The umpolung alkylation of silyl enol ethers with an iodonium(III) ylide proceeds under mild conditions to afford various 1,4-dicarbonyl compounds in high yields in the presence of a halogen-bonding catalyst. Unlike typical transition-metal activation processes of such ylideprecursors, which tend to proceed via carbenoid intermediates, experimental and computational studies indicate that halogen bonding
corresponding Mannich products by various dimethyl(methylene)ammonium salts under a range of reaction conditions. The several methods used to form these derivatives are compared. Excellent approaches to aldehyde derivatives involve treating the enol silyl ether of the carbonyl compound with methyllithium and then an iminium salt, or directly adding the iminium salt to the enol silyl ether. Ketones may be