Yttrium- and aluminum-bis(phenolate)pyridine complexes: Catalysts and model compounds of the intermediates for the stereoselective ring-opening polymerization of racemic lactide and β-butyrolactone

Article abstract of DOI:10.1021/om401047r

Yttrium and aluminum complexes of an original pyridine-bis(phenolate) ligand have been prepared. Reactions of {ONO^Me,Cumyl}Y[N(SiHMe ₂)₂](THF)(Et₂O) (1) with 1 equiv of methyl (R)-3-hydroxybutyrate and methyl (S,S)-lactyllactate afforded respectively {ONO^Me,Cumyl}Y((R)-OCH(CH₃)CH₂COOMe) (2) and {ONO^Me,Cumyl}Y((S,S)-OCH(CH₃)CO₂CH(CH ₃)CO₂Me) (3), which are rare models of the active species in the ring-opening polymerization (ROP) of β-butyrolactone (BBL) and lactide (LA), respectively. ¹³C NMR data for 2 and 3 indicate that, in solution, the carbonyl groups coordinate onto the yttrium centers, forming mononuclear species with six- and five-membered cycles, respectively. The aluminum compounds {ONO^Me,Cumyl}Al(iPr (S)-lactate) (5), {ONO ^Me,Cumyl}Al((R)-OCH(CH₃)CH₂COOCH₃) (6), and {ONO^Me,Cumyl}Al((rac)-OCH(CF₃)CH ₂CO₂C₂H₅) (7) were prepared analogously from the parent methyl complex {ONO^Me,Cumyl}AlMe (4). NMR data and the solid-state structure of 6 confirm the coordination of the carbonyl group. Yttrium compounds 1 and 2 are active initiators for the ROP of racemic LA and BBL. Their performances (activity, control of the molecular weights, tacticity) are much affected by the nature of the solvent or by the addition of just a few equivalents of pyridine. Optimal conditions are quite contrasted for the ROP of rac-LA and rac-BBL, highlighting fundamental differences between these two monomers. In the best cases, highly heterotactic PLAs (P_r up to 0.96) and syndiotactic-enriched PHB (P_r up to 0.86), with good control over the molecular weights, were obtained.

Full text of DOI:10.1021/om401047r

Article
pubs.acs.org/Organometallics
Yttrium− and Aluminum−Bis(phenolate)pyridine Complexes:
Catalysts and Model Compounds of the Intermediates for the
Stereoselective Ring-Opening Polymerization of Racemic Lactide and
β‑Butyrolactone
Joice S. Klitzke,†,‡ Thierry Roisnel,§ Evgeny Kirillov,*,† Osvaldo de L. Casagrande, Jr.,*,‡
and Jean-Franco̧
is Carpentier*,†
†Institut des Sciences Chimiques de Rennes, Organometallics: Materials and Catalysis Laboratories, UMR 6226 CNRS-Universite
Rennes 1, F-35042 Rennes Cedex, France
́
de
lves,
́
de Rennes 1, F-35042 Rennes
‡Instituto de Química, Laborator
9500, Porto Alegre, RS 90501-970, Brazil
́ ́
io de Catalise Molecular, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonca̧
§Institut des Sciences Chimiques de Rennes, Centre de Diffraction X, UMR 6226 CNRS-Universite
Cedex, France
S
* Supporting Information
ABSTRACT: Yttrium and aluminum complexes of an original
pyridine-bis(phenolate) ligand have been prepared. Reactions
of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1) with 1
equiv of methyl (R)-3-hydroxybutyrate and methyl (S,S)-
lactyllactate afforded respectively {ONOMe,Cumyl}Y((R)-OCH-
(CH3)CH2COOMe) (2) and {ONOMe,Cumyl}Y((S,S)-OCH-
(CH3)CO2CH(CH3)CO2Me) (3), which are rare models of
the active species in the ring-opening polymerization (ROP) of
β-butyrolactone (BBL) and lactide (LA), respectively. 13C
NMR data for 2 and 3 indicate that, in solution, the carbonyl groups coordinate onto the yttrium centers, forming mononuclear
species with six- and five-membered cycles, respectively. The aluminum compounds {ONOMe,Cumyl}Al(iPr (S)-lactate) (5),
{ONOMe,Cumyl}Al((R)-OCH(CH3)CH2COOCH3) (6), and {ONOMe,Cumyl}Al((rac)-OCH(CF3)CH2CO2C2H5) (7) were
prepared analogously from the parent methyl complex {ONOMe,Cumyl}AlMe (4). NMR data and the solid-state structure of 6
confirm the coordination of the carbonyl group. Yttrium compounds 1 and 2 are active initiators for the ROP of racemic LA and
BBL. Their performances (activity, control of the molecular weights, tacticity) are much affected by the nature of the solvent or
by the addition of just a few equivalents of pyridine. Optimal conditions are quite contrasted for the ROP of rac-LA and rac-BBL,
highlighting fundamental differences between these two monomers. In the best cases, highly heterotactic PLAs (Pr up to 0.96)
and syndiotactic-enriched PHB (Pr up to 0.86), with good control over the molecular weights, were obtained.
heterotactic polylactide (PLA) and syndiotactic polyhydroxy-
butyrate (PHB)).
INTRODUCTION
■
The stereoselective ring-opening polymerization (ROP) of
racemic cyclic esters such as lactide and β-butyrolactone (rac-
LA and rac-BBL, respectively) has attracted considerable
interest over the past decade, to access polyesters with
controlled microstructures.1 Among the variety of catalysts
that have been used for this purpose, of special note are
undoubtedly aluminum and rare earth compounds.1 Indeed,
some aluminum compounds, especially those based on salen,2
salan,3 or related {ONNO}2− ligand frameworks,4 do allow
accessing highly isotactic polylactides; these are of high interest
due to their possibility to form stereocomplexes with enhanced
thermal properties. On the other hand, some rare earth
compounds, with yttrium ones at the forefront,5 are highly
active and stereoselective initiators for the ROP of LA6 and
BBL7 (a quite reluctant monomer), enabling high levels of
racemic enchainments of these monomers (hence forming
Stereocontrol in those ROP reactions is accounted for by
enantiomorphic-site control (with chiral catalysts) or chain-end
control (usually with nonchiral catalysts) mechanisms.1
However, exact parameters that govern these stereocontrol
mechanisms, especially for chain-end control, and even the
nature of the catalytically active species involved in those
reactions, often remain unclear. In particular, little is under-
stood in the syndiotactic control during the ROP of rac-BBL
with yttrium complexes. Some of us have suggested that the
presence of remote aryl substituents on phenolate ligands of
some catalyst systems may play a significant role in this
stereocontrol, via weak attractive C−H···π interactions
Received: October 29, 2013
© XXXX American Chemical Society
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Chart 1. Examples of Group 3/13 Trivalent Metal−Lactate and −Lactyllactate Model Compounds
Scheme 1. Preparation of Proligand {ONOMe,Cumyl}H2
involving these aryl groups and methylenic moieties in the
growing polymer chain.7e However, such metal-β-alkoxy-ester
intermediates in the ROP of β-lactones remain thus far virtually
nonexistent.8 This contrasts with the ROP of LA, for which
several lactate and lactyllactate compounds of rare earths,9a,b
aluminum,9c−g gallium,9h,i and other metals (Zn, Sn, ...)10 have
been reported (Chart 1). Also, the subtle solvent effects in
these ROP reactions, with most often opposite trends between
LA and BBL, remain puzzling, nonrationalized observations.
In this work, we have prepared a series of well-defined Al and
Y complexes supported by a tridentate pyridine-bis(phenolate)
ligand having cumyl substituents. The latter cumyl substituents
have been selected since they proved earlier to have a
determining influence for achieving high stereoselectiv-
ities.6l,7a,c,e,11 Our aims were to assess possible interactions
between these substituents and α- or β-alkoxy ester moieties
coordinated onto the metal center. The latter α- or β-alkoxy
ester units have been selected to mimic the last monomer unit
inserted in the growing polymer chain in the ROP of lactide
and β-butyrolactone.
(see Figure S32 and Table S1 in the Supporting Information).
In the solid state, {ONOMe,Cumyl}H2 adopts a near (non-
crystallographic) C2 symmetry, in which the phenol groups are
twisted from the pyridine plane by ca. 23−24° in opposite
directions.
Synthesis of Yttrium Complexes Supported by the
{ONOMe,Cumyl 2−
Ligand. The amine elimination reaction
}
between the tris(amido) precursor [Y(N(SiHMe2)2)3(THF)]
and 1 equiv of the pro-ligand {ONOMe,Cumyl}H2 in diethyl ether
at room temperature afforded the amido complex
{ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1), with con-
comitant release of 2 equiv of bis(dimethylsilyl)amine (Scheme
2).
Single crystals of 1 suitable for an X-ray diffraction analysis
were obtained by recrystallization from a diethyl ether solution
at −30 °C. Crystallographic data are summarized in Table S1,
and important bond distances and angles are given in Figure 1.
In the solid state, 1 features a monomeric structure with the
RESULTS AND DISCUSSION
■
The diproteo proligand {ONOMe,Cumyl}H2 was efficiently
prepared following a route similar to that developed previously
for 2,6-bis(naphthol)pyridine proligands (Scheme 1).12 The
two-step protocol starts from the methoxymethyl-protected 2-
cumyl-4-methylphenol and involves (i) in situ generation of the
corresponding organozinc reagent and (ii) Negishi cross-
coupling of the latter intermediate with 2,6-dibromopyridine
using a Pd-SPhos catalyst. {ONOMe,Cumyl}H2 was recovered in
overall 64% yield as a colorless, crystalline compound readily
soluble in CHCl3, CH2Cl2, and THF, and in toluene (upon
heating at 80 °C). This compound was characterized by 1H and
13C NMR spectroscopy and an X-ray crystallographic study
yttrium center in a distorted octahedral geometry, six-
2−
coordinated by the {ONOMe,Cumyl
}
tridentate ligand, the
bis(dimethylsilyl)amido group, and THF and diethyl ether
B
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2−
Scheme 2. Synthesis of Yttrium Complexes Supported by the {ONOMe,Cumyl
}
Ligand
two oxygen donors of the 2,6-bis(phenolate)pyridine ligand are
in trans position. Because of the coordination onto the yttrium
center, the phenolate groups are orientated in the same
directions relative to the pyridine plane with a more
pronounced twisting (ca. 46° and 52°) than in the free ligand.
