New structurally interesting cyclopropane derivatives. A world of wonders and surprises

The structurally intriguing [4]- and [5]triangulanes have been prepared in enantiomerically pure form. Their surprisingly high specific rotations are well reproduced by DFT/SCI computations and stem from the fact that these hydrocarbons essentially are sigma-helicenes (i.e., rigidly held helical arrangements of sigma-bonds). Some light is shed on the properties of radical cations derived from [3]- and [4]rotanes. While the former adopts C-s or C-2v symmetry, the latter retains the D-4h symmetry of the neutral hydrocarbon, according to high-level computations. Experimental and computational evidence is also presented that the anti-aromatic cyclopentadienyl cation is stabilized as a singlet ground state by five cyclopropyl substituents. Yet, the three cyclopropyl groups in tricyclopropylamine do not favor the formation of its radical cation, because they are not in the proper orientation. When this amine radical cation is generated by cobalt gamma-irradiation in a Freon matrix, evidence for a significant conformational change is obtained by EPR spectroscopy. Finally, the conformational dynamics of the newly prepared crowded molecules tetracyclopropyl- and tetraisopropyl-methane are discussed.