Spectroscopic Studies on Pyrazaboles

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.

show abstract

Pyrazole‐Organoboranes, VI. Monomeric and Dimeric 9‐Pyrazolyl‐9‐borabicyclo[3.3.1]nonanes

Yalpani

Boese

Köster

1990

Chem. Ber.

View full text Add to dashboard Cite

Bis(9H-9-borabicyclo[3.3.l]nonane) (9H-Q-BBN)z reacts with (tb),mPz, and (tb),ePz, respectively] to give the 9-pyrazolyl-9-pyrazole (Pz) and its 4-bromo, 3-methyl, 3-phenyl, 3,5-dimethyl, borabicyclo[3.3.l]nonanes with the dimeric structures (1)2 -(3)2 3-methyl-5-phenyL 3S-dipheny1, 3S-di-tert-buty1, 3,5-diada-(X-ray, NMR, and MS analysis), and 4 -11 with monomeric mantyl, 3,5-di-terf-butyl-l-methyl, and 3,5-di-tert-butyl-4-ethyl structures (X-ray, NMR, MS In a previous publication*) we have shown that substituents in the 3-and/or 5-position of pyrazoles hinder the coordination of these N bases to the boron atom(s) of bis(l,5-cyclooctanediylboryl) oxide or sulfide. Dialkyl-pyrazolyl-boranes in the monomeric form I with a trigonal boron atom should become accessible by utilizing these steric interactions. This class of boron-nitrogen compounds has been extensively studied, and in all cases its members have been found to exist in the dimeric state U3-5) (X-ray'), NMR'), and MS8) analysis). The only monomeric pyrazolylborane species reported so far are the diamino-pyrazolylboranes 1119', in which the Lewis acidity of the trigonal boron atom can be assumed to be significantly reduced by n orbital interactions with the three neighbouring nitrogen atoms. In a preliminary account of this work") we have shown that pyrazoles with very bulky substituents in their 3-and 5-positions react with bis(9H-9-borabicyclo-[3.3.l]nonane) (9H-9-BBN)2 to form the corresponding monomeric 9-pyrazolyl-9-borabicyclo[3.3.1]nonanes. In this publication we show how the reaction of (9H-9-BBN)z with a number of pyrazoles with progressively increased number and sizes of the substituents gradually decreases the stability of the dimeric form and eventually leads to stable monomeric diorgano-pyrazolyl-boranes. Results and DiscussionThe dimeric 9-pyrazolyl-9-borabicyclo[3.3.l]nonane (1)* has previously been obtained in an indirect way by the reaction of the B20N2 heterocycle 1V with (9H-9-BBN);l).We have now found that (l)z is more readily accessible by

show abstract

Spectroscopic Studies on Pyrazaboles

Cited by 12 publications

References 8 publications

The Coordination Chemistry of Pyrazole‐Derived Ligands

The Coordination Chemistry of Pyrazole‐Derived Ligands

Pyrazabole: Elektrophile Substitutionen und Röntgenstrukturuntersuchungen

Pyrazole‐Organoboranes, VI. Monomeric and Dimeric 9‐Pyrazolyl‐9‐borabicyclo[3.3.1]nonanes

Contact Info

Product

Resources

About