O-Al
132.46(16), C9-N-Al 117.75(18).
The structure of complex 3b is very similar to that of 3a, and the ORTEP drawing is presented
in Fig. 3, along with selected bond lengths and angles. In the solid state, two ligands are
coordinated to the aluminum center through both of their oxygen and nitrogen atoms,
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respectively, and the geometry at the aluminium atom is also a distorted trigonal bipyramidal.
The corresponding bond angles (N1-Al1-N1’ and O1-Al1-O1) are 175.66(13) and 116.37(13)°,
respectively, these data compare well with those of complex 3a. Due to the steric hindrance
effect of 2, 6-diisopropyl benzene ring, the bond length of Al-O bond (1.7449(17) Å) is slightly
shorter while the bond length of Al-N bond (2.2725(19) Å) is slightly longer.
Fig. 3. ORTEP diagram of compound 3b. Thermal ellipsoids are drawn at the 20% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg):
Al1-O1 1.7449(17), Al1-C34 1.944(4), Al1-N1 2.2725(19), N1-C13 1.287(3), N1-C1 1.450(3),
O1-C21 1.407(3), C13-C20 1.510(3), C20-C21 1.551(3); O1-Al1-C34 121.82(6), O1-Al1-N1
91.72(7), C13-N1-Al1 115.83(17), N1-C13-C20 118.2(2), C13-C20-C21 116.2(2), O1-C21-C20
110.96(19).
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The molecular structure of 3c was also determined by single crystal X-ray diffraction
techniques and the ORTEP drawing is shown in Fig. 4, along with selected bond lengths and
angles. The aluminum center in complex 3c is coordinated by oxygen atoms of chelating β-
hydroxy-imino ligands and two methyl groups in a distorted tetrahedral manner. The central
Al2O2 ring is perfectly planar. The nitrogen atom of the ligand does not coordinate to the central
metal. The Al1-O1 (1.851(2) Å) is within the usual range found for similar alkoxide aluminum
derivatives [7]. The fluorene rings are in a plane nearly perpendicular to the Al2O2 plane,
dihedral angle is 89.78°.
Fig. 4. ORTEP diagram of compound 3c. Thermal ellipsoids are drawn at the 20% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg):
Al1-O1 1.851(2), O1-C17 1.453(3), N1-C9 1.279(4), N1-C1 1.420(4), C9-C16 1.507(4), C16-
C17 1.533(4); O1-Al1-O1’ 80.45(9), Al1-O1-Al1’ 99.55(9), C17-O1-Al1 132.67(16), C9-N1-C1
122.5(2), N1-C9-C16 118.9(3), C9-C16-C17 117.6(2), O1-C17-C16 105.7(2). Symmetry
elements for 3: ’–x, y, -z+1/2.
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The molecular structure of 3d is given in Figure 5. Like complex 3c, the Al centers of 3d also
exhibit a typically tetrahedral coordination, being bonded to two methyl groups and two
aryloxide bridging ligands in the solid state. Despite this slight distortion all the bond lengths and
angles compare well with complex 3c. The O1-C21and O1-Al distances in complex 3d (1.452(3)
and 1.8506(15) Å) are nearly equal with 3c (O1-C17 1.453(3) and O1-Al1 1.851(2) Å). It is
interesting to point out that the linear structure composed by N1, C13, C20, C21 and O1 atoms
present a ‘W’ shape due to the greater steric bulk of the 2, 6-diisopropyl benzene. While it
presents a ‘U’ shape in complex 3c.
Fig. 5. ORTEP diagram of compound 3d. Thermal ellipsoids are drawn at the 20% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg):
Al-O1 1.8506(15), O1-C21 1.452(3), C20-C21 1.535(3), C13-C20 1.513(3), N1-C13 1.266(3),
N1-C1 1.421(3); O1-Al-O1’ 79.86(7), Al-O1-Al’ 100.14(7), C21-O1-Al 127.85(13), C13-N1-C1
121.9(2), N1-C13-C20 117.4(2), O1-C21-C20-107.10(17). Symmetry elements for 4: ’–x+2, -
y+2, -z.
