Bora-aromatic systems.  Part 8.  The physical and chemical consequences of cyclic conjugation in boracyclopolyenes.  The antiaromatic character of pentaarylboroles

Eisch, John J.; Galle, James E.; Kozima, Sinpei

doi:10.1021/ja00263a006

Cited by 184 publications

(173 citation statements)

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“…Unsaturated boron-containing heterocycles such as boroles (I) [1] or borepines (II) [2] have been intensively studied, in particular with regard to the question of how the sp 2 -hybridized boron center affects the p-electron delocalization in these conjugated systems.…”

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confidence: 99%

“…[1,3] As a consequence of extended p conjugation across the boron center, substituted, electron-rich boroles have already attracted interest owing to their potentially useful photophysical and electrochemical properties. [4] Although delocalization of p electrons in boroles and borepines was demonstrated, the degree of aromatic stabilization, or antiaromatic destabilization in the case of of I, appears to be less pronounced than in the respective carbon analogues, that is, cyclopentadienyl-and tropylium cations.…”

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confidence: 99%

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Synthesis and Electronic Structure of a Ferroborirene

et al. 2007

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confidence: 99%

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confidence: 99%

Synthesis and Electronic Structure of a Ferroborirene

et al. 2007

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“…Compound 1 and Li 2 C 4 Ph 4 were prepared according to published procedures. [15,17] Elemental analysis was obtained on an Elementar Vario MICRO cube instrument. NMR spectra were recorded on a Bruker Avance 500 NMR spectrometer (500 121.66, 131.04, 131.80, 135.47, 136.38 (br), 138.37, 139.50, 141.96, 141.99, 142.41, 143.12, 154.51 (C q ), 125.37, 125.79, 125.96, 126.26, 126.71, 126.73, 126.76, 127.14 (br), 127.37 (br), 127.56, 127.79, 127.83, 127.87, 127.92, 127.95, 128.05, 128.07, 128.84, 129.53, 130.32, 130.43, 130.92, 131.00, 131.15, 131.22, 131.44 (br) …”

Section: Methodsmentioning

confidence: 99%

“…[16] Herein, we report the synthesis and structural characterization of a novel 2,3,4,5-tetracarba-1,6-nido-hexaborane(6) derivative, 2, which was obtained from the reaction of 1 and an equimolar amount of dilithio-1,2,3,4-tetraphenylbuta-1,3-diene. [17] The reaction proceeded with high selectivity, and the exocyclic nido-carborane derivative 2 was isolated in excellent yield (Scheme 1). The nature of the carbaborane body in 2 was deduced from 11 B NMR spectroscopy in solution, which revealed two distinct resonances at d = À40.3 ppm and d = 13.4 ppm in the typical range for 2,3,4,5-tetracarba-1,6-nido-hexaboranes(6) (d = À51 to À37 ppm and d = 9 to 28 ppm) for apical and basal boron atoms, respectively.…”

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High‐Yield Synthesis of a Hybrid 2,3,4,5‐Tetracarba‐1,6‐nido‐hexaborane(6) Cluster with an exo‐Polyhedral Boracycle