The Y−O(phenolate) (2.143(2) and 2.144(2) Å) and the Y−
N(silylamido) (2.268(3) Å) bond distances fall into the range
of distances observed in related amido-yttrium complexes
supported by tridentate bis(naphtholate)pyridine and tetra-
dentate amino-alkoxy-bis(phenolate) ligands.6a−f,j,7e The
Si(1)−N(3)−Si(2) angle of 122.63(16)°, which is just slightly
larger than for an ideal sp2 hybridization, and long, equivalent
Y···Si (3.428(1) and 3.416(1) Å) distances exclude a β(Si−
H)···Y agostic interaction in complex 1.13
Compound 1 is readily soluble in common organic solvents
(THF, toluene, benzene). The 1H and 13C NMR spectra of 1 in
toluene-d8 or C6D6 at room temperature all contain one set of
resonances indicative of the presence of one C2-symmetric
species, with one THF and one diethyl ether coordinated
molecule. The resonances for the latter amido, Et2O, and THF
moieties are broadened at room temperature. Upon heating,
those resonances sharpen, but concomitant dissociation of the
labile Et2O molecule then occurs (Figure S7). Compound 1
slowly transforms in toluene and benzene at room temperature
into another, yet unidentified bis(dimethylsilyl)amido species
(ca. 13% after 15 min at 25 °C) and even more so at higher
temperature (27% conversion after 4 h at 90 °C) (Figure S7).
In pyridine-d5, the THF and diethyl ether ligands are, as
expected, displaced from the yttrium coordination sphere, and
the resulting species remains intact, at least for several days, at
room temperature (see the Experimental Section). The Si−H
resonance in 1 (δ 4.80 ppm at 25 °C in C6D6) is only slightly
shifted upfield as compared to the corresponding resonance in
Y{N(SiHMe2)2}3(THF) (δ 4.99 ppm), which argues against a
strong β(Si−H) agostic interaction with the yttrium center in
hydrocarbon solutions;14 this is in line with the observations
made in the solid state.
Figure 1. ORTEP drawing of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)-
(Et2O) (1) (thermal ellipsoids drawn at the 50% probability level; all
hydrogen atoms are omitted for clarity). Selected bond distances (Å)
and angles (deg): Y(1)−O(11), 2.1427(19); Y(1)−O(51), 2.144(2);
Y(1)−O(61), 2.366(2); Y(1)−O(71), 2.358(2); Y(1)−N(2),
2.615(2); Y(1)−N(3), 2.268(3); Y(1)−Si(1), 3.4276(10); Y(1)−
Si(2), 3.4158(11); N(2)−Y(1)−N(3), 176.57(9); O(11)−Y(1)−
O(51), 153.77(7); O(71)−Y(1)−O(61), 162.75(9); O(11)−Y(1)−
N(2), 76.32(7); O(11)−Y(1)−N(3), 106.16(8); O(11)−Y(1)−
O(61), 90.60(8); O(11)−Y(1)−O(71), 85.44(8); O(51)−Y(1)−
N(2), 77.76(7); O(51)−Y(1)−N(3), 99.91(9); O(51)−Y(1)−
O(61), 88.43(8); O(51)−Y(1)−O(71), 87.77(8); O(71)−Y(1)−
N(2), 83.58(8); O(71)−Y(1)−N(3), 98.90(9); O(61)−Y(1)−N(2),
79.17(7); O(61)−Y(1)−N(3), 98.33(9); Si(2)−N(3)−Y(1),
118.58(14); Si(1)−N(3)−Y(1), 118.78(14), Si(2)−N(3)−Si(1),
122.63(16).
molecules. The latter THF and diethyl ether ligands are located
trans to each other and cis to the silylamido ligand, while the
C
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Scheme 3
Model compounds of putative intermediates involved in the
polymerization of lactide and β-butyrolactone were then sought
(Scheme 2). Attempts to generate an yttrium O-lactate complex
from the reaction of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)-
(Et2O) and 1 equiv of isopropyl (S)-lactate, under various
conditions, were unsuccessful; only intractable mixtures of
compounds were obtained.
NMR spectroscopy in THF-d8 at ambient temperature using a
freshly prepared sample (see the Experimental Section).
The 13C{1H} NMR spectra of 3 showed that the two signals
for the carbonyl groups appear at δ 190.0 (internal,
OCH(CH3)C(O)OCH(CH3)CO2Me) and 169.0 (terminal,
OCH(CH3)C(O)OCH(CH3)CO2Me) ppm. In comparison,
in the 13C{1H} NMR spectrum of the initial methyl (S,S)-
lactyllactate reagent, the carbonyl carbons appear at δ 175.1
(internal) and 170.0 (terminal) ppm. Thus, the carbonyl
carbon of complex 3 (δ 190.0 ppm) significantly shifted
downfield from the free reagent (δ 175.1 ppm), suggesting that
this internal carbonyl group is coordinated to the Y center,
forming a five-membered ring, as observed for a few Al
compounds bearing such a lactyllactate moiety (Chart 1;
Scheme 2).2h,9c,e,f
Gratifyingly, the β-alkoxy-butyrate yttrium derivative 2 was
prepared by treatment of 1 with 1 equiv of methyl (R)-3-
hydroxybutyrate in toluene or benzene at room temperature
and isolated in almost quantitative yield (Scheme 2). To our
knowledge, compound 2 is the first example of a well-defined
yttrium alkoxy-butyrate complex. It is soluble and quite stable
in aromatic hydrocarbons at room temperature. Despite
repeated efforts, no suitable crystal for X-ray diffraction analysis
could be obtained, and this compound was therefore
characterized by combustion analysis and solution NMR. The
Synthesis of Aluminum Complexes Supported by the
{ONOMe,Cumyl 2−
Ligand. To gain a better understanding of
}
1
resonances in the H and 13C{1H} NMR spectra of 2 in
the structure and reactivity of such putative intermediates
involved in the polymerization of lactide and β-butyrolactone,
we also prepared aluminum complexes supported by the
tridentate ligand {ONOMe,Cumyl}2−. Although aluminum com-
pounds typically show much lower reactivity than rare earth
compounds, they do indeed exhibit structural similarities.2−7
The methyl-aluminum complex 4 was synthesized selectively
by reaction of the pro-ligand {ONOMe,Cumyl}H2 with AlMe3 at
80 °C (Scheme 3). This product, isolated as colorless crystals in
77% yield, is soluble in CH2Cl2 at room temperature and in
aromatic hydrocarbons (benzene, toluene) at 60 °C, and its
toluene-d8 at room temperature are noticeably broadened,
reflecting the existence of a dynamic behavior in these
conditions. Decrease of the temperature to −15 °C allowed
freezing this dynamic process, resulting in a set of sharp
resonances, consistent with a single C1-symmetric species. The
asymmetric center in the β-alkoxy-butyrate moiety makes the
1
chelated {ONOMe,Cumyl} unit asymmetric. Thus, the H NMR
spectrum of 2 in toluene-d8 at −15 °C displays six singlets (δ
2.31, 2.27, 2.02, 1.99, 1.66, and 1.58 ppm) for the six
nonequivalent methyl groups in the {ONOMe,Cumyl} ligand unit.
The 13C NMR resonance for the carbonyl group in the methyl
(R)-β-alkoxy-butyrate moiety was observed at δ 180.1 ppm,
significantly shifted from the corresponding resonance in
methyl 3-hydroxybutyrate (δ 172.5 ppm). This is consistent
with coordination of the carbonyl group onto the yttrium
center and existence of a five-coordinate species (as established
for the parent Al compound; vide infra). In addition, the DOSY
NMR experiments, carried out in toluene-d8,15 allowed
determining the hydrodynamic radius of 2 (5.89 Å), which
suggests the latter complex to retain a mononuclear structure.
Also, an yttrium-O[methyl (S,S)-lactyllactate] compound
(3), which mimics the product of the first insertion of the LA
molecule, was generated on the NMR scale by treatment of 1
with 1 equiv of methyl (S,S)-lactyllactate in THF at room
temperature (Scheme 2). However, upon concentration of the
solution, compound 3 decomposes into a mixture of
unidentified species. This compound also slowly decomposes
in THF-d8 solution over a few hours at room temperature, to
yield complicated mixtures. We suspect that these decom-
position products result from transesterification reactions
within the lactyllactate moiety, but this is not clear yet. Due
to the instability of 3, it was characterized by multinuclear
1
identity was established on the basis of H and 13C NMR
spectroscopy (see the Experimental Section). The correspond-
ing O-lactate (5), β-alkoxy-butyrate (6), and 1,1,1-trifluoro β-
alkoxy-butyrate analogue (7) derivatives were prepared by
methane elimination from the reaction of 4 with 1 equiv of the
corresponding α- or β-hydroxy-ester at 80 °C in toluene
(Scheme 3). Complexes 5−7 were recovered as air-sensitive,
colorless solids in >60% yields. Complete purification of 6
proved difficult, but small amounts of analytically pure crystals
were successfully isolated from a concentrated CD2Cl2 solution
layered by hexane at room temperature. Complexes 5−7 were
characterized by solution NMR spectroscopy, combustion
analysis, and an X-ray diffraction study for 6.
1
The H and 13C{1H} NMR data for the aluminum(III) O-
(S)-lactate (5) and (R)- and (rac)-β-alkoxy-butyrate (6, 7)
complexes in toluene-d8 and CD2Cl2, at room temperature, are
indicative of the existence of a single C1-symmetric species (see
the Experimental Section). Of note, the 13C NMR resonance
for the carbonyl group in the O-(S)-lactate and fluorinated (R)-
β-alkoxy-butyrate moiety of 5 and 7 was observed at δ 189.5
and 178.9 ppm, respectively; these values are, as for yttrium
compounds 2 and 3, significantly shifted downfield from the
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Figure 2. ORTEP drawing of the two independent molecules observed in the unit cell of {ONOMe,Cumyl}Al((R)-OCH(Me)CH2CO2Me) (6)
(thermal ellipsoids drawn at the 50% probability level; all solvent molecules and hydrogen atoms are omitted for clarity). Selected bond distances
(Å) and angles (deg); on the top, with C−H···π interactions: Al−O(74), 1.745(2); Al−O(51), 1.757(2); Al−O(11), 1.762(2); Al−O(71), 2.006(2);
Al−N, 2.017(2); O(74)−Al−O(51), 118.16(12); O(74)−Al−O(11), 118.18(11); O(51)−Al−O(11), 122.96(11); O(71)−Al−N, 171.00(11);
O(51)−Al−O(71), 84.58(10); O(74)−Al−O(71), 93.40(10); O(11)−Al−O(71), 84.05(10); O(74)−Al−N, 95.44(10); O(51)−Al−N, 92.72(10);
O(11)−Al−N, 90.26(10). On the bottom, without C−H···π interactions: Al(2)−O(9), 1.743(2); Al(2)−O(8), 1.757(2); Al(2)−O(7), 1.771(2);
Al(2)−N(2), 1.996(3); Al(2)−O(10), 2.057(2); O(9)−Al(2)−O(8), 120.76(12); O(9)−Al(2)−O(7), 119.58(11); O(8)−Al(2)−O(7),
119.06(11); O(9)−Al(2)−N(2), 92.25(10); O(8)−Al(2)−N(2), 92.62(11); O(7)−Al(2)−N(2), 92.83(11); O(9)−Al(2)−O(10), 91.27(10);
O(8)−Al(2)−O(10), 85.07(10); O(7)−Al(2)−O(10), 85.95(10); N(2)−Al(2)−O(10), 176.44(10).
