Ring-opening polymerization of rac-lactide with aluminum chiral anilido-oxazolinate complexes

Article abstract of DOI:10.1021/om401226j

A series of dimethylaluminum complexes (L^1a-i)AlMe₂ (2a-i, where HL^1a-i = 2-(2′-ArNH)phenyl-4-R₁- oxazoline) bearing chiral, bidentate anilido-oxazolinate ligands have been prepared and characterized. Six of the

Full text of DOI:10.1021/om401226j

Article
pubs.acs.org/Organometallics
Ring-Opening Polymerization of rac-Lactide with Aluminum Chiral
Anilido-Oxazolinate Complexes
Shi Bian,† Srinivas Abbina,† Zhengliang Lu,†,§ Edward Kolodka,‡ and Guodong Du*,†
†Department of Chemistry, University of North Dakota, 151 Cornell Street Stop 9024, Grand Forks, North Dakota 58202, United
States
‡Department of Chemical Engineering, University of North Dakota, Grand Forks, North Dakota 58203, United States
S
* Supporting Information
ABSTRACT: A series of dimethylaluminum complexes (L1a−i)AlMe2
(2a−i, where HL1a−i = 2-(2′-ArNH)phenyl-4-R1-oxazoline) bearing
chiral, bidentate anilido-oxazolinate ligands have been prepared and
characterized. Six of the complexes, in the presence of an alcohol
cocatalyst, are shown to be active initiators for the stereoselective ring-
opening polymerization of rac-lactide in toluene solution and under
bulk conditions, yielding polylactides with a range of tacticity from
slightly isotactic to moderately heterotactic. The reactivity and
selectivity of these catalysts are discussed on the basis of the effect of their substituents.
microstructure showed that a chiral salen aluminum complex
(Scheme 1; I, where R = (R)-2,2′-binaphthyl, R1 = R2 = H, X =
INTRODUCTION
■
Synthetic aliphatic polyesters, such as polylactide (PLA), and
their copolymers have attracted considerable attention due to
their biocompatibility and biodegradability.1 These features are
important in biomedical applications, such as drug delivery and
tissue engineering,2 and in agricultural and packaging
applications.3 Furthermore, PLA is derived from renewable
resources and is considered a viable alternative to petrochem-
ical-based materials. Although PLA can be produced by the
polycondensation of lactic acids, the ring-opening polymer-
ization (ROP) of lactide (LA) with an initiator/catalyst is the
method of choice because it offers a higher degree of reaction
control.1 Due to the presence of two chiral centers in a lactide
monomer, a range of microstructures, including atactic,
isotactic, heterotactic, and syndiotactic, are possible for PLA
derived from lactide. The tacticity has a significant effect on the
physical and thermal properties of bulk materials. For example,
atactic PLA is amorphous, while isotactic PLA is a crystalline
polymer that melts at ∼170 °C.4 Alongside the structure of
lactide monomers, catalysts play a vital role in determining the
stereo outcome of ROP. Much effort has currently been
devoted to the design and synthesis of single-site catalysts/
initiators for the ROP of rac-lactide with high activities and
stereoselectivities.5
A wide array of catalytic systems for the ROP of lactides have
been developed in the literature, ranging from homoleptic
metal salts such as Sn(Octanoate)2, Ca[N(SiMe3)2]2, and
Sc(OTf)3 in combination with alcohols as chain transfer
reagents6 to well-defined single-site catalysts such as Zn and Mg
complexes supported by β-diketiminate7 and Y and Sc
complexes supported by pyridine-diamide type ligands.8 In
particular, group 13 metal complexes are of special interest
because of their effective stereo control and low toxicity.9 The
pioneering works by Spassky10 in the control of PLA
Scheme 1. Examples of Stereoselective Al-Based Initiators
for the ROP of rac-Lactide
OMe) was highly isoselective for the ROP of rac-lactide. This
system was further exploited11 and expanded to other salen
species,12 their reduced derivatives such as salan (II; Scheme
1)13 and salalen,14 and related N2O2 (III; Scheme 1) and N4
type ligands.15 These initiators typically feature a five-
coordinate aluminum center supported by dianionic, tetraden-
tate ligands. A range of stereoselectivity has been achieved,
from highly isospecific to highly heterospecific, even by simply
varying the substituent groups in the same ligand framework.13a
Al complexes supported by related bidentate half-salen ligands
are usually four-coordinate (IV; Scheme 1) and exert less
stereocontrol for the ROP of rac-lactide.16 Al complexes
supported by bidentate ligands can take on a pentacoordinate
Received: December 23, 2013
© XXXX American Chemical Society
A
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geometry and induce high isoselectivity.17 It should also be
mentioned that initiators based on heavier group 13 elements
have been shown to be stereoselective for the ROP of rac-
lactide.18
variations of the conventional β-diketiminate ligands. The
oxazoline ring is conveniently formed from chiral amino
alcohols and 2-bromobenzaldehyde, and the coupling between
oxazolines and anilines using a palladium-catalyzed amination
protocol gave the ligands as orange oily or crystalline materials
in good to high yields (69−88%). In addition to addressing the
question if the chiral version could induce stereoselectivity in
the ROP of rac-lactide, this particular series of ligands was
targeted to investigate the steric and electronic influences of the
substituents (see Scheme 2), because such factors may
drastically affect the stereoselectivity toward the ROP of rac-
lactide.13a The stereogenic center is introduced at the oxazoline
R1 position and is expected to exert a substantial steric
influence. The aniline R3 and R4 positions are mostly electronic.
On the other hand, the R2 position is in closer proximity to the
metal center, in comparison to the ortho position in related
bidentate half-salen ligands (see the R1 group in IV in Scheme
1), and it can also influence the metal center electronically. On
the basis of these considerations, (S)-iPr, (R)-Ph, and (R)-iBu at
the R1 position and a methyl group at the R2 position were
incorporated to test the effect of steric factors on the reactivity
and selectivity of the catalysts. A trifluoromethyl group on the
R3 position and a methoxy group on the R4 position were used
to probe the electronic effect.
