Synthetic and Mechanistic Studies of Organoscandium Compounds. Dimerization and Branching of Alkenes Catalyzed by Scandocene Hydrides

Abstract

Mixed-ring scandocene alkyl and hydride derivatives of structure Cp*CpSc(CH₃)(PMe₃), Cp*CpSc(η¹-η⁵-C₅H₄(Cp*Sc(H), Cp*(η⁵-1,3,4-C₅Me₃H₂)Sc(CH₃)(PMe₃), and Cp*(η⁵-1,3,4-C₅Me₃H₂)Sc(H)(PMe₃) were prepared (Cp* = η⁵-C₅Me₅, Cp = η⁵-C₅H₅. Dimethylsilicon bridged scandocene alkyl and hydride derivatives Me₂Si(η⁵-C₅Me₄)₂ScCH(SiMe₃)₂, Me₂Si(η⁵-C₅Me₄)₂Sc(H)(PMe₃), meso-Me₂Si(η⁵-t-butylC₅H₃)₂ScCH₂SiMe₃, and (meso-Me₂Si(η⁵-t-butylC₅H₃)₂Sc(H))₂ were also prepared. The activation energy for phosphine dissociation in Cp*CpSc(CH₃)(PMe₃) and Cp*(η⁵-1,3,4-C₅Me₃H₂)Sc(H)(PMe₃) were measured. The crystal structures of Me₂Si(η⁵-C₅Me₄)₂ScCH(SiMe₃)₂ and Me₂Si(η⁵-C₅Me₄)₂Sc(H)(PMe₃) were determined. The hydride derivatives, Me₂Si(η⁵-C₅Me₄)₂Sc(H)(PMe₃) and (meso-Me₂Si(η⁵-t-butylC₅H₃)₂Sc(H))₂ catalyze the following carbon-carbon bond forming reactions: (1) The catalytic dimerization of α-olefins to head-to-tail dimers, (2) The catalytic cyclization of α, ω-diolefins to methylenecycloalkanes with ring sizes between 5 and 9 carbon atoms, (3) The catalytic formation of six-membered ring nitrogen and sulfur heterocycles by catalytic cyclization of bisallyl amines and bisallyl sulfides respectively and (4) The catalytic formation of the spiro hydrocarbons 2-methylene spiro[4.4] nonane and 2-methylene dispiro [4.1.4.2] tridecane by catalytic cyclization of 5-methylene-1,8-nonadiene and 5,8-dimethylene-1,11-dodecadiene. (meso-Me₂Si(η⁵-t-butylC₅H₃)₂Sc(H))₂ catalyzes the ring opening reaction of methylenecyclopropane to butadiene and methylenecyclobutane to 1,4-pentadiene. Labelling experiments show that intramolecular olefin insertion in ω-alkenyl scandium complexes and β-alkyl elimination in cycloalkylmethyl scandium complexes is reversible when rings containing 3 or 4 carbon atoms are involved. The hydride derivative (meso-Me₂Si(η⁵-t-butylC₅H₃)₂Sc(H))₂ also catalyzes the isomerization of 1,4-pentadiene to isoprene and 2-methyl-1,4-pentadiene to 2,3-dimethylbutadiene and the isomerization of 3-methyl-1,4-pentadiene to 1,5-hexadiene, which is then cyclized to methylenecyclopentane.