- Structural evidence for antiaromaticity in free boroles
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(Chemical Equation Presented) Pentaphenylborole and a ferrocenylborole were synthesized and structurally characterized. Both experimental and theoretical data reveal that rather weak intermolecular interactions are able to significantly alter the bond lengths in the borole ring of pentaphenylborole (see picture). Moreover, the high Lewis acidity of boroles is demonstrated by a significant Fe...B interaction in the ferrocenylborole in the solid state.
- Braunschweig, Holger,Fernandez, Israel,Frenking, Gernot,Kupfer, Thomas
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- Investigating the ring expansion reaction of pentaphenylborole and an azide
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The reaction between trimethylsilyl azide and pentaphenylborole was recently shown to produce the corresponding 1,2-azaborine. Investigating this transformation theoretically suggests that the reaction proceeds via coordination of the azide to the borole, rearrangement to a bicyclic species, and conversion to a kinetically favoured eight-membered BN3C4 heterocycle or expulsion of N2 to furnish the thermodynamically favoured 1,2-azaborine. The eight-membered species was structurally characterized as a borole adduct and represents an unusual analogue of cyclooctatetraene. This journal is
- Couchman, Shannon A.,Thompson, Trevor K.,Wilson, David J. D.,Dutton, Jason L.,Martin, Caleb D.
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supporting information
p. 11724 - 11726
(2015/05/20)
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- Thermal generation and transformation of the borepin ring system: A paradigm of pericyclic processes
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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 cis-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]indene. 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 2pz orbital 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.
- Eisch, John J.,Galle, James E.,Shafii, Babak,Rheingold, Arnold L.
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p. 2342 - 2349
(2008/10/08)
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- DERIVATIVE DES BOROLS II. PENTAPHENYLBOROL: SYNTHESE, REDUKTION ZUM DIANION UND KOMPLEXE VON KOBALT UND PLATIN
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Analytically pure, dark blue pentaphenylborole (PPB) can be synthesized from 1,1-dibutyl-2,3,4,5-tetraphenylstannole and C6H5BCl2 in toluene in 95percent yield.Reduction of PPB with elemental potassium in THF produces brownish red K2 which is derived from the new 6?-electron anion 2-.The new complexes (C5H5)Co(PPB) and (COD)Pt(PPB) (COD=1,5-cyclooctadiene) are obtained by irradiation of a toluene solution of (C5H5)Co(CO)2 and PPB and by ligand substitution reaction from Pt(COD)2 and PPB respectively.
- Herberich, G. E.,Buller, B.,Hessner, B.,Oschmann, W.
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p. 253 - 260
(2007/10/02)
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