Physical and Chemical Consequences of Cyclic Conjugation in Boracyclopolyenes. The Antiaromatic Character of Pentaarylboroles

摘要

Pentasubstituted boroles or boracyclopentadienes have been prepared from 1,4-dilithio-1,3-butadienes or from stannoles by interaction with aryl(dihalo)boranes. The boroles are unexpectedly strong Lewis acids, complexing with amines, ethers and nitriles, and very prone to oxidation, solvolytic cleavage and Diels-Alder reactions. The foregoing behavior, taken together with their unusual nuclear magnetic and electronic spectral properties, can be understood in terms of the Hueckelantiaromatic character of the four pi-electron borole nucleus. By treating such nuclei as perturbed cyclopentadienyl pi-systems, an understanding of the spectral and chemical properties can be attained. From such considerations, it is evident that the conjugation between the tricoordinate boron's 2P sub 2 -orbital and the four carbon butadienylidene array is destabilizing and the source of the high reactivity of these unique, high-energy organoboranes. Keywords: Lewis acid, Molecular orbital calculations, Oxidizability, Bathochromic spectral shifts, Electron transfers.