Photochemistry of N-heterocycles. Part 1. Synthesis and photochemistry of some 2(4),5-dihydro-1,2,4-triazines. X-Ray molecular structure of 1-(4-methyl-3,5,6-triphenyl-1,4,5,6-tetrahydro-1,2,4-triazin-6-yl)ethanol

Abstract

Ethanolic solutions of the hydrochlorides of the mostly new 2(4),5-dihydro-1,2,4-triazines (1)–(4) were irradiated with a high-pressure mercury immersion lamp (λ 300 nm). The salts (1a)–(3a) underwent reductions, ring contractions, and dehydrogenations upon irradiation to yield pyrazoles (5) and (6) along with phenanthroimidazoles (9) and (10). In addition, the 4-methyl-3,5,6-triphenyl-4,5-dihydro-1,2,4-triazinium chloride (3a) furnished the 6-(1-hydroxyethyl) adduct (11) whose structure was assigned on the basis of its ¹H and ¹³C n.m.r. spectra, and an X-ray diffraction study. The 4-(3-hydroxypropyl)dihydrotriazinium chloride (4a) led, in addition to the expected pyrazole (5), to two new products: the aromatic triazine (15) and the oxazinotriazine (21) by photoelimination and intramolecular photosubstitution, respectively.