Synthesis, structure, and catalytic activity of a new zinc complex derived from a chiral hydroxyazetidine ligand

Article abstract of DOI:10.1016/j.inoche.2012.03.015

A new chiral tetrameric zinc complex [(1)ZnEt]₄ (2) has been readily prepared via alkane elimination between ZnEt₂ and a chiral azetidine derivative, (2R,3R)-1-[(1S)-1-(4-methoxyphenyl)ethyl]-2-phenyl-3- hydroxyazetidine (1H). Complex 2 has been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analysis. Complex 2 has a cubane-like Zn₄O₄ core structure, and is an active catalyst for the polymerization of rac-lactide, leading to the heterotactic-rich polylactides.

Full text of DOI:10.1016/j.inoche.2012.03.015

Inorganic Chemistry Communications 20 (2012) 234–237
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, structure, and catalytic activity of a new zinc complex derived from a chiral
hydroxyazetidine ligand
Zhanbin Zhang a, Ning Zhao a, Wenshan Ren a, Liang Chen a, Haibin Song b, Guofu Zi a,
⁎
a
Department of Chemistry, Beijing Normal University, Beijing 100875, China
b
State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new chiral tetrameric zinc complex [(1)ZnEt]4 (2) has been readily prepared via alkane elimination between
ZnEt2 and a chiral azetidine derivative, (2R,3R)-1-[(1S)-1-(4-methoxyphenyl)ethyl]-2-phenyl-3-hydroxyazetidine
(1H). Complex 2 has been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffrac-
tion analysis. Complex 2 has a cubane-like Zn4O4 core structure, and is an active catalyst for the polymerization of
rac-lactide, leading to the heterotactic-rich polylactides.
Received 14 February 2012
Accepted 8 March 2012
Available online 16 March 2012
Keywords:
Chiral azetidine ligand
Zinc complex
© 2012 Elsevier B.V. All rights reserved.
Synthesis
Structure
Polymerization of lactide
Chiral zinc complexes have been widely studied for the past decades.
One of the driving forces for this work is the growing interest in the de-
velopment of catalysts for lactide polymerization [1–9], because the poly-
lactide finds increasing use as a commodity polymer due to its
biodegradability and the possibility of deriving the monomer from re-
newable resources [10,11]. The physical and mechanical properties of
polylactides, as well as the rate of degradation, are intimately related to
the chain stereochemistry [12,13]. For example, a stereocomplex polymer
formed by an equivalent mixture of poly(L-lactide) and poly(D-lactide)
has many advantages such as higher melting temperature (230 °C) com-
paring with the enantiopure polylactide (180 °C) [14,15]. Thus, the
directing polymerization of rac-lactide via stereoselective catalysts offers
a significative challenge and opportunity for chemists. To date, the chiral
zinc catalysts have been shown to be promising for this transformation,
however, even within this class, successful catalysts affording signifi-
cant stereoselectivity are rare [1–9,16–19]. Thus, the preparation of
polylactide remains an open area of research. The development of
new zinc catalysts for polymerization of rac-lactide is still a desirable
and challenging goal. Ligand modification plays a key role in the devel-
oping new catalyst precursors for optimizing polymerization activity as
well as polymer properties such as stereoregularity, molecular weight,
bulky and polar comonomer incorporation, and microstructure. In recent
years, we have developed a series of chiral ligands, and their Ta(IV),
Ti(IV), Zr(IV), Zn(II) and lanthanide complexes are useful catalysts for a
range of transformations [20–31]. For example, the ligand (2R,3R)-1-
[(1S)-1-(4-methoxyphenyl)ethyl]-2-phenyl-3-hydroxyazetidine (1H) is
a useful auxiliary for the asymmetric diethylzinc addition to aldehydes,
in some cases, the enantioselectivity is as high as 97% ee [31]. Encouraged
by the attractive feature of this ligand, we have recently started exploring
the 1H in polymerization of rac-lactidine. Herein, we report on some ob-
servations concerning the chemistry of ligand 1H with ZnEt2, and the use
of the resulting complex as catalyst in the polymerization of rac-lactidine.
The chiral ligand, (2R,3R)-1-[(1S)-1-(4-methoxyphenyl)ethyl]-
2-phenyl-3-hydroxyazetidine (1H), can be easily prepared by a
three-step procedure (Scheme 1) [31]. Condensation of (S)-1-(4-
methoxyphenyl)ethylamine with the benzaldehyde in dichloro-
methane in the presence of anhydrous MgSO4 affords the corresponding
aldimine I. Subsequently, performing the Staudinger reaction of imine I
with acetoxyketene, derived from acetoxyacetyl chloride and Et3N, gives
a crude mixture of cis-β-lactams IIa and IIb, which can be separated by re-
crystallization from an ethyl acetate solution. Reduction of cis-β-lactams
IIa by AlH2Cl, prepared from LiAlH4 and AlCl3 in ratio of 1:1 in dry THF,
gives the 3-hydroxyazetidine 1H after purification by recrystallization
from an n-hexane and ethyl acetate mixture (2:3). The configuration of
phenyl and hydroxyl groups in 1H is assigned as cis by its crystal structure
(Fig. 1). The hydroxyl group and electron lone pair on N atom are situated
on the same side of the azetidine backbone, indicating that a metal ion
would easily chelated/coordinate to both of them.
Organozinc complexes can be easily prepared by alkane elimina-
tion of dialkylzinc with protic reagents. For example, treatment of
1H with 1 equiv of ZnEt2 gives a desired chiral tetrameric zinc com-
plex [(1)ZnEt]4 (2) in 68% yield (Scheme 1) [32]. Complex 2 is stable
in dry nitrogen atmosphere, while it is very sensitive to moisture. It is
soluble in organic solvents such as THF, DME, pyridine, toluene, and
benzene. Complex 2 has been characterized by various spectroscopic
techniques, elemental analysis, and X-ray diffraction analysis [33].
⁎
Corresponding author. Tel.: +86 10 5880 6051; fax: +86 10 5880 2075.
E-mail address: gzi@bnu.edu.cn (G. Zi).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.03.015

