Heterocyclic polyfluoro-compounds. Part 37. Diels–Alder reactions of trichloro- and trifluoro-1,2,4-triazine

Abstract

Trichloro-1,2,4-triazine undergoes reaction at 70 °C with the olefins: ethylene, (Z)-but-2-ene, cyclopentene, and (Z)-cyclo-octene to give the 3,4-substituted 2,6-dichloropyridine derivatives, 3,4-R₂C₅HCl₂N, where R₂= H₂(1%), Me₂(75%), (CH₂)₃(77%), and (CH₂)₆(80%), respectively, in a reaction which appears to involve initial Diels–Alder addition, and loss of nitrogen, to form an intermediate dihydropyridine. This dihydropyridine then undergoes a [1,5] sigmatropic hydrogen shift and loss of hydrogen chloride to give the pyridine, but adds a second molecule of cyclopentene to the extent of 3% in a further Diels–Alder reaction, and this is the sole pathway with bicyclo [2.21] hept-2-ene.

With trifluoro-1,2,4-triazine and the olefins cyclopentene, (Z)-cyclo-octene, and bicyclo[2.2.1]hept-2-ene, only products derived by addition of a second molecule of olefin to an intermediate dihydropyridine are obtained in 55, 13, and 52% yield respectively. Bis(trimethylstannyl)acetylene gives 3,4-bis(trimethylstannyl)-2,5,6-trifluoropyridine (11%), and bicyclo[2.2.1]hepta-2,5-diene gives 2,3,6-trifluoropyridine (43%).