Bidentate and monoanionic β-diketiminate ligands are
analogous to the half-salen ligands and have been successfully
employed in ROP reactions.5a,19 We have been interested in a
chiral variation derived from the anilido-oxazolinate frame-
work20 and turned our attention to aluminum complexes
incorporating these chiral ligands. The achiral version of the
anilido-oxazolinate aluminum complexes in the ROP of L-
lactide has been reported.21 It is expected that the new chiral
complexes are active initiators for the ROP of rac-lactide and
may induce appreciable stereoselectivity to control the
microstructure of the product. Herein we report a series of
dimethylaluminum complexes and their application as initiators
for the ROP of rac-lactide. Both hetero- and isotactic
selectivities could be achieved by modulation of the substituent
groups.
RESULTS AND DISCUSSION
■
Ligands. As mentioned earlier, the bidentate anilido-
oxazolinate ligands (HL1; Scheme 2) can be viewed as chiral
Scheme 2. Preparation of Ligands and Their Aluminum
Complexes
Synthesis and Characterization of Aluminum Com-
plexes. Reaction of the free ligands HL1 with 1.2 equiv of
AlMe3 in toluene at ambient temperature yielded the
dimethylaluminum complexes 2a−i (Scheme 2). The com-
pounds were normally isolated as yellow powders in good
1
yields (68−87%). Characterization by H NMR and 13C NMR
revealed the formation of the expected dimethylaluminum
complexes and was consistent with the mononuclear structures.
For instance, the NH peak of the free ligand (around 10 ppm)
1
disappeared upon reaction, and the H NMR signals for the
oxazoline moiety shift downfield in comparison to that in the
free ligands, indicative of the formation of a six-membered
chelate ring. Furthermore, two separate singlets in the upfield
region around −1 ppm were observed and attributed to the
aluminum methyl (AlMe2) protons. These observations are in
agreement with the unsymmetrical nature of the complexes,
and the two methyl groups are nonequivalent. The latter is
a
Table 1. Polymerization of rac-Lactide with Al Complexes 2a−i
b
c
d
d
e
entry
cat.
cat. loading (mol %)
time (h)
conversn (%)
Mn
6.92
28.0
Mn
Mw/Mn
Pr/Pm
1
2
3
4
2a
2a
2b
2c
2c
2d
2e
2e
2e
2f
2
24
48
24
24
20
24
24
48
48
24
24
24
24
96
97
15
95
95
12
95
98
40
12
95
93
98
7.7 (4.4)
1.20
1.45
Pr = 0.69
Pr = 0.51
0.5
2
12.1 (7.0)
2
6.85
6.85
10.8 (6.3)
7.7 (4.5)
1.23
1.29
Pm = 0.60
Pm = 1.0
Pr = 0.53
Pr = 0.74
f
5
2
6
7
8
2
2
6.85
8.3 (4.8)
1.28
1.69
1
14.1
23.6 (13.7)
g
9
1
10
11
12
13
2
2g
2h
2i
2
6.85
6.70
7.06
4.9 (2.8)
9.7 (5.6)
9.0 (5.2)
1.64
1.25
1.53
Pm = 0.57
Pr = 0.62
Pr = 0.61
2
2
a
b
c
Reaction conditions: see the Experimental Section for details. Monomer conversion determined by 1H NMR spectroscopy. Calculated molecular
d
weight based on conversion: [LA]0/[Al]0 × (conversn %) × 144. Experimental molecular weight determined by GPC vs polystyrene standards. The
values in parentheses are corrected by a factor of 0.58.23 Probability of racemic (Pr) or meso (Pm) enchainment, determined by homonuclear
e
fL-Lactide was used. Reaction was carried out in THF at 60 °C.
g
1
decoupled H NMR spectroscopy.
B
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a
Table 2. Polymerization of rac-Lactide with Al Complexes under Bulk Conditions
b
c
d
d
e
entry
cat.
cat. loading (mol %)
time (h)
conversn (%)
Mn
5.33
46.7
Mn
Mw/Mn
Pr/Pm
1
2
3
4
5
6
7
2a
2a
2c
2e
2f
2
1.5
1.0
1.0
1.0
2.0
2.0
1.5
74
81
65
61
31
78
3.9 (2.3)
23.1 (13.4)
13.1 (7.6)
4.2 (2.5)
1.74
1.45
2.02
1.82
1.95
2.99
Pr = 0.52
Pr = 0.52
Pm = 0.61
Pr = 0.57
Pr = 0.60
Pm = 0.57
Pr = 0.58
0.25
0.25
0.25
0.25
0.25
0.25
37.5
35.2
17.9
44.9
43.2
19.6 (11.3)
35.6 (20.6)
2g
2i
75
207 (120)
1.84
a
b
c
Reaction conditions: see the Experimental Section for details. Monomer conversion determined by 1H NMR spectroscopy. Calculated molecular
d
weight based on conversion: [LA]0/[Al]0 × (conversn %) × 144. Experimental molecular weight determined by GPC vs polystyrene standards. The
values in parentheses are corrected by a factor of 0.58. Probability of racemic (Pr) or meso (Pm) enchainment, determined by homonuclear
decoupled H NMR spectroscopy.
e
1
relevant in the context of stereoselective catalysis, as the
growing polymer chain will occupy one of the two likely
coordination sites during polymerization. In complexes 2b,d,f,
two singlets corresponding to the two o-methyl groups on the
aniline phenyl moiety were observed, indicative of restricted
rotation due to the steric bulk of o-methyl groups. In line with
temperatures (entry 9). When an L-lactide was employed with
2c, a perfectly isotactic PLA (PLLA) with Pm = 1 was obtained
and no epimerization was detected (entry 5). The observed
molecular weights of PLA are typically lower than or close to
the calculated values based on conversions, and the molecular
weight distributions are somewhat broad (1.2−1.7). MALDI-
TOF mass spectrometry (Figure S7, Supporting Information)
analysis of the polymer generated by 2c revealed the presence
of the benzyloxy end group, supporting the alkoxide as the
initiating group. Furthermore, the presence of a series of peaks
separated by a mass unit of 72 indicates considerable
transesterification during the polymerization.
