Photolysis and Pyrolysis of Phenyltetrazoles: Formation of Phenylcarbodiimide, N‐Phenylnitrile Imine, Phenylnitrene, Indazole, and Fulvenallene

Abstract

Photolysis of 1‐phenyltetrazole 2b at either 193 or 266 nm in a Ne matrix as well as flash vacuum pyrolysis (FVP) of the same compound with isolation of the products in Ne matrix generate phenylcarbodiimide PhN=C=NH 6 very efficiently as the principal product in an almost pure state. Minor amounts of phenyl azide 8 and 1‐azacyclohepta‐1,2,4,6‐tetraene 10, the product of ring expansion of phenylnitrene, are also formed, and the amounts of both substances increase as a function of photolysis time. The photolysis of 2‐phenyltetrazole 1b in a Ne matrix at 266 nm for 0.5 min generates the allenic N‐phenylnitrile imine PhN=N(+)=CH(–) 3b, absorbing strongly and cleanly at 2021 cm–1 in the IR spectrum. At longer photolysis times peaks ascribed to phenyl azide, phenylnitrene, 1‐azacycloheptatetraene, and phenylcarbodiimide 6 become prominent. FVP of 2‐phenyltetrazole yields indazole 15 as the major product together with a small amount of phenylcarbodiimide 6. Indazole formation is ascribed to cyclization of the nitrile imine in the ground state. It is not formed on FVP of 1‐phenyltetrazole in agreement with the computational result that the thermodynamically most favourable path for rearrangement of the putative N‐phenylimidoylnitrene 5b yields phenylcarbodiimide 6 rather than phenylnitrile imine 3b. Further pyrolysis of indazole affords a minor yield of fulvenallene 18.