The first facile stereoselectivity switch in the polymerization of rac-lactide—from heteroselective to isoselective dialkylgallium alkoxides with the help of N-heterocyclic carbenes

Horeglad, Paweł; Szczepaniak, Grzegorz; Dranka, Maciej; Zachara, Janusz

doi:10.1039/c2cc16072b

Cited by 81 publications

(75 citation statements)

References 41 publications

(6 reference statements)

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“…The reaction of 6-enyl-9-aryl-, 6,10-dienyl-13-aryl-, or 6,10,14-trienyl-17-aryl-propargylic silyl ethers in the presence of InI 3 (20 mol%) afforded the corresponding tricyclic, tetracyclic, or pentacyclic allylic silyl ethers (54). 82 The use of InBr 3 and other metal-based Lewis acids led to lower catalytic activity.…”

Section: Cascade Polycyclization Of Polyenyl Propargylic Silyl Ethersmentioning

confidence: 99%

Gallium and Indium Compounds in Homogeneous Catalysis

Dagorne

Fliedel

Frémont

2016

Encyclopedia of Inorganic and Bioinorganic Chemistry

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The present contribution highlights the most recent and representative developments on the use of Ga and In species in homogeneous catalysis. Recent studies of various ligand‐supported Ga(III) and In(III) compounds in cyclic esters polymerization catalysis for the production of biodegradable polyester materials are also included. Various organic transformations may now be catalyzed using Ga(III) and In(III) Lewis acids allowing access to a wide range of valuable organic molecules through diverse reaction sequences. The recent use of stable and robust N ‐heterocyclic carbene Ga(III) and In(III) compounds as strong π‐Lewis acid catalysts for various organic reactions is also discussed: such species are certainly promising for the future developments of Ga(III)‐ and In(III)‐mediated reactions. Although Lewis acid‐assisted reactions certainly dominate the field in Ga(III) and In(III) chemistry, discrete low‐valent In(I) species, presently emerging in catalysis, clearly display a distinct reactivity allowing their exploitation both as soft Lewis acids and alkyl‐/allyl‐/arylating agents.

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Section: Cascade Polycyclization Of Polyenyl Propargylic Silyl Ethersmentioning

confidence: 99%

Gallium and Indium Compounds in Homogeneous Catalysis

Dagorne

Fliedel

Frémont

2016

Encyclopedia of Inorganic and Bioinorganic Chemistry

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“…Previously, we have shown that 1 reacts with weak Bronsted acid CDCl 3 to form (S,S)-[Me 2 Ga(μ-OCH(Me)C(O)OMe)] 2 and SIMesD(CCl 3 ) adducts. 19 Moreover, the equimolar reaction between 1 and Me 3 Ga resulted in the formation of Me 3 Ga(SIMes) and (S,S)- 26g The analogous reaction for 5 allowed for the isolation of Me 3 Ga(IMes) (8) (Scheme 2). The structure of 8 was determined both in the solid state by X-ray analysis ( Figure 7) and in solution, and is discussed later in the text along with the coordination properties of NHC toward akylgallium complexes.…”

Section: ■ Introductionmentioning

confidence: 97%

“…It represents the first example of a facile switch of stereoselectivity in the polymerization of rac-LA. 19 In the above case, Me 2 GaOR-(NHC) complexes were also the very rare example of isoselective catalysts highly active at low temperatures. 7a,20 Therefore, the detailed understanding of the influence of NHCs on the structure and activity of Me 2 GaOR(NHC) should be important for the design of new catalysts based on main group metal alkoxides for the controlled and stereoselective polymerization of PLA and other heterocyclic monomers.…”

Section: ■ Introductionmentioning

confidence: 97%

“…For 1, the possibility of chelate interaction was demonstrated by FTIR in solution. Although initial FTIR studies showed one band at 1737 cm −1 corresponding to the free CO group, 19 the deconvolution of FTIR spectra of 1, in CH 2 Cl 2 , showed two bands, a major band at 1737 cm −1 (79%) in line with the free CO of the (S)-melac ligand and a minor one at 1717 cm −1 (21%) corresponding to the CO coordinated to gallium. This observation indicates weak and labile coordination of the (S)-melac ligand to gallium due to the Ga···OC interaction.…”

