Use of lithium aryloxides as promoters for preparation of α-hydroxy acid esters

Article abstract of DOI:10.1039/c9dt03631h

In this work, a hexanuclear lithium compound, [Li₆(MesalO)₆] (1), supported by a chelating ligand, namely methyl salicylato (MesalOH), was used as a precursor for preparation of the monomeric lithium aryloxides [Li(MesalO)(MesalOH)] (2) and [Li(MesalO)(MeOH)₂] (3) via reactions with MesalOH or MeOH. These aryloxides were characterized by single-crystal X-ray diffraction, and spectroscopic and other analytical methods. The diffusion-ordered ¹H NMR measurements revealed the retention of solid-state structures of 1 and 2 in THF-d₈ solution. Experimental data obtained for 3 showed its decomposition into compound 1 and free MeOH. Compound 1 generated from 3 was also used as a catalyst for the alcoholysis of l-lactide (l-LA) and glycolide (GA) for the preparation of α-hydroxy acid esters. We established that during methanolysis in the presence of 1, l-LA was selectively transformed into methyl (S,S)-O-lactyllactate (MeL₂), and GA was converted to methyl glycolate (MeG₁) and oligoglycolate esters MeG_n (n = 2, 3, and 4).

Full text of DOI:10.1039/c9dt03631h

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Use of lithium aryloxides as promoters for
preparation of α-hydroxy acid esters†
Cite this: DOI: 10.1039/c9dt03631h
b
Rafał Petrus, *a Patryk Fałata and Piotr Sobota
*
In this work, a hexanuclear lithium compound, [Li6(MesalO)6] (1), supported by a chelating ligand, namely
methyl salicylato (MesalOH), was used as a precursor for preparation of the monomeric lithium aryloxides
[Li(MesalO)(MesalOH)] (2) and [Li(MesalO)(MeOH)2] (3) via reactions with MesalOH or MeOH. These aryl-
oxides were characterized by single-crystal X-ray diffraction, and spectroscopic and other analytical
methods. The diffusion-ordered 1H NMR measurements revealed the retention of solid-state structures of
1 and 2 in THF-d8 solution. Experimental data obtained for 3 showed its decomposition into compound 1
and free MeOH. Compound 1 generated from 3 was also used as a catalyst for the alcoholysis of L-lactide
(L-LA) and glycolide (GA) for the preparation of α-hydroxy acid esters. We established that during methano-
lysis in the presence of 1, L-LA was selectively transformed into methyl (S,S)-O-lactyllactate (MeL2), and GA
was converted to methyl glycolate (MeG1) and oligoglycolate esters MeGn (n = 2, 3, and 4).
Received 10th September 2019,
Accepted 13th December 2019
DOI: 10.1039/c9dt03631h
rsc.li/dalton
lyst or reagent in organic synthesis,7 and an initiator in the
polymerizations of lactones,8,9 acrylamides,10 hydroxyalkyl
Introduction
Lithium alkoxides are the largest and most important group of acrylates, and methacrylates.11 It is also an effective promotor
alkali–metal alkoxo and aryloxo derivatives. They have numer- for C–C bond formation by α-alkylation of ketones with ROH
ous practical applications in organic, polymer, and materials under transition-metal-free conditions.12 Lithium(quinolin-8-
chemistry. Simple lithium alkoxides [Li(OR)] (R = Et, iPr, sec- olato), which has strong blue-light electroluminescence, good
Bu, and tBu) are commonly used as precursors for chemical film formability, and excellent electron mobility or a low turn-
vapor deposition of lithium-containing layers1 and sol–gel on voltage, has been investigated as a promising emitter13 or
processes.2–4 They have been used as electrolyte additives to interfacial material for the electron-transporting14 and elec-
improve the low-temperature performances of rechargeable tron-injecting layers15 in organic light-emitting diodes.
lithium-ion electrochemical cells, and enable extension of Lithium pyridylphenolate complexes have similar appli-
operating temperatures to as low as −40 °C.5 Lithium alkoxides cations.16 Tetranuclear lithium aryloxide cubane clusters are
are also important components of the superbases commonly effective building blocks for the construction of crystalline
used in synthetic and polymer chemistry, which are syn- porous materials for gas storage.17 Lithium aryloxides sup-
thesized by metal interchange reactions between organo- ported by aminophenolate or bis(phenolate) ligands have been
lithium compounds (LiR) and heavier-alkali–metal alkoxides intensively investigated as efficient initiators in the ring-
([M(OR′)], M = Na, K, Rb, and Cs). The reactivities of such opening polymerization (ROP) of heterocyclic monomers to give
systems are considerably greater than those of the parent com- biodegradable polymers with numerous applications in the bio-
pounds, and depend on the type of reaction, the solvents used, medical and food-packaging industries.18,19 In addition to their
and the superbase structures, i.e., monomeric or mixed-metal use in ROP, cyclic esters can be used as precursors for the syn-
aggregated species.6 [Li(OtBu)] is particularly useful as a cata- thesis of a wide range of α-hydroxy acid esters, which are impor-
tant eco-friendly chemicals, i.e., green solvents.20,21 They have
many commercial applications, e.g., as high-grade clean sol-
aFaculty of Chemistry, Wrocław University of Science and Technology,
23 Smoluchowskiego, 50-370 Wrocław, Poland. E-mail: rafal.petrus@pwr.edu.pl;
vents, paint and ink additives, preservatives and flavors, and
ingredients in pharmaceutical or skin-care formulations.22,23
Tel: +48 71 320 32 08
bŁukasiewicz Research Network – PORT Polish Center For Technology Development,
147 Stablowicka, 54-066 Wrocław, Poland. E-mail: piotr.sobota@port.org.pl;
Tel: +48 71 734 73 59
Alkyl lactates and glycolates are particularly useful and are
promising alternatives to classical petrochemical solvents.
