NMR Studies of Hindered Rotation. The Diels-Alder Adduct of Phencyclone withN-n-Propylmaleimide: Restricted Motion of Bridgehead Phenyls

Abstract

TheDiels-Alder adduct of phencyclone and N-npropylmaleimide has been studied in CDC1, solution at ambient temperatures by one-and two-dimensional 'H NMR (300 MHz) and "C NMR ( 7 5 MHz) techniques. Clear evidence is presented from slow exchange limit (SEL) spectra for hindered rotation of the bridgehead phenyls in the adduct. Full 'H spectral assignments have been made via selective homonuclear decoupling and high resolution COSY experiments. The number of signals in the aryl region of 13C NMR spectra also indicated slow rotation about the C (sp') -C (sp') bond to the unsubstituted bridgehead phenyls.

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

“…General synthetic procedures and spectral methods followed techniques described earlier [2][3][4][5][6][7][8][9]. Reagents were obtained from Aldrich Chemical (Sigma-Aldrich, Milwaukee, WI) and used directly without further purification.…”

“…For example, the unsubstituted bridgehead phenyls in many Diels-Alder adducts of 1 have been shown to exhibit slow rotations about the C(sp 3 )-C(aryl sp 2 ) bonds on the NMR timescales based on slow exchange limit (SEL) proton or carbon-13 spectra at ambient temperatures (at 7 and 4.7 tesla, i.e., 300 or 200 MHz for 1 H, 75 or 50 MHz for 13 C). Selected references include studies of the phencyclone adducts from N-n-propylmaleimide [3]; N-n-butylmaleimide [4]; N-carbamoylmaleimide [5]; N-pentafluorophenyl maleimide [6]; Nphenylmaleimide and N-(2-trifluoromethylphenyl)-maleimide [7]. Anisotropic effects on the bridgehead phenyls of the adducts are suggested by dispersion in proton chemical shifts of 1 ppm or more, a rather wide range for phenyl rings bearing no substituents or conjugated groups and connected only to a quaternary sp 3 [3,5-bis(trifluoromethyl)phenyl]maleimide, 2e.…”

“…Selected references include studies of the phencyclone adducts from N-n-propylmaleimide [3]; N-n-butylmaleimide [4]; N-carbamoylmaleimide [5]; N-pentafluorophenyl maleimide [6]; Nphenylmaleimide and N-(2-trifluoromethylphenyl)-maleimide [7]. Anisotropic effects on the bridgehead phenyls of the adducts are suggested by dispersion in proton chemical shifts of 1 ppm or more, a rather wide range for phenyl rings bearing no substituents or conjugated groups and connected only to a quaternary sp 3 [3,5-bis(trifluoromethyl)phenyl]maleimide, 2e. Geometry minimizations for the corresponding adducts, 3a-e, were performed at the Hartree-Fock level with the 6-31G* basis sets.…”

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

“…Slow exchange limit (SEL) 1 H and 13 C NMR spectra at ambient temperatures were observed for hindered rotations about the C(sp 2 )-C(sp 3 ) bonds to the unsubstituted bridgehead phenyls in the adducts, using medium field strength NMR spectrometers, e.g., ca. 200-300 MHz for proton and 50-75 MHz for carbon-13 [3][4][5][6][7]. When the adducts are formed from N-aryl maleimides, potential hindered rotations about the N(sp 2 )-C(aryl sp 2 ) bonds in the N-aryl moieties could be examined [8].…”

“…Phencyclone is well known as an effective diene component for Diels-Alder cycloaddition reactions [1] and N-substituted maleimides (2), are excellent www.elsevier.com/locate/fluor Journal of Fluorine Chemistry 125 (2004) [1893][1894][1895][1896][1897][1898][1899][1900][1901][1902][1903][1904][1905][1906][1907] dienophiles with (1) [2]. The normally expected endo stereochemistry for the Diels-Alder adducts (3), results in highly hindered systems. (Fig.…”