corresponding resonance in the free reagent (isopropyl lactate,
δ 175.0 ppm; methyl 3-hydroxybutyrate, δ 172.5 ppm). This is
again indicative of coordination of the carbonyl group onto the
aluminum center and is consistent with the existence of
mononuclear species in solution.
carbonyl oxygen atom with N−Al−O angles of 171.0(1)° and
176.4(1)°. The phenolate and alkoxide oxygen atoms occupy
the equatorial positions with O−Al−O angles ranging from
118.2(1)° to 123.0(1)°. The Al−O(carbonyl) bond distances
(Al−O(71), 2.006(2) Å; Al(2)−O(10), 2.057(2) Å) in 6 are
shorter than that (Al−O, 2.147(4) Å) observed in Lewinski’s
five-coordinated neutral complex9d and similar to that (Al−O,
2.018 Å) observed in Dagorne’s cationic Al complex,9f which
both contain a chelating lactate moiety; this trend likely reflects
lower steric constraints within the six-membered metallacycle
of 6, as compared to the five-membered metallacycle in the
The solid-state structure of 6 features a monomeric molecule
with an aluminum center in a slightly distorted trigonal
bipyramidal geometry (τ = 0.85−0.97),16 five-coordinated by
2−
the {ONOMe,Cumyl
}
ligand and the O,O-chelating (R)-β-
alkoxy-butyrate moiety (Figure 2 and Table S1). The axial
positions are occupied by the pyridine nitrogen atom and the
E
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latter two lactate compounds. These are expectedly much
shorter than those observed in Yb3+-β-alkoxy-butyrate com-
plexes (Yb−O, 2.382−2.482 Å),8 due to the much larger ionic
radius of the latter metal center (six-coordinate Yb3+ = 0.86 vs
five-coordinate Al3+ = 0.48 Å).17 In fact, two independent
molecules are observed in the crystal unit of 6, and, despite that
they share the same first coordination sphere described above,
they are quite different: in one, the phenyl groups of the two
cumyl substituents point in the direction of the (R)-β-alkoxy-
butyrate moiety, while in the second molecule those phenyl
groups point in the opposite direction with respect to the (R)-
β-alkoxy-butyrate group (Figure 2). Thus, in the first molecule,
short contacts (∼2.6−2.8 Å) between the phenyl π-system and
one methylene C−H of the butyrate moiety are observed. This
is reminiscent of similar attractive C−H···π interactions18 in
yttrium complexes supported by tetradentate amino-alkoxy-
bis(phenolate) with aryl-containing ortho-phenolate substitu-
ents and their possible beneficial involvement (as compared to
systems with no aryl ortho substituents) in the syndioselective
ROP of racemic β-butyrolactone.7e Yet, the observation of
another molecule in the unit cell of 6 with no such C−H···π
interactions indicates that their strength, if any, is quite low, at
least in these aluminum complexes.19
the OMe group of the β-alkoxy-butyrate moiety to phenyl
hydrogens of the cumyl substituents on the NMR time scale.
Ring-Opening Polymerization of rac-Lactide and rac-
β-Butyrolactone with Yttrium Pyridine-Bis(phenolate)
Complexes. The performance of the new yttrium pyridine-
bis(phenolate) complexes 1 and 2 was explored in the ring-
opening polymerization of racemic lactide and β-butyrolactone
(Scheme 5). These two compounds have potentially active
Scheme 5
nucleophilic groups (i.e., amido and β-alkoxy-butyrate,
respectively), and β-alkoxy-butyrate complex 2 is assumed to
be an early intermediate in the ROP of β-butyrolactone or,
more exactly, to mimic the active species with a growing PHB
chain in such a process. Representative results are summarized
in Table 1.
In a previous study,7e we have demonstrated by means of
DFT computations that, in yttrium amino-alkoxy-bis-
(phenolate) complexes, C−H···π interactions between methyl-
enic hydrogens of the alkoxy-butyrate moieties and certain
carbon atoms (typically, ortho and meta) of phenyl rings of the
cumyl substituents can be remarkably stabilizing (by 5−12 kcal·
mol−1). Herein, we have used the same computational
approach to assess the stabilization ability of cumyl groups in
complexes 2 and 6. Thus, the molecular structures of yttrium
and aluminum complexes adopting both geometries observed
for 6 in the solid state (Figure 2, Scheme 4) were optimized
Both 1 and 2 are active in the ROP of rac-LA at ambient
temperature. The activity and degree of control are quite
dependent on the nature of the solvent. Complex 1 is
significantly more active in THF solution than in toluene or
in pyridine; polymerizations of 1000 equiv of rac-LA in THF
reached 70% conversion within only 15 min (entry 7). Yet,
complex 2 proved to be even more active than 1 in toluene
solution (compare entry 1 vs entry 9) but less active than 1 in
THF solution (compare entry 5 vs entry 10). In toluene, we
assume that this difference in apparent catalytic activities
reflects the greater nucleophilicity and eventual higher initiating
efficiency of the β-alkoxy-butyrate group as compared to the
amido; narrower polydispersities in the former case (Mw/Mn =
1.3−1.5 for 2 vs 1.7−2.0 for 1) are consistent with this
hypothesis. Actually, a much narrower polydispersity (Mw/Mn =
1.14) was observed when 2-propanol was added to 1 to act as a
co-initiator (entry 8).20
Scheme 4. Model Geometries Computed to Assess Possible
C−H···π Interactions in 2 and 6
The number-average molecular weight values of PLAs
produced with 1, as determined by GPC (Mn,GPC), were
systematically higher than the calculated Mn,calc values,
indicating an incomplete initiation efficiency. Nonetheless, the
Mn,GPC values varied quite linearly with the monomer
conversions for different monomer loadings (compare entries
5−7). The molecular weight distributions, although unimodal,
were all rather broad. These observations are probably
connected to the above-mentioned instability of 1 in toluene
and in THF solutions. Indeed, when reactions were performed
in pyridine, a solvent in which 1 is stable, a narrower
polydispersity was observed (entry 4).
On the other hand, in the polymerizations of rac-LA
mediated by complex 2, the experimental Mn,GPC values of
the PLAs were in quite good agreement with the calculated
ones and those determined by NMR (Mn,NMR), taking into
account the β-alkoxy-butyrate and 2-hydroxy-ethyl polymer
end-groups (Figures 3 and S26; see also Figure S27 for MS
analysis); also, the polydispersities were narrower (Mw/Mn =
1.3−1.5).
(see the Experimental Section for details). Unexpectedly, the
only close contacts observed in the computed structures
involved hydrogens of the OMe group of the alkoxy-butyrate
moiety and the meta-carbon atoms of phenyl rings of the cumyl
substituents (2.74−2.77 Å) (see optimized geometries 2-I and
6-I). However, only insignificant differences were found
between the total electron energy values calculated for
geometries I and II of complexes 2 and 6 (0.3−0.7 kcal·
mol−1), which argues against strong stabilizing interactions in
these molecules. This is in line with the observation of two
conformations in the solid-state structure of 6.
Interestingly though, the NOESY data obtained for complex
2 in toluene-d8 at 253 K (Figure S10; see the Experimental
Section) are consistent with spatial proximity of hydrogens of
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a
Table 1. ROP of rac-Lactide and rac-β-Butyrolactone Initiated by Complexes 1 and 2
b
c
d
e
d
f
entry complex monomer [M]0/[Y]0
solvent
time (min) conv (%) Mn,calc (kDa) Mn,GPC (kDa) Mn,NMR (kDa) Mw/Mn
Pr
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
2
2
2
1
1
1
1
1
1
1
1
2
2
2
2
2
rac-LA
100
500
100
100
100
500
1000
100
100
100
500
100
250
500
100
100
200
100
500
100
100
250
250
500
toluene
toluene
toluene
pyridine
THF
10
50
41
67
55
90
94
75
70
64
87
94
65
43
39
25
82
100
100
89
62
35
81
28
93
81
5.9
48.2
7.9
10.9
60.6
9.0
nd
1.91
1.88
1.94
1.47
1.98
2.02
1.75
1.14
1.49
1.38
1.29
1.15
1.43
2.10
1.11
1.28
2.40
nd
0.60
0.60
0.79
0.77
0.96
0.96
0.94
0.87
0.55
0.84
0.88
0.81
nd
rac-LA
nd
g
rac-LA
13
nd
rac-LA
30
13.0
13.5
54.0
100.8
9.2
18.3
30.6
79.0
165.9
4.2
nd
rac-LA
6
nd
rac-LA
THF
10
nd
rac-LA
THF
15
nd
h
rac-LA
THF
13
6.4
13.9
15.8
nd
rac-LA
toluene
THF
10
12.5
13.5
46.8
3.7
9.4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
rac-LA
90
8.8
rac-LA
THF
330
45
25.1
20.1
44.3
41.7
23.4
27.3
28.1
nd
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
rac-BBL
toluene
toluene
toluene
pyridine
pyridine
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
nd
90
8.4
nd
230
10
10.7
7.0
nd
nd
nd
nd
130
1
8.6
nd
0.59
0.86
nd
g
17.2
7.6
nd
h
h
45
8.1
nd
230
45
26.7
3.0
27.6
6.4
1.07
1.05
1.13
1.05
1.10
1.15
0.81
0.80
0.80
nd
3.2
11.5
7.5
26.5
40.2
i
1080
90
7.0
16.1
11.0
35.0
66.0
6.0
i
i
1080
1080
20.0
34.8
nd
nd
a
General conditions: reactions performed at [rac-LA]0 = 2.0 mol·L−1 or [rac-BBL]0 = 3.0 mol·L−1, at room temperature; the reaction time was not
b
c
necessarily optimized. Conversion of monomer as determined by 1H NMR on the crude reaction mixture. Calculated Mn values considering one
polymer chain per Y center. Experimental Mn and Mw/Mn values determined by GPC in THF vs PS standards; Mn values are corrected with a 0.58
factor for PLAs and are uncorrected for PHBs. Experimental Mn values determined by H NMR analysis of the reprecipitated polymer. Pr is the
probability of racemic linkage, as determined by H NMR homodecoupled experiments. The polymerization was carried out in the presence of 5
equiv of pyridine vs Y. The polymerization was carried out in the presence of 1 equiv of iPrOH vs Y. The reaction mixture turned rapidly highly
viscous, hampering effective stirring.
d
e
f
1
g
1
h
i
1
Figure 3. H NMR spectrum (500 MHz, CDCl3, 298 K) of a PLA produced from {ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2) (Table 1,
entry 10).
