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Heteroscorpionate Magnesium Alkyls Bearing Unprecedented Apical σ-C(sp 3 )–Mg Bonds: Heteroselective Ring-Opening Polymerization of rac -Lactide

2013; American Chemical Society; Volume: 52; Issue: 21 Linguagem: Inglês

10.1021/ic401915b

1520-510X

Andrés Garcés, Luis F. Sánchez‐Barba, Juan Fernández‐Baeza, Antonío Otero, Manuel Honrado, Agustín Lara‐Sánchez, Ana M. Rodrı́guez,

Organometallic Complex Synthesis and Catalysis

The previously described reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li(κ3-pbpamd)(THF)] and [Li(κ3-tbpamd)(THF)] with a series of commercially available Grignard reagents RMgCl in an equimolecular ratio yielded the magnesium monoalkyls [Mg(R)(κ3-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)(κ3-NNN)] with two additional equivalents of the same RMgCl in tetrahydrofuran gave rise to dinuclear dialkyls of the type [RMg(κ3-N,N,N;κ2-C,N)MgR(thf)] (κ3-N,N,N;κ2-C,N = pbpamd–, R = CH2SiMe3 (5), Et (6); κ3-N,N,N;κ2-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(κ3-N,N,N;κ2-C,N)MgR}2{μ-O,O-(C4H8)}] (κ3-N,N,N;κ2-C,N = pbpamd–, R = CH2SiMe3 (9), Et (10), Bn (11); κ3-N,N,N;κ2-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(sp3)–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.