Chiral N, N, O-scorpionate zinc alkyls as effective and stereoselective initiators for the living ROP of lactides

Article abstract of DOI:10.1021/om300146n

The chiral and the enantiopure bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of the alcohol compounds (bpzbeH) [bpzbe = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide], (bpzteH) [bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-para-tolylethoxide], and (R,R)-bpzmmH {(R,R)-bpzmm = (1R)-1-[(1R)-6,6-dimethylbicyclo[3.1.1]-2-hepten-2-yl]-2,2-bis(3, 5-dimethylpyrazol-1-yl)ethoxide} have been utilized to obtain new NNO-scorpionate zinc alkyl complexes. The reaction of bpzbeH, bpzteH (racemic mixture), or (R,R)-bpzmmH (enantiopure) with [ZnMe₂] in a 1:1 molar ratio in toluene afforded the mononuclear and monoalkyl zinc complexes [Zn(Me)(κ³-NNO)] (1-3), respectively. However, when the same reaction was carried out with [ZnEt₂], [Zn^tBu ₂], or [Zn(CH₂SiMe₃)₂], new dinuclear complexes of the type [Zn(R)(κ-NNμ-O)]₂ (κ-NNμ-O = bpzbe, R = Et 4, ^tBu 5, CH₂SiMe ₃6; bpzte, R = Et 7, ^tBu 8, CH₂SiMe ₃9; (R,R)-bpzmm, R = Et 10, ^tBu 11, CH ₂SiMe₃12) were obtained. The single-crystal X-ray structure of derivative 1 confirms a monomeric 4-coordinative structure in which the metal center is in a distorted tetrahedral geometry with the heteroscorpionate ligands in a κ³ coordination mode, whereas 4?2C₇H₈, 6, and 7 reveal an asymmetric κ-NNμ-O arrangement in a dimeric molecular disposition. Interestingly, alkyl-containing zinc complexes 1, 4, 5, 6, 9, and 12 can act as single-component initiators for the ring-opening polymerization of lactides at 50 °C. Thus, lactides were polymerized to afford PLA materials with low molecular weights in a few hours. The polymerizations are living, as evidenced by the narrow polydispersities (M_w/M_n = 1.03) of the isolated polymers in conjunction with the linear nature of the number-average molecular weight versus conversion plot. Inspection of the kinetic parameters for l-LA showed that propagations present the usual pseudo-first-order dependence on monomer and catalyst concentration. ¹H NMR and MALDI-TOF mass spectra confirmed that the initiation occurs through nucleophilic attack of alkyl on the lactide monomer. Microstructural analysis of poly(rac-lactide) by ¹H NMR spectroscopy revealed that the myrtenal substituent on the alkoxide fragment has a moderate influence on the degree of stereoselectivity, producing at mild temperatures enriched-heterotactic PLAs with a P_r value of up to 0.77.

Full text of DOI:10.1021/om300146n

Article
pubs.acs.org/Organometallics
Chiral N,N,O-Scorpionate Zinc Alkyls as Effective and Stereoselective
Initiators for the Living ROP of Lactides
Antonio Otero,*,† Juan Fernandez-Baeza,*,† Luis F. Sanchez-Barba,*,‡ Juan Tejeda,† Manuel Honrado,†
́
́
Andres Garces,‡ Agustín Lara-Sanchez,† and Ana M. Rodríguez†
́
́
́
†Departamento de Química Inorgan
13071-Ciudad Real, Spain
‡Departamento de Química Inorgan
́
ica, Organ
́
ica y Bioquímica, Universidad de Castilla-La Mancha, Campus Universitario,
́
ica y Analítica, Universidad Rey Juan Carlos, Mos
́
toles-28933-Madrid, Spain
S
* Supporting Information
ABSTRACT: The chiral and the enantiopure bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of
the alcohol compounds (bpzbeH) [bpzbe = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide], (bpzteH) [bpzte = 2,2-
bis(3,5-dimethylpyrazol-1-yl)-1-para-tolylethoxide], and (R,R)-bpzmmH {(R,R)-bpzmm = (1R)-1-[(1R)-6,6-dimethylbicyclo-
[3.1.1]-2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethoxide} have been utilized to obtain new NNO-scorpionate zinc alkyl
complexes. The reaction of bpzbeH, bpzteH (racemic mixture), or (R,R)-bpzmmH (enantiopure) with [ZnMe2] in a 1:1 molar
ratio in toluene afforded the mononuclear and monoalkyl zinc complexes [Zn(Me)(κ3-NNO)] (1−3), respectively. However,
when the same reaction was carried out with [ZnEt2], [ZntBu2], or [Zn(CH2SiMe3)2], new dinuclear complexes of the type
[Zn(R)(κ-NNμ-O)]2 (κ-NNμ-O = bpzbe, R = Et 4, tBu 5, CH2SiMe3 6; bpzte, R = Et 7, tBu 8, CH2SiMe3 9; (R,R)-bpzmm, R =
t
Et 10, Bu 11, CH2SiMe3 12) were obtained. The single-crystal X-ray structure of derivative 1 confirms a monomeric 4-
coordinative structure in which the metal center is in a distorted tetrahedral geometry with the heteroscorpionate ligands in a κ3
coordination mode, whereas 4·2C7H8, 6, and 7 reveal an asymmetric κ-NNμ-O arrangement in a dimeric molecular disposition.
Interestingly, alkyl-containing zinc complexes 1, 4, 5, 6, 9, and 12 can act as single-component initiators for the ring-opening
polymerization of lactides at 50 °C. Thus, lactides were polymerized to afford PLA materials with low molecular weights in a few
hours. The polymerizations are living, as evidenced by the narrow polydispersities (Mw/Mn = 1.03) of the isolated polymers in
conjunction with the linear nature of the number-average molecular weight versus conversion plot. Inspection of the kinetic
parameters for L-LA showed that propagations present the usual pseudo-first-order dependence on monomer and catalyst
concentration. 1H NMR and MALDI-TOF mass spectra confirmed that the initiation occurs through nucleophilic attack of alkyl
1
on the lactide monomer. Microstructural analysis of poly(rac-lactide) by H NMR spectroscopy revealed that the myrtenal
substituent on the alkoxide fragment has a moderate influence on the degree of stereoselectivity, producing at mild temperatures
enriched-heterotactic PLAs with a Pr value of up to 0.77.
of organometallics to aldehydes to afford chiral secondary
alcohols.4 When this type of reaction was carried out with an
enantiopure aldehyde [(1R)-(−)-myrtenal] as a possible chiral
substrate to control the stereochemistry of the newly created
asymmetric center,5 we obtained an enantiopure scorpionate
ligand in one step with high stereoselectivity. Furthermore,
INTRODUCTION
■
In the important field of ligand design, we have previously
reported the synthesis of new “heteroscorpionate” ligands
based on bis(pyrazol-1-yl)methane.1 More recently, we have
been interested in the introduction of chirality into these
systems in an effort to obtain new enantiopure scorpionate
ligands.2 For example, we have reported3 the preparation of
new chiral bis(pyrazol-1-yl)methane-based NNO-donor scorpi-
onate ligands in a single high-yielding step by the 1,2-addition
Received: February 22, 2012
Published: May 24, 2012
© 2012 American Chemical Society
4191
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
these alcohol compounds were found to be excellent reagents
for the introduction of scorpionate ligands into group 4 metal
complexes, and a series of neutral amide and alkoxide
complexes were prepared by treatment of M(NMe2)4 or
M(OR)4 with these alcohol ligands.3
CH2SiMe3) to give different scorpionate zinc alkyl complexes.
Thus, the reaction of bpzbeH, bpzteH (racemic mixture), or
(R,R)-bpzmmH (enantiopure) with [ZnMe2] in a 1:1 molar
ratio in toluene afforded, after the appropriate workup, the
mononuclear monoalkyl zinc complexes [Zn(Me)(κ3-NNO)]
(1−3). However, when this reaction was carried out with
[ZnEt2], [ZntBu2], or [Zn(CH2SiMe3)2], dinuclear complexes
of the type [Zn(R)(κ-NNμ-O)]2 (κ-NNμ-O = bpzbe, R = Et 4,
In the past few years, our research group has been interested
in the preparation of efficient catalysts for the well-controlled
ring-opening polymerization (ROP) of ε-caprolactone and
lactide, an inexpensive annually renewable natural feedstock.6
The biocompatible nature and the nontoxicity to living tissue of
the bioassimilable polylactides (PLAs)7,8 have attracted our
attention in the employment of biocompatible metals for the
design of these catalysts, such as magnesium9 and zinc,10 since
the resulting PLAs are widely utilized in biomedical/
pharmaceutical applications.11−13 In particular, we have ex-
plored the reactivity of amidinate-based14 and scorpionate/
cyclopentadienyl hybrid15 ligands for the synthesis of well-
defined alkyl magnesium16−18 and alkyl17,19-amide20 zinc
complexes of the type [M(R)(κ3-NNN)] as highly effective
single-component living initiators for the ROP of lactides.
Unfortunately, appreciable levels of heterotacticity in the
polymerizaton of rac-lactide were only reached when the
more sterically hindered heteroscorpionate alkyl magnesium
complexes (Pr = 0.79) were employed,18 while the analogous
less sterically demanded zinc alkyls19 did not promote the
formation of heterotactic bias effectively (Pr = 0.68) and also
showed limited catalytic activity. In this sense, the possibility of
designing more efficient catalyst systems that would be capable
of exerting higher degrees of stereo/enantioselectivity stimu-
lated our interest in the preparation of alternative alkyl zinc
initiators supported by these new chiral NNO-scorpionate
ligands.3 In this context, several highly effective organo-zinc
initiators21 have been described for the efficient ROP of cyclic
esters, highlighting, that is, the alkoxo zinc β-diketinimates
reported by Coates,22 the alkoxo tris(pyrazol-1-yl)borates,
communicated by Chisholm,23 and the alkoxo amidophenols,
published by Hillmyer and Tolman,24 where almost complete
conversions are reached in minutes at 20 °C exerting
remarkable levels of stereoselectivity. However, only a few
examples of organo−zinc scorpionate complexes containing
stereogenic centers have been reported,25 and to the best of our
knowledge, studies on the ROP of any cyclic ester have not
been carried out to date.
t
tBu 5, CH2SiMe3 6; κ-NNμ-O = bpzte, R = Et 7, Bu 8,
t
CH2SiMe3 9; κ-NNμ-O = (R,R)-bpzmm, R = Et 10, Bu 11,
CH2SiMe3 12) were obtained (see Scheme 1). Complexes 1−
Scheme 1. Synthesis of NNO-Scorpionate Alkyl Zinc
Complexes (1−12)
12 were isolated as white or colorless crystalline solids in good
yields. The mononuclear [Zn(Me)(R,R-bpzmm)] (3) and
dinuclear [Zn(R)(R,R-bpzmm)]2 (10−12) are the first
enantiopure scorpionate zinc complexes where the chirality
stems from the substituent bound to the carbon in the α
position (Ca) to the methyne bridge.
