Unusual C鈥揅 Bond Cleavage in the Formation of Amine-Bis(phenoxy) Group 4 Benzyl Complexes: Mechanism of Formation and Application to Stereospecific Polymerization

详细信息    查看全文

• 作者：Ravikumar R. Gowda ; Lucia Caporaso ; Luigi Cavallo ; Eugene Y.-X. Chen
• 刊名：Organometallics
• 出版年：2014
• 出版时间：August 11, 2014
• 年：2014
• 卷：33
• 期：15
• 页码：4118-4130
• 全文大小：578K
• ISSN：1520-6041

文摘

Group 4 tetrabenzyl compounds MBn₄ (M = Zr, Ti), upon protonolysis with an equimolar amount of the tetradentate amine-tris(phenol) ligand N[(2,4-^tBu₂C₆H₂(CH₂)OH]₃ in toluene from 鈭?0 to 25 掳C, unexpectedly lead to amine-bis(phenoxy) dibenzyl complexes, BnCH₂N[(2,4-^tBu₂C₆H₂(CH₂)O]₂MBn₂ (M = Zr (1), Ti (2)) in 80% (1) and 75% (2) yields. This reaction involves an apparent cleavage of the >NCH₂鈥揂rOH bond (loss of the phenol in the ligand) and formation of the >NCH₂鈥揅H₂Bn bond (gain of the benzyl group in the ligand). Structural characterization of 1 by X-ray diffraction analysis confirms that the complex formed is a bis(benzyl) complex of Zr coordinated by a newly derived tridentate amine-bis(phenoxy) ligand arranged in a mer configuration in the solid state. The abstractive activation of 1 and 2 with B(C₆F₅)₃路THF in CD₂Cl₂ at room temperature generates the corresponding benzyl cations {BnCH₂N[(2,4-^tBu₂C₆H₂(CH₂)O]₂MBn(THF)}⁺[BnB(C₆F₅)₃]^{鈭?/sup> (M = Zr (3), Ti, (4)). These cationic complexes, along with their analogues derived from (imino)phenoxy tri- and dibenzyl complexes, [(2,6-iPr₂C₆H₃)N鈺怌(3,5-tBu₂C₆H₂)O]ZrBn₃ (5) and [2,4-Br₂C₆H₂(O)(6-CH₂(NC₅H₉))CH₂N鈺怌H(2-adamantyl-4-MeC₆H₂O)]ZrBn₂ (6), have been found to effectively polymerize the biomass-derived renewable 尾-methyl-伪-methylene-纬-butyrolactone (_尾MMBL) at room temperature into the highly stereoregular polymer P_尾MMBL with an isotacticity up to 99% mm. A combined experimental and DFT study has yielded a mechanistic pathway for the observed unusual C鈥揅 bond cleavage in the present protonolysis reaction between ZrBn₄ and N[(2,4-tBu₂C₆H₂(CH₂)OH]₃ for the formation of complex 1, which involves the benzyl radical and the Zr(III) species, resulting from thermal and photochemical decomposition of ZrBn₄, followed by a series of reaction sequences consisting of protonolysis, tautomerization, H-transfer, oxidation, elimination, and radical coupling.}