The reaction of Me3CCOOBEt2 with pyrazole led to the formation of the stable addition complex 1, which was found to be monomeric in solution. The reaction of 9-H-9-borabicyclo[3.3.1]nonane (9-H-9-BBN)(2) with pyrazole and pivalic acid yielded a similar complex Me2CCOO(9-BBN)-pzH 2. Crystallographic analysis of 2 revealed a dimeric structure due to the strong hydrogen bonding between azole hydrogen and carbonyl oxygen. The one boron-carbon bond of compound 1 cleaves readily with triethylborane at 20 degrees C with the evolution of ethane, giving a species Me2CCOO(BEt2)(2)pz 3 with a seven-membered heterocyclic ring as the main structural feature. (C) 1998 Elsevier Science S.A. All rights reserved.
Stereospecific Transformations of Alkylboronic Esters Enabled by Direct Boron-to-Zinc Transmetalation
作者:Hao Liang、James P. Morken
DOI:10.1021/jacs.3c01677
日期:2023.5.10
Chiral secondary organoboronic esters, when activated with t-butyllithium, are shown to undergo efficient stereoretentive transmetalation with either zinc acetate or zinc chloride. This reaction provides chiral secondary alkylzinc reagents that are configurationally stable under practical experimental conditions. The organozinc compounds were found to engage in stereospecific reactions with difluorocarbene
The reaction of triethylborane with pyrazole leading to 4,4,8,8-tetraethylpyrazabole was found to be catalyzed by carboxylic acids. 2,2'-Dimethylpropanoic (pivalic) or benzoic acids act as catalysts in this reaction. Intermediates occurring in the catalytic cycle have been isolated and characterized by NMR as well as IR spectroscopy, and in one case also by X-ray crystallography. The mechanism of the reaction is proposed. (C) 2000 Elsevier Science S.A. All rights reserved.