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Catalytic activities of complexes 3a, 3b, 3c and 3d for the Tishchenko reaction
The Tishchenko reaction, a good typical atom-economic reaction, has been known for more
than a century [8]. During this period, the Tishchenko reaction has been studied throughly and
many types of catalysts have been developed to obtain the object esters in high yields. For
example, traditional aluminum alkoxides [9], metal catalysts including alkaline metals [10],
alkaline metal oxides or alkaline earth metal oxides [11], transition metals [12], lanthanides [13],
actinides [14] and various other catalysts [15] have been developed towards the improvement of
Tishchenko chemistry. Very recently, we have reported the aminopyrrolyl dilithium [16],
guanidinatozinc [17], amidinatoaluminum [3] and N, O-bidentate aluminum complexes [5a]
exhibit excellent catalytic activities for the dimerization of aldehydes to the corresponding esters
under mild conditions. Inspired by these works, the catalytic behaviors of aluminum complexes
incorporating β-hydroxy-imino ligands about the Tishchenko reaction were assessed.
The initially studied of the reaction was carried out using benzaldehyde as a model substrate
with obtained aluminum complexes. We were pleased to find that in the presence of 0.5% mol
3a, 3b, 3c and 3d, the dimerization of benzaldehyde can proceed smoothly to afford ester with
o
97.1%, 95.7%, 90.7% and 89.9% isolated yield at 60 C after 6 h, respectively (Table 1, entries
1-4). The activity of the complex 3a or 3b was higher than that of the complex 3c or 3d. This is
attributed to the mononuclear and discrete complexes are superior to aggregated polynuclear
species. Among them, complex 3a gave the highest yield under the same reaction conditions.
When complex 3a was used as catalyst and the temperature increased to 80 oC, the reaction went
to completion in 6 h with 99.2% isolated yield, while the reaction only afford 48.1% isolated
o
yield at 40 C (Table 1, entries 5-6). This indicates that temperature plays a key role in this
reaction system. The yield decrease to 87.8% when catalyst loading decreases to 0.1% (Table 1,
entry 8), and the optimized results show six hours are reasonable reaction time.
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Having realized the appropriate conditions for benzaldehyde dimerization, we continued our
quest with a variety of aromatic substrates. As summarized in Table 2. These aldehydes
containing electron donating substituents and electron withdrawing group in the para substituted
aromatic ring were converted to the corresponding esters in good isolated yields in reasonable
time, while the strongly donating methoxy substitutent dramatically reduces reactivity (Table 2,
entries 3). These general trends are consistent with the previous reports. And the Tishchenko
reaction of o-phthalaldehyde also gave a moderate yield of the corresponding ester (Table 2,
entries 10).
Table 1. Optimization of Tishchenko reaction catalyzed by aluminum alkoxide complexes
containing β-hydroxy-imino ligands
Catalyst loading
Entry
Cat.
Temp. (oC)
Time (h)
Yield(%)a
(mol%)
1
2
3
4
5
6
7
8
9
0.5
60
60
60
60
80
40
80
80
80
6
6
6
6
6
6
6
6
4
97.1
95.7
90.7
89.9
99.2
48.1
93.7
87.8
92.6
3a
3b
3c
3d
3a
3a
3a
3a
3a
0.5
0.5
0.5
0.5
0.5
0.3
0.1
0.5
aIsolated yield.
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Table 2. Tishchenko reactions of aromatic aldehydes under 3a.
Yield
(%)
Entry Aldehyde
1
Product
99.2
87.3
2
3
4
5
49.6
96.4
97.1
6
80.9
73.5
O
10
O
The catalyst amount is 0.5 mol%, the yield is isolated yield, Solvent-free, 80 oC, 6 hour.
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CONCLUSION
In summary, four aluminum alkoxide complexes containing β-hydroxy-imino ligands were
prepared, and these complexes were well characterized, including X-ray structure determination.
The catalytic behavior of these aluminum alkoxide complexes about the Tishchenko reaction was
assessed. It was found that all of these complexes can catalyze the Tishchenko reaction with a
range of aromatic aldehydes with high efficiency under mild reaction conditions, and the desired
esters can be obtained in good to excellent isolated yields (up to 99%).
EXPERIMENTAL
General Remarks
Unless otherwise noted, all syntheses and manipulations of air-sensitive materials were
performed under a purified nitrogen atmosphere using the standard Schlenk techniques.
Tetrahydrofuran and diethyl ether were distilled from sodium-benzophenone under nitrogen.