The different complexes were characterized by spectroscopic
1
methods. The H and 13C{1H} NMR spectra of 1−12 exhibit
two distinct sets of pyrazole resonances, indicating the existence
1
of two types of pyrazole rings. The H NMR spectra of these
complexes show two singlets for the H4, Me3, and Me5 pyrazole
protons, one broad singlet for each of the methine groups (CHb
bridge of pyrazole rings and CHa), and the signals
corresponding to the “R′” moieties of the scorpionate ligands
as well as the alkyl ligands. The results are consistent with a
tetrahedral environment for the zinc atoms in which the two
pyrazole rings are located in cis and trans positions with respect
to the tert-butyl group, para-tolyl group, or bicyclic moiety
(Myr) (see Scheme 1). The 1H NOESY-1D experiments
permitted the unequivocal assignment of all 1H resonances, and
the assignment of the 13C{1H} NMR signals was made on the
The work described here concerns two areas of our research.
We initially utilized the new chiral NNO-scorpionate ligands,
one of which was enantiopure, to prepare novel organo−zinc
complexes of the types [Zn(R)(NNO)] and [Zn(R)(NNO)]2.
This work included a study of the different synthetic
accessibility and a discussion of the structural arrangements
observed. Second, we assessed these new materials as single-
component living initiators for the ROP of L-/rac-lactide under
well-controlled conditions and also carried out an analysis of
the polymer microstructures.
1
basis of H−13C heteronuclear correlation (g-HSQC) experi-
RESULTS AND DISCUSSION
■
The alcohol-scorpionate compounds, prepared in our research
group, bpzbeH3a [bpzbe = 1,1-bis(3,5-dimethylpyrazol-1-yl)-
3,3-dimethyl-2-butoxide], bpzteH3a [bpzte = 2,2-bis(3,5-
dimethylpyrazol-1-yl)-1-para-tolylethoxide], and (R,R)-
ments. The mononuclear and dinuclear structure of complexes
1−3 and 4−12, respectively, in the solid state, has been
confirmed by X-ray diffraction studies (see below). However,
according to the NMR data, it is not possible to confirm
unequivocally this nuclearity in solution, and therefore, the
possibility that there is an equilibrium between a dimeric and a
monomeric species in solution cannot be ruled out. In addition,
the presence in solution of only one enantiomer for complexes
bpzmmH3 b {(R,R)-bpzmm
= (1R)-1-[(1R)-6,6-
dimethylbicyclo[3.1.1]-2-hepten-2-yl]-2,2-bis(3,5-dimethylpyr-
azol-1-yl)ethoxide}, were reacted at low temperature with
t
diverse dialkyl zinc complexes [ZnR2] (R = Me, Et, Bu, and
4192
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
3 and 10−12 was confirmed by the addition of a chiral shift
reagent as this addition did not modify the 1H NMR spectra of
these compounds. However, in the rest of the complexes, which
are racemic mixtures, the addition of the shift reagentnamely,
(R)-(−)-(9-anthryl)-2,2,2-trifluoroethanolgave rise to the
1
appearance in the H NMR spectra of two signals for each
proton, resulting from the two diastereoisomers of the
corresponding two enantiomers. Furthermore, the specific
rotations of 3 and 10−12 were measured by polarimetry (see
the Experimental Section). Additionally, the crystal structures
of complexes 1, 4, 6, and 7 were determined by X-ray
diffraction studies, and the ORTEP diagrams are shown in
Figures 1−4. These studies confirmed that the presence in
Figure 3. ORTEP view of [Zn(CH2SiMe3)(bpzbe)]2 (6). Hydrogen
atoms have been omitted for clarity. Thermal ellipsoids are drawn at
the 30% probability level.
Figure 1. ORTEP view of the S enantiomer of [Zn(Me)(bpzbe)] (1).
Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are
drawn at the 30% probability level.
Figure 4. ORTEP view of [Zn(Et)(bpzte)]2 (7). Hydrogen atoms
have been omitted for clarity. Thermal ellipsoids are drawn at the 30%
probability level.
solution of the corresponding two enantiomers for these
compounds is maintained in the solid state. The most
representative bond lengths and angles are presented in
Table 1, and the crystallographic details are reported in Table
S1 in the Supporting Information. Complex 1 has a monomeric
structure that consists of a heteroscorpionate ligand bonded to
the zinc atom through the two nitrogen atoms and the oxygen
atom of the alkoxide group in a κ3-NNO-coordination mode. In
addition, the zinc center is coordinated to one alkyl ligand. This
center has a distorted tetrahedral environment due to the κ3-
NNO coordination of the scorpionate ligand, with major
distortions in the O(1)−Zn(1)−N(1) and O(1)−Zn(1)−
C(17) angles, which have values of 89.03(5)° and 135.29(8)°,
respectively. The Zn−N and Zn−C distances [2.125(1),
2.155(1), and 1.963(2) Å] are in good agreement with others
determined for zinc scorpionate complexes, such as [Zn(CH3)-
(pbptamd)], prepared by our research group.19 The Zn−O
Figure 2. ORTEP view of [Zn(Et)(bpzbe)]2 (4·2C7H8). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at the 30%
probability level.
4193
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
d
Table 1. Selected Geometrical Parameters from the X-ray Studies of Compounds 1, 4·2C7H8, 6, and 7
1
4·2C7H8
6
7
bond lengths (Å)
1.958(3) Zn(1)−C(17)
Zn(1)−O(1)
Zn(1)−C(17)
Zn(1)−O(1)
Zn(1)−N(1)
Zn(1)−N(3)
O(1)−C(12)
C(11)−C(12)
1.963(2)
1.933(1)
2.125(1)
2.155(1)
1.381(2)
1.554(2)
Zn(1)−C(17)
Zn(1)−O(1)
Zn(1)−O(1)
1.96(2)
1.985(3)
2.023(3)
2.174(4)
2.9920(9)
1.411(5)
1.536(5)
Zn(1)−C(20)
Zn(1)−O(1)
Zn(1)−O(1)
1.978(2)
2.001(1)
2.110(1)
2.074(2)
3.0136(4)
1.391(2)
1.553(3)
1.975(2)
2.002(2)
2.207(2)
2.9710(7)
1.414(3)
1.536(3)
a
b
c
Zn(1)−O(1)
Zn(1)−N(1)
Zn(1)−N(1)
Zn(1)−N(1)
a
b
c
Zn(1)−Zn(1)
O(1)−C(12)
Zn(1)−Zn(1)
O(1)−C(12)
C(11)−C(12)
Zn(1)−Zn(1)
O(1)−C(12)
C(11)−C(12)
a
C(11)−C(12)
bond angles (deg)
a
b
c
O(1)−Zn(1)−C(17)
C(12)−O(1)−Zn(1)
O(1)−Zn(1)−N(1)
O(1)−Zn(1)−N(3)
135.29(8)
120.4(1)
89.03(5)
91.42(6)
O(1)−Zn(1)−O(1)
83.31(7)
96.69(7)
112.0(2)
O(1)−Zn(1)−O(1)
Zn(1)−O(1)−Zn(1)
Si(1)−C(17)−Zn(1)
83.4(1)
96.6(1)
121(1)
O(1)−Zn(1)−O(1)
85.76(5)
94.24(5)
108.5(1)
a
b
c
Zn(1)−O(1)−Zn(1)
C(18)−C(17)−Zn(1)
Zn(1)−O(1)−Zn(1)
C(21)−C(20)−Zn(1)
a
b
Symmetry transformations used to generate equivalent atoms: −x + 2, −y, −z + 1. Symmetry transformations used to generate equivalent atoms:
−x + 2, −y, −z + 1. Symmetry transformations used to generate equivalent atoms: −x + 2, −y + 1, −z + 1. Distances in Å, angles in deg.
c
d
a
Table 2. Polymerization of L-Lactide and rac-Lactide Catalyzed by 1, 4, 5, 6, 9, and 12
b
c
d
d
e
entry
initiator
monomer
temp (°C)
time (h)
conv (%)
Mn(theor) (Da)
Mn (Da)
Mw/Mn
Pr
1
2
1
4
5
6
6
6
9
L-LA
20
20
20
20
50
65
20
50
50
50
50
50
50
50
65
20
50
65
50
20
50
50
65
50
65
14.5
14.5
14.5
14.5
3.5
1.0
14.5
6.0
0.9
1.8
2.7
3.6
4.5
9.0
1.0
14.5
3.5
1.0
3.5
3.5
3.5
3.5
1.0
3.5
1.0
11
21
15
70
68
87
79
70
25
42
63
83
97
96
96
60
60
77
78
1600
3000
2100
3400
1.07
1.09
1.10
1.09
1.06
1.12
1.03
1.21
1.05
1.08
1.10
1.10
1.11
1.14
1.15
1.08
1.09
1.21
1.09
L-LA
3
L-LA
2100
2400
4
L-LA
10 000
9800
9700
5
L-LA
9500
6
L-LA
12 500
11 300
40 200
3500
12 700
11 500
32 000
4100
7
L-LA
f
8
9
L-LA
9
9
9
9
9
9
L-LA
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
L-LA
6000
7400
L-LA
9000
10 300
12 400
14 100
27 900
13500
8800
L-LA
11 900
14 000
28 200
13 800
8700
L-LA
g
9
L-LA
9
12
12
12
6
L-LA
L-LA
L-LA
8700
8900
L-LA
11 100
11 200
11 300
11 500
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
0.67
9
9
85
62
81
73
72
12 200
8900
12 500
9100
1.08
1.09
1.12
1.08
1.09
0.72
0.64
0.69
0.77
0.70
h
9
9
12
12
11 700
10 500
10 300
12 000
10 600
10 200
a
b
Polymerization conditions: 90 μmol of initiator, [L-lactide]0/[Zn]0 = 100 and 10 mL of tetrahydrofuran as solvent. Percentage conversion of the
c
monomer [(weight of polymer recovered/weight of monomer) × 100]. Theoretical Mn = (monomer/initiator) × (% conversion) × (Mw of lactide).
d
Determined by size exclusion chromatography relative to polystyrene standards in tetrahydrofuran. Experimental Mn was calculated considering
Mark−Houwink’s corrections41 for Mn [Mn(obsd) = 0.56 × Mn(GPC)]. Pr is the probability of racemic linkages between monomer units and is
e
determined from the relative intensity in the tretrads obtained in the decoupled 1H NMR by Pr = 2I1/(I1 + I2), with I1 = δ 5.20−5.25 ppm (sis, sii/iis)
and I2 = δ 5.13−5.20 ppm (iis/sii, iii, isi).42 [L-Lactide]0/[9]0 = 400 and 40 mL of tetrahydrofuran as solvent. Double-feed experiment of 100 equiv
f
g
h
of L-LA each one. 30 mL of toluene as solvent.
distance [1.933(1) Å] is similar to that in another NNO-
alkoxide scorpionate zinc complex.26
of each Zn atom can be described as a distorted tetrahedron
with a “heteroscorpionate” ligand that acts in a bidentate
fashion (one coordinated pyrazole ring and an oxygen atom
from the alkoxide fragment bridging the two zinc atoms). The
dimeric aggregate is based on Zn2O2 four-membered rings,
which have previously been observed in other zinc compounds
that contain, for example, thiolate-oxo,27 alkoxide-imino,28a
aryloxide,28b or aminoalcoholate29 ligands. However, it should
be noted that compounds of zinc with this type of metallacycle
Furthermore, the X-ray structure analyses of 4·2C7H8
(obtained from a 3:1 toluene/n-hexane solution), 6, and 7
reveal that these centrosymmetric molecules have a rhomboidal
(ZnO)2 core with clearly different Zn(1)−O(1) and Zn(1A)−
O(1A) bond lengths, ranging from 1.975(2) to 2.110(1) Å,
with the Zn···Zn diagonal (nearly 3 Å) much longer than the
O···O diagonal (nearly 2.7 Å). The coordination environment
4194
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
containing a scorpionate ligand have not been yet reported in
the literature. Additionally, the Zn−O bond distances in these
complexes are much longer than those in complex 1, where the
oxygen atom is coordinated to only one zinc atom [1.933(1) Å
for 1, 1.975(2) Å for 4·2C7H8, 1.985(3) Å for 6, and 2.001(1)
Å for 7]. The Zn(1)−N(1) bond distance is significantly
shorter for 7 [2.207(2) Å for 4·2C7H8, 2.174(4) Å for 6, and
2.074(2) Å for 7], probably as a result of the different steric
hindrance and electronic effects of the R and R′ substituents in
the molecules.
