Degenerate and nondegenerate rearrangements of the 7-borabicyclo[..1]heptadiene system: Twofold [1,3] supra-facial sigmatropic migrations and anionic [1,] carbon-boron shifts

Abstract

Two notable skeletal rearrangements of the 7-borabicyclo[..1]heptadiene system (III) have been uncovered: (1) a facile, degenerate, net twofold [1,] suprafacial sigmatropic migration of the 7-substituted boro group, observable by NMR spectroscopy when the carbon centers of the borole (I) and alkyne (II) bear suitable groups; and () a nondegenerate, anionic [1,] aryl shift from carbon to boron, converting the 7,7-dimethylborate salt of III into an aryl(dimethyl)pentaarylborate salt. Thus, the degenerate rearrangement was revealed when pentaphenylborole (Ia) was allowed to react with di- p-tolylacetylene (IIb) at 5°C; the resulting 7-borabicyclo[..1] heptadiene (III) formed displayed NMR methyl signals characteristic of p-tolyl groups located at both C 5 and C 6, as well as C 4 and C 5. Similarly, the nondegenerate, disruptive anionic isomerization of the 7,7-dimethylborate salt of IIIe or IVe was found to involve migration of either phenyl or p-tolyl groups from carbon to boron, showing that both phenyl (IIIe) and p-tolyl groups (IVe) were located at the bridgeheads in the precursors to VI.