Abstract:Abstract:The reaction of MgCOT(thf), with tBuBF, or PhBCl, affords the first 9-borabarbaralanes 2 (C,H,BR, a: R = tBu; b: R =Ph). With the aminoboron dihalides BCI,NiPr, and BCI,N(SiMe,)tBu 9-borabicyclo[4.2.1]-nona-2,4,7-trienes 3 (a: R = NiPr,, b: R = N(SiMe,)tBu) and the trans-9-borabicyclo[4.3.0]nona-2,4,7-triene 4 are obtained. The bicyclic compounds 3 a and 3 b are converted into 9-borabarbaralanes
“…The six carbon atoms (1–6) in the front of the C 2v TSs can be divided into two equivalent groups weakly bonded together, with two effective parallel CCC multicenter bonds (C1C3C5 and C2C4C6) along the two long edges of the C 8 octagon and two weak C--C interactions (C1--C2 and C5-C6) on the top and at the bottom between them. C 10 H 10 , C 8 H 8 , C 9 H 10 , and C 8 BH 9 possess the calculated free energy barriers of ΔG ≠ = 13.32 kcal/mol at 100 °C, 5.94 kcal/mol at −143 °C, 7.86 kcal/mol at −77 °C, and 10.84 kcal/mol at 27 °C at PBE0 level, respectively, well in line with the corresponding values previously reported for these species at finite temperatures 11,17 .Figure 1Optimized structures of the ground states (GSs/GSs′) and transition states (TSs) of ( a ) C 10 H 10 , ( b ) C 8 H 8 , ( c ) C 9 H 10 , and ( d ) C 8 BH 9 , with the lowest vibrational frequencies ν min and relative energies Δ E a indicated at PBE0 and CCSD(T) levels, respectively. Typical calculated C-C bond lengths are indicated in Å.…”
Section: Resultssupporting
confidence: 89%
“…A topological analysis of experimental electron densities of the ground-state C 3 v bullvalene was reported in 1996 18 . C 10 H 10 , C 8 H 8 , and C 9 H 10 have the experimental free energy barriers of ΔG ≠ = 12.8 kcal/mol at 100 °C, 5.5 kcal/mol at −143 °C, and 7.8 kcal/mol at −77 °C in NMR measurements, respectively 11 , while the model compound C 8 BH 9 has the calculated ΔG ≠ = 10.36 kcal/mol at 27 °C 17 . Semibullvalene C 8 H 8 has proven to have the lowest fluxional energy barrier, fastest rearrangement rate, and lowest fluctuating temperature in the series 11 .…”
Section: Introductionmentioning
confidence: 92%
“…Prototypical fluxional molecules in organic chemistry include the norcaradiene-cycloheptatriene system, various annulenes, and homotropilidenes. Bridged homotropilidenes with degenerate valence-bond tautomerisms, such as the cage-like bullvalene C 10 H 10 , semibullvalene C 8 H 8 , barbaralane C 9 H 10 , and 9-borabarbaralane C 8 BH 9 , are of particular interest which exhibit reversible fluxionalities in rapid Cope rearrangements through a transition state with a bis-homoaromatic array of orbitals 5–17 . A topological analysis of experimental electron densities of the ground-state C 3 v bullvalene was reported in 1996 18 .…”
Bullvalene C10H10 and its analogs semibullvalene C8H8, barbaralane C9H10, and 9-Borabarbaralane C8BH9 are prototypical fluxional molecules with rapid Cope rearrangements at finite temperatures. Detailed bonding analyses performed in this work reveal the existence of two fluxional π-bonds (2 2c-2e π → 2 3c-2e π → 2 2c-2e π) and one fluxional σ-bond (1 2c-2e σ → 1 4c-2e σ → 1 2c-2e σ) in their ground states and transition states, unveiling the universal π + σ double fluxional bonding nature of these fluctuating cage-like species. The highest occupied natural bond orbitals (HONBOs) turn out to be typical fluxional bonds dominating the dynamics of the systems. The 13C-NMR and 1H-NMR shielding tensors and chemical shifts of the model compound C8BH9 are computationally predicted to facilitate future experiments.