(SEC) data for the resulting polyesters show a monomodal
weight distribution, with polydispersities ranging from 1.03 to
1.21. The zinc alkyl derivatives 1, 4, 5, 6, 9, and 12 polymerized
L-LA at room temperature, and for example, 6 and 9 gave rise
to 70% and 79% conversion, respectively, of 100 equiv after
14.5 h (entries 4 and 7), whereas in the case of 12, 60% of the
monomer was converted for the same time (entry 16). In
contrast, catalysts 1, 4, and 5 show markedly decreased
productivity values (entries 1, 2, and 3). The polymerizations
were well-controlled and produced low-molecular-weight
materials with narrow molecular weight distributions, a finding
that suggests living behavior (Mn = 11 500, Mw/Mn = 1.03,
entry 7). Not unexpectedly, heating the reaction mixture up to
50 °C led to an increase in the activity for all catalysts (entries 5
and 6), and a slight increase in the molecular weight
distribution (entry 6, Mw/Mn = 1.12). Inspection of the
experimental Mn values reveals, as a common trend, that, at this
particular initial catalyst concentration ([L-LA]0/[Zn]0 = 100),
the molecular weights of the resulting polymer samples closely
approximate the expected theoretical calculated values.
However, at lower concentrations of catalyst, the molecular
weight drops off significantly. For example, a number-average
molecular weight slightly greater than 32 000 Da, which is only
80% of the calculated value, was obtained when [L-LA]0/[9]0 =
400 (entry 8). This observation is consistent with the presence
of monomer impurities acting as chain-transfer agents.
It is well known that Grignard reagents exist in solution as a
complex mixture of species due to facile ligand redistribution
reactions, for example, the Schlenk equilibrium.30 This behavior
has previously been observed by Parkin31 in similar alkyl
homoscorpionate magnesium complexes and, more recently, in
magnesium heteroscorpionates with low steric hindrance
reported by our group.16 As a result, we considered it of
interest to investigate the possibility of similar ligand-exchange
processes for the heteroscorpionate alkyl zinc complexes
synthesized here [Zn(R)(NNO)] (1−12) in order to prepare
the corresponding potential 6-coordinate sandwich species of
the type [Zn(NNO)2]. The formation of zinc sandwich
complexes from heteroscorpionates based on a carboxylate
system has also been described by Burzlaff,32 and a species of
stoichiometry [ZnL2] in a tetrahedral arrangement based on
sulfur donor atoms has been reported by Carrano.33 It is worth
noting that solutions of derivatives 1−12 are stable for several
days even in refluxing toluene, and a ligand redistribution
process to give the 6-coordinate sandwich complexes [Zn-
(NNO)2] was not observed in any case. Furthermore, when the
reaction was carried out with 2 equiv of ligand versus Zn, even
under reflux in toluene, formation of the sandwich species was
not detected. Additionally, the reaction of complexes 1−12
with aliphatic alcohols (ROH) or primary amines (RNH2)
proved fruitless, and the starting material was recovered in both
cases.
Polymerization Studies. Complexes 1, 4, 5, 6, 9, and 12
were assessed in the ring-opening polymerization of the polar
monomers L-/rac-lactide in tetrahydrofuran under a nitrogen
atmosphere. These studies were carried out in order to probe
the potential of these compounds as initiators for these cyclic
esters and to determine kinetic parameters, such as the reaction
rate and the order dependence on monomer and catalyst
concentration. These reactivity studies also allowed a
comparison of these alkoxo-based heteroscorpionate Zn alkyls
with the recently published analogous heteroscorpionate
amidinate-based zinc−alkyl19−amide20 systems as initiating
groups and also with other interesting and efficient systems
reported to date.22−24
In accordance with the aforementioned data, the difference in
activity between these alkyl derivatives, which, under the
present polymerization conditions, decreases in the order
t
CH2SiMe3 ≫ Et > Bu > Me, is presumably a consequence of
the M−C bond strength; this trend is consistent with the
increase in the strength of the Zn−C bond.34 This behavior has
also been observed in analogous amidinate- and hybrid
scorpionate/cyclopentadienyl-based magnesium16−18 and
zinc17,19 alkyl derivatives. Additionally, the nature of the
substituent R′ in the asymmetric center from the NNO-donor
heteroscorpionate ligand also affects the catalytic activity, which
decreases in the order p-MeC6H4 > tBu > Myr, possibly due to
electronic, rather than steric, effects (entries 7, 4, and 16).
The polymerization occurred without observable epimeriza-
tion reactions at the stereogenic centers of the poly(L-lactide),
as evidenced by the homonuclear decoupled 1H NMR
spectrum, which only shows a single resonance at δ 5.16
ppm in the methine region. This reaction afforded highly
crystalline, isotactic polymers (Tm = 168−176 °C).35,36 The
high level of control afforded by this initiator in the
polymerization of L-lactide is further exemplified by the narrow
molecular weight distributions (Table 2, entries 9−13) in
conjunction with the linear correlations between Mn and
percentage conversion (R2 = 0.985) (see Figure 5). These
results are characteristic of well-controlled living propagations
and the existence of a single type of reaction site. At this stage,
the livingness of the L-LA polymerization employing these
catalytic systems was further investigated. For this propose, a
double-feed experiment was carried out to demonstrate the
living behavior of catalyst 9 (Table 2, entries 13 and 14), which
resulted in a polymer chain extension,37 with very similar
molecular features, and the molecular weight increased by the
expected value based on monomer conversion.
L-/rac-Lactide Polymerization Initiated by NNO-Het-
eroscorpionate Alkyl Zinc Complexes. Initiators 1, 4, 5, 6,
9, and 12 were systematically examined for the production of
poly(lactides) (PLAs), and they acted as single-component
catalysts for the polymerization of lactides (LAs) in
tetrahydrofuran to give low-molecular-weight polymers without
the need for a cocatalyst or an activator; the results of these
experiments are collected in Table 2. A variety of polymer-
ization conditions were explored, with the ratios [monomer]0/
[Zn]0 always referred per Zn center, for both monomer and
dinuclear species. In all cases, the PLAs produced had
molecular weights in close agreement with calculated values
for one polymer chain per metal center [Mn(calcd)PLA100 = 14
400 g·mol−1] (Table 2). The size exclusion chromatography
Inspection of low-molecular-weight materials by matrix-
assisted laser desorption/ionization time-of-flight mass spec-
trometry (MALDI-TOF MS) revealed the presence of a main
distribution in the spectrum, indicating the existence of a single
4195
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
increments of 72 Da (Figure 6),38 but no cyclic oligomer was
identified.
Initiators 6, 9, and 12 were also tested for the polymerization
of rac-lactide, in tetrahydrofuran at 20, 50, and 65 °C. PLAs
were again produced with molecular weights in good agreement
with calculated values [Mn(calcd)PLA100 = 14 400 g·mol−1]
(Table 2, entries 19−25). Catalysts 6, 9, and 12 proved to be
active at 50 and 65 °C, with conversions close to 80% achieved
after 3.5 and 1 h of reaction, respectively (entries 19, 21, 24, 23,
and 25, respectively), and low-molecular-weight PLA materials
produced with a very narrow polydispersity (Mn = 12 500, Mw/
Mn = 1.08, entry 21). By contrast, at 20 °C, the catalytic activity
was drastically reduced in all cases and only traces of polymers
were recovered under identical polymerization conditions
(entry 20).
Figure 5. Plot of PLA Mn and molecular weight distribution values
(PDI) as a function of monomer conversion (%) for the polymer-
ization of L-LA initiated by 9; [L-LA]0/[9]0 = 100, tetrahydrofuran, 50
°C.
The new heteroscorpionate alkoxo-based alkyl zinc deriva-
tives 6, 9, and 12 proved to be more active for the ROP of
lactides than the analogous heteroscorpionate amidinate-based
zinc alkyl19-amides20 [Zn(R)(κ3-NNN)] and the hybrid
scorpionate/cyclopentadienyl-based alkyl zinc initiators [Zn-
(R)(κ2-π-bpzcp)],17 previously described by our group, or the
recently published robust zinc guanidine complexes of the type
[Zn(guanidine)2OTf]OTf,40 which need more energetic
conditions and longer periods of time to produce lower
conversions of material.
Solution Kinetic Studies on the Ring-Opening
Polymerization of L-Lactide. Kinetic studies were conducted
at 50 and 65 °C to establish the reaction order with respect to
monomer and catalyst concentration as well as the reaction
constant rate at each temperature. The polymerization kinetics
were initially studied for catalysts [Zn(CH2SiMe3)(bpzbe)]2
(6) and [Zn(CH2SiMe3)(bpzte)]2 (9) with [L-LA]0/[Zn]0 =
90, [L-LA]0 = 0.72 M, and [Zn]0 = 8 mM, using
tetrahydrofuran as solvent and without cocatalyst or activator,
according to the standard literature procedures.18 The standard
error associated with the kinetic parameters was calculated by
family of polymer chains capped by −CH(CH3)OH and
(CH3)3CC(O)− termini, corresponding to oligomers of the
formula H(OCHMeCO)2nCMe3·Na+ (n = 8−16) with
consecutive peaks separated by increments of 144 Da (Figure
6).38 (See Figures S1 and S2 in the Supporting Information for
additional MALDI-TOF MS corresponding to higher and
similar [L-LA]0/[Zn]0 ratios, respectively, and alternative alkyl
termini.) Moreover, the end group analysis by 1H NMR of the
poly(L-lactide) oligomer obtained by the reaction of 4 with 20
equiv of L-LA exhibited characteristic resonances that are
consistent with the presence of one −CH2CH3 end group per
−CH2OH hydroxyl chain end (see Figure S3 in the Supporting
Information). These two findings provide evidence that the
ring-opening of L-LA occurs by the initial addition of the alkyl
fragment to the monomer, following a nucleophilic route, with
cleavage of the acyl−oxygen bond,39 followed by further
monomer addition to the (macro)alcohols. In addition, a
certain extension of undesirable intermolecular ester-exchange
(transesterification) reactions could be detected since a low
intensity distribution was observed with peaks separated by
Figure 6. MALDI-TOF mass spectrum of PLA sample obtained with [L-LA]0/[5]0 = 10, 98% conversion; theoretical molecular weights calculated
according to the equation: Mn = (DPn × MwLA) + MwtBuH + MwNa, where DPn is the degree of polymerization, MwLA = 144.13 g·mol−1, MwtBuH
57.07 g·mol−1, and MwNa = 23.09 g·mol−1.
