Aluminum complexes containing salicylbenzoxazole ligands and their application in the ring-opening polymerization of: Rac -lactide and ?-caprolactone

Article abstract of DOI:10.1039/c6dt00990e

Two series of four-coordinate aluminum (1a-9a) and five-coordinate aluminum (1b-9b) complexes were successfully synthesized via the reactions between the corresponding salicylbenzoxazole ligands and 1 or 0.5 equivalents of AlMe₃, respectively. The synthesized aluminum complexes were characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis. The solid-state structures of complexes 7a and 1b were determined using single crystal X-ray diffraction. Upon addition of 1 equivalent of benzyl alcohol, all complexes were efficient initiators for the ring-opening polymerization (ROP) of rac-lactide (rac-LA) and ?-caprolactone (?-CL). The polymerizations were living with a good control over molecular weights and molecular weight distributions. Under immortal polymerization conditions, all four-coordinate aluminum complexes (1b-9b) exhibited a living polymerization with the obtained molecular weights proportional to the ratio of monomer/benzyl alcohol and the PDIs were narrow. Kinetic studies revealed that both rac-LA and ?-CL polymerizations mediated by all complexes were first-order in monomers. The effects of ligand structure and coordination geometry on the catalytic activity and stereoselectivity were discussed. A good isoselectivity control was achieved for the polymerizations mediated by complexes 4b (P_m = 0.75), 5b (P_m = 0.74), and 9b (P_m = 0.74).

Full text of DOI:10.1039/c6dt00990e

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Aluminum complexes containing salicylbenzoxazole
ligands and their application in the ring-opening
polymerization of rac-lactide and ε-caprolactone†
Pattarawut Sumrit,a Pitak Chuawong,b Tanin Nanok,c Tanwawan Duangthongyouc
and Pimpa Hormnirun*a
Cite this: DOI: 10.1039/c6dt00990e
Two series of four-coordinate aluminum (1a–9a) and five-coordinate aluminum (1b–9b) complexes were
successfully synthesized via the reactions between the corresponding salicylbenzoxazole ligands and 1 or
0.5 equivalents of AlMe3, respectively. The synthesized aluminum complexes were characterized by
1H and 13C NMR spectroscopy and elemental analysis. The solid-state structures of complexes 7a and 1b
were determined using single crystal X-ray diffraction. Upon addition of 1 equivalent of benzyl alcohol, all
complexes were efficient initiators for the ring-opening polymerization (ROP) of rac-lactide (rac-LA) and
ε-caprolactone (ε-CL). The polymerizations were living with a good control over molecular weights and
molecular weight distributions. Under immortal polymerization conditions, all four-coordinate aluminum
complexes (1b–9b) exhibited a living polymerization with the obtained molecular weights proportional to
the ratio of monomer/benzyl alcohol and the PDIs were narrow. Kinetic studies revealed that both rac-LA
and ε-CL polymerizations mediated by all complexes were first-order in monomers. The effects of ligand
structure and coordination geometry on the catalytic activity and stereoselectivity were discussed. A good
isoselectivity control was achieved for the polymerizations mediated by complexes 4b (Pm = 0.75), 5b
(Pm = 0.74), and 9b (Pm = 0.74).
Received 13th March 2016,
Accepted 2nd May 2016
DOI: 10.1039/c6dt00990e
www.rsc.org/dalton
regarding molecular weight, molecular weight distribution,
and stereoselectivity.2 A number of metal complexes, including
Introduction
Due to the environmental concerns and sustainability issues Al,3 Y,4 Ga,5 Zn,6 Sn,7 Ti,8 and lanthanide9 complexes, have
associated with petroleum-based polymers, the demand for been developed as efficient initiators for the ROP of lactide
biodegradable polymers has increased significantly in recent (LA) and ε-caprolactone (ε-CL). The increasing interest in
years. Polylactide (PLA) and polycaprolactone (PCL) are two this area is evidenced by the number of reviews recently
well-known biodegradable aliphatic polyesters. They have published.10
received much interest as high potential materials for medical
Among the various metal initiators, aluminum complexes
and pharmaceutical applications due to their biodegradable are well-suited initiators owing to their strong Lewis acidity
and biocompatible properties.1 Ring-opening polymerization and low toxicity.10a,11 The catalytic performances of aluminum
(ROP) of the cyclic esters initiated by metal complexes via initiators are influenced by the structure of the ancillary
a coordination–insertion mechanism is the most efficient ligand. Most interest was devoted to tetradentate ligands due
method to produce aliphatic polyesters with good control to the ease of fine-tuning their electronic and steric properties.
Many aluminum complexes containing tetradentate ligands,
such as Salen,12 Salan,12h,13 and Salalen,14 have been exten-
aLaboratory of Catalysts and Advanced Polymer Materials, Department of Chemistry
sively studied and some of them have displayed excellent
and Center of Excellence for Innovation in Chemistry, Faculty of Science, Kasetsart
stereocontrol in the catalytic ROP of rac-LA. In the past decade,
University, Bangkok 10900, Thailand. E-mail: fscipph@ku.ac.th
bDepartment of Chemistry and Center of Excellence for Innovation in Chemistry,
Faculty of Science, and Special Research Unit for Advanced Magnetic Resonance
(AMR), Kasetsart University, Bangkok 10900, Thailand
the use of bidentate ligands has gained increasing attention.
However, low stereoselectivity was observed in the polymeri-
zation of rac-lactide due to a limited influence of steric
hindrance on the ligand structure. Bidentate ligands can
bind to the metal center in the form of four-coordinate and
five-coordinate geometries. For example, four-coordinate
aluminum complexes are stabilized by phenoxy-imine (I),3e,15
cDepartment of Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900,
Thailand
†Electronic supplementary information (ESI) available. CCDC 1429409 and
1429410. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6dt00990e
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Fig. 1 Structures of monoanionic bidentate ligands.
β-diketiminate (II),16 amidinate (III),17 and other ligands18 and Currently, to the best of our knowledge, there is no report on
five-coordinate aluminum complexes are stabilized by pyrro- the use of salicylbenzoxazole aluminum complexes as
lylaldiminate (IV),19 benzotriazole-phenoxide (V)20 and 8-hydroxy- initiators for the ROP of rac-LA and ε-CL. Herein, we report the
quinoline (VI) (Fig. 1).3b Besides, aluminum complexes synthesis of a series of four-coordinate and five-coordinate
containing other bidentate ligands have been demonstrated to aluminum complexes supported by salicylbenzoxazole ligands,
be efficient initiators for the ROP of cyclic esters.21
and their catalytic behavior for the polymerizations of rac-LA
The search for new ancillary ligands is an important key to and ε-CL.
success for the development of new efficient catalyst systems.
The 2-(2′-hydroxyphenyl)benzoxazole (HBO) or salicylbenzoxa-
zole (VII) derivatives are an interesting class of ligands due to
Results and discussion
Synthesis and characterization of aluminum complexes 1a–9a
their strong conjugative effect that might have an influence on
the metal center. These ligands were reported to be excellent
monoanionic bidentate [N,O] ligands for various metals for
and 1b–9b
the main application as luminescent materials.22 Jin and co- The salicylbenzoxazole proligands HL1–HL9 were synthesized
worker reported the synthesis of half-sandwich titanium com- by a two-step reaction according to a published procedure
plexes bearing salicylbenzoxazole ligands for ethylene (Scheme 1).25 The first step is a condensation reaction between
polymerization.23 All complexes showed moderate to high 2-aminophenol and the corresponding salicylaldehyde deriva-
activities in the presence of MAO. Recently, the same group tive in ethanol to produce compounds 1–9 (see the ESI† for
designed and synthesized a series of titanium and zirconium the synthetic procedure), followed by an oxidative cyclization
salicylbenzoxazole complexes for olefin polymerization.24 using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as
The electronic and steric influences on the catalytic activities an oxidant. After purification by column chromatography,
of zirconium precatalysts were found to be remarkable. the ligands were obtained as off-white solids (11–78%).
Scheme 1 Synthetic route for proligands HL1–HL9.
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The four-coordinate aluminum complexes 1a–9a were obtained
Single crystals of complexes 7a and 1b suitable for X-ray
in moderate yields (40–63%) via alkane elimination from diffraction analysis were grown from their DCM/hexane mixed
trimethylaluminum (AlMe3) and the corresponding proligands solutions at −20 °C and their molecular structures are illus-
in the molar ratio of 1 : 1 in toluene at room temperature trated in Fig. 2 and 3, respectively (see Tables S1–S4 in the
(Scheme 2). Treatment of AlMe3 with the appropriate proligands ESI† for the crystallographic details). The molecular structure
in the molar ratio of 1 : 2 in toluene at 100 °C afforded of 7a features a monomeric molecule with a four-coordinate
five-coordinate aluminum complexes 1b–9b (20–61%). All aluminum center in a geometry best described as distorted
ligands and aluminum complexes were characterized by tetrahedral as seen in the bond angles for O(1)–Al(1)–C(19)
1H NMR and 13C NMR spectroscopy and elemental analysis.