These esters can be synthesized by alcoholysis of L-lactide (L-LA)
and glycolide (GA) initiated by simple metal alkoxides.
The development of inexpensive methods for the synthesis
of stable and well-defined lithium aryloxide compounds is
†Electronic supplementary information (ESI) available: NMR, IR, and crystallo-
graphic data for 1–3; NMR data for time monitoring of alcoholysis reactions and
obtained esters (PDF). CCDC 1944070–1944072. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9dt03631h
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therefore particularly important to enable their practical use in and chairs;29 Li4O4 cubanes30 and ladders,31 Li4O6 double-
catalytic reactions.
open dicubanes;32 Li5O5 aggregates, and Li6O6 hexagonal
Here, we report the preparation and characterization of the prisms33 and dicubanes34 (Scheme 1a–i).
hexanuclear aryloxylithium compound [Li6(MesalO)6] (1),
Scheme 2 shows that the reaction of MesalOH (1 equiv.)
which is supported by the industrially used methyl salicylato with nBuLi (1 equiv.) under butane evolution in toluene
ligand (MesalOH).24,25 We also examined the stability of 1 by afforded a new lithium compound, [Li6(MesalO)6] (1, 64%).
using MesalOH and MeOH as solvating agents. In this study,
Single-crystal X-ray diffraction data (XRD, ESI, Table S1†)
we obtained two new monomeric lithium aryloxides, namely show that in the solid form compound 1 is a centrosymmetric
[Li(MesalO)(MesalOH)] (2) and [Li(MesalO)(MeOH)2] (3). We hexanuclear cluster based on a hexagonal-prismatic Li6(μ3-O)6
also showed that 1 is an effective catalyst for the alcoholysis of core structure. The structure of 1 (Fig. 1) consists of two
heterocyclic esters, i.e., L-LA and GA, to give a family of envir- stacked Li3O3 units; this is a common structural motif in
onmentally friendly α-hydroxy acid esters in excellent yields lithium coordination chemistry and has been found in
and under ambient conditions.
[Li6(OPh)6(THF)6]35, [Li6(OAr)6] (ArO− = 2,6-dimethoxypheno-
lato36 or quinolin-8-olato37 anions), and [Li6(L)2] (L3− = cage-
shaped triphenolato anions38). The Li3O3 unit is nearly planar
with a maximum deviation of 0.058 Å from a plane drawn
through this six-membered ring. The lithium ions are four-co-
ordinated with O4 donor sets from three MesalO ligands.
Continuous-shape measurement (CShM)39 analysis of the LiO4
polyhedrons confirmed the presence of axially vacant trigonal
bipyramidal coordination environments around the Li1 and
Li3 atoms or a tetrahedral environment around Li2, as con-
firmed by the shape parameters: S(vTBPY-4) = 2.431 for Li1
and 2.326 for Li3, or S(T-4) = 2.444 for Li2. The lengths of the
Li–O(aryloxo) bonds within the Li3O3 ring, i.e., 1.883(5)–1.946(5)
Å, are much shorter than those between the rings, which range
from 2.043(5) to 2.075(5) Å.
Results and discussion
Synthesis and characterization of lithium compounds 1–3
In the solid state, lithium aryloxides usually exist as various
aggregates with structures varying from dimers to hexamers,
as shown in Scheme 1. The structural diversity of lithium aryl-
oxides results from the coordinating ability, and steric or elec-
tronic effects, of the ligands used. Boyle and colleagues
studied ortho-substituted lithium phenoxides and reported
that steric hindrance at this position strongly affected the final
[Li(OAr)] structure by reducing the bridging capacity of the
OAr ligands.26 Other factors responsible for the degree of
aggregation were the donor solvent basicity and solvation with
strong bases, which result in the formation of smaller aggre-
gates. Most previously reported structures are based on various
types of central core structure: Li2O2 rings;27 Li3O3 hexagons28
In lithium chemistry, the use of strong donor solvents gen-
erally leads to the formation of smaller aggregates. For
example, [Li(OPh)] crystallizes as a hexamer or tetramer from
THF and as a dimer from pyridine.26 However, it has never
Scheme 1 Common lithium aryloxide core structures: Li2O2 ring (a);
Li3O3 hexagon (b), and chair (c); Li4O4 cubane (d) and ladder (e); Li4O6
defective dicubane (f); Li5O5 aggregate (g); and Li6O6 hexagonal prism
(h) and dicubane (i).
Scheme 2 Synthesis of compounds 1–3.
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Fig. 3 Molecular structure of [Li(MesalO)(MeOH)2] (3). Displacement
ellipsoids are drawn at 30% probability level.
MeOH molecules; this is crucial for the formation of non-
aggregated lithium species. CShM analysis showed a large
departure from an ideal tetrahedron for the coordination
environment around Li1 in 2, as confirmed by the shape para-
meters, i.e., S(T-4) = 3.121 for 2 and 0.784 for 3. This is also
clearly reflected in the O–Li–O bond angles, which range from
Fig. 1 Molecular structure of [Li6(MesalO)6] (1). Displacement ellipsoids
are drawn at 30% probability level. Hydrogen atoms are omitted for
clarity. Symmetry code: (i) −x + 1, −y + 1, −z + 1.
