From meso-Lactide to Isotactic Polylactide: Epimerization by B/N Lewis Pairs and Kinetic Resolution by Organic Catalysts

Article abstract of DOI:10.1021/jacs.5b08658

B/N Lewis pairs have been discovered to catalyze rapid epimerization of meso-lactide (LA) or LA diastereomers quantitatively into rac-LA. The obtained rac-LA is kinetically polymerized into poly(l-lactide) and optically resolved d-LA, with a high stereoselectivity k_L/k_D of 53 and an ee of 91% at 50.6% monomer conversion, by newly designed bifunctional chiral catalyst 4 that incorporates three key elements (β-isocupreidine core, thiourea functionality, and chiral BINAM) into a single organic molecule. The epimerization and enantioselective polymerization can be coupled into a one-pot process for transforming meso-LA directly into poly(l-lactide) and d-LA.

Full text of DOI:10.1021/jacs.5b08658

Communication
pubs.acs.org/JACS
From meso-Lactide to Isotactic Polylactide: Epimerization by B/N
Lewis Pairs and Kinetic Resolution by Organic Catalysts
Jian-Bo Zhu and Eugene Y.-X. Chen*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States
S
* Supporting Information
and any incorporation of meso-LA into the PLLA product will
ABSTRACT: B/N Lewis pairs have been discovered to
catalyze rapid epimerization of meso-lactide (LA) or LA
diastereomers quantitatively into rac-LA. The obtained rac-
LA is kinetically polymerized into poly(L-lactide) and
optically resolved D-LA, with a high stereoselectivity kL/kD
of 53 and an ee of 91% at 50.6% monomer conversion, by
newly designed bifunctional chiral catalyst 4 that
incorporates three key elements (β-isocupreidine core,
thiourea functionality, and chiral BINAM) into a single
organic molecule. The epimerization and enantioselective
polymerization can be coupled into a one-pot process for
transforming meso-LA directly into poly(L-lactide) and D-
LA.
significantly alter or deteriorate the properties of the PLLA
materials such as crystallinity and biodegradation rate.11
The ROP of meso-LA typically forms predominantly atactic,
amorphous PLA,12 but the stereoselective ROP of meso-LA has
led to syndiotactic PLA (st-PLA)13 or amorphous heterotactic
PLA.14 The crystalline st-PLA has a melting-transition temper-
ature (Tm) of ∼150 °C,13f which is about 20−30 °C lower than
that of it-PLA, thus an inferior material. Hence, it is of great
interest to be able to catalytically isomerize meso-LA to rac-LA.15
However, the epimerization under base-catalyzed homogeneous
conditions is controlled by the equilibrium between meso-LA and
rac-LA, achieving only an equilibrium mixture;8 citing the state-
of-the-art example here, epimerization of meso-LA by sodium
ethylhexanoate (0.05%) in bulk at 160 °C for 15 h afforded a
mixture containing 36% L-LA + 36% D-LA + 28% meso-LA, plus
0.5 wt % linear oligomers.15
In view of the above challenges facing effective utilization of
meso-LA, we devised a strategy for conversion of meso-LA directly
into the desired product it-PLA by organic catalysts, through a
two-step process that can be carried out in a one-pot fashion:
epimerization of meso-LA to rac-LA, followed by stereoselective
or enantioselective polymerization (Scheme 1). To realize this
strategy, one must first develop an epimerization reaction that
can achieve quantitative conversion of meso-LA to rac-LA and
oly(lactide) (PLA) is one of the most commercially
Pimportant biodegradable and biocompatible polymers,
with a wide array of applications in packaging, microelectronics,
and biomedical fields.1 Crystalline isotactic PLA (it-PLA) is
produced by either ring-opening polymerization (ROP) of S,S-
lactide (L-LA) to isotactic poly(L-lactide) (PLLA) or the
stereoselective ROP of rac-LA by metal-based catalysts or
initiators.2 The emerging organopolymerization of LA offers a
metal-free alternative to the PLA products that are free of residual
metal contaminates as desired for biomedical or microelectronic
applications.3 The stereoselective organopolymerization of rac-
LA has been achieved by a phosphazene superbase (tBu-P2) and
bulky N-heterocyclic carbenes (NHCs) at low temperatures,4
and enantioselective or kinetic resolution polymerization of rac-
LA by chiral organic catalysts has also been made possible by a
cinchona alkaloid5 and chiral phosphoric acids.6
Scheme 1. Proposed New Strategy for Utilization of meso-LA
The current industrial PLLA production relies on the large-
scale production of L-LA, which is furnished via metal (tin)-
catalyzed depolymerization of a low molecular weight (MW)
condensation prepolymer of L-lactic acid.7 However, this process
affords a mixture of LA stereoisomers that are predominately L-
LA but also a considerable amount of meso-LA as a byproduct or
waste that needs to be removed from the rest of the LA stream,
seriously affecting the economy of the manufacturing of PLLA.8
In addition, as a possible feedstock-recycling pathway, thermal
degradation of PLLA produced many kinds of degradation
products including LA diastereomers (rac-LA/meso-LA = 2/1),
cyclic oligomers, and their diastereomers as well as CO2, CO,
CH3CHO, and CH2CHCOOH.9 Hence, racemization causes
a serious problem with the feedstock recycling and the
reproduction of PLLA.10 The optical purity of LA significantly
affects the materials properties of the PLLA produced by ROP,
Received: August 15, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b08658
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Journal of the American Chemical Society
Communication
then discover an enantioselective catalyst that can perform the
effective kinetic resolution polymerization of rac-LA.
