Ultrasound-promoted selective formation of optically active cyclopentadienyl ligands

Abstract

In order to prepare optically active cyclopentadienyl ligands, the chiral version of Bercaw’s synthesis of 1,2,3,4,5-pentamethylcyclopentadiene was carried out between the Grignard- or the lithium-derivatives of 1- or 2-bromopropene and the methyl ester of (-)-pinane-3-carboxylic acid I. Under conventional conditions, the condensation of the vinylic Grignard reagents with I yielded, as the major products, the unsaturated ketones VI and VII (resulting from 1,4 addition to the intermediate vinylic ketones VIII and IX) instead of the expected dienic alcohols II and III. In the case of the lithium derivatives, obtained by halogen–metal exchange between LiBu ^t and 1- or 2-bromopropene, the same reaction gives a better selectivity towards 1,2 addition. However, the reactions are rather tedious and a large excess of reactants is needed to achieve a total reaction. When a one-pot reaction was conducted under ultrasound irradiation with lithium wire, 1- or 2-bromopropene and I, the dienic alcohols, resulting from 1,2 addition to VIII and IX are the major products. Furthermore, under these conditions, addition of LiBr affords II and III almost quantitatively. Dehydration of II or III gives optically active trisubstituted cyclopentadienes IV or V as mixtures of isomers. Single-crystal X-ray structures of the corresponding cyclopentadienyl molybdenum complexes 1 and 2 have been determined.