“…The six carbon atoms (1–6) in the front of the C 2v TSs can be divided into two equivalent groups weakly bonded together, with two effective parallel CCC multicenter bonds (C1C3C5 and C2C4C6) along the two long edges of the C 8 octagon and two weak C--C interactions (C1--C2 and C5-C6) on the top and at the bottom between them. C 10 H 10 , C 8 H 8 , C 9 H 10 , and C 8 BH 9 possess the calculated free energy barriers of ΔG ≠ = 13.32 kcal/mol at 100 °C, 5.94 kcal/mol at −143 °C, 7.86 kcal/mol at −77 °C, and 10.84 kcal/mol at 27 °C at PBE0 level, respectively, well in line with the corresponding values previously reported for these species at finite temperatures 11,17 .Figure 1Optimized structures of the ground states (GSs/GSs′) and transition states (TSs) of ( a ) C 10 H 10 , ( b ) C 8 H 8 , ( c ) C 9 H 10 , and ( d ) C 8 BH 9 , with the lowest vibrational frequencies ν min and relative energies Δ E a indicated at PBE0 and CCSD(T) levels, respectively. Typical calculated C-C bond lengths are indicated in Å.…”
Section: Resultssupporting
confidence: 89%
“…A topological analysis of experimental electron densities of the ground-state C 3 v bullvalene was reported in 1996 18 . C 10 H 10 , C 8 H 8 , and C 9 H 10 have the experimental free energy barriers of ΔG ≠ = 12.8 kcal/mol at 100 °C, 5.5 kcal/mol at −143 °C, and 7.8 kcal/mol at −77 °C in NMR measurements, respectively 11 , while the model compound C 8 BH 9 has the calculated ΔG ≠ = 10.36 kcal/mol at 27 °C 17 . Semibullvalene C 8 H 8 has proven to have the lowest fluxional energy barrier, fastest rearrangement rate, and lowest fluctuating temperature in the series 11 .…”
Section: Introductionmentioning
confidence: 92%
“…Prototypical fluxional molecules in organic chemistry include the norcaradiene-cycloheptatriene system, various annulenes, and homotropilidenes. Bridged homotropilidenes with degenerate valence-bond tautomerisms, such as the cage-like bullvalene C 10 H 10 , semibullvalene C 8 H 8 , barbaralane C 9 H 10 , and 9-borabarbaralane C 8 BH 9 , are of particular interest which exhibit reversible fluxionalities in rapid Cope rearrangements through a transition state with a bis-homoaromatic array of orbitals 5–17 . A topological analysis of experimental electron densities of the ground-state C 3 v bullvalene was reported in 1996 18 .…”
Bullvalene C10H10 and its analogs semibullvalene C8H8, barbaralane C9H10, and 9-Borabarbaralane C8BH9 are prototypical fluxional molecules with rapid Cope rearrangements at finite temperatures. Detailed bonding analyses performed in this work reveal the existence of two fluxional π-bonds (2 2c-2e π → 2 3c-2e π → 2 2c-2e π) and one fluxional σ-bond (1 2c-2e σ → 1 4c-2e σ → 1 2c-2e σ) in their ground states and transition states, unveiling the universal π + σ double fluxional bonding nature of these fluctuating cage-like species. The highest occupied natural bond orbitals (HONBOs) turn out to be typical fluxional bonds dominating the dynamics of the systems. The 13C-NMR and 1H-NMR shielding tensors and chemical shifts of the model compound C8BH9 are computationally predicted to facilitate future experiments.
“…As indicated in Figure 3, compound 2 is a chiral spiro compound that features one five‐membered and one six‐membered B‐ chloroboracycle. Since the C2C3, C5C6, and C7C8 bond lengths are typical for a CC bond, the six‐membered ring can be described as a boracyclohexadiene,17 while the five‐membered ring is a borolene 11b. Both boron atoms adopt a trigonal‐planar geometry as indicated by the sum of angles around the boron atom of 359.8° and 360.0°, respectively.…”
As neutral isoelectronic analogues of the elusive cyclopentadienyl cation, boroles have been of interest for their prospective applications as strong Lewis acids, chromophores, and electron acceptors. Recently our group discovered a π-nucleophilic boryl anion based on the borole system. In an effort to extend borole chemistry, we now report the molecular structure of 1-chloro-2,3,4,5-tetraphenylborole (1) and its corresponding borole dianion resulting from the two-electron reduction of 1 with KC(8). The thermally induced dimerization of 1 yields an unprecedented boracyclohexadiene/borolene spiro-bicyclic compound and the resulting dimer was fully characterized including a single-crystal X-ray analysis.
“…This 'reversal' is attributed to the ability of Me3Si in the 'inverse' TS to stabilise the developing + charge on C2 of the enophile; in addition, the 'normal' TS would involve some unfavourable steric interaction^.^^ Activation barriers for the [3,3] sigmatropic rearrangement (Cope) of the 9borabarbaralanes 37 (Scheme 16) have been determined to be 45.7 kJ mol-' (10.9 kcal mol-') (R = Bur) and 40.5 kJ mol-' (9.7 kcal mol-') (R = Ph) by NMR spectroscopy; barriers for other substituents (R = H, Me, NH,) were calculated (Becke3LYP/6-31 lG**) and found to be 41.1-35.0 kJ mol-(9.8-8.4 kcalmol-1). 54 A dynamic NMR investigation of the related rearrangements in substituted bullvalenes 38 (R = F, CN, C0,H) gives values of E,,, for interconversion of the major isomers similar to those found for the parent bullvalene, ca. 54.8kJmol-' (1 3.1 kcal mol -I).'…”
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