1
Homodecoupled H NMR spectra showed that some of the
PLAs formed with these systems had highly heterotactic
microstructures (Table 1 and Figure S29). As we observed with
tetradentate amino-alkoxy-bis(phenolato)lanthanide6b and lan-
thanide-amido complexes supported by tridentate bis(ortho-
silyl-substituted naphtholate)pyridine/thiophene ligands,6j the
stereoselectivity strongly depended on the nature of the
solvent: all the PLAs produced in toluene with 1 or 2 had
poorly heterotactic microstructures (probability of racemic
linkage, Pr = 0.55−0.60), whereas those obtained in THF with
complex 1 had highly heterotactic microstructures (Pr = 0.94−
0.96; Figure 4); these are improved stereoselectivities over
those achieved under identical conditions with {ONOSiR3}La-
[N(SiHMe2)2](THF) compounds (Pr = 0.84−0.90).6j Surpris-
ingly yet, the PLAs produced in THF with alkoxy-butyrate
complex 2 were less heterotactic (Pr = 0.86−0.88). The
G
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1
1
Figure 4. Methine region of the H NMR and H homodecoupled NMR spectra of a PLA obtained in THF with complex 1 (Table 1, entry 6).
1
Figure 5. H NMR spectrum (500 MHz, CDCl3, 298 K) of a PHB produced from {ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2) (Table 1,
entry 21).
presence of pyridine, either as a few equivalents added to 1 in
THF solution (entry 3) or as solvent (entry 4), afforded PLAs
with intermediate heterotacticities (Pr = 0.77−0.79).21
higher than that in neat pyridine (entry 15). This suggests that
a few equivalents of pyridine are needed to enhance the
intrinsic reactivity of the propagating species but, most likely,
that excess pyridine is detrimental because its coordination on
the metal center then overwhelms that of rac-BBL. Spassky and
co-workers have previously noted that addition of 1 equiv of
pyridine to an aluminum-alkoxide initiator used in the ROP of
rac-LA, besides limiting transesterification reactions,23 also
greatly increases the rate of polymerization.24 One can
speculate here that coordination of pyridine to the yttrium
center may enhance, e.g., via its trans influence, the
nucleophilicity of the propagating alkoxide. Compounds 1
and 2 grossly featured the same activity in a given solvent
(compare, e.g., entries 12 vs 20 and 13 vs 22).
The degree of control over the molar masses proved,
however, better with 2. With this compound as initiator, the
Mn,NMR values determined by 1H NMR from the PHB β-alkoxy-
butyrate end-groups (Figures 5 and S26) were in close
agreement with the calculated ones. The Mn,GPC values
(uncorrected) for PHBs produced with 1 were, for a given
Similarly, the ROP of rac-BBL promoted by 1 and 2 allowed
the formation of poorly to highly syndiotactic PHBs, with Pr
values of ca. 0.60 for PHBs produced in pyridine and ca. 0.80
for those produced in toluene (Figure S30, entries 12−24).
Such superiority of toluene as solvent for stereocontrol in rac-
BBL polymerizations was previously noted.6j,7a−c,e Unexpect-
edly enough, when 5 equiv of pyridine was added to 1 in
toluene, an increased Pr value of ca. 0.86 was observed (entry
17), while a value intermediate between those in neat pyridine
and in neat toluene may have been anticipated. Those
observations highlight how elusive these solvent effects are
and how difficult it is to rationalize them at this moment.19
As reflected in entries 12 and 15, polymerization of rac-BBL
with 1 proceeded faster in pyridine than in toluene. Yet, in line
with the above comments, rac-BBL was best polymerized with
1 in the presence of 1−5 equiv of pyridine (entry 17);22 the
apparent activity of this system is at least 1 order of magnitude
H
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Figure 6. MALDI-ToF mass spectrum of a PHB sample produced from 2 using IAA as matrix (Table 1, entry 20). The two observed distributions
correspond to a single distribution of macromolecules, i.e., MeOC(O)CH2CH(CH3)O−(BBL)n−H, ionized by Na+ and K+, respectively. Calculated
m/z for n = 44 − H + Na+ = 3928.05 Da; K+ = 3944.1 Da.
number of rac-BBL equivalents converted, about 2−3 times
larger than those of PHBs produced with 2. This indicates
imperfect initiation efficiency with amido complex 1, in line
with that noted in the ROP of rac-LA (vide supra); in contrast,
excellent initiation efficiency was observed with β-alkoxy-
butyrate complex 2, as also attested by MALDI-TOF mass
spectrometry (Figures 6 and S30). In most cases, the molecular
weight distributions of the PHBs were unimodal and much
narrower (Mw/Mn = 1.05−1.15) than those observed for PLAs
(Table 9). The main exception to this trend appeared with the
catalyst system made of 1 and 5 equiv of pyridine (Mw/Mn =
2.4, entry 17).
The aluminum-O-lactate complex (S)-5 also acts as a single-
site initiator in the ROP of rac-lactide. As well-established for
aluminum complexes, its reactivity proved much lower than
that of parent group 3 metal complexes, and significant
polymerization rates were observed only from 80 °C. Despite a
fairly good control over the molecular weights (e.g., in a typical
experiment, [rac-LA]0/[(S)-5] = 100, T = 80 °C, t = 17 h; conv
= 20%; Mn,calc = 2,880 g·mol−1; Mn,GPC(corr) = 2740 g·mol−1;
Mw/Mn = 1.15), only atactic polymers (Pr = 0.5) were
recovered.
equivalents of pyridine, but surprisingly enough, this effect was
not noted in the ROP of rac-LA.
Overall, these results confirm the valuable contribution of
ortho-cumyl substituents in tri/tetradentate bis(phenolate)
ligands, when associated to group 3 metal centers, especially
yttrium, for achieving the hetero/syndiotacticity in ROP of rac-
LA and rac-BBL. The β-alkoxy-butyrate yttrium 2 and
aluminum 6 and 7 complexes constitute very rare examples
of models of putative active species involved in the ROP of
BBL. The crystal structure of 2 could not be established to
gauge the pertinence of C−H···π interactions that have been
proposed to account for a part of the high syndiotacticity in the
ROP of rac-BBL.7a,c,e Interestingly, such interactions appear at
best quite weak in the β-alkoxy-butyrate aluminum complex 6,
as revealed by an X-ray diffraction study and confirmed by DFT
computations.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under a purified argon atmosphere using standard Schlenk techniques
or in a glovebox. Solvents were distilled from Na/benzophenone
(THF, Et2O) and Na/K alloy (toluene, pentane, hexanes) under
argon, degassed thoroughly, and stored under argon prior to use.
Deuterated solvents (>99.5% D, Eurisotop) were vacuum-transferred
from Na/K alloy (benzene-d6, toluene-d8) or from CaH2 (CD2Cl2)
into storage tubes. Pyridine-d5 was distilled under argon and kept over
activated 4 Å molecular sieves. Isopropyl alcohol (Acros) was distilled
over Mg turnings under argon atmosphere and kept over activated 4 Å
molecular sieves. Y[N(SiHMe2)2]3(THF)13 was prepared using
reported procedures. rac-Lactide (rac-LA) was received from Acros;
its purification required a three-step procedure involving first a
recrystallization from a hot, concentrated iPrOH solution (80 °C),
followed by two subsequent recrystallizations in hot toluene (100 °C).
After purification, rac-LA was stored at a temperature of −30 °C in the
glovebox. Racemic β-butyrolactone (rac-BBL, Aldrich) was freshly
distilled from CaH2 under nitrogen and degassed thoroughly by
freeze−vacuum−thaw cycles prior to use. Other starting materials and
reagents were purchased from Acros, Strem, Aldrich, and Alpha Aesar
and used as received.
CONCLUSIONS
■
The amido and β-alkoxy-butyrate yttrium complexes 1 and 2
supported by the new tridentate bis(ortho-cumyl-substituted
phenolate)pyridine ligand feature high performance in the
stereoselective ROP of rac-LA and rac-BBL. The heterotacticity
values observed in the ROP of rac-LA (Pr = 0.94−0.96) are
noticeably higher than those achieved under identical
conditions with {ONOSiR3}Y[N(SiHMe2)2](THF) compounds
(Pr = 0.84−0.90; SiR3 = SiPh3 or SitBuMe2).6j The level of
syndiotacticity in the ROP of rac-BBL is similar for these two
classes of compounds (Pr = 0.80−0.86; o-cumyl or o-SiPh3).
The activity and stereoselectivity of 1 in toluene toward rac-
BBL were significantly enhanced upon addition of a few
I
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NMR spectra of complexes were recorded on Bruker AC-300,
The resulting solid was dissolved in a mixture of 36% aqueous HCl (20
mL), CHCl3 (30 mL), and EtOH (40 mL), and the solution was
refluxed for 24 h. The reaction mixture was cooled to 0 °C and
carefully reacted with a concentrated solution of NaOH (30 mL).
Then, a concentrated solution of NH4Cl was added to adjust the pH
value to 7−8. The product was extracted with CH2Cl2 (3 × 20 mL),
and the combined organic extracts were dried over MgSO4 and finally
evaporated to afford {ONOMe,Cumyl}H2 as a white solid (5.80 g, 97%
Avance DRX 400, and AM-500 spectrometers in Teflon-valved NMR
1
tubes at 25 °C unless otherwise indicated. H and 13C chemical shifts
are reported in ppm vs SiMe4 and were determined by reference to the
residual solvent peaks. 19F NMR chemical shifts were determined by
external reference to an aqueous solution of NaBF4. Assignment of
1
resonances for complexes was made from 2D H−13C HMQC and
HMBC NMR experiments. Elemental analyses (C, H, N) were
performed using a Flash EA1112 CHNS Thermo Electron apparatus
and are the average of two independent determinations.
1
second step, 64% overall). H NMR (500 MHz, CDCl3, 298 K): δ
9.80 (br s, 2H, OH), 7.82 (t, 3J = 7.8 Hz, 1H, Haro), 7.55 (d, 3J = 7.65
Hz, 2H, Haro), 7.38−7.22 (m, 14H, Haro), 2.44 (s, 6H, CH3), 1.78 (s,
12H, CH3). 13C{1H} NMR (125 MHz, CDCl3, 298 K): δ 156.53,
152.53, 150.71, 139.03, 137.22, 129.65, 128.04, 127.91, 127.07, 125.73,
125.43, 122.42, 120.11 (Caro), 42.28 (C(CH3)2), 29.77 (C(CH3)2),
21.17 (CH3). Anal. Calcd for C37H37NO2: C, 84.21; H, 7.07; N, 2.65.
Found: C, 84.15; H, 7.21; N, 2.58.
