Synthesis of Hafnacyclopentanes from Hafnocene Alkyne Complexes: Influence of Styrene Substituents on the C–C Coupling Regioselectivity

Abstract

The alkyne complex rac‐(ebthi)Hf(η2‐Me3SiC2SiMe3) (1)[ebthi = 1,2‐ethylene‐1,1′‐bis(η5‐tetrahydroindenyl)] reacts with styrene and p‐(trifluoromethyl)styrene to give hafnacyclopentanes 2 and 3, respectively. C–C coupling proceeds in different ways: whereas for complex 2 a hafnacyclopentane with substituents in the α,β‐position is formed, in the latter case only α,α coupling was observed. Complexes 2 and 3 can be activated with B(C6F5)3 to form active catalysts for the polymerization of ethylene.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Modern applications of low-valent early transition metals in synthesis and catalysis

“…For example, the Negishi reagent, Cp 2 Zr(η 2 -butene) ( 1 ), is synthesized by the treatment of Cp 2 ZrCl 2 with n BuLi or n BuMgBr, wherein the Cp 2 Zr n Bu 2 intermediate undergoes β-H abstraction to liberate one equivalent of butane along with the product 44–46 . Cp 2 Ti(TMS 2 C 2 ) ( 2 ) (TMS 2 C 2 = bistrimethylsilylacetylene) and related Ti and Zr analogues can be synthesized by reduction of the corresponding high-valent chlorides with Mg in the presence of the π -accepting alkyne TMS 2 C 2 (REF 47 ), while substituted hafnocene analogues have been synthesized using Li as a reductant 48,49 .…”

“…In a similar vein, stoichiometric Cp 2 Zr(TMS 2 C 2 ) has been used as a general platform for C-C coupling and can be used to couple tethered diynes to make polycyclic aromatic hydrocarbons 130–133 . Cp 2 M II fragments can also be used to construct metallacyclocumulenes and other highly strained metallacycles 47,49,134–142 . Here, the steric bulk of the Cp* ligands imbue stability, which enables easy isolation of the metal complexes that feature exotic bonding motifs.…”