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Chiral (1,2)-diphenylethylene-salen complexes of triel metals: coordination patterns and mechanistic considerations in the isoselective ROP of lactide.

Abstract : The synthesis of enantiomerically pure aluminium, gallium and indium complexes supported by chiral (R,R)-((HH)ONNO(HH)) (1), (R,R)-((MeH)ONNO(HMe)) (2), (R,R)-((tButBu)ONNO(tButBu)) (3), (R,R)-((MeNO2)ONNO(MeNO2)) (4), (R,R)-((HOMe)ONNO(HOMe)) (5) and (R,R)-((ClCl)ONNO(ClCl)) (6) (1,2)-diphenylethylene-salen ligands is described. Several of these complexes have been crystallographically authenticated, which highlights a diversity of coordination patterns. Whereas all Ga complexes form [Ga2(CH2SiMe3)4(ONNO)] bimetallic species (ONNO = 1-3), aluminium [AlR(ONNO)] (R = Me, CH2SiMe3) and indium [In(CH2SiMe3)(ONNO)] derivatives are monometallic for ONNO = 1, 2 and 4-6, and only form the bimetallic complexes [Al2R4(ONNO)] and [In2(CH2SiMe3)4(ONNO)] for the most sterically crowded ligand 3. The [AlMe(ONNO)] complexes react with iPrOH to give [AlOiPr(ONNO)] complexes that are robust towards further iPrOH. The [In(CH2SiMe3)(ONNO)] congeners are inert towards excess alcohol, whereas the Ga compounds decompose easily. All these alkyl complexes, as well as the [AlOiPr(ONNO)] derivatives, catalyse the ring-opening polymerisation (ROP) of racemic lactide (rac-LA). The [AlMe(ONNO)] complexes require additional alcohol to afford controlled reactions, but [AlOiPr(ONNO)] complexes are single-component catalysts for the isoselective ROP of rac-LA, with values of Pm in the range 0.80-0.90. Experimental evidence unexpectedly shows that chain-end control leads to the isoselectivity of these aluminium catalysts; also, the more crowded the coordination sphere, the higher the isoselectivity. The bimetallic Ga complexes do not afford controlled reactions, but the binary [In(ONNO)(CH2SiMe3)/(PhCH2OH)] systems competently mediate non-stereoselective ROP; evidence is given that an activated monomer mechanism is at work. Kinetic studies show that catalytic activity decreases when electronic density and steric congestion at the metal atom increase.
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https://hal-univ-rennes1.archives-ouvertes.fr/hal-01114344
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Submitted on : Monday, February 9, 2015 - 11:43:56 AM
Last modification on : Thursday, March 5, 2020 - 2:06:31 PM

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Nicolas Maudoux, Thierry Roisnel, Vincent Dorcet, Jean-François Carpentier, Yann Sarazin. Chiral (1,2)-diphenylethylene-salen complexes of triel metals: coordination patterns and mechanistic considerations in the isoselective ROP of lactide.. Chemistry - A European Journal, Wiley-VCH Verlag, 2014, 20 (20), pp.6131-47. ⟨10.1002/chem.201304788⟩. ⟨hal-01114344⟩

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