Hydrovinylation of styrene derivatives to 3-aryl-1-butenes catalysed by nickel complexes

Abstract

The hydrovinylation of styrene and ring-substituted styrene molecules by the ionic nickel precursor trans-[Ni(2,4,6-Me₃C₆H₂) (CH₃CN) (P(CH₂Ph)₃)₂]BF₄ in THF solution has been investigated. The chemoselectivity towards hydrovinylation products is about 99%. For styrene the conversion is nearly complete and the regioselectivity towards the vinylation in the α-position in the conjugated aryl olefin derivatives is 100%. The yield of 3-phenyl-1-butene reaches 97% with minor amounts of the isomerization products cis- and trans-2-phenyl-2-butene. The turnover rate of the reaction at 25°C, 15 bar (initial pressure) of ethylene with a styrene/[Ni] ratio 1000/1 is 1915 h⁻¹. With styrene derivatives substituted in the vinyl fragment, the hydrovinylation reaction was very limited; however those substituted in the phenyl fragment (ArCH=CH₂; Ar:p- and m-Me(C₆H₄), p- and m-Et(C₆H₄), p-and m-vinyl(C₆H₄), p- and m-Cl(C₆H₄), p-MeO(C₆H₄) and m-NO₂(C₆H₄)) showed similar selectivity to styrene but different turnover rates. Activity decreases with the substituent in the sequence Cl∼OMe∼CH=CH₂∼H∼CH₂CH₃&>;Me&>;NO₂ related with the coordination ability of the olefin. Activity of the para derivatives is slightly higher than that of the meta analogues. The ionic compound [Ni(2,4,6-Me₃C₆H₂) (CH₃CN) ((PhCH₂)₂PCH₂CH₂P(CH₂Ph)₂)]BF₄ and the neutral trans-[NiBr(2,4,6-Me₃C₆H₂) (P(CH₂Ph)₃)₂] do not show any activity in the codimerization reaction of styrene and ethylene in the same conditions.