2,2′‐Bis‐azonia‐Cope Rearrangements of 2,3‐Homo‐6 H ‐1,4‐diazepinium Dications

The condensation of cis ‐1,2‐cyclopropanediamines with 1,3‐dicarbonyl compounds and 1,5‐diazapentadienium salts afforded 2,3‐homo‐1 H ‐1,4‐diazepinium salts, which were protonated by very strong acids to yield 2,3‐homo‐6 H ‐1,4‐diazepinium dications. As measures of basicity, the mole fractions of water in trifluoromethanesulfonic acid at half‐protonation were determined for 2,3‐homo‐ and 2,3‐dihydro‐1 H ‐1,4‐diazepinium salts. 2,3‐Homo‐6 H ‐1,4‐diazepinium dications underwent 2,2′‐bis‐azonia‐Cope rearrangements, as can be inferred from the product structures, or H/D exchange involving the methylene group of the cyclopropane rings. Apparently, in the Cope rearrangement, this group is interchanged with the methylene group in the seven‐membered ring (C‐6), which is susceptible to H/D exchange in strong deuteriated acids. Rapid, degenerate, as well as very slow, strongly biased Cope rearrangements have been uncovered in this way. 1 H NMR spectra, recorded at elevated temperatures for a solution of the parent dication in trifluoromethanesulfonic acid, reveal broadening of the interchanging 2‐H, 3‐H and 5‐H, 7‐H signals and eventually the onset of their coalescence. Surprisingly, the estimated free enthalpy of activation is higher (Δ G ‡ 110 °C ≈ 73 ± 2 kJ/mol) than that of 3,4‐homotropilidene (Δ G ‡ 110 °C = 61 kJ/mol), not lower, as would be expected on the basis of the well‐known charge‐acceleration of Cope rearrangements.