Fiche publication
Date publication
janvier 2007
Auteurs
Membres identifiés du Cancéropôle Est :
Dr BELLEMIN-LAPONNAZ Stéphane
Tous les auteurs :
Dagorne S, Le Bideau F, Welter R, Bellemin-Laponnaz S, Maisse-Francois A
Lien Pubmed
Résumé
We describe the synthesis, structure, and reactivity of low-coordinate Al-alkyl and -alkoxide cationic complexes incorporating the sterically bulky aminophenolate bidentate ligand 6-(CH2NMe2)-2-CPh3-4-Me-C6H2O- (N,O). These complexes are derived from the ionization of neutral dialkyl Al complexes (NO)AIR(2) (1a, R=Me; 1b, R=iBu), readily obtained by alkane elimination between AIR(3) and the corresponding aminophenol ligand, with the alkyl abstracting reagents B(C6F5)(3) and [Ph3C][B(C6F5)(4)]- ne reactions of 1a,b with B(C6F5)(3) yield complicated mixtures or decomposition products, however the ionization of the Al-diisobutyl derivative 1b with [Ph3C][B(C6F5)4] affords a stable four-coordinate Al-PhBr cationic adduct [(N,O)Al(iBu)(PhBr)](+) (3(+)), as deduced from elemental analysis data. Complex 3(+) readily coordinates Lewis bases such as THF to form the corresponding adduct [(N,O)Al(iBu)(thf)](+) (4(+)), and also rapidly chain-transfers with 1-hexene to yield the three-coordinate Al-hexyl cation [(NO)Al-hexyl](+) (5(+)). Both cations 31 and 5(+) slowly dimerize to form unprecedented organoaluminum dications [(NO)AIR(+)](2) (3'(++), R=iBu; 5'(++), R=hexyl) as deduced from X-ray crystallographic analysis. Cation 3(+) reacts quickly with iPrOH to form a stable Lewis acid/base adduct [(NO)AI(iBu)(HOiPr)](+) (61), which constitutes the first X-ray characterized adduct between an Al-alkyl complex and a simple ROH. The Al-ROH proton in 61 is readily abstracted by NMe2Ph to form the neutral isopropoxide Al complex [(N,O)Al(iBu)(OiPr)] (7). Upon reaction with THF, cation 6(+) undergoes an intramolecular proton transfer to yield the ammonium Al-THF complex [(eta(1)-HN,O)Al(iBu)(OiPr)(thf)] (8b(+)), in which the aminophenolate is eta(1)-coordinated to the Al center. Cation 8b(+) can then be converted to the desired Alalkoxide derivative [(NO)AI(OiPr)(thf)](+) (10(+)) by an intramolecular protonolysis reaction, as confirmed by X-ray crystallography. The synthesized Al-alkyl cations form robust four-coordinate adducts in the presence of cyclic esters such as E-caprolactone and (D,L)lactide, but no insertion chemistry occurs, illustrating the poor ability of the Al-R+ moiety to ring-open. In contrast, the Al-alkoxide cation 10(+) polymerizes E-caprolactone in a controlled manner with excellent activity, but is inactive in the polymerization of (D,L)-lactide and L-lactide. Control experiments with L-lactide show that cation 10(+) ring-openS L-lactide to yield a robust five-coordinated Al-lactate cation [(N, O)Al(eta(2) -L-lactate-OiPr)(thf)](+) (13(+)), derived from a monoinsertion Of L-lactide into the Al-OiPr bond of 10+, that does not further react. Cation 13+ may be regarded as a structurally characterized close mimic of the initial intermediate in the ring opening polymerization (ROP), of lactides by [{LX}M(OR)(L)] (where LX-=bidentate monoanionic ligand and L=labile ligand) metal complex initiators.
Référence
Chemistry. 2007;13(11):3202-17.