=
4196
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
the standard deviation in slope and intercept for each
regression analysis. The semilogarithmic plots of ln([L-LA]0/
[L-LA]t) versus reaction time for catalysts 6 and 9 at both 50
and 65 °C, employing different initial catalyst concentrations,
are shown in Figure 7a for 6 and Figure 7b for 9. As expected,
Table 3. Rate Constant Dependence on the Concentration
of the Catalysts and Temperature
[Zn]0 × 103
T
(°C)
kapp × 104
(s−1
kp × 102
catalyst
(M)
)
(M−1·s−1
)
n
6
8
12
16
20
8
50
65
50
65
2.8 0.1
3.9 0.1
6.0 0.2
7.2 0.1
7.4 0.2
11.0 0.4
16.3 0.4
18.8 0.5
3.4 0.1
4.5 0.2
7.2 0.1
8.3 0.3
12.8 0.3
18.0 0.7
23.2 0.4
35.2 0.9
2.9 0.3
1.07 0.04
8.4 0.7
3.6 0.4
1.05 0.03
1.05 0.04
1.05 0.06
12
16
20
8
9
12
16
20
8
14
2
12
16
20
and kapp20
= 33.33 × 10−3 s−1 for [Mg(CH2SiMe3)(κ3-
°C
tbptamd)]).
The kinetic dependence on the catalyst concentration (n)
and the propagation rate constant (kp) were also determined18
(see the Supporting Information, Figure S4), confirming that
the reaction is first-order in catalysts 6 and 9 at both
temperatures (Table 3). These values prove that the polymer-
ization of L-LA mediated by [Zn(CH2SiMe3)(bpzbe)]2 (6) and
[Zn(CH2SiMe3)(bpzte)]2 (9) obeys an overall second-order
rate kinetic law of the form
−d[L‐LA]/dt = kp[Zn]1[L‐LA]1
Figure 7. Pseudo-first-order kinetic plots for L-LA polymerizations in
tetrahydrofuran with [L-LA]0 = 0.72 M, employing (a) [Zn-
(CH2SiMe3)(bpzbe)]2 (6) and (b) [Zn(CH2SiMe3)(bpzte)]2 (9) as
catalysts.
(1)
Poly(rac-lactide) Microstructure Analysis. Microstruc-
tural analysis by homonuclear decoupled H NMR spectra of
1
the polymers22−24 revealed that initiators 6 and 9 exert a
moderate level of heterotacticity, while initiator 12, despite its
enantiopure nature, did not impart catalytic site control,44
although it shows moderate preference for a heterotactic dyad
enchainment. This behavior means a relevant improvement in
comparison with the previous results obtained with alkyl zinc19
initiators supported by sterically hindered amidinate-based
heteroscorpionates, such as [Zn(Et)(κ3-tbptamd)], where the
levels of heterotacticity reached were significantly lower, with a
Pr = 0.68 (Figure S5a, Supporting Information). The influence
of the solvent and temperature on the level of stereoselectivity
was also studied.18,45 For instance, initiator 9 produced an
appreciable heterotactic-enriched PLA with a Pr = 0.72 in
tetrahydrofuran at 50 °C (Figure S5b, Supporting Information;
Table 2, entry 21), whereas in toluene at the same temperature,
the Pr value was lower (Pr = 0.64, Table 2, entry 22).18 The
temperature was also investigated, and initiator 12 exhibited a
level of heteroactivity at 65 °C of Pr = 0.70 (Figure S5c,
Supporting Information; Table 2, entry 25), while on cooling
the reaction mixture to 50 °C, the probability value improved
up to Pr = 0.77 (Figure S5d, Supporting Information; Table 2,
entry 24). This step forward obtained for 12, under these
conditions, is most probably the result of the high steric
demand of the myrtenal fragment in the heteroscorpionate
ligand.
in all cases, the linearity of the plots shows that the
propagations were first-order with respect to lactide monomer
(square correlation coefficients ≥ 0.97), and the kapp data are
presented in Table 3.
The pseudo-first-order rate constant, kapp, for initiator 6 at 50
°C (7.2 0.1 × 10−4 s−1, [Zn]0 = 20 mM) was found to be
comparable to the kapp value for 9 (8.3 0.3 × 10−4 s−1; [Zn]0
= 20 mM), as a result of the similar catalytic activity observed at
this temperature for both catalysts. However, a similar trend
was not observed when polymerizations were carried out at 65
°C at this catalyst concentration, with a kapp value of 35.2 0.9
× 10−4 s−1 for 9, that is, much higher (almost double) than that
observed for 6, 18.8 0.5 × 10−4 s−1, possibly as a result of the
t
higher steric effect of the Bu group in the scorpionate ligand
for complex 6, but electronic effects cannot either be discarded.
For comparison, the kapp values for the L-LA polymerization
at 50 °C are significantly higher than that observed for an
enantiopure heteroscorpionate neodymium amide initiator
previously reported by our group (kapp50 °C = 1.55 × 10−5 s−1,
[L-LA]0/[Nd]0 = 200, 20 μmol of initiator),43 but still far from
that recently published for amidinate-based sterically hindered
heteroscorpionate alkyl magnesium18 complexes at 20 °C
(kapp20 °C = 36.97 × 10−3 s−1 for [Mg(CH2SiMe3)(κ3-pbptamd)]
4197
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
G3000H column and an ELSD-LTII light-scattering detector.
The size exclusion chromatography column was eluted with
THF at 45 °C at 1 mL min−1 and was calibrated using eight
monodisperse polystyrene standards in the range of 580−483
000 Da. The microstructures of PLA samples were determined
by examination of the methine region in the homodecoupled
1H NMR spectrum of the polymers recorded at room
temperature in CDCl3 on a Varian Inova FT-500 spectrometer
with concentrations in the range of 1.0−2.0 mg·mL−1. MALDI-
TOF MS spectra were acquired with a Bruker Autoflex II ToF/
ToF spectrometer, using a nitrogen laser source (337 nm, 3 ns)
in linear mode with a positive acceleration voltage of 20 kV.
Samples were prepared as follows: PLA (20 mg) was dissolved
in HPLC quality THF with DCTB (trans-2-[3-(4-tert-
butylphenyl)-2-methyl-2-propenylidene]malononitrile) as the
matrix, and NaI in a 100:5:5 ratio. Before evaporation, 10 μL of
the mixture solution was deposited on the sample plate.
External calibration was performed by using Peptide Calibra-
tion Standard II (covered mass range: 700−3 200 Da) and
Protein Calibration Standard I (covered mass range: 5000−17
500 Da).
CONCLUSIONS
■
In conclusion, we have explored the reactivity of our previously
described both the chiral and the enantiopure bis(pyrazol-1-
yl)methane-based NNO-donor scorpionate ligands in the form
of the alcohol compounds with ZnR2. These reactions yield
monomeric heteroscorpionate zinc alkyl complexes [Zn(Me)-
(NNO)], where the heteroscorpionate ligands act in a κ3-
NNO-coordination mode, and dinuclear complexes [Zn(R)-
(NNO)]2 with the heteroscorpionate in a κ-NNμ-O style.
Some of these complexes reported are the first enantiopure
scorpionate zinc complexes, which contain the chirality in the
substituent bound to the methyne bridge. The single-crystal X-
ray diffraction studies on derivatives 1, 4, 6, and 7 confirm the
structures with the heteroscorpionate ligands arranged in both
coordination modes.
Interestingly, the alkyl nucleophilicity of these zinc
complexes means that they can act as single-site living initiators
for the well-controlled polymerization of lactides at mild
conditions. The polymerization of LA also occurs within hours
to give low-molecular-weight polymers and offers very good
control, as evidenced by the living behavior as well as the low
polydispersity indexes measured in the materials. Regardless of
the mechanism of polymerization, initial kinetic parameter
studies for L-LA revealed that propagations present the usual
first-order dependence on monomer and catalyst concentration
for the ROP of lactides, without the need for a cocatalyst or
activator. Additionally, the most sterically hindered initiator 12
promotes a heterotactic bias in the polymerization of rac-lactide
in tetrahydrofuran at 50 °C, producing enhanced degrees of
heteroactivity and resulting in heterotactic-enriched PLAs with
Pr up to 0.77. End group analysis and MALDI-TOF mass
spectra suggest that zinc catalysts initiate the polymerization
process by alkyl transfer to the monomer.
Preparation of Compounds 1−12. Synthesis of [Zn-
(Me)(bpzbe)] (1). In a 250 cm3 Schlenk tube, bpzbeH (1.0 g,
3.4 mmol) was dissolved in dry toluene (60 mL), and the
solution was cooled to −70 °C. A solution of ZnMe2 (2.0 M in
toluene) (1.7 mL, 3.4 mmol) was added, and the mixture was
allowed to warm up to room temperature and stirred during 2
h. The solvent was evaporated to dryness under reduced
pressure to yield a white product. The product was washed with
n-hexane (1 × 25 mL) and recrystallized from toluene at −26
°C to give compound 1 as colorless crystals. Yield: (1.14 g,
90%). Anal. Calcd for C17H28N4OZn: C, 55.21; H, 7.83; N,
1
15.15. Found: C, 55.49; H, 7.91; N, 15.14. H NMR (C6D6,
297 K): δ 6.07 (bs, 1H, CHb), 5.38 (s, 1H, H4′), 5.34 (s, 1H,
H4), 4.11 (bs, 1H, CHa), 2.06 (s, 3H, Me5), 2.06 (s, 3H, Me5′),
1.73 (s, 3H, Me3′), 1.69 (s, 3H, Me3), 1.03 (s, 9H, −C-(CH3)3),
0.09 (s, 3H, Zn-CH3). 13C{1H} NMR (C6D6, 297 K): δ 148.5−
137.2 (C3,3′, C5,5′), 105.1 (C4′), 105.6 (C4), 88.1 (Ca), 64.9
(Cb), 36.8 (C-(CH3)3), 26.3 (C-(CH3)3), 12.5 (Me3,3′), 10.3
(Me5), 10.1 (Me5′), −2.4 (Zn-Me).