90.75(18)° to 126.89(19)° for
2 and from 93.97(14)° to
115.41(19)° for 3. The Li–O distances, i.e., 1.887(3)–1.949(4) Å,
are typical and are in good agreement with previously reported
values for lithium aryloxides.40,41
been observed that the use of simple donor solvents leads to
the formation of well-defined monomeric species. We there-
fore chose cluster 1 as the starting material for the synthesis of
monomeric lithium aryloxides via reactions with MesalOH and
MeOH. The addition of MesalOH (6 equiv.) to a THF solution
of 1 led to breakage of the hexanuclear structure and for-
mation of a monomeric lithium compound, [Li(MesalO)
(MesalOH)] (2, 57%). The XRD data for 2 (Fig. 2) indicate that
the Li1 atom is tetrahedrally coordinated with O4 donor sets
from one MesalO and one MesalOH ligand. The Li–O bond
lengths, i.e., 1.892(4)–1.931(4) Å, are similar to those in 1.
When a similar reaction was performed in the presence of
MeOH, the monomeric in solid state lithium aryloxide
[Li(MesalO)(MeOH)2] (3, 61%) was obtained (Fig. 3). The struc-
tures of 2 and 3 are based on the same motif, in which a
lithium ion coordinated with a bidentate aryloxide ligand is
solvated by two oxygen donor atoms from one MesalOH or two
In the field of structural chemistry, the presence of mono-
meric tetracoordinated lithium centers surrounded by four
oxygen donors is typical of ionic compounds such as t-butanol
solvates of lithium halides, i.e., [Li(tBuOH)4]X (X = Cl, Br, and
I).42 It is worth noting that formation of solvated mononuclear
lithium aryloxides [Li(OAr)(solv)x] is relatively limited. They
have often been proposed as catalysts or reagents but well-
defined examples of such compounds are rare. The only
examples of monomeric lithium aryloxides are those obtained
when bulky phenolato,43,44 biphenolato,45 bis- and tris-
phenolato,46,47 or calixarene48 ligands have been used in the
presence of strong N/O-donor solvents such as THF, Et2O,
acetone, tetramethylethylenediamine and pentamethyldiethyl-
enetriamine. Several mononuclear lithium compounds sup-
ported by aminophenolate49,50 or aminobisphenolate51 ligands
have been reported.
Compounds 1–3 were characterized by 1H, 13C, and 7Li
NMR, and FTIR-ATR spectroscopies, and elemental analysis
(ESI, Fig. S1–S26†). The 1H NMR spectrum of 1 shows only one
set of signals at 7.70–6.24 and 3.76 ppm, arising from the aro-
matic and methyl protons of MesalO ligands (ESI, Fig. S3†).
The 7Li NMR spectrum of 1 exhibits one sharp singlet at δLi
1.29 ppm indicating the presence of one-equivalent environ-
ments for the lithium atoms (ESI, Fig. S5†). These NMR obser-
vations suggested that compound 1 had a C3 symmetry, con-
sistent with the XRD structure. The performed variable con-
centration 1H and 7Li NMR study did not provide any
1
additional information (ESI, Fig. S12 and S13†). When VT H
Fig. 2 Molecular structure of [Li(MesalO)(MesalOH)] (2). Displacement
ellipsoids are drawn at 30% probability level.
NMR study was performed for 1, at −80 °C the split of reso-
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nances of protons of MesalO ligands for two set of signals at temperature lithium ion lost coordinated MeOH molecules,
7.15/7.08, 6.97/6.45, 6.26/6.16 and 3.82/3.77 with 1 : 1 ratio was and hexanuclear cluster 1 and free MeOH were generated, as
observed (ESI, Fig. S18†). The 7Li NMR spectrum of 1 at verified by the presence of two different D coefficients equal to
−80 °C also showed two main peaks at 2.13 and 1.53 ppm 27.2 × 10−10 m2 s−1 for free MeOH and 8.83 × 10−10 m2 s−1 for
(ESI, Fig. S19†). The VT 1H and 7Li NMR spectra of 2 and 3 did 1 (ESI, Fig. S31 and S32, Table S2†). However VT 1H and 7Li
not show any significant differences in the resonances of NMR spectra of 3 at temperature below −80 did not show
MesalO ligand or lithium ion, suggesting the presence of only characteristic resonances pattern observed for 1. When DOSY
one equivalent species in solution (ESI, Fig. S20–S23†).
1H NMR spectrum of 3 was recorded in methanol-d4 also the
Finally, the diffusion-ordered 1H NMR spectroscopy (DOSY) presence of signals associated with hexanuclear compound 1
was used to estimate the formula weight (FW) of the species was observed (ESI, Fig. S33 and S34, Table S2†).
present in solution, by use of known internal references and
correlate their molecular weights with relative diffusion coeffi-
Synthesis of α-hydroxy acid esters
cients (D) through the linear regression plot of the logarithms
of D against the FWs of the references. The DOSY 1H NMR
Alcoholysis reactions of heterocyclic esters, namely L-LA and
GA, were investigated for the synthesis of various lactyl and gly-
colyl derivatives (Schemes 3 and 4). Typical reactions were per-
formed in MeOH solution at room temperature using 25 equiv.
