Pyrolysis of benzotriazoles. 1-Acyl- and 1-alkoxycarbonylbenzotriazoles: Hetero-Wolff rearrangement to N-acyl- and N-alkoxycarbonyl-fulvenimines and free radical routes to cyanocyclopentadienes

Abstract

The flash vacuum pyrolysis (FVP) of 1H-benzotriazoles is an important and high-yielding source of cyanocyclopentadienes (cyclopentadienecarbonitriles). Early research on the pyrolysis of 1-acylbenzotriazoles only revealed formation of benzoxazoles, while the important formation of cyanocyclopentadienes was missed. Similarly, previous studies of the pyrolysis of 1-alkoxycarbonylbenzotriazoles revealed the formation of 2-alkoxybenzoxazoles, 1- and 2-alkylbenzotriazoles and a variety of decomposition products, but again formation of cyanocyclopentadienes was not reported. We now report the formation of methylcyanocyclopentadienes 7–9 and unsubstituted 1-cyanocyclopentadiene 4 as well as the benzoxazoles 33 and 24 in FVP reactions of 1-acetylbenzotriazole 32 and 1-methoxycarbonylbenzotriazole 21a, respectively. Similarly, 1-benzoylbenzotriazole 18 affords 2-phenylbenzoxazole 20 and phenylcyanocyclopentadienes 27. The reactions are interpreted in terms of Wolff-type ring contraction to N-acylfulvenimines, which may take place from either 2-diazocyclohexadienimine valence isomers of the benzotriazoles or from singlet iminocyclohexadienylidene diradicals/carbenes. Homolysis of the NCO bond in the N-acylfulvenimines generates free radicals, which can recombine to form the methylcyanocyclopentadienes 7–9, undergo hydrogen abstraction to afford the unsubstituted 4, and in the case of 1-benzoylbenzotriazole 18 dimerize to form biphenyl. The FVP of 1-ethoxycarbonylbenzotriazole 21b yields ethylcyanocyclopentadienes 45 as well as the unsubstituted cyanocyclopentadiene 4, which results from elimination of ethene. FVP of 1-methylbenzotriazole 22a does not lead to methylcyanocyclopentadienes 7–9; instead a 1,4-hydrogen shift in the putative N-methyliminocyclohexadienylidene diradical 47 yields N-phenylmethanimine 48. The proposed reaction mechanisms are supported by DFT calculations.