Phosphine-initiated general base catalysis: facile access to benzannulated 1,3-diheteroatom five-membered rings via double-Michael reactions of allenes.

Judy Szeto, Vardhineedi Sriramurthy, Ohyun Kwon

Index: Org. Lett. 13(20) , 5420-3, (2011)

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Abstract

General base-catalyzed double-Michael reactions of allenes with various dinucleophiles are described. The reactions are facilitated most efficiently by a catalytic amount of trimethylphosphine, affording six types of C2-functionalized benzannulated five-membered heterocycles: benzimidazolines, benzoxazolines, benzothiazolines, 1,3-benzodioxoles, 1,3-benzoxathioles, and 1,3-benzodithioles. This atom-economical reaction is operationally simple and provides the product heterocycles in good to excellent yields. Careful mechanistic studies unveiled the phosphine-triggered general base catalysis pathway. Furthermore, the double-Michael reaction can serve as an alternative method for the selective monoketalization of β-diketones.© 2011 American Chemical Society