Towards benign synthesis of indenes from indanones: Zinc-mediated allylation of ketones in aqueous media as a source of substituted indenyl ligand precursors

Substituted indenes are valuable ligand precursors for transition-metal complexes. Previously, most of the methods employed for the preparation of alkyl-substituted indenes have involved the use of air-sensitive organometallic lithium or Grignard reagents, often in combination with expensive metal catalysts. The present work evaluates an approach to the synthesis of 2- and 3-allyl-substituted indenes by employing a simple, environmentally benign organometallic zinc-mediated Barbier-type allylation of 1- and 2-indanones in aqueous media. A large series of new achiral and racemic indenyl ligand precursors have been prepared in variable yields by reacting substituted and unsubstituted indanones with allyl-, crotyl-, and cinnamyl halides using metallic zinc as the mediating metal in THF/NH4Claq followed by acid-catalyzed dehydration. The method described is applicable also for indanones containing unprotected halide- and hydroxyl substituents. As an example of extension of the approach, some indenes have been further hydrosilylated with achiral Silanes and disilanes in the presence of Karstedt's catalyst to provide new silaalkyl-substituted indenes and bis(indenes). Hydrosilylation with a chiral silane, (+)-(R)-methyl-1-naphthalenyl-phenylsilane, provides access to new chirally substituted indenes. ((c) Wiley-VCH Verlag GmbH S Co. KGaA, 69451 Weinheim, Germany, 2005).