EXPERIMENTAL SECTION
■
All manipulations were performed under nitrogen using
standard Schlenk techniques. Solvents were predried over
sodium wire (toluene, n-hexane, THF, n-pentane, diethyl ether)
or calcium hydride (dichloromethane) and distilled under
nitrogen from sodium (toluene, n-hexane, and n-pentane),
sodium-benzophenone (THF, diethyl ether), or calcium
hydride (dichloromethane). Deuterated solvents were stored
over activated 4 Å molecular sieves and degassed by several
freeze−thaw cycles. Microanalyses were carried out with a
Synthesis of [Zn(Me)(bpzte)] (2). The synthesis of 2 was
carried out in an identical manner to 1, but 2 was obtained as a
white crystalline solid (using bpzteH (1.0 g, 2.9 mmol) and
ZnMe2 (2.0 M in toluene) (1.5 mL, 2.9 mmol)). Yield: (1.10 g,
93%). Anal. Calcd for C20H26N4OZn: C, 59.98; H, 6.49; N,
1
PerkinElmer 2400 CHN analyzer. H and 13C NMR spectra
1
13.87. Found: C, 60.21; H, 6.82; N, 13.62. H NMR (C6D6,
3
297 K): δ 7.37 (d, 2H, JH−H = 5 Hz, o-H-Ph), 7.00 (d, 2H,
were recorded on a Varian Inova FT-500 spectrometer and
referenced to the residual deuterated solvent. The H NMR
3JH−H = 5 Hz, m-H-Ph), 5.89 (bs, 1H, CHb), 5.86 (bs, 1H,
CHa), 5.42 (s, 1H, H4′), 5.22 (s, 1H, H4), 2.13 (s, 3H, Me5′),
2.12 (s, 3H, Me5), 2.11 (s, 3H, Me-Ph), 1.65 (s, 3H, Me3′), 1.18
(s, 3H, Me3), 0.19 (s, 3H, Zn-CH3). 13C{1H} NMR (C6D6, 297
K): δ 148.9−137.2 (C3,3′, C5,5′), 144.7 (p-C-Ar), 136.6 (ipso-C-
Ar), 128.6 (m-C-Ar), 126.5 (o-C-Ar), 105.3 (C4′), 104.5 (C4),
81.9 (Ca), 70.7 (Cb), 20.9 (Me3′), 13.5 (Me-Ar), 12.5 (Me3),
10.1 (Me5′), 9.9 (Me5), −2.5 (Zn-Me).
1
homodecoupled and the NOESY-1D spectra were recorded on
the same instrument with the following acquisition parameters:
irradiation time 2 s and number of scans 256, using standard
VARIAN-FT software. Two-dimensional NMR spectra were
acquired using standard VARIAN-FT software and processed
using an IPC-Sun computer. The zinc alkyls ZnR2 (R = Me, Et)
were used as purchased (Aldrich), and ZnR2 (tBu, CH2SiMe3)
were prepared according to literature procedures.19,46 The
starting materials bdmpzm,47 bpzbeH,3a bpzteH,3a and (R,R)-
bpzmmH3 were also prepared according to literature
procedures. L-Lactide and rac-lactide were sublimed twice,
recrystallized from THF, and finally sublimed again prior to
use. Size exclusion chromatography measurements were
performed on a Polymer Laboratories PLsize exclusion
chromatography-220 instrument equipped with a TSK-GEL
Synthesis of [Zn(Me)(bpzmm)] (3). The synthesis of 3 was
carried out in an identical manner to 1, but 3 was obtained as a
white crystalline solid (using (R,R)-bpzmmH (1.0 g, 2.8 mmol)
and ZnMe2 (2.0 M in toluene) (1.4 mL, 2.8 mmol)). Yield:
(1.09 g, 89%). Anal. Calcd for C22H32N4OZn: C, 60.89; H,
1
7.43; N, 12.91. Found: C, 61.15; H, 7.55; N, 13.02. H NMR
(C6D6, 297 K): δ 5.89 (bs, 1H, CHb), 5.59 (bs, 1H, CHa), 5.42
(s, 1H, H4′), 5.40 (s, 1H, H4), 5.24 (bs, 1H, Hd), 2.41 (m, 4H,
4198
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
He,f,i), 2.33 (bs, 2H, Hh), 1.95 (s, 3H, Me5′), 1.87 (s, 3H, Me5),
1.77 (s, 3H, Me3′), 1.73 (s, 3H, Me3), 1.25 (s, 3H, Mej), 1.01 (s,
3H, Mek), 0.14 (s, 3H, Zn-CH3). 13C{1H} NMR (C6D6, 297
K): δ 152.3−138.5 (Me3,3′, Me5,5′), 148.8 (Cd), 139.2 (Cc),
105.6 (C4′), 105.2 (C4), 81.2 (Ca), 63.37 (Cb), 41.7 (Cf), 41.1
(Ci), 38.2 (Cg, Ce), 31.7 (Ch), 26.2 (Mek), 21.3 (Mej), 12.9
(Me3), 12.3 (Me3′), 10.5 (Me5′), 10.2 (Me5), −2.4 (Zn-Me).
[α]2D5 = +29.8 (c 10−3 g/mL, toluene).
Synthesis of [Zn(Et)(bpzbe)]2 (4). The synthesis of 4 was
carried out in an identical manner to 1 (using bpzbeH (1.0 g,
3.4 mmol) and ZnEt2 (1.0 M in n-hexane) (3.4 mL, 3.4
mmol)). Yield: (1.20 g, 91%). Anal. Calcd for C36H60N8O2Zn2:
C, 56.32; H, 7.88; N, 14.60. Found: C, 56.46; H, 8.01; N, 14.46.
1H NMR (C6D6, 297 K): δ 6.07 (bs, 2H, CHb), 5.43 (s, 2H,
H4′), 5.39 (s, 2H, H4), 4.07 (bs, 2H, CHa), 2.10 (s, 12H,
Me5,5′), 1.86 (t, 6H, 3JH−H = 10 Hz, Zn-CH2-CH3), 1.77 (s, 6H,
Me3′), 1.73 (s, 6H, Me3), 1.00 (s, 18H, −C-(CH3)3), 0.94 (q,
4H, 3JH−H = 10 Hz, Zn-CH2-CH3). 13C{1H} NMR (C6D6, 297
K): δ 148.3−137.4 (C3,3′, C5,5′), 108.0 (C4′), 107.6 (C4), 88.1
(Ca), 65.2 (Cb), 36.7 (C-(CH3)3), 26.4 (C-(CH3)3), 13.9 (Zn-
CH2-CH3), 12.6 (Me3,3′), 10.7 (Me5′), 10.5 (Me5), −2.4 (Zn-
CH2-CH3).
mmol)). Yield: (1.15 g, 94%). Anal. Calcd for C42H56N8O2Zn2:
C, 60.36; H, 6.75; N, 13.41. Found: C, 60.34; H, 6.91; N, 13.54.
1H NMR (C6D6, 297 K): δ 7.35 (d, 4H, 3JH−H = 5 Hz, o-H-Ar),
3
6.98 (d, 4H, JH−H = 5 Hz, m-H-Ar), 5.90 (bs, 2H, CHb), 5.83
(bs, 2H, CHa), 5.44 (s, 2H, H4′), 5.24 (s, 2H, H4), 2.16 (s, 6H,
Me5′), 2.13 (s, 6H, Me5), 2.12 (s, 6H, Me-Ar), 1.86 (t, 6H,
3JH−H = 10 Hz, Zn-CH2-CH3), 1.67 (s, 6H, Me3′), 1.22 (s, 6H,
3
Me3), 0.94 (q, 4H, JH−H = 10 Hz, Zn-CH2-CH3). 13C{1H}
NMR (C6D6, 297 K): δ 148.7−138.8 (C3,3′, C5,5′), 144.5 (p-C-
Ar), 136.1 (ipso-C-Ar), 128.6 (m-C-Ar), 126.5 (o-C-Ar), 105.4
(C4′), 104.7 (C4), 81.7 (Ca), 70.9 (Cb), 20.9 (Me3), 18.7
(Me3′), 13.8 (Zn-CH2-CH3), 13.6 (Me-Ar), 10.2 (Me5′), 9.5
(Me5), −2.4 (Zn-CH2-CH3).
Synthesis of [Zn(tBu)(bpzte)]2 (8). The synthesis of 8 was
carried out in an identical manner to 5 (using bpzteH (1.0 g,
2.9 mmol) and ZntBu2 (0.038 M in Et2O) (77.3 mL, 2.9
mmol)). Yield: (1.01 g, 77%). Anal. Calcd for C46H64N8O2Zn2:
C, 61.95; H, 7.23; N, 12.56. Found: C, 61.85; H, 7.35; N, 12.48.
1H NMR (C6D6, 297 K): δ 7.30 (d, 4H, 3JH−H = 5 Hz, o-H-Ar),
3
6.96 (d, 4H, JH−H = 5 Hz, m-H-Ar), 5.85 (bs, 2H, CHb), 5.74
(bs, 2H, CHa), 5.41 (s, 2H, H4′), 5.21 (s, 2H, H4), 2.25 (s, 6H,
Me5′), 2.16 (s, 6H, Me5), 2.11 (s, 6H, Me-Ar), 1.84 (s, 18H,
Zn-C-(CH3)3), 1.63 (s, 6H, Me3′), 1.15 (s, 6H, Me3). 13C{1H}
NMR (C6D6, 297 K): δ 148.7−138.2 (C3,3′, C5,5′), 144.5 (p-C-
Ar), 136.1 (ipso-C-Ar), 128.6 (m-C-Ar), 126.5 (o-C-Ar), 105.4
(C4′), 104.7 (C4), 81.1 (Ca), 70.9 (Cb), 25.2 (Zn-C-(CH3)3),
20.8 (Me3), 18.7 (Me3′), 13.6 (Me-Ar), 10.2 (Me5′), 9.6 (Me5),
8.2 (Zn-C-(CH3)3).
Synthesis of [Zn(tBu)(bpzbe)]2 (5). In a 250 cm3 Schlenk
tube, bpzbeH (1.0 g, 3.4 mmol) was dissolved in dry diethyl
ether (50 mL), and the solution was cooled to −70 °C. A
solution of ZntBu2 (0.038 M in Et2O) (90.5 mL, 3.4 mmol) was
added, and the mixture was allowed to warm up to room
temperature and stirred during 2 h. The solvent was evaporated
to dryness under reduced pressure to yield a white product.
The product was washed with cold hexane (1 × 20 mL) to give
compound 5 as a white crystalline solid. Yield: (1.02 g, 72%).
Anal. Calcd for C40H68N8O2Zn2: C, 58.32; H, 8.32; N, 13.60.
Found: C, 58.09; H, 8.33; N, 13.45. 1H NMR (C6D6, 297 K): δ
6.03 (bs, 2H, CHb), 5.43 (s, 2H, H4′), 5.36 (s, 2H, H4), 4.21
(bs, 2H, CHa), 2.05 (s, 6H, Me5), 2.05 (s, 6H, Me5′), 1.76 (s,
6H, Me3′), 1.84 (s, 18H, Zn-C-(CH3)3, 1.71 (s, 6H, Me3), 1.02
(s, 18H, −C-(CH3)3). 13C{1H} NMR (C6D6, 297 K): δ 148.2−
137.3 (C3,3′, C5,5′), 108.0 (C4′), 107.3 (C4), 88.1 (Ca), 64.9
(Cb), 35.2 (C-(CH3)3), 26.2 (C-(CH3)3), 25.2 (Zn-C-(CH3)3),
12.2 (Me3,3′), 10.2 (Me5, Me5′), 8.2 (Zn-C-(CH3)3).
