THERMAL GENERATION AND TRANSFORMATION OF THE BOREPIN RING-SYSTEM - A PARADIGM OF PERICYCLIC PROCESSES

Almost 15 years ago the synthesis of the borepin ring system was reported to result from the interaction of an alkyne with the highly reactive borole ring and the occurrence of an unusual sequence of skeletal rearrangements. In this manner, both heptaphenylborepin and 1-phenyl-2,3,4,5,6,7-hexa-p-tolylborepin were prepared as bright yellow solids, whose chemical degradation by successive treatment with HgCl2, n-butyllithium, and H2O yielded a mixture of the cts-1,2-dihydrohexaarylbenzenes and the hexaarylbenzenes. The isolation of these hydrocarbons was consistent with the cleavage of the carbon-boron bonds of the borepins and the disrotatory electrocyclic ring closure of the resulting (E, Z, E)-1,3,5-hexatrienes. Further heating of these borepins leads to electronic and NMR spectral changes which show that all the six C-aryl substituents become nonequivalent and that an isomer of the borepin has now been formed. These fluorescent green compounds undergo protodeboronation with glacial acetic acid to yield 5-benzyl-1,2,3,4,5-pentaarylcyclopentadienes. X-ray crystallographic analysis of the isomer formed from 1,2,3,4,5,6,7-heptaphenylborepin shows it to be 1,2,3,3a,4,5-hexaphenyl-5-bora-3a,4-dihydro-5H-benz[e]mdene. That the tricoordinate boron in the latter is conjugated with the cyclopentadiene ring is shown by the hypsochromic spectral shift caused by coordination with amines. The overall conversion of borole to this isomer of borepin involves a remarkable sequence of pericyclic reactions: a Diels-Alder cycloaddition, 1,3-sigmatropic suprafacial rearrangements of the resulting 7-borabicyclo[2.2.1]heptadiene, a 1,6-disrotatory electrocyclic ring opening to the borepin, a 1,3-sigmatropic ring expansion to a boracyclononatetraene, and a final ring-closing ene reaction yielding the 5-bora-3a,4-dihydro-5H-benz[e]indene system. The role of the unoccupied 2pzorbital on boron in facilitating these rearrangements and in influencing the spectral properties of the unsaturated boracarbocycles involved is discussed in terms of Hückel molecular orbital theory. © 1990, American Chemical Society. All rights reserved.