Molecular weights of PLAs were determined by size exclusion
chromatography in THF at 30 °C (flow rate = 1.0 mL·min−1) on a
Polymer Laboratories PL50 apparatus equipped with a refractive index
detector and two ResiPore 300 × 7.5 mm columns. The polymer
samples were dissolved in THF (2 mg·mL−1). The number average
molecular masses (Mn) and polydispersity indexes (Mw/Mn) of the
polymers were calculated with reference to a universal calibration vs
polystyrene standards. The Mn values of PLAs were corrected with a
Mark−Houwink factor of 0.58, to account for the difference in
hydrodynamic volumes between polystyrene and polylactide.25
The microstructure of PLAs was determined by homodecoupling
1H NMR spectroscopy at 25 °C in CDCl3, with a Bruker AC-500
spectrometer. The microstructure of PHBs was determined by
analyzing the carbonyl and methylene regions of 13C{1H} NMR
spectra at 40 °C in CDCl3 with a Bruker AC-500 operating at 125
MHz, using ca. 0.2 mol·L−1 solutions of PHB (reprecipitated with
pentane from a dichloromethane solution).7c
Chart 2. Numbering Scheme of the Bis(phenolate)pyridine
(Pro)ligand
MALDI-ToF mass spectra were recorded at the CESAMO
(Bordeaux, France) on a Voyager mass spectrometer (Applied
Biosystems) equipped with a pulsed N2 laser source (337 nm) and
a time-delayed extracted ion source. Spectra were recorded in the
positive-ion mode using the reflectron mode and with an accelerating
voltage of 20 kV. A THF solution (1 mL) of the matrix (trans-3-
indoleacrylic acid, IAA (Aldrich, 99%); 10 mg.mL−1) and possibly a
MeOH solution of the cationization agent (NaI, 10 mg·mL−1) were
prepared. A fresh solution of the polymer samples in THF (10 mg·
mL−1) was then prepared. The two (or three) solutions were then
rapidly combined in a 1:1(:10) volume ratio of matrix-to-sample-to-
cationization agent. Then 1−2 μL of the resulting solution was
deposited onto the sample target and vacuum-dried.
{ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1). A Schlenk flask was
charged with {ONOMe,Cumyl}H2 (331 mg, 0.63 mmol) and Y[N-
(SiHMe2)2]3(THF) (350 mg, 0.63 mmol), and diethyl ether (15 mL)
was vacuum transferred in. The reaction mixture was gently warmed to
room temperature and stirred overnight. After filtration, the clear
solution was concentrated and placed at −30 °C, which eventually
afforded 1 as yellow crystals (130 mg, 23%). Anal. Calcd for
C49H67N2O4Si2Y: C, 65.89; H, 7.56; N, 3.14. Found: C, 65.73; H, 7.62;
N, 3.05. 1H NMR (500 MHz, toluene-d8, 298 K): δ 7.37−7.35 (m, 6H,
3
1-(Methoxymethyl)-4-methyl-2-(2-phenylpropan-2-yl)benzene.
To a suspension of NaH (3.20 g, 133 mmol) in DMF (100 mL),
under argon flow, was added dropwise a solution of 4-methyl-2-(2-
phenylpropan-2-yl)phenol26 (20.0 g, 88.4 mmol) in DMF (100 mL) at
0 °C. After stirring for 4 h at room temperature, methoxymethyl
chloride (13.64 g, 141 mmol) was added slowly, and the mixture was
stirred for 18 h. The reaction mixture was carefully diluted with water
(ca. 1 L), and organic materials were extracted with CH2Cl2 (3 × 100
mL). The combined organic extracts were washed with water (2 × 500
mL) and brine and dried over MgSO4. The solution was evaporated to
dryness at 80 °C to give a colorless, oily product (21.0 g, 88%), which
Haro), 7.09−6.89 (m, 11H, Haro), 4.68 (br s, 2H, Si-H), 3.22 (q, J =
7.0 Hz, 4H, Et2O), 3.15 (br m, α-CH2, 4H, THF), 2.33 (s, 6H, Me),
1.93 (s, 12H, Me), 1.06 (br s, 6H, Et2O), 0.99 (br m, β-CH2, 4H,
THF), 0.26 (br s, 12H, SiHMe2). Note that this compound is,
however, not very stable in toluene-d8 and slowly transforms into
another, yet unidentified species at room temperature (ca. 13% after
15 min at 25 °C) and even more so at higher temperature (27%
conversion after 4 h at 90 °C). Characteristic signals for the
decomposition species formed: (500 MHz, toluene-d8, 298 K) δ
7.59 (2H, aro), 7.41 (2H, Haro), 7.28 (m, 2H, Haro), 7.17 (m, 2H,
3
Haro), 7.11 (m, 2H, Haro), 6.85 (m, 1H, Haro), 6.81 (d, J = 1.85 Hz,
1H, Haro), 6.76 (d, 3J = 2.3 Hz, 1H, Haro), 6.47 (dd, 3J = 2.4 Hz and 4J
= 0.5 Hz, 1H, Haro), 6.39 (dd, 3J = 6.6 Hz, 4J = 2.1 Hz, 1H, Haro), 4.77
(m, 2H, Si-H), 2.42 (s, 3H, Me), 2.33 (s, 3H, Me), 1.96 (s, 3H, Me),
1.90 (s, 3H, Me), 1.79 (s, 3H, Me), 0.04 (d, 3J = 3.0 Hz, 6H, SiHMe2),
−0.08 (d, 3J = 3.0 Hz, 6H, SiHMe2). Compound 1 is much more stable
in pyridine-d5, in which it remains intact, at least for several hours at
1
was used without further purification. H NMR (300 MHz, CDCl3,
298 K): δ 7.34 (s, 1H, Haro), 7.22 (br s, 4H, Haro), 7.13−6.91 (m, 3H,
Haro), 4.56 (s, 2H, OCH2OCH3), 3.00 (s, 3H, OCH2OCH3), 2.40 (s,
3H, CH3), 1.72 (s, 6H, CH3).
{ONOMe,Cumyl}H2. A solution of sec-BuLi (30.0 mL of a 1.3 M
solution in hexane, 39.0 mmol) was added dropwise over 20 min to a
stirred solution of 1-(methoxymethyl)-4-methyl-2-(2-phenylpropan-2-
yl)benzene (9.50 g, 35.1 mmol) in THF (100 mL) at −78 °C. After
stirring overnight at room temperature, a dark solution was obtained,
to which was added a solution of ZnCl2 (4.79 g, 35.1 mmol) in THF
(ca. 30 mL) at −78 °C over 30 min. Then, the resulting solution was
gently warmed to room temperature. To the resulting organozinc
solution was added a suspension of Pd2(dba)3 (0.16 g, 0.35 mmol), S-
Phos (0.326 g, 0.70 mmol), and 2,6-dibromopyridine (4.16 g, 17.6
mmol) in THF (10 mL). The reaction mixture was stirred for 72 h at
64 °C, then cooled to room temperature, diluted with water (200 mL),
and finally extracted with CH2Cl2 (3 × 50 mL). After evaporation to
dryness of the combined organic extracts, the crude material was
recrystallized in ethyl acetate to give a white powder (7.00 g, 65%).
1
room temperature. H NMR (500 MHz, pyridine-d5, 298 K): δ 7.63
(d,3J = 7.9 Hz, 4H, H o-cumyl), 7.48 (s, 2H, H4), 7.37−7.29 (m, 5H, H
m-cumyl + H3p), 7.18−7.16 (m, 2H, H p-cumyl), 7.06 (d,3J = 7.9 Hz,
2H, H2p), 6.9 (s, 2H, H6), 4.77 (br m, 2H, Si-H), 3.64 (br m, α-CH2,
3
4H, free THF), 3.35 (q, J = 7.0 Hz, 4H, free Et2O), 2.35 (s, 6H,
CH3), 2.13 (s, 12H, CH3 cumyl), 1.60 (br m, β-CH2, 4H, free THF),
1.11 (t, 3J = 7.0 Hz, 4H, free Et2O), 0.20 (d,3J = 2.8 Hz, 12H, SiHMe2).
13C{1H} NMR (125 MHz, pyridine-d5, 298 K): δ 161.74 (C−N),
158.79 (C−O), 158.77 (C−O), 153.17, 138.13, 137.51, 131.84, 130.11
(C6), 129.09 (C4), 128.11, 126.77, 125.06, 124.12, 67.84 (α-CH2, free
THF), 65.79 (OCH2, free Et2O), 43.19 (C(CH3)2), 31.34 (C(CH3)2),
25.82 (β-CH2, free THF), 21.05 (CH3), 15.52 (CH3, free Et2O), 4.15
(SiHMe2).
J
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Organometallics
Article
{ONOMe,Cumyl}Y((R)-OCH(CH3)CH2COOMe) (2). In the glovebox,
methyl (R)-3-hydroxybutyrate (1.4 μL, 13 μmol) was added to a
solution of {ONOMe,Cumyl}Y[N(SiHMe2)2](THF)(Et2O) (1, 11.6 mg,
13 μmol) in C6D6 (ca. 0.7 mL). After 10 min, the reaction was
complete, as evidenced by H NMR. Evaporation of volatiles under
vacuum left 2 as a pale yellow solid in quantitative yield (6.35 mg,
42.37 (C(CH3)2, 30.49 (C(CH3)2), 28.51 (C(CH3)2), 21.19 (CH3),
−17.25 (Al-CH3)).
{ONOMe,Cumyl}Al(iPr (S)-lactate) (5). A Schlenk flask was charged
with {ONOMe,Cumyl}AlMe (4, 0.114 g, 0.200 mmol) and toluene (10
mL). Then, in the glovebox, isopropyl (S)-2-lactate (27 μL, 0.202
mmol) was microsyringed in. The reaction mixture was then warmed
to 80 °C and stirred overnight. Evaporation of volatiles under vacuum
left 5 as a pale yellow powder in essentially quantitative yield (0.136 g,
99%). Anal. Calcd for C43H46AlNO5: C, 75.53; H, 6.78; N, 2.05.
1
99%). Anal. Calcd for C42H44NO5Y: C, 68.94; H, 6.06; N, 1.91.
1
Found: C, 68.6; H, 6.3; N, 2.1. H NMR (500 MHz, toluene-d8, 258
3
3
K): δ 7.48 (d, J = 7.6 Hz, 2H, Haro), 7.35 (d, J = 7.6 Hz, 2H, Haro),
7.20−6.93 (m, 13H, Haro), 2.96 (br m, 1H, OCH(Me)), 2.89 (s, 3H,
1
Found: C, 75.37; H, 6.86; N, 1.85. H NMR (500 MHz, toluene-d8,
3
CH2COOCH3), 2.43 (br dd, J = 14.4 Hz, 1H, CHHCOOMe), 2.31
298 K): δ 7.57 (m, 4H, H o-cumyl), 7.29 (m, 6H, H4 + H m-cumyl),
7.09 (m, 2H, H p-cumyl), 6.95 (s, 1H, H6), 6.91 (s, 1H, H6′), 6.55 (t,
3J = 8.0 Hz, 1H, H3p), 6.48 (d, 3J = 7.8 Hz, 1H, H2p), 6.42 (d, 3J = 7.8
(s, 3H, CH3), 2.27 (s, 3H, CH3), 2.03 (s, 3H, CH3), 1.99 (s, 3H, CH3),
1.74 (br dd, 2J = 14.4 Hz, 1H, CHHCOOMe), 1.66 (s, 3H, CH3), 1.58
3
(s, 3H, CH3), 0.87 (d, J = 5.7 Hz, 3H, OCH(CH3)). 13C{1H} NMR
3
Hz, 1H, H2p′), 5.37 (hept, J = 6.3 Hz, 1H, C(O)OCH(CH3)2), 4.00
3
(100 MHz, toluene-d8, 258 K): δ 180.06 (COO), 163.02, 162.90,
159.21, 158.48, 151.99, 151.53, 138.02 (C3p), 137.37, 136.95, 136.87,
136.64, 131.63, 131.36, 131.15, 131.03, 128.92, 128.0, 127.17, 127.03,
126.79, 125.1, 125.0, 124.28, 123.47, 123.30, 123.16, 122.89, 65.07
OCH(Me), 53.37 (COOCH3), 43.46 (C(CH3)2), 43.25 (C(CH3)2),
42.37 (CH2COOMe), 31.44 (CH3), 30.94 (CH3), 29.34 (CH3), 26.53
(CH3), 23.49 (CH3), 20.98 (CH3), 20.84 (CH3).
