Non-classical 1,2-Diboretanes

The classical form 35A of 1,2-diboretane is 34.5 kcal mol-1 higher in energy than the non-classical, homoaromatic isomer 36A [46]. However, the planar C2 form 35A is not a local minimum. This is reminiscent of the corresponding results for diborirane (see Section 3.2.2.1). The classical minimum is the Ci symmetric, 

The doso-carboranes with a C2 B3 skeleton are of particular interest because their structure and bonding have been described as classical and non-classical. Koster et al. [49] reported the preparation of pentaalkyl-1,5-dicarba-doso-pentaboranes by hydroboration of dialkyl(l-alkynyl)boranes with tetraalkyl-diboranes(6) (R2BH)2. By using a large excess of (Et2BH)2 as a hydride bath the route to pentaethyl-1,5 -d ic ar ba- doso-pe n tabo ra n e(5) was found to proceed via the 1-carba-aradwo-pentaborane(lO) [50] (see also Section 3.2.2.4 [33]). 

The molecular structures of 39c,e in the crystals show that the B-B and B-C distances of the C2B3 frameworks are considerably shorter than those of boron-amino-substituted derivatives [56], which are described as bicyclo-organoboranes. 

Many derivatives of isomeric 2,3-dicarba-nido-hexaborane(8) are known, available from the reaction of pentaborane(9) with alkynes [59, 60]. In contrast, the carboranes of type 43 with non-adjacent carbon atoms are rare. 

The endohedral hydrogen atom in 43 is readily removed as a proton by strong bases to give the carbaborate 44, structurally characterized as its sodium salt [45]. Treatment of 44 with CH3OH or CH3OD reforms the carborane 43 with an endohedral C-H or C-D bond, respectively. 

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