Z. Zhang et al. / Inorganic Chemistry Communications 20 (2012) 234–237
235
Zn1
01
05
Zn3
03
Zn4
Zn2
07
Fig. 2. Molecular structure of 2 (thermal ellipsoids drawn at the 35% probability level).
Selected bond lengths (Å) and bond angles (deg): Zn–C (av.) 1.974(4), Zn–O (av.)
2.099(3), Zn∙∙∙Zn (av.) 3.099(1), Zn–O–Zn (av.) 96.7(1), O–Zn–O (av.) 82.8(1).
which is topologically related to the structural fragments observed
in NaCl-like metal oxide ZnO [34]. Each zinc atom possesses highly dis-
torted tetrahedral coordination and lies at the corner of a tetrahedron. The
atom arrangement in 2 is very close to the structures observed for related
tetrameric alkylzinc alkoxides, such as [MeZnO(CMe3)]4 [35], and [Me3-
SiCH2Zn(O-1-Ad)]4 [36]. The entire molecule shows S4 quasi-symmetry
with a symmetry axis passing through the centers of two parallel cube
faces, Zn(1)-O(3)-Zn(2)-O(1) and Zn(3)-O(5)-Zn(4)-O(7). The distortion
of the Zn4O4 heterocubane (the average Zn–O–Zn and O–Zn–O angles are
96.7(1)° and 82.8(1)°, respectively) can be described in terms of outward
movements of zinc atoms along the S4 axis, indicating that the atomic or-
bitals of zinc used in forming bonds to oxygen are mostly p character. The
Zn∙∙∙Zn distances are in the range of 3.061(1) to 3.146(1) Å, with an aver-
age value of 3.099(1) Å. The Zn–O distances range from 1.992(3) to
2.276(2) Å, with an average value of 2.099(3) Å. The Zn–C distances are
equal within the experimental error and have an average value of
1.974(4) Å. These crystal data are comparable to those found in tetrameric
alkylzinc alkoxides [35,36].
The polymerization data show that 2 can initiate the ring opening
polymerization (ROP) of racemic-lactide [37] under mild conditions
(Table 1). For example, it allows the complete conversion of 250 equiv
Table 1
a
Polymerization of rac-lactide catalyzed by complex 2
.
Scheme 1. Synthesis of complex 2.
The solid-state structure of 2 shows that each substituted azeti-
dine group is far away from the metal center (Fig. 2). The main struc-
tural feature of 2 is the distorted Zn4O4 heterocubane central core
b
b
c
Entry
T (°C)
Solvent
Conv. (%)
Mn (kg/mol)
Mw/Mn
Pr (%)
1
2
3
4
5
6
25
70
25
10
10
40
40
40
40
THF
100
100
62
85
28
100
92
93
35.8
35.6
24.2
29.7
11.5
35.4
33.7
66.7
39.5
1.15
1.18
1.58
1.22
1.96
1.26
1.48
1.32
1.72
66
68
58
71
54
65
56
67
59
Toluene
CH2Cl2
THF
CH2Cl2
THF
CH2Cl2
THF
CH2Cl2
C11
C10
7
8d
9d
C3
N1
56
02
a
Conditions: [Zn]/LA (mol/mol)=1/250; polymerization time, 12 h; solvent, 5 mL;
[LA]=0.5 mol/L.
b
Measured by GPC (using polystyrene standards in THF).
Pr is the probability of racemic linkages between monomer units and is determined
C2
01
c
from the methine region of the homonuclear decoupled 1H NMR spectrum in CDCl3 at
25 °C [2,3].
d
Conditions: [Zn]/LA (mol/mol)=1/500; polymerization time, 12 h; solvent, 5 mL;
[LA]=1.0 mol/L.
Fig. 1. Molecular structure of 1 (thermal ellipsoids drawn at the 35% probability level).

