Boron containing boroles

Boron-containing Heterocycles.—The enthalpies of adduct formation between pyridine, 2-picoline, 4-picoline, or 2,4,6-collidine and a number of heterocyclic boron derivatives (2-Br- or 2-organo-l,3,2-dihetero-borolans, -borinans, and -boroles) have been measured.282 The presence of Br greatly increases the Lewis acidity of the B atom. 

Siebert and Herberich have systematically synthesized boron-containing electron-deficient rings, especially because they found that these rings were excellent stabilizers of the triple-and multiple-decker structures. For instance, borole C4H4BH (4e) and 2,3-dihydro-1,3-diborole (3e) are bonded to the metal in a pentahapto fashion as Cp, yet they have and bring a reduced number of k electrons. The tripledecker sandwich complexes formed herewith can have different oxidation states as known in transition-metal sandwich chemistry. - ... 

In contrast to furan, pyrrole, and thiophene, the heteroatom in borole does not contain an electron pair that can participate in cyclic delocalization. The p orbital in the sp -hybridized boron atom (see Figure 1-17) is empty and the molecule is isoelectronic with the antiaromatic cyclopentadienyl cation (Section 15-7). Indeed, the parent compound borole is not known. 

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