ChemInform Abstract: Stereospecific Synthesis of Olefins Through Sequential Cross‐Coupling Reactions

Abstract

A conceptually straightforward and useful procedure leading to (I)- or (g)-olefins is presented. The method is based upon the r eaction of the readily available (I)- or (E)-l-bromo-2-phenylthioethene (1 or 2) with Grignard reagents in the presence of Ni(II1 or Pd(I1) catalysts. Other ca- talysts, 5.2. iron(II1) derivatives, and other organometallics, 2.2. zinc reagents or copper derivatives, can also be successfully used during the substitution of the halogen. The stereospecificit y iss99% in the case of (El-isomers and slightly lower in the case of (I)-isomers. Enantioselecti- vTty is achieved by using chiral ligands. By appropriate choice of the reagent, alkyl- or a ryl-substituted ethenes, 1,3- or 1 ,6-dienes1 and con- jugated enynes can be prepared. Ally1 and alkadienyl silanes can also be obtained. The reaction of the latter compounds with palladium chloride leads to conjugated tetraenes. Oienyl bromides or ketones are prepared by reaction of the same compounds with aluminum chloride and cyanogen bromide or acyl chloride respectively. The versatility of this method has been demonstrated by the stereospecific synthesis of insect pheromones and a number of key synthetic intermediates. logeno-2-phenylethenes and 1 -halogeno-2-phenylsulphonylethenes. The two systems showed a different behaviour. A stereochemical pathway of retention of configuration was observed for the styrene system, whereas a variable stereochemical course depending upon the configuration of the substrate, the nature of the leaving group, and the type of cuprate was valid for the sulphones. Furthermore, a marked leaving group effect (Br>Cl>F) was observed for the first system whereas the halogeno-sulphones were found to be much less sensitive to the change of the leaving group (Br-Cl-F). These and other results were explained in terms of two main mechanistic pathways with blurred boundaries: a concerted substitution of the halogen should occur in the styrene system whereas the halogeno-sulphones should follow the more common addition-elimina tion mechanism (ref.