lowered the conversion to ∼20% or below. On the other hand,
use of the relatively nonpolar toluene, which enabled
precipitation of rac-LA from solution once formed markedly
enhanced the meso-to-rac-LA conversion to 95%. Increasing the
initial [meso-LA]0 concentration from 0.2 M to 0.5, 1.0, 1.5, and
2.0 M resulted in a steady increase in the conversion from 86% to
93, 95, 97, and 98%, respectively (Table S1). While keeping the
concentration at 2.0 M, we gradually lowered the LP loading
from 2.0 mol % to 1.0, 0.5, 0.2, 0.1, 0.05, 0.01, 0.005, and 0.002
mol % (i.e., 20 ppm) and found the meso-to-rac-LA conversion
still remained at an essentially quantitative level of 97−99%. The
epimerization is rapid, as shown by the reaction with 0.01 mol %
of the LP, which achieved 95.4% meso-to-rac-LA conversion in
just 5 min (Figure 1). To more accurately assess the rate of the
epimerization, the reaction was stopped at different times, and
the conversion−time plot gave a very high turnover frequency of
81.8 s−1 or 2.95 × 105 h−1 (Figure S1). The epimerization can
also be carried out in bulk at 70 °C, still achieving a high meso-to-
rac-LA conversion of 94−97%, depending on catalyst loading
and reaction time (Table S1). At this temperature, precipitation
of rac-LA from meso-LA shifts the equilibrium further toward rac-
LA. This LP-catalyzed epimerization worked similarly, irre-
spective of the initial meso-LA/rac-LA ratio in the feed (Table
S1). Isolation of the epimerization product was performed on a
5.76 g scale of the reaction with 20 ppm catalyst loading in
toluene at RT; upon completion of the reaction, simple filtration,
followed by washing with cold toluene and drying under vacuum,
afforded the pure rac-LA in 88.2% isolated yield. The filtrate was
a mixture of rac-LA (95%) and meso-LA (5%), which can be
reused for the epimerization. Alternatively, the crude epimeriza-
tion product was purified by flash chromatography on silica gel to
give rac-LA (>98% purity) in 98.6% yield.
To the first task, we reasoned that the dynamic meso-LA ⇔ rac-
LA equilibrium should be controlled by temperature, base,
solvent, and solubility difference. For instance, carrying out the
epimerization at lower temperatures (e.g., 25 °C or room
temperature, RT) should shift the equilibrium further toward
rac-LA8 and also avoid any LA oligomerization, and finding
conditions that can continuously remove the formed rac-LA from
the reaction mixture should drive the epimerization to
completion. However, an immediate challenge was the
epimerization rate at RT is too low to be practical. Indeed, our
initial screening of some potent base catalysts such as quinidine
(QD), β-isoquinidine (β-IQD), 1,4-diazabicyclo [2.2.2]octane
(DABCO), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) for
the meso-LA epimerization reaction with 2 mol % base at RT
rendered no or negligible epimerization after up to 2 days of
reaction, or just resulted in polymerization (by DBU). Inspired
by the “frustrated Lewis pair” (FLP) chemistry16 that takes
advantage of the unquenched, orthogonal Lewis acid and Lewis
base reactivity for FLP-type activation of small molecules or
tandem catalysis (FLP- and classical LP working in concert) in
polymerization of polar monomers,17 we turned our attention to
investigations into LPs for meso-LA epimerization. Just like the
above Lewis bases, the Lewis acids such as B(C6F5)3, when used
alone, exhibited no activity. Excitingly, when combined together,
the LP DABCO/B(C6F5)3 catalyzes rapid, essentially quantita-
tive epimerization of meso-LA to rac-LA (Figure 1).