Hexane and toluene is dried using sodium potassium alloy and distilled under nitrogen prior to
use. AlMe3 was purchased from J & K Scientific Ltd. and used as received. All aldehydes,
ketones and amines were recrystallized or distilled before use. And the imines (1a, 1b) were
1
synthesized according to the literature procedure. H NMR and 13C NMR spectra of the
compounds were recorded on a Bruker DRX 300 instrument or Bruker AVANCE III HD 600
instrument in CDCl3 or C6D6 at 298 K and referenced internally to the residual solvent
resonances (1H, 13C ). Elemental analyses were performed on a Vario EL-III instrument. Melting
points were determined on a STUART SMP10 melting point apparatus and uncorrected.
Preparation
(2, 6-Me2C6H3)NC(Ph)CH2C(Ph)2OH (2a)
The Et2O solution of LDA (1.07 g, 10.0 mmol) was added dropwise to an Et2O (20 mL)
solution of (2,6-Me2C6H3)NC(Ph)CH3 1a (2.23 g, 10.0 mmol) at -78 °C under nitrogen. The
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solution was allowed to room temperature and stirred for 2 h. then the Et2O solution of
benzophenone (1.82 g, 10.0 mmol) was added dropwise to the above reaction mixture at 0 °C
and stirred for 4 h. The solution was hydrolyzed by distilled water (50 mL), the aqueous phase
was then extracted with diethyl ether (3 x 20 mL), the organic phase was dried over anhydrous
magnesium sulfate and concentrates under reduced pressure to leave faint yellow reside, which
was recrystallized from Et2O solution to 2a as a colorless solid. The yield is 88%, Mp: 104-106
°C, 1H NMR (CDCl3, 600 MHz): 1.60 (s, 6H, CH3), 3.80 (s, 2H, CH2), 6.80 (m, 3H, C6H3), 7.05
(d, 2H, J=7.2, C6H5 ), 7.23 (m, 5H, C6H5), 7.29 (s, 1H, OH), 7.32(m, 4H, C6H5), 7.58 (d, 4H,
J=7.8, C6H5). 13C NMR (CDCl3, 150 MHz): 17.90 (CH3), 48.7 (CH2), 78.4 (COH), 123.5 (C6H5),
126.1, 126.3, 126.6, 126.7, 127.8, 128.3, 129.6, 138.3, 146.7, 147.4, 170.6 (-C=N).
(2, 6-iPr2C6H3)NC(Ph)CH2C(Ph)2OH (2b)
The reaction was carried out by a similar procedure as described for 2a, except that 2.79 g
(10.0 mmol) of (2,6-iPr2C6H3)NC(Ph)CH3 1b was used instead of 1a, and the product 2b was
recrystallized from saturated tetrahydrofuran and hexane solution as a yellow solid in 81% yield.
1
Mp: 126-127 °C, H NMR (CDCl3, 600 MHz): 0.73 (d, 6H, J= 5.4, CH3), 0.94 (d, 6H, J= 4.8,
CH3), 2.31(m, 2H, CH), 3.88 (s, 2H, CH2), 6.95 (m, 3H, C6H3), 7.09 (m, 2H, C6H5), 7.24 (m, 5H,
C6H5), 7.37 (m, 4H, C6H5), 7.50 (s, 1H, OH), 7.61 (s, 4H, C6H5 ). 13C NMR (CDCl3, 150 MHz):
22.1 (CH3), 24.6 (CH3), 27.9 (-CH(CH3)2), 48.5 (CH2), 78.5 (COH), 122.9 (C6H5), 124.3, 126.1,
126.8, 127.1, 128.2, 128.3, 129.6, 136.9, 137.7, 143.7, 147.6, 170.4 (-C=N).
(2, 6-Me2C6H3)NCPhCH2C(C12H8)OH (2c)
The reaction was carried out by a similar procedure as described for 2a, except that 1.80 g
(10.0 mmol) of fluorenone was used instead of benzophenone, and the product 2c was
recrystallized from saturated ethyl acetate and hexane solution as a yellow solid in 73.4%. Mp:
95~96 °C, 1H NMR (CDCl3, 600 MHz): 2.05 (s, 6H, CH3), 3.51 (s, 2H, CH2), 6.27 (s, 1H, OH),
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6.89 (m, 5H ), 7.19 (m, 2H), 7.28(m, 3H, C6H5), 7.38 (m, 2H), 7.66 (d, 2H, J=6.6), 7.79 (d, 2H,
13
J=6.6). C NMR (CDCl3, 150 MHz): 18.8 (CH3), 48.1(CH2), 81.8 (COH), 119.9, 123.6, 124.7,
126.6, 126.8, 127.8, 127.9, 128.1, 128.9, 129.1, 129.4, 137.7, 139.5, 147.0, 149.1, 170.1 (-C=N).
