Aromatic Allylsulfenylation with in Situ Generated Allylic Thiols under the Heck Conditions

Article abstract of DOI:10.1246/bcsj.70.445

A wide structural variety of S-allylic thiocarbamates 2 can be prepared in good yields by the rearrangement of O-allylic thiocarbamates 1 under three different conditions: thermal activation (neat, 120-150°C), palladium(0) (25-65 °C), and palladium(II) catalysis (25-65°C). Of the two possible regioisomers of unsymmetrical S-allylic thiocarbamates 2, those of higher thermodynamic stability can be prepared in high purity under either thermal activation or palladium(0) catalysis. Although the thermodynamically less stable regioisomers of 2 are, in general, hard to be prepared in high purity, some of them (e.g., 2d and 2h′) can be obtained with an exclusive or high selectivity by the catalysis of palladium(II). The stereoisomeric pair of 2j and 2j′ can be prepared selectively by the palladium(0)-catalyzed rearrangement of 1j and 1k, respectively. These reactions proceed with retention of configuration at the allylic stereocenters, S-Allylic (2) and S-alkyl thiocarbamates (7) undergo fragmentation to generate thiolates in the presence of inorganic bases (e.g., K₂CO₃, K₂CO₃·-Et₄N⁺I^-) by heating in an aprotic solvent; the thus-formed thiolates react with aromatic iodides and vinyl bromides in the presence of palladium(0) complexes to furnish aryl and vinyl sulfides, respectively. A wide variety of aryl sulfides can be prepared in good yields irrespective of the kind of substituents and their substitution positions (o-, m-, p-) under conditions B [Pd(OAc)₂, PPh₃, K₂CO₃·Et₄N⁺I^-, dioxane, 100°C]. Under conditions E [Pd(OAc)₂, PPh₃, Cs₂CO₃, dioxane, 100°C], better yields result specifically for the sulfenylation of aromatic iodides bearing substituents having large Hammett σ constants.