Ring opening of methylenecyclopropanes with halides in liquid sulfur dioxide

Methylenecyclopropanes (MCP) undergo ring opening reactions with group I and II metal halides and ammonium halides in liquid SO2 to afford homoallylic halides, which are versatile reagents in organic synthesis. The developed reaction conditions are compatible with acid-labile substrates such as N-Boc-protected compounds. Liquid SO2 is a polar reaction medium with Lewis acid properties that solubilizes inorganic salts. The unique properties of SO2 were demonstrated by control experiments: 1) upon performing the reactions with the aforementioned salts in conventional solvents, the MCP ring opening was not observed either in the presence of catalytic amounts of H3PO4 (similar pK(a) to that of H2SO3), or in the absence of H3PO4; 2) a solution of SO2 in THE exhibited similar properties to that of liquid SO2. (C) 2020 Elsevier Ltd. All rights reserved.