One of our main goals was to investigate if these chiral
catalysts could induce stereoselectivity in the ROP of rac-
lactide. Thus, the microstructure (or tacticity) of the resulting
PLA, in particular the probability of isotactic enchainment at
the diad level, Pm, was assessed by integration of the methine
region of the homonuclear decoupled 1H NMR spectrum
(Table 1). It is noteworthy that a range of stereoselectivities
was observed. While two of the initiators (2c,g) lead to isotactic
enrichment, the remaining initiators lead to heterotactic
enrichment. Among this series of catalysts, 2c showed the
highest isoselectivity (Pm = 0.61), while 2a (Pr = 0.69) and 2e
(Pr = 0.74) were heterotactically inclined. Apparently the
electron-withdrawing ability and the bulkiness of the phenyl
group at R1 may be responsible for the increase in isoselectivity.
Compound 2g also exhibited a slight isotactic bias (Pm = 0.57),
which again was attributed to the presence of a strong electron-
withdrawing group at R3. On the basis of these considerations,
2i with a phenoxy group at the R2 position was expected to
have high heteroselectivity. However, a Pr value of 0.58 was
observed. This might be accounted for by the presence of anti
and syn isomers of 2i. Although the stereoselectivity observed
was not particularly high, the findings here are interesting
because the change of selectivity was due to the variation of
substituents on R1, R3, and R4 in the similar initiator structures.
Such a strategy of modulation of tacticity by changing
substituents has been utilized in other aluminum13 and
zirconium24 systems.
1
these observations, two sets of H and 13C NMR signals were
observed for compound 2i, in which a bulky phenoxy group
occupies one of the ortho positions (R2 in Scheme 2),
presumably due to the presence of a pair of isomers with anti
and syn configurations. The ratio of ∼1.5 remains largely
unchanged upon dilution or heating to 60 °C.22 We were also
able to obtain crystals of 2d for single-crystal X-ray diffraction
analysis, which confirmed the mononuclear, distorted-tetrahe-
dral geometry around the aluminum center, despite the low
quality of the data (see Figure S14 and Table S1, Supporting
Information).22 Attempts to obtain pentacoordinate Al
complexes of the formula (L)2AlMe by using a 2:1 ratio of
ligand to AlMe3 have been unsuccessful.
Polymerization of rac-Lactide. The aluminum complexes
were tested as initiators for rac-lactide ROP, and the results are
summarized in Table 1. The polymerizations were typically
conducted in dry toluene at 80 °C with concentrations of rac-
lactide (0.50 M), catalyst (10 mM), and benzyl alcohol, BnOH
(10 mM). The reaction progress was monitored by taking
regular aliquots which were analyzed by 1H NMR spectroscopy.
Under catalytic conditions, it was generally assumed that benzyl
alcohol reacted with the dimethylaluminum compound to
generate an alkoxide complex that initiated the polymerization
reaction. However, such a (L)Al(Me)(OBn) complex could not
be clearly identified in the stoichiometric reaction between 1
equiv of benzyl alcohol and 2a. When 2 equiv of BnOH was
employed in the catalytic reaction, the conversion of rac-lactide
was very poor, likely due to demetalation of the Al catalyst by
excess alcohol. In the absence of an exogenous alcohol, these
complexes led to very little conversion of lactide. Mixing of
stoichiometric amounts of compound 2c and rac-LA showed no
sign of reaction. Complexes 2b,d,f with 2,6-dimethyl sub-
stituents were not effective; less than 15% conversion was
observed after 24 h (entries 3, 6, and 10, Table 1), while the
rest of the aluminum catalysts achieved >93% conversion under
identical conditions. Presumably the steric bulk played a
considerable role in inhibiting the polymerization.21 However,
in the case of 2i, where only one R2 substituent (see Scheme 2)
was present, reactivity was observed comparable to that of
unsubstituted complexes (entry 13). Furthermore, experiments
conducted in THF at 60 °C were much slower than in toluene,
likely due to the coordinating nature of THF and lower
We also tested representative catalysts under bulk/melt
conditions at 130 °C, since such conditions require no solvent
and allow high temperature and high monomer concentration,
leading to shorter reaction time and higher turnover
frequency.25 Selected results are summarized in Table 2.
Indeed, these aluminum compounds functioned as initiators for
ROP of rac-lactides with low catalyst loads (0.25 mol %) and
much shorter reaction times (1.0−1.5 h). In addition, the
alcohol cocatalyst was not required under bulk conditions and
C
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therefore not employed in the above runs. The actual nature of
initiating groups is likely to be external nucleophilic impurities.
Even 2f with the bulky 2,6-dimethylphenyl group showed a
higher conversion (31% vs 12% in solution). The molecular
weights were typically lower than the theoretical values (except
for 2i, where a much higher Mn was obtained) and the
molecular weight distributions tended to be broader than the
solution polymerizations. This may be due to diffusional
constraints imposed by the elevated viscosities found in bulk
polymerizations, particularly at high conversions. However, the
conversion of rac-lactide in these reactions seems to reach a
plateau of around 70−80%. One likely reason is that the
mobility of the monomer is reduced due to viscosity-induced
diffusional constraint, and the reaction slows down consid-
erably. Another interesting observation is the variation of
selectivity as judged by Pm. For example, 2a,e showed a
moderate heteroselectivity under normal solution conditions
but became essentially nonselective under bulk conditions. In
contrast, the isoselectivity for 2c,g remained the same under
both normal and bulk conditions.
