Ring Contraction of 4-Diazo-1-phenyl-5,6-dioxo-1,4,5,6-tetrahydropyridazine in the Solid Stave via “Pyrazolo-lactone”

The title compound 8a was generated by both flash vacuum pyrolysis and matrix photolysis of diazoquinolone 7 and identified by Ar-matrix IR spectroscopy and mass spectrometry.Trapping of the highly reactive a-0x0 ketene 8a with methanol leads to 0-keto ester 9, whereas dimerization of 8a furnishes spiro compound 10.Recently, much interest has been focused on the chemistry of a0x0 ketenes ["]. Usually, this type of conjugated ketenes is highly reactive and cannot be isolated or observed under ordinary reaction conditions, since in most cases rapid dimerization or reaction with atmospheric moisture takes place but also makes a six-membered transition state for 3,4-addition of nucleophiles[61 inaccessible. Cyclic a-0x0 ketenes, which are constrained to the s-Z conformation, are generally more reactive, even if substituted by bulky groups (e.g. 6)"'. Therefore, it is not surprising that heterocyclic a-0x0 ketenes are very unstable, and -with very few -have not been identified so far, although such ketenes were proposed as intermediates in a variety of reactions[81. Here we report on the generation and characterization of such a ketene, employing e.g. lowtemperature FT-IR spectroscopy which has proved to be a useful tool in the identification of other reactive a-0x0 ketenesP1.

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