Aromatic propellenes, part. 8 : semiempirical calculations and DNMR studies of hexakis (pyrazol-1-yl) benzenes

Abstract

The stability of all possible conformers of hexa(pyrazol-1-yl)benzene [(pz)6bz], hexakis(3,5-dimethylpyrazol-1-yl)benzene [(dmpz)6bz], 1,2,4,5-tetra(pyrazol-1-yl)-3,6-bis(3,5-dimethylpyrazol-1-yl)benzene [(pz)4(dmpz)2bz] and 1,4-di(pyrazol-1-yl)-2,3,5,6-tetrakis(3,5-dimethylpyrazol-1-yl)benzene [(pz)2- (dmpz)4bz] has been determined by semiempirical AM1 calculations. In all series the absolute minimum corresponds to the conformation where the nitrogen lone pair of consecutive pyrazole rings is alternatively oriented to opposite sides of the benzene plane. The case of (pz)2(dmpz)4bz has been studied by 1 H NMR studies in solution. Three isomers have been identified and all the signals assigned. The slow evolution to other isomers from those found in the solid state has been followed by 1 H NMR spectroscopy thus allowing determination of the barrier to rotation of a (dmpz) substituent (ÄG‡ = 21–22 kcal mol21 ).