Well-Defined Alkyl Heteroscorpionate Magnesium Complexes as Excellent Initiators for the ROP of Cyclic Esters

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• 作者：Luis F. S&aacute ; nchez-Barba ; André ; s Garcé ; s ; Mariano Fajardo ; Carlos Alonso-Moreno ; Juan Fern&aacute ; ndez-Baeza ; Antonio Otero ; Antonio Antiñ ; olo ; Juan Tejeda ; Agustí ; n Lara-
• 刊名：Organometallics
• 出版年：2007
• 出版时间：December 3, 2007
• 年：2007
• 卷：26
• 期：25
• 页码：6403 - 6411
• 全文大小：158K
• 年卷期：v.26,no.25(December 3, 2007)
• ISSN：1520-6041

文摘

The reaction of the 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 1 equiv of RMgCl proceeds to give very high yields ofthe neutral heteroscorpionate alkyl magnesium complexes [Mg(R)(NNN)] (NNN = tbpamd, R = C₃H₅1, ^tBu 2, CH₂SiMe₃ 3; NNN = pbpamd, R = C₃H₅ 4, ^tBu 5, CH₂SiMe₃ 6). On heating toluene solutionsof complexes 1-3, 5, and 6, a ligand redistribution process occurs to give the corresponding 6-coordinatedsandwich complexes [Mg(tbpamd)₂] (7) and [Mg(pbpamd)₂] (8). Interestingly, the allyl derivative 4 canbe easily transformed to 8 at room temperature. In addition, the cationic sandwich complex [Mg(tbpamdH)₂]Cl₂ (9) [tbpamdH = N-ethyl-N'-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidine] was obtained from7 by means of a protonation process. Finally, alkyl-containing complexes 1-6 can act as highly effectivesingle-component living initiators for the ring-opening polymerization of -caprolactone and lactidesover a wide range of temperatures. -Caprolactone is polymerized within seconds to give high molecularweight polymers with narrow polydispersities. Lactide afforded PLA materials with medium molecularweights and polydispersities as narrow as M_w/M_n = 1.05. Additionally, polymerization of L-lactide occurredwithout racemization in the propagation process and offered highly crystalline, isotactic poly(L-lactides)with high melting temperatures (T_m = 160 C). Polymer end-group analysis shows that the polymerizationprocess is initiated by alkyl transfer to the monomer.