Recyclable indium catalysts for additions of 1,3-dicarbonyl compounds to unactivated alkynes affected by structure and acid strength of solid supports

Indium has received much attention as a widely used rare metal, which has led to its consumption and cost increasing dramatically. Development of economical and ecological procedures for indium recycling is highly desired in all fields consuming indium. Here, we propose that indium species can be immobilized on solid acids by ion exchange, which increases their activity and enables the generation of the first recyclable heterogeneous indium catalysts for addition reactions of 1,3-dicarbonyl compounds to unactivated alkynes. This reaction was first reported in 2003, as a typical green protocol using abundant carbon sources for useful fine chemicals. The In3+-exchanged ultrastable Y zeolite (In3+-USY) and montmorillonite (In3+-mont) showed various unique catalyses. For example, the alkenylation reaction proceeded successfully without any additives except for the catalysts.