This observed remarkable performance of the LP is
presumably due to the cooperativity of the LP in that the [B]
Lewis acid activates the substrate meso-LA via carbonyl
coordination, which accelerates the depronation at the tertiary
carbon of the activated substrate by the [N] base (I), leading the
planar enolate intermediate (II) that can be reprotonated causing
epimerization. An alternative mechanism that possibly involves
reshuffling of the two halves of the lactide by transesterification
was excluded as the same mixture of rac/meso lactides in a ratio of
ca. 82:18 was formed when homochiral L-LA was treated under
the same epimerization conditions as described above for the
meso-LA. Noteworthy is that use of the preformed classical Lewis
adduct (insoluble) DABCO·B(C6F5)318 performed identically to
the current standard procedure that premixes the borane with
meso-LA, followed by addition of DABCO to start the reaction.
This result indicates that, under the current catalytic conditions,
this adduct can reversibly and rapidly dissociate into the
respective base and acid to catalyze the epimerization.
Figure 1. Left: NMR spectra (CDCl3) of the methyl region of: (a) pure
meso-LA; (b) meso-LA/rac-LA (90/10) from NatureWorks; and (c)
isolated pure rac-LA from epimerization of meso-LA. Right: pictorial
representation of fast epimerization of meso-LA to rac-LA (which
precipitates out of solution) by DABCO/B(C6F5)3 (2.88 g scale, 2.0 M
in toluene, 5 min, 95.4% conversion).
When carried out in CH2Cl2 (1.0 M) by DABCO/B(C6F5)3
(2.0 mol %), the meso-LA epimerization reaction achieved a
maximum meso-to-rac-LA conversion of 82%. Starting with rac-
LA under the same conditions, the same mixture of rac/meso
lactides formed in a ratio of ca. 82:18. Variations of acids and
bases showed that DABCO/B(C6F5)3 and Et3N/B(C6F5)3
performed similarly and appeared to give the best conversion
(Table S1). Thus, one of these LPs, DABCO/B(C6F5)3, was
used for further optimization. Changing the solvent to more
polar, donor solvents such as DMF and DMSO drastically
Having achieved the first task of quantitative epimerization of
meso-LA to rac-LA, we tackled the second task of developing an
enantioselective catalyst that can perform the effective kinetic
resolution polymerization of rac-LA. We reasoned that the
bifunctional β-isocupreidine (β-ICD), which represents the first
example of organocatalytic enantioselective polymerization of
B
DOI: 10.1021/jacs.5b08658
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Journal of the American Chemical Society
Communication
a
Table 1. Kinetic Resolution Polymerization of rac-LA at 25 °C by Chiral Organic Catalysts
b
c
d
e
e
run no.
cat.
solvent (rac-LA conc.)
[M]/[Cat]/[I]
time (h)
conv. (%)
ee (%)
kS/kR (s)
Mn (kg/mol)
Đ (Mw/Mn)
1
2
β-ICD
DCM (1.67M)
DCM (1.67M)
DCM (1.67M)
DCM (1.67M)
DCM (1.67M)
DCM (1.67M)
DCM (1.67M)
FB (0.31M)
50/1/1
50/1/1
100/1/1
50/1/1
50/1/1
50/1/1
100/1/1
50/1/1
50/1/1
100/1/1
100/1/1
100/1/1
100/1/1
100/1/1
100/1/1
6
131
7
47.6
42.4
47.8
53.5
52.4
50.1
50.2
50.8
50.6
50.5
51.0
49.5
51.0
48.8
49.0
40
33
63
25
3.8
3.6
10
5.53
5.37
11.3
6.72
6.88
6.38
10.3
6.10
7.21
11.0
10.4
11.0
10.9
10.0
11.1
1.14
1.14
1.13
1.10
1.11
1.16
1.14
1.12
1.14
1.12
1.13
1.13
1.15
1.12
1.13
CPD
1
3
4
2
36
22
7
1.9
0.4
32
f
5
3
−33
6
4
85
83
89
91
88
88
64
67
64
7
4
25
30
17
43
40
18
48
29
18
26
8
4
41
9
4
DFB (0.31M)
DFB (0.31M)
DFB (0.31M)
DCM (1.67M)
DCM (1.67M)
DCM (1.67M)
DCM (1.67M)
53
10
4
38
g
11
4
35
12
13
14
15
5
8.8
9.0
9.7
0.5
6
7
f
8
−24
a
Carried out at 25 °C unless indicated otherwise, using BnOH as initiator (I), in dichloromethane (DCM), toluene (TOL), fluorobenzene (FB), or
b
c
1
o-difluorobenzene (DFB). Determined by H NMR in CDCl3. Enantiomeric excess of the unreacted monomer measured by chiral HPLC.
