1.2.3- Benzotriazines thermolysis

In addition to the examples of 1,2,3-benzotriazine thermolysis reviewed earlier, the following cases are noteworthy ring contraction by loss of N2 transforms 4-phenyl-l,2,3-benzotriazine (36 R = Ph) into 2-phenylbenzazete 56, which is allowed to react with diphenylketene in a formal [4-1-2] cycloaddition (probably stepwise) followed by a hydrogen shift to generate the tetracyclic lactam 57 (Equation 4) <1976CC125>. 

The formation of transient benzazetidinones (251) in the photolysis and thermolysis of benzotriazin-4-ones (250) is well established (76AHC215) and the highly hindered adamantyl derivative has actually been isolated from flash pyrolysis of 4-adamantylbenzotriazinone (73JCS(Pl)868). A second route to such hindered benzazetidinones involves cyclization of the iminoketene valence tautomer (252 R = Bu ), the latter being generated by deprotonation of the anthranilium salts (253) (71JA1543). 

Results available from studies of the photochemical decomposition of benzotriazinones indicate that formation of the reactive species 146 during thermolysis of 10 is a reasonable postulate. Simple 3-alkyl-1,2,3-benzotriazin-4(3/f)-ones (10, R = alkyl) are apparently inert to photo-... 

Benzazetes 7 are obtained by thermolysis of 1,2,3-benzotriazines 6 (Scheme 5) <1975J(P1)45>. Photocycloaddition of 2-alkoxy-3-cyanopyridines 8 with methylacrylonitrile yields a bicyclic [2 + 2] cycloadduct intermediate followed by rearrangement to give product 9 in moderate yield (Scheme 6) <1996CC1349>. 

The thermolysis of 3-(l-naphttyl)-l,2,3-benzotriazin-4(3fl)-one (115) results in the loss of nitrogen and formation of benz[c]acrid-7-one, m.p. 365-367°C, together with the major product benzo[c]phenanthrid-6-one. The benzacridone is considered to arise by rearrangement of a naphthoazetinone (A.J. Barker et J. chem. Soc. 

Thermolysis of 4-anilino-l,2,3-benzotriazines 21 in hot formamide gave 3-arylquinazolin-4(3//)-ones 23 in high yields. 4-(4-Nitroanilino)-l,2,3-benzotriazine (21, Ar = 4-02NC6H4) afforded 3-substituted iV-(4-nitrophenyl)-l,2,3-benzotriazin-3(4/f)-imine 24 in high yield.215 4-Anilino-1,2,3-benzotriazines 21 decomposed in piperidine and related secondary amines to yield A ,/V,.V -trisubstituted 2-aminobenzimidamide 22 in excellent yields. 4-(2-Aminoanilino)-l,2,3-... 

Careful thermolysis of 3-(o-nitrophenyl)-4-phenyl-l,2,3,5-oxathiadiazole 2-oxide (476) in toluene led to the isolation of 2-phenyl-l,2,4-benzotriazin-3(2if)-one 1-oxide (54). In solution a reversible equilibrium of (54) with the ring-opened o-isocyanato-azoxybenzene (55) is observed (Scheme 87) <85JCS(P1)1471>. 

Both 1,2,4 or better 1,2,3-benzotriazines, 49d and 49c, respectively, loose Nj and nitriles on thermolysis to give arynes. The triazinones 49f undergo a similar minor fragmentation with expulsion of nitrogen and isocyanates. The major reaction leads to azetinones 54 which were shown not to be precursors of arynes. 

By analogy with the benzotriazines 49q and 49e (cf. Table 3), benzox-azinones might be expected to lose CO2 and a nitrile to give arynes. Thermolysis of three of the possible isomers has been investigated. The 2,3,1-isomers (81) undergo substantial fragmentation to arynes at 700°C, while the 2,4,1-... 

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