Catalysis of the Michael Reaction and the Vinylogous Michael Reaction by Ferric Chloride Hexahydrate

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Abstract
Ferric chloride hexahydrate catalyzes the Michael reaction of 1,3-dicarbonyl compounds with α,β-unsaturated ketones under mild and non-basic conditions with extraordinary efficiency. The chemoselectivity of this Fe(III)-catalyzed process is superior to that of the classic base-mediated Michael reaction, since the latter suffers from various side and subsequent reactions, namely drawbacks such as aldol cyclizations and retro-Claisen reactions. Excellent yields and chemoselectivities together with the environmentally friendly nature of the Fe(III) catalysis make this an important alternative to established methods. An enone-dienol tautomerization of 2-acceptor substituted cycloalkenones is the precondition for the iron-catalyzed vinylogous Michael reaction. Herein a γ-donor formally undergoes conjugate addition to a β-acceptor to furnish products with 1,7-dioxo constitutions. Actually, this process is a sequence initiated by the tautomerization followed by a [4 + 2]-cycloaddition and a retro-aldol reaction. When benzoquinone derivatives are employed as acceptors, the products of the vinylogous Michael reactions are highly functionalized biaryl precursor compounds.

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