Borirene, aromaticity

The resulting crystal proved amenable to a conclusive XRD analysis. As shown in Eq. (26), the C—C ring bond is lengthened over what it is in the structurally similar cyclopropene (1.304 A) and the B—C bonds shortened relative to the electronically analogous bond in trivinylborane (1.558 A). Thus, one can safely conclude that there is extensive 7r-electron delocalization and Hiickel aromatic character in the borirene ring. 

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. 

The disproportionation of borirene to acetylene and diboretene (58) is slightly endothermic (by 5 kcal/mol), whereas the activation barrier of this BH transfer reaction equals 14.6 kcal/mol (86JA3960) The estimation of the aromatic stabilization energy for isodesmic reaction (59) yields values of 47.5 (6-31G //6-31G ) (86JA3960), 47.1 kcal/mol (6-31G //STO-3G), [81JA( 103)2589] which makes about 70% of the stabilization energy... 

A compared to the same bond in cyclopropene. These features of the geometry of the borirene ring may be regarded, in accordance with the structural criteria, as evidence for the aromaticity of borirene. 

Unlike aromatic borirene, only thiirene of the antiaromatic three-membered ring species (100), (123), and (127) (see Table XII) could be experimentally detected by means of matrix isolation at low temperature [81JA(103)486]. The IR spectrum of thiirene is consistent with the data of ab initio calculation [80JA2507 also see 83JA( 105)396]. 

---