Stereoselective ring-opening polymerization of rac -lactides catalyzed by chiral and achiral aluminum half-salen complexes

Article abstract of DOI:10.1021/om100518e

A new series of aluminum half-salen complexes have been synthesized from either chiral or achiral tridentate Schiff base ligands derived from amino alcohols or amino acids. All aluminum complexes have been shown to be active catalysts for the ring-opening

Full text of DOI:10.1021/om100518e

Organometallics 2010, 29, 5627–5634 5627
DOI: 10.1021/om100518e
Stereoselective Ring-Opening Polymerization of rac-Lactides Catalyzed
by Chiral and Achiral Aluminum Half-Salen Complexes†
Donald J. Darensbourg* and Osit Karroonnirun
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Received May 26, 2010
A new series of aluminum half-salen complexes have been synthesized from either chiral or achiral
tridentate Schiff base ligands derived from amino alcohols or amino acids. All aluminum complexes have
been shown to be active catalysts for the ring-opening polymerization (ROP) of rac-lactide in toluene at
70 °C, producing polylactides with controlled molecular weights and narrow molecular weight distribu-
tions. Both chiral and achiral aluminum complexes showed moderate selectivity to the ROP of rac-lactide
to produce isotactic polylactide with a Pm value up to 0.76. In addition, epimerization of rac-lactide to
meso-lactide was observed during the polymerization process for some of the complexes studied.
Introduction
The use of metal-based catalysts, especially those derived
from a biocompatible metal, for the ring-opening polymeri-
zation (ROP) of cyclic esters has been the subject of numer-
ous reviews.2c,6 Relevant to this topic, we have recently
reported the ROP of rac-lactide using zinc-based half-salen
complexes derived from chiral natural amino acids as cata-
lysts.7 Although these complexes were chiral, our observa-
tions revealed these zinc complexes underwent ROP of
rac-lactide via a chain-end control mechanism to provide
heterotactic polylactide.8 While chiral ligands bound to
active metal centers are typically expected to play a major
role in stereoselectivity via an enantiomorphic site control
mechanism,9 it is generally not true for the ROP of cyclic
esters by zinc-catalyzed systems. Indeed, zinc complexes
derived from chiral7,10 or achiral ligands11 which have been
Polylactides are biodegradable polymers derived from re-
newable resources such as corn, wheat, and sugar beets.1
These polymeric materials have received much attention over
the past decade because of their attractive physical and
mechanical properties, which lend them to having numerous
applications in medical2 and microelectronic areas.3 Of im-
portance, polylactides and various copolymers thereof are
readily metabolized in the human body by normal metabolic
pathways.4 The thermal properties of polylactides are highly
dependent on the microstructures of the polylactides. There-
fore, researchers have focused their studies on synthesizing
stereocomplex polylactidesfromrac-lactide, utilizingcatalytic
systems which can control the tacticity of the polymers
formed. Stereocomplexed polylactides can thereby be pro-
duced from a blend of poly-L-lactide and poly-D-lactide which
have melting temperatures up to 230 °C.5
(7) Darensbourg, D. J.; Karroonnirun, O. Inorg. Chem. 2010, 49,
2360–2371.
(8) (a) Zhang, Z.; Xu, X.; Sun, S.; Yao, Y.; Zhang, Y.; Shen, Q. Chem.
Commun. 2009, 7414–7416. (b) Du, H.; Velders, A. H.; Dijkstra, P. J.;
Zhong, Z.; Chen, X.; Feijen, J. Macromolecules 2009, 42, 1058–1066.
(9) (a) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 1316–
1326. (b) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. J. Am. Chem. Soc.
2002, 124, 5938–5939.
(10) (a) Wu, J.-C.; Huang, B.-H.; Hsueh, M.-L.; Lai, S.-L.; Lin, C.-C.
Polymer 2005, 46, 9784–9792. (b) Labourdette, G.; Lee, D. J.; Patrick, B. O.;
Ezhova, M. B.; Mehrkhodavandi, P. Organometallics 2009, 28, 1309–1319.
(c) Drouin, F.; Oguadinma, P. O.; Whitehorne, T. J. J.; Prud'homme, R. E.;
Schaper, F. Organometallics 2010, 29, 2139–2147.
(11) (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W.
J. Am. Chem. Soc. 1999, 121, 11583–11584. (b) Chamberlain, B. M.;
Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W.
J. Am. Chem. Soc. 2001, 123, 3229–3238. (c) Chisholm, M. H.; Huffman,
J. C.; Phomphrai, K. Dalton Trans. 2001, 222–224. (d) Chisholm, M. H.;
Gallucci, J.; Phomphrai, K. Inorg. Chem. 2002, 41, 2785–2794. (e) Chen,
H.-Y.; Huang, B.-H.; Lin, C.-C. Macromolecules 2005, 38, 5400–5405.
(f) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg. Chem. 2005, 44,
8004–8010. (g) Chen, H.-Y.; Tang, H.-Y.; Lin, C.-C. Macromolecules 2006,
39, 3745–3752. (h) Bunge, S. D.; Lance, J. M.; Bertke, J. A. Organometallics
2007, 26, 6320–6328. (i) Huang, Y.; Hung, W.-C.; Liao, M.-Y.; Tsai, T.-E.;
Peng, Y.-L.; Lin, C.-C. J. Polym. Sci., Part A: Polym. Chem. 2009, 47,
2318–2329. (j) Jones, M. D.; Davidson, M. G.; Keir, C. G.; Hughes, L. M.;
Mahon, M. F.; Apperley, D. C. Eur. J. Inorg. Chem. 2009, 2009, 635–642.
(k) Xu, X.; Chen, Y.; Zou, G.; Ma, Z.; Li, G. J. Organomet. Chem. 2010, 695,
1155–1162.
† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar
Seyferth for his numerous contributions to organometallic chemistry and role
as founding editor of Organometallics.
*To whom correspondence should be addressed. E-mail: djdarens@
mail.chem.tamu.edu. Fax (979)845-0158.
(1) (a) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater.
2000, 12, 1841–1846. (b) Dove, A. P. Chem. Commun. 2008, 6446–6470.
(c) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Dalton Trans. 2009, 4832–
4846. (d) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 486–494.
(2) (a) Penco, M.; Donetti, R.; Mendichi, R.; Ferruti, P. Macromol.
Chem. Phys. 1998, 199, 1737–1745. (b) Ikada, Y.; Tsuji, H. Macromol.
Rapid Commun. 2000, 21, 117–132. (c) Platel, R. H.; Hodgson, L. M.;
Williams, C. K. Polym. Rev. 2008, 48, 11–63.
(3) (a) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff,
K. M. Chem. Rev. 1999, 99, 3181–3198. (b) Auras, R.; Harte, B.; Selke, S.
Macromol. Biosci. 2004, 4, 835–864.
(4) Ghosh, S. J. Chem. Res. 2004, 241–246.
(5) Ovitt, T. M.; Coates, G. W. J. Polym. Sci., Part A: Polym. Chem.
2000, 38, 4686–4692.
(6) (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B.
