Alkaline earth metal complexes of a chiral polyether as initiator for the ring-opening polymerization of lactide

Article abstract of DOI:10.1039/c2dt31309j

A chiral, tetradentate polyether ligand with a trans-1,2-cyclohexanediyl backbone, bis(methoxyethoxy)-trans-1,2-cyclohexane (5), was synthesized as both a racemate and the (S,S) enantiomer. 5 was found to form stable adducts with alkaline earth metal amides [M{N(SiMe₃)₂} ₂(thf)_x] (M = Mg (x = 0), Ca (x = 2) and Sr (x = 2/3)), [Ca{N(SiHMe₂)₂}₂(thf)] as well as with hydrocarbyl compounds [Mg(CH₂SiMe₃)₂] and [Ca(η³-C₃H₅)₂]. X-ray diffraction study of the bis(amide) [((S,S)-5)Ca{N(SiMe₃) ₂}₂] and of the bis(allyl) [(rac-5)Ca(η³- C₃H₅)₂] was performed. The complexes obtained were tested as initiators for the ring-opening polymerization of meso-, racemic and l-lactide.

Full text of DOI:10.1039/c2dt31309j

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PAPER
Alkaline earth metal complexes of a chiral polyether as initiator for the
ring-opening polymerization of lactide†
Julien P. Davin, Jean-Charles Buffet,‡ Thomas P. Spaniol and Jun Okuda*
Received 18th June 2012, Accepted 21st August 2012
DOI: 10.1039/c2dt31309j
A chiral, tetradentate polyether ligand with a trans-1,2-cyclohexanediyl backbone, bis(methoxyethoxy)-
trans-1,2-cyclohexane (5), was synthesized as both a racemate and the (S,S) enantiomer. 5 was found to
form stable adducts with alkaline earth metal amides [M{N(SiMe3)2}2(thf)x] (M = Mg (x = 0), Ca (x = 2)
and Sr (x = 2/3)), [Ca{N(SiHMe2)2}2(thf)] as well as with hydrocarbyl compounds [Mg(CH2SiMe3)2]
and [Ca(η3-C3H5)2]. X-ray diffraction study of the bis(amide) [((S,S)-5)Ca{N(SiMe3)2}2] and of the
bis(allyl) [(rac-5)Ca(η3-C3H5)2] was performed. The complexes obtained were tested as initiators for the
ring-opening polymerization of meso-, racemic and L-lactide.
polyamine (NNNN)-type ligand.13 We report here a new type of
chiral alkaline earth metal polyether complexes for the ROP of
Introduction
In 1967, Pedersen discovered that crown ethers readily coordi-
nate a wide variety of metal cations.1 They influence the solubi-
lity of metal complexes and crown ethers are now well
established in phase-transfer catalysis.2 Much attention has been
focused on the binding properties of crown ethers to alkali
metals, but to a lesser extent also to alkaline earth metals.3,4 In
their reactivity, the latter metals can sometimes be compared to
rare earth metals which led to their implication as catalysts
within the last decade.5 Furthermore, alkaline earth metals are
inexpensive due to their abundance and are non-toxic. Group
2 metals have been employed in ring-opening polymerization
(ROP) of cyclic esters,6 hydroamination,7 polymerization of
styrene and dienes,8 terminal alkyne coupling,9 hydrogenation,10
and pyridine activation.11 Nonetheless, little is known about
crown ether coordinated alkaline earth metals as catalysts,12 and
the use of alkaline earth metal catalysts with non-cyclic poly-
ethers as ligands is unknown.
Alkaline earth metal compounds often undergo facile ligand
exchange in Schlenk-type equilibria. To avoid such redistribu-
tions we have been searching for a chiral and neutral ligand
which coordinates alkaline earth metal fragments. This led us to
the idea of preparing “half-crown ethers” which are flexible
enough to coordinate metals of different size but are still suffi-
ciently rigid to control the stereoselective ROP of lactide mono-
mers. Recently, we reported the ROP of meso-lactide monomers
with alkaline earth metal initiators that contain a cyclic
lactide.
Results and discussion
Synthesis of alkaline earth metal complexes
The polyether rac-5 was synthesized according to modified litera-
ture procedures.14 The first step, a trans-selective nucleophilic
ring opening of cyclohexene oxide (1) with 2-methoxyethanol (2),
led to the intermediate racemic alcohol rac-3 (Scheme 1, top).
This compound was functionalized by nucleophilic substi-
tution using MeSO3CH2CH2OMe (4) which was obtained by
esterification of 2 with methanesulfonyl chloride in a parallel
reaction. The polyether rac-5 was obtained as a colorless,
viscous liquid after vacuum distillation.
To study the influence of chirality of the initiator on lactide
polymerization, the dimethoxy ligand 5 was also prepared in
enantiomerically pure form ((S,S)-5) following a synthetic route
developed by Aspinall et al., which involves a one step functio-
nalization of (S,S)-trans-1,2-cyclohexanediol.15
Alkaline earth metal bis(trimethylsilyl)amides [M{N-
(SiMe3)2}2(thf)x] (M = Mg, x = 0; Ca,16 x = 2; Sr,17 x = 2/3) as
well as bis(dimethylsilyl)amide [Ca{N(SiHMe2)2}2(thf)]18 were
reacted with rac-5 (Scheme 1, bottom). The hydrocarbyl com-
pounds [Mg(CH2SiMe3)2]19 and [Ca(η3-C3H5)2]8c were also
reacted with rac-5 to give racemic complexes. All alkaline earth
metal precursors form stable, THF-free complexes with rac-5 to
give alkaline earth metal complexes rac-6–11. They were
purified by crystallization at low temperature and isolated in 56
to 88% yield. The optically active calcium complex (S,S)-8 was
prepared in analogous manner using enantiopure (S,S)-5 and iso-
lated in 84% yield.
