1H, 13C, and 19F NMR studies of phencyclone adducts of N-(polyhalophenyl)maleimides: evidence for dynamic NMR in maleamic acids: Ab initio calculations for optimized structures

NMR methods, including one- and two-dimensional techniques (at 7.05 T) for 1H, 13C and 19F, have been applied to studies of hindered rotations and magnetic anisotropy in some crowded Diels–Alder adducts of phencyclone (1). Symmetrically substituted N-aryl maleimides (2) bearing numerous halogens on the N-aryl ring, were employed as dienophiles to form the target adducts (3). The maleimides included: N-(4-bromo-2,6-difluorophenyl)maleimide (2a); N-(2,3,5,6-tetrafluorophenyl)maleimide (2b); N-(4-bromo-2,3,5,6-tetrafluorophenyl)maleimide (2c); N-(2,3,4,5,6-pentachlorophenyl)maleimide (2d); and N-(2,4,6-tribromophenyl)maleimide (2e). Maleimides (2a–2c) were prepared from the precursor N-aryl maleamic acids (5a–5c). Ambient temperature fluorine-19 NMR of these maleamic acids in d6-acetone showed substantial unusual peak broadening consistent with intermediate exchange rate processes, which may correspond to the N-aryl rotation process. Maleimides (2d) and (2e) were produced in one step from pentachloroaniline or 2,4,6-tribromoaniline, respectively, and maleic anhydride with anhydrous ZnCl2 at ca. 200 °C. For the adducts (3), we observed slow exchange limit spectra on the 1H, 13C, [and 19F, for (3a–3c)] NMR timescales for the rotation of the unsubstituted bridgehead phenyls about the C(sp3)C(sp2) bonds, and for the rotations of the N-aryl rings about the N(sp2)C(aryl sp2) bonds. Ab initio calculations for geometry optimizations at the Hartree–Fock level with 6-31G* (or LACVP*) basis sets were performed for the adducts. We believe that this is the first report of detailed 1H, 13C, and 19F NMR data for a substantial collection of N-aryl maleamic acids, maleimides and their phencyclone adducts bearing multiple fluorines or other halogens directly on the N-aryl ring, together with complementary quantitative geometric parameters from high-level HF/6-31G* (or LACVP*) calculations.