Theory and Structural Aspects

Trost and co-workers used proton and carbon NMR to show that the 

2-diazocine they synthesized (see Section 11,A,2) had structure 20 (R = R1 = R = H), not 21, and consisted almost exclusively of monocyclic, as opposed to diazabicyclo[4.2.0]octatriene (see 86, Section II,C,3), valence tautomers. This latter observation is also supported by estimated enthalpies of isomerization to bicyclic tautomers, which are less favorable than in the carbocyclic series (79JOC1264). 

a) its dipole moment, IR, UV, and NMR spectra were distinguishable from (Z)-isomer 40 (R = H) (72TL4565). NMR studies were particularly relevant. Shift reagent experiments showed the europium complex coordinated more strongly with (Z)-compound 40 (R = H) than with ( )-isomer 12 (R1 - R4 = H) (72TL4565). Also, the (E)-isomer and its 3,8-disubstituted derivatives (12) showed nonequivalent methylene and meth-ine protons, in contrast to (Z)-isomers 40 (81JOC303). 

Quinkert and co-workers, using X-ray diffraction, showed that ( )-compound 12 (R1 = R4 = Ph, R2 = R3 = H) had a slightly distorted crown shape. The configuration of epimeric 12 (R1 = R3 = Ph, R2 = R4 = H) was established by nonempirical NMR. cis and tra/w-Diphenyl (Z)-isomers 40 (R = Ph) were adequately characterized as photoisomers of corresponding ( )-compounds 12 (76CB518) (see Section II,A,3,c). 

Kao and Huang employed molecular mechanics and ab initio molecular orbital (MO) theory to determine the structure, energies, and conformations of the (Z)- (40, R = H) and ( )- (12, R1 - R4 = H) isomers. For the (Z)-isomer, an unsymmetrical conformation (67) is favored by at least 6 kcal over symmetrical chair, boat, and twist forms, which suffer from ethane-type and other H-H repulsions. In the case of ( )-isomer, the twist 

---