(2, 6-iPr2C6H3)NCPhCH2C(C12H8)OH (2d)
The reaction was carried out by a similar procedure as described for 2b, except that 1.80 g
(10.0 mmol) of fluorenone was used instead of benzophenone, and the product was recrystallized
1
from saturated ether solution as a faint yellow solid in 87%. Mp. 97-100 °C. H NMR (CDCl3,
600 MHz): 0.85 (d, 6H, J= 5.4, CH3), 1.26 (d, 6H, J= 4.8, CH3), 2.95(m, 2H, CH), 3.53 (s, 2H,
CH2), 6.76 (s, 1H, OH), 6.99 (m, 2H), 7.13 (m, 5H), 7.20 (m, 1H), 7.29 (m, 2H), 7.39 (m, 2H),
13
7.68 (d, 2H, J=6.6), 7.78 (d, 2H, J=6.6). C NMR (CDCl3, 150 MHz): 22.4 (CH3), 24.9 (CH3),
28.4 (-CH(CH3)2), 48.1 (CH2), 81.7 (COH), 120.0, 123.5, 124.6, 124.7, 127.8, 127.9, 128.1,
128.9, 129.5, 1396.8, 136.9, 139.5, 143.8, 149.4, 170.1 (-C=N).
[(2, 6-Me2C6H3)NCPhCH2CPh2O]2AlMe (3a)
The AlMe3 (1.0 mmol, 1.0 M solution in hexane) was added dropwise to an Et2O (20 mL)
solution of 2a (0.405 g, 1.0 mmol) at -78 °C under nitrogen. The reaction mixture was stirred at
room temperature for 8 h. The white solid was isolated by filtration and recrystallized from a
saturated hexane solution to yield colorless crystals of 3a (0.243 g, 57%). Mp. 84°C (decomp.).
1H NMR (C6D6, 300 MHz): -0.29 (s, 3H, AlCH3), 1.89 (s, 6H, CH3), 2.52 (s, 6H, CH3), 3.81-3.97
(q, 2H, CH2), 6.69-7.45 (m, 36H, C6H5, C6H3). 13C NMR (C6D6, 75 MHz): -14.9 (AlCH3), 11.7,
12.1 (CH3), 41.8 (CH2), 71.7 (COAl), 117.4-143.4 (Ar), 170.3 (C=N). Anal. Calc. for
C59H55AlN2O2: C, 83.26; H, 6.51; N, 3.29. Found: C, 83.01; H, 6.59; N, 3.03.
[(2, 6-iPr2C6H3)NCPhCH2CPh2O]2AlMe (3b)
The synthesis of compound 3b was carried out following the procedures similar to those used
for the preparation of 3a. The white solid was recrystallized from a saturated diethyl ether and
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1
hexane solution to yield colorless crystals of 3b (0.371 g, 77%). H NMR(C6D6, 300 MHz): 0.90
(s, 3H, AlCH3), 0.79-0.81, 1.37-1.41, 1.55-1.57, 2.23-2.33 (d, 24H, CH3), 3.16~3.21 (m, 2H,
CH), 5.19-5.24 (m, 2H, CH), 4.28-4.30 (q, 4H, CH2), 7.30-8.40 (m, 36H, C6H5). 13C NMR (C6D6,
75 MHz): -17.2 (AlCH3), 13.5-19.4 (CH3), 39.6 (CH2), 69.9 (COAl), 114.6-143.7 (Ar), 168.0
(C=N). Anal. Calc. for C67H71AlN2O2: C, 83.54; H, 7.43; N, 2.91. Found: C, 82.84; H, 7.28; N,
2.87.