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg
1, D-52056 Aachen, Germany. E-mail: jun.okuda@ac.rwth-aachen.de;
Fax: (+49) 241 80 92 644
†Electronic supplementary information (ESI) available: Fig. S1–S14.
CCDC 884447–884450. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt31309j
In this work, we have also established the composition
of “[Ca{N(SiHMe2)2}2(thf)]” that crystallizes as a dimer
‡Current address: Chemistry Research Laboratory, Mansfield Road,
Oxford, OX1 3TA, United Kingdom
12612 | Dalton Trans., 2012, 41, 12612–12618
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Scheme 1 Top: Synthesis of racemic polyether ligand rac-5. Bottom:
Complexation of alkaline earth metal precursors with polyether ligand
rac-5. *Synthesis performed with enantiopure ligand (S,S)-5.
[C{N(SiHMe2)2}2(thf)]2 from concentrated pentane solution.
The crystal structure obtained by X-ray diffraction shows that
two of the amide groups are bridging the metal centers (ESI,
Fig. S13†). The solid-state structure of complex rac-9, obtained
from the calcium amide [Ca{N(SiHMe2)2}2(thf)]2 shows a
similar coordinative environment to the amide complex rac-8
(ESI, Fig. S14†).
Fig. 1 Molecular structures of (S,S)-8 (a) and rac-10 (b). Hydrogen
atoms were omitted for the sake of clarity. Selected bond lengths (Å)
and angles (°): (a) Ca1–N1 2.393(2), Ca1–N2 2.377(2), Ca1–O1 2.4235(19),
Ca1–O2 2.5358(19), Ca1–O3 2.509(2), Ca1–O4 2.4739(18); N1–
Ca1–N2 135.84(6), O1–Ca1–O4 172.37(9), O2–Ca1–O3 63.57(5). (b)
Ca1–C2 2.691(3), Ca1–O1 2.439(2), Ca1–O2 2.511(3); O1–Ca1–O1′
161.89(15), O2–Ca1–O2′ 65.26(11), C2–Ca1–C2′ 150.8(2).
1
The H NMR spectrum of 5 in C6D6 shows two multiplets at
3.37–3.48 and 3.61–3.74 ppm due to the OCH2CH2O moieties.
A further multiplet at 3.20–3.29 ppm is assigned to the protons
of the CHO group within the cyclic backbone. Coordination to a
metal center leads to significant change in the chemical shifts as
well as in multiplicity, suggesting that coordination had
occurred. The more rigid orientation in the metal compound
renders the protons of the OCH2CH2O moieties diastereotopic.
The complexation of [Ca{N(SiMe3)2}2(thf)2] by rac-5 to give
rac-8 leads to four signals for the ΟCH2CH2O moieties of equal
intensity spread out over a large chemical shift range (see experi-
mental section and Fig. S1 and S8 in the ESI†).
The structures of (S,S)-8 and rac-10 in the solid state were
established by X-ray diffraction and are shown in Fig. 1. The
latter adopts crystallographic C2 symmetry. Calcium is coordi-
nated by four oxygen atoms of the polyether ligand and by either
the two amido nitrogen atoms in (S,S)-8 or by both η3-coordi-
nated allyl ligands in rac-10. The arrangement of oxygen atoms
can roughly be considered as planar. The arrangement of the two
remaining ligands on opposite sides of this plane (with a
N1–Ca1–N2 angle of 135.84(6)° and a C2–Ca1–C2′ angle of
150.8(2)°) leads to a coordination geometry around calcium
that can be described as pentagonal bipyramid with a vacant site
in the equatorial plane. In (S,S)-8, the Ca1–N1 distance
(2.393(2) Å) is 0.12 Å longer than the Ca–N distance to the
terminal amido ligands in the four coordinated calcium com-
16b
pound [Ca{N(SiMe3)2}2]2
(2.27 Å). The structure of rac-10
is similar to that of the triglyme adduct [(triglyme-κ4)Ca(η3-
C3H5)2].8c The coordination by the polyether hardly affects the
bond distances between the allyl fragment and the metal center.
The distance Ca1–C2 in rac-10 (2.691(3) Å) is only 0.02 Å
larger than in [(triglyme-κ4)Ca(η3-C3H5)2] (2.671 Å).
Polymerization of lactide monomers
Compounds rac-6–11 (Scheme 1, bottom) were used in the ROP
of meso-lactide leading to full conversion in toluene in less than
30 min at room temperature (Scheme 2). The data in Table 1
show the formation of syndiotactically enriched polylactide,
albeit with low control of polymerization (1.49 < Mw/Mn
<
This journal is © The Royal Society of Chemistry 2012
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led to higher syndiotacticity (rac-8: from Ps of 0.73 to Ps of
0.81; rac-9: from Ps of 0.72 to Ps of 0.79). These syndiotacticity
values are higher than those obtained previously using alkaline
earth complexes containing a (NNNN)-type macrocycle, whilst
the rate of the polymerization is similar (Fig. 2).13
No pronounced effect on the polydispersity or syndiotacticity
of the PLA was noted when the metal was changed from mag-
nesium (rac-6) to calcium (rac-8) and strontium (rac-11)
(Table 1, entries 1, 3 and 6). When the enantiopure complex
(S,S)-8 was used to polymerize meso-lactide, the syndiotacticity
(0.73 for rac-8 and 0.74 for (S,S)-8) was similar to that observed
for rac-8.
