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

Abstract

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 d 6-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 ZnCl 2 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(sp 3) C(sp 2) bonds, and for the rotations of the N-aryl rings about the N(sp 2) C(aryl sp 2) 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.