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,... 

In Ref. 125, Wrackmeyer and Berndt reported the SSCC results computed for 1,2-diboretane-3-ylidene (1) and related compounds. Calculation of the coupling constants, based on optimized geometries, gave data in agreement with the experiments. Furthermore, the calculations predicted for the first time a negative B) between C(3) and B(l) in 1, although so far no examples... 

Berndt et al. demonstrated that 1,2-diboretanes with electron-deficient boron atoms prefer the non-classical 125a to the classical 125b structure. The nonclassical structure is characterized by two 3c/2e interactions, one for the hydrogen-bridged B—H—B and one for the homoaromatic B—C—B three-membered ring (isoelectronic with the homocyclo-propenium cation). Bemdt et al. prepared three derivatives (126) of 1,2-diboretane. They obtained an... 

Jbc and Jcsi couplings have been derived by Wrackmeyer and Berndt for l,2-diboretane-3-ylidene, a homoaromatic system with Jt and a 3c/2e bonds, from the low temperature NMR spectra. The results shown in Fig. 2 allowed an insight into the electronic structure of this unusual molecule. 

In a more complex example, reaction of tris(TMS)phosphine and the tris(dichloroboryl)alkane 6 provides the interesting zwit-terionic l-phosphonium-2,4-diboretane-3-ide 7, which is an unusual neutral monohomoaromatic (eq 20). 

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