Pyrazabole: Elektrophile Substitutionen und Röntgenstrukturuntersuchungen

Abstract

Elektrophil induzierte Substitutionsreaktionen an den Boratomen der Pyrazabole [R2BpzI2 mit "BBr, erfolgen ohne Austausch von Boratomen. Alle vier Substituenten R werden gegen Br substituiert (Ausnahme R = C,H,), wobei "B-NMR-spektroskopisch verschieden substituierte Pyrazabole als Zwischenprodukte nachgewiesen und in einigen Fallen isoliert wurden. Dabei weisen die NMR-Daten der Umsetzung von [(CzH5)2Bpz]z mit BBr, auf eine ionische Zwischenstufe fur diese Austauschreaktion hin. Eine schrittweise Substitution erfolgt auch bei der Umsetzung des Pyrazabols l a mit Pyrazol, wie die Isolierung von Monopyrazol-1-ylpyrazabol l e zeigt. -Nach Rontgenstrukturuntersuchungen an fiinf Pyrazabolen liegt der zentrale B2N,-Ring in der Sessel-, Boot-oder planaren Konformation vor. Danach scheinen Packungseffekte die jeweilige Konformation im festen Zustand zu bestimmen. Pyrazaboles: Electrophilic Substitutions and Studies of Their Crystal and Molecular StructuresElectrophilically induced substitution reactions at the boron atoms of pyrazaboles, [R2BpzI2, using "BBr, proceed without exchange of the boron atoms. All four substituents R may be exchanged for Br, with the exception of R = C2H,, and variously substituted pyrazaboles are detected by "B NMR as intermediates. Some of these were isolated. Based on NMR data obtained for the reaction of [(CZH5)zBpz]2 with BBr,, ionic intermediates are proposed for these exchange reactions. A stepwise interaction also occurs on treatment of the pyrazabole l a with pyrazole; this is documented by the isolation of the monopyrazol-1-ylpyrazabole l e . -The central B2N, ring of pyrazaboles adopts a chair, a boat, or a planar conformation in the solid state as shown by Xray structural studies on five different pyrazaboles. Therefore, packing effects seem to determine the ring conformation.

Investigations on the reactivity of arylboronic acid with phenolic pyrazole

“…In general, pyrazaboles with symmetrical 4,8-disubstitutions at the boron atom have been synthesized by the reaction of amine complexes of borane or organoborane reagents with pyrazole under reflux conditions in toluene [27,28]. The other synthetic route employed is by functionalizing the hydrogen coordinated to boron of pyrazabole by other groups [42,43]. In the case of gem boron disubstituted or asymmetrically boron monosubstituted materials, the synthesis is challenging due to unpredictable reaction of the bis(pyrazolyl)borate salt with the amine complexes of the boranes bearing good leaving group [44,45].…”

The electronic structure of molecules with the BF and BCl bond in light of the topological analysis of electron localization function: Possibility of multiple bonds?

“…The experimental BCl bond length in seven molecules with the CSD Refcode: CAQNEN, FUMXAO, ACNBCL, TUWMEF, BUGGUF, ADAJIA, and ADAJEW is between 1.793 and 1.917Å (see Tables and ).…”

Formation and structural characterization of a europium(II) mono(scorpionate) complex and a sterically crowded pyrazabole

“…The parent pyrazabole, {H 2 B(-pz)} 2 has been known since 1966 when it was reported by Trofimenko contemporaneously with the discovery of Tp ligands (Trofimenko, 1966). Since then, numerous substituted pyrazaboles have been prepared and structurally investigated (Alcock & Sawyer, 1974;Cavero et al, 2008;Niedenzu & Niedenzu, 1984;Niedenzu & Nö th, 1983;Trofimenko, 1966). In a number of recent studies, it has been demonstrated that certain substituted pyrazaboles possess unique photophysical and electrochemical properties and could thus find promising applications in organic photovoltaics (OPVs) and non-linear optics (Jadhav et al, 2013(Jadhav et al, , 2015Misra et al, 2013Misra et al, , 2014Patil et al, 2017).…”

“…For the chemistry of divalent lanthanide scorpionate complexes, see: Davies et al (1985) For general information on the chemistry and structures of pyrazaboles, see: Cavero et al (2008), Niedenzu & Niedenzu (1984), Niedenzu & Nö th (1983), Trofimenko (1966).…”