Dalton Trans. 2001, 2215–2224. (b) Dechy-Cabaret, O.; Martin-Vaca, B.;
Bourissou, D. Chem. Rev. 2004, 104, 6147–6176. (c) Kamber, N. E.; Jeong,
W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem.
Rev. 2007, 107, 5813–5840. (d) Thomas, C. M. Chem. Soc. Rev. 2010, 39,
165–173.
r
2010 American Chemical Society
Published on Web 08/11/2010
pubs.acs.org/Organometallics

5628 Organometallics, Vol. 29, No. 21, 2010
Darensbourg and Karroonnirun
Scheme 1
reported in the literature thus far undergo a chain-end con-
trol mechanism to produce heterotactic polylactides. On the
other hand, chiral5,12 and achiral8b,9b,13 complexes of alumi-
num have shown significant stereocontrol for the ROP of
rac-lactide to afford polylactides with high degrees of iso-
tactic enrichment. Chisholm and co-workers have shown
that, in addition to chiral ligands bound to aluminum, other
factors such as the chirality of the alkoxide initiator and
solvents can contribute to the stereoselectivity in the ROP of
lactides when utilizing aluminum salen catalysts.12h,i
Herein we have synthesized and characterized structurally
a series of aluminum half-salen complexes containing both
chiral and achiral ligands and report some of our preliminary
observations on their use as catalysts for the ring-opening
polymerization of lactides.
Results and Discussion
(12) (a) Spassky, N.; Wisniewski, M.; Pluta, C.; Borgne, A. L. Macro-
mol. Chem. Phys. 1996, 197, 2627–2637. (b) Wisniewski, M.; Borgne, A. L.;
Spassky, N. Macromol. Chem. Phys. 1997, 198, 1227–1238. (c) Ovitt, T. M.;
Coates, G. W. J. Am. Chem. Soc. 1999, 121, 4072–4073. (d) Radano, C. P.;
Baker, G. L.; Smith, M. R. J. Am. Chem. Soc. 2000, 122, 1552–1553.
(e) Zhong, Z.;Dijkstra, P. J.;Feijen, J. Angew. Chem., Int. Ed. 2002, 41, 4510–
4513. (f) Zhong, Z.; Dijkstra, P. J.; Feijen, J. J. Am. Chem. Soc. 2003, 125,
11291–11298. (g) Majerska, K.; Duda, A. J. Am. Chem. Soc. 2004, 126,
1026–1027. (h) Chisholm, M. H.; Patmore, N. J.; Zhou, Z. Chem. Commun.
2005, 127–129. (i) Chisholm, M. H.; Gallucci, J. C.; Quisenberry, K. T.; Zhou,
Z. Inorg. Chem. 2008, 47, 2613–2624. (j) Lian, B.; Ma, H.; Spaniol, T. P.;
Okuda, J. Dalton Trans. 2009, 9033–9042.
(13) (a) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; White,
A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2004, 126, 2688–2689.
(b) Tang, Z.; Chen, X.; Pang, X.; Yang, Y.; Zhang, X.; Jing, X. Biomacro-
molecules 2004, 5, 965–970. (c) Nomura, N.; Ishii, R.; Yamamoto, Y.;
Kondo, T. Chem. Eur. J. 2007, 13, 4433–4451. (d) Pang, X.; Chen, X.;
Du, H.; Wang, X.; Jing, X. J. Organomet. Chem. 2007, 692, 5605–5613.
(e) Bouyahyi, M.; Grunova, E.; Marquet, N.; Kirillov, E.; Thomas, C. M.;
Roisnel, T.; Carpentier, J.-F. O. Organometallics 2008, 27, 5815–5825.
(f) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. O. Orga-
nometallics 2009, 28, 1469–1475. (g) Phomphrai, K.; Chumsaeng, P.;
Sangtrirutnugul, P.; Kongsaeree, P.; Pohmakotr, M. Dalton Trans. 2010,
39, 1865–1871.
The ligands used in our studies were derived from either
amino alcohols or amino acids by condensation reactions
with the corresponding aldehydes to afford compounds
1a-j. Reactions of these ligands with triethylaluminum in
dry toluene resulted in the formation of complexes 2a-j, as
depicted in Scheme 1. In this manner three series of closely
related aluminum half-salen complexes were synthesized
whose ligand backbones were easily modified by chiral
amino alcohols (2a-d), aliphatic amino alcohols (2e-h),
and amino acids (2i,j). The reactivity and selectivity of
aluminum complexes 2a-j for catalyzing the ROP of rac-
lactide are provided in Table 1.
As noted in Table 1, complex 2a did not polymerize rac-
lactide in CDCl3 after 66 h at 60 °C (entry 2); however, in
toluene a 57% conversion to polylactide was observed over
the same time period at 70 °C (entry 4). For complexes in the
series 2a-d and the substituents (R2) on the phenoxide of the
half-salen ligand increase in size (R2 = SiPh3) or are more

Article
Organometallics, Vol. 29, No. 21, 2010 5629
Table 1. Reactivity and Selectivity of Aluminum Complexes 2a-j
for the ROP of rac-Lactidea
Table 2. ROP of rac- and L-Lactide Using Aluminum Complexes
2e-g,i,ja
Mn
Mn
time conversn
(%)b
conversn
(%)b
e
f
e
f
entry
M
(h)
mesoc theord 0.58Mn,GPC PDI Pm
entry
M
lactide
mesoc theord 0.58Mn,GPC PDI Pm
1
2
3
4
5
6
7
8
9
2ag
2ah
2a
2a
2b
2b
2c
20
66
15
66
15
69
15
168
15
15
15
15
15
15
15
0
0
0
57
0
43
0
45
0
64
0
34
50
36
42
no
yes
yes
yes 4107
yes
yes 3243
yes
yes 3207
yes
yes 4614
yes
no
no
no
no
1
2
3
4
5
6
7
8
9
2e
2e
2f
2fg
2g
2g
2i
rac
L
64
77
0
36
34
44
34
54
42
45
yes 4614
yes 4107
yes
6987
7938
1.04
1.05
62
52
rac
L
7938
3844
4962
6987
2456
1.05
70
yes 2600
3726
2456
N/Ah
3916
N/Ah
4043
N/Ah
1.07 <50
1.07 76
N/Ah 100
1.07 74
N/Ah 100
1.03 72
N/Ah 100
rac
L
no
no
no
no
no
no
2446
3164
2594
3891
3026
3243
1.08 <50
1.09 <50
rac
L
2c
2d
2i
2j
rac
L
10 2e
1.04
1.07
62
10 2j
11 2f
12 2g
13 2h
14 2i
15 2j
a Unless otherwise specified, the polymerization reactions were per-
formed in sealed reaction tubes under the following conditions:
[LA]/[Al] = 50, in toluene at 70 °C for 15 h. b Obtained from 1H
NMR spectroscopy. c meso-Lactide was obtained from epimerization
of L- or D-lactide during the polymerization process. d Theoretical Mn =
(M/I) ꢀ (% conversion) ꢀ (mol wt of lactide). e Mn values were corrected
2446
76
73
74
72
2594
3026
3916
4043
1.07
1.03
a Unless otherwise specified, the polymerization reactions were per-
formed in sealed reaction tubes under the following conditions: [rac-
LA]/[Al] = 50, in toluene at 70 °C. b Obtained from 1H NMR spectros-
copy. c meso-Lactide was obtained from epimerization of L- or D-lactide
during the polymerization process. d Theoretical Mn = (M/I) ꢀ (% con-
version) ꢀ (mol wt of lactide). e Mn values were corrected by the equation
by the equation Mn = 0.58Mn,GPC.