(q, J = 6.6 Hz, 1H, OCH(CH3), 2.24 (s, 3H, CH3 aryl), 2.23 (s, 3H,
CH3 aryl), 2.00 (s, 3H, CH3 cumyl), 1.99 (s, 3H, CH3 cumyl), 1.83 (s,
3
3H, CH3 cumyl), 1.82 (s, 3H, CH3 cumyl), 1.12 (2d overlapped, J =
6.3 Hz, 6H, C(O)OCH(CH3)2), 0.91 (d, 3J = 6.7 Hz, 3H,
OCH(CH3)). 13C{1H} NMR (125 MHz, toluene-d8, 298 K): δ
189.50 (COO), 157.27 (Caro), 157.17 (Caro), 154.53 (CN), 154.46
(C′N), 151.49 (CO), 151.38 (C′O), 139.99 (Caro), 139.78 (Caro),
139.61 (C3p), 130.71 (C4), 130.56 (C4′), 129.18 (Caro), 128.26 (Caro),
127.56 (C6), 127.43 (C6′), 126.87 (Caro), 126.86 (Caro), 122.91 (Caro),
122.83 (Caro), 120.79 (C2p), 120.57 (C′2p), 73.79 (C(O)OCHMe2),
68.76 (OCH(CH3)), 42.64 (C(CH3)2), 42.56 (C′(CH3)2), 32.41
(CH3 cumyl), 32.39 (CH3 cumyl), 27.58 (CH3 cumyl), 27.47 (CH3
cumyl), 21.82 (OCH(CH3), 21.63 (C(O)OCHMe2), 21.57 (C(O)-
OCHMe2), 21.19 (CH3 aryl, 2 signals overlapping).
NMR Scale Synthesis of {ONOMe,Cumyl}Y((S,S)-OCH(CH3)CO2CH-
(CH3)CO2Me) (3). This product was prepared as described above for 2,
starting from 1 (0.030 g, 33.6 μmol) and methyl (S,S)-lactyllactate (6.0
mg, 33.6 μmol) in THF-d8 (ca. 0.7 mL) for 30 min at room
temperature. 1H NMR spectroscopy revealed the release of HN-
3
(SiHMe2)2 (δ 1H: 4.44 (m, J = 3.0 Hz, 2H, HN(SiHMe2)2), 0.09 (d,
1
3
3J = 3.1 Hz, 12H, HN(SiHMe2)2) and Et2O (δ H: 3.36 (q, J = 7.0
3
{ONOMe,Cumyl}Al((R)-OCH(CH3)CH2COOCH3) (6). In the glovebox, a
Hz, 3H, OCH2CH3, free Et2O), 1.09 (t, J = 7.0 Hz, 3H, OCH2CH3,
-
J-Young NMR tube was charged with a solution of {ONOMe,Cumyl
}
free Et2O)) and formation of 3 along with another species yet
unidentified (33−40%). Compound 3 is not stable in solution at room
temperature and transforms over 18 h into other species, probably
AlMe (4, 12.7 mg, 22.4 μmol) in toluene-d8 (ca. 0.4 mL), and a
solution of methyl (R)-3-hydroxybutyrate (2.5 μL, 22.4 μmol) in
toluene-d8 (ca. 0.3 mL) was added at room temperature. The tube was
closed, shaken, and put in a preset oil bath at 80 °C for 15 h. 1H NMR
spectroscopy revealed the release of methane and completion of the
reaction. Evaporation of volatiles under vacuum left 6 as a pale yellow
solid. Analytically pure crystals of 6 suitable for X-ray diffraction were
obtained by recrystallization from a concentrated CH2Cl2 solution
layered with hexane at room temperature. Anal. Calcd for
C42H44AlNO5: C, 75.32; H, 6.62; N, 2.09. Found: C, 75.0; H, 6.9;
N, 2.2. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 7.83 (t, 3J = 8.15 Hz,
1H, H3p), 7.65 (m, 2H, Haro), 7.45 (m, 2H, Haro), 7.26−7.07 (m, 12H,
Haro), 3.91 (s, 3H, CH2COOCH3), 3.47 (m, 1H, OCH(CH3)), 2.35
1
arising from transesterification reactions. Characteristic data for 3: H
3
NMR (500 MHz, THF-d8, 298 K): δ 7.63 (t, J = 7.9 Hz, 1H, H3p),
7.22−6.77 (m, 16H, Haro), 4.90−4.85 (two quadruplets overlapped, 3J
= 6.9 Hz, 2H, −OCH(CH3)CO2CH(CH3)CO2Me), 3.67 (s, 3H,
COOCH3), 2.22−2.20 (m, 6H, p-CH3 phenolate), 1.69−1.65 (m, 12
H, CH3 of cumyl), 1.56 (d, 3J = 7.0 Hz, 3H, CH(CH3)), 1.49 (d, 3J =
6.9 Hz, 3H, CH(CH3)). 13C{1H} NMR (125 MHz, THF-d8, 298 K): δ
190.0 (OCH(CH3)CO2CH(CH3)CO2Me), 169.0 (OCH(CH3)-
CO2CH(CH3)CO2Me), 162.1 and 162.0 (CN), 158.8 and 158.7
(CO), 154.7 and 154.6 (C quat aro), 153.5 and 153.4 (C quat aro),
136.5 (CH3p), 130.6 and 130.5 (CH), 129.15 and 129.1 (C quat aro),
128.0 and 127.8 (CH), 126.2 and 126.15 (C quat aro), 125.2 and
125.15 (CH), 125.1 and 125.0 (CH), 123.15 and 123.1 (CH), 122.5
and 122.3 (CH2p), 73.0 (OCH(CH3)CO2CH(CH3)CO2Me), 69.9
(OCH(CH3)CO2CH(CH3)CO2Me), 42.1 and 41.9 (C(CH3)2), 51.0
(OCH3), 29.1 and 29.0 (C(CH3)2), 23.2 (OCH(CH3)CO2CH(CH3)-
CO2Me), 20.1 and 20.3 (CH3), 16.0 (OCH(CH3)CO2CH(CH3)-
CO2Me). Characteristic signals for the other species observed in THF
(500 MHz, THF-d8, 298 K): δ 7.73 (t, 3J = 8.0 Hz, 1H, H3p), 7.30 (d,
3J = 8.0 Hz, 2H, H2p), 4.57 (br m, 2H, SiH), 1.81 (s, 12H, CH3 of
3
2
(s, 3H, CH3), 2.32 (s, 3H, CH3), 1.95 (dd, J = 2.9 Hz and J = 16.4
Hz, 1H, CHHCOOCH3), 1.85 (dd, 3J = 2.9 Hz and 2J = 16.4 Hz, 1H,
CHHCOOCH3), 1.77 (s, 3H, CH3), 1.72 (s, 3H, CH3), 1.68 (s, 3H,
3
1
CH3), 1.63 (s, 3H, CH3), 0.65 (d, J = 6.2 Hz, 3H, OCH(CH3)). H
NMR (400 MHz, C6D6, 298 K): 7.40−6.90 (m, 17H, Haro), 3.66 (m,
1H, OCH(CH3)), 3.44 (s, 3H, CH2COOCH3), 2.28 (s, 3H, CH3),
2.25 (s, 3H, CH3), 2.05 (s, 3H, CH3), 1.98 (s, 3H, CH3), 1.84 (s, 3H,
CH3), 1.81 (s, 3H, CH3), 1.75 (dd, 3J = 2.9 Hz and 2J = 16.0 Hz, 1H,
CHHCOOCH3), 1.52 (dd, 3J = 8.4 Hz and 2J = 16.0 Hz, 1H,
3
3
cumyl), 0.12 (d, J = 3.1 Hz, 12H, N(SiHMe2)2; some other signals
CHHCOOCH3), 0.71 (d, J = 6.1 Hz, 3H, OCH(CH3)). 13C{1H}
overlapped with those of 3.
NMR (100 MHz, C6D6, 258 K): δ 181.16 (COO), 158.57, 158.39,
156.20, 156.17, 152.53, 152.27, 139.60, 139.55, 138.34 (C3p), 131.99,
131.96, 129.34, 128.1, 127.95, 127.59, 127.49, 126.45, 126.35, 125.70,
125.25, 125.15, 125.05, 122.89, 122.65, 120.17, 63.48 (OCH(Me)),
53.50 (COOCH3), 43.07 (C(CH3)2), 41.76 (CH2COOMe), 32.36
(CH3), 31.50 (CH3), 29.41 (CH3), 28.85 (CH3), 25.59 (OCH(CH3)),
21.30 (CH3), 21.25 (CH3).
{ONOMe,Cumyl}AlMe (4). A Schlenk flask was charged with
{ONOMe,Cumyl}H2 (0.300 g, 0.57 mmol), and toluene (ca. 10 mL)
was vacuum transferred in at −78 °C. To this solution was added
AlMe3 (0.80 mL of a 1.0 M solution in heptane, 0.80 mmol, 1.4 equiv).
The reaction mixture was warmed to 80 °C and stirred overnight. The
solution was cooled to room temperature, and a pale yellow
microcrystalline solid precipitated. The latter solid was filtrated and
washed with cold hexane (5 mL) to give 4 (0.248 g, 77%). Anal. Calcd
for C38H38AlNO2: C, 80.40; H, 6.75; N, 2.47. Found: C, 80.34; H,
6.80; N, 2.19. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 7.89 (t, 3J = 8.1
Hz, 1H, H3p), 7.54 (d, 3J = 8.1 Hz, 2H, H2p), 7.46 (s, 2H, H4), 7.32 (s,
2H, H6), 7.24 (m, 8H, H o- + m-cumyl), 7.14 (m, 2H, H p-cumyl),
2.39 (s, 6H, CH3 aryl), 1.70 (s, 6H, CH3 cumyl), 1.65 (s, 6H, CH3
cumyl), −1.87 (s, 3H, Al-CH3). 13C{1H} NMR (125 MHz, CD2Cl2,
298 K): δ 155.12 (C−O), 154.86 (C−N), 151.39, 141.94, 141.70,
131.81, 128.02, 127.66, 127.49, 126.11, 125.17, 122.62, 121.58 (Caro),
{ONOMe,Cumyl}Al((rac)-OCH(CF3)CH2CO2C2H5) (7). This compound
was prepared following the same procedure as that described above for
6, starting from {ONOMe,Cumyl}AlMe (25.6 mg, 45.1 μmol) and
( )-ethyl 3-hydroxy-4,4,4-trifluorobutyrate (9.2 mg, 45.1 μmol) in
1
toluene-d8 (ca. 0.7 mL) for 17 h at 80 °C. H NMR spectroscopy
revealed the release of methane gas and completion of the reaction.
