The three-center, two-electron (3c,2e) bonding of bishomoaromatic cations of type 1 [1±4] are known to be weakened by R C 6 H 5 [2b, 3a] and displaced by R p-MeOC 6 H 4 and R OH.[4] Evidently, cations 1 are stabilized by the 3c,2e bonds, whereas cations 2 are stabilized by donor substituents R. According to computational estimates, 1 u (R H) is more stable than 2 u (R H) by 15 kcal mol
À1. [2a] The energy difference between the prototypes 3 u and 4 u, neither of which is known experimentally, is only 10.1 kcal mol We now report on bishomoaromatic dianions 5 of 1,2,4-triboracyclopentanes that have skeletons isoelectronic to that of 3 u. [6,7] The 3c,2e bonds of dianions of type 5 remain intact even in the presence of strong donor substituents like R NR 2 . Hence, the energy difference between 5 and 6 must be larger than the 26 kcal mol À1 stabilization afforded to a tricoordinate boron atom by a dialkylamino substituent. [8] Calculations at the MP2/6-31G* level [9] confirm this conclusion.The dianions 5 a ± c (Dur 2,3,5,6-tetramethylphenyl) are obtained by reduction of 7 a ± c with lithium in diethyl ether. Diederich, Chem. Eur. J. 1998, 4, 1351 ± 1361 [15] The steric influences of the dendrons are also mirrored in the long reaction times: The mixtures had to be stirred for up to four weeks before all of the starting materials had reacted while usually the reaction is complete after a few days. Furthermore, between 77 % (in case of 9 b) and 94 % (in case of 10 b) of the free axles were formed here.[16] To make sure that under these severe conditions the spectral changes were caused by mechanical deslipping and not by chemical decomposition, we exposed the free components of the rotaxanes, that is the tetralactam 1 and the axles, to the same conditions. Indeed, there was a slow color change of the solutions and slight changes in the spectra were observed, however, these processes were significantly slower than the changes observed in the spectra of the rotaxanes.