spectrum of 1 revealed the retention of hexanuclear solid-state
structure in THF-d8 solution (ESI, Fig. S27†). The estimated
molecular weights of 961 g mol−1 for 1 was practically identical
with the value of 948.5 g mol−1 calculated from the molecular
of alcohol per equivalent of lactone. For both diesters, we first
conducted control experiments in the absence of a catalyst
formula (ESI, Fig. S28 and Table S2†). The calculated mole-
1
cular weights of 301 g mol−1 for 2 correspond to its solid-state
source. H NMR spectroscopic monitoring of L-LA alcoholysis
over time showed that after 94 h, 84% conversion of L-LA to
structure, and confirmed the presence of mononuclear species
1
in solution (ESI, Fig. S29 and S30, Table S2†). The DOSY 1H
methyl (S,S)-O-lactyllactate (MeL2) had been achieved. The H
NMR spectra show characteristic resonance signals at 4.98,
NMR analysis of 3 revealed that in THF-d8 solution at room
4.16, 1.41, and 1.12 ppm from the methine and methyl
protons of MeL2, and the resonances for the methyl ester and
hydroxyl groups at 3.20 and 2.47 ppm (Fig. 4, and ESI Fig. S35
and S36†). Electro-spray ionization mass spectrometry
(ESI-MS) studies confirmed the presence in the reaction mix-
tures of lactic acid oligomers separated by increments of 72
Da, with methyl ester end-groups (Fig. 5). Graphical analysis of
these results (ESI, Fig. S37a†) showed that 98% conversion was
achieved after 241 h. When the reaction was continued for
another 122 h, a slight increase in L-LA conversion to MeL2
was observed and no other lactate derivatives were detected
(ESI, Table S3†).
This suggests that under catalyst-free conditions MeOH
reacts with L-LA to give only the ring-opened product MeL2. An
alternative method was reported by Claborn, who synthesized
alkyl lactyllactates (AL2) from crude LA, anhydrous alcohols,
aromatic solvents, and acid catalysts at 70–90 °C and reaction
times of 6–8 h.52,53 Formation of AL2 as one of the main pro-
Scheme 3 Catalytic switching of L-LA alcoholysis reaction.
Scheme 4 Proposed courses of GA alcoholysis performed under catalyst-free conditions and with catalytic amount of 1.
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esters mediated by group 1 metal alkoxides synthesized in situ
from metal bis(trimethylsilyl)amides.55 They observed that the
final conversions of L-LA to MeL2 after 60 min at room temp-
erature were 48%, 60%, and 84% for MN(SiMe3)2, where M =
Li, Na, and K, respectively. However, the reactions stopped
after 10 to 30 min because of the formation of metal alkoxide
aggregates of the type [M(OR)]n, and increasing the reaction
time did not increase the L-LA conversion. When we increased
the amount of 1 in the reaction system five-fold, complete con-
version of L-LA to a mixture of methyl (S)-lactate (MeL1) (90%)
and MeL2 (10%) was achieved in 0.25 h. After 2.5 h the
Fig. 4 1H NMR spectrum in C6D6 of products of L-LA alcoholysis reac-
tion performed under catalyst-free conditions.
1
amount of synthesized MeL1 increased to 99%. The H NMR
spectrum of MeL1 in Fig. 6 shows resonance signals from the
methine and methyl protons of the lactate unit at 4.03 and
1.21 ppm, and from the methyl ester and hydroxyl groups at
3.20 and 2.47 ppm. The increased amount of catalyst enables
the methanolysis reaction to switch toward the synthesis of
MeL1 as the main product, as shown in Scheme 3 (ESI,
Fig. S39 and S40†). This reaction step is similar to those pre-
viously reported by us in the magnesium aminophenolate-
mediated transesterification of AL2 to AL, which occurs after
complete conversion of L-LA.56 This is the first time that con-
trolled switching of the reaction course toward the synthesis of
MeL2 or MeL1 caused by changing the amount of catalyst has
been observed.
When we examined GA alcoholysis in the absence of a cata-
lyst, a mixture of methyl glycolylglycolate (MeG2) and methyl
tris(glycolyl)glycolate (MeG4) was obtained (ESI, Table S3†).
For example, after 80 h, the GA conversions to MeG2/MeG4
were 82%/18% and these values did not change when the reac-
1
Fig. 5 ESI mass spectrum of sodium-cationized methyl oligolactates
formed during L-LA alcoholysis under catalyst-free conditions.
tion time was extended to 200 or even 556 h. H NMR spectro-
scopic analysis of the obtained products showed six different
methylene peaks, at 4.19 and 3.93 ppm, from MeG2, and at
4.32, 4.31, 4.16, and 3.90 ppm, from MeG4, as shown in Fig. 7.
ducts, in yields of up to ca. 40%, has also been observed ESI-MS showed the presence of glycolic acid oligomers termi-
during solvothermal alcoholysis of poly(lactic acid).54 Once we nated by methyl ester end-groups, separated by a mass of
had identified a suitable system for the alcoholysis of L-LA to
MeL2, we investigated the use of 1 as a catalyst to achieve high
product yields in short periods of time. However when we care-
fully investigated the synthesis of 1 in situ by the reaction of
n
MesalOH with BuLi (1 : 1) the formation of several species in
solution was observed. Therefore, we used compound 3 as
catalyst precursor because of its easy synthesis in crystalline
form and generation of 1 in pure form in solution. An L-LA
alcoholysis study performed at a reactant stoichiometry of
L-LA/MeOH/Li = 1/25/0.01 showed that in the first 22 h only
selective formation of MeL2 occurred (ESI, Table S4†). The
results (ESI, Fig. S37b†) show that L-LA conversion greater than
90% was achieved after 14.5 h. We did not observe the for-
mation of any other products, even when the reaction time was
extended to 242 h (ESI, Fig. S38†).