order 2a > 2h > 2e ≈ 2g ≈ 2i > 2c. The trend can be
understood, at least in part, on the basis of steric and electronic
considerations. In the 2a,g−i series, where the oxazoline moiety
is the same, both the electron-withdrawing groups at R3 and
electron-donating groups at R4 result in lowered reactivity,
which may suggest a delicate sensitivity of reactivity vs Lewis
acidity of the metal center.26 The enhanced Lewis acidity
induced by electron-withdrawing groups may lead to preferred
coordination of lactide monomer, but it could also inhibit the
subsequent insertion step. It is also worth noting that the two
catalysts (2c,g) with a preference for isotacticity are less active
catalysts. In addition, kinetic measurements for the polymer-
ization of L-lactide by 2c revealed rates very similar to those for
rac-lactide. Analogous kinetic results have been observed for a
highly isoselective aluminum complex.12a
CONCLUSIONS
■
In conclusion, a series of aluminum complexes containing
chiral, bidentate anilido-oxazolinate ligands have been synthe-
sized and characterized. These complexes were tested in the
ring-opening polymerization of rac-lactide in the presence of
benzylic alcohol, and six of them were effective in promoting
the polymerization, while the other three with 2,6-dimethyl-
phenyl substituents showed low catalytic reactivity. The
microstructure of the resulting polylactides ranged from slightly
isotactic to moderately heterotactic. The reactivity and
selectivity can be roughly understood on the basis of steric
and electronic factors of substituents on several specific
positions of the ligand framework. Taken together, these
results indicate that electron-withdrawing groups on this series
of aluminum compounds tend to give slower, but isoselective,
catalysts for the ROP of rac-LA. Studies further delineating
these factors in catalysis are underway.
To further probe the catalyst factors that may influence the
activity in ROP of rac-lactide, the polymerization kinetics were
monitored for the six active initiators (2a,c,e,g−i) in
combination with 1 equiv (vs Al) of BnOH. All of them
showed a first-order dependence on lactide concentration up to
80% conversion, as judged by the linear relationship of
ln([LA]0/[LA]t) versus time (Figure 1). The pseudo-first-
EXPERIMENTAL SECTION
■
General Considerations. All reactions that involved compounds
sensitive to air and/or moisture were carried out under a dry nitrogen
atmosphere using freshly dried solvents and standard Schlenk line and
glovebox techniques. All chemicals were purchased from commercial
sources. Toluene, hexanes, and THF were distilled under nitrogen
from Na/benzophenone. CDCl3 was dried over CaH2, distilled, and
degassed prior to use. rac-LA was recrystallized from dry toluene,
sublimed under vacuum, and stored under a dry nitrogen atmosphere.
NMR spectra were recorded on a Bruker AVANCE-500 NMR
spectrometer (1H, and 13C) and referenced to the residual solvent
peak. J values are given in Hz. Gel permeation chromatography (GPC)
analysis was performed on a Varian Prostar instrument, using a PLgel 5
μm Mixed-D column, a Prostar 355 RI detector, and THF as eluent at
a flow rate of 1 mL/min (20 °C). Polystyrene standards were used for
calibration. Matrix-assisted laser desorption/ionization time-of-flight
mass spectrometry (MALDI-TOF MS) spectra were obtained on
Applied Biosystems/MD SCIEX 4800 equipment using α-cyano-4-
hydroxycinnamic acid as a matrix and 5 mM sodium acetate as an
ionization agent. The HR-MS was performed using high-resolution
time-of-flight G1969A instrumentation (Agilent). The optical rotation
was measured on a Jasco P-2000 polarimeter with a 589 nm light
source at 23 °C.
Figure 1. Semilogarithmic plot of rac-lactide conversion vs time
catalyzed by complexes 2 in toluene at 80 °C.
order rate constants, kobs, were obtained from the slope of these
linear plots and are given in Table 3. Among the six Al
complexes, the apparent first-order rate constants follow the
Table 3. Apparent Rate Constants (kobs) for the
a
Polymerization of rac-Lactide
entry
cat.
kobs (h−1
)
entry
cat.
kobs (h−1
)
Synthesis of Ligands. Similar ligands have been obtained
previously;20 the following example is typical. To a round-bottom
flask were added 2-bromobenzaldehyde (6.5 mmol), 1 equiv of amino
alcohol, and 25 mL of toluene. After the mixture was stirred for 24 h,
K3PO4 (19.5 mmol) and NBS (13.0 mmol) were added and stirring
was continued for 5 h at room temperature. After filtration, the
mixture was washed three times with NaHCO3 and H2O. The organic
fraction was dried with Na2SO4 and purified by column to the give the
oxazoline precursors. The product was mixed with 120 mol % of
1
2
2a
2c
2c
2e
0.248 ( 0.009)
0.11 ( 0.04)
0.09 ( 0.02)
0.16 ( 0.01)
5
6
7
2g
2h
2i
0.173 ( 0.006)
0.21 ( 0.04)
0.17 ( 0.06)
b
3
4
a
The kinetic experiments were run in the presence of 1 equiv of
BnOH (vs Al catalysts); see Experimental section for details.
Lactide was used.
bL-
D
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aniline, 5 mol % of Pd(OAc)2, 5 mol % of rac-BINAP, 140 mol % of
sodium tert-butoxide, and 15 mL of dry toluene in a Schlenk flask
under nitrogen. The mixture was heated to 100 °C for 48 h and then
filtered and purified by column. The desired ligands were usually
obtained as yellow-orange oils (HL1a−c and HL1e−h) or crystalline
solids (HL1d and HL1i) in ∼66−89% yields. Characterizations of
HL1b,d,f have been reported previously.20
K): δ 10.63 (1H, s, NH), 7.79 (1H, d, ArH), 7.63 (3H, d, ArH),
7.46(1H, d, ArH), 7.29(2H, m, ArH), 7.11(1H, m, ArH), 7.03(2H, m,
ArH), 6.97(4H, m, ArH), 6.78(1H, m, ArH), 4.31 (1H, t,
NCH(R)CH2O), 4.00 (2H, m, NCH(R)CH2O), 1.65 (1H, m,
CHCH3), 0.93 (3H, d, CHCH3), 0.85 (3H, d, CHCH3). 13C NMR
(CDCl3, 298 K): δ 163.33 (CN), 157.78, 148.29, 144.95, 133.89,
131.84, 130.12, 129.69, 124.08, 122.89, 122.71, 120.92, 120.40, 118.20,
117.50, 113.75, 111.59, 73.15 (NCH(R)CH2O), 69.18 (NCH(R)
CH2O), 33.29 (CHMe2), 19.17 (CHMe2), 18.86 (CHMe2). GC/MS:
m/z 372 [M]+, 329, 299, 286 (100), 245, 209, 167. HRMS (EI+): m/z
calcd for C24H25N2O2 [M + H]+ 373.19160; found 373.18851.
