Stereo-regulating compounds

Y. Yamamoto and K. Maruyama, Organometallic Compounds for Stereo-regulated Synthesis of Acyclic Systems. Their Application to the Synthesis of the Prelog-Djerassi Lactonic Acid , Heterocycles, 1982, 18, 357. 

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

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