14 f Pm values were calculated from
the ratio of the (area of iii)/(total area in methine proton region). g The
reaction time was 76 h. h The molecular weight was not measured
because of the polymer’s insolubility in THF, due to its high crystallinity.
Mn = 0.58 Mn, GPC.
14 f Pm values were calculated from the ratio of the
(area of iii)/(total area in methine proton region). g CDCl3 was used
as the solvent at ambient temperature. h CDCl3 was used as the solvent at
60 °C.
Figure 2. DSC curves (second heating run) of polylactide from
rac-lactide catalyzed by complex 2i.
complexes with achiral aliphatic backbone with the exception
of 2f, i.e., complexes 2e,g,h, were found to catalyze the ROP
of rac-lactide at rates faster than that of complex 2a to give
isotactic polylactides with Pm values of 0.62, 0.76, and 0.73,
respectively. Similarly, the complexes derived from rac-amino
acids, 2i,j, were also found to be active for the ring-opening
polymerization of rac-lactide, providing substantially isotactic
polymers with Pm values of 0.74 and 0.72, respectively.
A noteworthy point of importance is that for several of
these aluminum-catalyzed systems, namely 2a-f, a 1H NMR
signal at 1.70 ppm appears during the polymerization pro-
cess, while polymerization reactions catalyzed by complexes
Figure 1. 1H NMR spectrum (CDCl3, room temperature) of the
reaction mixtures during the ROP of L-lactide in the presence of
complex 2a (A), 2e (B), 2g (C) (entries 3, 10, and 12 in Table 1,
respectively). The polymerization reactions were performed
in toluene at 70 °C for 15 h. Methyl protons of meso-lactide,
L-lactide, and polylactide were observed at 1.70 (a), 1.66 (b), and
1.57 (c), respectively.
1
2g-j did not exhibit such a H NMR signal (see resonance
labeled a in Figure 1). This H NMR peak is assigned to
1
intermediate formation of meso-lactide from epimerization
of L- and D-lactide during the ROP process. In order to
further confirm this occurrence, select aluminum complexes
2e-g,i,j were used to catalyze the ROP of L-lactide. In these
instances, if there is no epimerization occurring during the
electron donating (R2 = OMe), the rate of lactide polymeri-
zation decreases (entries 6 and 8). Similar observations have
been previously reported by Normura and co-workers and
Gibson and co-workers.13a,c Complex 2a afforded a moder-
ately isotactic polymer with a Pm value of 0.70, while complexes
2b,c yielded polylactides with Pm values less than 0.50. The
(14) The Mn values for polylactide were corrected from the Mn values
determined by GPC vs polystyrene standards, according to the equation
Mn = 0.58Mn,GPC, as previously reported in the literature: (a) Barakat,
ꢀ ^
ꢀ
I.; Dubois, P.; Jerome, R.; Teyssie, P. J. Polym. Sci., Part A: Polym.
Chem. 1993, 31, 505–514. (b) Baran, J.; Duda, A.; Kowalski, A.; Szymanski,
R.; Penczek, S. Macromol. Rapid Commun. 1997, 18, 325–333.

5630 Organometallics, Vol. 29, No. 21, 2010
Darensbourg and Karroonnirun
Figure 3. X-ray crystal structures of (a) 2f (cis), (b) 2f (trans). Thermal ellipsoids represent the 50% probability surfaces. Hydrogen
atoms are omitted for the sake of clarity.
1
Figure 4. Variable-temperature H NMR spectrum (500 MHz) of complex 2f in deuterated toluene taken sequentially at different
temperatures: (A) room temperature; (B) 40 °C; (C) 70 °C; (D) 100 °C; (E) sample cooled from 100 °C to room temperature, spectrum
taken 2 days later.
polymerization process, only isotactic polylactide will be
formed. As anticipated, complexes 2e,f polymerized L-lactide
to afford atactic polylactide with Pm values of 0.52 and
<0.50, respectively, while complexes 2g,i,j provided isotactic
polylactide with a Pm value of 1, indicative of an absence of
epimerization in these instances (Table 2).
The observed molecular weights of all the polymer pro-
duced via catalysis with the aluminum complexes employed
in this study, i.e., complexes 2a-j, were found to closely
parallel the theoretical values. In addition, the polymers thus
afforded had polydispersity indices ranging from 1.03 to
1.08. The ring-opening polymerization was shown to be first
order in [monomer] and [catalyst]. Hence, these catalytic
systems have the characteristic of well-controlled polymeri-
zation processes. The thermal properties of a purified poly-
lactide sample produced from the ROP of rac-lactide by
complex 2i (Pm = 74%) were determined by differential
scanning calorimetry (DSC). The Tm and Tg values of the poly-
mer were found to be 158 and 52 °C, respectively (Figure 2).
The observed Tm value is consistent with a moderately isotactic
polylactide.
In an effort to understand the catalytic differences noted
for the ROP of lactides by the closely related aluminum
complexes 2e-h, we report here the structural characteri-
zation of one of these derivatives. Specifically, complex 2f
was synthesized and isolated according to Scheme 1, except

Article
Organometallics, Vol. 29, No. 21, 2010 5631
1
Figure 5. Variable-temperature H NMR spectra (500 MHz) of complex 2g in deuterated toluene taken sequentially at different
temperatures: (A) room temperature; (B) 40 °C; (C) 70 °C; (D) 100 °C; (E) sample cooled from 100 °C to room temperature, spectrum
taken 2 days later.
pentane was used as solvent, which allowed the complex to
precipitate out of solution during its preparation. Single
crystals suitable for X-ray structural analysis were obtained
upon recrystallization from dichloromethane at -10 °C. In
this manner both platelike crystals and blocklike crystals
were obtained. X-ray crystallography revealed the blocklike
crystals to be dimeric, with bridging oxygen atoms. The two
five-coordinate aluminum centers possess a distorted-bipyr-
amidal geometry with the ethyl group of each metal cis to one
another, as shown in Figure 3a. Similarly, the solid-state
structure of the platelike crystals was shown to be dimeric,
except that the ethyl groups on the aluminum centers were
trans to one another (Figure 3b).
distribution of the isomeric complex mixture was reached at
100 °C, this complex mixture remained the same after the
solution stood at ambient temperature for 2 days (spectrum
E in Figure 4). As indicated in Figure 4, we propose the
isomeric mixture of aluminum complexes favors the trans
arrangement of ethyl groups. That is, due to the decrease in
steric repulsion of the bulky phenyl ring, the trans isomer is
thermodynamically more stable.