Synthesis of [Zn(CH2SiMe3)(bpzte)]2 (9). The synthesis of 9
was carried out in an identical manner to 6, but 9 was obtained
as a white crystalline solid (using bpzteH (1.0 g, 2.9 mmol) and
Zn(CH2SiMe3)2 (0.025 M in Et2O) (117.6 mL, 2.9 mmol)).
Yield: (1.21 g, 87%). Anal. Calcd for C46H68N8O2Si2Zn2: C,
58.03; H, 7.20; N, 11.77. Found: C, 58.28; H, 7.40; N, 11.71.
1H NMR (C6D6, 297 K): δ 7.30 (d, 4H, 3JH−H = 5 Hz, o-H-Ar),
3
6.96 (d, 4H, JH−H = 5 Hz, m-H-Ar), 5.84 (bs, 2H, CHb), 5.75
(bs, 2H, CHa), 5.41 (s, 2H, H4′), 5.20 (s, 2H, H4), 2.17 (s, 6H,
Me5′), 2.14 (s, 6H, Me5), 2.11 (s, 6H, Me-Ar), 1.63 (s, 6H,
Me3′), 1.15 (s, 6H, Me3), 0.64 (s, 18H, Zn-CH2-SiMe3), −0.11
(s, 4H, Zn-CH2-SiMe3). 13C{1H} NMR (C6D6, 297 K): δ
148.8−138.6 (C3,3′, C5,5′), 144.6 (p-C-Ar), 136.0 (ipso-C-Ar),
128.6 (m-C-Ar), 126.5 (o-C-Ar), 105.4 (C4′), 104.5 (C4), 81.8
(Ca), 70.5 (Cb), 20.8 (Me3′), 13.6 (Me-Ar), 12.8 (Me5′), 10.0
(Me5), 9.5 (Me3), 3.3 (Zn-CH2-SiMe3), −9.7 (Zn-CH2-SiMe3).
Synthesis of [Zn(Et)(bpzmm)]2 (10). The synthesis of 10
was carried out in an identical manner to 4, but 10 was
obtained as a white crystalline solid (using (R,R)-bpzmmH (1.0
g, 2.8 mmol) and ZnEt2 (1.0 M in n-hexane) (2.8 mL, 2.8
mmol)). Yield: (1.07 g, 85%). Anal. Calcd for C46H68N8O2Zn2:
C, 61.67; H, 7.65; N, 12.51. Found: C, 61.49; H, 7.91; N, 12.44.
1H NMR (C6D6, 297 K): δ 5.75 (bs, 2H, CHb), 5.58 (bs, 2H,
CHa), 5.38 (s, 2H, H4′), 5.33 (s, 2H, H4), 5.09 (s, 2H, Hd),
2.26−2.23 (m, 8H, He,f,i), 2.06 (s, 6H, Me5′), 2.04 (s, 6H, Me5),
Synthesis of [Zn(CH2SiMe3)(bpzbe)]2 (6). In a 250 cm3
Schlenk tube, bpzbeH (1.0 g, 3.4 mmol) was dissolved in dry
diethyl ether (50 mL), and the solution was cooled to −70 °C.
A solution of Zn(CH2SiMe3)2 (0.025 M in Et2O) (137.6 mL,
3.4 mmol) was added, and the mixture was allowed to warm up
to room temperature and stirred during 2 h. The solvent was
evaporated to dryness under reduced pressure to yield a white
product. The product was washed with cold hexane (1 × 20
mL) and recrystallized from hexane to give compound 6 as
colorless crystals. Yield: (1.29 g, 85%). Anal. Calcd for
C40H72N8O2Si2Zn2: C, 54.34; H, 8.21; N, 12.68. Found: C,
1
54.55; H, 8.46; N, 12.61. H NMR (C6D6, 297 K): δ 6.02 (bs,
2H, CHb), 5.40 (s, 2H, H4′), 5.34 (s, 2H, H4), 4.02 (bs, 2H,
CHa), 2.11 (s, 12H, Me5,5′), 1.69 (s, 6H, Me3′), 1.67 (s, 6H,
Me3), 0.97 (s, 18H, −C-(CH3)3), 0.58 (s, 18H, Zn-CH2-
SiMe3), −0.23 (s, 4H, Zn-CH2-SiMe3). 13C{1H} NMR (C6D6,
297 K): δ 148.4−137.3 (C3,3′, C5,5′), 105.6 (C4), 105.2 (C4′),
87.6 (Ca), 65.1 (Cb), 37.2 (C-(CH3)3), 26.2 (C-(CH3)3), 12.7
(Me3,3′), 10.5 (Me5), 10.3 (Me5′), 3.4 (Zn-CH2-SiMe3), −10.4
(Zn-CH2-SiMe3).
3
1.96 (bs, 2H, Hh), 1.87 (bs, 2H, Hh′), 1.71 (t, 6H, JH−H = 10
Hz, Zn-CH2-CH3), 1.69 (s, 6H, Me3′), 1.66 (s, 6H, Me3), 1.27
(s, 6H, Mej), 0.98 (s, 6H, Mek), 0.85 (q, 4H, Zn-CH2-CH3).
13C{1H} NMR (C6D6, 297 K): δ 152.1−138.4 (Me3,3′, Me5,5′),
148.9 (Cd), 139.4 (Cc), 105.3 (C4′), 105.1 (C4), 80.9 (Ca), 69.2
(Cb), 41.8 (Cf), 41.1 (Ci), 38.2 (Cg), 32.3 (Ce), 31.7 (Ch), 26.2
(Mek), 21.3 (Mej), 13.4 (Zn-CH2-CH3), 12.8 (Me3), 12.4
Synthesis of [Zn(Et)(bpzte)]2 (7). The synthesis of 7 was
carried out in an identical manner to 4 (using bpzteH (1.0 g,
2.9 mmol) and ZnEt2 (1.0 M in n-hexane) (2.9 mL, 2.9
(Me3′), 10.4 (Me5′), 10.2 (Me5), −2.1 (Zn-CH2-CH3). [α]2D5
+30.1 (c 10−3 g/mL, toluene).
=
4199
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
Synthesis of [Zn(tBu)(bpzmm)]2 (11). The synthesis of 11
was carried out in an identical manner to 5 (using (R,R)-
bpzmmH (1.0 g, 2.8 mmol) and ZntBu2 (0.038 M in Et2O)
(74.2 mL, 2.8 mmol)). Yield: (0.98 g, 73%). Anal. Calcd for
C50H76N8O2Zn2: C, 63.08; H, 8.05; N, 11.77. Found: C, 62.85;
and transferred to a Bruker X8 APEX II CCD-based
diffractometer equipped with a graphite monochromated Mo
Kα radiation source (λ = 0.71073 Å). The data sets were
integrated using SAINT48 and corrected for Lorentz and
polarization effects. The absorption correction was performed
with the program SADABS.49 The software package
SHELXTL, version 6.1250, was used for space group
determination, structure solution, and refinement by full-matrix
least-squares methods based on F2. A successful solution by the
direct methods provided most non-hydrogen atoms from the E-
map. The remaining non-hydrogen atoms were located in an
alternating series of least-squares cycles and difference Fourier
maps. All non-hydrogen atoms were refined with anisotropic
displacement coefficients unless specified otherwise. Hydrogen
atoms were placed using a “riding model” and included in the
refinement at calculated positions. In the case of compound 6,
the SiMe3 group is disordered and was modeled over two static
sites. The occupancies of the disorder components were refined
initially and then fixed to a value of 60:40.
1
H, 7.85; N, 11.58. H NMR (C6D6, 297 K): δ 5.71 (bs, 2H,
CHb), 5.50 (bs, 2H, CHa), 5.39 (s, 2H, H4′), 5.33 (s, 2H, H4),
5.06 (s, 2H, Hd), 2.26−2.23 (m, 8H, He,f,i), 2.07 (s, 6H, Me5′),
2.03 (s, 6H, Me5), 1.96 (bs, 2H, Hh), 1.87 (bs, 2H, Hh′), 1.83
(s, 18H, Zn-C-(CH3)3), 1.66 (s, 6H, Me3′), 1.63 (s, 6H, Me3),
1.25 (s, 6H, Mek), 0.98 (s, 6H, Mej). 13C{1H} NMR (C6D6,
297 K): δ 151.9−138.3 (Me3,3′, Me5,5′), 148.9 (Cd), 139.4 (Cc),
105.3 (C4′), 105.2 (C4), 80.9 (Ca), 69.2 (Cb), 41.8 (Cf), 41.1
(Ci), 38.2 (Cg), 32.3 (Ce), 31.7 (Ch), 26.2 (Mek), 25.2 (Zn-C-
(CH3)3), 21.3 (Mej), 12.8 (Me3), 12.4 (Me3′), 10.5 (Me5′),
10.31 (Me5), 8.2 (Zn-C-(CH3)3). [α]2D5 = +28.0 (c 10−3 g/mL,
toluene).
Synthesis of [Zn(CH2SiMe3)(bpzmm)]2 (12). The synthesis
of 12 was carried out in an identical manner to 6, but 12 was
obtained as a white crystalline solid (using (R,R)-bpzmmH (1.0
g, 2.8 mmol) and Zn(CH2SiMe3)2 (0.025 M in Et2O) (112.8
mL, 2.8 mmol)). Yield: (1.18 g, 83%). Anal. Calcd for
C50H80N8O2Si2Zn2: C, 59.33; H, 7.97; N, 11.07. Found: C,
ASSOCIATED CONTENT
* Supporting Information
■
S
Details of data collection, refinement, atom coordinates,
anisotropic displacement parameters, and bond lengths and
angles for complexes 1, 4·2C7H8, 6, and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
59.48; H, 8.20; N, 11.11. H NMR (C6D6, 297 K): δ 5.76 (bs,
2H, CHb), 5.51 (bs, 2H, CHa), 5.37 (s, 4H, H4,4′), 5.09 (s, 2H,
Hd), 2.26−2.23 (m, 8H, He,f,i), 2.18 (bs, 2H, Hh), 2.12 (s, 6H,
Me5′), 2.08 (s, 6H, Me5), 1.90 (bs, 2H, Hh′), 1.69 (s, 6H, Me3′),
1.65 (s, 6H, Me3), 1.36 (s, 6H, Mek), 1.12 (s, 6H, Mej), 0.60 (s,
18H, Zn-CH2-SiMe3), −0.25 (s, 4H, Zn-CH2-SiMe3). 13C{1H}
NMR (C6D6, 297 K): δ 151.9−137.2 (Me3,3′, Me5,5′), 148.8
(Cd), 139.3 (Cc), 105.3 (C4′), 105.2 (C4), 80.9 (Ca), 69.2 (Cb),
41.7 (Cf), 41.1 (Ci), 38.2 (Cg), 32.2 (Ce), 31.7 (Ch), 26.2
(Mek), 21.3 (Mej), 12.8 (Me3), 12.4 (Me3′), 10.5 (Me5′), 10.3
AUTHOR INFORMATION
Corresponding Author
*E-mail: antonio.otero@uclm.es (A.O.), juan.fbaeza@uclm.es
(J.F.-B.), luisfernando.sanchezbarba@urjc.es (L.F.S.-B.).
