Diboriranes

Electronically related to such borirenes are the salts of the dianion of 1,2-di-tert-butyl-3-[bis(trimethylsilyl)]methyl]-l, 2-diborirane.85 An XRD analysis of the dipotassium salt (86a,b) uncovered shortened B—B (1.58 A) and B—C (1.50 A) bonds, as suggested by resonance contributions 86a and 86b. Salt 86 can be considered to have largely a 2- r-electron Hiickel aromatic ring. 

Scheme 3.2-2. 2e aromatic diboriranes IB and 1C are stabilized to a considerable extent compared with classical 1A. la-c are experimentally known derivatives of 1C. 

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,... 

Treatment of anionic four-membered rings with electrophiles leads to boron-bridged diboriranes (Equation 26). The specific structure varies with the nature of the electrophile, but three-membered rings are always formed <2003AGE669, 2003AGE671>. 

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