The product is slightly soluble in toluene, and it precipitates with
progress of the reaction. The supernatant solution was then removed,
and the recovered solid was dried under vacuum to give 7 as a pale
yellow solid (20.0 mg, 60%). Anal. Calcd for C43H43AlF3NO5: C,
K
dx.doi.org/10.1021/om401047r | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
70.00; H, 5.87; N, 1.90. Found: C, 69.86; H, 6.05; N, 1.87. H NMR
(500 MHz, CD2Cl2, 298 K): δ 7.81 (t, 3J = 8.3 Hz, 1H, H3p), 7.65 (d,
3J = 8.3 Hz, 1H, H2p), 7.63 (d, 3J = 8.2 Hz, 1H, H2p′), 7.45 (2 singlets,
2H, H6), 7.31−7.07 (m, 12H, H4, H4′ and other Haro), 4.34 (2 quartets
overlapping, 3J = 7.2 Hz, 2H, COOCH2CH3), 3.56 (m, 3J = 3.1 and 9.1
Hz, 1H, OCH(CF3)), 2.37 (s, 3H, CH3), 2.34 (s, 3H, CH3), 1.98 (dd,
J = 3.1 and 16.5 Hz, 1H, CHHCOOEt), 1.84 (dd, J = 9.1 and 16.5 Hz,
1H, CHHCOOEt), 1.81 (s, 3H, CH3), 1.75 (s, 3H, CH3), 1.67 (s, 3H,
REFERENCES
■
(1) For leading reviews, see: (a) O’Keefe, B. J.; Hillmyer, M. A.;
Tolman, W. B. J. Chem. Soc., Dalton Trans. 2001, 2215. (b) Dechy-
Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104,
6147. (c) Williams, C. K.; Hillmyer, M. A. Polym. Rev. 2008, 48, 1.
(d) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165. (e) Stanford, M. J.;
Dove, A. P. Chem. Soc. Rev. 2010, 39, 486. (f) Carpentier, J.-F.
Macromol. Rapid Commun. 2010, 31, 1696. (g) Ajellal, N.; Carpentier,
J.-F.; Guillaume, C.; Guillaume, S. M.; Helou, M.; Poirier, V.; Sarazin,
Y.; Trifonov, A. Dalton Trans. 2010, 39, 8363. (h) Dijkstra, P. J.; Du,
H.; Feijen, J. Polym. Chem. 2011, 2, 520.
(2) (a) Le Borgne, A.; Vincens, V.; Jouglard, M.; Spassky, N.
Makromol. Chem., Macromol. Symp. 1993, 73, 37. (b) Spassky, N.;
Wisniewski, M.; Pluta, C.; Le Borgne, A. Macromol. Chem. Phys. 1996,
197, 2627. (c) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124,
1316. (d) Zhong, Z.; Dijkstra, P. J.; Feijen, J. Angew. Chem., Int. Ed.
2002, 41, 4510. (e) Tang, Z.; Chen, X.; Pang, X.; Yang, Y.; Zhang, X.;
Jing, X. Biomacromolecules 2004, 5, 965. (f) Nomura, N.; Ishii, R.;
Akahura, M.; Aoi, K. J. Am. Chem. Soc. 2002, 124, 5938. (g) Zhong, Z.;
Dijkstra, P. J.; Feijen, J. J. Am. Chem. Soc. 2003, 125, 11291.
(h) Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Chem.Eur. J.
2007, 13, 4433. (i) Darensbourg, D. J.; Karroonnirun, O. Organo-
metallics 2010, 29, 5627. (j) Darensbourg, D. J.; Karroonnirun, O.;
Wilson, S. J. Inorg. Chem. 2011, 50, 6775. (k) Zhao, W.; Wang, Y.; Liu,
X.; Chen, X.; Cui, D.; Chen, E. Y.-X. Chem. Commun. 2012, 48, 6375.
(3) (a) Hormniru, P.; Marshall, E. L.; Gibson, V. C.; White, A. J. P.;
Williams, D. J. J. Am. Chem. Soc. 2004, 126, 2688. (b) Hormnirun, P.;
Marshall, E. L.; Gibson, V. C.; Pugh, R. I.; White, A. J. P. Proc. Natl.
Acad. Sci. U.S.A. 2006, 103, 15343. (c) Du, H.; Velders, A. H.; Dijkstra,
P. J.; Sun, J.; Zhong, Z.; Chen, X.; Feijen, J. Chem.Eur. J. 2009, 15,
9836.
(4) (a) Pang, X.; Chen, X.; Du, H.; Wang, X.; Jing, X. J. Organomet.
Chem. 2007, 692, 5605. (b) Bouyahi, M.; Grunova, E.; Marquet, N.;
Kirillov, E.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Organo-
metallics 2008, 27, 5815. (c) Alaaeddine, A.; Thomas, C. M.; Roisnel,
T.; Carpentier, J.-F. Organometallics 2010, 29, 491. (d) Whitelaw, E.
L.; Loraine, G.; Mahon, M. F.; Jones, M. D. Dalton Trans. 2011, 40,
11469.
(5) For pioneering work in this field, see ref 2c and: (a) McClain, S.
J.; Ford, T. M.; Drysdale, N. E. Polym. Prep. 1992, 463. (b) Le Borgne,
A.; Pluta, C.; Spassky, N. Macromol. Rapid Commmun. 1994, 15, 955.
(c) Stevels, W. M.; Ankone, M. J. K.; Dijkstra, P. J.; Feijen, J.
Macromol. Chem. Phys. 1995, 196, 1153. (d) Simic, V.; Spassky, N.;
Hubert-Pfalzgraf, L. G. Macromolecules 1997, 30, 7338. (e) Chamber-
lain, B. M.; Sun, Y.; Hagadorn, J. R.; Hemmesch, E. W.; Young, V. G.,
Jr.; Pink, M.; Hillmyer, M. A.; Tolman, W. B. Macromolecules 1999, 32,
2400. (f) Chamberlain, B. M.; Jazdzewski, B. A.; Pink, M.; Hillmyer,
M. A.; Tolman, W. B. Macromolecules 2000, 33, 3970.
CH3), 1.63 (s, 3H, CH3), 1.49 (t, 3J
= 7.2 Hz, 3H,
CH2COOCH2CH3). 13C{1H} NMR (125 MHz, CD2Cl2, 298 K): δ
178.86 (COO), 157.36 and 157.05 (CO), 156.30 and 156.27 (CN),
152.40 and 152.10 (quat Caro), 139.5 (CH3p), 139.38 and 139.33 (quat
Caro), 132.11 and 132.02 (CH4), 129.39 and 128.58 (C5), 128.00
(CHaro), 127.44 and 127.36 (CH6), 126.20 and 126.17 (CH cumyl),
126.14 and 125.98 (CH cumyl), 125.28 and 125.21 (CH cumyl),
122.53 and 122.01 (quat Caro), 121.02 and 120.82 (CH2p), 67.25 (q, 2J
= 31.2 Hz, OCH(CF3)), 64.84 (COOCH2CH3), 42.80 and 42.70
(C(CH3)2), 33.64 (CH2COOEt), 32.26 and 30.84 (C(CH3)2), 29.87
and 28.65 (C(CH3)2′), 21.17 and 21.14 (CH3 phenyl), 13.79
(COOCH2CH3); the CF3 signal was not observed due to its low
intensity. 19F{1H} NMR (185 MHz, CD2Cl2, 298 K): δ −81.87 ppm.
Typical Polymerization Procedure. Polymerizations of rac-LA
and rac-BBL were described as reported earlier.7c
X-ray Diffraction Analyses. Suitable single crystals were mounted
onto a glass fiber using the “oil-drop” method. Diffraction data were
collected at 100(2) K using an APEXII Bruker-AXS diffractometer
with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). A
combination of ω and ϕ scans was carried out to obtain at least a
unique data set. The crystal structures were solved by direct methods,
and the remaining atoms were located from difference Fourier
synthesis followed by full-matrix least-squares refinement based on F2
(programs SIR97 and SHELXL-97)27 with the aid of the WINGX
program.28 Many hydrogen atoms could be found from the Fourier
difference analysis. Other hydrogen atoms were placed at calculated
positions and forced to ride on the attached atom. The hydrogen atom
contributions were calculated but not refined. All non-hydrogen atoms
were refined with anisotropic displacement parameters. The locations
of the largest peaks in the final difference Fourier map calculation as
well as the magnitude of the residual electron densities were of no
chemical significance. Crystal data and details of data collection and
structure refinement for the different compounds are given in Table
S1. The crystallographic data (excluding structure factors) are available
as Supporting Information, as cif files.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data as Table S1 and CIF files;
1
representative H, 13C{1H}, and 19F{1H} NMR spectra for
(6) (a) Cai, C.-X.; Amgoune, A.; Lehmann, C. W.; Carpentier, J.-F.
Chem. Commun. 2004, 330. (b) Amgoune, A.; Thomas, C. M.; Roisnel,
T.; Carpentier, J.-F. Chem.Eur. J. 2006, 12, 169. (c) Ma, H.; Spaniol,
T. P.; Okuda, J. Angew. Chem., Int. Ed. 2006, 45, 7818. (d) Liu, X.;
Shang, X.; Tang, T.; Hu, N.; Pei, F.; Cui, D.; Chen, X.; Jing, X.
Organometallics 2007, 26, 2747. (e) Ma, H.; Spaniol, T. P.; Okuda, J.
Inorg. Chem. 2008, 47, 3328. (f) Dyer, H. E.; Huijser, S.; Schwarz, A.
D.; Wang, C.; Duchateau, R.; Mountford, P. Dalton Trans. 2008, 32.
(g) Hodgson, L. M.; Platel, R. H.; White, A. J. P.; Williams, C. K.
Macromolecules 2008, 41, 8603. (h) Platel, R. H.; White, A. J. P.;
Williams, C. K. Chem. Commun. 2009, 4115. (i) Buchard, A.; Auffrant,
A.; Ricard, L.; Le Goff, X. F.; Platel, R. H.; Williams, C. K.; Le Floch, P.