A comparison of these results with those for catalyst-free
reactions shows that the use of 0.167 mol% of 1 enabled 99%
conversion to be achieved 16 times faster (ESI, Fig. S37†). The
obtained results are consistent with those previously reported
Fig. 6 1H NMR spectrum in C6D6 of products of L-LA alcoholysis per-
formed in presence of 0.834 mol% of 1. *Assigned signals from aromatic
by Phomphrai and colleagues for the alcoholysis of cyclic protons of 1.
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Fig. 7 1H NMR spectrum in C6D6 of products of GA alcoholysis per-
formed under catalyst-free conditions.
Fig. 9 1H NMR spectrum in C6D6 of products of GA alcoholysis per-
formed in presence of 1.
Fig. 9 (ESI, Table S4†). After 10 min, GA conversion was com-
plete, with formation of 48% MeG1, 35% MeG2, 13% MeG3,
and 4% MeG4. These results (ESI, Fig. S41b†) show that as the
reaction progressed, the amount of MeGn (n = 2–4) esters
decreased, with simultaneous increases in the amount of the
final product MeG1 (ESI, Fig. S42†). In this reaction, com-
pound 1 acts as a catalyst in ring opening of GA through alco-
holysis and actively participates in the transesterification of
methyl oligoglycolate esters to MeG1 (ESI, Fig. S42 and S43†).
We also found that during the investigated reaction, the
slowest step was the transformation of MeG2 to MeG1; for
example, after 1 h, the GA conversions to MeG1/MeG2 were
69%/28%, after 4 h they were 85%/12%, and after 9 h the
values were 97%/2%. This synthesis of MeG3 is analogous to
that previously described for MeG4, and is achieved by alcoho-
lysis of GA by MeG1. When we increased the amount of 1 in
the reaction system two-fold, conversion of GA to a mixture of
MeG1/MeG2 reached 81%/17% after 5 min, 97%/2% after
10 min and 98%/1% after 15 min.
Fig. 8 ESI mass spectrum of sodium-cationized methyl oligoglycolates
formed during GA alcoholysis under catalyst-free conditions.
58 Da. The most intensive peak, with a value of 287.0317,
corresponds to an adduct of MeG4 with sodium ions, i.e., Na
{H[O(CH2)CO]4OCH3}+ (Fig. 8).
These results show that under similar conditions GA alco-
holysis to MeG2 is faster than L-LA alcoholysis to MeL2 (ESI,
Fig. S37a and S41a†). The GA conversion reached 47% after
9 h, whereas 54% conversion of L-LA was achieved after 53 h.
The main difference between the reactions is that L-LA was
selectively ring opened to give only MeL2, whereas GA metha-
nolysis led to a mixture of MeG2 and MeG4 esters. The MeG4
The main difference between the alcoholysis reactions of
the two studied cyclic esters in the presence of 0.167 mol% of
1 is that L-LA is selectively ring opened to give only MeL2,
whereas for GA, after ring opening, MeG2 reacts with MeOH to
give MeG1. Both MeG2 and MeG1 participate actively in ring
opening of GA, which leads to formation of MeG3 and MeG4.
Conclusion
ester was formed by the reaction of GA with MeG2, which pro- In this study, we demonstrated the synthesis of well-defined,
ceeds as long as GA is present in the reaction mixture. When monomeric in solid state lithium aryloxides, i.e., [Li(MesalO)
this reaction ends, neither MeOH nor MeG2 can attack the (MesalOH)] (2) and [Li(MesalO)(MeOH)2] (3), by reaction of the
linear MeG4, as shown by its constant presence in the reaction hexanuclear precursor [Li6(MesalO)6] (1) with MesalOH or
mixture over a wide time period, i.e., from 39 to 556 h.
MeOH. We revealed the retention of solid-state structures of 1
When the same reaction was performed with compound 1, and 2 in THF-d8 solution. We showed that mononuclear in
with a stoichiometry of GA/MeOH/Li = 1/25/0.01, methyl glyco- solid state compound 3 in THF-d8 or methanol-d4 solution
late (MeG1) and several oligoglycolate esters, including MeG2, undergoes transformation into 1 and free MeOH. We showed
MeG3 and MeG4, were formed, as shown in Scheme 4 and that compound 1 generated from 3 is an effective catalyst for
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the alcoholysis of L-LA and GA. We established that during Synthesis of [Li6(MesalO)6] (1)
methanolysis in the presence of 0.167 mol% of 1, L-LA was
LiBu (5 mL, 8 mmol) was added dropwise to a solution of
MesalOH (1.04 mL, 8 mmol) in 20 mL toluene. The mixture
was stirred at room temperature for 2 h. Then, the volume was
reduced to 10 mL and the white precipitate was filtered off,
washed with hexane (3 × 10 mL), and dried under vacuum.
Single crystals of 1 for XRD analysis were obtained as a result
of recrystallization from CH2Cl2 during the two-month crystal-
lization period. The crystals were filtered off, washed with
hexane (3 × 5 mL), and dried under vacuum. Yield 0.81 g
(64%). Anal. calcd for C48H42O18Li6: C, 60.78; H, 4.46. Found:
selectively ring opened to give only methyl (S,S)-O-lactyllactate
(MeL2), whereas GA was converted to the methyl glycolate
(MeG1) and oligoglycolate esters MeGn (n = 2, 3, and 4). A five-
fold increase in the amount of catalyst in the L-LA alcoholysis
switched the reaction course toward the synthesis of methyl
(S)-lactate (MeL1) as the main product. In the absence of a
catalyst source, L-LA was transformed to MeL2, whereas GA was
converted to a mixture of MeG2 and MeG4 esters. This is the
first time that this has been observed.
