Pyrrole Ring Opening in 5-Nitroso- and 5-Phenylazo-1H-pyrrolotetrazoles --- An Unexpected Valence Isomerism
作者:Dietrich Moderhack、Dirk Decker
DOI:10.3987/com-93-s68
日期:——
1,6-Disubstituted 5-nitroso-1H-pyrrolotetrazoles ring open below 20-degrees-C to give the isomeric acrylonitrile oxides. The 5-phenylazo analogs as well as 5-nitroso derivatives having in addition an acceptor group at C-7 are stable at 20-degrees-C, but heating with the dipolarophile DMAD leads to pyrazoles and isoxazoles.
Aromatic azapentalenes: 1H- and (mesoionic) 2H-pyrrolotetrazoles. Part 1. Synthesis and spectral characteristics
Two separate series of the title systems have been prepared by cyclisation of tetrazolium salts having acylmethyl functions attached to both the ring carbon and the adjacent nitrogen atom (3, 4): (i) working in an acetate buffer led to 7-acyl derivatives (5, 6; Scheme 3), and (ii) treatment with anhydride–base gave 5,7-diacyl compounds by a deviating ring closure mechanism (11, 12; Scheme 4). These materials could be defunctionalised to afford pyrrolotetrazoles (7, 8) which were earlier approached in vain from the respective 5-methyltetrazolium salts (Tschitschibabin reaction). Regarding characterisation data, attention is drawn to the conspicuous spectroscopic differences between the 1H- and the 2H-system. 2H-Pyrrolotetrazoles (6, 8, 12) represent a novel class of Ramsden’s ‘type C’ heteropentalene mesoions.
AM1 calculations. 1,3-Dipolar cycloaddition behaviour of 2 towards DMAD, a conceivable process, could not be detected; only linear addition was observed. Nitroso derivatives of the series 3, 4 and 8 were not isolated as such but as the ring-opened nitrile oxides 11 and 12; at elevated temperature analogous valence isomers arise also from the nitroso derivative 7e and the azo compounds 3e and 4e.