Diversity of reaction of azoalkenes in cycloadditions

Abstract

Reactions of the azoalkenes (E,E)-4-methyl-N-phenyl-2,3-diazapenta-2,4-diene-5-carboxamide 2 and methyl (E,Z)-3-methyl-4-phenylcarbamoyl-1,2-diazabuta-1,3-diene-1-carboxylate 3, having respectively an electron-rich and an electron-poor group on nitrogen, are described. Compound 2, is a stable compound and its stereochemistry was established as E,Eby X-ray crystallography; compound 3 is much less stable and was assigned the E,Z stereochemistry on the basis of its ¹H NMR spectrum and the crystal structure of a derivative 17.

Of a wide variety of substrates, only azo esters react with 2, which behaves as a heterodiene. The 1,2,3,6-tetra hydro-1,2,3,4-tetrazine 7 obtained from 2 by reaction with dimethyl azodicarboxylate is unstable and on work up yields a 4,5-dihydro-1H-1,2,3-triazole 9a(whose structure was established by X-ray crystallography). This isomerisation is probably acid catalysed since, by inclusion of acid, 7 tautomerises to a 1,2,3,4-tetrahydro-1,2,3,4-tetrazine 8 which then ring contracts to 9a, followed by a decarboxylative elimination of methyl carbamate to yield the 1,2,3-triazole 10.

The azoalkene 3 is unreactive to electron-poor substrates, but reacts with cyclopentadiene both as a heterodiene and as a dienophile (at CC) to give as Diels–Alder adducts the tetrahydro-1H-cyclopenta[c]pyridazine 16 and the bicyclo[2.2.1]hept-2-ene 17 respectively. With ethyl vinyl ether 3 gave the ethoxytetrahydropyridazine 18 which eliminated EtOH in acid to the dihydro tautomers 19 and 20 which on basic hydrolysis and oxidation give the pyridazine 21.