We believe that the current results represent an important
advance in the synthesis of new alkyl lactate and glycolate
derivatives via cyclic ester alcoholysis mediated by metal
alkoxides/aryloxides. Our study will be helpful in the design of
new efficient catalysts for the selective transformation of
lactones or hydroxy acid oligomers to alkyl α-hydroxy acid
esters for industrial applications.
1
C, 60.82; H, 4.49. H NMR (THF-d8, 600 MHz): δ 7.70 (dd, J =
8.0, 1.9 Hz, 6H, ArH), 7.09 (ddd, J = 8.7, 6.8, 1.9 Hz, 6H, ArH),
6.67 (dd, J = 8.7, 1.1 Hz, 6H, ArH), 6.24 (ddd, J = 8.0, 6.8, 1.1
Hz, 6H, ArH), 3.76 (s, 18H, CH3). 13C NMR (THF-d8, 151 MHz)
δ 173.6 (6C, CvO), 171.0 (6C, C–OLi), 134.7 (6C, ArH), 131.9
(6C, ArH), 124.5 (6C, ArH), 115.1 (6C, ArH), 111.7 (6C, Ar), 51.1
(6C, CH3). 7Li NMR (THF-d8, 155 MHz): δ 1.29. FTIR-ATR
(cm−1): 3352 (vw), 3018 (vw), 2949 (w), 2870 (w), 2653 (vw),
2323 (vw), 2194 (vw), 2162 (vw), 2050 (vw), 1981 (vw), 1917 (vw),
1681 (vs), 1601 (m), 1547 (s), 1508 (vw), 1473 (s), 1446 (s),
1437 (s), 1379 (vw), 1335 (s), 1319 (s), 1266 (m), 1226 (vs),
1194 (s), 1162 (m), 1146 (m), 1085 (s), 1043 (m), 989 (vw),
964 (w), 933 (vw), 884 (vw), 860 (s), 819 (m), 797 (w), 760 (vs),
708 (s), 661 (m), 589 (s), 541 (w), 519 (m), 472 (m), 436 (m),
405 (w).
Experimental section
Materials and methods
All syntheses were performed under a dry nitrogen atmo-
sphere, using standard Schlenk techniques. Reagents were
purified by standard methods: toluene, hexane, tetrahydro-
furan were distilled over Na; CH2Cl2 was distilled over P2O5,
CH3OH was distilled over Mg. L-Lactide and glycolide were
three times recrystallized from toluene and kept under P2O5.
All chemical reagents were purchased from commercial
sources: THF-d8, methyl salicylate, L-lactide, glycolide and
nBuLi solution 1.6 M in hexanes (Aldrich, St Louis, MO, USA),
toluene, hexane, THF, THF-d8, CH2Cl2, CH3OH, CD3OD and
Synthesis of [Li(MesalO)(MesalOH)] (2)
LiBu (5 mL, 8 mmol) was added dropwise to a solution of
MesalOH (2.08 mL, 16 mmol) in 20 mL toluene. The mixture
was stirred at room temperature for 6 h. Then, the volume was
reduced to 5 mL and the colorless oil was precipitated with
hexane (10 mL), and then filtered of, washed with hexane (3 ×
10 mL), and dried under vacuum. Single crystals of 2 for XRD
analysis were obtained as a result of recrystallization from THF
during the one week crystallization period. The crystals were
filtered off, washed with hexane (3 × 5 mL), and dried under
vacuum. Yield 1.41 g (57%). Anal. calcd for C16H15O6Li: C,
61.95; H, 4.81. Found: C, 62.02; H, 4.89. Compound 2 were
also easily synthesized by recrystallization of 1 from MesalOH
solution. 1H NMR (THF-d8, 600 MHz): δ 10.70 (s, 1H, OH), 7.77
(dd, J = 8.0, 1.8 Hz, 2H, ArH), 7.32 (ddd, J = 8.7, 7.1, 1.8 Hz,
2H, ArH), 6.81 (dd, J = 8.7, 1.0 Hz, 1H, ArH), 6.63 (m, 2H, ArH),
1
C2H5OH (Carl Roth). H and 13C NMR spectra were recorded at
room temperature using Bruker Avance 600 MHz and Jeol
JNM-ECZ 400 MHz spectrometers. Chemical shifts are reported
in ppm and referenced to the residual protons in deuterated sol-
vents. The 7Li spectra were referenced to a 0.1 M solution of
LiNO3 in D2O. Fourier-transform infrared attenuated total reflec-
tance (FTIR-ATR) spectra were recorded on a Bruker Vertex 70
Vacuum spectrometer. Elemental analysis was performed with a
PerkinElmer 2400 CHN elemental analyzer.
