Angewandte Chemie International Edition
10.1002/anie.201804368
COMMUNICATION
Renewable polyethers via GaBr
polyesters
3
catalyzed reduction of
[a]
[b]
[a]
[b]
Patrick-Kurt Dannecker, Ursula Biermann,* Marc von Czapiewski, Jürgen O. Metzger and
Michael A. R. Meier*[a]
Abstract: Herein, we demonstrate a novel approach for the synthesis
of middle and long chain aliphatic polyethers 2 by applying the GaBr
great variety of chain lengths of the diacid and the diol. Herein,
we thus report on the application of the GaBr /TMDS system to
3
-
3
catalyzed reduction with TMDS as reducing agent to polyesters 1.
Thus, various linear and branched aliphatic polyesters 1 were
prepared and systematically investigated for this reduction strategy,
demonstrating the applicability and versatility of this new polyether
synthesis protocol. Middle and long methylene chain polyethers were
obtained from the respective polyesters without or with minor chain
degradation, whereas short chain polyesters, such as poly-L-lactide
predominantly renewable polyesters 1a-l as a new route for the
synthesis of polyethers 2a-l (Scheme 1). Polyesters 1a-h were
prepared from the respective dimethyl esters and their
corresponding diols, polyesters 1i and 1k from L-lactide and
ε-caprolactone by ring-opening polymerization and 1l from methyl
10-hydroxyundecanoate. Poly[(R)-3-hydroxybutanoate] 1j is
produced by cupriavidus necator bacteria and was obtained
commercially. The respective polymerizations of diol and dimethyl
ester were carried out without solvent at elevated temperatures
1i and poly[(R)-3-hydroxybutanoate] 1j, showed major chain
degradation. In this way, yet unavailable and uncommon polyethers
were obtained and studied.
and reduced pressure using either titanium isopropoxide
i
(
Ti(OPr)
4
)
or 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as
catalyst. Ring-opening polymerizations of L-lactide 1i and
ε caprolactone 1j were carried out in CH Cl with pyrene butanol
as initiator and TBD as catalyst. Since polyester 1e is insoluble in
most common solvents at room temperature, polyesters 1f-h
were synthesized representing soluble variants with a long
distance in between the ester groups. Thus, polyesters with 4 to
Aliphatic polyethers, such as poly(ethylene oxide), poly(propylene
2
2
oxide) and poly(tetramethylene oxide), constitute
a well-
established and industrially important class of polymers being
produced by very efficient ring-opening polymerization of the
respective cyclic ethers.[1] Procedures for the synthesis of other
polyethers, for instance the acid catalysed polycondensation of
glycols[2] or polycondensation of glycols and dihalides,[3] are
severely limited in terms of the achievable molecular weight. On
the other hand, some polyethers bearing longer aliphatic
segments have been synthesized by ADMET polymerizations of
22 methylene groups (as well as heteroatom linkages in the case
of 1h) between the ester moieties were obtained. The structure of
1
these polyesters was confirmed by H-NMR and IR-spectroscopy.
GPC analysis showed number average molecular weights up to
-1
3
1,700 g mol (Table 1).
,-diene ethers.[4] However, the synthesis of the ,-diene
[4d, 4e]
ethers was not straightforward.
Most recently, we reported
on a general protocol for the high-yielding reduction of aliphatic
esters to the respective ethers using GaBr as catalyst and
3
tetramethyldisiloxane (TMDS) as reducing agent.[5] This protocol
could be applied successfully also to the reduction of ,-diene
esters giving easy access to the respective ,-diene ethers.[6]
Triglycerides could be reduced as well to give the respective
glyceryl trialkyl ethers.[7] Further methods were recently
reviewed[ and the most relevant reductions of esters to ethers
with silanes involve lewis acids catalysts based on indium,[9]
iron[10] or ruthenium.[11]
8]
A novel, powerful and generally applicable method for the
synthesis of middle and long chain aliphatic polyethers would be
of high interest and is demonstrated herein by the successful and
up to now unknown reduction of polyesters, being available in a
[
[
a]
b]
P.-K. Dannecker, Dr. M. von Czapiewski, Prof. Dr. M. A. R. Meier
Institute of Organic Chemistry (IOC)
Karlsruhe Institute of Technology (KIT)
Straße am Forum 7
E-mail: m.a.r.meier@kit.edu
Dr. U. Biermann, Prof. Dr. J. O. Metzger
Institute of Chemistry
Scheme 1. Catalytic reduction of renewable polyesters 1a-h (obtained from the
respective dimethyl esters and diols), 1i and 1k (obtained by ring-opening
polymerization of L-lactide and ε-caprolactone, respectively), 1j (natural origin)
and 1l (obtained from methyl 10-hydroxyundecanoate) to polyethers 2a-l.
University of Oldenburg
Carl-von-Ossietzky-Straße 9-11
While we already demonstrated the successful reduction of
esters to ethers on small organic molecules,[5, 7] a transfer of this
reaction to polymers is highly demanding: For instance, even a
Supporting information for this article is given via a link at the end of
the document.
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