Apparently, a chiral backbone in the ligand does not affect the
stereoselectivity during ROP of lactide monomers. The polymer-
ization of meso-lactide led to similar results when using a
complex with a ligand that contains a chiral backbone (rac-8) or
with an achiral one such as the triglyme complex [(triglyme-κ4)-
Ca{N(SiMe3)2}2], as shown by the syndiotacticity (Ps of
0.73 vs. 0.71).
Scheme 2 Polymerization of lactide monomers.
Complexes rac-6, 8–10 and (S,S)-8 were used in the ROP of
rac- and L-lactide. The polymerization data are collated in
Table 2. The amido and alkyl alkaline earth metal complexes led
to full conversion of rac- or L-lactide in less than 1 h at room
temperature in THF that is among the highest rate reported for
alkaline earth metal complexes.6c–f
Table 1 Polymerization of meso-lactide using complexes rac-6–11a
Conv.b [%]
Mn,expc [g mol−1
]
Mw/Mn
Ps
c
d
Entry
Init.
1
2
3
4
5
6
rac-6
rac-7
rac-8
rac-9
rac-10
rac-11
>99
>99
98
>99
>99
95
12 500
10 250
18 000
11 500
7500
1.58
1.49
1.63
2.29
1.98
1.58
0.69
0.61
0.73
0.72
0.68
0.70
The molecular weight of the poly(rac-lactide) decreased with
the steric bulk of the initiating group from N(SiMe3)2 (rac-8,
16 000
a Polymerization conditions: [mon]0/[initiator]0 = 100, [LA]0 = 0.520 M,
30 min, toluene, 25 °C, 2 mL. b Conversion of monomer (([mon]0
−
[mon]t)/[mon]0). c Measured by GPC, calibrated with PS standards.20
d Ps is the probability for the formation of a new s dyad, calculated from
1H{1H} NMR spectra.21
2.29). This can be explained by the high speed of the polymeriz-
ation resulting in chain transfer reactions such as transesterifica-
tion. The experimental molecular weights indicate that one chain
was initiated using complexes rac-6–9 and 11 (10 250 < Mn,exp
<
18 000 g mol−1) and two using rac-10 (Mn,exp of 7500 g mol−1).
Variation of [LA]0/[init]0 for meso-lactide polymerization with
rac-8 shows a linear dependence (ESI, Fig. S15†). For high
monomer initiator ratio (400), lower Mn,exp was observed which
is most probably caused by increasing viscosity and tendency to
undergo transesterification.
Fig. 2 NMR spectra of the methine region of poly(meso-lactide)
obtained at 25 °C with rac-8: (a) 1H{1H} and (b) 13C{1H}.
Table 2 Polymerization of rac- and L-lactide (LA) using complexes
rac-6, rac-8–10 and (S,S)-8a
The polymerization of meso-lactide by the amido magnesium
complex rac-6 (25 °C, toluene, initiator–monomer ratio of
1 : 100) led to syndiotactically enriched PLA (Ps = 0.69) with
broad polydispersity (Mw/Mn = 1.58, Table 1, entry 1).
The polymerization of meso-lactide proceeds with higher syn-
dioselectivity (Ps from 0.61 to 0.69) when the initiating group at
the magnesium center was changed from the CH2SiMe3 group
(rac-7, entry 2) to the N(SiMe3)2 group (rac-6, entry 1).22 At
25 °C, similar results were obtained with calcium complexes.
The initiating group N(SiMe3)2 in rac-8 led to the highest syn-
diotacticity (Ps = 0.73) comparable to N(SiHMe2)2 in rac-9 (Ps
= 0.72) and to C3H5 in rac-10 (Ps = 0.68). For the calcium
amido complexes, decreasing the temperature from 25 to 10 °C
c
Conv.b
[%]
Mn,exp
c
d
Entry
Init.
LA
[g mol−1
]
Mw/Mn
Pi
1
2
3
4
5
6
rac-8
rac-9
rac-10
rac-6
rac-8
(S,S)-8
rac-
rac-
rac-
L-
L-
L-
>99
>99
>99
>99
>99
>99
21 500
13 250
7500
19 000
10 750
9000
1.34
1.41
1.29
1.62
1.68
1.62
0.51
0.48
0.53
0.97
0.99
0.96
a Polymerization conditions: [mon]0/[initiator]0 = 100, [LA]0 = 0.520 M,
60 min, THF, 25 °C, 2 mL. b Conversion of monomer (([mon]0
−
[mon]t)/[mon]0). c Measured by GPC, calibrated with PS standards.20
d Pi is the probability for the formation of a new s dyad, calculated from
1H{1H} NMR spectra.
12614 | Dalton Trans., 2012, 41, 12612–12618
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Mn,exp = 21 500 g mol−1) to N(SiHMe2)2 (rac-9, Mn,exp
=
Synthesis of 4. 2-Methoxyethanol (2) (7.61 g, 100 mmol) and
methanesulfonyl chloride (12.6 g, 110 mmol) were dissolved in
200 mL THF. Triethylamine (11.1 g, 110 mmol) was slowly
added via a syringe and the mixture was stirred for 18 h. The
reaction mixture was filtered, the organic phase was washed with
dilute hydrochloric acid and dried over MgSO4. After removal of
the solvent, a yellow oil was obtained. Vacuum distillation
(75 °C, 5 × 10−4 mbar) gave 4 as a colorless liquid. Yield:
12.7 g (82.0 mmol, 82%).
13 250 g mol−1) and to C3H5 (rac-10, Mn,exp = 7500 g mol−1).
