Novel chemoselective and diastereoselective iron(III)-catalysed Michael reactions of 1,3-dicarbonyl compounds and enones

Abstract

Iron(III) chloride hexahydrate catalyses the Michael reaction of 1,3-dicarbonyl compounds with α,β-unsaturated ketones under mild and neutral conditions with extraordinary efficiency. The chemoselectivity of this Fe^III-catalysed process is superior to that of the classic base-catalysed Michael reaction, since the latter suffers from various side reactions, namely drawbacks such as aldol cyclisations and ester solvolysis. Excellent yields and chemoselectivities together with the environmentally friendly nature of Fe^III catalysis makes this an important alternative to classic base catalysis. Moreover, the reaction procedure is reasonably easy: Fe^III catalysis does not require inert or anhydrous conditions, and in most cases no solvent is needed. In terms of diastereoselectivity, the Fe^III-mediated reaction may also prove superior to a base-catalysed one. In at least one example, Fe^III catalysis forms a diastereoisomer as the major kinetic product, which is disfavoured in the base-mediated Michael reaction, where a thermodynamic mixture is obtained. The relative configuration of the diastereoisomeric Michael products has been determined for two examples by synthesis and structure elucidation of the cyclic aldol derivatives.