The photochemical decomposition of alkyl azides

Abstract

Abstract A study has been made of the products resulting from the solution photolyses of n-butyl azide, n-ocytl azide, 4-phenyl-1-butyl azide, α-azidobutyric acid, α-azidoisobutyric acid, azidodiphenylacetic acid, ethyl α-azidobutyrate, α-azidovaleric acid, and ethyl α-azidovalerate. The effect of variations in solvent and temperature has been investigated as well as the influence of benzophenone photosensitization and iodine catalysis. In the case of the first three azides the predominant product (besides nitroen) under all photolytic conditions studied was the aldimine formed by 1,2-hydrogen migration. Hydrogen abstraction probably from solvent accounted for small amounts of primary amine. 1,2 Alkyl migration accounted for the formation of some of the lower homologous amine. Also secondary amine (1–2%) formed by insertion into the solvent cyclohexane was observed. Photolysis of the α-azido acids in methanol solution resulted in efficient decarboxylation and aldimine formation. Minor amounts of amino acid formed presumably via hydrogen abstraction from solvent were also obtained. In the case of azidodiphenylacetic acid, phenyl group migration occurs in competition with direct loss of esters. α-Deuterio-α-azidobutyric acid has been synthesized and photolysed for the purpose of determining the sequence of steps in the photodecarboxylation reaction. Intramolecular photocyclization of alkyl azides to form pyrrolidine derivatives has been found to be a highly irreproducible process. 1 2 In initial experiments in our laboratory we succeeded in obtaining 2-n-butylpyrrolidine by irradiation of octyl azide. Subsequently we were unable to reproduce this result although as far as possible essentially identical reaction conditions were employed. In a private communication from Prof. D. H. R. Barton we have learned that he so far had been unsuccessful in repeating the original photolytic cyclization of azides. In addition to the work reported in this paper, research continues in our laboratory directed towards resolving these ambiguities. In the systems studied this pathway accounted for only a very minor amount of product or none at all. The stability of pyrrolidine under the photolytic and work up conditions was established. In spite of the large number of experimental variations employed, the yield of cyclic product could not be improved. Conditions for high yield cyclization have not been defined in the present investigation; and based upon results obtained, the photochemical decomposition of alkyl azides does not appear to be a generally useful method for pyrrolidine synthesis. The mechanism of the photolyses is discussed in terms of the possible formation of an alkyl nitrene and subsequent intervention of other intermediates. For the α-azido acids, a dual mechanism for decarboxylation is discussed involving the α-nitreno acid and α-imino acid.