Synthesis of Aluminum Complexes. In a typical procedure,
ligand HL1 (1.0 mmol) was mixed with 1.2 or 1.5 equiv of AlMe3 in
toluene. The mixture was stirred under nitrogen at room temperature
for 12 h. After removal of toluene in vacuo, the resulting yellow-orange
residue was extracted multiple times with dry hexanes and combined.
The solvent was then removed in vacuo at low temperature, affording
the product as a yellow powder. The products can be further purified
by recrystallization from a dichloromethane−hexane solution.
(4S)-4,5-Dihydro-2-(2′-anilinophenyl)-4-isopropyloxazole (HL1a).
1
Yield: 1.0 g (3.57 mmol), 86%. H NMR (CDCl3, 298 K): δ 10.55
(1H, s, NH), 7.73 (1H, d, ArH), 7.28 (4H, m, ArH), 7.20 (2H, d,
ArH), 6.97 (1H, t, ArH), 4.29 (1H, m, NCH(R)CH2O), 4.05 (1H, m,
NCH(R)CH2O), 3.96 (1H, m, NCH(R)CH2O), 1.71 (1H, m,
CHCH3), 0.97 (3H, d, CHCH3), 0.88 (3H, d, CHCH3). 13C NMR
(CDCl3, 298 K): δ 163.75 (CN), 145.73, 141.74, 132.09, 130.12,
129.51, 122.77, 121.55, 117.10, 113.28, 110.59, 73.16 (NCH(R)-
CH2O), 69.28 (NCH(R)CH2O), 33.59 (CHMe2), 19.24 (CHMe2),
19.02 (CHMe2). GC/MS: m/z 280.0 [M]+, 237.0 (100), 206.9, 193.9,
179.9, 166.9. HRMS (EI+): m/z calcd for C18H21N2O [M + H]+
281.16539; found 281.16457.
(L1a)AlMe2 (2a).28 Yield: 315 mg (0.935 mmol), 69%. [α]D = +359°
(4R)-4,5-Dihydro-2-(2′-anilinophenyl)-4-phenyloxazole (HL1c).27
Yield: 304 mg (0.968 mmol), 88%. 1H NMR (CDCl3, 298 K): δ
10.39 (1H, s, NH), 7.80 (1H, d, ArH), 7.22 (6H, m, ArH), 7.17−7.15
(3H, m, ArH), 7.08 (2H, t, ArH), 6.96 (1H, t, ArH), 6.69 (1H, t, ArH),
5.39 (1H, m, NCH(R)CH2O), 4.63 (1H, m, NCH(R)CH2O), 4.06
(1H, m, NCH(R)CH2O). 13C NMR (CDCl3, 298 K): δ 165.25,
146.25, 142.69, 141.44, 132.48, 130.40, 129.45, 128.97, 127.78, 126.73,
123.21, 122.32, 117.11, 113.36, 110.10, 73.31 (NCH(R)CH2O), 70.32
(NCH(R)CH2O). GC/MS: m/z 314 [M]+, 283, 205, 194 (100), 167,
91. HRMS (EI+): m/z calcd for C21H18N2O [M]+ 314.14191; found
314.14866.
1
(c 1.5, toluene). H NMR (CDCl3, 298 K): δ 7.67 (1H, d, J = 10,
ArH), 7.39 (2H, m, ArH), 7.19 (1H, t, J = 10, ArH), 7.09 (3H, m,
ArH), 6.45 (2H, m, ArH), 4.44 (3H, m, NCH(R)CH2O), 2.33 (1H, m,
CHMe2), 0.99 (3H, d, J = 10, CH(CH3)2), 0.91 (3H, d, J =10, J =
CH(CH3)2), −0.86 (3H, s, AlMe), −0.89 (3H, s, AlMe). 13C NMR
(CDCl3, 298 K): δ 169.87 (CN), 157.07, 147.12, 135.32, 130.74,
129.94, 128.33, 124.81, 117.21, 113.75, 105.04, 67.90 (NCH(R)
CH2O), 67.57 (NCH(R)CH2O), 29.94 (CHMe2), 19.51 (CHMe2),
14.33 (CHMe2), −7.77 (AlCH3), −9.93 (AlCH3).
(L1b)AlMe2 (2b).28 Yield: 252 mg (0.690 mmol), 69%. [α]D
=
+96.7° (c 0.667, toluene). 1H NMR (CDCl3, 298 K): δ 7.70 (1H, d, J
= 10, ArH), 7.12 (2H, m, ArH), 7.08 (2H, m, ArH), 6.45 (1H, t, J = 5,
ArH), 6.05 (1H, d, J = 5, ArH), 4.48 (3H, m, NCH(R)CH2O), 2.35
(1H, m, CHMe2), 2.12 (3H, s, ArMe), 2.10 (3H, s, ArMe), 0.99 (3H,
d, J = 10, CH(CH3)2), 0.90 (3H, d, J = 10, CH(CH3)2), −0.88 (3H, s,
AlMe), −0.93 (3H, s, AlMe). 13C NMR (CDCl3, 298 K): δ 169.99,
(CN), 156.16, 143.21, 137.05, 136.82, 135.77, 130.98, 128.92,
128.87, 125.41, 116.19, 113.63, 104.50, 67.75 (NCH(R)CH2O), 67.54
(NCH(R)CH2O), 29.89 (CHMe2), 19.44 (CHMe2), 18.69 (ArCH3),
18.64 (ArCH3), 14.27 (CHMe2), −8.25 (AlCH3), −8.96 (AlCH3).
(L1c)AlMe2 (2c). Yield: 200 mg (0.539 mmol), 53%. [α]D = −422°
(4R)-4,5-Dihydro-2-(2′-anilinophenyl)-4-isobutyloxazole (HL1e).