236
Z. Zhang et al. / Inorganic Chemistry Communications 20 (2012) 234–237
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of lactide within 12 h at room temperature in THF at [rac-LA]=0.5 -
mol L−1, and the molecular weight distribution is very narrow (1.15)
over the entire monomer-to-initiator ratio range (Table 1, entry 1), in
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.
Our results show that the catalytic activity of 2 resemble that of zinc
complex [(S)-PhCH(Me)N(2-CH2-4,6-tBu2C6H2O)2]Zn2Et2 [2], while
the microstructure of the resulting polylactides is similar to those initi-
ated by [{(S)-PhCH(Me)(N=CMe)}2CH]Zn(OPri) [3].
In conclusion, a new chiral tetrameric zinc complex [(1)ZnEt]4 (2) has
been readily prepared via alkane elimination reaction between ZnEt2 and
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a
chiral azetidine derivative, (2R,3R)-1-[(1S)-1-(4-methoxyphenyl)
ethyl]-2-phenyl-3-hydroxyazetidine (1H). Complex 2 has a cubane-like
Zn4O4 core structure, and is an active catalyst for the polymerization of
rac-lactide, leading to the heterotactic-rich polylactides.
Acknowledgements
This work was supported by the National Natural Science Foundation
of China (Grant Nos. 20972018, 21074013 and 21172022), the Program
for New Century Excellent Talents in University (NCET-10-0253), and
the Fundamental Research Funds for the Central Universities.
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Appendix A. Supplementary data
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CCDC 865690 and 865691 contain the supplementary crystallo-
graphic data for 1H and 2. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.
cam.ac.uk. Supplementary data with this article can be found, in the
online version, at doi: 10.1016/j.inoche.2012.03.015.
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with chiral biaryldiamine-based ligands, Dalton Trans. 40 (2011) 1547–1566.
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1
[37] General procedure for polymerization of rac-lactide. In a glovebox, a Schlenk flask
was charged with a solution of 2 (typically 0.025 mmol) in CH2Cl2 (0.2 mL),
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