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Dialkylgallium Alkoxides Stabilized with N‑Heterocyclic Carbenes: Opportunities and Limitations for the Controlled and Stereoselective Polymerization of rac-Lactide

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posted on 2015-07-27, 00:00 authored by Paweł 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.

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