All polymers obtained were atactic. Poly(rac-lactide) that was
synthesized previously using group 2 metal complexes gave
heterotactic,23 isotactic,24 or atactic25 PLAs.
The polymerization of L-lactide with the magnesium com-
pound rac-6 led to polymers with higher molecular weight
(Mn,exp = 19 000 g mol−1) as compared to those obtained using
the calcium homologue rac-8 (Mn,exp = 10 750 g mol−1). All
resulting polymers are highly isotactic indicating the absence of
epimerization.23d,26
1H NMR (400 MHz, CDCl3): δ = 3.04 (s, 3H, CH3SO3); 3.38
(s, 3H, OCH3); 3.63–3.65 (m, 2H, OCH2CH2O); 4.34–4.36 (m,
2H, OCH2CH2O) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ =
37.6 (OCH3); 59.0 (CH3SO3); 68.9 (OCH2CH2O); 70.2
(OCH2CH2O) ppm. Anal. Calcd for C4H10O4S (154.18 g mol−1):
C 31.16, H 6.54. Found: C 31.20, H 6.71%.
Conclusions
The synthesis of a tetradentate chiral polyether and its com-
plexation with alkaline earth metals magnesium, calcium, and
strontium are reported. The resulting complexes are relatively
inert, suggesting that the 1,2-trans-cyclohexandediyl backbone
is capable of generating a rigid coordination sphere even for
the large metal calcium. Whilst relatively high activity toward
ROP of meso-, rac- and L-lactide using the described com-
plexes was observed, they failed to show pronounced stereose-
lectivity during the formation of isotactic and syndiotactic
PLA.
Synthesis of rac-3. 2-Methoxyethanol (2) (7.61 g, 100 mmol)
was slowly added to NaH (0.24 g, 10 mmol) giving a yellow
slurry solution. After addition of cyclohexene oxide (1) (9.81 g,
100 mmol) the mixture was heated to 140 °C for 3 h. Then the
crude product was dissolved in 50 mL of CH2Cl2 and the
organic phase was washed with brine and water. After a second
extraction of the aqueous phase with CH2Cl2 the combined
organic phases were dried over MgSO4 and the solvent was
removed under reduced pressure yielding a yellow oil which was
purified via vacuum distillation (60 °C, 5 × 10−4 mbar) to obtain
rac-3 as a colorless, viscous liquid. Yield: 11.4 g (65.5 mmol,
65%).
Experimental
1H NMR (400 MHz, CDCl3): δ = 1.10–1.30 (m, 4H, C6H10
CH2); 1.59–1.74 (m, 2H, C6H10 CH2); 1.92–2.04 (m, 2H, C6H10
CH2); 2.97–3.06 (m, 1H, C6H10 CHO); 3.36 (s, 3H, OCH3);
3.37–3.45 (m, 1H, C6H10 CHO); 3.48 (s, 1H, OH); 3.50–3.54
(m, 2H, OCH2CH2O); 3.54–3.59 (m, 1H, OCH2CH2O);
3.81–3.87 (m, 1H, OCH2CH2O) ppm. 13C{1H} NMR (100 MHz,
CDCl3): δ = 24.0 (C6H10 CH2); 24.4 (C6H10 CH2); 29.8 (C6H10
CH2); 32.1 (C6H10 CH2); 58.9 (OCH3); 68.6 (OCH2CH2O);
72.2 (OCH2CH2O); 73.9 (C6H10 CHO); 84.9 (C6H10 CHOH)
ppm. EI-MS: m/z 174 (0.2) [M]+, 157 (0.7) [M − OH]+,
115 (20) [C6H11O2]+. Anal. Calcd for C9H18O3 (174.24 g mol−1):
C 62.04, H 10.41. Found: C 61.54, H 10.57%.
General procedures
All operations were performed under an inert atmosphere of
argon using standard Schlenk-line or glove-box techniques.
Glassware and vials used in the polymerization were dried in an
oven at 120 °C overnight and exposed to a vacuum–argon cycle
three times. Toluene, pentane, THF, [D8]THF and C6D6 were
distilled under argon from sodium–benzophenone ketyl prior to
use. CDCl3 was distilled from CaH2 prior to use. NMR spectra
were recorded on a Varian Mercury 200 BB (1H, 200.0 MHz) or
Bruker DRX 400 spectrometer (1H, 400.1 MHz; 13C{1H},
100.6 MHz) at 25 °C unless otherwise stated. Chemical shifts
for 1H and 13C{1H}NMR spectra were referenced internally
using the residual solvent resonances and reported relative to
tetramethylsilane. Specific rotation was measured using a
JASCO P-2000 Digital Polarimeter (path length 10 cm, volume
1 mL, λ = 589.3 nm at 24 °C). Mass spectrometry data was
obtained with a Finnigan MAT95 mass spectrometer. meso-
Lactide (Uhde Inventa-Fischer), rac- and L-lactide were recrystal-
lized from isopropanol at −30 °C, washed with diethyl ether,
and dried under vacuum. [Mg{N(SiMe3)2}2] was purchased
from Sigma-Aldrich and used without further purification. [Mg-
Synthesis of rac-5. To NaH (864 mg, 36 mmol) suspended in
50 mL of THF was added neat rac-3 (5.23 g, 30 mmol) via a
syringe and the mixture was heated to reflux for 30 min. After a
short cooling period, 4 (5.09 g, 33 mmol) was added and the
mixture was heated to reflux for a further 18 h. The reaction was
quenched by addition of a small amount of water, the organic
phase was washed with water and brine and the aqueous phase
was extracted with diethyl ether. The combined organic phases
were dried with MgSO4, filtered and the solvent was removed in
vacuum. The crude product was obtained as yellow oil. Vacuum
distillation (65 °C, 5 × 10−4 mbar) afforded rac-5 as a colorless
liquid. Yield: 5.09 g (21.9 mmol, 73%).