[111.99(11)°], N(1)–Al(1)–C(19) [107.97(11)°], O(1)–Al(1)–C(18)
The 1H and 13C NMR spectra of complexes 1a–9a, 1b–3b [114.56(11)°] and O(1)–Al(1)–N(1) [92.08(9)°] (Fig. 2). The
and 5b–9b in CDCl3 solution at room temperature contain a aluminum center coordinated with the phenoxy atom O(1) and
single set of resonances, consistent with the existence of a the atom N(1) forms a six-membered N,O-chelated ring with
highly symmetric species on the NMR time scale. It is noted co-planarity. The Al–C bond lengths Al(1)–C(18) [1.956(3) Å]
that complex 4b is insoluble in CDCl3, CD2Cl2, C6D6, and and Al(1)–C(19) [1.951(3) Å] are typical.15a,b,27 The Al(1)–O(1)
DMSO-d6 at room temperature. Therefore, the 1H and 13C bond length is 1.7697(18) Å which is characteristic of σ-
NMR spectra of 4b were obtained in toluene-d8 at 70 °C. The bonding11a,21b,27 while the Al(1)–N(1) distance is 1.951(2) Å dis-
disappearance of the O–H signal of the ligand precursors and playing the coordinative covalent bond character.11a,21b,27
the appearance of the resonance for the protons of aluminum–
Fig. 3 illustrates the molecular structure of the five-coordi-
methyl groups in the high field region are consistent with the nate aluminum complex 1b with a distorted trigonal bipyrami-
structure proposed in Scheme 2 (see Fig. S2–S19 in the ESI† dal geometry around the aluminum center. The amount of
1
for the H NMR spectra of the complexes).26 The four-coordi- distortion can be quantified using the geometric criterion τ =
nate aluminum complexes 1a–9a display only one sharp (β − α)/60.28 The τ value of 0.75 was obtained for complex 1b,
singlet of two magnetically equivalent aluminum methyl indicating the distorted trigonal bipyramidal character. The
groups which can be attributed to the symmetric environment diaxial angle, N(2)–Al(1)–N(1), is close to linear [166.35(5)°]
1
around the aluminum center. For example, the H NMR spec- and the angles at the aluminum center fall within the narrow
trum of complex 3a shows the resonance of two methyl groups range from 119.15(7)° for O(3)–Al(1)–C(27) to 121.02(6)° for
bound to the aluminum center as a singlet at δ −0.62 ppm. O(3)–Al(1)–O(1). The Al–O bond distances [1.787(12) and
Signals assigned to the tert-butyl groups on the phenoxide 1.789(12) Å] and the Al–N bond lengths [2.047(12) and
moiety appeared as two singlets at δ 1.47 and 1.38 ppm. The 2.067(13) Å] are similar to those reported in 7a.
1H NMR spectra for the five-coordinate aluminum complexes
Ring-opening polymerization of rac-lactide
are all similar in that both salicylbenzoxazole ligands are mag-
netically equivalent on the NMR time scale. For example, in
complex 3b, the two aromatic protons from each phenoxide
moiety occurred as two doublet resonances at δ 7.88 and
7.40 ppm. Signals ascribed to tert-butyl protons appeared as
two singlets at δ 1.32 and 0.90 ppm, and the aluminum-methyl
resonance was observed at δ −0.11 ppm.
The four-coordinate aluminum complexes 1a–9a and the five-
coordinate aluminum complexes 1b–9b were tested as
initiators for the ROP of rac-LA in the presence of benzyl
alcohol. The in situ alcoholysis is a typical protocol for
polymerizations employing aluminum initiators.3b,12–15 In
this study, the formation of the true initiating species was
Scheme 2 Synthetic routes for four-coordinate aluminum complexes (1a–9a) and five-coordinate aluminum complexes (1b–9b) from HL1–HL9.
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zations were carried out in toluene at 70 °C. The molar ratio
of rac-LA to initiator was fixed at 100 : 1 ([LA]0/[Al] = 100; [LA]0 =
0.83 M; [Al] = 8.33 mM; Mn (theory) = 14 400). The polymeri-
zation progress was monitored by taking regular aliquots which
1
were subsequently analyzed by H NMR spectroscopy to deter-
mine the conversion. The molecular weights and molecular
weight distributions were determined by gel permeation
chromatography (GPC) using the Mark–Houwink correction of
0.58.29 The results are summarized in Table 1. The aluminum
complexes were all active initiators in the polymerization of
rac-LA. All of the initiator systems exhibited the molecular
weights in good agreement with the theoretical values and
narrow molecular weight distributions in accord with con-
trolled living polymerizations (entries 1–18, Table 1). The poly-
merizations using four-coordinate aluminum complexes 1a–8a
proceeded to ca. 90% within 12 h except the polymerization
using complex 9a, which required 36 h to achieve 88% conver-
sion. For the five-coordinate aluminum complexes 1b–9b, the
polymerizations were relatively slower than those using four-
coordinate aluminum analogs. For instance, complexes 3b, 7b
and 9b required 144 h to reach 90%, 88% and 90%, respect-
ively. The living characteristic of the polymerization initiated
by 1a was also illustrated by a linear relationship between the
number-average molecular weights (Mn) and monomer conver-
sion with narrow molecular weight distributions as shown
in Fig. 4.
Fig. 2 ORTEP representation of 7a with the thermal ellipsoids drawn at
50% probability level. Selected bond lengths (Å) and angles (°) are as
follows: Al(1)–O(1), 1.7697(18); Al(1)–N(1), 1.951(2); Al(1)–C(19), 1.951(3);
Al(1)–C(18), 1.956(3); C(13)–O(1), 1.334(3); C(7)–N(1), 1.317(3); O(1)–Al(1)–
N(1), 92.08(9); O(1)–Al(1)–C(19), 111.99(11); N(1)–Al(1)–C(19), 107.97(11);
O(1)–Al(1)–C(18), 114.56(11); N(1)–C(7)–C(8), 128.2(2); N(1)–C(7)–O(2),
113.3(2); C(13)–C(8)–C(7), 120.0(2); O(1)–C(13)–C(12), 120.0(2); C(1)–
N(1)–Al(1), 131.08(16); C(19)–Al(1)–C(18), 118.06(12).
Polymerizations using four-coordinate aluminum com-
plexes 1a–9a were also performed under “immortal” condi-
tions,3e,15d,30 in which excess benzyl alcohol molecules were
added, which acted as a chain transfer agent (CTA). As shown
in Table 2, when two equivalents of benzyl alcohol were added
(entries 1–9, Table 2), complexes 1a–9a catalyzed the ROP of
rac-LA to PLAs with the observed molecular weights close to
the theoretical values with narrow PDIs, indicating the living
and immortal polymerization. The “immortal” character of all
complexes was also observed when increasing the ratio of
[LA]0 : [Al] : [PhCH2OH] from 100 : 1 : 2 to 100 : 1 : 10. The mole-
cular weights of the resulting polymers decreased with the Mn
value proportional to the monomer/benzyl alcohol ratio, and
the molecular weight distributions were still narrow. These
results suggested that a fast reversible exchange between
dormant hydroxyl-end-capped polymer chains/free alcohol and
the active alkoxy-type polymer chain coordinated onto the
aluminum center occurred significantly faster than the chain
propagation.3e,11b,30d Therefore, the ROP of rac-LA mediated by
complexes 1a–9a under “immortal” conditions is an effective
method for the synthesis of low molecular weight PLAs using
only a small amount of catalyst.
Fig. 3 ORTEP representation of 1b with the thermal ellipsoids drawn at
50% probability level. A molecule of co-crystallized toluene is omitted
for clarity. Selected bond lengths (Å) and angles (°) are as follows: Al(1)–
O(3), 1.789(12); Al(1)–N(2), 2.047(12); Al(1)–O(1), 1.787(12); Al(1)–C(27),
1.957(16); Al(1)–N(1), 2.067(13); C(27)–Al(1)–N(2), 98.15(6); N(2)–Al(1)–
N(1), 166.35(5); O(3)–Al(1)–O(1), 121.02(6); O(3)–Al(1)–C(27), 119.15(7);
O(1)–Al(1)–N(1), 85.15(5); O(3)–Al(1)–N(1), 87.46(5); O(3)–Al(1)–N(2),
85.89(5); O(1)–Al(1)–C(27), 119.79(8); C(27)–Al(1)–N(2), 98.15(6); C(27)–
Al(1)–N(1), 95.50(6).
Kinetic studies of rac-LA polymerization
Kinetic studies of rac-LA polymerization initiated by complexes
1a–9a and 1b–9b in the presence of one equivalent of benzyl
confirmed by a successful synthesis of aluminum benzyloxide alcohol were conducted in toluene at 70 °C ([LA]0/[Al] = 50; [Al]
complex 10 via a stoichiometric reaction between complex 3b = 8.33 mM; [LA]0 = 0.42 M). Conversions of rac-LA over time
and benzyl alcohol (see the synthesis and characterization in were monitored by taking regular aliquots which were then
the ESI and Fig. S20† for the 1H NMR spectrum). All polymeri- analyzed by H NMR spectroscopy. The semilogarithmic plots
1
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Table 1 Polymerization of rac-LA using complexes 1a–9a and 1b–9b in the presence of benzyl alcohola
f
Time
(h)
Conv.b
(%)
Mn (theory)c
Mn (GPC)d
kapp
Entry
Complex
(g mol−1
)
(g mol−1
)
Mw/Mn
Pm
(105 s−1
)
d
e
1
2
3
4
5
6
7
8
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
9a
9b
12
36
12
72
12
144
12
36
12
36
12
36
12
144
12
36
36
144
91
90
90
95
90
90
92
92
92
92
89
84
86
88
90
87
88
90
13 200
13 100
13 100
13 800
13 100
13 100
13 400
13 400
13 400
13 400
12 900
12 200
12 500
12 800
13 100
12 600
12 800
13 100
13 500
12 500
12 800
14 300
11 000
13 000
12 400
12 200
12 000
13 500
10 000
11 000
11 200
10 900
11 300
11 900
12 500
11 100
1.23
1.19
1.23
1.14
1.10
1.05
1.25
1.06
1.12
1.16
1.22
1.11
1.20
1.06
1.23
1.16
1.14
1.04
0.60
0.71
0.54
0.71
0.68
0.67
0.54
0.75
0.55
0.74
0.54
0.67
0.69
0.64
0.55
0.71
0.70
0.74
6.0 0.1
1.7 0.1
5.8 0.2
1.1 0.1
5.6 0.2
0.52 0.01
8.0 0.4
3.5 0.2
8.3 0.3
3.6 0.1
5.7 0.1
1.4 0.1
5.6 0.1
0.54 0.01
7.8 0.2
2.5 0.1
2.6 0.1
0.78 0.01
9
10
11
12
13
14
15
16
17
18
a [LA]0/[Al] = 100, [Al]/[PhCH2OH] = 1, [LA]0 = 0.83 M, [Al] = 8.33 mM, toluene, 70 °C. b As determined via integration of the methine resonances
(1H NMR) of LA and PLA (CDCl3, 500 MHz). c Calculated by [([LA]0/[Al]) × 144.13 × conversion] + 108.14. d Determined by gel permeation chrom-
atography (GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.58 for PLA. e Pm is the probability of the meso linkage
between monomer units and was calculated from the homonuclear decoupled 1H NMR spectra of the obtained poly(rac-LA): [mmm] = Pm
+
2
(1 − Pm)Pm/2; [mmr] = [rmm] = (1 − Pm)Pm /2; [rmr] = (1 − Pm)2/2; [mrm] = [(1 − Pm)2 + (1 − Pm)Pm]/2. f [LA]0/[Al] = 50, [Al]/[PhCH2OH] = 1, [LA]0 =
