Discrete Heteroscorpionate Lithium and Zinc Alkyl Complexes. Synthesis, Structural Studies, and ROP of Cyclic Esters

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• 作者：Carlos Alonso-Moreno ; Andr&#xe9 ; s Garc&#xe9 ; s ; Luis F. S&#xe1 ; nchez-Barba ; Mariano Fajardo ; Juan Fern&#xe1 ; ndez-Baeza ; Antonio Otero ; Agust&#xed ; n Lara-S&#xe1 ; nchez ; Antonio Anti&#xf1 ; olo ; Lewis Broom
• 刊名：Organometallics
• 出版年：2008
• 出版时间：March 24, 2008
• 年：2008
• 卷：27
• 期：6
• 页码：1310 - 1321
• 全文大小：257K
• 年卷期：v.27,no.6(March 24, 2008)
• ISSN：1520-6041

文摘

The reaction of bis(3,5-di-tert-butylpyrazol-1-yl)methane (bdtbpzm) with BuⁿLi and carbodiimide derivatives, namely, N,N′-diisopropyl and 1-tert-butyl-3-ethyl carbodiimides, gives rise to the new sterically hindered lithium acetamidinate [Li(tbp^tamd)(THF)] (1) [tbp^tamd = N-ethyl-N′-tert-butylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate] and [Li(pbp^tamd)(THF)] (2) [pbp^tamd = N,N′-diisopropylbis(3,5-di-tert-butylpyrazol-1-yl)acetamidinate]. Subsequent hydrolysis of 1 and 2, and the recently reported heteroscorpionate lithium salts [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N′-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(pbpamd)(THF)] [pbpamd = N,N′-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with NH₄Cl/H₂O in ether cleanly affords the corresponding amidine ligands Htbpamd (3), Hpbpamd (4), Htbp^tamd (5), and Hpbp^tamd (6) in very good yields. The X-ray diffraction molecular structure of 3 was obtained. Reaction of the amidine-heteroscorpionate ligands 3–6 with 1 equiv of ZnR′₂ proceeds in very high yields to give the neutral heteroscorpionate alkyl zinc complexes [Zn(R′)(NNN)] (NNN = tbpamd, R′ = Me 7, Et 8, CH₂SiMe₃ 9; NNN = pbpamd, R′ = Me 10, Et 11, CH₂SiMe₃ 12; NNN = tbp^tamd, R′ = Me 13, Et 14; NNN = pbp^tamd, R′ = Me 15, Et 16). The single-crystal X-ray structures of the derivatives 8, 12, 15, and 16 confirm a four-coordinative structure with the zinc metal center in a distorted tetrahedral geometry and the heteroscorpionate ligands arranged in κ³-coordination mode. The new lithium salts 1 and 2 and the alkyls 7–9, 13, and 14 can act as efficient single-component initiators for the ring-opening polymerization of ε-caprolactone and lactides over a wide range of temperatures. ε-Caprolactone is polymerized within minutes to give high-medium molecular weight polymers with medium broad values of polydispersities. Lactide afforded PLA materials with medium molecular weights and polydispersities as narrow as M_w/M_n = 1.05. Additionally, polymerization of L-lactide occurred without racemization in the propagation process and offered highly crystalline, isotactic poly(L-lactides) with high melting temperatures (T_m = 165 °C). rac-Lactide polymerization also produces enriched levels of heterotactic poly(lactide). Polymer end group analysis shows that the polymerization mediated by alkyl zinc complexes is initiated by alkyl transfer to monomer.