(CH2SiMe3)2],19 [Ca{N(SiMe3)2}2(thf)2],16 [Ca{N(SiHMe2)2}2-
(thf)],18 [Ca(η3-C3H5)2]8c and [Sr{N(SiMe3)2}2(thf)2/3
]
were
17
prepared following procedures reported in the literature. (S,S)-
trans-1,2-Cyclohexanediol was obtained from resolution of
racemic trans-1,2-cyclohexanediol.27 All alkaline earth com-
plexes are air sensitive. As a result complete satisfactory elemen-
tal analyses could not be obtained, even upon glovebox
handling. This is a frequent problem for alkaline earth metal
compounds.28
1H NMR (400 MHz, CDCl3): δ = 1.03–1.30 (m, 4H, C6H10
CH2); 1.49–1.69 (m, 2H, C6H10 CH2); 1.82–2.02 (m, 2H, C6H10
CH2); 3.08–3.21 (m, 2H, C6H10 CHO); 3.32 (s, 6H, OCH3);
3.39–3.56 (m, 4H, OCH2CH2O); 3.62–3.77 (m, 4H,
1
OCH2CH2O) ppm. H NMR (400 MHz, C6D6): δ = 1.03–1.13
(m, 2H, C6H10 CH2); 1.24–1.40 (m, 2H, C6H10 CH2); 1.44–1.57
Dalton Trans., 2012, 41, 12612–12618 | 12615
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Synthesis of [(rac-5)Mg{N(SiMe3)2}2] (rac-6). [Mg{(NSiMe3)2}2]
(345.1 mg, 1.0 mmol) was dissolved in 8 mL of pentane and a
solution of rac-5 (232.3 mg, 1.0 mmol) in pentane (2 mL) was
added. After one minute of stirring a colorless precipitate formed
and the mixture was kept at −30 °C overnight to complete the
precipitation. The mother liquor was removed and the remaining
solid was dried in a vacuum to yield rac-6 as a colorless powder.
Yield: 413 mg (0.72 mmol, 72%).
(m, 2H, C6H10 CH2); 1.84–1.98 (m, 2H, C6H10 CH2); 3.20–3.29
(m, 2H, C6H10 CHO); 3.18 (s, 6H, OCH3); 3.37–3.48 (m, 4H,
OCH2CH2O); 3.61–3.74 (m, 4H, OCH2CH2O) ppm. 13C{1H}
NMR (100 MHz, CDCl3): δ = 23.6 (C6H10 CH2); 30.3 (C6H10
CH2); 58.8 (OCH3); 69.3 (OCH2CH2O); 72.3 (OCH2CH2O);
82.1 (C6H10 CHO) ppm. 13C{1H} NMR (100 MHz, C6D6): δ =
23.7 (C6H10 CH2); 30.1 (C6H10 CH2); 59.1 (OCH3); 69.9
(OCH2CH2O); 73.2 (OCH2CH2O); 81.6 (C6H10 CHO) ppm.
EI-MS: m/z 233 (0.3) [M + H]+. Anal. Calcd for C12H24O4
(232.32 g mol−1): C 62.04, H 10.41. Found: C 61.50, H 9.92%.
1H NMR (400 MHz, C6D6): δ = 0.44 (s, 36 H, Si(CH3)3);
0.85–0.95 (m, 2H, C6H10 CH2); 1.00–1.10 (m, 2H, C6H10 CH2);
1.35–1.45 (m, 2H, C6H10 CH2); 2.07–2.16 (m, 2H, C6H10CH2);
3.07–3.15 (m, 2H, C6H10CHO); 3.11 (s, 6H, OCH3); 3.32–3.48
(m, 8H, OCH2CH2O) ppm. 13C{1H} NMR (100 MHz, C6D6):
δ = 7.0 (Si(CH3)3); 24.5 (C6H10 CH2); 30.5 (C6H10 CH2); 59.8
(OCH3); 65.1 (OCH2CH2O); 71.9 (OCH2CH2O); 81.5 (CHO)
ppm. Anal. Calcd for C24H60N2O4Si4Mg (577.39 g mol−1):
C 49.92, H 10.47, N 4.85. Found: C 48.41, H 11.57, N 5.69%.
Synthesis of enantiomerically pure (S,S)-5
Tosylation of 2-methoxyethanol. 2-Methoxyethanol (7.61 g,
0.1 mol) was dissolved in 50 mL of CH2Cl2 and 20 mL of pyri-
dine and cooled to 0 °C with an ice bath. 4-Methylbenzene-
1-sulfonyl chloride (20.97 g, 0.11 mol) in 100 mL of CH2Cl2
was added to the stirred mixture via a dropping funnel. Stirring
was maintained for 1 h at 0 °C and then overnight at ambient
temperature. The organic phase was washed with diluted NaOH
and dried over MgSO4 before the solvent was evaporated. The
remaining yellowish liquid was fractionally distilled under
vacuum (110 °C, 6 × 10−3 mbar). The product was isolated as a
clear, viscous liquid. Yield: 14.87 g (64.6 mmol, 65%).