Yield: 254 mg (0.861 mmol), 86%. 1H NMR (CDCl3, 298 K): δ
10.48 (1H, s, NH), 7.73 (1H, d, ArH), 7.26 (4H, m, ArH), 7.18 (2H, t,
ArH), 6.97 (1H, t, ArH), 6.67 (1H, t, ArH), 4.35 (2H, m,
NCH(R)CH2O), 3.82 (1H, t, NCH(R)CH2O), 1.80 (1H, m
CH2CH), 1.59 (1H, m, CH2CH), 1.36 (1H, m, CH2CH), 0.93 (6H,
m, CHCH3). 13C NMR (CDCl3, 298 K): δ 163.45, 145.54, 141.49,
131.86, 129.90, 129.27, 122.66, 121.56, 116.93, 113.11, 110.44, 71.48
(NCH(R)CH2O), 65.17 (NCH(R)CH2O), 45.65 (CH2CHMe2),
25.84 (CH2CHMe2), 22.98 (CH2CHMe2), 22.64 (CH2CHMe2).
GC/MS: m/z 294 [M]+ (100), 263, 237, 194, 167, 139. HRMS
(EI+): m/z calcd for C19H23N2O [M + H]+ 295.18104; found
295.18182.
1
(c 3.86, toluene). H NMR (CDCl3, 298 K): δ 7.77 (1H, d, J = 10,
ArH), 7.5−7.3 (7H, m, ArH), 7.15 (2H, m, ArH); 7.05 (2H, m, ArH),
6.50 (1H, t, J = 10, ArH), 6.43 (1H, d, J = 10, ArH), 5.36 (1H, m,
NCH(R)CH2O), 4.96 (1H, m, NCH(R)CH2O), 4.57 (1H, m,
NCH(R)CH2O), −0.94 (3H, s, AlMe), −1.55 (3H, s, AlMe). 13C
NMR (CDCl3, 298 K): δ 170.18 (CN), 157.33, 146.94, 138.47,
135.68, 130.88, 129.95, 129.43, 129.35, 128.24 128.05, 124.86, 117.32,
113.84, 104.59, 75.28 (NCH(R)CH2O), 66.98 (NCH(R)CH2O),
−9.00 (MeAl), −9.79 (MeAl). Anal. Calcd for C23H23N2OAl: C, 74.58;
H, 6.26; N, 7.56. Found: C, 74.20; H 6.33; N, 7.55.
(4S)-4,5-Dihydro-2-(2′-(3,5-bis(trifluoromethyl)anilino)phenyl)-4-
1
isopropyloxazole (HL1g). Yield: 670 mg (1.61 mmol), 87%. H NMR
(CDCl3, 298 K): δ 11.00 (1H, s, NH), 7.74 (1H, d, ArH), 7.50 (2H, s,
ArH), 7.30−7.27 (3H, m, ArH), 6.80 (1H, t, ArH), 4.28 (1H, m,
NCH(R)CH2O), 4.04 (1H, m, NCH(R)CH2O), 3.95 (1H, m,
NCH(R)CH2O), 1.70 (1H, m,CHCH3), 0.93 (3H, d, CHCH3),
0.84 (3H, d, CHCH3). 13C NMR (CDCl3, 298 K): δ 163.66, 143.79,
143.49, 133.01, 132.74, 132.40, 130.50, 124.66, 122.48, 119.65, 118.89,
114.82, 114.30, 112.59, 72.99 (NCH(R)CH2O), 69.47 (NCH(R)
CH2O), 33.47 (CHMe2), 19.14 (CHMe2), 18.88 (CHMe2). GC/MS:
m/z 416 [M]+, 373 (100), 345, 316, 291, 234, 182. HRMS (EI+): m/z
calcd for C20H19N2OF6 [M + H]+ 417.14016; found 417.14115.
(4S)-4,5-Dihydro-2-(2′-(4-methoxyanilino)phenyl)-4-isopropylox-
azole (HL1h). Yield: 558 mg (1.79 mmol), 84%. 1H NMR (CDCl3, 298
K): δ 10.36 (1H, s, NH), 7.80 (1H, d, ArH), 7.22 (3H, d, ArH), 7.11
(1H, d, ArH); 6.93 (2H, d, ArH), 6.70 (1H, t, ArH), 4.38 (1H, t,
NCH(R)CH2O), 4.15 (1H, m, NCH(R)CH2O), 4.06 (1H, t,
NCH(R)CH2O), 3.83 (3H, s, ArOMe), 1.82 (1H, m, CHCH3), 1.06
(3H, d, CHCH3), 0.96 (3H, d, CHCH3). 13C NMR (CDCl3, 298 K): δ
163.36 (CN), 156.28, 147.33, 134.47, 132.13, 130.03, 125.10,
116.19, 114.76, 112.46, 109.58, 73.08 (NCH(R)CH2O), 69.10
(NCH(R)CH2O), 55.75 (ArOMe), 33.50 (CHMe2), 19.26
(CHMe2), 18.97 (CHMe2). GC/MS: m/z 310 [M]+ (100), 267,
237, 209, 182, 154. HRMS (EI+): m/z calcd for C19H23N2O2 [M +
H]+ 311.17595; found 311.17612.
(L1d)AlMe2 (2d). Yield: 98.4 mg (0.247 mmol), 50%. [α]D = −162°
1
(c 2.62, toluene). H NMR (CDCl3, 298 K): δ 7.80 (1H, d, J = 10,
ArH), 7.40 (3H, m, ArH), 7.35 (2H, m, ArH), 7.09 (3H, m, ArH), 7.04
(1H, m, ArH), 6.49 (1H, t, J = 10, ArH), 6.06 (1H, d, J = 10, ArH),
5.37 (1H, m, NCH(R)CH2O), 5.01 (1H, m, NCH(R)CH2O), 4.58
(1H, m, NCH(R)CH2O), 2.11 (3H, s, ArMe), 2.04 (3H, s, ArMe),
−0.92 (3H, s, AlMe), −1.62 (3H, s, AlMe). 13C NMR (CDCl3, 298 K):
δ 170.30 (CN), 156.42, 143.05, 138.86, 136.97, 136.68, 136.00,
131.06, 129.34, 128.83, 127.91, 125.38, 116.26, 113.68, 104.11, 75.38
(NCH(R)CH2O), 66.94 (NCH(R)CH2O), 18.58 (MePh), −9.08
(MeAl), −9.46 (MeAl). Anal. Calcd for C25H27N2OAl: C, 75.35; H,
6.83; N, 7.03. Found: C, 77.33; H, 7.18; N, 7.08.
