Nickel-Catalyzed Annulative Synthesis of 1,2-Benzothiazine-1,1(2H)-dioxides

Significance

Based on previous work on the enantioselective synthesis of 3,4-dihydroisoquinolin-1(2H)-ones by nickel-catalyzed denitrogenative annulation of allenes to 1,2,3-benzotriazin-4(3H)-ones (M. Yamauchi, M. Morimoto, T. Miura, M. Murakami J. Am. Chem. Soc. 2010, 132, 54), Murakami and co-workers report the regio- and enantioselective synthesis of 3,4-dihydro-1,2-benzothiazine-1,1(2H)-dioxides 3 from 1,2,3,4-benzothiatriazine-1,1(2H)-dioxides 1. Ni(cod)₂ as catalyst and (R)-quinap as ligand afforded stereoselectively the optically active products 3 along with small amounts of regioisomers 4. The reaction is compatible with various primary and secondary alkyl and aromatic groups R^¹ on the nitrogen (except R^¹ = t-Bu probably due to steric hindrance) as well as with a variety of allene substituents R^³. The scope of R^² variation was poorly evaluated. A mechanism involving oxidative addition of the N-N bond to nickel(0) and extrusion of N₂ to form a five-membered azanickelacycle, followed by allene insertion and allylic amidation at the more substituted carbon is assumed.