Reexamination of Products and the Reaction Mechanism of the Chalcogeno-Baylis–Hillman Reaction: Chalcogenide–TiCl4-mediated Reactions of Electron-Deficient Alkenes with Aldehydes

Reactions of p-nitrobenzaldehyde (4) with methyl vinyl ketone (5) were conducted in the presence of TiCl4 and dimethyl sulfide (3) or selenopyranone 6. When the raw product was purified by column chromatography on silica gel, α-chloromethyl aldol 8 was obtained as a mixture of diastereoisomers 8a and 8b. In contrast, purification of the raw product by preparative TLC on silica gel gave α-methylene aldol 7. The mechanism for the formation of α-chloromethyl aldol 8 and diasteroselection for the syn-isomer 8a and anti-isomer 8b are discussed.