X-ray Structures of Cubylcubane and 2-tert-Butylcubylcubane: Short Cage-Cage Bonds. (Reannouncement with New Availability Information).

摘要

As described in the preceeding communications about cubane, the major isolable products from the reaction of 1,2-diiodocubane with tert-butyllithium are tert-butylcubane, a liquid at room temperature, and 2-tert-butylcubylcubane. While attempting to recrystallize the latter by slow evaporation of an n-octane solution we fortuitously obtained some crystals in which 2-tert-butylcubylcubane and cubylcubane had crystallized. Apparently, there was a small amount of cubylcubane formed in the reaction, and this was focused in the recrystallization procedure. The centric triclinic unit cell contains three molecules, two molecules of 2-tert-butylcubylcubane (related by a center of symmetry) and one molecule of cubylcubane, it being on a center of symmetry located at the midpoint of the bond linking the two cubane cages. To within experimental error, the cubylcubane molecule displays (noncrystallographic) D3d symmetry. The cage bond lengths and angles in the two perfectly staggered cubyl portions are not significantly different from the values in cubane itself. In 2-tert-butylcubylcubane although the substituent is nearly perfectly staggered relative to the cage , there are still distortions about it due to crowding, most notably lengthening of the Cla-C2a cubane edge to 1.606 (6) A and opening of the Clb-Cla-C2a angle to 133. More interesting are the intercage bond lengths: 1,458 (8) A in cubylcubane and 1.464 (5) A in the tert-butyl derivative. These carbon-carbon bond lengths are significantly shorter than any recorded to date between carbons fully substituted by other carbon atoms. This phenomenon can be understood by considering a simple bonding picture for cubanes.