Bonding along a Linear B···N···B Triad

Livant, P.; Northcott, D. J.; Shen, Yu-An; Webb, Thomas R.

doi:10.1021/jo040199o

Cited by 22 publications

(24 citation statements)

References 49 publications

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“…[26] A similar borate structure, with its phenolic rings connected to nitrogen, was reported, but its coordination to boron resulted in nearly perpendicular aromatic rings. [27] The bond length of B À N in 1 aB·Py is 1.628 (5) , and the sum of the angles for O-B-O and N-B-O around boron are 342.7 and 312.68, respectively. The Br-sub-stituted compounds 1 b,cB·Py and F-substituted compound 1 dB·Py have structures that are similar to 1 aB·Py and their THCs also are very similar, as shown in Table 3.…”

Section: Resultsmentioning

confidence: 99%

Cage‐Shaped Borate Esters with Tris(2‐oxyphenyl)methane or ‐silane System Frameworks Bearing Multiple Tuning Factors: Geometric and Substituent Effects on Their Lewis Acid Properties

Yasuda

Nakajima

Takeda

et al. 2011

Chemistry A European J

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Boron complexes that contain new tridentate ligands, tris(o-oxyaryl)methanes and -silanes, were prepared. These complexes had a cage-shaped structure around a boron center and showed higher Lewis acidity and catalytic activity than open-shaped boron compounds. The cage-shaped ligands determined the properties of the borates by altering the geometry and were consistently bound to the metal center by chelation. The synthesized compounds were L⋅B(OC(6)H(4))(3)CH, L⋅B(OC(6)H(4))(3)SiMe, and its derivatives (L=THF or pyridine as an external ligand). Theoretical calculations suggested that the cage-shaped borates had a large dihedral angle (C(ipso)-O-B-O) compared with open-shaped borates. The geometric effect due to the dihedral angle means that compared with open-shaped, the cage-shaped borates have a greater Lewis acidity. The introduction of electron-withdrawing groups on the aryl moieties in the cage-shaped framework increased the Lewis acidity. Substitution of a bridgehead Si for a bridgehead C decreased the Lewis acidity of the boron complexes because the large silicon atom reduces the dihedral angle of C(ipso)-O-B-O. The ligand-exchange rates of the para-fluoro-substituted compound B(OC(6)H(3)F)(3)CH and the ortho-phenyl-substituted compound B(OC(6)H(3)Ph)(3)CH were less than that of the unsubstituted borate B(OC(6)H(4))(3)CH. The ligand-exchange rate of B(OC(6)H(4))(3)SiMe was much faster than that of B(OC(6)H(4))(3)CH. A hetero Diels-Alder reaction and Mukaiyama-type aldol reactions were more effectively catalyzed by cage-shaped borates than by the open-shaped borate B(OPh)(3) or by the strong Lewis acid BF(3) ⋅OEt(2). The cage-shaped borates with the bulky substituents at the ortho-positions selectively catalyzed the reaction with less sterically hindered substrates, while the unsubstituted borate showed no selectivity.

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Section: Resultsmentioning

confidence: 99%

Cage‐Shaped Borate Esters with Tris(2‐oxyphenyl)methane or ‐silane System Frameworks Bearing Multiple Tuning Factors: Geometric and Substituent Effects on Their Lewis Acid Properties

Yasuda

Nakajima

Takeda

et al. 2011

Chemistry A European J

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“…However, the compound showed no NMR spectroscopic signal. [4] Herein, we report the preparation, structures, and properties of the neutral 2 and the radical cation 2C…”

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confidence: 99%

2,2′:6′,2′′:6′′,6‐Trioxytriphenylamine: Synthesis and Properties of the Radical Cation and Neutral Species