Characterization of ligand precursor methyl salicylate (MesalOH)
1H NMR (THF-d8, 600 MHz): δ 10.72 (s, 1H, OH), 7.81 (dd, J = 3.86 (s, 6H, CH3). 13C NMR (THF-d8, 151 MHz): δ 171.3 (2C,
8.0, 1.6 Hz, 1H, ArH), 7.45 (ddd, J = 8.5, 7.4, 1.6 Hz, 1H, ArH), CvO), 166.7 (2C, C–OLi), 135.9 (2C, ArH), 131.1 (2C, ArH),
6.93 (dd, J = 8.5, 1.2 Hz, 1H, ArH), 6.86 (ddd, J = 8.0, 7.4, 1.2, 120.5 (2C, ArH), 117.0 (2C, ArH), 113.9 (2C, ArH), 52.1 (2C,
1H, ArH), 3.92 (s, 3H, CH3). 13C NMR (THF-d8, 151 MHz) δ CH3). 7Li NMR (THF-d8, 155 MHz): δ 1.31. FTIR-ATR (cm−1):
171.2 (1C, CvO), 162.7 (1C, C–OH), 136.3 (1C, ArH), 130.4 (1C, 3334 (m), 3037 (vw), 2996 (vw), 2954 (m), 2937 (m), 2870 (w),
ArH), 119.6 (1C, ArH), 118.1 (1C, ArH), 113.1 (1C, Ar), 52.4 (1C, 2715 (vw), 2655 (vw), 2165 (vw), 2054 (vw), 1925 (vw), 1825 (vw),
CH3). FTIR-ATR (cm−1): 3186 (m), 2955 (w), 2901 (vw), 2854 1674 (vs), 1624 (m), 1590 (m), 1558 (w), 1505 (vw), 1483 (w),
(vw), 2049 (vw), 1937 (vw), 1674 (vs), 1614 (s), 1585 (m), 1485 1466 (m), 1454 (m), 1436 (s), 1376 (w), 1314 (s), 1250 (s), 1190
(s), 1465 (m), 1439 (s), 1404 (w), 1327 (s), 1302 (vs), 1251 (s), (m), 1143 (w), 1119 (vw), 1088 (m), 1037 (w), 967 (w), 954 (vw),
1211 (vs), 1194 (s), 1156 (vs), 1133 (s), 1089 (vs), 1032 (m), 962 884 (w), 855 (w), 823 (w), 772 (m), 751 (m), 706 (m), 659 (m),
(m), 865 (w), 848 (s), 800 (m), 754 (vs), 721 (s), 699 (vs), 665 (s), 641 (w), 616 (vw), 575 (m), 549 (w), 533 (m), 470 (w), 450 (m),
562 (m), 529 (s), 511 (m), 438 (m).
428 (m).
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Synthesis of [Li(MesalO)(MeOH)2] (3)
CH3). 13C NMR (C6D6, 101 MHz): δ 175.3 (1C, CvO), 170.5 (1C,
CvO), 69.3 (1C, CH), 66.9 (1C, CH), 51.8 (1C, OCH3), 20.7 (1C,
CH3), 16.5 (1C, CH3). MS-ESI found 199.0691, C7H12O5Na+
requires 199.0582. FTIR-ATR (cm−1): 3472 (m), 2991 (w), 2957 (w),
2885 (w), 2850 (vw), 1739 (vs), 1451 (m), 1378 (w), 1356 (w),
1308 (w), 1274 (m), 1197 (s), 1122 (vs), 1095 (vs), 1046 (s), 977 (m),
937 (w), 902 (vw), 865 (w), 848 (w), 748 (w), 666 (vw), 449 (vw).
LiBu (5 mL, 8 mmol) was added dropwise to a solution of
MesalOH (2.08 mL, 16 mmol) in 20 mL toluene/methanol
mixture (1 : 1). The mixture was stirred at room temperature
for 5 h. Then, the volume was reduced to 10 mL and the color-
less crystals were obtained from concentrated mother liquor.
The crystals were filtered off, washed with hexane (3 × 5 mL),
and dried under vacuum. Yield 1.08 g (61%). Anal. calcd for
C10H15O5Li: C, 54.06; H, 6.81. Found: C, 54.11; H, 6.84.
Compound 3 can be also easily synthesized by recrystallization
Methyl glycolate (MeG1)
1H NMR (C6D6, 400 MHz): δ 3.77 (s, 2H, CH2), 3.18 (s, 3H,
OCH3), 2.42 (s, 1H, OH). 13C NMR (C6D6, 101 MHz): δ 173.9
(1C, CvO), 60.5 (1C, CH2), 51.4 (1C, OCH3). FTIR-ATR (cm−1):
3424 (m), 3005 (vw), 2957 (w), 2913 (vw), 2848 (vw), 1736 (vs),
1436 (m), 1376 (w), 1279 (m), 1212 (s), 1089 (vs), 977 (m), 888
(w), 847 (w), 697 (w), 576 (w), 449 (w), 410 (vw).
1
of 1 from MeOH. H NMR (THF-d8, 600 MHz): δ 7.68 (dd, J =
8.1, 1.9 Hz, 1H, ArH), 7.08 (ddd, J = 8.7, 6.8, 2.0 Hz, 1H, ArH),
6.65 (dd, J = 8.7, 1.1 Hz, 1H, ArH), 6.22 (m, 1H, ArH), 3.96 (s,
2H, OHMeOH), 3.74 (s, 3H, CH3), 3.28 (s, 6H, CH3MeOH). 13C
NMR (THF-d8, 151 MHz) δ 173.6 (1C, CvO), 171.0 (1C, C–OLi),
134.7 (1C, ArH), 131.9 (1C, ArH), 124.5 (1C, ArH), 115.1 (1C,
ArH), 111.7 (1C, Ar), 51.1 (1C, CH3), 49.6 (2C, CH3MeOH). 7Li
NMR (THF-d8, 155 MHz): δ 1.57. FTIR-ATR (cm−1): 3141 (m),
2954 (m), 2935 (m), 2851 (m), 2821 (m), 2652 (vw), 2603 (vw),
2565 (vw), 2323 (vw), 2194 (vw), 2166 (vw), 2050 (vw), 1978 (vw),
1917 (vw), 1825 (vw), 1783 (vw), 1744 (vw), 1672 (vs), 1598 (m),
1545 (m), 1466 (s), 1445 (s), 1437 (s), 1326 (s), 1261 (m), 1223
(vs), 1199 (s), 1153 (s), 1140 (m), 1121 (w), 1084 (m), 1037 (s),
1025 (s), 966 (w), 861 (m), 818 (w), 802 (vw), 761 (s), 711 (s),
660 (w), 584 (m), 556 (w), 544 (vw), 485 (m), 447 (m), 434 (m),
415 (w).
