Olefin platinum complexes, stereoselection

Platinum complexes show none of the catalytic activity found in palladium and nickel complexes. This chemical inertness makes platinum a useful model for the more active catalysts. There has been a suggestion that platinum complexes of chiral a-diimines might lead to stereoselective olefin polymerization. Chiral camphor-based ligands have been employed in palladium complexes for ethylene polymerization, but there was no mention of stereoselectivity in hexene polymerizations. 

These facts may well imply that the stereoselectivity for addition of a hydrosilane to the enantiotopic faces of a ketone is different from that of an olefin which is undoubtedly tr-coordinated to the chiral catalyst the difference is again explained in terms of the intermediacy of an a-siloxyalkyl-platinum complex similar to the a-siloxyalkyl-rhodium complex described in the hydrosilylation of terpene ketones (see Scheme 4). 

Several workers have studied platinum(II) complexes of olefins containing chiral amine or amino-acid ligands. Panunzi(9) observed stereoselectivity in the reaction between cis-(S - a-methylbenzylamine)dichloroplatinum (II) and trans-2-butene with the major diastereomer formed to the extent of 70% of total complex. More recently (10), it has been shown that the replacement of coordinated trans-2-butene by free olefin in (S-prolinato) dichloroplatinum (II) complexes takes place more easily with retention than with inversion. Addition of a large excess of trans-2-butene to solutions of the corresponding ethylene complexes produced first an increase and then a gradual decrease in their circular dichroism. The kinetic stereoselectivity in this reaction (that is, the differing reaction rates of the two prochiral faces of trans-but-2-ene) was 3 1, but at equilibrium the ratio of major and minor diastereomers was 64 36 in the cis-isomer and 59 41 in the trans-isomer. 

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