2005

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A planar stable triphenylamine radical cation is a fascinating p-electron system that is potentially applicable to electronic and magnetic materials. Hellwinkel and co-workers synthesized an interesting compound 1, [1] from which the radical cation 1C+ was generated in concentrated sulfuric acid [1a] or by oxidation with lead tetraacetate in trifluoroacetic acid.[1c] The triphenylamine framework of 1C + is most likely planar. The dimethylmethylene bridges contribute to its stabilization; however, they disrupt the CT-type intermolecular interaction that is crucial for the construction of electronic and magnetic materials. To overcome this disadvantage and to improve stability, we designed a synthetic route to the new oxygenbridged analogue 2. [2,3] Quite recently, Livant and co-workers described a product fraction showing a molecular ion of m/z = 287 in the thermolysis of tris(2,6-dimethoxyphenyl)amine. They assumed the structure 2 for the MS peak. However, the compound showed no NMR spectroscopic signal.[4] Herein, we report the preparation, structures, and properties of the neutral 2 and the radical cation 2C+ .The synthesis of 2 is outlined in Scheme 1. Sequential aromatic nucleophilic substitution reactions of 2,6-difluoronitrobenzene with 2-bromo-3-methoxyphenolate and then with 2-bromo-3-fluorophenolate gave 4 (71 % yield in two steps). The reduction of 4 proceeded selectively in the presence of p-bromophenol (10 equiv) to avoid a competing reduction of aromatic bromide. Intramolecular cyclization of 5 was performed under Pd 0 -mediated cross-coupling reaction conditions [5] to afford 6 in 35 % yield. Treatment of 6 with BBr 3 gave 7 in good yield. Intramolecular nucleophilic substitution of 7 in DMF with K 2 CO 3 as a base proceeded efficiently under remarkably mild conditions to give the desired compound 2 in good yield. Compound 2 had a reversible oxidation wave at + 0.59 V versus SCE in DMF (SCE = saturated calomel electrode). The chemical oxidation of 2 was performed by using tris(p-bromophenyl)aminium hexafluorophosphate as an oxidant in methylene chloride. The salt 2CÀ can be recrystallized from acetonitrile/ diethyl ether. Figure 1 shows the molecular structures determined by Xray crystallographic analysis of 2 and 2C + .[6] The neutral compound 2 has a shallow bowl structure, whereas the radical cation 2C + has a planar structure. The CÀN bond lengths become shorter and the C-N-C bond angles approach 1208 in the radical cation 2C+ . The bond-length difference is in qualitative accordance with the HOMO shape of 2; that is, the C À N and C À O bonds have an antibonding nature in the HOMO.Triphenylamine radical cations without para substituents are generally unstable because of the large spin densities of Scheme 1. Reaction conditions: a) 2-bromo-3-methoxyphenol, NaH/ DMSO, 130 8C; b) 2-bromo-3-fluorophenol, NaH/DMSO, 130 8C; c) hydrazine hydrate, Pd/C, p-bromophenol/ethanol, reflux; d) NaOtBu, [Pd-(dba) 2 ], P(tBu) 3 /toluene, reflux; e) BBr 3 /CH 2 Cl 2 , À78 8C!room temperature; f) K 2 CO 3 /DMF, ...

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“…The more electron-donating substituent gives rise to lower activation barriers and faster rates of exchange. As the activation barrier for exchange falls in line with the expected bond strength of a B-N dative interaction, [10] the mechanism for pyrazolyl exchange, presumably involves pyrazolyl bond dissociation. In this context, electron-withdrawing para-aniline substituents render the aniline nitrogen electron-deficient and the nitrogen lone pair is less able to stabilize three-coordinate boron by conjugation with the empty porbital on boron.…”

Section: Resultsmentioning

confidence: 95%

Molecular Motion and Performance Enhancement of BORAZAN Fluorescent Dyes

2008

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The preparation of three 2,6‐dipyrazolyl‐4‐X‐anilines, H(pz2AnX) (X = p‐CF3, Cl, tBu) using CuI‐catalyzed amination is described. Subsequent reactions of H(pz2AnX) with triphenylboron proceeds with benzene elimination to give the corresponding Ph2B(pz2AnX) compounds in high yields. The Ph2B(pz2AnX) are more highly emissive in the solid state than the previously reported BORAZAN fluorophores, Ph2B(pzAnX), their monopyrazolyl counterparts. As with the Ph2B(pzAnX), the color of emission in Ph2B(pz2AnX) can be tuned simply by varying the para‐aniline substituent where the emission of Ph2B(pz2AnX) is red‐shifted relative to the corresponding Ph2B(pzAnX) derivatives. The electronic properties were studied by cyclic voltammetry and electronic absorption/emission spectroscopy as well as by density functional calculations (B3LYP/6‐31G*). The di‐pyrazolyl derivatives exhibit greater stability toward solvolysis and higher photoluminescent quantum yields (despite the red‐shift in emission) compared to their monopyrazolyl counterparts presumably due to kinetic stabilization of the chromophore imparted by the second pyrazolyl ligand. For Ph2B(pz2AnX), evidence for intramolecular motion of the diphenylboryl moiety traversing both pyrazolyl groups was detected by variable temperature 1H NMR spectroscopy. The rate increases with increasing electron‐donor abilities of the para‐aniline substituent.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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Bonding along a Linear B···N···B Triad

Cited by 22 publications

References 49 publications

Cage‐Shaped Borate Esters with Tris(2‐oxyphenyl)methane or ‐silane System Frameworks Bearing Multiple Tuning Factors: Geometric and Substituent Effects on Their Lewis Acid Properties

Cage‐Shaped Borate Esters with Tris(2‐oxyphenyl)methane or ‐silane System Frameworks Bearing Multiple Tuning Factors: Geometric and Substituent Effects on Their Lewis Acid Properties

2,2′:6′,2′′:6′′,6‐Trioxytriphenylamine: Synthesis and Properties of the Radical Cation and Neutral Species

Molecular Motion and Performance Enhancement of BORAZAN Fluorescent Dyes

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