Dissecting the Stereocontrol Elements of a Catalytic Asymmetric Chlorolactonization: <i>Syn</i> Addition Obviates Bridging Chloronium
作者:Roozbeh Yousefi、Kumar Dilip Ashtekar、Daniel C. Whitehead、James E. Jackson、Babak Borhan
DOI:10.1021/ja4072145
日期:2013.10.2
absolute and relative stereochemistry of addition in enantioselective chlorolactonizations of 4-phenyl-4-pentenoic acid and its related t-butyl ester, catalyzed by (DHQD)2PHAL. Predominant syn addition of the chlorenium and the nucleophile across the olefin is observed. As shown by isotopic labeling, NMR spectroscopy, and derivative studies, the two new stereocenters formed by addition across the double
AbstractThe main fragmentation pathway of ionized hydroxyallenes (1) consists of a methyl loss. Extensive deuterium‐labelling experiments indicate that the terminal allenic carbon is implied in this fragmentation. Collisional activation spectra indicate a propenyl‐acylium structure (a) for these [M – CH3]+ ions which can originate from a 1,4‐hydroxyl migration followed by hydrogen rearrangements. Isomeric hydroxyacetylenes (2) behave similarly, also giving rise, by methyl loss, to acylium ions a. It is proposed that 2+ ˙ is irreversibly isomerized into 1+ ˙ by a 1,3‐hydrogen transfer ‘catalysed’ by the hydroxy group. The proposed internal proton‐bound complex justifies also the easier loss of water from 2+˙. Ethyl loss is also a prominent fragmentation for the hydroxyallene and hydroxy‐acetylene homologues.
作者:Marcileia Zanatta、Francisco P. dos Santos、Cristina Biehl、Graciane Marin、Gunter Ebeling、Paulo A. Netz、Jairton Dupont
DOI:10.1021/acs.joc.6b03029
日期:2017.3.3
for the H/Dexchange reaction of various substrates using CDCl3 as D source, without the addition of any extra bases or metal. High deuterium incorporation (up to 49%) in acidic C–H bonds of ketone and alkyne substrates (pKa from 18.7 to 28.8) was found at room temperature. The reaction proceeds through the fast and reversible deuteration of the 2-methyl H of the imidazolium cation followed by D transfer
与碱性阴离子(例如碳酸氢盐,脯氨酸盐和咪唑酸盐)缔合的简单1,2,3-三烷基咪唑鎓阳离子是使用CDCl 3作为D源进行各种底物H / D交换反应的活性催化剂,而无需添加任何多余的碱或金属。酮和炔烃底物的酸性C–H键中氘的高掺入量(最高49%)(p K a在室温下发现(从18.7到28.8)。反应通过咪唑鎓阳离子的2-甲基H的快速可逆氘化进行,然后D转移至底物。IL充当中性碱催化剂,其中在反应过程中保持接触离子对。碱性活性位点归因于阴离子中存在一个偏远的碱性位点,即碳酸氢根的OH,脯氨酸根的NH和咪唑基阴离子中的活化水。详细的动力学实验表明,由于涉及溶剂对离子液体进行氘化的快速可逆反应,因此该反应在底物上相对于离子液体为一级伪零级。
Regiospecific and Diastereoselective C−H and C−Si Bond Activation of ω-Silyl-Substituted Alkane Nitriles by “Bare” Co<sup>+</sup> Cations in the Gas Phase
作者:Georg Hornung、Detlef Schröder、Helmut Schwarz
DOI:10.1021/ja963254m
日期:1997.3.1
The unimolecular decompositions of 5-methyl-7-trimethylsilylheptanenitrile (1) and 6-methyl-8-trimethylsilyloctanenitrile (2) complexed to “bare” Co+ cations have been investigated by means of tandem mass spectrometry. The Co+-mediated bond activations display a high degree of selectivity in that the major (>90%) neutral product formed is trimethylsilane. Regio- and diastereospecific labeling experiments