Dalton Trans. 2009, 10219. (j) Grunova, E.; Kirillov, E.; Roisnel, T.;
Carpentier, J.-F. Dalton Trans. 2010, 39, 6739. (k) Dyer, H. E.;
Huijser, S.; Susperregui, N.; Bonnet, F.; Schwarz, A. D.; Duchateau, R.;
Maron, L.; Mountford, P. Organometallics 2010, 29, 3602. (l) Buffet, J.-
C.; Kapelski, A.; Okuda, J. Macromolecules 2010, 43, 10201. (m) Clark,
L.; Cushion, M. G.; Dyer, H. E.; Schwarz, A. D.; Duchateau, R.;
Mountford, P. Chem. Commun. 2010, 46, 273. (n) Buchard, A.; Platel,
R. H.; Auffrant, A.; Le Goff, X. F.; LeFloch, P.; Williams, C. K.
Organometallics 2010, 29, 2892. (o) Buffet, J.-C.; Okuda, J. Polym.
ligands, complexes, and some polymers; details of DFT
computations. This information is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: evgueni.kirillov@univ-rennes1.fr.
*E-mail: osvaldo.casagrande@ufrgs.br.
*E-mail: jean-francois.carpentier@univ-rennes1.fr. Fax: (+33)
(0)223-236-939.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by the Brazilian CAPES,
French MESR, and CNRS. The authors are grateful to CAPES-
COFECUB for joined action Ph556-07_2007-2010 and
CAPES-CNRS for joined action PICS05923.
L
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Article
Chem. 2011, 2, 2758. (p) Buffet, J.-C.; Okuda, J. Dalton Trans. 2011,
40, 7748. (q) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem.
2011, 50, 7718. (r) Kapelski, A.; Buffet, J.-C.; Spaniol, T. P.; Okuda, J.
Chem. Asian J. 2012, 7, 1320. (s) Nie, K.; Fang, L.; Yao, Y.; Zhang, Y.;
Shen, Q.; Wang, Y. Inorg. Chem. 2012, 51, 11133. (t) Li, G.; Lamberti,
M.; Mazzeo, M.; Pappalardo, D.; Roviello, G.; Pellecchia, C.
Organometallics 2012, 31, 1180. (u) Bakewell, C.; Cao, T.-P.-A.;
Long, N. J.; Le Goff, X. F.; Auffrant, A.; Williams, C. K. J. Am. Chem.
Soc. 2012, 134, 20577. (v) Nie, K.; Gu, W.; Yao, Y.; Zhang, Y.; Shen,
Q. Organometallics 2013, 32, 2608. (w) Bakewell, C.; Cao, T.-P.-A.; Le
Goff, X. F.; Long, N. J.; Auffrant, A.; Williams, C. K. Organometallics
2013, 32, 1475. For isotactic enrichment from enantiopure initiators,
see: (x) Heck, R.; Schulz, E.; Collin, J.; Carpentier, J. F. J. Mol. Catal.
A: Chem. 2007, 268, 163. (y) Arnold, P. L.; Buffet, J.-C.; Blaudeck, R.
P.; Sujecki, S.; Blake, A. J.; Wilson, C. Angew. Chem., Int. Ed. 2008, 47,
semiaxes of the prolate ellipsoid, respectively), estimated from the
solid-state structure of the parent complex 1, were used for calculation
of the hydrodynamic radius (rH,PGSE). The corresponding value of
rH,DRX for 1 is 6.84 Å.
(16) Calculated following the equation τ = (β − α)/60, where α and
β are the O−Al−O and O(carbonyl)−Al−N angles in the present
case. The τ value ranges from 0 (perfectly square pyramidal) to 1
(perfectly trigonal bipyramidal); see: Addison, A. W.; Rao, T. N.;
Reedjik, J.; Van Rijn, L.; Verschoor, G. C. J. Chem. Soc., Dalton. Trans.
1984, 1349.
(17) Shannon, R. D. Acta Crystallogr. A 1976, A32, 751.
(18) Umezawa, Y.; Tsuboyama, S.; Takahashi, H.; Uzawa, J.; Nishio,
M. Tetrahedron 1999, 55, 10047.
(19) DFT computations performed on yttrium-β-alkoxybutyrate
complexes supported by an amino-alkoxy-bis(phenolate) ligand
bearing cumyl ortho-substituents suggested a stabilization of ca. 10−
12 kcal mol−1 for these C−H···π interactions. See ref 7e.
(20) In this experiment, it is assumed that iPrOH reacts fast with
amido complex 1 to release inert HN(SiMe3)2 and generate
concomitantly the corresponding isopropoxy species, which is the
real initiator of the polymerization, as we earlier established for closely
related complexes (see ref 6b). The observed slightly lower
experimental Mn values (as compared to the calculated one) likely
arise from the use of a slightly larger amount (than strictly 1 equiv) of
iPrOH, which in this case acts as a reversible chain-transfer agent in a
so-called immortal polymerization (see ref 1g). The lower Mn values
(as compared to other PLAs prepared in this study) and hence the
larger proportion of end-groups relative to monomer repeat units in
the main chain likely account for the apparent decrease in
heterotacticity.
(21) The presence of a molecule of a coordinating solvent within the
coordination sphere of the metal center in the active species during the
course of a ROP may affect the stereochemistry of monomer
coordination/insertion; see: Chisholm, M. H.; Choojun, K.; Chow,
A. S.; Fraenkel, G. Angew. Chem., Int. Ed. 2013, 52, 3264.
(22) Essentially the same results as those of entry 17 were obtained
when 1 equiv of pyridine was used.
6033. (z) 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.
(7) (a) Amgoune, A.; Thomas, C. M.; Ilinca, S.; Roisnel, T.;
Carpentier, J.-F. Angew. Chem., Int. Ed. 2006, 45, 2782. (b) Ajellal, N.;
Lyubov, D. M.; Sinenkov, M. A.; Fukin, G. K.; Cherkasov, A. V.;
Thomas, C. M.; Carpentier, J.-F.; Trifonov, A. A. Chem.Eur. J. 2008,
14, 5440. (c) Ajellal, N.; Bouyahyi, M.; Amgoune, A.; Thomas, C. M.;
Bondon, A.; Pillin, I.; Grohens, Y.; Carpentier, J.-F. Macromolecules
2009, 42, 987. (d) Kramer, J. W.; Treiter, D. S.; Dunn, E. W.; Castro,
P. M.; Roisnel, T.; Thomas, C. M.; Coates, G. W. J. Am. Chem. Soc.
2009, 131, 16042. (e) Bouyahyi, M.; Ajellal, N.; Kirillov, E.; Thomas,
C. M.; Carpentier, J. F. Chem.Eur. J. 2011, 17, 1872. (f) Pappalardo,
D.; Bruno, M.; Lamberti, M.; Pellecchia, C. Macromol. Chem. Phys.
2013, 214, 1965. For an example of isoselective ROP of rac-BBL, see:
(g) Ajellal, N.; Durieux, G.; Delevoye, L.; Tricot, G.; Dujardin, C.;
Thomas, C. M.; Gauvin, R. M. Chem. Commun. 2010, 46, 1032.
(8) Steudel, A.; Stehr, J.; Siebel, E.; Fischer, R. D. J. Organomet. Chem.
1998, 570, 89.
(9) (a) Scandium: ref 6c. (b) Yttrium: Grunova, E.; Kirillov, E.;
Roisnel, T.; Carpentier, J.-F. Organometallics 2008, 27, 5691.
́
Aluminum: (c) Ref 2h. (d) Lewinski, J.; Horeglad, P.; Wojcik, K.;
(23) (a) LeBorgne, A.; Vincens, V.; Jouglard, M.; Spassky, N.
Makromol.Chem., Macromol. Symp. 1993, 173, 37. See also:
Justyniak, I. Organometallics 2005, 24, 4588. (e) Phomphrai, K.;
Chumsaeng, P.; Sangtrirutnugul, P.; Kongsaeree, P.; Pohmakotr, M.
Dalton Trans. 2010, 39, 1865. (f) Dagorne, S.; Le Bideau, F.; Welter,
(b) Dubois, P.; Jer
Am. Chem. Soc. 1994, 35, 596.
́ ̂ ́
ome, R.; Teyssie, P. Polym. Prepr. Div. Polym. Sci.,
R.; Bellemin-Laponnaz, S.; Maisse-Franco
13, 3202. (g) Normand, M.; Dorcet, V.; Kirillov, E.; Carpentier, J.-F.
Organometallics 2013, 32, 1694. Gallium: (h) Horeglad, P.; Kruk, P.;
̧
is, A. Chem.Eur. J. 2007,
(24) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F.;
Spassky, N.; LeBorgne, A.; Wisniewski, M. Macromolecules 1996, 29,
6461.
(25) Barakat, I.; Dubois, P.; Jerome, R.; Teyssie, P. J. Polym. Sci., A:
Polym. Chem. 1993, 31, 505.
́
Pecaut, J. Organometallics 2010, 29, 3729. (i) Horeglad, P.;
Szczepaniak, G.; Dranka, M.; Zachara, J. Chem. Commun. 2012, 48,
1171.
(26) Kochnev, A. I.; Oleynik, I. I.; Oleynik, I. V.; Ivanchev, S. S.;
Tolstikov, G. A. Russ. Chem. Bull., Int. Ed. 2007, 56, 1125.
(27) (a) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (b) Sheldrick, G. M.
SHELXS-97, Program for the Determination of Crystal Structures;
University of Goettingen: Germany, 1997. (c) Sheldrick, G. M.
SHELXL-97, Program for the Refinement of Crystal Structures; University
of Goettingen: Germany, 1997. (d) Sheldrick, G. M. Acta Crystallogr.
2008, 64, 112.
(10) For other examples of discrete alkyl lactate metal complexes
used as initiators for/models of ROP of lactide, see the following.
Zinc: (a) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229.
(b) Sn: Dove, A. D.; Gibson, V. C.; Marshall, E. L.; Rzepa, H. S.;
White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2006, 128, 9834. (c)
Sn: Wang, L.; Kefalidis, C. E.; Sinbandhit, S.; Dorcet, V.; Carpentier,
J.-F.; Maron, L.; Sarazin, Y. Chem.Eur. J. 2013, 19, 13463.
(11) (a) Chen, H.-L.; Dutta, S.; Huang, P.-Y.; Lin, C.-C.
Organometallics 2012, 31, 2016. (b) Wang, L.; Ma, H. Macromolecules
2010, 43, 6535.
(28) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(12) Kirillov, E.; Roisnel, T.; Razavi, A.; Carpentier, J.-F. Organo-
metallics 2009, 28, 5036.
(13) Hieringer, W.; Eppinger, J.; Anwander, R.; Herrmann, W. A. J.
Am. Chem. Soc. 2000, 122, 11983−11994.
(14) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.;
Herdtweck, E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847.
(15) The PGSE NMR measurements were performed on a 50 mM
solution of complex 2 in toluene-d8 at room temperature using
tetrakis(trimethylsilyl)silane as internal reference. The average value of
the translational diffusion coefficient (Dt), determined for three
separate signals in the 1H PGSE NMR spectrum, is 6.82 × 10−10 m2·s.
The values a (7.22 Å) and b (6.15 Å) (the major and the minor
M
dx.doi.org/10.1021/om401047r | Organometallics XXXX, XXX, XXX−XXX