0.42 M, [Al] = 8.33 mM, toluene, 70 °C.
concentrations of the catalyst were performed. For example,
the semilogarithmic plots of rac-LA conversions versus time for
the polymerizations using six different concentrations of 1a
are shown in Fig. 6. The gradient obtained from the plot of
ln kapp and ln [Al] was equal to 1.07 (ca. 1.0), signifying a first-
order dependence on the aluminum concentration (Fig. 7). In
addition, the propagation rate constant (kp) of 7.38 × 10−3 s−1
mol−1 L was obtained from the slope of the plot between kapp
and [1a] (Fig. 8). Thus, for the 1a/PhCH2OH system, the overall
rate equation is −d[LA]/dt = kp[LA][1a]. The same treatment
was applied to the determination of the overall rate expression
of the 1b/PhCH2OH system (see Fig. S29–S31 in the ESI†). The
order x was equal to 1.05 (ca. 1.0) and the kp value of 2.22 ×
10−3 s−1 mol−1 L was determined. Therefore, the overall rate
●
○
)
Fig. 4 Plot of PLA Mn
(
) (versus polystyrene standards) and PDI (
law in the form of −d[LA]/dt = kp[LA][1b] was established.
Based on the overall rate law, there are two species involved
stoichiometrically in the polymerization process, i.e. the active
aluminum benzyloxide complex and the lactide monomer.
as a function of monomer conversion for a rac-LA polymerization using
1a/PhCH2OH ([LA]0/[Al] = 50, toluene, 70 °C).
of ln [LA]0/[LA]t versus time for the polymerizations using 1a Therefore, the ROP of rac-LA follows a monometallic coordi-
and 1b are shown in Fig. 5 (see also Fig. S21–S28 in the ESI† nation–insertion mechanism with the coordination of rac-LA
for the plots of other complexes). In all cases, the semilogarith- to the aluminum center followed by the ring-opening process.
mic plots are linear and intercept the origin, indicating that
The apparent rate constants (kapp) of all complexes deter-
the polymerizations are first-order in monomer concentration mined from the gradient of ln [LA]0/[LA]t versus time plots are
and proceed without an induction period. The absence of an collected in Table 1. It can be seen that the catalytic activity of
induction period indicated that the polymerization took place each four-coordinate aluminum complex was higher than that
immediately after the active aluminum alkoxide species was of its five-coordinate aluminum analog. This can be explained
generated via in situ alcoholysis between the aluminum methyl by an increase in steric congestion at the aluminum center
complex and benzyl alcohol. Therefore, the polymerization when two ligands bind to the metal. For example, the largest
rate law can be expressed as −d[LA]/dt = kapp[LA], where kapp
=
difference in catalytic activity was observed for complexes 7a
kp[Al]x, in which kp is the propagation rate constant. To deter- and 7b in which the kapp value of 7a ((5.6 0.1) × 10−5 s−1) was
mine the order in aluminum (x), polymerizations with various ca. 10 times higher than that of 7b ((0.54 0.1) × 10−5 s−1).
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Table 2 Polymerization of rac-LA using complexes 1a–9a in the presence of benzyl alcohola
[LA]0 : [Al] :
[PhCH2OH]
Time
(h)
Conv.b
(%)
Mn (theory)c
Mn (GPC)d
(g mol−1
)
(g mol−1
)
Mw/Mn
d
Entry
Complex
1
2
3
4
5
6
7
8
1a
2a
3a
4a
5a
6a
7a
8a
9a
1a
2a
3a
4a
5a
6a
7a
8a
9a
1a
2a
3a
4a
5a
6a
7a
8a
9a
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 5
100 : 1 : 10
100 : 1 : 10
100 : 1 : 10
100 : 1 : 10
100 : 1 : 10
100 : 1 : 10
100 : 1 : 10
100 : 1 : 10
100 : 1 : 10
8
8
8
8
8
8
8
8
24
8
8
8
8
8
8
8
8
24
8
8
8
8
8
8
8
8
24
92
90
80
95
91
87
80
88
88
86
71
77
90
90
70
79
90
84
93
77
85
91
81
74
80
80
84
6700
6600
5900
7000
6700
6400
5900
6400
6400
2600
2100
2300
2700
2700
2100
2400
2700
2400
1400
1200
1300
1400
1300
1200
1300
1300
1300
6500
6500
5800
6600
6200
6100
5400
6200
6100
2300
1700
2000
2400
2300
2100
2200
2500
2200
1200
1100
1100
1300
1100
1100
1200
1100
1100
1.20
1.19
1.08
1.18
1.14
1.14
1.08
1.15
1.08
1.19
1.13
1.17
1.06
1.10
1.12
1.11
1.16
1.11
1.16
1.16
1.14
1.13
1.10
1.13
1.15
1.16
1.13
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
a [LA]0/[Al] = 100, [LA]0 = 0.83 M, [Al] = 8.33 mM, toluene, 70 °C. b As determined via integration of the methine resonances (1H NMR) of LA and
PLA (CDCl3, 500 MHz). c Calculated by ([([LA]0/[Al]) × 144.13 × conversion]/[PhCH2OH]) + 108.14. d Determined by gel permeation chromatography
(GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.58 for PLA.
Fig. 6 Semilogarithmic plots of the rac-lactide conversion versus time
in toluene at 70 °C with complex 1a/PhCH2OH as an initiator ([LA]0
Fig. 5 Semilogarithmic plots of rac-lactide conversion versus time in
■
●
=
toluene at 70 °C with complexes 1a ( ) and 1b ( ) ([LA]0/[Al] = 50, [Al]/
0.42 M: I, [Al] = 24.99 mM, [LA]0/[Al] = 17; II, [Al] = 20.82 mM,
[LA]0/[Al] = 20; III, [Al] = 16.66 mM, [LA]0/[Al] = 25; IV, [Al] = 12.50 mM,
[LA]0/[Al] = 34; V, [Al] = 10.41 mM, [LA]0/[Al] = 40; VI, [Al] = 8.33 mM,
[LA]0/[Al] = 50).
[PhCH2OH] = 1, [LA]0 = 0.42 M, [Al] = 8.33 mM).
It was also found that the catalytic activity decreased when the
size of the alkyl phenoxy substituents at the ortho- and para-
t
positions increased from H to Me to Bu, i.e. the kapp values
diminished in the order 1 (H) > 2 (Me) > 3 (tBu) (entries 1–6, complexes (entries 7 and 8, Table 1) were higher than those
Table 1). Additionally, when the chlorine atoms were intro- of their dimethyl substituted counterparts (2a and 2b),
duced at the ortho- and para-positions of the phenoxy ring suggesting an increase in Lewis acidity at the aluminum center
(complexes 4a and 4b), the catalytic activities of these via the incorporation of electron withdrawing substituents.31
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catalysts. Despite having the same H substituted atom at the
para-position of the phenoxy ring, the catalytic activity of com-
plexes 6–8 decreased in the order 8 > 6 > 7 which corresponded
i
t
to the increase of steric bulkiness from Ph (8) to Pr (6) to Bu
(7) (entries 11–16, Table 1). Introducing the more sterically
congested CMePh2− moiety to the ortho-position (complexes
9a and 9b) appeared to reduce the catalytic activity (entries 17
and 18, Table 1). For the four-coordinate aluminum complexes
1a–9a, the activities decreased in the order 5a > 4a > 8a > 1a >
2a > 6a > 3a ≈ 7a > 9a while the activities of five-coordinate
aluminum counterparts 1b–9b followed the order 5b > 4b > 8b
> 1b > 6b > 2b > 9b > 3b ≈ 7b. The highest catalytic activity in
this study was obtained from complex 5a with a kapp value of
(8.3 0.3) × 10−5 s−1, whereas complex 3b displayed the lowest
Fig. 7 Plot of ln kapp versus ln [Al] for the polymerization of rac-lactide
kapp value (0.52 0.01) × 10−5 s−1
.
with complex 1a/PhCH2OH as an initiator (toluene, 70 °C, [LA]0
=
0.42 M).
Stereoselectivity of rac-LA polymerization
The polymer microstructures of the PLAs were determined by
the inspection of the methine region of the homonuclear
decoupled 1H NMR spectra of the resultant polymers.34 All
homonuclear decoupled 1H NMR spectra produced by com-
plexes 1a–9a and 1b–9b are shown in Fig. S32–S49.† All
initiators gave rise to PLA which has some degrees of isotactic
enchainment. In comparison, the degree of isoselectivity of
five-coordinate aluminum complexes 1b–9b (Pm = 0.64–0.75)
was higher than that of four-coordinate aluminum analogs 1a–
9a (Pm = 0.54–0.70). The lower stereoselectivity observed in the
four-coordinate aluminum complexes can be attributed to the
large coordination sphere for the lactide to coordinate to the
aluminum center, which leads to a decrease in the catalyst’s
ability to select the desired stereoisomer of the lactide. In the
cases of four-coordinate aluminum complexes, it is apparent
that the isoselectivity increases with the steric encumbrance of
the ortho phenoxy substituent. Complexes 3a (o-tBu), 7a
(o-tBu), and 9a (o-CMePh2) produced moderate isotactic PLAs
with the Pm values of 0.68, 069, and 0.70, respectively while the
other complexes in the series polymerized rac-LA with a slight
Fig. 8 Plot of kapp versus [Al] for the polymerization of rac-lactide with
complex 1a/PhCH2OH as an initiator (toluene, 70 °C, [LA]0 = 0.42 M).