(L1e)AlMe2 (2e). Yield: 245 mg (0.698 mmol), 67%. [α]D = −388°
1
(c 1.73, toluene). H NMR (CDCl3, 298 K):): δ 7.68 (1H, d, J = 10,
ArH), 7.38 (2H, m, ArH), 7.19 (1H, m, ArH), 7.11 (3H, m, ArH), 6.45
(2H, m, ArH), 4.72 (1H, m, NCH(R)CH2O), 4.41 (1H, m,
NCH(R)CH2O), 4.23 (1H, m, NCH(R)CH2O), 1.95 (1H, m,
CHCH3), 1.62 (2H, m, CH2CH), 0.99 (6H, m, CH(CH3)2), −0.85
(3H, s, AlMe), −0.90 (3H, s, AlMe). 13C NMR (CDCl3, 298 K): δ
(4S)-4,5-Dihydro-2-(2′-(2-phenoxyanilino)phenyl)-4-isopropylox-
azole (HL1i). Yield: 659 mg (1.77 mmol), 88%. 1H NMR (CDCl3, 298
E
dx.doi.org/10.1021/om401226j | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
169.72, 157.12, 147.17, 135.36, 130.71, 129.99, 128.34, 124.84, 117.25,
113.81, 105.22, 73.21 (NCH(R)CH2O), 62.01 (NCH(R)CH2O),
44.06 (CH2CHMe2), 25.94 (CH2CHMe2), 23.91 (CHMe2), 21.70
(CHMe2), −6.85 (MeAl), −9.98 (MeAl). Anal. Calcd for
C21H27N2OAl: C, 71.98; H, 7.77; N, 7.99. Found: C, 70.64; H,
7.34; N, 7.59.28
Polymerization of rac-Lactide. A typical solution polymerization
procedure was exemplified by the following: rac-lactide (0.71 g, 4.9
mmol), benzyl alcohol (10.3 μL, 0.10 mmol), Al complex (0.10
mmol), and toluene (8.0 mL) were placed in a Schlenk flask. The
reaction mixture was heated in an oil bath preset at 80 °C. The
conversion of rac-lactide was monitored by periodically taking samples
1
(L1f)AlMe2 (2f). Yield: 343 mg (0.905 mmol), 90%. [α]D = −150° (c
2.1, toluene). 1H NMR (CDCl3, 298 K): δ 7.71 (1H, d, J = 10, ArH),
7.15 (3H, m, ArH), 7.08 (1H, d, J = 10, ArH), 6.45 (1H, t, J = 10,
ArH), 6.05 (1H, d, J = 10, ArH), 4.73 (1H, m, NCH(R)CH2O), 4.42
(1H, m, NCH(R)CH2O), 4.25 (1H, m, NCH(R)CH2O), 2.12 (3H, s,
ArMe), 2.10 (3H, s, ArMe), 1.96 (1H, m, CHCH3), 1.62 (2H,
m,CH2CH), 1.00 (6H, m, CHCH3), −0.88 (3H, s, AlMe), −0.89 (3H,
s, AlMe). 13C NMR (CDCl3, 298 K): δ 169.79 (CN), 156.05,
143.15, 137.08, 136.73, 135.69, 130.81, 128.93, 128.83, 125.37, 116.13,
113.59, 104.66, 73.18 (NCH(R)CH2O), 61.94 (NCH(R)CH2O),
44.05 (CCHCH3), 25.91 (CHCH2CH), 24.00 (CHMe), 21.70
(CHMe), 18.76 (MePh), −7.17 (MeAl), −9.28 (MeAl). Anal. Calcd
for C23H31N2OAl: C, 72.99; H, 8.26; N, 7.40. Found: C, 72.76; H,
8.47; N, 7.19.
via H NMR spectroscopic analyses. Typical conversions were 92−
99% after 24 h (except for compounds 2b,d,f). At the end of the
reaction, the polymer was isolated by precipitation from a CH2Cl2
solution. The tacticity was determined by a homonuclear decoupled
1H NMR spectrum in the methine region (5.25−5.15 ppm), which can
be assigned according to the literature.29
The bulk polymerization was performed by heating rac-lactide (2.84
g, 19.8 mmol) and an Al catalyst (0.050 mmol) in a Schlenk flask
under nitrogen with an oil bath preset at 130 °C. The heating was
discontinued when the melt became very viscous and stirring stopped,
usually within 2 h. The resulting reaction mixture was analyzed by 1H
NMR spectroscopy to determine the conversion of lactide and tacticity
of PLA.
Kinetic Runs. A Schlenk flask was loaded with rac-lactide, Al
catalyst (2 mol %), benzyl alcohol (2 mol %, 1 equiv vs catalyst), and
toluene such that [rac-LA] = 0.50 M under nitrogen. The flask was
then heated to 80 °C via an oil bath. At preset time intervals, an
aliquot (0.1 mL) was withdrawn and quenched with MeOH or
CH2Cl2. The solvent was then removed in vacuo, and the conversion
(L1g)AlMe2 (2g).28 Yield: 224 mg (0.474 mmol), 95%. [α]D = +304°
1
(c 1.77, toluene). H NMR (CDCl3, 298 K): δ 7.65 (1H, d, J = 5,
ArH), 7.52 (1H, s, ArH), 7.46 (2H, s, ArH), 7.14 (1H, m, ArH), 6.55
(1H, t, J = 5, ArH), 6.48 (1H, d, J = 5, ArH), 4.44 (2H, m,
NCH(R)CH2O), 4.34 (1H, m, NCH(R)CH2O), 2.23 (1H, m,
CHCH3), 0.91 (3H, d, J = 5, CHCH3), 0.82 (3H, d, J = 5,
CHCH3), −0.92 (3H, s, AlMe), −0.97 (3H, s, AlMe). 13C NMR
(CDCl3, 298 K): δ 169.96 (CN), 155.60, 150.27, 135.90, 133.20
(quartet, CF3), 131.14, 127.58, 124.81, 122.53, 117.84, 117.41, 116.20,
107.31, 68.14 (NCH(R)CH2O), 68.08 (NCH(R)CH2O), 30.03
(CHMe2), 19.37 (CHMe2), 14.38 (CHMe2), −7.88 (AlCH3), −10.30
(AlCH3).
