Taming Highly Unstable Radical Anions and 1,4-Organodilithiums by Flow Microreactors: Controlled Reductive Dimerization of Styrenes
作者:Yiyuan Jiang、Hideki Yorimitsu
DOI:10.1021/jacsau.2c00375
日期:2022.11.28
dimerization, and the resulting 1,4-organodilithiums are trapped with various electrophiles. Trapping with divalent electrophiles affords precursors for useful yet less accessible cyclic structures, for example, siloles from dichlorosilanes. Thus, we highlight the power of single-electron reduction of unsaturated compounds in flowmicroreactors for organic synthesis.
Comparison of the Regiochemical Behavior of Zirconium and Hafnium in the Polyinsertion of Styrenes
作者:Nunzia Galdi、Lorella Izzo、Leone Oliva
DOI:10.1021/om1001342
日期:2010.10.25
The hafnocene-based catalyst ethylenebis(1-indenyl)hafnium dichloride/methylalumoxane, as well as its zirconium analogue, is able to oligomerize styrene, p-methylstyrene, and p-tert-butylstyrene in the presence of hydrogen to produce hydrooligomers. The composition of the product mixture compared to that obtained using zirconium-based catalysts indicates that the primary insertion occurs with greater frequency. This difference in regioselectivity is likely to be related to the electronic differences between the two metal centers. Previous experimental and theoretical evidence suggests that increasing the electron density at the incoming metal carbon bond decreases the preference for the secondary insertion; so by exploiting the contribution from the electron-releasing substituent of the monomer and changing the metal center, from zirconium to hafnium, the regiochemistry of insertion can be inverted from secondary to prevailingly primary. The styrene regiochemistry of insertion is engraved into the structure of ethylene styrene copolymers. Some relevant differences can be observed by comparing the C-13 NMR spectra of copolymer obtained with zirconium- and hafnium-based catalysts. In fact the structure of the chains obtained with hafnium, compared to zirconium, shows higher styrene uptake and the presence of styrene homosequences and of regioirregularly arranged units.
Regiochemistry of the Styrene Insertion with CH<sub>2</sub>-Bridged <i>a</i><i>nsa</i>-Zirconocene-Based Catalysts
作者:Lorella Izzo、Mariagrazia Napoli、Leone Oliva
DOI:10.1021/ma034818r
日期:2003.12.1
eMethylenebis(indenyl)zirconium dichloride substituted in C(3), activated by methylalumoxane, is able to give polystyrene and ethylene-styrene copolymers. In this study hydrooligomers, whose structure, determined by C-13 NMR and GC-MS techniques, gives information about the regiochemistry and the stereochemistry of styrene insertion, have been purposefully prepared. The regiochemistry of the styrene insertion is related to the encumbrance of substituents in C(3). rac-[Methylene-(3-R-1-indenyl)(2)]ZrCl2 with R = H, CH3, or CH2CH3 induces a prevailingly secondary styrene insertion into the zirconium-carbon bond. With increasing the substituent's steric hindrance (R = CH(CH3)(2)), regiochemistry inversion occurs and the primary insertion becomes prevailing. The analysis of ethylene-styrene copolymers obtained in the presence of the different catalysts allows confirming the correlation between regiochemistry and comonomers' reactivity. Besides, also the stereospecificity can be evaluated from the structure of the hydrotrimers, when the insertion is primary. Whereas the isospecificity in the absence of substituents (secondary insertion) and in the presence of the tert-butyl substituent (primary insertion) is well-known, a surprising syndiospecificity is observed when the indenyl ligand bears the isopropyl substituent in C(3).