Sterically facilitated meta-lithiation of arenes, containing electron-donating groups
摘要:
The influence of the bulky trimethylsilyl substituent on the selectivity of metallation of dimethylaniline, anisole and 1-dimethylaminonaphthalene is studied. The neighboring SiMe3 group forces dimethylamino and methoxy groups to occupy a conformation with an unshared electron pair turned towards silicon atom. This forced conformation prevents NMe2 and OMe groups from providing the DOM-effect, thus facilitating meta-metallation with the use of bulky LiCKOR. While the inverted NMe2 group completely suppresses ortho-metallation, the less bulky and more electron withdrawing OMe group demonstrates more rotation freedom allowing selective ortho-metallation with smaller reagents such as n-BuLi or tert-BuLi. (C) 2019 Elsevier B.V. All rights reserved.
Fluorine- and Trimethylsilyl-Containing Phenoxy−Imine Ti Complex for Highly Syndiotactic Living Polypropylenes with Extremely High Melting Temperatures
A fluorine- and trimethylsilyl-containing phenoxy-imine titanium complex was synthesized and the structure was determined by an X-ray analysis. The complex on activation with MAO initiates highly controlled syndiospecific living propylene polymerization to form extremely high Tm syndiotactic polypropylenes (Mw/Mn = 1.05-1.08, Tm = 156-152 degrees C) at 0 or 25 degrees C. Moreover, at 50 degrees C, the complex afforded monodisperse syndiotactic polypropylene with very high Tm's of 149, 150 degrees C. In contrast, complexes having a t-Bu group instead of the silyl group provided lower tacticity polymers with much lower Tm's. In addition, we revealed the substituent effect that plays a key role for the highly controlled syndiospecific polymerization displayed by the catalyst.