Methyl glycolylglycolate (MeG2)
1H NMR (C6D6, 400 MHz): δ 4.18 (s, 2H, CH2), 3.94 (s, 2H,
CH2), 3.16 (s, 3H, OCH3), 2.42 (s, 1H, OH). 13C NMR (C6D6,
101 MHz): δ 172.7 (1C, CvO), 167.5 (1C, CvO), 60.6 (1C,
CH2), 60.4 (1C, CH2), 51.6 (1C, OCH3). MS-ESI found 171.0296,
C5H8O5Na+ requires 171.0269. FTIR-ATR (cm−1): 3484 (m), 3009
(vw), 2959 (w), 2857 (vw), 1740 (vs), 1440 (m), 1427 (m), 1384 (m),
1293 (w), 1171 (s), 1094 (s), 1047 (m), 1005 (m), 970 (m), 881 (w),
850 (w), 716 (w), 695 (w), 570 (w), 510 (vw), 444 (vw).
Methyl bis-(glycolyl)glycolate (MeG3)
1H NMR (C6D6, 400 MHz): δ 4.33 (s, 2H, CH2), 4.16 (s, 2H,
CH2), 3.91 (s, 2H, CH2), 3.15 (s, 3H, OCH3), 2.42 (s, 1H, OH).
MS-ESI found 229.0366, C7H10O7Na+ requires 229.0324.
Cyclic esters alcoholysis procedure
The typical alcoholysis procedure for synthesis of various alkyl
lactate or glycolate derivatives was as follows. For the catalyst
free reactions GA (1.22 g, 10.5 mmol) or L-LA (1.51 g,
10.5 mmol) was added to a 10.7 mL of MeOH at a stoichio-
metry of GA(L-LA)/MeOH = 1/25 (ESI, Table S2†). For GA and
L-LA alcoholysis reaction in the presence of catalyst a solution
of 3 (0.0233 g, 0.105 mmol) in MeOH (1 mL) was added to a
solution of GA (1.22 g, 10.5 mmol) or L-LA (1.51 g, 10.5 mmol)
in MeOH (9.7 mL) at a stoichiometry of GA(L-LA)/MeOH/Li = 1/
25/0.01 (SI, Table S3†). The reaction mixture was stirred for the
prescribed time. Next MeOH was removed under vacuum and
the conversion yields of GA and L-LA were determined by 1H
NMR spectroscopy.
Methyl tris-(glycolyl)glycolate (MeG4)
1H NMR (C6D6, 400 MHz): δ 4.32 (s, 2H, CH2), 4.31 (s, 2H,
CH2), 4.15 (s, 2H, CH2), 3.88 (s, 2H, CH2), 3.14 (s, 3H, OCH3),
2.42 (s, 1H, OH). MS-ESI found 287.0317, C9H12O9Na+ requires
287.0379.
Crystallography
XRD data were collected at 100 K using a KUMA KM4 CCD or
Agilent SuperNova Dual, Atlas diffractometer.57 The experi-
mental details and the crystal data are given in Table S1.† The
structures were solved by direct methods and refined by full-
matrix least-squares on F2, using the SHELXTL package.58
Non-hydrogen atoms were refined with anisotropic thermal
parameters. All hydrogen atoms were positioned geometrically
and added to the structure factor calculations, but were not
refined. The molecular graphics were created using Diamond,
version 3.1e.59 Crystallographic data for the structural analyses
reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre (CCDC), numbers
CCDC 1944070–1944072.†
Methyl (S)-lactate (MeL1)
1H NMR (C6D6, 400 MHz): 4.03 (q, J = 6.9 Hz, 1H, CH), 3.21 (s,
3H, OCH3), 2.45 (s, 1H, OH), 1.21 (d, J = 6.9 Hz, 3H, CH3). 13C
NMR (C6D6, 101 MHz): δ 176.1 (1C, CvO), 66.8 (1C, CH), 51.7
(1C, OCH3), 20.3 (1C, CH3). FTIR-ATR (cm−1): 3437 (m), 2986
(w), 2957 (w), 2892 (vw), 2850 (vw), 1732 (vs), 1452 (m), 1438
(m), 1372 (w), 1269 (m), 1214 (s), 1124 (vs), 1084 (m), 1046 (s),
978 (m), 917 (w), 842 (w), 757 (w), 678 (vw), 492 (vw).
Methyl (S,S)-O-lactyllactate (MeL2)
1H NMR (C6D6, 400 MHz): δ 4.98 (q, J = 7.1 Hz, 1H, CH), 4.16
(q, J = 6.9 Hz, 1H, CH), 3.20 (s, J = 1.1 Hz, 3H, OCH3), 2.47 (s,
Conflicts of interest
1H, OH), 1.41 (d, J = 6.9 Hz, 3H, CH3), 1.12 (d, J = 7.1 Hz, 3H, The authors declare no competing financial interests.
Dalton Trans.
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Paper
12 Y.-F. Liang, X.-F. Zhou, S.-Y. Tang, Y.-B. Huang, Y.-S. Feng
and H.-J. Xu, Lithium tert-butoxide mediated α-alkylation
of ketones with primary alcohols under transition-metal-
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Author contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding sources
This work was supported by the Polish National Science
Center grant numbers 2017/26/D/ST5/01123 and 2018/29/B/
ST5/00341.
15 J. Kido and T. Matsumoto, Bright organic electrolumines-
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