Synthesis of [(rac-5)Mg(CH2SiMe3)2] (rac-7). [Mg(CH2SiMe3)2]
(128.3 mg, 0.65 mmol) was placed in a J. Young NMR tube and
rac-5 (150.0 mg, 0.65 mmol) was added dissolved in 0.5 mL of
C6D6. The volatiles were removed in vacuum and the remaining
solid was dissolved in pentane (1 mL) and kept at −30 °C to
afford rac-7 as colorless crystals. The mother liquor was
removed and the residue was dried under vacuum. Yield:
181 mg (0.42 mmol, 65%).
1H NMR (400 MHz, CDCl3): δ = 2.43 (s, 3H, p-CH3); 3.29
(s, 3H, OCH3); 3.55–3.575 (m, 2H, OCH2CH2O); 4.13–4.155
(m, 2H, OCH2CH2O); 7.31–7.35 (m, 2H, C6H4 CH); 7.77–7.81
(m, 2H, C6H4 CH) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ =
21.6 (p-CH3); 59.0 (OCH3); 69.0 (OCH2CH2O); 69.9
(OCH2CH2O); 127.9 (C6H4 CH); 129.8 (C6H4 CH); 133.0
(C6H4 CCH3); 144.8 (C6H4 CSO3) ppm. Anal. Calcd for
C10H14O4S (230.28 g mol−1): C 52.16, H 6.13. Found: C 52.01,
H 5.85%.
1H NMR (400 MHz, CDCl3): δ = −1.40 (s, 4 H, CH2Si-
(CH3)3); 0.43 (s, 18 H, CH2Si(CH3)3); 0.76–1.00 (m, 4H,
C6H10CH2); 1.35–1.48 (m, 2H, C6H10CH2); 1.82–1.95 (m, 2H,
C6H10CH2); 3.18–3.25 (m, 2H, OCH2CH2O); 3.21 (s, 6H,
OCH3); 3.32–3.43 (m, 4H, OCH2CH2O); 3.32–3.43 (m, 2H,
1
C6H10 CHO); 3.58–3.68 (m, 2H, OCH2CH2O) ppm. H NMR
(400 MHz, C6D6): δ = −1.42 (s, 4 H, CH2Si(CH3)3); 0.42 (s, 18
H, CH2Si(CH3)3); 0.61–0.77 (m, 2H, C6H10CH2); 0.80–0.97 (m,
2H, C6H10CH2); 1.16–1.30 (m, 2H, C6H10CH2); 1.68–1.82 (m,
2H, C6H10CH2); 3.05 (s, 6H, OCH3); 3.06–3.11 (m, 2H,
OCH2CH2O); 3.13–3.18 (m, 2H, OCH2CH2O); 3.19–3.26 (m,
2H, OCH2CH2O); 3.26–3.33 (m, 2H, C6H10CHO);
3.46–3.54 (m, 2H, OCH2CH2O) ppm. 13C{1H} NMR
(100 MHz, CDCl3): δ = −6.0 (CH2Si(CH3)3); 5.53 (CH2Si
(CH3)3); 23.9 (C6H10CH2); 29.2 (C6H10CH2); 59.2 (OCH3);
65.6 (OCH2CH2O); 71.0 (OCH2CH2O); 82.0 (CHO) ppm. Anal.
Calcd for C20H46O4Si2Mg (431.05 g mol−1): C 55.73, H 10.76.
Found: C 53.08, H 9.53%.
Synthesis of (S,S)-5 from (S,S)-trans-1,2-cyclohexanediol. To a
suspension of NaH (455 mg, 18.9 mmol) in 50 mL of THF stir-
ring at 0 °C, (S,S)-trans-1,2-cyclohexanediol (1.0 g, 8.61 mmol)
was added via a syringe in 40 mL of THF. After the gas evolu-
tion stopped, 15-crown-5 (3.8 g, 17.22 mmol) was added fol-
lowed by the addition of neat TsO(CH2)2OCH3 (4.36 g,
18.94 mmol). The mixture was stirred at 0 °C for an hour before
it was refluxed for an additional 20 h. The reaction was
quenched with brine and extracted with three portions of Et2O
(200 mL). The combined organic phases were dried over
MgSO4, filtered and the volatile material was removed under
vacuum leaving a yellowish liquid, which was distilled under
vacuum (65 °C, 5 × 10−4 mbar) to obtain a colorless liquid.
Yield: 985 mg (4.2 mmol, 49%).
Synthesis of [(rac-5)Ca{N(SiMe3)2}2] (rac-8) and [((S,S)-5)Ca-
{N(SiMe3)2}2] ((S,S)-8). [Ca{(NSiMe3)2}2(thf)2] (252.5 mg,
0.5 mmol) was dissolved in 2 mL of pentane and rac-5 or
(S,S)-5 (116.2 mg, 0.5 mmol) was added in 1 mL of pentane.
After one minute of stirring a colorless precipitate formed and
the mixture was kept at −30 °C overnight to complete the pre-
cipitation. The mother liquor was removed and the precipitate
was dried in vacuum to afford the complex (rac-8 or (S,S)-8) as
a colorless powder. Yield: rac-8: 257 mg (0.43 mmol, 87%);
(S,S)-8: 246 mg (0.41 mmol, 84%).
[α]2D4 = 28.075° in THF (c = 0.022 g mL−1). 1H NMR
(400 MHz, CDCl3): δ = 1.03–1.30 (m, 4H, C6H10 CH2);
1.49–1.69 (m, 2H, C6H10 CH2); 1.82–2.02 (m, 2H, C6H10 CH2);
3.08–3.21 (m, 2H, C6H10 CHO); 3.32 (s, 6H, OCH3); 3.39–3.56
(m, 4H, OCH2CH2O); 3.62–3.77 (m, 4H, OCH2CH2O) ppm.