Section: ■ Introductionmentioning

confidence: 98%

“…Compounds Me 2 Ga(OCH 2 CH 2 OMe)(SIMes) (3) ( Figure 1) and Me 2 Ga(OCH(Me)CH 2 OMe)(SIMes) (4) (Figure 2) were synthesized, analogously to previously obtained 1 and 2, (19).…”

Section: ■ Introductionmentioning

confidence: 99%

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

et al. 2015

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The structure of a series of Me 2 GaOR(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−C NHC bond (32.6− 39.6 kcal mol −1 ) is strong enough to form stable Me 2 GaOR-(NHC) complexes in the form of monomeric species. The reactivity of Me 2 Ga((S)-OCH(Me)CO 2 Me)(SIMes) (1) and Me 2 Ga((S)-OCH(Me)CO 2 Me)(IMes) (5) toward Lewis acids such as CO 2 and GaMe 3 has resulted in breaking of the Ga−C NHC bond with the formation of (NHC)CO 2 and Me 3 Ga(NHC) (8 and 10) and [Me 2 Ga(μ-(S)-OCH(Me)CO 2 Me)] 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−C NHC bond strength for model Me 3 GaSIMes, Me 3 GaIMes (8), and Me 3 GaSIPr (10) adducts. The reaction of SIPr with [Me 2 Ga(μ-OR)] 2 has not allowed for the breaking of Ga 2 O 2 bridges and the formation of monomeric Me 2 GaOR(SIPr) complexes, contrary to SIMes and IMes. In the case of the reaction with [Me 2 Ga(μ-(S)-OCH(Me)CO 2 Me)] 2 , the ionic compound [Me 2 Ga(OCH(Me)-CO 2 )] − [SIPrH] + (9) has been isolated. The investigated Me 2 GaOR(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−O alkoxide bond, leading to isotactically enriched polylactide (PLA) (P m = 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)CO 2 Me or (OCH(Me)C(O)) 2 OR resulting from the primary insertion of rac-LA into the Ga−O alkoxide 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 Me 2 Ga((PLA) n OR)(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 Me 2 GaOR(SIMes) and Me 2 GaOR(IMes) complexes, only the latter allowed for the immortal ring opening polymerization of rac-LA in the presence of i PrOH.

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Stereoselective Ring‐Opening Polymerization of Heterocyclic Monomers

Takeuchi

2014

Encyclopedia of Polymer Science and Technology

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Recent progress in stereoselective ring‐opening polymerization of heterocyclic monomers, especially polymerization and copolymerization of epoxides and lactones, is summarized. Zn, Al, Co, and Cr complexes promote stereospecific ring‐opening polymerization of epoxides as well as copolymerization of epoxides with carbon dioxide. Enantioselective polymerization of epoxide is also achieved to afford optically active polyethers. Cr and lanthanide complexes promote stereoselective polymerization of β‐butyrolactone. Stereospecific polymerization of lactide has been achieved by using varieties of metal catalysts, including Al, Ga, In, Mg, Ca, Sn, Zn, Ti, Zr, Hf, Ge, and rare earth metals. Organic catalysts such as carbenes and phosphazene bases are also utilized for the synthesis of stereoregular polylactide. By utilizing those stereoselective polymerizations, more stereogradient and stereoblock copolymers have been synthesized. Polylactides with high degree of stereoregularity show high melting points. One‐to‐one mixtures of isotactic poly( l ‐lactide) and poly( d ‐lactide) or stereoblock copolymer of l ‐lactide and d ‐lactide show higher melting points compared with the polymer of homochiral monomer, due to the formation of stereocomplex.

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The first facile stereoselectivity switch in the polymerization of rac-lactide—from heteroselective to isoselective dialkylgallium alkoxides with the help of N-heterocyclic carbenes

Cited by 81 publications

References 41 publications

Gallium and Indium Compounds in Homogeneous Catalysis

Gallium and Indium Compounds in Homogeneous Catalysis

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

Stereoselective Ring‐Opening Polymerization of Heterocyclic Monomers

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