However, replacing the dichloro substituents with the less elec- isotactic bias (Pm = 0.54–0.60). In the series of five-coordinate
tronegative dibromo atoms (complexes 5a and 5b) resulted in aluminum complexes, the variation of the ortho substituent at
a slight increase of activity (entries 9 and 10, Table 1). An the phenoxy ring has no influence on the stereoselectivity. The
increase in catalytic activity as the halogen substituent isotactic-enriched PLAs were produced by complexes 4b (Pm
=
becomes less electronegative was also observed in other 0.75), 5b (Pm = 0.74), and 9b (Pm = 0.74) as evidenced by the
systems.32 This was attributed to the different sensitivities observed strong mmm peak illustrated in Fig. S39, S41 and
between the coordination and insertion steps to the Lewis S49,† respectively. In addition, the influence of polymerization
acidity of the metal center.33 The increased Lewis acidity temperature on the degree of isoselectivity was investigated
caused by electron withdrawing substituents may result in using complex 4b. It was found that the degree of isoselectivity
stronger lactide coordination and activation, but it may also increased with the decrease of the polymerization temperature
induce stronger binding of the growing alkoxide chain to the from 70 °C to 50 °C ([LA]0/[Al]/[PhCH2OH] = 100/1/1, [Al] =
metal, decelerating the subsequent insertion step. In addition, 8.33 mM, toluene, 168 h, 96% conversion, Mn = 14 900, PDI =
the effect of ortho- and para-substituents on the phenoxy ring 1.03). A highly isotactic PLA with a Pm value of 0.80 was pro-
was also studied. In comparison with the di-tert-butyl substi- duced (see Fig. S50 in the ESI†).
tuted complexes (3a and 3b), the catalytic activities of the
In general, the stereoselectivity control cannot be achieved
ortho-tert-butyl substituted complexes (7a and 7b, respectively) directly from a bidentate ligand system due to a restricted
were comparable (entries 13 and 14, Table 1). These results influence of steric hindrance. Therefore, it is noteworthy that
demonstrated that the steric factor from the para-substituent these bidentate aluminum complexes (4b, 5b, and 9b) can
has no significant impact on the catalytic performance of the produce isotactic PLAs that are among the highest reported so
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far for these type of aluminum complexes.3b,e,8f,19a For 5b, and 8b were much more active and reached completion in
example, the aluminum complexes supported by 8-hydroxyqui- 30 min.
noline ligands (VI) produced isotactic-biased PLAs with the
highest Pm values of 0.70–0.76.3b,8f The dialkylaluminum com-
plexes stabilized by phenoxy-imine ligands (I) gave rise to PLAs
with different degrees of selectivity (Pm = 0.29–0.80).8f For
aluminum pyrrolylaldiminate complexes (IV), the highest Pm
value of 0.74 was reported.19a
Kinetic studies of ε-CL polymerization
The kinetics of the ROP of ε-CL initiated by complexes 1a–9a
and 1b–9b in the presence of one equivalent of benzyl alcohol
were studied in more detail. The polymerizations were carried
out in C6D6 at 40 °C and the molar ratio of [ε-CL]0/[Al]/
[PhCH2OH] was fixed at 50/1/1 ([Al] = 16.67 mM; [ε-CL]0 = 0.83 M).
The conversion of ε-CL was monitored by 1H NMR spec-
troscopy and the semilogarithmic plots of ln [ε-CL]0/[ε-CL]t
versus time for the polymerizations using 1a and 1b are shown
in Fig. 9 (see also Fig. S51–S58 in the ESI† for the plots of
Ring-opening polymerization of ε-caprolactone
The ring-opening polymerizations of ε-caprolactone using all
aluminum complexes (1a–9a and 1b–9b) were also investigated
under identical conditions for the polymerization of rac-LA.
The results are summarized in Table 3. The molecular weights
and molecular weight distributions were determined by gel
permeation chromatography (GPC) using the Mark–Houwink
correction factor of 0.56.35 All complexes were found to be
active initiators. GPC analysis of the PCL samples obtained
from all complexes displayed monomodal molecular weight
distributions ranging from 1.02–1.56. The number-average
molecular weights were in good agreement with the theoretical
values, indicating the single-site nature of these initiators.
Similar to the ROP of rac-LA, the four-coordinate aluminum
complexes (1a–9a) exhibited higher catalytic activity than their
five-coordinate aluminum counterparts (1b–9b). As shown in
Table 3, the polymerizations using complexes 1a–9a proceeded
to more than 99% monomer conversion within 15 min
whereas a longer polymerization time was required for the
polymerization using complexes 1b–9b. The polymerizations
using complexes 3b, 7b, and 9b required 180 min to reach
>99% conversion. Complexes 1b, 2b, and 6b polymerized rac-
LA to >99% conversion within 120 min whereas complexes 4b,
Fig. 9 Semilogarithmic plots of rac-lactide conversion versus time in
■
●
C6D6 at 70 °C with complexes 1a ( ) and 1b ( ) ([ε-CL]0/[Al] = 50, [Al]/
[PhCH2OH] = 1, [ε-CL]0 = 0.83 M, [Al] = 16.67 mM).
Table 3 Polymerization of ε-caprolactone using complexes 1a–9a and 1b–9b in the presence of benzyl alcohola
Time
(min)
Conv.b
(%)
Mn (theory)c
Mn (GPC)d
kapp
(104 s−1
)
e
Entry
Complex
(g mol−1
)
(g mol−1
)
PDId
1
2
3
4
5
6
7
8
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
8a
8b
9a
9b
15
120
15
120
15
180
15
30
15
30
15
120
15
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
14 500
11 600
10 600
10 000
13 400
12 500
13 000
10 600
12 200
12 400
12 600
12 400
11 300
11 000
11 900
10 600
9500
1.47
1.54
1.42
1.19
1.56
1.35
1.28
1.07
1.29
1.02
1.17
1.21
1.42
1.40
1.54
1.14
1.24
1.04
2.1 0.1
0.4 0.1
4.2 0.3
1.0 0.1
4.5 0.1
0.6 0.1
5.2 0.3
1.5 0.1
12.0 0.1
1.4 0.1
3.8 0.2
0.9 0.1
4.7 0.2
0.6 0.1
5.9 0.3
1.9 0.1
3.8 0.2
0.6 0.1
9
10
11
12
13
14
15
16
17
18
180
15
30
15
180
9400
a [ε-CL]0/[Al] = 100, [Al]/[PhCH2OH] = 1, [ε-CL]0 = 1.67 M, [Al] = 16.7 mM, toluene, 70 °C. b As determined via integration of the methylene reson-
ances (1H NMR) of ε-CL and PCL (CDCl3, 500 MHz). c Calculated by [([ε-CL]0/[Al]) × 114.13 × conversion] + 108.14. d Determined by gel permeation
chromatography (GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.56 for PCL. e [ε-CL]0/[Al] = 50, [Al]/[PhCH2OH]
= 1, [ε-CL]0 = 0.83 M, [Al] = 16.67 mM, C6D6, 40 °C.
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other complexes). In all cases, the polymerization followed 2a > 6a ≈ 9a > 1a. In the cases of the five-coordinate aluminum
first-order kinetics in ε-CL concentrations and no induction complexes, the catalytic activity decreased in the order 8b > 4b
period was observed, indicating that the active species formed > 5b > 2b > 6b > 3b ≈ 7b ≈ 9b > 1a. To a certain extent, the
instantaneously by an in situ alcoholysis reaction. In addition, trend can be understood on the basis of steric and electronic
the living character of the polymerization process was demon- considerations. The presence of electron-withdrawing groups
strated by a linear increase of the Mn values with monomer at the ortho- and para- positions of the phenoxy ring resulted
conversion and the observed narrow molecular weight distri- in an increase of catalytic activity whereas a diminished cata-
butions, as illustrated in Fig. 10 (see also Fig. S59 in the ESI†).
lytic activity was observed by increasing steric bulkiness at the
The apparent first-order rate constants (kapp) are collected ortho- and para- positions of the phenoxy ring. For instance,
in Table 3. For the four-coordinate aluminum complexes 1a– the low kapp value of (0.6 0.1) × 10−4 s−1 was obtained for the
9a, the kapp values followed the order 5a > 8a > 4a > 7a > 3a > polymerizations using complexes 3b (tBu), 7b (tBu), and 9b
(CMePh2) with the bulky ortho-phenoxy substituent. In each
series of aluminum complexes, complexes 1a and 1b showed
the lowest kapp value. Given the unsubstituted phenoxy moiety,
the decrease in the catalytic activity was rather unexpected.
Further studies will be required to understand these
observations.
End-group analysis by 1H NMR spectroscopy
In order to investigate the polymerization mechanism, end-
group analyses of the low molecular weight polymer samples
were performed using 1H NMR spectroscopy. Fig. 11 and 12
1
show the H NMR spectra of PLA-40 and PCL-40, respectively
(number 40 indicates the designed [LA]0/[Al] or [ε-CL]0/[Al] =
40). According to the 1H NMR spectrum of PLA-40, the
polymer chain is capped with a benzyl ester group on one end
and a hydroxyl group on the other end with the integration
ratio of He : Hc = 5 : 1 (Hc = –CH(CH3)OH; He = –OCH2C6H5).
●
(
○
)
Fig. 10 Plot of PLA Mn
) (versus polystyrene standards) and PDI (
These results indicated that the back-biting reaction leading to
the formation of cyclic polyesters did not take place and the
as a function of monomer conversion for a ε-CL polymerization using
1a/PhCH2OH ([ε-CL]0/[Al] = 50, C6D6, 40 °C).
Fig. 11 1H NMR spectrum of PLA-40 initiated by 1b in CDCl3 at 298 K (* = solvent residue peak).
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Fig. 12 1H NMR spectrum of PCL-40 initiated by 1b in CDCl3 at 298 K (* = solvent residue peak).
ring-opening of rac-LA occurred via an acyl-oxygen bond. In the ortho phenoxy position increased the degree of isoselectiv-
the case of PCL-40, the 1H NMR spectrum displays an inte- ity at the metal center. End-group analyses of the low mole-
gration ratio close to 1 between Hb (–OCH2Ph) and Hg cular weight polymers verified that the polymerizations of rac-
(–CH2OH), signifying that the polymer chain is also capped LA and ε-CL followed a coordination–insertion mechanism.
with one benzyl ester and one hydroxyl end. This observation The influence of the ligand substituent and the metal coordi-
implied that the terminal alkoxide OCH2Ph was the initiating nation geometry over the catalytic activity and stereochemistry
group in the polymerization process. Thus, a coordination– found in this system could be useful for the development
insertion mechanism, proceeding through acyl-oxygen cleav- of new metal complexes containing monoanionic bidentate
age of the monomer, should be operative in this system.
ligands in the future.
