1,3-Butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene complexes of Zr and Hf were prepared by the 1:l reaction of ZrClzCp2 or HfClzCpz with the corresponding enediylmagnesium [MgCHzCR=CRCH2],, where R is H or CH3 The variable 'H NMR spectra of the zirconium-diene complexes showed a limiting s-cis-q4-1,3-diene structure below -40 "C, although they are fluxional at higher temperatures. The 'H NMR parameters for these Zr complexes at -70 "C were determined by computer simulation. The corresponding Hf complexes showed a strong preference for the metallacyclopentene structure in the temperature range of -90 to +80 O C . A clear difference between the Zr and Hf complexes was also observed in the electronic spectra. In contrast, the Cp2Zr(l,4-diphenyl-l,3-butadiene) complex favored the s-tram-q4-diene structure at -80 to +50 "C, although the Hf analogue preferred the s-cis-q4-diene structure. All these complexes released the coordinated l,&dienes quantitatively on reaction with oxygen. 3,CDideuterated butene (or 1,4) or its methyl or phenyl derivatives were formed on treatment of these complexes with DzO.

The catalyst systems which consist of MCl2(C5R5)2(M=Ti, Zr; R=H, CH3)/NaC10H8, i-C3H7MgBr, n-C4H9Li or LiAlH4 in 1 : 2 ratio were highly effective for the stereoselective isomerization of monosubstituted 1-alkenes to (E)-2-alkenes. Unconjugated dienes were converted to (E)- or (E,E)-dienes. The catalysis of (C5Me5)TiCl2/NaC10H8 was extremely high and resulted in the complete isomerization of 1-alkenes in >99% stereoselectivity within a short period. The use of the bulky C5Me5 ligand is essential to find out the excellent stereoselectivity. Systems, (C5H5)2TiCl2/NaC10H8 and (C5H5)2TiCl2/i-C3H7MgBr showed less selectivity and the catalysis of the corresponding zirconium systems was very poor irrespective of the reducing agents and substrates.

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ZrCp2(isoprene) reacts with aliphatic aldehydes, ketones or nitriles regioselectively on C1-carbon of the isoprene moiety to give novel 2-oxa- or 2-aza-metallacycles which release alcohols or ketones respectively on hydrolysis. In contrast, ZrCH3(η1-methylallyl)Cp2, Ti(η3-methylallyl)Cp2, and [MgCH2C(CH3)=CHCH2]n reacted with acetone on the C3-carbon of the methylallyl or the isoprene unit.