Regioselective [3 + 2]/[4 + 2] Annulations of α-Nitrosostyrenes with Triazines.

Abstract

Reported herein are the regioselective [3 + 2]/[4 + 2] annulations of α-nitrosostyrenes with hexahydro-1,3,5-triazines, which provide five- and six-membered heterocycles in moderate yields under mild reaction conditions. Compared to previous reports, this unusual [3 + 2] annulation features the use of rhodium catalysts to control the regioselectivity of α-nitrosostyrenes, generated in situ from α-bromooximes 1. Meanwhile, the [4 + 2] annulation process was primarily regulated by 1,4-dioxane and temperature. In addition, DFT calculations reveal that the activation barrier of the [3 + 2] annulation is lower than the [4 + 2] process (1.7 kcal/mol) in the presence of a Rh catalyst. Finally, synthetic transformation of cyclic nitrones 3 and oxadiazines 4 was also demonstrated.