As indicated by the variable-temperature 1H NMR study,
complex 2f exists as both the cis and trans isomers in toluene
solution at the ROP temperature of 70 °C. We propose that
one of these isomeric forms of the aluminum complex 2f is
responsible for meso-lactide formation: i.e., the cis form (vide
1
Since this difference in dimeric aluminum structures may
be related to the disparity in catalytic reactivity for the ROP
of lactides noted above, and the ROP processes are carried
out in solution at 70 °C, it is important to examine the
structures of these aluminum complexes in solution. We
therefore performed variable-temperature 1H NMR spectro-
scopic studies of complex 2f in deuterated toluene. As
observed in Figure 4, the 1H NMR spectrum of complex 2f
at ambient temperature reveals two sets of methyl group
protons from the tert-butyl substituents of the phenoxide
ligands (resonances a and a0 and b and b0). As the tempera-
ture was increased, the intensity of the proton signals a0 and
infra). In much the same way, the H NMR spectrum of
complex 2e exhibited two sets of tert-butyl group methyl
protons at ambient temperature. However, in this case, upon
increasing the temperature to 100 °C with periodic monitor-
ing of the spectra, no spectral changes were observed. This
result strongly suggests that the structural rearrangements
seen in these aluminum dimers are related to the length of the
carbon chain backbone in the amino-alkoxide bridging
ligand, with the five-membered ring (2e) being, as expected,
more stable than the six-membered ring (2f). In support of
this contention, upon examination of the variable-tempera-
ture 1H NMR spectra of complex 2g, which contains a longer
four-carbon-chain backbone leading to a seven-membered
amino-alkoxide bridging ligand, a facile isomeric exchange
reaction was observed (Figure 5). That is, at ambient
1
b0 increased with a concomitant decrease in the H NMR
signals at a and b. Both set of resonances were noted at
100 °C, with those of a0 and b0 being more intense. Once the

5632 Organometallics, Vol. 29, No. 21, 2010
Darensbourg and Karroonnirun
temperature the 1H NMR spectrum revealed the presence of
both isomers in solution, with the cis isomer in larger quantity.
As the temperature was increased, the distribution of iso-
mers converted to all trans by 70 °C, with no further change
occurring with either an increase in temperature to 100 °C or a
decrease down to ambient temperature.15 As evident from
Table 1, when complexes 2g,h (four- or five-membered carbon
chain backbone) were employed as catalysts for the ROP
of rac-lactide, no meso-lactide formation was observed. These
observations confirm our suggestion that the cis isomer is
responsible for the epimerization of lactide prior to the ring-
opening polymerization process.
3-(tert-butyldimethylsilyl)-2-hydroxy-5-methylbenzaldehyde were
prepared according to the published procedure.16 All other com-
pounds and reagents were obtained from Sigma-Aldrich and were
used without further purification. Analytical elemental analysis
was provided by Canadian Microanalytical Services Ltd.
1
Measurements. H NMR spectra were recorded on Unityþ
300 or 500 MHz and VXR 300 or 500 MHz superconducting
NMR spectrometers. Molecular weight determinations were
carried out with a Viscotek Modular GPC apparatus equipped
with ViscoGEL I-series columns (H þ L) and Model 270 dual
detector comprised of refractive index and light scattering
detectors. DSC measurements were performed with a Polymer
DSC instrument by Mettler Toledo. The samples were scanned
from -100 to 200 °C under a nitrogen atmosphere. The glass
transition temperature (Tg), the crystallization temperature
(Tc), and the melting temperature (Tm) of polylatides were
determined from the second heating at a heating rate of 5 °C/
min. X-ray crystallography was done on a Bruker GADDS
X-ray diffractometer under a nitrogen cold stream maintained
at 110 K. Crystal data and details of the data collection for
complexes 2f (cis) and 2f (trans) are provided in the Supporting
Information (Table S1).
Conclusions
Herein we have reported the synthesis of a series of
tridentate NOO Schiff base ligands along with their alumi-
num complexes. These metal complexes were dimeric and
exhibited two isomeric structures, one with the initiators on
the two aluminum centers cis to one another and the other
with the trans arrangement. The latter form was shown to be
thermodynamically more stable by variable-temperature 1H
NMR studies. All complexes were shown to polymerize rac-
lactide in toluene at 70 °C. The molecular weights of the
afforded polylactides correlated well with monomer/initia-
tor and conversion level and displayed narrow distributions
with PDI values ranging from 1.03 to 1.08. It was established
that complexes which existed in both cis and trans forms
under the conditions of the ring-opening polymerization
reaction led to partial epimerization of D- and L-lactide
to meso-lactide prior to polymerization. On the other hand,
complexes which exist in the trans isomeric form led to
polymerization of rac-lactide with no epimerization and
concomitantly a polylactide with a high degree of isotacticity
(Pm = 76%). We are in the process of performing more
detailed studies of the mechanistic aspects of the ROP of
lactides, employing a complete series of structurally well-
characterized aluminum dimeric complexes.
General Procedure for Synthesis of Tridentate Schiff Base
Ligands 1a-d. trans-2-Aminocyclohexanol hydrochloride (1.1
equiv) and triethylamine (1.09 equiv) were added to the corre-
sponding benzaldehyde (1.0 equiv) in MeOH (30 mL). The
solution mixture was heated to reflux overnight, and the solvent
was removed under reduced pressure to give a yellow solid. The
resulting yellow solid was washed with water (3 ꢀ 20 mL) to
remove excess trans-2-aminocyclohexanol hydrochloride and
triethylamine. The organic layer was separated, dried over
NaSO4, and concentrated to dryness under reduced pressure
to afford the crude products; these were crystallized in pentane
to give a yellow powder in 81-95% yield.
(E)-2,4-Di-tert-butyl-6-(((trans-2-hydroxycyclohexyl)imino)-
methyl)phenol (1a; L1-H). Following the general procedure for
synthesis of the tridentate Schiff base ligands 1a-d, trans-2-
aminocyclohexanol (0.711 g, 4.69 mmol, 1.1 equiv) and triethyl-
amine (0.470 g, 4.65 mmol, 1.09 equiv) were added to 3,5-di-tert-
butyl-2-hydroxybenzaldehyde (1.00 g, 4.26 mmol, 1.0 equiv) in
MeOH (30 mL). The solution mixture was heated to reflux
overnight, and the solvent was removed under reduced pressure
to give a yellow solid. The resulting yellow solid was washed with
water (3 ꢀ 20 mL) to remove excess trans-2-aminocyclohexanol
hydrochloride and triethylamine. The organic layer was sepa-
rated, dried over NaSO4, and concentrated to dryness under
reduced pressure to afford product 1a, which was crystallized
in pentane to give a yellow powder in 95% yield. 1H NMR (300
MHz, CDCl3): δ 1.31 (s, 9H, C(CH3)3), 1.45 (s, 9H, C(CH3)3),
1.35-2.4 (bm, 8H, CH(CH2)4CH), 2.99 (m, 1H, NCHCH2),
3.69 (m, 1H, OCHCH2), 7.12 (d, J = 2.38 Hz, 1H, C6H2), 7.40
(d, J = 2.38 Hz, 1H, C6H2), 8.45 (s, 1H, CHdN), 13.60 (s, 1H,
OH). 13C NMR (300 MHz, CDCl3): δ 24.28 (NCHCH2-
CH2CH2), 24.43 (OCHCH2CH2CH2), 29.42 (C(pCH3)3), 31.47
(C(oCH3)3), 32.51 (NCHCH2CH2), 32.83 (OCHCH2CH2),
34.12 (pCCH3), 35.01(oCCH3), 73.68 (NCHCH2), 75.55 (OCH-
CH2), 117.73, 126.04, 127.06, 136.66, 140.22, 157.97 (Ar), 166.59
(CdN). Anal. Calcd for C21H33NO2: C, 76.09; H, 10.03; N, 4.23.
