Dialkylgallium Alkoxides Stabilized with N‑Heterocyclic Carbenes: Opportunities and Limitations for
the Controlled and Stereoselective Polymerization of rac-Lactide
posted on 2015-07-27, 00:00authored byPaweł Horeglad, Martyna Cybularczyk, Bartosz Trzaskowski, Grażyna
Zofia Żukowska, Maciej Dranka, Janusz Zachara
The structure of a series of Me2GaOR(NHC) complexes
with N-heterocyclic carbenes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene
(SIMes) and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes))
have been characterized using spectroscopic and X-ray techniques and
discussed in view of their reactivity in the polymerization of rac-lactide (rac-LA). Both structure studies
and density functional theory (DFT) calculations show the significant
influence of NHC and OR on the structure of investigated complexes
and has indicated that the Ga–CNHC bond (32.6–39.6
kcal mol–1) is strong enough to form stable Me2GaOR(NHC) complexes in the form of monomeric species. The
reactivity of Me2Ga((S)-OCH(Me)CO2Me)(SIMes) (1) and Me2Ga((S)-OCH(Me)CO2Me)(IMes) (5) toward
Lewis acids such as CO2 and GaMe3 has resulted
in breaking of the Ga–CNHC bond with the formation
of (NHC)CO2 and Me3Ga(NHC) (8 and 10) and [Me2Ga(μ-(S)-OCH(Me)CO2Me)]2. Different results have been obtained for
l,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene (SIPr), which
coordinates more weakly to gallium, as demonstrated by the Ga–CNHC bond strength for model Me3GaSIMes, Me3GaIMes (8), and Me3GaSIPr (10) adducts. The reaction of SIPr with [Me2Ga(μ-OR)]2 has not allowed for the breaking of Ga2O2 bridges and the formation of monomeric Me2GaOR(SIPr)
complexes, contrary to SIMes and IMes. In the case of the reaction
with [Me2Ga(μ-(S)-OCH(Me)CO2Me)]2, the ionic compound [Me2Ga(OCH(Me)CO2)]−[SIPrH]+ (9)
has been isolated. The investigated Me2GaOR(NHC) complexes
are highly active and stereoselective in the ring-opening polymerization
of rac-lactide from −20 °C to room temperature,
due to the insertion of rac-LA exclusively into the
Ga–Oalkoxide bond, leading to isotactically enriched
polylactide (PLA) (Pm = 0.65–0.78).
It has been shown that the polymerization of lactide at low temperature
is influenced by the chelate interaction of (S)-OCH(Me)CO2Me or (OCH(Me)C(O))2OR resulting from the primary
insertion of rac-LA into the Ga–Oalkoxide bond, with the Ga center, which can be responsible for the low control
over the molecular weight of the obtained PLA. The latter effect can
be eliminated by the initial synthesis of Me2Ga((PLA)nOR)(NHC) with short PLA chains, which allows
for controlled polymerization. Although the adverse chelate effect
can be also eliminated by the polymerization of rac-LA at room temperature, the stereoselectivity of rac-LA polymerization is strongly affected by transesterification reactions.
Out of investigated Me2GaOR(SIMes) and Me2GaOR(IMes)
complexes, only the latter allowed for the immortal ring opening polymerization
of rac-LA in the presence of iPrOH.