Multinuclear NMR studies and ab initio structure calculations of hindered phencyclone Diels-Alder adducts from symmetrical cyclic dienophiles: Cyclohexene, vinylene carbonate, vinylene trithiocarbonate and two N-aryl maleimides

摘要

A series of hindered Diels-Alder adducts have been prepared from phencyclone, 1, with various unusual symmetrical cyclic dienophiles, including cyclohexene, 2a; vinylene carbonate, 2b; vinylene trithiocarbonate, 2c and the N-aryl maleimides: N-(4-dimethylamino-3,5-dinitrophenyl)maleimide ("Tuppy's maleimide"), 2d; and N-[3,5-bis(trifluoromethyl)phenyl]maleimide, 2e. The highly hindered adducts, 3a-e, respectively, were extensively characterized by one- and two-dimensional NMR methods, observing proton, carbon-13 and fluorine-19. High resolution COSY45 spectra permitted rigorous proton NMR assignments. The 2D heteronuclear C-H chemical shift correlation spectra (HETCOR, XHCCRR) were obtained for adducts 3a-d, allowing specific assignments for protonated carbons. Corrections to earlier proton NMR assignments for the vinylene carbonate adduct are given; results of the gated decoupling C-13 NMR experiment for this adduct supported endo adduct stereochemistry. Relative proton chemical shifts for bridgehead phenyls of adduct 3c appeared anomalous relative to other adducts, suggesting possible special anisotropic interactions (with endocyclic sulfur or other anisotropic groups in the product) due to the unusual calculated orientation of the phenyls. The unsubstituted bridgehead phenyls in all adducts were shown to exhibit slow exchange limit (SEL) H-1 and C-13 spectra on the NMR timescales at ambient temperatures (7 tesla) showing slow rotations about the C(sp(3))-C(aryl sp(2)) bonds. The rapid rotation of the N-aryl rings of the maleimide adducts was indicated by fast exchange limit spectra, suggesting that ortho substitution of the N-aryl ring may be necessary to slow this rotation to the SEL regime. Ab initio geometry optimizations at the Hartree-Fock level were carried out for each adduct, with the 6-31G* basis sets. Appreciable geometry differences were seen in calculated structures, and significant NMR chemical shift differences were experimentally observed, depending on the nature of the groups attached to the (Z)-HC=CH moiety of the dienophiles.