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Pyrazole—Organoboranes, VIII Thermal C —Borylation in Crowded Pyrazolylboranes
Author(s) -
Yalpani Mohamed,
Köster Roland,
Boese Roland
Publication year - 1991
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19911240805
Subject(s) - chemistry , triethylborane , boranes , borylation , medicinal chemistry , pyrazole , borane , alkyl , hydroboration , adduct , stereochemistry , organic chemistry , catalysis , boron , aryl
Crowded 3,5‐disubstituted pyrazoles, e. g. 3,5‐di‐ tert ‐butyl‐,3,5‐di‐ tert ‐butyl‐ 4 ‐methyl‐, or 3,5‐di‐ tert ‐butyl‐4‐ethylpyrazole [ (tb) 2 Pz, (tb) 2 mPz, or (tb) 2 ePz , respectively], fail to react with activated trialkylboranes (e. g. BEt 3 * or BPr 3 *) at 110°C, even after prolonged heating. At 170°C the reaction of (tb) 2 Pz with activated triethylborane gives the monomeric diethyl(3,5‐di‐ tert ‐butyl‐1‐pyrazolyl)borane (3) . At 220–230°C the dimeric boracycles (4) 2 are mainly formed [X‐ray structure of (4a) 2 ]. In contrast, the reactions of these crowded pyrazoles with tetraalkyldiboranes(6) commence just above room temperature. Diethylhydroborane adducts ( 5 ) of 3 and dimeric alkyl(3,5‐di‐ tert ‐butyl‐1‐pyrazolyl)‐boranes (2b) 2 and (2c) 2 are the main products.