Found: C, 75.93; H, 10.13; N, 4.16. HRMS (ESI): m/z 332.2581
[M þ Hþ], calcd for C21H33NO2 331.25.
Experimental Section
Methods and Materials. All manipulations were carried out
using a double-manifold Schlenk vacuum line under an argon
atmosphere or an argon-filled glovebox unless otherwise stated.
Toluene was freshly distilled from sodium/benzophenone before
use. Methanol and dichloromethane were purified by an
MBraun Manual Solvent Purification System packed with
Alcoa F200 activated alumina desiccant. Pentane was freshly
distilled from CaH2. Deuterated chloroform and deuterated
benzene from Cambridge Isotope Laboratories Inc. were stored
in the glovebox and used as received. L- and D-lactide and rac-
lactide were gifts from PURAC America Inc. These lactides
were recrystallized from toluene, dried under vacuum at 40 °C
overnight, and stored in the glovebox. 4-Amino-1-butanol and
triethylaluminium were purchased from TCI America and
Sigma-Aldrich, respectively, and used without further purifica-
tion. Ethanolamine, 3-amino-1-propanol, 5-amino-1-pentanol,
trans-2-aminocyclohexanol hydrochloride, 2-hydroxy-3-meth-
oxybenzaldehyde, rac-methionine, rac-phenylalanine, and tert-
butyldimethylchlorosilane were purchased from Alfa Aesar and
used as received. 3,5-Di-tert-butyl-2-hydroxybenzaldehyde and
(E)-2-(((trans-2-Hydroxycyclohexyl)imino)methyl)-6-methoxy-
phenol (1b; L2-H). Following the general procedure for synthe-
sis of the tridentate Schiff base ligands 1a-d, trans-2-amino-
cyclohexanol (1.09 g, 7.22 mmol, 1.1 equiv) and triethylamine
(15) Complex 2g was recrystallized by heating the complex in a sealed
test tube at 50 °C in dichloromethane until all of the complex was
dissolved. The crystal tube was kept at -10 °C and provided crystals of
the trans form exclusively, as evidenced by X-ray crystallographic
studies.
(16) (a) Darensbourg, D. J.; Choi, W.; Karroonnirun, O.; Bhuvanesh,
N. Macromolecules 2008, 41, 3493–3502. (b) Darensbourg, D. J.; Choi, W.;
Richers, C. P. Macromolecules 2007, 40, 3521–3523. (c) Hansen, T. V.;
Skattebøl, L. Tetrahedron Lett. 2005, 46, 3829–3830. (d) Thadani, A. N.;
Huang, Y.; Rawal, V. H. Org. Lett. 2007, 9, 3873–3876.

Article
Organometallics, Vol. 29, No. 21, 2010 5633
(0.724 g, 7.16 mmol, 1.09 equiv) were added to 2-hydroxy-3-
methoxybenzaldehyde (1.00 g, 6.57 mmol, 1.0 equiv) in MeOH
(30 mL). Compound 1b was obtained as a yellow powder in 87%
yield. 1H NMR (300 MHz, CDCl3): δ 1.38, 1.62, 1.81, 2.08
(bm, 8H, CH(CH2)4CH), 3.03 (m, 1H, NCHCH2), 3.66 (m, 1H,
OCHCH2), 3.90 (s, 3H, OCH3) 6.78, 6.81, 6.84, 6.88, 6.91, 6.94
(m, 3H, C6H3), 8.42 (s, 1H, CHdN), 13.84 (s, 1H, OH). 13C
NMR (300 MHz, CDCl3): δ 24.12 (NCHCH2CH2CH2), 24.28
(OCHCH2CH2CH2), 32.58 (NCHCH2CH2), 32.62 (OCHCH2-
CH2), 73.59 (NCHCH2), 74.76 (OCHCH2), 114.08, 117.97,
118.42, 123.00, 148.50, 151.94 (Ar), 165.41 (CdN). Anal. Calcd
for C14H19NO3: C, 67.45; H, 7.68; N, 5.62. Found: C, 67.63; H,
7.73; N, 5.60. HRMS (ESI): m/z 250.1515 [M þ Hþ], calcd for
C14H19NO3 249.14.
(E)-2-(((trans-2-Hydroxycyclohexyl)imino)methyl)-4-methyl-
6-(triphenylsilyl)phenol (1c; L3-H). Following the general pro-
cedure for the synthesis of tridentate Schiff base ligands 1a-d,
trans-2-aminocyclohexanol (0.338 g, 2.23 mmol, 1.1 equiv) and
triethylamine (0.223 g, 2.21 mmol, 1.09 equiv) were added to
2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde16d (0.800 g,
2.02 mmol, 1.0 equiv) in MeOH (30 mL). Compound 1c was
obtained as a yellow powder in 90% yield. 1H NMR (300 MHz,
CDCl3): δ 1.31, 1.56, 1.75, 2.03 (bm, 8H, CH(CH2)4CH), 2.19 (s,
3H, CCH3), 2.97 (m, 1H, NCHCH2), 3.59 (m, 1H, OCHCH2),
3.90 (s, 3H, OCH3) 7.07-7.64 (m, 18H, Ar), 8.41 (s, 1H,
CHdN), 13.20 (s, 1H, OH). 13C NMR (300 MHz, CDCl3): δ
20.43 (CCH3), 24.25 (NCHCH2CH2CH2), 24.40 (OCHCH2-
CH2CH2), 32.59 (NCHCH2CH2), 32.76 (OCHCH2CH2),
73.42 (NCHCH2), 75.71 (OCHCH2), 117.73, 121.32, 127.40,
127.67, 129.24, 129.76, 134.23, 134.68, 135.16, 135.42, 136.35,
142.04, 163.98 (Ar), 165.54 (CdN). Anal. Calcd for C32H33NO2-
Si: C, 78.17; H, 6.76; N, 2.85. Found: C, 76.99; H, 6.67; N, 2.16.
HRMS (ESI): m/z 492.2625 [M þ Hþ], calcd for C32H33NO2Si
491.23.