1
of lactide was determined by H NMR spectroscopy. The apparent
first-order rate constants were obtained by the slope of linear fit of
semilogarithmic plot of rac-lactide conversion vs time, using the
KaleidaGraph program (version 4.0).
ASSOCIATED CONTENT
* Supporting Information
■
S
(L1h)AlMe2 (2h). Yield: 308 mg (0.839 mmol), 84%. [α]D = +187°
Text, figures, and tables giving experimental details, representa-
tive 1H and 13C NMR spectra of aluminum complexes, MALDI
MS of a PLA sample, and structural data for 2d. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
(c 2.03, toluene). H NMR (CDCl3, 298 K): δ 7.66 (1H, d, J = 10,
ArH), 7.11 (1H, m, ArH), 6.98 (4H, m, ArH), 6.43 (1H, t, J = 10,
ArH), 6.38 (1H, d, J = 10, ArH), 4.48 (3H, m, NCH(R)CH2O), 3.86
(3H, s, ArOMe), 2.33 (1H, m, CHCH3), 0.99 (3H, d, J = 10, CHCH3),
0.90 (3H, d, J = 10, CHCH3), −0.87 (3H, s, AlMe), −0.89 (3H, s,
AlMe). 13C NMR (CDCl3, 298 K): δ 170.22 (CN), 157.67, 156.22,
141.25, 135.69, 130.75, 129.56, 117.12, 116.91, 113.68, 104.89, 68.06
(NCH(R)CH2O), 67.64 (OCH3), 58.28 (NCH(R)CH2O), 30.01
(CCHCH3), 19.57 (CHMe), 14.37 (CHMe), −8.37 (MeAl), −9.90
(MeAl). Anal. Calcd for C21H27N2O2Al: C, 68.83; H, 7.43; N, 7.64.
Found: C, 68.36; H, 7.62; N, 7.50.
AUTHOR INFORMATION
Corresponding Author
*G.D.: tel, +1-701-777-2241; fax, +1-701-777-2331; e-mail,
gdu@chem.und.edu.
■
Present Address
§School of Chemistry and Chemical Engineering, University of
Jinan, Jinan 250022, People’s Republic of China.
(L1i)AlMe2 (2i). Yield: 381 mg (0.888 mmol), 86%. [α]D = +468° (c
2.5, toluene). The compound exists as two isomers in ∼1.5:1.0 ratio,
presumably the anti and syn isomers due to the orientation of the o-
Notes
1
PhO group. H NMR (CDCl3, 298 K): anti-2i, δ 7.47 (1H, d, J = 5,
The authors declare no competing financial interest.
ArH), 6.58 (1H, d, J = 10, ArH), 6.52 (2H, d, J = 10, o-C6H5O), 6.48
(m, 1H, ArH), 4.02 (m, 1H, oxazoline−CH), 2.12 (m, 1H, CHMe2),
0.85 (d, 3H, J = 5, CHMe2), 0.76 (d, 3H, J = 5, CHMe2), −0.91 (s, 3H,
AlMe), −1.06 (s, 3H, AlMe); syn-2i, δ 7.56 (1H, d, J = 10, ArH), 6.80
(1H, d, J = 5, ArH), 6.70 (2H, d, J = 5, o-C6H5O), 6.48 (m, 1H, ArH),
2.18 (m, 1H, CHMe2), 0.85 (d, 3H, J = 5, CHMe2), 0.66 (d, 3H, J = 5,
CHMe2), −0.87 (s, 3H, AlMe), −1.06 (s, 3H, AlMe). The rest of the
signals at δ 7.22−6.82 for aromatic protons and at δ 4.42−4.20 for
oxazoline protons show considerable overlap and are not resolved. 13C
NMR (CDCl3, 298 K): anti-2i, δ 169.29 (CN), 157.08 (ipso-C),
155.54 (ipso-C), 151.32 (ipso-C), 139.04 (ipso-C), 134.58, 129.87,
129.03 (m-C6H5O), 128.81, 124.88, 124.68, 122.17, 121.07, 119.13,
118.38 (o-C6H5O), 115.03, 107.22 (ipso-C), 67.95, 67.81, 30.05
(CHMe2), 19.35 (CHMe2), 14.49 (CHMe2), −7.51 (AlMe), −11.16
(AlMe); syn-2i, δ 169.37 (CN), 156.54 (ipso-C), 156.03 (ipso-C),
152.79 (ipso-C), 138.90 (ipso-C), 134.65, 130.16, 129.61 (m-C6H5O),
127.92, 124.35, 124.07, 123.47, 119.80 (o-C6H5O), 119.03, 118.63,
114.91, 107.07 (ipso-C), 68.16, 67.69, 29.80 (CHMe2), 19.48
(CHMe2), 14.30 (CHMe2), −8.78 (AlMe), −9.53 (AlMe). Anal.
Calcd for C26H29N2O2Al: C, 72.88; H, 6.82; N, 6.54. Found: C, 72.70;
H, 6.76; N, 6.43.
ACKNOWLEDGMENTS
■
Financial support from the ND EPSCoR through an NSF grant
(EPS-0814442) and the Office of the Vice President for
Research at UND through an FRSM award is greatly
appreciated. We thank Dr. A. Kubatova for HR-MS analysis,
Dr. Q. Chu and Dr. L. Stahl for help with X-ray crystallography,
and Dr. J. B. Shabb and W. W. Muhonon of UND Proteomics
Core, supported by the NIH (grant number P20 RR016741)
from the INBRE program of the National Center for Research
Resources, for MALDI-MS.
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dx.doi.org/10.1021/om401226j | Organometallics XXXX, XXX, XXX−XXX