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We demonstrate here that even the most electron-poor olefins undergo Os0,-catalyzed oxidations. The reduced reactivity of chloroolefins relative to their fluorine counterparts is explained by a size effect; in all other respects the chloroolefins behave analog~usly.~~] The procedure is also applicable to strained fluoroolefins, for example norbornene derivatives.Our results cannot be rationalized with the frequently held conception of the mechanism of the osmylation as the electrophilic attack of osmium tetroxide on the olefin. The pyridine base effect discussed by Corey et al.

4] The twist angle I and the pyramidalization terms xc and lN were obtained from the primary torsion angle data 0,. w2. and w, as follows. 7 = (w, + w,)12; zN = (w2-(o, + x)mod2rr; xc = (w,-wJ + n)mod2x.Here we use the modified twist angle. 1;5 = (T) mod T I [Sa]. i maximizes at +90 and can he interpreted as the angle between the idealized positions of the px orbitals on C and N. The pyramidalization terms maximize at + 6 0 .[S] The other two classes are (1)cases where nonplanar amido-N ligands are thermodynamically favored over planar analogues accessible by isomeriza-

Ethylene and propene were homopolymerized with 11 different metallocene-methylaluminoxane (MAO) catalysts 1-11 using a constant set of parameters. The catalytic activity of the two unbridged and 9 ansa-metallocenes as well as the properties of the polymers, such as molecular weight, width of molecular weight distribution, melting point, and, with polypropene, chain microstructure, were investigated. It was found that structure variations of the catalysts cause rather different catalytic behaviour, which further diversifies the performances with the two different monomers. In some cases this comparison allows insight into the nature of the influencing factors. 0 1992, Huthig & Wepf Verlag, Basel CCC 0025-1 16)3/92/$05,00 Standardized polymerizations of ethylene and propene with . . . Scheme 1: 6 : M t z Z r 7 : Mt=Hf 9 4-R=CH, (2 isomers) 5: R-H

Structure‐performance relationships in the propene polymerization behaviour of a number of silicon bridged bisindenyl zirconocenes show a rational pattern. Prognosis of the polymerization behaviour of different type structures, however, often fails, which is demonstrated by three examples of new zirconocenes. These include two bisindenyl complexes with two‐membered silicon bridges and one isospecific bridged fluorenyl cyclopentadienyl species. It is thus demonstrated that the scope of a “rational catalyst design” in the field of metallocene catalysts is still limited.

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