Benzazetes (1-azabenzocyclobutenes)

Abstract

Flash vacuum pyrolysis of 4-phenyl-1,2,3-benzotriazine (1) at 420° gives the bright red 2-phenylbenzazete (2) which is stable up to ca. –40° but which dimerises at higher temperatures. The azete (2) reacts with dilute aqueous acid and with phenylhydrazine to give o-aminobenzophenone and its phenylhydrazone, respectively. It undergoes cycloaddition reactions with conjugated dienes to give a variety of new heterocyclic systems; since these contain the labile azetine ring they mostly rearrange spontaneously to other heterocyclic systems. Thus diphenylisobenzofuran gives a primary adduct (16) which is isolable, but cyclopentadiene gives a hydrated product (18); carbon monoxide is extruded spontaneously from the tetraphenylcyclopentadienone adduct (20) to give the benzazocine (21), and nitrogen is similarly lost from the dipyridyltetrazine adduct to give the 1,3,4-benzotriazocine (22), the first example of this ring system. 4-Chloro-2-phenylbenzazete, 2-(p-methoxyphenyl)-benzazete, and 2-phenylnaphth[2,3-b]azete (24) are formed similarly from the appropriate triazines; the last two are, as expected, much more stable than 2-phenylbenzazete (2), compound (24) even having appreciable stability in the solid state at room temperature.

Flash pyrolysis of 4-phenyl-1,2,3-benzotriazine 3-oxide (27) gave 3-phenyl-2,1-benzisoxazole and acridone, possibly via 2-phenylbenzazete N-oxide (28), together with 3-phenylindazole, probably formed by rearrangement of (27) to 2-nitroso-3-phenylindazole and thermal denitrosation.