■
Notes
The authors declare no competing financial interest.
(Me5), 3.3 (Zn-CH2-SiMe3), −9.9 (Zn-CH2-SiMe3). [α]2D5
=
+33.5 (c 10−3 g/mL, toluene).
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from the Minister-
■
Typical Polymerization Procedures. Polymerizations of
L-lactide and rac-lactide (LA) were performed on a Schlenk line
in a flame-dried round-bottom flask equipped with a magnetic
stirrer. The Schlenk tubes were charged in the glovebox with
the required amount of lactide and initiator, separately, and
then attached to the vacuum line. The initiator and monomer
were dissolved in the appropriate amount of solvent, and
temperature equilibration was ensured in both Schlenk flasks by
stirring the solutions for 15 min in a bath. The appropriate
amount of initiator was added by syringe, and polymerization
times were measured from that point. Polymerizations were
stopped by injecting wet methanol. Polymers were precipitated
in methanol, filtered off, redissolved and reprecipitated in
methanol, and dried in vacuo to a constant weight.
General Kinetic Procedure. A solution of catalyst in
tetrahydrofuran (2.5 mL) was added to a tetrahydrofuran
solution of monomer (10 mL) to give a [L-LA]0 = 0.72 M. The
mixture was then stirred at 50 or 65 °C under N2. At
appropriate time intervals, 0.5 mL aliquots were removed using
a syringe and quickly quenched into 5 mL vials with wet
methanol (three drops). The aliquots were then dried to a
constant weight in vacuo and analyzed by 1H NMR
spectroscopy.
io de Ciencia e Inonovacion
́
(MICINN), Spain (Grant No.
CTQ2011-22578/BQU and Consolider-Ingenio 2010 ORFEO
CSD2007-00006), and the Junta de Comunidades de Castilla-
La Mancha (Grant No. PCI08-0010).
REFERENCES
(1) (a) Otero, A.; Fernan
chez, A. Dalton Trans. 2004, 1499−1510. (perspective article).
■
́
dez-Baeza, J.; Antinolo, A.; Tejeda, J.; Lara-
̃
San
́
(b) Pettinari, C.; Pettinari, R. Coord. Chem. Rev. 2005, 249, 663−691.
(2) Otero, A.; Fernandez-Baeza, J.; Lara-Sanchez, A.; Tejeda, J.;
Sanchez-Barba, L. F. Eur. J. Inorg. Chem. 2008, 5309−5326
(microreview).
(3) (a) Otero, A.; Fernan
Sanchez-Molina, M.; Franco, S.; Lopez-Solera, M. I.; Rodríguez, A. M.;
Sanchez-Barba, L. F.; Morante-Zarcero, S.; Garces, A. Inorg. Chem.
2009, 48, 5540−5554. (b) Otero, A.; Fernandez-Baeza, J.; Antinolo,
A.; Tejeda, J.; Lara-Sanchez, A.; Sanchez-Barba, L.; Sanchez-Molina,
́
́
́
́
́
dez-Baeza, J.; Tejeda, J.; Lara-Sanchez, A.;
́
́
́
́
̃
́
́
M.; Franco, S.; Lopez-Solera, M. I.; Rodríguez, A. M. Inorg. Chem.
2007, 46, 8475−8477.
(4) (a) Gennari, M.; Tegoni, M.; Lanfranchi, M.; Pellinghelli, M. A.;
̀
Marchio, L. Inorg. Chem. 2007, 46, 3367−3377. (b) Goldfuss, B.
Synthesis 2005, 14, 2271−2280. (c) Hodgson, D. M., Ed. Organo-
lithiums in Enantioselective Synthesis; Springer: Heidelberg, 2003; Vol. 5.
(d) Nogradi, M. Stereoselective Synthesis; VCH: Weinheim, Germany,
́ ́
1995. (e) Noyori, R. Asymmetric Catalysis in Organic Synthesis; John
Wiley & Sons: New York, 1994.
X-ray Crystallographic Structure Determination for
Complexes 1, 4·2C7H8, 6, and 7. A summary of crystal data
collection and refinement parameters for all compounds is
given in Table S1 in the Supporting Information. The single
crystals for 1, 4·2C7H8, 6, and 7 were mounted on a glass fiber
(5) See, for example: (a) Vargas-Díaz, M. E.; Joseph-Nathan, P.;
́
Tamariz, J.; Zepeda, L. G. Org. Lett. 2007, 9, 13−16. (b) Alvaro, E.; de
4200
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
la Torre, M. C.; Sierra, M. A. Chem. Commun. 2006, 985−987.
(c) Vargas-Díaz, M. E.; Lagunas-Rivera, S.; Joseph-Nathan, P.;
Tamariz, J.; Zepeda, L. G. Tetrahedron Lett. 2005, 46, 3297−3300.
F.; San
́
chez-Molina, M.; Rodríguez, A. M.; Bo, C.; Urbano-Cuadrado,
M. Organometallics 2007, 26, 4310−4320. (c) Otero, A.; Fernan
Baeza, J.; Lara-Sanchez, A.; Antinolo, A.; Tejeda, J.; Martínez-
Caballero, E.; Marquez-Segovia, I.; Lop
L. F.; Alonso-Moreno, C. Inorg. Chem. 2008, 47, 4996−5005.
(16) Sanchez-Barba, L. F.; Garces, A.; Fajardo, M.; Alonso-Moreno,
C.; Fernandez-Baeza, J.; Otero, A.; Antinolo, A.; Tejeda, J.; Lara-
Sanchez, A.; Lop
(17) Garces, A.; San
M.; Fernandez-Baeza, J.; Otero, A.; Lara-San
Rodríguez, A. M. Inorg. Chem. 2010, 49, 2859−2871.
(18) Sanchez-Barba, L. F.; Garces, A.; Fernandez-Baeza, J.; Otero, A.;
Alonso-Moreno, C.; Lara-Sanchez, A.; Rodríguez, A. M. Organo-
metallics 2011, 30, 2775−2789.
(19) Alonso-Moreno, C.; Garces
M.; Fernandez-Baeza, J.; Otero, A.; Antinolo, A.; Lara-San
Broomfield, L.; Lop
2008, 27, 1310−1321.
(20) Sanchez-Barba, L. F.; Alonso-Moreno, C.; Garces
M.; Fernandez-Baeza, J.; Otero, A.; Lara-Sanchez, A.; Rodríguez, A.
M.; Lopez-Solera, I. Dalton Trans. 2009, 8054−8062.
́
dez-
́
̃
́
(d) Silva, M. M. C.; Sae Melo, M. L.; Parolin, M.; Tessaro, D.; Riva, S.;
́
ez-Solera, I.; Sanchez-Barba,
́ ́
Danieli, B. Tetrahedron: Asymmetry 2004, 15, 21−27. (e) Fleming, F.
F.; Shook, B. C. J. Org. Chem. 2002, 67, 3668−3672. (f) Martinez-
́
́
́
Ramos, F.; Vargas-Díaz, M. E.; Chacon-García, L.; Tamariz, J.; Joseph-
́
̃
Nathan, P.; Zepeda, L. G. Tetrahedron: Asymmetry 2001, 12, 3095−
3103.
́
́
ez-Solera, M. I. Organometallics 2007, 26, 6403−6411.
chez-Barba, L. F.; Alonso-Moreno, C.; Fajardo,
chez, A.; Lopez-Solera, I.;
́
́
(6) Biela, T.; Kowalski, A.; Libiszowski, J.; Duda, A.; Penczek, S.
Macromol. Symp. 2006, 47.
́
́
́
(7) For reviews in this area, see: (a) Cameron, D. J. A.; Shaver, M. P.
Chem. Soc. Rev. 2011, 40, 1761−1776. (b) Kiesewetter, M. K.; Shin, E.
J.; Hedrick, J. L.; Waymouth, R. M. Macromolecules 2010, 43, 2093−
2107. (c) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 165−
173. (d) Carpentier, J.-F. Macromol. Rapid Commun. 2010, 31, 1696−
1705. (e) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165−173.
(f) Ajellal, N.; Carpentier, J.-F.; Guillaume, C.; Guillaume, S. M.;
Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010,
8363−8366. (g) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Dalton
Trans. 2009, 4832−4846. (h) Place, E. S.; George, J. H.; Williams, C.
K. Chem. Soc. Rev. 2009, 38, 1139−1151. (i) Platel, R. H.; Hodgson, L.
M.; Williams, C. K. Polym. Rev. 2008, 48, 11−63. (j) Williams, C. K.;
Hillmyer, M. A. Polym. Rev. 2008, 48, 1−10. (k) Dove, A. P. Chem.
Commun. 2008, 6446−6470. (l) Amgoune, A.; Thomas, C. M.;
Roisnel, T.; Carpentier, J.-F. Chem. Eur. J. 2006, 12, 169−179.
(m) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Coord. Chem. Rev. 2006,
250, 602−626. (n) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D.
Chem. Rev. 2004, 104, 6147−6176. (o) Chisholm, M. H.; Zhou, Z. J.
Mater. Chem. 2004, 14, 3081−3092.
(8) For books in this area, see: (a) Auras, R., Lim, L.-T., Selke, S. E.
M., Tsuji, H., Eds. Poly(lactic acid): Synthesis, Structures, Properties,
Processing, and Applications; John Wiley & Sons, Inc.: Hoboken, NJ,
2010; pp xvii−xviii and 4. (b) Dubois, P., Coulembier, O., Raquez, J.-
M., Eds. Handbook of Ring-Opening Polymerization; Wiley-VCH:
Weinheim, Germany, 2009. (c) Chisholm, M. H.; Zhou, Z. In
Stereoselective Polymerization with Single Site Catalysts; Baugh, L. S.,
Canich, J. A. M., Eds.; CRC Press: Boca Raton, FL, 2008; Chapter 25.
(9) (a) Cowan, J. A., Ed. The Biological Chemistry of Magnesium;
VCH: New York, 1995. (b) Campbell, N. A. Biology, 3rd ed.;
Benjamin/Cummings Publishing Company: Redwood City, CA, 1993;
pp 718 and 811.
́
́
́
́
́
, A.; San
́
chez-Barba, L. F.; Fajardo,
chez, A.;
ez-Solera, I.; Rodríguez, A. M. Organometallics
́
́
̃
́
́
́
, A.; Fajardo,
́
́
́
(21) (a) Piedra-Arroni, E.; Brignou, P.; Amgoune, A.; Guillaume, S.
M.; Carpentier, J.-F.; Bourissou, D. Chem. Commun. 2011, 47, 9828−
9830. (b) Darensbourg, D. J.; Karroonnirun, O. Inorg. Chem. 2010, 49,
2360−2371. (c) Chisholm, M. H. Pure Appl. Chem. 2010, 82, 1647−
1662. (d) Drouin, F.; Oguadinma, P. O.; Whitehorne, T. J. J.;
Prud’homme, R. E.; Schaper, F. Organometallics 2010, 29, 2139−2147.