13C{1H} NMR (100 MHz, CDCl3): δ = 23.6 (C6H10 CH2); 30.3
(C6H10 CH2); 58.8 (OCH3); 69.3 (OCH2CH2O); 72.3
(OCH2CH2O); 82.1 (C6H10 CHO) ppm. EI-MS: m/z 233 (0.3)
[M + H]+. Anal. Calcd for C12H24O4 (232.32 g mol−1): C 62.04,
H 10.41. Found: C 61.72, H 10.68%.
(S,S)-8: [α]2D4 = 14.503° in THF (c = 0.0096 g mL−1). 1H
NMR (400 MHz, C6D6): δ = 0.41 (s, 36 H, Si(CH3)3);
0.60–0.75 (m, 4H, C6H10CH2); 1.24–1.31 (m, 2H, C6H10CH2);
1.48–1.56 (m, 2H, C6H10CH2); 2.71–2.78 (m, 2H, OCH2CH2O);
12616 | Dalton Trans., 2012, 41, 12612–12618
This journal is © The Royal Society of Chemistry 2012

View Article Online
2.84–2.90 (m, 2H, OCH2CH2O); 3.23–3.28 (m, 2H, C6H10-
CHO); 3.33 (s, 6H, OCH3); 3.59–3.65 (m, 2H, OCH2CH2O);
3.87–3.94 (m, 2H, OCH2CH2O) ppm. 13C{1H} NMR
(100 MHz, C6D6): δ = 7.2 (Si(CH3)3); 23.5 (C6H10CH2); 27.7
(C6H10CH2); 61.2 (OCH3); 63.5 (OCH2CH2O); 68.6 (OCH2-
CH2O); 78.7 (CHO) ppm. Elemental analysis was carried out for
rac-8: Anal. Calcd for C24H60N2O4Si4Ca (593.16 g mol−1): C
48.60, H 10.20, N 4.72. Found: C 48.25, H 9.85, N 4.78%.
1H NMR (400 MHz, C6D6): δ = 0.41 (s, 36 H, Si(CH3)3);
0.57–0.72 (m, 4H, C6H10 CH2); 1.25–1.32 (m, 2H, C6H10CH2);
1.47–1.53 (m, 2H, C6H10CH2); 2.73–2.79 (m, 2H,
OCH2CH2O); 2.91–2.98 (m, 2H, OCH2CH2O); 3.14–3.19 (m,
2H, C6H10CHO); 3.32 (s, 6H, OCH3); 3.37–3.43 (m, 2H,
OCH2CH2O); 3.64–3.70 (m, 2H, OCH2CH2O) ppm. 13C{1H}
NMR (100 MHz, C6D6): δ = 6.7 (Si(CH3)3); 23.5 (C6H10CH2);
27.7 (C6H10CH2); 60.8 (OCH3); 63.2 (OCH2CH2O); 69.3
(OCH2CH2O); 78.4 (CHO) ppm. EI-MS: m/z 567 (0.4) [M −
SiMe3]+, 335 (10) [Sr(N(SiMe3)2)(NSiMe3)]+. Anal. Calcd for
C24H60N2O4Si4Sr (640.71 g mol−1): C 44.99, H 9.44, N 4.37.
Found: C 44.15, H 10.69, N 4.47%.
Synthesis of [(rac-5)Ca{N(SiHMe2)2}2] (rac-9). [Ca{(NSiH-
Me2)2}2(thf)] (188 mg, 0.5 mmol) was dissolved in 2 mL of
pentane and rac-5 (116.2 mg, 0.5 mmol) was added in 1 mL of
pentane under stirring. After adding a few drops of THF the
mixture was kept at −30 °C overnight which led to the formation
of block shaped colorless crystals. The mother liquor was
removed and the precipitate was dried in vacuum to afford rac-9
as a colorless powder. Yield: 161 mg (0.30 mmol, 60%).
Crystallography. Low temperature single crystal X-ray dif-
fraction experiments were performed with Mo-Kα radiation (λ =
0.71073 Å) INCOATEC microsource using ω scans and multi-
layer optics monochromator. Analysis of diffraction data col-
lected with the Bruker Apex II CCD was performed by using
SAINT+ within the SMART software package.29a Empirical
absorption corrections were applied to all data using
MULABS.29b The structures were solved by direct methods
using SIR-9229c and refined against F2 using all the reflections
with the SHELXL-9729d software within the graphical interface
WIN-GX.29e Crystals of (S,S)-8 were twinned by lattice mero-
hedry emulating the space group P43212 (no 96); the twinning
matrix 0 1 0 1 0 0 0 0 −1 was used and the refinement resulted
in a twin fraction of 0.49. Crystal data for (S,S)-8: C24H60-
CaN2O4Si4, Mr = 593.18, a = 11.5590(13) Å, b = 11.5507(13)
Å, c = 26.697(3) Å, U = 3564.4(7) Å3, P212121, Z = 4, 50 821
refln. measured, 9351 unique (Rint = 0.0669), R1 (I > 2σ(I)) =
0.0340, wR2 (all data) = 0.0648, GoF = 0.917, CCDC 884448;
for rac-10: C18H34CaO4, Mr = 354.53, a = 11.0925(10) Å, b =
13.1506(12) Å, c = 13.4576(12) Å, U = 1963.1(3) Å3, C2221,
Z = 4, 13 645 refln. measured, 2501 unique (Rint = 0.1070), R1 (I
> 2σ(I)) = 0.0625, wR2 (all data) = 0.1731, GoF = 1.135, CCDC
884449; for [Ca{N(SiHMe2)2}2(thf)]: C24H72Ca2N4O2Si8, Mr =
753.74, a = 18.1714(18) Å, b = 23.153(2) Å, c = 21.826(2) Å,
β = 97.8750(15)°, U = 9095.8(16) Å3, P21/n, Z = 8, 123 697
refln. measured, 22 744 unique (Rint = 0.0811), R1 (I > 2σ(I)) =
0.0366, wR2 (all data) = 0.0733, GoF = 0.0850, CCDC 884449;
for rac-9: C20H52CaN2O4Si4, Mr = 537.08, a = 9.321(2) Å, b =
16.216(4) Å, c = 21.088(5) Å, U = 3187.4(13) Å3, P212121,
Z = 4, 40 012 refln. measured, 7992 unique (Rint = 0.1201),
R1 (I > 2σ(I)) = 0.0907, wR2 (all data) = 0.2200, GoF = 1.097,
CCDC 884450.
