No abstract

No abstract

To remove uncertainties in the apparent C=C and B-C bond lengths of the borirene ring, as previously estimated from an X-ray crystallographic analysis of trimesitylborirene, the unsymmetrically substituted 2-(2,6-dimethylphenyI)-1,3dimesitylborirene was prepared by the photorearrangement of ((2,6-dimethylphenyl)ethynyl)dimesitylborane. With such an unsymmetrical borirene there was no disorder in the identity of the ring atoms, and all three ring atom separations could be unambiguously assigned as follows: C-C, 1.380 A; Cl-B, 1.450 A; and C,-B, 1.464 A. Compared with the reported C = C bond length in cyclopropene (1.304 A), the C=C bond in the borirene has been lengthened by 0.08 A, and compared with the reported C-B bond length in trivinylborane (1.558 A), ring C-B bonds have been shortened by an average of 0.10 A.These altered bond lengths are fully consistent with those expected if the two ring *-electrons are extensively delocalized among the three sp*-hybridized ring atoms. Therefore, the borirene ring can truly be considered as a Huckel aromatic nucleus. In order to corroborate that cyclic conjugation led to C -C bond lengthening, the open-chain precursor to the borirene ring, namely dimesityl(mesitylethyny1)borane was also examined by X-ray crystallography. In this system the bond showed no unusual lengthening and hence gave no indication of significant conjugation with the tricoordinate boron. When the tricoordinate boron of the borirene ring becomes tetracoordinate by ligation with an amine, such as pyridine, or a sterically suitable alcohol like MeOH or EtOH, the borirene ring is promptly ruptured and, in the presence of a proton source, irreversibly converted into an acyclic borinic ester. Such rupture of the borirene ring by pyridine can be monitored by electronic and multinuclear NMR spectroscopy. These observations corroborate the role of the available Zp,-orbital on sp7-hybridized boron in stabilizing the borirene ring.

No abstract