[(2, 6-Me2C6H3)NCPhCH2C(C12H8)OAlMe2]2 (3c)
The synthesis of compound 3c was carried out following the procedures similar to those used
for the preparation of 3a. The white solid was recrystallized from a saturated tetrahydrofuran and
1
hexane solution to yield colorless crystals of 3c (0.662 g, 72%). H NMR (C5D5N, 300 MHz): -
1.23- -0.99 (m, 12H, AlCH3), 1.45-1.53 (m, 12H, CH3), 1.80-2.05 (m, 8H, THF), 3.50-3.57 (m,
8H, THF), 4.12 (s, 4H, CH2), 6.55-7.50 (m, 32H, C6H5, C6H3, fluoryl ring). 13C NMR (C5D5N, 75
MHz): -22.2 (AlCH3), 6.7, 7.2 (CH3), 33.2 (THF), 41.7 (CH2), 56.5 (COAl), 70.7 (THF), 108.4-
140.2 (Ar), 156.9 (C=N). Anal. Calc. for C70H76Al2N2O4: C, 79.07; H, 7.20; N, 2.63. Found: C,
78.56; H, 7.25; N, 2.47.
[(2, 6-iPr2C6H3)NCPhCH2C(C12H8)OAlMe2]2 (3d)
The synthesis of compound 3d was carried out following the procedures similar to those used
for the preparation of 3a. The white solid was recrystallized from a saturated tetrahydrofuran and
1
hexane solution to yield colorless crystals of 3d (0.711 g, 69%). H NMR(C6D6, 300 MHz): -
1.02- -0.77 (m, 12H, AlCH3), 0.28-1.39 (m, 24H, CH3), 1.97-2.01 (m, 4H, CH), 4.43 (s, 4H,
CH2), 6.55-7.48(m, 32H, C6H5, C6H3, fluoryl ring). 13C NMR (C6D6, 75 MHz): -14.8 (AlCH3),
14.2, 16.5(CH3), 19.4 (CH(CH3)2), 40.4 (CH2), 76.6 (COAl), 111.9-137.2 (Ar), 155.0 (C=N).
Anal. Calc. for C78H98Al2N2O4: C, 79.29; H, 8.36; N, 2.37. Found: C, 78.94; H, 8.11; N, 2.13.
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General procedure for Tishchenko reaction catalyzed by aluminum alkoxide complexes
aldehyde (10 mmol) was added to a 10 mL Schlenk flask charged with 0.05 mmol aluminum
alkoxide complexes under nitrogen. Immediately, a yellow solution was obtained with stirring at
room temperature. After the reaction was completed, it was diluted with HCl solution (0.5 M),
and the mixture was extracted 3 times with CH2Cl2. The combined organic layers were dried
over MgSO4. Removal of all volatiles in vacuo, the crude product was purified by column
chromatography on silica gel (Hexane-EtOAc).
X-ray crystallography
Single X-ray diffraction data of the compounds were collected on a Bruker Smart Apex CCD
diffractometer using monochromated Mo Kα radiation, λ = 0.71073 Å. A total of N reflections
were collected by using ω scan mode. Corrections were applied for Lorentz and polarization
effects as well as absorption using multi-scans (SADABS) [18]. Each structure was solved by
direct method and refined on F2 by full matrix least squares (SHELX97) [19] using all unique
data. Then the remaining non-hydrogen atoms were obtained from the successive difference
Fourier map. All non-hydrogen atoms were refined with anisotropic displacement parameters,
whereas the hydrogen atoms were constrained to parent sites, using a riding mode (SHELXTL)
[20]. Details of the modelling of disorder in the crystals can be found in their CIF files.
ACKNOWLEDGMENT
Financial supports from the National Natural Science Foundation of China (No. 20572065),
the Coal-based Key Scientific and Technological Project (No. MH2014-07), the Special Fund for
Agro-scientific Research in the Public Interest (No. 201303106) and the Natural Science
Foundation of Shanxi Province (No. 2011021011-1, 2014011049-26) are gratefully
acknowledged.
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Appendix A. Supplementary material
CCDC (1444854-1444858) contains the supplementary crystallographic data for 2a, 3a, 3b, 3c
and 3d, respectively. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Highlights
ꢀ A series of aluminum complexes of β-hydroxy-imino ligands were synthesized.
ꢀ Their structures were characterized by NMR, elemental analysis and X-ray study.
ꢀ Those compounds as catalysts for Tishchenko reaction have been investigated.
1