1H NMR (400 MHz, C6D6): δ = 0.46 (d, 12 H, SiH-
(CH3)2,3JHH = 3.0 Hz); 0.48 (d, 12 H, SiH(CH3)2, 3JHH = 3.0 Hz);
0.64–0.73 (m, 4H, C6H10CH2); 1.21–1.29 (m, 2H, C6H10CH2);
1.51–1.60 (m, 2H, C6H10CH2); 2.75 (m, 2H, OCH2CH2O); 3.06
(m, 2H, OCH2CH2O); 3.29–3.36 (m, 2H, C6H10CHO);
3.29–3.36 (m, 2H, OCH2CH2O); 3.48 (s, 6H, OCH3); 3.75 (m,
3
2H, OCH2CH2O); 5.06 (sept, 4H, SiH(CH3)2, JHH = 3.0 Hz)
ppm. 13C{1H} NMR (100 MHz, C6D6): δ = 5.43 (Si(CH3)3);
5.46 (Si(CH3)3); 23.8 (C6H10CH2); 28.2 (C6H10CH2); 61.0
(OCH3); 65.1 (OCH2CH2O); 69.8 (OCH2CH2O); 80.6
(CHO) ppm. EI-MS: m/z 537 (0.01) [M]+, 404 (13) [M − N
(SiHMe2)2]+. Anal. Calcd for C20H52N2O4Si4Ca (537.06 g mol−1):
C 44.73, H 9.76, N 5.22. Found: C 42.44, H 12.34, N 4.83%.
Synthesis of [(rac-5)Ca(η3-C3H5)2] (rac-10). [Ca(η3-C3H5)2]
(50 mg, 0.414 mmol) was dissolved in 1.5 mL of THF and rac-5
(95 mg, 0.414 mmol) in 0.5 mL THF was added under stirring.
The mixture was filtered and stored at −30 °C which led to the
formation of a colorless precipitate after 12 h. The mother liquor
was removed; the precipitate was washed with pentane and dried
in a vacuum to afford rac-10 as a colorless solid. Yield: 83 mg
(0.23 mmol, 56%).
1H NMR (400 MHz, [D8]THF): δ = 1.15–1.35 (m, 4H, C6H10
CH2); 1.56–1.66 (m, 2H, C6H10 CH2); 1.87–1.98 (m, 2H,
3
C6H10CH2); 2.30 (d, 8H, C3H5CH(CH2)2, JHH = 12.0 Hz);
3.19–3.27 (m, 2H, C6H10CHO); 3.32 (s, 6H, OCH3); 3.40–3.52
(m, 4H, OCH2CH2O); 3.63–3.68 (m, 4H, OCH2CH2O); 6.25
3
(quint, 2H, C3H5 CH(CH2)2, JHH = 12.0 Hz) ppm. 13C{1H}
NMR (100 MHz, [D8]THF): δ = 24.0 (C6H10 CH2); 30.0 (C6H10
CH2); 57.4 (C3H5 CH(CH2)2); 59.3 (OCH3); 69.1 (OCH2-
CH2OCH3); 73.1 (OCH2CH2OCH3); 81.6 (C6H10 CHO); 147.7
(C3H5 CH(CH2)2) ppm. EI-MS: m/z 313 (0.01) [M − C3H5]+, 157
(20) [C9H17O2]+. Anal. Calcd for C18H34O4Ca (354.54 g mol−1):
C 60.98, H 9.67. Found: C 60.73, H 7.67%.
Polymerization procedure. A solution of a specified amount
of the initiator in 0.5 mL of toluene was added to a solution of
150 mg (1.04 mmol) of meso-lactide dissolved in 1.5 mL of
toluene. After the desired time the polymerization mixture was
quenched with moist hexanes and was added slowly to cool
hexanes with stirring. The polymer was filtered over a Büchner
funnel, washed with diethyl ether, and dried in vacuo.
Synthesis of [(rac-5)Sr{N(SiMe3)2}2] (rac-11). To a solution
of [Sr{(NSiMe3)2}2(thf)2/3] (276.3 mg, 0.5 mmol) in 4 mL of
pentane was added rac-5 (116.2 mg, 0.5 mmol) in 1 mL of
pentane. After one minute of stirring a colorless precipitate
formed and the mixture was kept at −30 °C overnight to com-
plete the precipitation. The mother liquor was removed and the
precipitate was dried in a vacuum to afford rac-11 a colorless
powder. Yield: 280 mg (0.44 mmol, 88%).
Acknowledgements
We thank the Fonds der Chemischen Industrie for financial
support and Uhde Inventa-Fischer for a gift of meso-lactide.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12612–12618 | 12617

View Article Online
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