Abstract
Chiral group 4 NHC-metal complexes were prepared in good yields by amine elimination from M(NR2)4 (M = Ti, Zr, Hf; R = Me, Et) and chiral pincer NHC-ligands, L4(L4a and L4b), L5 and L6, which are derived from (S,S)-diphenyl-1,2-ethanediamine. Treatment of M(NR2)4 with 1 equiv. of L4 in THF gives, after recrystallization from a benzene solution, the chiral titanium amides (L4)Ti(NMe2)(Br)(THF) (7) and (L4)Ti(NMe2)(Cl)(THF) (11), zirconium amides (L4)Zr(NMe2)(Br)(THF) (8), (L4)Zr(NEt2)(Br)(THF) (10), (L4)Zr(NMe2)(Cl)(THF) (12) and (L4)Zr(NEt2)(Cl)(THF) (14), and hafnium amides (L4)Hf(NMe2)(Br)(THF) (9) and (L4)Hf(NMe2)(Cl)(THF) (13), respectively. Similarly, the reactions of L5 or L6 with 1 equiv. of M(NR2)4 yield the titanium amide (L6)Ti(NMe2)(Cl)(THF) (16), the zirconium amides (L5)Zr(NMe2)(Cl)(THF) (15), (L6)Zr(NMe2)(Cl)(THF) (17) and (L6)Zr(NEt2)(Cl)(THF) (19), and the hafnium amide (L6)Hf(NMe2)(Cl)(THF) (18), respectively. Complexes 7 - 19 were characterized by various spectroscopic techniques and elemental analyses. The molecular structures of 10 and 14 - 19 were also established by X-ray diffraction analyses, which represent the first example of the structurally characterized group 4 chiral NHC-metal complex. Furthermore, 7 - 19 are active catalysts for the polymerization of rac-lactide in the presence of isopropanol, leading to the heterotactic-rich polylactides.
Highlights
Biodegradable polymers derived from renewable resources such as polylactides have received much attention over the past decade because of their attractive physical and mechanical properties.[1]
Numerous reviews have covered catalyst systems for the ring-opening polymerization (ROP) of cyclic esters based on metals such as aDepartment of Chemistry, Beijing Normal University, Beijing 100875, China
An additional driving force is the longstanding interest in catalysts for enantioselective reactions such as olefin metathesis,[7] conjugate addition of enones,[8] allylic alkylations,[9] olefin hydrogenations[10] and hydrosilylations.[11]
Summary
Biodegradable polymers derived from renewable resources such as polylactides have received much attention over the past decade because of their attractive physical and mechanical properties.[1]. This compound was prepared as pale yellow microcrystals from the reaction of L4a (0.26 g, 0.50 mmol) with Zr(NMe2)[4] (0.14 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7.
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