title: Heteroscorpionate magnesium alkyls bearing unprecedented apical σ-C(sp3)-mg bonds: heteroselective ring-opening polymerization of rac-lactide.
abstract: The previously described reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li(κ(3)-pbpamd)(THF)] and [Li(κ(3)-tbpamd)(THF)] with a series of commercially available Grignard reagents RMgCl in an equimolecular ratio yielded the magnesium monoalkyls [Mg(R)(κ(3)-NNN)] (NNN = pbpamd, R = CH2SiMe3, Et (1), Bn (2); NNN = tbpamd, R = CH2SiMe3, Et (3), Bn (4)). However, subsequent reaction of these monoalkyls [Mg(R)(κ(3)-NNN)] with two additional equivalents of the same RMgCl in tetrahydrofuran gave rise to dinuclear dialkyls of the type [RMg(κ(3)-N,N,N;κ(2)-C,N)MgR(thf)] (κ(3)-N,N,N;κ(2)-C,N = pbpamd(-), R = CH2SiMe3 (5), Et (6); κ(3)-N,N,N;κ(2)-C,N = tbpamd(-), R = CH2SiMe3 (7), Et (8)). Furthermore, when the reaction was carried out in a mixture of tetrahydrofuran/dioxane with the same stoichiometry, a new family of tetranuclear tetraalkyl magnesium complexes [{RMg(κ(3)-N,N,N;κ(2)-C,N)MgR}2{μ-O,O-(C4H8)}] (κ(3)-N,N,N;κ(2)-C,N = pbpamd(-), R = CH2SiMe3 (9), Et (10), Bn (11); κ(3)-N,N,N;κ(2)-C,N = tbpamd(-), R = CH2SiMe3 (12), Et (13), Bn (14)) was obtained. In both families, an apical methine C-H activation process on the heteroscorpionate takes place. The single-crystal X-ray structures of 4, 8, 9, and 12 confirm the nuclearity of each family, with 4-coordinative arrangements for all magnesium atoms. More importantly, the presence in the di- and tetranuclear complexes of unprecedented apical carbanions with a direct σ-C(sp(3))-Mg covalent bond, and as a result, the existence of stereogenic magnesium centers, have been unambiguously confirmed. Interestingly, the dinuclear dialkyls 5 and 7, as well as the tetranuclear tetraalkyls 9, 10, and 12, can act as highly efficient single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides. Lactide (LA) polymerizations afforded polylactide (PLA) materials with medium molecular weights in only a few minutes even at 20 °C for L-LA and in a few hours at 50 °C for rac-LA propagations. More importantly, microstructural analysis of the poly(rac-lactide) materials revealed that the tetranuclear tetra-alkyl 12 exerts enhanced levels of heteroselectivity on the PLAs under mild conditions, with Ps values up to 0.78.
authors.affiliation: Departamento de Química Inorgánica y Analítica, Universidad Rey Juan Carlos , Móstoles-28933-Madrid, Spain.
authors.forename: Andrés
authors.initials: A
authors.lastname: Garcés
authors.forename: Luis F
authors.initials: LF
authors.lastname: Sánchez-Barba
authors.forename: Juan
authors.initials: J
authors.lastname: Fernández-Baeza
authors.forename: Antonio
authors.initials: A
authors.lastname: Otero
authors.forename: Manuel
authors.initials: M
authors.lastname: Honrado
authors.forename: Agustín
authors.initials: A
authors.lastname: Lara-Sánchez
authors.forename: Ana M
authors.initials: AM
authors.lastname: Rodríguez
mesh_terms: D002208:Caproates / Q000737:chemistry; D002384:Catalysis; D060326:Chemistry Techniques, Synthetic; D004146:Dioxanes / Q000737:chemistry; D005663:Furans / Q000737:chemistry; D007783:Lactones / Q000737:chemistry; D008274:Magnesium / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011091:Polyesters / Q000737:chemistry*; D058105:Polymerization
chemical_list: D002208:Caproates; D004146:Dioxanes; D005663:Furans; D007783:Lactones; D009942:Organometallic Compounds; D011091:Polyesters; C018674:tetrahydrofuran; C033616:poly(lactide); C121056:caprolactone; D008274:Magnesium; C025223:1,4-dioxane