(E)-2-(tert-Butyldimethylsilyl)-6-(((trans-2-hydroxycyclohexyl)-
imino)methyl)-4-methylphenol (1d; L4-H). Following the general
procedure for synthesis of tridentate Schiff base ligands 1a-d,
trans-2-aminocyclohexanol (0.166 g, 1.09 mmol, 1.1 equiv) and
triethylamine (0.110 g, 1.08 mmol, 1.09 equiv) were added to
2-hydroxy-5-methyl-3-(triphenylsilyl)benzaldehyde16d (0.250 g,
0.99 mmol, 1.0 equiv) in MeOH (30 mL). Compound 1d was
obtained as a yellow powder in 81% yield. 1H NMR (300 MHz,
CDCl3): δ 0.32 (s, 6H, Si(CH3)2), 0.92 (s, 9H, C(CH3)3), 1.37,
1.63, 1.80, 2.08 (bm, 8H, CH(CH2)4CH), 2.97 (m, 1H, NCH-
CH2), 3.69 (m, 1H, OCHCH2), 7.05 (d, J = 1.78 Hz, 1H, C6H2),
7.20 (d, J = 2.09 Hz, 1H, C6H2), 8.37 (s, 1H, CHdN), 12.98 (s,
1H, OH). 13C NMR (300 MHz, CDCl3): δ -4.71 (Si(CH3)2),
17.61 (SiC(CH3)3), 20.43 (CCH3), 24.28 (NCHCH2CH2CH2),
24.45 (OCHCH2CH2CH2), 27.11 (SiC(CH3)3), 32.53 (NCH-
CH2CH2), 32.83 (OCHCH2CH2), 73.62 (NCHCH2), 75.74
(OCHCH2), 117.42, 124.72, 126.81, 133.06, 140.20, 163.73
(Ar), 165.90 (CdN). Anal. Calcd for C20H33NO2Si: C, 69.11;
H, 9.57; N, 4.03. Found: C, 69.34; H, 9.81; N, 4.01. HRMS
(ESI): m/z 348.2482 [M þ Hþ], calcd for C20H33NO2Si, 347.23.
General Procedure for Synthesis of Tridentate Schiff Base
Ligands 1e-h. The corresponding benzaldehyde (1.0 equiv)
was added to the corresponding amino alcohol (1.0 equiv) in
MeOH (30 mL). The solution mixture was heated to reflux
overnight and dried over Na2SO4 followed by filtration. The
volatile component was removed in vacuo to give tridentate
Schiff base ligands in quantitative yield.
C(CH3)3), 1.37 (s, 9H, C(CH3)3), 3.68 (m, 2H, OCH2CH2), 3.86
(m, 2H, NCHCH2), 7.03 (d, J = 2.68 Hz, 1H, C6H2), 7.32 (d,
J = 2.68 Hz, 1H, C6H2), 8.35 (s, 1H, CHdN), 13.49 (s, 1H, OH).
13C NMR (300 MHz, CDCl3): δ 29.40 (C(pCH3)3), 31.48
(C(oCH3)3), 34.35 (pCCH3), 35.01 (oCCH3), 61.82 (OCH2CH2),
62.33 (NCH2CH2), 117.73, 126.05, 127.14, 136.71, 140.21,
157.95 (Ar), 168.14 (CdN). Anal. Calcd for C17H27NO2: C,
73.61; H, 9.81; N, 5.05. Found: C, 73.94; H, 10.16; N, 4.70.
HRMS (ESI): m/z 278.2213 [M þ Hþ], calcd for C17H27NO2,
277.20.
(E)-2,4-Di-tert-butyl-6-(((3-hydroxypropyl)imino)methyl)-
phenol (1f; L6-H). Following the general procedure for synthesis
of tridentate Schiff base ligands 1e-h, 3,5-di-tert-butyl-2-hy-
droxybenzaldehyde (0.800 g, 3.41 mmol, 1.0 equiv) was added to
3-amino-1-propanol (0.256 g, 3.41 mmol, 1.0 equiv) in MeOH
(30 mL). The solution mixture was heated to reflux overnight
and dried over Na2SO4, followed by filtration. The volatile
component was removed in vacuo to give the tridentate Schiff
base ligand 1f in quantitative yield. 1H NMR (300 MHz,
CDCl3): δ 1.31 (s, 9H, C(CH3)3), 1.45 (s, 9H, C(CH3)3), 1.97
(m, 2H, CH2CH2CH2), 3.71 (t, J = 6.84 Hz, 2H, OCH2CH2),
3.76 (t, J = 6.25 Hz, 2H, NCHCH2), 7.09 (d, J = 2.38 Hz, 1H,
C6H2), 7.38 (d, J = 2.38 Hz, 1H, C6H2), 8.39 (s, 1H, CHdN),
13.85 (s, 1H, OH). 13C NMR (300 MHz, CDCl3): δ 29.40
(C(pCH3)3), 31.48 (C(oCH3)3), 33.50 (CH2CH2CH2) 34.11 (pC-
CH3), 35.00 (oCCH3), 55.84 (OCH2CH2), 60.31 (NCH2CH2),
117.77, 125.78, 126.83, 136.65, 139.97, 158.08 (Ar), 166.35
(CdN). Anal. Calcd for C18H29NO2: C, 74.18; H, 10.03; N,
4.81. Found: C, 74.05; H, 10.20; N, 4.84. HRMS (ESI): m/z
292.2394 [M þ Hþ], calcd for C18H29NO2, 291.22.
(E)-2,4-Di-tert-butyl-6-(((4-hydroxybutyl)imino)methyl)phenol
(1g; L7-H). Following the general procedure for synthesis of
tridentate Schiff base ligands 1e-h, 3,5-di-tert-butyl-2-hydro-
xybenzaldehyde (1.00 g, 4.26 mmol, 1.0 equiv) was added to
4-amino-1-butanol (0.380 g, 4.26 mmol, 1.0 equiv) in MeOH
(30 mL). The solution mixture was heated to reflux overnight
and dried over Na2SO4, followed by filtration. The volatile
component was removed in vacuo to give the tridentate Schiff
base ligand 1g in quantitative yield. 1H NMR (300 MHz,
CDCl3): δ 1.31 (s, 9H, C(CH3)3), 1.45 (s, 9H, C(CH3)3), 1.70
(m, 2H, CH2CH2CH2), 1.79 (m, 2H, CH2CH2CH2), 3.62 (t, J =
6.55 Hz, 2H, OCH2CH2), 3.70 (t, J = 6.24 Hz, 2H, NCHCH2),
7.08 (d, J = 2.38 Hz, 1H, C6H2), 7.37 (d, J = 2.38 Hz, 1H,
C6H2), 8.36 (s, 1H, CHdN), 13.90 (s, 1H, OH). 13C NMR (300
MHz, CDCl3): δ 27.19 (CH2CH2CH2), 29.39 (C(pCH3)3),
30.29 (CH2CH2CH2), 31.48 (C(oCH3)3), 34.08 (pCCH3), 34.98
(oCCH3), 59.17 (OCH2CH2), 62.56 (NCH2CH2), 117.77, 125.70,
126.72, 136.62, 139.88, 158.13 (Ar), 166.83 (CdN). Anal. Calcd
for C19H31NO2: C, 74.71; H, 10.23; N, 4.59. Found: C, 74.73; H,
10.32; N, 4.58. HRMS (ESI): m/z 306.2501 [M þ Hþ], calcd for
C19H31NO2, 305.24.
