NMR studies of hindered rotation and magnetic anisotropy: The Diels-Alder adducts of phencyclone with N-phenylmaleimide and N-(2-trifluoromethylphenyl)maleimide. Ab initio calculations for optimized structures

摘要

N-Phenylmaleimide, 2, and N-(2-trifluoromethylphenyl)maleimide, 3, were separately added to phencyclone, 1, to yield the corresponding phen-cyclone Diels-Alder adducts, 4 and 5. The resulting adducts (and some precursors) have been characterized by one- and two-dimensional H-1 and C-13 NMR at 300 and 75 MHz, and by F-19 NMR at 282 MHz, at ambient temperatures. The NMR data are consistent, for both adducts, with: (a) hindered rotation of the bridgehead unsubstituted phenyl groups about the C(SP2)-C(SP3) bonds, based on slow exchange limit (SEL) spectra and (b) endo adduct configuration based on magnetic ani-sotropic effects in the H-1 NMR. The NMR spectra of the phencyclone adduct, 4, of N-phenylmaleimide, indicate free rotation on the NMR time-scales (fast exchange limit, FEL spectra) about the N-phenyl bond. The spectra for the adduct, 5, of N-(2-trifluoroniethylphenyl)maleimide are interpreted as consistent with SEL regimes, for the N-aryl rotations, with a single rotamer present in which the trifluoromethyl group is directed "out of" the adduct cavity, and away from the phenanthrenoid moiety. This conclusion is based, in part, on NMR data suggesting the apparent slow N-aryl bond rotation in a pair of atropisomers corresponding to the acetic acid addition products from the N-(2-trifluoronlethylphenyl)maleimide. Evidence of magnetic anisotropic effects due to the phenanthrenoid moiety and proximal carbonyls is discussed. H-1, C-13, and F-19 assignments are presented and interpreted. Molecular modeling calculations at the Hartree-Fock level, 6-31G* basis set, were performed to provide geometry optimizations for energy-minimized structures of selected compounds.