Conclusions
Experimental section
Materials and methods
In conclusion, two series of aluminum complexes supported
by salicylbenzoxazole ligands (complexes 1a–9a and 1b–9b) All the manipulations with air- and/or water-sensitive com-
were successfully synthesized and characterized. All complexes pounds were carried out under a dry nitrogen atmosphere
were found to be effective initiators for the polymerizations of using standard Schlenk and cannula techniques in oven-dried
rac-LA and ε-CL in the presence of benzyl alcohol. A linear cor- glassware or in a glove box. Toluene and hexane were distilled
relation between Mn and percentage conversion was observed, from Na-benzophenone and CaH2 prior to use, respectively.
and the PDI values were narrow throughout the polymeri- Benzyl alcohol was dried over sodium and then freshly dis-
zation. An effective immortal polymerization was achieved tilled onto activated 4 Å molecular sieves. All the solvents were
when excess alcohol was added. Kinetic studies revealed that degassed before use, unless stated otherwise. The NMR sol-
the four-coordinate aluminum complex exhibited higher vents were dried over 4 Å molecular sieves and degassed prior
activity than its five-coordinate counterpart. An increase of to use. A 2.0 M solution of trimethylaluminum in toluene
steric bulk at the ortho phenoxy substituent resulted in a (Aldrich) was used without purification. 2-hydroxybenzalde-
decrease in catalytic activity while the substitution with an hyde (98%), 3,5-dichloro-2-hydroxybenzaldehyde (99%), 3,5-
electron withdrawing halogen atom at the ortho position dibromo-2-hydroxybenzaldehyde (98%), 3,5-di-tert-butyl-2-
exerted higher activity. Analysis of polymer microstructure hydroxybenzaldehyde (99%), 2-aminophenol (99%) and 2,3-
revealed that more isoselectivity control was achieved in the dichloro-5,6-dicyano-1,4-benzoquinone (98%) were purchased
five-coordinate aluminum complexes. For the series of four- from Aldrich and used as received. 3,5-dimethyl-2-hydroxy-
coordinate aluminum complexes, the steric encumbrance at benzaldehyde, 3-phenyl-2-hydroxybenzaldehyde, 3-isopropyl-
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2-hydroxybenzaldehyde and 3-tert-butyl-2-hydroxybenzalde-
2-(2′-Hydroxy-3′,5′-di-tert-butylphenyl)benzoxazole (HL3).
1
hyde were synthesized using a standard method described in Yield: 1.50 g, 75%. H NMR data (500.13 MHz, CDCl3, 300 K):
the literature.36 The procedure for preparation of 3-(1,1-diphe- δ 11.90 (br s, 1H, OH), 7.93 (d, 4JHH = 2.5 Hz, 1H, ArH), 7.72–7.70
4
nylethyl)-5-methyl-2-hydroxybenzaldehyde is provided in the (m, 1H, ArH), 7.64–7.61 (m, 1H, ArH), 7.52 (d, JHH = 2.5 Hz, 1H,
ESI.† rac-Lactide (Aldrich) was sublimed three times prior to ArH), 7.38 − 7.36 (m, 2H, ArH), 1.51 (s, 9H, C(CH3)3), 1.40 (s, 9H,
use. ε-Caparolactone (Aldrich) was distilled from CaH2. All C(CH3)3). 13C NMR data (125.77 MHz, CDCl3, 300 K): δ 164.3
other chemicals are commercially available and were used as (CvN), 156.1 (ArC), 149.3 (ArC), 141.4 (ArC), 140.3 (ArC), 137.5
received unless otherwise stated. 1H NMR (500.13 MHz) and (ArC), 128.7 (ArCH), 125.3 (ArCH), 125.1 (ArCH), 121.5 (ArCH),
13C NMR (125.77 MHz) spectra and the homonuclear 119.2 (ArCH), 110.8 (ArCH), 110.0 (ArC), 35.6 (C(CH3)3), 34.7
decoupled 1H NMR spectra were recorded on a Bruker Advance (C(CH3)3), 31.8 (C(CH3)3), 29.8 (C(CH3)3). Anal. calcd for
500 MHz spectrometer at 300 K. The 1H NMR spectra were C21H25NO2: C 77.98, H 7.79, N 4.33; found: C 77.89, H 7.78, N 4.30.
referenced internally to the residual proton impurity peaks
2-(2′-Hydroxy-3′,5′-dichlorophenyl)benzoxazole (HL4). Yield:
according to the literature.37 Elemental analysis data (C, H, 0.50 g, 25%. 1H NMR (500.13 MHz, CDCl3, 300 K): δ 12.09
4
and N) were obtained using a Thermo Scientific™ FLASH™ (s, 1H, OH), 7.92 (d, JHH = 2.5 Hz, 1H, ArH), 7.77 − 7.74
2000 Organic Elemental Analyzer. Gel permeation chromato- (m, 1H, ArH), 7.64–7.61 (m, 1H, ArH), 7.50 (d, JHH = 2.5 Hz,
4
graphy (GPC) measurements were conducted on a Polymer 1H, ArH), 7.44–7.42 (m, 2H, ArH). 13C NMR (125.77 MHz,
Laboratories PL-GPC-220 instrument equipped with PLgel CDCl3, 300 K): δ 161.3 (CvN), 153.5 (ArC), 149.4 (ArC), 139.6
5 μm MIXED-D 300 × 7.5 mm columns, and tetrahydrofuran (ArC), 133.4 (ArCH), 126.5 (ArCH), 125.7 (ArCH), 125.1 (ArCH),
(THF) was used as the eluent (flow rate: 1 mL min−1 at 40 °C). 124.5 (ArC), 123.4 (ArC), 119.8 (ArCH), 112.5 (ArC), 111.1
The number-average molecular weights (Mn) and polydis- (ArCH). Anal. calcd for C13H7Cl2NO2: C 55.74, H 2.52, N 5.00;
persity indices (Mw/Mn) were calibrated against polystyrene found: C 55.74, H 2.33, N 4.94.
(PS) standards.
2-(2′-Hydroxy-3′,5′-dibromophenyl)benzoxazole (HL5). Yield:
2.78 g, 59%. 1H NMR (500.13 MHz, CDCl3, 300 K): δ 12.21
General protocol for the synthesis of ligands HL1–HL9
4
4
(s, 1H, OH), 8.09 (d, JHH = 2.4 Hz, 1H, ArH), 7.78 (d, JHH
=
The reaction was performed according to the procedure pre- 2.4 Hz, 1H, ArH), 7.75–7.73 (m, 1H, ArH), 7.63–7.61 (m, 1H,
viously reported in the literature, and the synthesis of com- ArH), 7.44–7.41 (m, 2H, ArH). 13C NMR (125.77 MHz, CDCl3,
pounds 1–9 is described in the ESI.†25 The following example 300 K): δ 161.0 (CvN), 154.7 (ArC), 149.5 (ArC), 139.6 (ArC),
is typical. To a stirred solution of 1 (56.28 mmol) in dichloro- 138.9 (ArCH), 128.8 (ArCH), 126.5 (ArCH), 125.7 (ArCH), 119.8
methane (100 mL), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (ArCH), 112.9 (ArC), 112.5 (ArC), 111.4 (ArC), 111.1 (ArCH).
(67.53 mmol) was added. The reaction mixture was stirred at Anal. calcd for C13H7Br2NO2: C 42.31, H 1.91, N 3.80; found:
room temperature for 15 h. After removal of the volatiles, the C 42.11, H 1.60, N 3.61.
dark color residue was purified by column chromatography
(CH2Cl2 : hexane = 60 : 40). The desired ligands were obtained 4.72 g, 74%. 1H NMR (500.13 MHz, CDCl3, 300 K): δ 11.76
2-(2′-Hydroxy-3′-isopropylphenyl)benzoxazole (HL6). Yield:
4
3
as white solids.
(s, 1H, OH), 7.90 (dd, JHH = 1.6 Hz, JHH = 7.8 Hz, 1H, ArH),
2-(2′-Hydroxyphenyl)benzoxazole (HL1). Yield: 5.10 g, 45%. 7.73–7.71 (m, 1H, ArH), 7.62–7.59 (m, 1H, ArH), 7.40–7.36
1H NMR (500.13 MHz, CDCl3, 300 K): δ 11.47 (s, 1H, OH), 8.03 (m, 3H, ArH), 6.98 (t, JHH = 7.9 Hz, 1H, ArH), 3.50 (sept,
3
4
3
3
(dd, JHH = 1.7 Hz, JHH = 7.9 Hz, 1H, ArH), 7.74–7.72 (m, 1H, 1H, CH(CH3)2), 1.32 (d, JHH = 6.9 Hz, 6H, CH(CH3)2).
ArH), 7.62–7.60 (m, 1H, ArH), 7.46–7.42 (m, 1H, ArH), 13C NMR (125.77 MHz, CDCl3, 300 K): δ 163.7 (CvN),
7.40–7.37 (m, 2H, ArH), 7.14–7.12 (m, 1H, ArH), 7.03–6.99 156.6 (ArC), 149.4 (ArC), 140.3 (ArC), 137.0 (ArC), 130.4 (ArCH),
(m, 1H, ArH). 13C NMR (125.77 MHz, CDCl3, 300 K): δ 163.1 125.4 (ArCH), 125.1 (ArCH), 124.8 (ArCH), 119.5 (ArCH),
(CvN), 159.0 (ArC), 149.3 (ArC), 140.2 (ArC), 133.8 (ArCH), 119.4 (ArCH), 110.8 (ArCH), 110.2 (ArC), 27.2 (CH(CH3)2),
127.3 (ArCH), 125.6 (ArCH), 125.2 (ArCH), 119.8 (ArCH), 119.4 22.6 (CH(CH3)2). Anal. calcd for C16H15NO2: C 75.87, H 5.97,
(ArCH), 117.6 (ArCH), 110.9 (ArCH), 110.8 (ArC). Anal. calcd for N 5.53; found: C 75.98, H 6.00, N 5.53.