(E)-2,4-Di-tert-butyl-6-(((5-hydroxypentyl)imino)methyl)phenol
(1h; L8-H). Following the general procedure for synthesis of
tridentate Schiff base ligands 1e-h, 3,5-di-tert-butyl-2-hydro-
xybenzaldehyde (1.00 g, 4.26 mmol, 1.0 equiv) was added to
5-amino-1-pentanol (0.440 g, 4.26 mmol, 1.0 equiv) in MeOH
(30 mL). The solution mixture was heated to reflux overnight
and dried over Na2SO4, followed by filtration. The volatile
component was removed in vacuo to give the tridentate Schiff
base ligand 1h in quantitative yield. 1H NMR (300 MHz,
CDCl3): δ 1.31 (s, 9H, C(CH3)3), 1.45 (s, 9H, C(CH3)3), 1.48
(m, 2H, CH2CH2CH2), 1.63 (m, 2H, CH2CH2CH2), 1.74 (m,
2H, CH2CH2CH2), 3.59 (t, J = 6.84 Hz, 2H, OCH2CH2), 3.67
(t, J = 6.54 Hz, 2H, NCHCH2), 7.07 (d, J = 2.38 Hz, 1H,
C6H2), 7.37 (d, J = 2.38 Hz, 1H, C6H2), 8.34 (s, 1H, CHdN),
13.95 (s, 1H, OH). 13C NMR (300 MHz, CDCl3): δ 23.39
(CH2CH2CH2), 29.40 (C(pCH3)3), 30.66 (CH2CH2CH2), 31.49
(C(oCH3)3), 33.44 (CH2CH2CH2), 34.10 (pCCH3), 35.00 (oC-
CH3), 59.43 (OCH2CH2), 62.80 (NCH2CH2), 117.80, 125.67,
126.70, 136.63, 139.86, 158.17 (Ar), 158.17 (CdN). Anal. Calcd
(E)-2,4-Di-tert-butyl-6-(((2-hydroxyethyl)imino)methyl)phenol
(1e; L5-H). Following the general procedure for synthesis of
tridentate Schiff base ligands 1e-h, 3,5-di-tert-butyl-2-hydro-
xybenzaldehyde (1.00 g, 4.26 mmol, 1.0 equiv) was added to
ethanolamine (0.260 g, 4.26 mmol, 1.0 equiv) in MeOH (30 mL).
The solution mixture was heated to reflux overnight and dried
over Na2SO4, followed by filtration. The volatile component
was removed in vacuo to give the tridentate Schiff base ligand 1e
in quantitative yield. 1H NMR (300 MHz, CDCl3): δ 1.24 (s, 9H,

5634 Organometallics, Vol. 29, No. 21, 2010
Darensbourg and Karroonnirun
for C20H33NO2: C, 75.19; H, 10.41; N, 4.38. Found: C, 75.10; H,
10.41; N, 4.38. HRMS (ESI): m/z 320.2708 [M þ Hþ], calcd for
C20H33NO2, 319.25.
methanol. The solid polymer was collected and dried under
vacuum to constant weight.
General Procedure for Synthesis of Tridentate Schiff Base
Aluminum Complexes (2e-g; L5-7AlEthyl). A solution of tri-
ethylaluminum (1.0 equiv) in pentane was cannulated to a
solution of tridentate Schiff base ligand (1 equiv) in pentane.
The reaction mixture was stirred until a yellow precipitate was
formed, and this mixture was stirred at room temperature for an
additional 3 h. The resulting yellow precipitate was washed with
cold pentane (3 ꢀ 1 mL). The volatile component was removed
under reduced pressure to give light yellow solids of complexes
2e-g.
General Procedure for Synthesis of Tridentate Schiff Base
Ligands 1i,j. The corresponding amino acid (2 equiv) and tri-
ethylamine (2 equiv) were added to 3,5-di-tert-butyl-2-hydro-
xybenzaldehyde16a-c (1.0 equiv) in MeOH (30 mL). The solu-
tion mixture was heated to reflux for overnight, and the solvent
was removed under reduced pressure to obtain a yellow solid.
The resulting yellow solid was washed with water (3 ꢀ 20 mL) to
remove excess amino acid and triethylamine. The organic layer
was separated, dried over NaSO4, and concentrated to dryness
under reduced pressure to afford the crude products with 0.5
equiv of triethylamine, which were crystallized in pentane to give
a yellow powder in 81-85% yield.
(E)-2-((3,5-Di-tert-butyl-2-hydroxybenzylidene)amino)-4-(methyl-
thio)butanoic acid (1i; L9-H). Following the general procedure
for synthesis of tridentate Schiff base ligands 1i,j, rac-methio-
nine (1.273 g, 8.535 mmol, 2 equiv) and triethylamine (0.863 g,
8.535 mmol, 2 equiv) were added to 3,5-di-tert-butyl-2-hydro-
xybenzaldehyde (1.00 g, 4.26 mmol) in MeOH (30 mL). Com-
pound 1i was obtained as a yellow powder in 81% yield.
1H NMR (300 MHz, CDCl3): δ 1.20 (t, J = 7.44 Hz, 4.5H,
CH2CH3), 1.30 (s, 9H, C(CH3)3), 1.43 (s, 9H, C(CH3)3), 2.08 (s,
3H, SCH3), 2.24 (m, 2H, CHCH2CH2), 2.52 (m, 2H, CH2CH2S),
3.05 (q, J = 7.73 Hz, 3H, CH2CH3) 4.08 (m, 1H, NCHCH2) 7.10
(d, J = 2.67 Hz, 1H, C6H2), 7.37 (d, J = 2.38 Hz, 1H, C6H2),
8.48 (s, 1H, CHdN). 13C NMR (300 MHz, CDCl3): δ 8.41
(CH2CH3), 15.19 (SCH3), 29.25 (CH2CH2S), 29.42 (C(pCH3)3),
31.30 (CHCH2CH2), 31.49 (C(oCH3)3), 34.10 (pCCH3), 35.01
(oCCH3), 44.99 (CH2CH3), 71.76 (NCHCH2), 117.90, 126.24,
127.02, 136.52, 139.84, 158.22 (Ar), 167.41 (CdN), 175.78
(CdO). Anal. Calcd for C46H77N3O6S2: C, 66.39; H, 9.33;
N, 5.05; S, 7.71. Found: C, 66.14; H, 9.28; N, 5.07; S, 8.00.
HRMS (ESI): m/z 366.2241 [M þ Hþ], calcd for C46H77N3O6S2,
365.20.
Synthesis of [L5AlEthyl] (2e). A solution of triethylaluminum
(0.463 g, 4.06 mmol, 1.0 equiv) in pentane (2 mL) was cannu-
lated to a suspended solution of the tridentate Schiff base ligand
1e (1.12 g, 4.06 mmol, 1 equiv) in pentane (2 mL). Complex 2e
was obtained in 62% yield. 1H NMR (300 MHz, CDCl3): δ 0.37
(bm, 2H, CH2CH3), 0.79, 0.90 (2 sets of t, 3H, CH2CH3), 1.29,
1.31 (2 set of s, 9H, C(CH3)3), 1.45, 1.48 (2 set of s, 9H, C(CH3)3),
3.52, 3.83 (m, 2H, OCH2CH2), 4.10, 4.34 (m, 2H, NCHCH2),
7.01, 7.04 (2 set of d, J = 2.67 and 2.67 Hz, 1H, C6H2), 7.45, 7.48
(d, J = 2.67 and 2.38 Hz, 1H, C6H2), 8.27, 8.22 (2 set of s, 1H,
CHdN). 13C NMR (300 MHz, CDCl3): δ 10.10, 14.07, 22.35,
29.25, 31.43, 33.98, 35.39, 55.40, 55.80,59.31, 60.23, 118.66,
119.19, 126.91, 127.07, 129.74, 137.67, 137.93, 139.90, 139.97,
161.66, 161.93, 166.25, 166.55. Anal. Calcd for C19H30AlNO2:
C, 68.85; H, 9.12; N, 4.23. Found: C, 68.63; H, 9.40; N, 4.20.
