Formation of an unexpected rearrangement product using Grubbs’ second generation catalyst: 2-allyl-3,4-dihydro-2H-1,4-benzothiazines from diene precursors

Application of sub-stoichiometric amounts of Grubbs’ second generation catalyst to the substrate N-allyl-N-[2-(allylsulfanyl)phenyl]-4-methylbenzenesulfonamide afforded the ring-closed compound 6-[(4-methylphenyl)sulfonyl]-5,6-dihydro-2H-1,6-benzothiazocine, as well as the unexpected 2-allyl-4-[(4-methylphenyl)sulfonyl]-3,4-dihydro-2H-1,4-benzothiazine. Use of similar conditions on an analogous sulfoxide resulted in the expected product, 6-[(4-methylphenyl)sulfonyl]-5,6-dihydro-2H-1,6-benzothiazocine 1-oxide, indicating that the sulfide was playing a key role in this novel transformation. Furthermore, the use of N-allyl-4-methyl-N-{2-[(2-methyl-2-propenyl)sulfanyl]phenyl}-benzenesulfonamide in the same reaction gave 2-(2-methyl-2-propenyl)-3,4-dihydro-2H-1,4-benzothiazine.