Anomalous shielding and hidden partner chemical exchange in the 1H NMR spectra of the bisurethane diazetidines, the 1,2-diaryl-3,5-dialkyl-6,7-dialkoxycarbonyl-4-oxo-6,7-diazabicyclo [3.2.0]hept-2-enes
The bicyclic diazetidines (3), (11), and (17), have trans-pyramidal nitrogens, the ester group on N(7) being sterically protected and very unreactive. Its alkyl group is close to the face of the aryl ring attached to C(1) and shows remarkable shielding effects in its 1H NMR spectrum, while the alkyl group of the N(6) ester shows a more normal resonance. In the methyl esters the OMe group of the N(7) ester is ca. 1 ppm upfield of the other ester OMe group and in each of the oxymethylene compounds one of the protons of the N(7) ester group is upfield of its (unshielded) diastereotopic partner by ca. 2 ppm.
These shielded resonances are extremely broadened at room temperature, especially at high field. DNMR studies show that on cooling, the shielded absorption in each case broadens further and then sharpens into two peaks, an effect due to the interconversion of two major conformations with one or more of very low population, an example of ‘hidden partner exchange.’ The exchange processes responsible, which occur between protons in the N(7) ester group in very shielded (major conformations) and normal (minor) environments, must involve either N(7) inversion or N(7)–CO rotation.
The di-methyl and -trichloroethyl compounds (3a) and (3c), were investigated in detail in CD2Cl2 and C7D8. In (3a) the N(7) ester OMe resonance of the minor partner was found at δ 3.65 (ca. 3%, 200 K, CD2Cl2), its identity being confirmed by low-temperature examination of the deuteriated isomers (20) and (21). In (3c) there are two minor conformations, ca. 2% each.
Two major and at least one minor conformation were observed for the dibenzyl and dimethyl esters (3d) and (11); the latter also showed a very shielded Me in the ring C(5) ethyl group. The presence of a minor conformation was not conclusively detected in the diethyl ester (3b), but was inferred from the broadening of the upfield signals of its two major conformations. The acecylcone derived compound (17) showed three major conformations (46 : 36 : 18%), the second and third together (unresolved) being the most shielded in the N(7) group, though less so than in the other diazetidines.
Rotation of the C(1) Ph ring has also been identified as a conformational process in all the diazetidines [except (17)] but is quite independent of the processes leading to anomalous shielding of the ester groups.