C13H9NO2: C 73.92, H 4.29, N 6.63; found: C 73.92, H 4.26,
N 6.59.
2-(2′-Hydroxy-3′-tert-butylphenyl)benzoxazole (HL7). Yield:
1
3.80 g, 38%. H NMR (500.13 MHz, CDCl3, 300 K): δ 12.07 (br
3
2-(2′-Hydroxy-3′,5′-dimethylphenyl)benzoxazole (HL2). Yield: s, 1H, OH), 7.94 (d, JHH = 7.8 Hz, 1H, ArH), 7.73–7.71 (m, 1H,
0.86 g, 11%. 1H NMR (500.13 MHz, CDCl3, 300 K): δ 11.46 ArH), 7.61–7.60 (m, 1H, ArH), 7.47 (d, JHH = 7.7 Hz, 1H, ArH),
(s, 1H, OH), 7.72–7.70 (m, 1H, ArH), 7.68 (s, 1H, ArH), 7.38–7.37 (m, 2H, ArH), 6.95 (t, JHH = 7.7 Hz, 1H, ArH), 1.51
3
3
7.60–7.58 (m, 1H, ArH), 7.38–7.35 (m, 2H, ArH), 7.13 (s, 1H, (s, 9H, C(CH3)3). 13C NMR (125.77 MHz, CDCl3, 300 K): δ 163.9
ArH), 2.34 (s, 3H, CH3), 2.33 (s, 3H, CH3). 13C NMR (CvN), 158.2 (ArC), 149.3 (ArC), 140.2 (ArC), 138.1 (ArC), 131.0
(125.77 MHz, CDCl3, 300 K): δ 163.7 (CvN), 155.2 (ArC), (ArCH), 125.4 (ArCH), 125.1 (ArCH), 119.3 (ArCH), 119.1
149.4 (ArC), 140.4 (ArC), 135.9 (ArCH), 128.3 (ArC), 126.5 (ArC), (ArCH), 110.9 (ArC), 110.8 (ArCH), 35.3 (C(CH3)3), 29.6
125.4 (ArCH), 125.1 (ArCH), 124.7 (ArCH), 119.4 (ArCH), (C(CH3)3). Anal. calcd for C17H17NO2: C 76.38, H 6.41, N 5.24;
110.8 (ArCH), 109.6 (ArC), 20.7 (CH3), 16.2 (CH3). Anal. calcd found: C 76.25, H 6.10, N 5.14.
for C15H13NO2: C 75.30, H 5.48, N 5.85; found: C 75.27, H 5.55,
N 5.88.
2-(2′-Hydroxy-3′-phenylphenyl)benzoxazole (HL8). Yield:
3.90 g, 78%. H NMR (500.13 MHz, CDCl3, 300 K): δ 8.06 (dd,
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4JHH = 1.7 Hz, JHH = 7.9 Hz, 1H, ArH), 7.73–7.69 (m, 3H, ArH), (m, 2H, ArH), 1.47 (s, 9H, C(CH3)3), 1.38 (s, 9H, C(CH3)3),
7.64–7.62 (m, 1H, ArH), 7.54–7.47 (m, 3H, ArH), 7.42–7.38 −0.62 (s, 6H, Al(CH3)2). 13C NMR (125.77 MHz, CDCl3, 300 K):
3
(m, 3H, ArH), 7.10 (t, JHH = 7.7 Hz, 1H, ArH), 4.91 (br s, 1H, δ 166.8 (CvN), 161.9 (ArC), 148.7 (ArC), 141.1 (ArC), 139.6
OH). 13C NMR (125.77 MHz, CDCl3, 300 K): δ 163.4 (CvN), (ArC), 135.7 (ArC), 131.8 (ArCH), 126.4 (ArCH), 126.3 (ArCH),
156.3 (ArC), 149.4 (ArC), 140.1 (ArC), 137.9 (ArC), 134.8 (ArCH), 121.8 (ArCH), 116.7 (ArCH), 111.4 (ArCH), 109.2 (ArC), 35.7
130.6 (ArC), 129.8 (ArCH), 128.5 (ArCH), 127.6 (ArCH), 126.7 (C(CH3)3), 34.6 (C(CH3)3), 31.6 (C(CH3)3), 29.6 (C(CH3)3), −9.7
(ArCH), 125.7 (ArCH), 125.3 (ArCH), 119.8 (ArCH), 119.5 (Al(CH3)2). Anal. calcd for C23H30AlNO2: C 72.80, H 7.97,
(ArCH), 111.1 (ArC), 110.9 (ArCH). Anal. calcd for C19H13NO2: N 3.69; found: C 72.56, H 8.23, N 3.66.
C 79.43, H 4.56, N 4.88; found: C 79.41, H 4.49, N 4.82.
L4AlMe2 (4a). Yield: 0.65 g, 54%. 1H NMR (500.13 MHz,
4
2-(2′-Hydroxy-3′-(1,1-diphenylethyl)-5′-methylphenyl)benz- CDCl3, 300 K): δ 7.89 (d, JHH = 2.7 Hz, 1H, ArH), 7.73–7.70
1
4
oxazole (HL9). Yield: 1.02 g, 20%. H NMR data (500.13 MHz, (m, 2H, ArH), 7.58 (d, JHH = 2.7 Hz, 1H, ArH), 7.56–7.53
CDCl3, 300 K): δ 11.78 (s, 1H, OH), 7.81–7.80 (m, 1H, ArH), (m, 2H, ArH), −0.57 (s, 6H, Al(CH3)2). 13C NMR (125.77 MHz,
7.61–7.58 (m, 2H, ArH), 7.36–7.33 (m, 2H, ArH), 7.32–7.30 CDCl3, 300 K): δ 163.8 (CvN), 158.4 (ArC), 148.9 (ArC), 136.0
4
(m, 4H, ArH), 7.26–7.21 (m, 6H, ArH), 6.67 (d, JHH = 2.1 Hz, (ArCH), 135.1 (ArC), 127.8 (ArC), 127.6 (ArCH), 127.2 (ArCH),
1H, ArH), 2.42 (s, 3H, CH3), 2.24 (s, 3H, CH3). 13C NMR data 125.6 (ArCH), 122.0 (ArC), 117.3 (ArCH), 111.8 (ArCH), 111.2
(125.77 MHz, CDCl3, 300 K): δ 163.6 (CvN), 155.7 (ArC), 149.4 (ArC), −9.6 (Al(CH3)2). Anal. calcd for C15H12AlCl2NO2: C 53.60,
(ArC), 148.5 (ArC), 140.2 (ArC), 136.9 (ArC), 135.7 (ArCH), 128.7 H 3.60, N 4.17; found: C 53.50, H 3.57, N 3.97.
(ArCH), 128.1 (ArCH), 127.8 (ArC), 126.1 (ArCH), 126.0 (ArCH),
125.5 (ArCH), 125.2 (ArCH), 119.3 (ArCH), 110.8 (ArC), 110.7 CDCl3, 300 K): δ 8.08 (d, JHH = 2.5 Hz, 1H, ArH), 7.88
L5AlMe2 (5a). Yield: 0.48 g, 42%. 1H NMR (500.13 MHz,
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(ArCH), 52.2 (C), 28.0 (CH3), 21.1 (CH3). Anal. calcd for (d, JHH = 2.5 Hz, 1H, ArH), 7.73–7.70 (m, 2H, ArH), 7.56–7.53
C28H23NO2: C 82.94, H 5.72, N 3.45; found: C 82.93, H 5.73, (m, 2H, ArH), −0.58 (s, 6H, Al(CH3)2). 13C NMR (125.77 MHz,
N 3.32.
CDCl3, 300 K): δ 163.6 (CvN), 159.4 (ArC), 148.9 (ArC), 141.5
(ArCH), 135.1 (ArC), 129.4 (ArCH), 127.6 (ArCH), 127.2 (ArCH),
118.0 (ArC), 117.2 (ArCH), 111.8 (ArCH), 111.6 (ArC), 108.8
(ArC) −9.6 (Al(CH3)2). Anal. calcd for C15H12AlBr2NO2: C 42.39,
General protocol for the synthesis of aluminum
complexes 1a–9a
In a typical procedure, to a stirred solution of HL1 (4.73 mmol) H 2.80, N 3.30; found: C 42.70, H 3.08, N 3.70.
in toluene (30 mL) trimethylaluminum (TMA) (2.37 mL of a
2.0 M solution in toluene, 4.73 mmol) was slowly added at CDCl3, 300 K): δ 7.85 (d, JHH = 8.0 Hz, 1H, ArH), 7.68–7.66
room temperature. The reaction mixture was stirred at room (m, 2H, ArH), 7.48–7.46 (m, 2H, ArH), 7.43 (d, JHH = 7.3 Hz,
L6AlMe2 (6a). Yield: 0.74 g, 61%. 1H NMR (500.13 MHz,
3
3
3
temperature for 24 h. The volatiles were removed under 1H, ArH), 6.85 (t, JHH = 7.7 Hz, 1H, ArH), 3.49 (sept, 1H,
reduced pressure to leave a pale yellow solid, which was then CH(CH3)2), 1.26 (d, 3JHH = 6.9 Hz, 6H, CH(CH3)2), −0.60 (s, 6H,
recrystallized in hexane at −20 °C. The desired complexes were Al(CH3)2). 13C NMR (125.77 MHz, CDCl3, 300 K): δ 166.2
obtained as off-white solids.
