Bimanes. 24. Synthesis, Structure, and Dynamic Properties of Zero-Bridged Bimanes, 3,7-Dimethyl- and 3,7-Dichloro-4,6-(l',2'-dimethylene)-1,5-diazabicyclo[3.3.0]-octa-3,6-diene-2,8-diones [μ-0-syn-(CH₂,CH₃ or C1)B]

Conversion of the dibromo compound, 4,6-bis(bromomethyl)-3,7-dimethyl-1,5-diazabicyclo[3.3.0]octa-3,6-diene-2,8-dione (syn-(BrCH₂,CH₃)B) or the related 3,7-dichloro derivative (syn-(BrCH₂,Cl)B) to a 4,6-sulfone-bridged bimane via oxidation of a 4,6-thiatrimethylene derivative is described. Elimination of SO₂ via thermolysis of the sulfones at 525 ° C in a quartz reactor yields the strained tricyclic bimanes containing a 4,6-dimethylene group (“zero-bridge”) μ-0-syn-(CH₂,CH₃)B (3e) and μ-0-syn-(CH₂,Cl)B (3a). The structure of the 3,7-dimethyl derivative, μ-0-syn-(CH₂,CH₃)B, has been established by X-ray crystallography. The absorption maxima (3a, 324 nm; 3e, 315 nm) are at much shorter wavelengths than those of compounds with longer “bridges”, but the positions of the fluorescence maxima are similar. The NMR spectrum at room temperature shows only one signal for CH₂, the hydrogens being equilibrated by flipping of the bimane ring. The coalescence temperature for the 3,7-dimethyl derivative is at 205 K; that for the 3,7-dichloro derivative is somewhat below 181 K. The rate constant ((6-8) ⨯ 10⁵ s^-1) and activation energy (11 ± 1 kcal/mol) for the flipping process in 3e have been estimated by simulation of NMR spectra at different temperatures. The structure of μ-(S)-syn-(CH₂,Cl)(5a) has been confirmed by X-ray analysis. Thermolysis of the sulfone at temperatures somewhat higher than 525 ° C leads to the loss of a CH₂=S fragment and formation of syn-(H,C1)(CH₃,C1)B.