• Allenes, highly strained;
  • Strain — structure correlation;
  • Hybridization effects, 13C, 13C coupling constants;
  • Calculations

Bis(tetramethylcyclopropylidene)methane (5) was prepared by treating 1-(dichloroethenylidene)-2,2,3,3-tetramethylcyclopropane (3) with tert-butyllithium in the presence of 2,3-dimethyl-2-butene. The parent allene 6 was obtained along a new route from bis(1-bromocyclopropyl) ketone (11) with low-valent titanium. Both 5 and 6 show unusually intense allene stretching bands in the IR spectra, and the signals of their central carbon atoms appear at remarkably high field in the 13C-NMR spectra. The unique structural features of 5 and 6, as evidenced by an X-ray crystal structural analysis of 5 and 1Jcc values, agree remarkably well with theoretical predictions made by semiempirical (MNDO) and ab initio (4-31G) calculations and on the basis of the simple picture offered by the hybridization model using the iterative maximum overlap (IMO) approach.