Braunschweig

Ghosh

Kupfer

et al. 2011

Chemistry A European J

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Dedicated to Prof. Dr. Hansgeorg Schnçckel on the occasion of his 70th birthdaySince the pioneering work of Stock et al., [1] the chemistry of boron hydrides has evolved into a fascinating area comprising different classes of compounds such as organoboranes, polyhedral boranes, and carborane clusters, to name only a few. [2][3][4][5] In polyhedral carborane chemistry, several approaches towards the construction of clusters have been developed. [6, 7] For example, i) cage or polyhedron growth by carbon or boron insertion, [8] ii) intercluster fusion, with two or more atoms connecting the individual subclusters, [9] and iii) selective cage degradation and cage coupling.[10] Even though several reports on the synthesis of carboranes and their derivatives have appeared in the literature, the number of carborane clusters prepared by organoboranes still remains rather limited. With the appropriate choice of the substituents at boron, which allows a selective fine-tuning of the electronic and/or steric properties, small organoboranes can serve as useful and versatile starting materials for the generation of carborane clusters.[11] However, these protocols tend to provide the targeted species only in moderate yields and with limited selectivity, experimental drawbacks that have been observed so far when novel cluster types are to be generated.As part of our ongoing studies on boroles and their derivatives, [12][13][14] we recently described the unusual dimerization/ rearrangement of 1-chloro-2,3,4,5-tetraphenylborole at slightly elevated temperatures (40 8C) to afford the [4.5]decatriene derivative 1.[15] Since boroles still represent a rather uncommon class of main group element species with only few structurally characterized examples, we became interested in the reactivity of 1 towards different reducing agents, which might be useful in the generation of new structural motifs.[16] Herein, we report the synthesis and structural characterization of a novel 2,3,4,5-tetracarba-1,6-nido-hexaborane(6) derivative, 2, which was obtained from the reaction of 1 and an equimolar amount of dilithio-1,2,3,4-tetraphenylbuta-1,3-diene.[17] The reaction proceeded with high selectivity, and the exocyclic nido-carborane derivative 2 was isolated in excellent yield (Scheme 1). The nature of the carbaborane body in 2 was deduced from 11 B NMR spectroscopy in solution, which revealed two distinct resonances at d = À40.3 ppm and d = 13.4 ppm in the typical range for 2,3,4,5-tetracarba-1,6-nido-hexaboranes(6) (d = À51 to À37 ppm and d = 9 to 28 ppm) for apical and basal boron atoms, respectively. [18, 19] The 11 B NMR chemical shifts of 2 proved temperature independent in the inspected range between 75 and À80 8C. The number of signals determined by 13 C NMR spectroscopy indicates rotational hindrance of several phenyl groups around the cluster framework, which results in chemical inequivalence of the respective phenyl carbon atoms and concomitant line broadening of some resonances.Single crystals of 2 suitable for X-ray diffraction anal...

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“…60°) which reduces the overlap of the lone pair on nitrogen with the empty p-orbital on boron, an effect that is better reflected by the multicenter indices. A comparison between boroles with different aryl substituents (1: R = phenyl; [7] 7: R = 2,4,6-trimethylphenyl; [17] 8: R = 2,3,5,6-tetramethylphenyl; [50] 9: R = 2,4,6-triisopropylphenyl) 50 The data also suggests that the antiaromaticity of the boroles correlates with the steric demand of the B-aryl substituent: the higher the steric bulk of the R substituent, the higher the antiaromaticity. This can be understood by an increasingly weaker π-overlap of the aryl group with the borole unit due to the steric crowding around the exocyclic B−C bond.…”

Section: Monoborolesmentioning

confidence: 99%

A theoretical study of the aromaticity in neutral and anionic borole compounds

et al. 2015

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Abstract:In this contribution, we have evaluated the (anti)aromatic character of thirty-four different borole compounds in their neutral and reduced states based on two aromaticity indices, namely nucleus-independent chemical shift (NICS) and multicenter indices (MCI), calculated at the PBE0/6-31+G(d,p) level of theory. Both indices corroborate the notion that neutral borole compounds are antiaromatic and become increasingly aromatic upon addition of electrons. Effects of the ring substituents on the degree of (anti)aromaticity are discussed together with differences in the two theoretical methods, which are on the one hand based on magnetic (NICS) and on the other hand based on electronic criteria (MCI).

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Bora-aromatic systems. Part 8. The physical and chemical consequences of cyclic conjugation in boracyclopolyenes. The antiaromatic character of pentaarylboroles

Cited by 184 publications

References 0 publications

Synthesis and Electronic Structure of a Ferroborirene

Synthesis and Electronic Structure of a Ferroborirene

High‐Yield Synthesis of a Hybrid 2,3,4,5‐Tetracarba‐1,6‐nido‐hexaborane(6) Cluster with an exo‐Polyhedral Boracycle

A theoretical study of the aromaticity in neutral and anionic borole compounds

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