(CvN), 162.3 (ArC), 148.8 (ArC), 141.1 (ArC), 135.6 (ArC), 132.8
L1AlMe2 (1a). Yield: 0.65 g, 52%. 1H NMR (500.13 MHz, (ArCH), 126.5 (ArCH), 125.5 (ArCH), 117.6 (ArCH), 116.8
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3
CDCl3, 300 K): δ 7.99 (dd, JHH = 1.8 Hz, JHH = 8.0 Hz, 1H, (ArCH), 111.4 (ArCH), 109.2 (ArC), 27.3 (CH(CH3)2), 22.4
ArH), 7.69–7.67 (m, 2H, ArH), 7.51–7.46 (m, 3H, ArH), (CH(CH3)2), −9.6 (Al(CH3)2). Anal. calcd for C18H20AlNO2:
7.02–7.00 (m, 1H, ArH), 6.89–6.85 (m, 1H, ArH), −0.60 (s, 6H, C 69.89, H 6.52, N 4.53; found: C 69.75, H 6.52, N 4.50.
Al(CH3)2). 13C NMR (125.77 MHz, CDCl3, 300 K): δ 165.6
L7AlMe2 (7a). Yield: 0.65 g, 54%. 1H NMR (500.13 MHz,
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(CvN), 164.4 (ArC), 148.8 (ArC), 136.9 (ArCH), 135.4 (ArC), CDCl3, 300 K): δ 7.92 (dd, JHH = 1.8 Hz, JHH = 8.0 Hz, 1H,
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128.2 (ArCH), 126.8 (ArCH), 126.6 (ArCH), 122.5 (ArCH), 118.0 ArH), 7.70–7.68 (m, 2H, ArH), 7.75 (dd, JHH = 1.8 Hz, JHH
=
(ArCH), 116.9 (ArCH), 111.6 (ArCH), 109.8 (ArC), −9.2 7.7 Hz, 1H, ArH), 7.51–7.49 (m, 2H, ArH), 6.84 (t, 3JHH = 7.9 Hz,
(Al(CH3)2). Anal. calcd for C15H14AlNO2: C 67.41, H 5.28, 1H, ArH), 1.48 (s, 9H, C(CH3)3), −0.57 (s, 6H, Al(CH3)2.
N 5.24; found: C 67.34, H 5.41, N 5.23.
13C NMR (125.77 MHz, CDCl3, 300 K): δ 166.4 (CvN), 163.7
L2AlMe2 (2a). Yield: 0.78 g, 63%. 1H NMR (500.13 MHz, (ArC), 148.7 (ArC), 141.8 (ArC), 135.6 (ArC), 123.6 (ArCH), 126.5
CDCl3, 300 K): δ 7.67–7.64 (m, 2H, ArH), 7.63 (s, 1H, ArH), (ArCH), 126.4 (ArCH), 126.3 (ArCH), 117.4 (ArCH), 116.7
7.47–7.44 (m, 2H, ArH), 7.22 (s, 1H, ArH), 2.31 (s, 3H, CH3), (ArCH), 111.4 (ArCH), 110.2 (ArC), 35.5 (C(CH3)3), 29.5
2.27 (s, 3H, CH3), −0.60 (s, 6H, Al(CH3)2). 13C NMR (C(CH3)3), −9.8 (Al(CH3)2). Anal. calcd for C19H22AlNO2:
(125.77 MHz, CDCl3, 300 K): δ 166.2 (CvN), 161.4 (ArC), 148.8 C 70.57, H 6.86, N 4.33; found: C 70.67, H 6.86, N 4.23.
(ArC), 138.7 (ArCH), 135.6 (ArC), 130.8 (ArC), 126.4 (ArCH),
124.9 (ArCH), 116.8 (ArCH), 111.4 (ArCH), 108.3 (ArC), CDCl3, 300 K): δ 8.02 (dd, JHH = 1.8 Hz, JHH = 8.0 Hz, 1H,
L8AlMe2 (8a). Yield: 0.53 g, 44%. 1H NMR (500.13 MHz,
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3
4
3
20.6 (CH3), 16.7 (CH3), −9.2 (Al(CH3)2). Anal. calcd for ArH), 7.70–7.67 (m, 4H, ArH), 7.60 (dd, JHH = 1.8 Hz, JHH
=
C17H18AlNO2: C 69.14, H 6.14, N 4.74; found: C 69.08, H 6.12, 7.4 Hz, 1H, ArH), 7.51–7.44 (m, 4H, ArH), 7.38–7.34 (m, 1H,
3
N 4.58.
ArH), 6.96 (t, JHH = 7.7 Hz, 1H, ArH), −0.60 (s, 6H, Al(CH3)2).
L3AlMe2 (3a). Yield: 0.48 g, 41%. 1H NMR (500.13 MHz, 13C NMR (125.77 MHz, CDCl3, 300 K): δ 165.8 (CvN), 161.8
CDCl3, 300 K): δ 7.86 (d, JHH = 2.6 Hz, 1H, ArH), 7.70–7.64 (ArC), 148.8 (ArC), 138.6 (ArC), 137.5 (ArCH), 135.5 (ArC), 134.1
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(m, 2H, ArH), 7.60 (d, JHH = 2.6 Hz, 1H, ArH), 7.48–7.45 (ArC), 129.7 (ArCH), 128.2 (ArCH), 127.6 (ArCH), 127.2 (ArCH),
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126.8 (ArCH), 126.6 (ArCH), 118.0 (ArCH), 117.0 (ArCH),
111.5 (ArCH), 110.5 (ArC), −9.4 (Al(CH3)2). Anal. calcd for toluene-d8, 343 K): δ 8.23 (d, JHH = 8.3 Hz, 2H, ArH), 7.62
C21H18AlNO2: C 73.46, H 5.28, N 4.08; found: C 73.46, H 5.25, (d, JHH = 2.7 Hz, 2H, ArH), 7.29 (d, JHH = 2.7 Hz, 2H, ArH),
L42AlMe (4b). Yield: 0.60 g, 56%. 1H NMR (500.13 MHz,
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4
4
N 4.02. 7.23–7.16 (m, 4H, ArH), 7.08–7.06 (m, 2H, ArH), −0.20 (s, 3H,
L9AlMe2 (9a). Yield: 0.71 g, 62%. 1H NMR (500.13 MHz, AlCH3). 13C NMR (125.77 MHz, toluene-d8, 343 K): δ 134.6
CDCl3, 300 K): δ 7.76 (m, 1H, ArH), 7.67–7.65 (m, 1H, ArH), (ArCH), 126.6 (ArCH), 125.7 (ArCH), 125.6 (ArCH), 122.1 (ArC),
7.59–7.58 (m, 1H, ArH), 7.47–7.44 (m, 2H, ArH), 7.30–7.27 121.4 (ArCH), 110.7 (ArCH). It is noted that some signals in the
(m, 4H, ArH), 7.24–7.19 (m, 6H, ArH), 6.91 (m, 1H, ArH), 2.34 13C NMR spectrum could not be assigned because they were
(s, 3H, CH3), 2.25 (s, 3H, CH3), −0.93 (s, 6H, Al(CH3)2). overlapped with the dominant toluene-d8 peaks. Anal. calcd
13C NMR (125.77 MHz, CDCl3, 300 K): δ 166.1 (CvN), 161.2 for C27H15AlCl4N2O4: C 54.03, H 2.52, N 4.67; found: C 54.33,
(ArC), 148.7 (ArC), 148.3 (ArCH), 140.8 (ArC), 138.7 (ArCH), H 2.52, N 4.39.
135.6 (ArC), 128.6 (ArCH), 127.9 (ArCH), 126.5 (ArC), 126.4
(ArCH), 126.3 (ArCH), 125.8 (ArC) 125.7 (ArCH), 116.7 (ArCH), CDCl3, 300 K): δ 8.14 (d, JHH = 2.5 Hz, 2H, ArH), 8.13–8.12
L52AlMe (5b). Yield: 0.64 g, 61%. 1H NMR (500.13 MHz,
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111.3 (ArCH), 109.9 (ArC), 52.1 (C), 28.2 (CH3), 21.0 (CH3), (m, 2H, ArH), 7.75 (d, JHH = 2.5 Hz, 2H, ArH), 7.68–7.66
−10.1 (Al(CH3)2). Anal. calcd for C30H28AlNO2: C 78.07, H 6.11, (m, 4H, ArH), −0.56 (s, 3H, AlCH3). 13C NMR (125.77 MHz,
N 3.03; found: C 78.05, H 6.17, N 2.87.
CDCl3, 300 K): δ 161.3 (CvN), 149.1(ArC), 140.5 (ArC), 139.9
(ArCH), 136.7 (ArC), 129.0 (ArCH), 126.6 (ArCH), 125.6 (ArCH),
121.3 (ArCH), 117.6 (ArC), 113.0 (ArC), 110.7 (ArCH), 108.6
(ArC). Anal. calcd for C27H15AlBr4N2O4: C 41.68, H 1.94,
General protocol for the synthesis of aluminum
complexes 1b–9b
In a typical procedure, to a stirred solution of HL1 (4.73 mmol) N 3.60; found: C 41.88, H 1.96, N 3.60.
in toluene (30 mL), trimethylaluminum (1.18 mL of a 2.0 M
L62AlMe (6b). Yield: 0.55 g, 51%. 1H NMR (500.13 MHz,
solution in toluene, 2.36 mmol) was slowly added at room CDCl3, 300 K): δ 8.07–8.06 (m, 2H, ArH), 7.88 (d, 3JHH = 8.0 Hz,
temperature. The reaction mixture was stirred at 100 °C for 2H, ArH), 7.68–7.65 (m, 2H, ArH), 7.47–7.43 (m, 4H, ArH), 7.27
3
3
15 h. After cooling to room temperature, a solid precipitated. (d, JHH = 7.9 Hz, 2H, ArH), 6.80 (t, JHH = 7.7 Hz, 2H, ArH),
The desired complexes were obtained as off-white solids.
