Heteroscorpionate Magnesium Alkyls Bearing Unprecedented Apical 蟽-C(sp3)鈥揗g Bonds: Heteroselective Ring-Opening Polymerization of rac-Lactide

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• 作者：Andr茅s Garc茅s ; Luis F. S谩nchez-Barba ; Juan Fern谩ndez-Baeza ; Antonio Otero ; Manuel Honrado ; Agust铆n Lara-S谩nchez ; Ana M. Rodr铆guez
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：November 4, 2013
• 年：2013
• 卷：52
• 期：21
• 页码：12691-12701
• 全文大小：454K
• 年卷期：v.52,no.21(November 4, 2013)
• ISSN：1520-510X

文摘

The previously described reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li(魏³-pbpamd)(THF)] and [Li(魏³-tbpamd)(THF)] with a series of commercially available Grignard reagents RMgCl in an equimolecular ratio yielded the magnesium monoalkyls [Mg(R)(魏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)(魏³-NNN)] with two additional equivalents of the same RMgCl in tetrahydrofuran gave rise to dinuclear dialkyls of the type [RMg(魏³-N,N,N;魏²-C,N)MgR(thf)] (魏³-N,N,N;魏²-C,N = pbpamd^{鈥?/sup>, R = CH₂SiMe₃ (5), Et (6); 魏3-N,N,N;魏2-C,N = tbpamd鈥?/sup>, 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(魏3-N,N,N;魏2-C,N)MgR}₂{渭-O,O-(C₄H₈)}] (魏3-N,N,N;魏2-C,N = pbpamd鈥?/sup>, R = CH₂SiMe₃ (9), Et (10), Bn (11); 魏3-N,N,N;魏2-C,N = tbpamd鈥?/sup>, R = CH₂SiMe₃ (12), Et (13), Bn (14)) was obtained. In both families, an apical methine C鈥揌 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 蟽-C(sp3)鈥揗g 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 蔚-caprolactone and lactides. Lactide (LA) polymerizations afforded polylactide (PLA) materials with medium molecular weights in only a few minutes even at 20 掳C for l-LA and in a few hours at 50 掳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 P_s values up to 0.78.}