(e) Schofield, A. D.; Barros, M. L.; Cushion, M. G.; Schwarz, A. N. D.;
Mountford, P. Dalton Trans. 2009, 85−96.
(22) 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.
(23) (a) Chisholm, M. H.; Huffman, J. C.; Phomphrai, K. J. Chem.
Soc., Dalton Trans. 2001, 222−224. (b) Chisholm, M. H.; Eilerts, N.
W.; Huffman, J. C.; Iyer, S. S.; Pacold, M.; Phomphrai, K. J. Am. Chem.
Soc. 2000, 122, 11845−11854.
(24) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young,
V. G., Jr.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125,
11350−11359.
(25) (a) Godau, T.; Bleifuss, S. M.; Mueller, A. L.; Roth, T.;
Hoffmann, S.; Heinemann, F. W.; Burzlaff, N. Dalton Trans. 2011,
6547−6554. (b) Godau, T.; Platzmann, F.; Heinemann, F. W.;
Burzlaff, N. Dalton Trans. 2009, 254−255.
(26) Schofield, A. D.; Barros, M. L.; Cushion, M. G.; Schwarz, A. D.;
Mountford, P. Dalton Trans. 2009, 85−96.
(27) Timothy, J. B.; Pratt, H. D., III; Alam, T. M.; Headley, T.;
Rodriguez, M. A. Eur. J. Inorg. Chem. 2009, 855−865.
(10) (a) Parkin, G. Chem. Commun. 2000, 1971−1985. (b) Mills, C.
F. Zinc in Human Biology; Springer-Verlag: New York, 1989.
(11) (a) Pego, A. P.; Siebum, B.; VanLuyn, M. J. A.; Gallego, X. J.;
̂
Van Seijen, Y.; Poot, A. A.; Grijpma, D. W.; Feijen, J. Tissue Eng. 2003,
9, 981−994. (b) Marler, J. J.; Upton, J.; Langer, R.; Vacanti, J. P. Adv.
Drug Delivery Rev. 1998, 33, 165−182. (c) Frazza, E. J.; Schmitt, E. E.
J. Biomed. Mater. Res. Symp. 1971, 1, 43−58. (d) Dexon and Vicryl are
products of Davis & Geek Corp., Wayne, NJ, and Ethicon, Inc.,
Somerville, NJ, respectively.
(12) (a) Penco, M.; Donetti, R.; Mendichi, R.; Ferruti, P. Macromol.
Chem. Phys. 1998, 199, 1737−1745. (b) Leupron Depot is a product
of Takeda Chemical Industries, Ltd., Japan, for drug delivery purposes.
(c) Kim, H.; Sung, Y. K.; Jung, J.; Baik, H.; Min, T. J.; Kim, Y. S. J. Kor.
Chem. Soc. 1990, 34, 203−210. (d) Smith, A.; Hunneyball, I. M. Int. J.
Pharm. 1986, 30, 215−220.
(13) (a) Darensbourg, D. J.; Choi, W.; Richers, C. P. Macromolecules
2007, 40, 3521−3523. (b) Beiser, I. H.; Konat, I. O. J. Am. Podiatr.
Med. Assoc. 1990, 80, 272−275.
(28) (a) Grunova, E.; Roisnel, T.; Carpentier, J.-F. Dalton Trans.
2009, 9010−9019. (b) Poirier, V.; Roisnel, T.; Carpentier, J.-F.;
Sarazin, Y. Dalton Trans. 2009, 9820−9027.
(29) Johnson, A. L.; Hollingsworth, N.; Kociok-Kohn, G.; Molloy, K.
̈
C. Inorg. Chem. 2008, 47, 12040−12048.
(30) The simple model of the Schlenk equilibrium (2RMgX ⇌ MgR2
+ MgX2) for describing the composition of Grignard reagents is
complicated by a variety of factors, including (i) the formation of
complexes of each component with either solvent, reactant, or
product, (ii) the formation of dimeric (or higher-order) species, and
(iii) the presence of ionic species. (a) Ashby, E. C. Pure Appl. Chem.
1980, 52, 545−569. (b) Ashby, E. C.; Laemmle, J.; Neumann, H. M.
Acc. Chem. Res. 1974, 7, 272−280. (c) Ashby, E. C. Q. Rev. Chem. Soc.
1967, 259−285. (d) Kharasch, M. S.; Reinmuth, O. Grignard Reactions
of Nonmetallic Substances; Prentice-Hall: New York, 1954. (e) Schlenk,
W., Jr. Ber. Dtsch. Chem. Ges. 1929, 62, 920−924.
(14) (a) Otero, A.; Fernan
Lara-Sanchez, A.; Sanchez-Barba, L. F.; Lop
M. Inorg. Chem. 2007, 46, 1760−1770. (b) Otero, A.; Fernan
Baeza, J.; Antinolo, A.; Tejeda, J.; Lara-Sanchez, A.; Sanchez-Barba, L.;
Rodríguez, A. M. Eur. J. Inorg. Chem. 2004, 260−266.
(15) (a) Otero, A.; Fernandez-Baeza, J.; Antinolo, A.; Tejeda, J.;
Lara-Sanchez, A.; Sanchez-Barba, L.; Rodríguez, A. M.; Maestro, M. A.
J. Am. Chem. Soc. 2004, 126, 1330−1331. (b) Otero, A.; Fernandez-
Baeza, J.; Antinolo, A.; Tejeda, J.; Lara-Sanchez, A.; Sanchez-Barba, L.
́
dez-Baeza, J.; Antinolo, A.; Tejeda, J.;
̃
́
́
́
ez-Solera, I.; Rodríguez, A.
dez-
́
́
́
̃
(31) (a) Kisko, J. L.; Fillebeen, T.; Hascall, T.; Parkin, G. J.
Organomet. Chem. 2000, 596, 22−26. (b) Han, R.; Parkin, G. J. Am.
Chem. Soc. 1992, 114, 748−757. (c) Han, R.; Parkin, G. Organo-
metallics 1991, 10, 1010−1020. (d) Han, R.; Parkin, G. J. Am. Chem.
Soc. 1990, 112, 3662−3663. (e) Han, R.; Brachrach, M.; Parkin, G.
́
̃
́
́
́
́
́
̃
4201
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202

Organometallics
Article
Polyhedron 1990, 9, 1775−1778. (f) Han, R.; Looney, A.; Parkin, G. J.
Am. Chem. Soc. 1989, 111, 7276−7278.
(32) Beck, A.; Weibert, B.; Burzlaff, N. Eur. J. Inorg. Chem. 2001,
521−527.
(33) (a) Hammes, B. S.; Carrano, C. J. Inorg. Chem. 1999, 38, 4593−
4600. (b) Hammes, B. S.; Carrano, C. J. J. Chem. Soc., Dalton Trans.
2000, 3304−3309.
(34) (a) Martinho, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629−
́ ́
688. (b) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.;
́
́
Echeverría, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans.
2008, 2832−2838.
(35) (a) Zhong, Z.; Dijkstra, J. P.; Feijen, J. J. Am. Chem. Soc. 2003,
125, 11291−11298. (b) Radano, C. P.; Baker, G. L.; Smith, M. R. J.
Am. Chem. Soc. 2000, 122, 1552−1553.
(36) Becker, J. M.; Pounder, R. J.; Dove, A. P. Macromol. Rapid
Commun. 2010, 31, 1923−1937.
(37) (a) Sarazin, Y.; Liu, B.; Roisnel, T.; Laurent Maron, L.;
Carpentier, J.-F. J. Am. Chem. Soc. 2011, 133, 9069−9087. (b) Ajellal,
N.; Carpentier, J.-F.; Guillaume, C.; Guillaume, S. M.; Helou, M.;
Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39, 8363−
8376.
(38) Poirier, V.; Roisnel, T.; Carpentier, J.-F.; Sarazin, Y. Dalton
Trans. 2011, 40, 523−524.
́
(39) (a) Sanchez-Barba, L. F.; Hughes, D. L.; Humphrey, S. M.;
́
Bochmann, M. Organometallics 2006, 25, 1012−1020. (b) Sanchez-
Barba, L. F.; Hughes, D. L.; Humphrey, S. M.; Bochmann, M.
Organometallics 2005, 24, 5329−5334.
(40) Borner, J.; Santos, I.; Pawlis, A.; Doring, A.; Kuckling, D.;
̈
̈
Herres-Pawlis, S. Chem.Eur. J. 2011, 17, 4507−4512.
(41) (a) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S.
Macromol. Rapid Commun. 1997, 18, 325−333. (b) Barakat, I.; Dubois,
Ph.; Jer
31, 505−514.
́ ̂ ́
ome, R.; Teyssie, Ph. J. Polym. Sci., Part A: Polym. Chem. 1993,
(42) (a) Drouin, F.; Oguadinma, P. O.; Whitehorne, T. J. J.;
Prud’homme, R. E.; Schaper, F. Organometallics 2010, 29, 2139−2147.
For the Pr value calculations, see also ref 41a and: (b) Chisholm, M.
H.; Gallucci, J. C.; Phomphrai, K. Inorg. Chem. 2005, 44, 8004−8010.
(c) Cai, C.-X.; Amgoune, A.; Lehmann, C. W.; Carpentier, J.-F. Chem.
Commun. 2004, 330−331.
(43) Otero, A.; Fernan
́
dez-Baeza, J.; Lara-San
́
chez, A.; Alonso-
Moreno, C.; Marquez-Segovia, I.; San
́
́
chez-Barba, L. F.; Rodríguez, A.
M. Angew. Chem., Int. Ed. 2009, 48, 2176−2179.
(44) (a) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. J. Am. Chem. Soc.
2002, 124, 5938−5939. (b) Ovitt, T. M.; Coates, G. W. J. Am. Chem.
Soc. 2002, 124, 1316−1326.
(45) Zhang, L.; Nederberg, F.; Messman, J. M.; Pratt, R. C.; Hedrick,
J. L.; Wade, C. G. J. Am. Chem. Soc. 2007, 129, 12610−12611.
(46) Hartwell, F. E.; Brown, T. L. Inorg. Chem. 1966, 5, 1257−1263.
(47) (a) Sebastian, J.; Sala, P.; Del Mazo, J.; Sancho, M.; Ochoa, C.;
Elguero, J.; Fayet, J. P.; Vertut, M. C. J. Heterocycl. Chem. 1982, 19,
́
1141−1145. (b) Díez-Barra, E.; de la Hoz, A.; Sanchez-Migallon, A.;
Tejeda, J. J. Chem. Soc., Perkin Trans. 1 1993, 1079−1083.
(48) SAINT+: Area-Detector Integration Program, v7.12a; Bruker-
Nonius AXS: Madison, WI, 2004.
(49) Sheldrick, G. M. SADABS: A Program for Empirical Absorption
Correction, version 2004/1; University of Gottingen: Gottingen,
̈
̈
Germany, 2004.
(50) SHELXTL-NT: Structure Determination Package, version 6.12.;
Bruker-Nonius AXS: Madison, WI, 2001.
4202
dx.doi.org/10.1021/om300146n | Organometallics 2012, 31, 4191−4202