d
e
Calculated from {ln[(1 − conv.)(1 − ee)]}/{ln[(1 − conv.)(1 + ee)]}. Number-average molecular weight (Mn) and polydispersity index (Đ =
fL-LA
Mw/Mn) determined by gel-permeation chromatography (GPC) at 40 °C in DMF relative to poly(methyl methacrylate) (PMMA) standards.
was left. One-pot reaction from meso-LA.
g
rac-LA but with a low stereoselectivity factor (s = kS/kR or kL/kD)
of 4.4 and a low ee value of 45.1% (kinetically resolved D-LA) at
48.4% monomer conversion,5 could be modified or redesigned
into advanced chiral organic catalysts with potentially much
higher stereoselectivity. While both β-ICD and cupreidine
(CPD) are effective for partial kinetic resolution of rac-LA
(runs 1 and 2, Table 1), their methylated derivatives β-IQD and
QD had no activity for ROP of rac-LA, which reveals the phenol
OH in β-ICD and CPD is crucial as an H-bonding donor in this
bifunctional catalysis. Replacing the OH group in β-ICD with a
thiourea group, a double H-bonding donor, furnished catalyst 1
(Figure 2),19 which significantly enhanced not only the ROP
and QN precursors, the stereoselectivity of 2 and 3 (runs 4 and
5) was much inferior to 1, β-ICD, or CPD.
The observed much improved activity and stereoselectivity by
1 pointed us in the right direction for developing better catalysts
based on derivatizing β-ICD. To this end, we reasoned that
introduction of a chiral double H-bonding donor based on the
binaphthyl-amine (BINAM) framework should exert double
stereodifferentiation due to asymmetric activation of both the
monomer (by the chiral acid) and the propagating P−OH
species (by the chiral amine). It has recently been reported that a
BINOL-based phosphoric acid achieved a high s factor of kD/kL =
28 at 49% monomer conversion.6 Accordingly, we designed and
synthesized new catalyst 4, which combines three essential
elements (β-ICD core, thiourea functionality, and BINAM) into
a single molecule. Indeed, 4 exhibited drastically enhanced the
stereoselectivity over catalyst 1, now achieving a high s factor of
32 and 85% ee at 50.1% monomer conversion (run 6). Lowering
the catalyst loading from 2 to 1 mol % increased the PLA Mn to
1.03 × 104 g/mol (Đ = 1.14, run 7) while maintaining similar
stereoselectivity. Optimization of the reaction by using
fluorobenzene as the solvent further enhanced the s factor to
41 and the ee value to 89% at 50.8% monomer conversion (run
8). When o-difluorobenzene (DFB) was used, the s factor
reached the highest of 53 and the ee value of 91% at 50.6%
monomer conversion (run 9). Changing the X group on the
BINAM from −NHAc (4) to −NHCOCF3 (5), −NHCOCH
CHMe (6), and −NHCOAr (7, Ar = 4-CF3C6H4) drastically
reduced catalyst stereoselectivity (runs 12−14). Flipping the
BINAM from R to S (8) yielded a much inferior catalyst (run 15).
With both tasks now having been met, we set out to examine
the possibility of carrying out meso-LA to rac-LA epimerization
and enantioselective polymerization in a one-pot fashion. We
first confirmed that the isolated rac-LA from the epimerization
reaction was polymerized into it-PLA (Pm = 0.94, at −75 °C,
Figure 3), the result of which is nearly identical to the ROP of rac-
LA (Pm = 0.95, at −75 °C, or Pm = 0.72, at 20 °C) reported in the
Figure 2. Structures of chiral organic catalysts used for kinetic resolution
polymerization of rac-LA in this study.
activity but also the stereoselectivity with s = 10 and ee = 63% at
47.8% conversion in dichloromethane (run 3). Substituting the
OH group in QD and quinine (QN) with the thiourea group
gave the corresponding pseudoenantiomers 2 and 3;20 although
this substitution turned on the ROP activity of the inactive QD
literature,4a using the same catalyst phosphazene Bu-P2. In
t
comparison, the chiral catalyst 4 produced it-PLA with a higher
Pm value of 0.82 at 25 °C (the ee of the unreacted monomer was
C
DOI: 10.1021/jacs.5b08658
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Journal of the American Chemical Society
Communication
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (NSF-1300267) and
Keck Foundation for financial support. We also thank Nature-
Works and Boulder Scientific Co. for the research gifts of meso-
LA and B(C6F5)3, respectively.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b08658.
■
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Experimental details and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Author
*eugene.chen@colostate.edu
■
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/jacs.5b08658
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX