Bishomoaromatic 1,2,4-Triboracyclopentane Dianions: Strong Three-Center, Two-Electron Bonds between Three Boron Atoms

Abstract

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.

Reductions of 1,3,5-Triboracyclohexanes and Related 1,3,5-Triboraalkanes with Alkali Metals to Trishomoaromatic Aggregates — Tetrameric Lithium and Sodium, and Polymeric Sodium and Potassium Trishomoaromatic Cyclotriborate Compounds

“…[1] Remarkable results such as the synthesis of the doubly-aromatic anion 1 with σ and π 3c,2e bonding have been reported by Berndt et al [2,3] Recently we obtained the new, trishomoaromatic system 3 by reducing 1,3,5-triboracyclohexanes 2 with lithium. [4,5] The empty p-orbitals of 2 are able to accept two electrons to form the dianion 3, which is isoelectronic with the trishomocyclopropenylium cation 6.…”

“…Reduction of 3 with sodium; in the formula of {(Na ϩ ) 2 (thf) 3 ([18]cr-6) 2 } 2ϩ {[3(Na ϩ )(thf)] 4 (Na ϩ ) 2 } 2Ϫ the hydrogen atoms are omitted…”

“…The low solubility of both these compounds precluded spectroscopic characterization. An attempt to obtain 3[Li(OEt 2 ) 2 ] 3 ϩ by reducing 2 with lithium in Et 2 O in the presence of LiCl failed to give any monomeric product.…”

“…X-ray structure Scheme 3 Scheme 4 analyses revealed a polymeric aggregation of 3(Na ϩ ) 2 (THF) 3 and, in the presence of [18]crown-6, an interesting tetramer. In contrast the potassium structure forms only a zigzag polymer (see below).…”

“…The structure of [3(Li ϩ )] 4 {Li( [12]cr-4) 2 ϩ } 4 with [12]crown-4 as ligand refined well ( Figure 2). In both structures four (MeCH) 3 (MeB) 3 Air-and moisture-sensitive crystals of {3[(Na ϩ ) 2 -(thf) 3 ]} m were obtained after partial removal of the THF solvent. Surprisingly, the X-ray structure analysis (Figure 3a,b) revealed a polymeric arrangement of {3(Na ϩ )[Na(thf) 3 ϩ ]} m , consisting of a two-dimensional, meander-like chain pattern.…”