Silver(1+) ion catalyzed rearrangements. XXXII. Unsaturated heterocyclic systems. XCIII. Probing of the interrelation between heteroatomic substitution and equilibrium imbalance in 2,8-trimethylenesemibullvalene derivatives

4,6‐Barbaralanedicarboxy‐2,8‐dicarboxylic anhydride and 1,5‐dimethyl‐4,6‐semibullvalenedicarboxy‐2,8‐dicarboxylic anhydride: examples of unusual barbaralanes and semibullvalenes that do not undergo the Cope rearrangement. They are locked as the closed tautomers

“…Most importantly, they found that for 19 , 20 , 21 , which are analogous to our monoanhydrides, the closed forms, as shown with a 2–8 bond, dominated the Cope equilibria in every case with the closest analog, to our systems, 19 being nearly 100% at (skewed) equilibrium. The sulfur bridged 20 was determined to be ~90% in the closed form 20 loc and the corresponding distribution for nitrogen bridged 21 being between that for 19 and 20 . It is interesting to note that for the trimethylene annelated semibullvalene 22 the NMR spectra are invariant from −91 to +116 °C indicating no measureable preference for one tautomer or the other …”

“…In another study in which we investigated the role of strain in the homoaromatization of semibullvalenes, we found that 2,8‐ and/or 4,6‐substitution by carboxylic acid groups on the semibullvalene nucleus resulted in anomalously large increases in strain energies (determined using isodesmic reactions). This suggests that 5 and 12 exclusively adopt the closed forms in order to minimize the strong steric and dipolar interactions between the carboxyl functions and to avoid the severe anti‐Bredt destabilization that would result from the anhydride bridge in the open forms 1d,20 …”

“…The sulfur bridged 20 was determined to be~90% in the closed form 20 loc and the corresponding distribution for nitrogen bridged 21 being between that for 19 and 20. [20] It is interesting to note that for the trimethylene annelated semibullvalene 22 the NMR spectra are invariant from À91 to +116°C indicating no measureable preference for one tautomer or the other.…”