BORA-AROMATIC SYSTEMS .11. AROMATIC STABILIZATION OF THE TRIARYLBORIRENE RING-SYSTEM BY TRICOORDINATE BORON AND FACILE RING-OPENING WITH TETRACOORDINATE BORON

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-dimethylphenyl)-1,3-dimesitylborirene 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 Å; C1-B, 1.450 Å; and C2-B, 1.464 Å. Compared with the reported C=C bond length in cyclopropene (1.304 Å), 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 Å), 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 sp2-hybridízed ring atoms. Therefore, the borirene ring can truly be considered as a Hückel 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(mesitylethynyl)borane was also examined by X-ray crystallography. In this system the C≡C 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 2pz-orbital on sp2-hybridized boron in stabilizing the borirene ring. © 1990, American Chemical Society. All rights reserved.