3
2.97 (sept, 2H, CH(CH3)2), 0.98 (d, JHH = 6.9 Hz, 6H, CH
L12AlMe (1b). Yield: 0.62 g, 57%. 1H NMR (500.13 MHz, (CH3)2), 0.60 (d, JHH = 6.8 Hz, 6H, CH(CH3)2), −0.39 (s, 3H,
CDCl3, 300 K): δ 8.11–8.09 (m, 2H, ArH), 8.06–8.04 (m, 2H, AlCH3). 13C NMR (125.77 MHz, CDCl3, 300 K): δ 164.1 (CvN),
ArH), 7.69–7.66 (m, 2H, ArH), 7.49–7.45 (m, 4H, ArH), 161.5(ArC), 149.0 (ArC), 140.6 (ArC), 137.8 (ArC), 131.1 (ArCH),
7.44–7.39 (m, 2H, ArH), 6.92–6.86 (m, 4H, ArH), −0.76 (s, 3H, 125.6 (ArCH), 125.0, (ArCH), 124.9 (ArCH), 119.8 (ArCH), 117.1
AlCH3). 13C NMR (125.77 MHz, CDCl3, 300 K): δ 164.0 (CvN), (ArCH), 110.7 (ArCH), 110.1 (ArC), 26.2 (CH(CH3)2), 22.8
163.8 (ArC), 149.2 (ArC), 137.8 (ArC), 135.3 (ArCH), 127.6 (CH(CH3)2), 22.2 (CH(CH3)2). Anal. calcd for C33H31AlN2O4:
(ArCH), 125.7 (ArCH), 125.6 (ArCH), 122.0 (ArCH), 119.7 C 72.51, H 5.72, N 5.13; found: C 72.55, H 5.73, N 5.13.
3
(ArCH), 117.7 (ArCH), 111.2 (ArC), 110.9 (ArCH), 0.24 (AlCH3).
Anal. calcd for C27H19AlN2O4: C 70.13, H 4.14, N 6.06; found: CDCl3, 300 K): δ 8.06–8.04 (m, 2H, ArH), 7.92 (dd, JHH
L72AlMe (7b). Yield: 0.59 g, 55%. 1H NMR (500.13 MHz,
4
=
3
C 70.26, H 4.10, N 6.01.
1.8 Hz, JHH = 8.0 Hz, 2H, ArH), 7.66–7.64 (m, 2H, ArH),
L22AlMe (2b). Yield: 0.56 g, 52%. 1H NMR (500.13 MHz, 7.46–7.44 (m, 4H, ArH), 7.33 (dd, JHH = 1.8 Hz, JHH = 7.5 Hz,
4
3
3
CDCl3, 300 K): δ 8.06–8.05 (m, 2H, ArH), 7.69 (s, 2H, ArH), 2H, ArH), 6.74 (t, JHH = 7.8 Hz, 2H, ArH), 0.88 (s, 18H,
7.65–7.63 (m, 2H, ArH), 7.43–7.41 (m, 4H, ArH), 7.09 (s, 2H, C(CH3)3), −0.08 (s, 3H, AlCH3). 13C NMR (125.77 MHz, CDCl3,
ArH), 2.30 (s, 6H, CH3), 1.86 (s, 6H, CH3), −0.60 (s, 3H, AlCH3). 300 K): δ 164.4 (CvN), 162.8 (ArC), 149.0 (ArC), 140.9 (ArC),
13C NMR (125.77 MHz, CDCl3, 300 K): δ 164.1 (CvN), 160.7 137.6 (ArC), 132.1 (ArCH), 126.0 (ArCH), 125.6 (ArCH), 123.1
(ArC), 149.1 (ArC), 137.8 (ArC), 137.1 (ArCH), 130.4 (ArC), 125.8 (ArCH), 120.2 (ArCH), 116.9 (ArCH), 111.3 (ArC), 110.6 (ArCH),
(ArC), 125.5 (ArCH), 125.1 (ArCH), 124.4 (ArCH), 119.9 (ArCH), 34.6 (C(CH3)3), 28.9 (C(CH3)3). Anal. calcd for C35H35AlN2O4:
110.6 (ArCH), 109.5 (ArC), 20.6 (CH3), 17.0 (CH3), −6.2 (AlCH3). C 73.15, H 6.14, N 4.87; found: C 73.20, H 6.47, N 4.78.
Anal. calcd for C31H27AlN2O4: C 71.80, H 5.25, N 5.40; found:
L82AlMe (8b). Yield: 0.56 g, 52%. 1H NMR (500.13 MHz,
CDCl3, 300 K): δ 8.10 (dd, JHH = 1.8 Hz, JHH = 7.9 Hz, 2H,
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3
C 71.83, H 5.27, N 5.39.
L32AlMe (3b). Yield: 0.45 g, 42%. 1H NMR (500.13 MHz, ArH), 7.58 (td, 4JHH = 0.9 Hz, 3JHH = 7.1 Hz, 2H, ArH), 7.48 (dd,
CDCl3, 300 K): δ 8.06–8.04 (m, 2H, ArH), 7.88 (d, 4JHH = 2.6 Hz, 4JHH = 1.9 Hz, JHH = 7.3 Hz, 2H, ArH), 7.29–7.25 (m, 8H, ArH),
3
4
4
3
2H, ArH), 7.68–7.65 (m, 2H, ArH), 7.45–7.43 (m, 4H, ArH), 7.40 6.96 (t, JHH = 3.7 Hz, 2H, ArH), 6.87 (td, JHH = 1.0 Hz, JHH
=
4
(d, JHH = 2.6 Hz, 2H, ArH), 1.32 (s, 9H, C(CH3)3), 0.90 (s, 9H, 7.8 Hz, 2H, ArH), 6.74–6.65 (m, 6H, ArH), −0.73 (s, 3H, AlCH3).
C(CH3)3), −0.11(s, 3H, AlCH3). 13C NMR (125.77 MHz, CDCl3, 13C NMR (125.77 MHz, CDCl3, 300 K): δ 163.3 (CvN), 161.2
300 K): δ 164.7 (CvN), 161.0 (ArC), 149.0 (ArC), 140.1 (ArC), (ArC), 148.6 (ArC), 138.6 (ArC), 137.5 (ArC), 135.8 (ArCH), 134.2
138.9 (ArC), 137.8 (ArC), 130.2 (ArCH), 125.4 (ArCH), 125.0 (ArC), 129.3 (ArCH), 127.0 (ArCH), 126.9 (ArCH), 126.6 (ArCH),
(ArCH), 121.6 (ArCH), 120.2 (ArCH), 110.5 (ArCH), 110.3 (ArC), 125.3 (ArCH), 125.1 (ArCH), 119.5 (ArCH), 117.5 (ArCH),
34.9 (C(CH3)3), 34.5 (C(CH3)3), 31.6 (C(CH3)3), 29.0 (C(CH3)3), 111.5 (ArC), 110.0 (ArCH), −6.0 (AlCH3). Anal. calcd for
−3.8 (AlCH3). Anal. calcd for C43H51AlN2O4: C 75.19, H 7.48, C39H27AlN2O4: C 76.21, H 4.43, N 4.56; found: C 76.12, H 4.25,
N 4.08; found: C 75.51, H 7.73, N 4.43.
N 4.20.
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L92AlMe (9b). Yield: 0.42 g, 40%. 1H NMR (500.13 MHz, solved by direct methods with SIR200438 and refined with
CDCl3, 300 K): δ 7.68–7.66 (m, 4H, ArH), 7.56–7.55 (m, 2H, Olex2.refine.39 The structure of C34H27AlN2O4 (1b) was solved
ArH), 7.36 (td, 4JHH = 1.1 Hz, 3JHH = 7.4 Hz, 2H, ArH), 7.31–7.28 by XT40 and refined with XL.41 Olex242 is used for molecular
(m, 2H, ArH), 7.16–7.13 (m, 4H, ArH), 7.09–7.06 (m, 2H, ArH), graphic. Crystallographic data have been deposited at the
6.90 (d, 3JHH = 7.6, 4H, ArH), 6.74 (d, 3JHH = 7.0, 2H, ArH), 6.61 Cambridge Crystallographic Data Centre under reference
3
3
(t, JHH = 7.1, 4H, ArH), 6.51 (d, JHH = 7.6, 4H, ArH), 6.40 numbers CCDC 1429409 (1b) and 1429410 (7a).
(s, 2H, ArH), 2.20 (s, 6H, CH3), 1.98 (s, 6H, CH3), −0.57 (s, 3H,
AlCH3). 13C NMR (125.77 MHz, CDCl3, 300 K): δ 163.3 (CvN),
160.0 (ArC), 149.5 (ArC), 148.7 (ArC), 147.4 (ArC), 139.7 (ArC),
137.4 (ArC), 137.0 (ArCH), 128.9 (ArCH), 127.9 (ArCH), 127.0
Acknowledgements
(ArCH), 126.7 (ArCH), 126.8 (ArC), 125.7 (ArCH), 125.2 (ArCH), We gratefully acknowledge financial support from Kasetsart
125.1 (ArCH), 124.8 (ArCH), 124.4 (ArCH), 120.1 (ArCH), 111.6 University Research and Development Institute (KURDI) (grant
(ArC), 110.3 (ArCH), 51.8 (C), 26.1 (CH3), 21.2 (CH3), −3.7 no. 26.58), the National Research Council of Thailand (NRCT)
(AlCH3). Anal. calcd for C57H47AlN2O4: C 80.45, H 5.57, N 3.29; and the Faculty of Science, Kasetsart University (grant no.
found: C 80.47, H 5.61, N 3.29.
ScRF-S21-2558). This research is also supported in part by the
Graduate Program Scholarship from the Graduate School,
Kasetsart University (P.T.). P. H. and P. C. thank Kasetsart Uni-
General polymerization procedure for rac-LA
In a nitrogen-filled glove box, rac-lactide (720 mg, 5.0 mmol) versity and the Center of Excellence for Innovation in Chem-
and benzyl alcohol (5.17 μL, 0.05 mmol) were placed in a istry (PERCH-CIC), the Office of the Higher Education
polymerization ampoule. To this ampoule, a solution of the Commission, Ministry of Education for financial support. We
initiator (0.05 mmol) in toluene (6.00 mL) ([monomer] : [Al] = also gratefully acknowledge Dr Kittiponk Chainok for his
100 : 1) was added. The reaction was stirred for the desired useful discussion on X-ray structure determination.
reaction time at 70 °C. Subsequently, the reaction was
quenched with methanol (2–3 drops). The polymer was preci-
pitated from excess methanol, collected by filtration and dried
in vacuo to a constant mass. Conversions were determined by
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