Synthesis of [L6AlEthyl] (2f). A solution of triethylaluminum
(0.156 g, 1.37 mmol, 1.0 equiv) in pentane (2 mL) was cannu-
lated to a suspended solution of the tridentate Schiff base ligand
1f (0.400 g, 1.37 mmol, 1 equiv) in pentane (2 mL). Complex 2f
1
was obtained in 20% yield. H NMR (300 MHz, CDCl3): δ
-0.32, -0.14 (2 sets of m, 2H, CH2CH3), 0.83, 0.92 (2 set of t,
3H, CH2CH3), 1.28, 1.38 (2 set of s, 9H, C(CH3)3), 1.31, 1.48
(2 set of s, 9H, C(CH3)3), 1.83, 1.93, 2.21, 3.50, 3.86, 4.11, 4.39
(m, 6H, CH2CH2CH2), 6.95, 7.00 (2 set of d, J = 2.38 and 2.67
Hz, 1H, C6H2), 7.40, 7.47 (d, J = 2.68 and 2.68 Hz, 1H, C6H2),
8.04, 8.09 (2 set of s, 1H, CHdN). 13C NMR (300 MHz, CDCl3):
δ 10.22, 10.34, 29.44, 30.36, 31.40, 31.82, 33.89, 35.02, 35.16,
57.45, 60.77, 61.30, 63.52, 118.22, 118.66, 127.37, 127.64, 129.57,
129.69, 137.54, 137.80, 139.24, 139.51, 161.57, 161.68, 167.61,
167.92. Anal. Calcd for C20H32AlNO2: C, 69.54; H, 9.34; N,
4.05. Found: C, 68.81; H, 9.54; N, 4.01.
(E)-2-((3,5-Di-tert-butyl-2-hydroxybenzylidene)amino)-3-phe-
nylpropanoic acid (1j; L10-H). Following the general procedure
for synthesis of tridentate Schiff base ligands 1i,j, rac-phenyl-
alanine (1.409 g, 8.532 mmol, 2 equiv) and triethylamine (0.863 g,
8.532 mmol, 2 equiv) were added to 3,5-di-tert-butyl-2-hydro-
xybenzaldehyde (1.00 g, 4.26 mmol) in MeOH (30 mL). Com-
pound 1j was obtained as a yellow powder in 85% yield.
1H NMR (300 MHz, CDCl3): δ 1.18 (t, J = 7.23 Hz, 4.5H,
CH2CH3), 1.25 (s, 9H, C(CH3)3), 1.43 (s, 9H, C(CH3)3), 3.00 (q,
J = 7.37 Hz, 3H, CH2CH3) 3.14 (m, 1H, CHCH2Ph), 3.37 (m,
1H, CHCH2Ph), 4.10 (m, 1H, NCHCH2), 6.93 (d, J = 2.46 Hz,
1H, C6H2), 7.10-7.21 (m, 5H, C6H5), 7.33 (d, J = 2.46 Hz, 1H,
C6H2), 8.06 (s, 1H, CHdN). 13C NMR (300 MHz, CDCl3): δ
8.37 (CH2CH3), 29.42 (C(pCH3)3), 31.46 (C(oCH3)3), 34.05
(pCCH3), 35.00 (oCCH3), 40.24 (CHCH2Ph), 45.10 (CH2-
CH3), 74.61 (NCHCH2), 117.87, 126.17, 126.82, 128.23, 129.66,
136.39, 138.33, 139.61, 158.20 (Ar), 166.95 (CdN), 175.23
(CdO). Anal. Calcd for C54H77N3O6: C, 75.05; H, 8.98; N,
4.68. Found: C, 75.28; H, 9.14; N, 4.90. HRMS (ESI): m/z
382.2631 [M þ Hþ], calcd for C54H77N3O6, 381.23.
Synthesis of [L7AlEthyl] (2g). A solution of triethylaluminum
(0.196 g, 1.71 mmol, 1.0 equiv) in pentane (2 mL) was canulated
to a suspended solution of the tridentate Schiff base ligand 1g
(0.525 g, 1.71 mmol, 1 equiv) in pentane (2 mL). Complex 2g was
obtained in 21% yield. 1H NMR (300 MHz, CDCl3): δ -0.34,
-0.15 (m, 2H, CH2CH3), 0.82, 0.88 (2 sets of m, 3H, CH2CH3),
1.31 (s, 9H, C(CH3)3), 1.50 (s, 9H, C(CH3)3), 1.72 (m, 2H,
CH2CH2CH2), 2.17 (m, 2H, CH2CH2CH2), 3.44 (m, 2H,
OCH2CH2), 4.05 (m, 2H, NCHCH2), 6.99 (d, J = 2.68 Hz,
1H, C6H2), 7.45 (d, J = 2.68 Hz, 1H, C6H2), 8.16 (s, 1H,
CHdN), 13.90 (s, 1H, OH). 13C NMR (300 MHz, CDCl3): δ
10.08, 10.52, 14.07, 22.34, 27.73, 28.68, 29.50, 31.38, 33.85,
33.94, 34.73, 35.44, 58.06, 58.35, 61.56, 62.18. Anal. Calcd for
C21H34AlNO2: C, 70.16; H, 9.53; N, 3.90. Found: C, 69.12; H,
9.91; N, 3.57.
Preparation of Aluminum Complexes 2a-j. A solution of
triethylaluminum in toluene (0.10 M, 0.25 mL, 1 equiv) was
added to a solution of the corresponding ligands 1a-j in
0.75 mL of toluene in a sealed tube, and the reaction mixture
was stirred for 3 h at room temperature.
Acknowledgment. We gratefully acknowledge financial
support from the National Science Foundation (No. CHE
05-43133) and the Robert A. Welch Foundation (No.
A-0923). We also thank PURAC America, Inc., for their
generous gift of lactides.
Polymerization Procedure. In a typical experiment carried out
in the argon-filled glovebox, a Teflon-screw-capped heavy-
walled pressure vessel containing the corresponding aluminum
complex 2a-j and 50 equiv of rac-lactide (per aluminum center)
in 1.00 mL of toluene was stirred at 70 °C for the designated time
period. Upon removal of a small sample of the crude product via
Supporting Information Available: A table and CIF files
giving crystallographic data for complexes 2f (cis) and 2f
(trans). This material is available free of charge via the Internet
at http://pubs.acs.org.
1
syringe, it was analyzed by H NMR spectroscopy in CDCl3.
The product was isolated and purified by precipitation from
dichloromethane by the addition of 5% hydrochloric acid in