Asymmetric Cyclopropanation of Vinyl Fluorides: Access to Enantiopure Monofluorinated Cyclopropane Carboxylates
作者:Oliver G. J. Meyer、Roland Fröhlich、Günter Haufe
DOI:10.1055/s-2000-7103
日期:——
The transition metal catalyzed cyclopropanation with alkyl diazoacetates of aliphatic or aromatic vinyl fluorides, prepared from the corresponding alkenes by bromofluorination and subsequent dehydrobromination, provides a smooth access to racemic 1 : 1 mixtures of cis/trans isomeric monofluorinated cyclopropane carboxylates. The application of enantiopure bis(oxazoline) ligands and copper(I) triflate makes the reaction trans-diastereoselective and enantioselective. For example, treatment of α-fluorostyrene (3a) with tert-butyl diazoacetate in the presence of 2 mol% of the catalyst prepared from (S)-tert-leucine-based 11b and CuOTf gave a 4 : 1 mixture of trans-2-fluoro-2-phenylcyclopropanecarboxylate (4e) with 93% ee and the corresponding cis-isomer 5e with 89% ee. The absolute configuration of the trans-isomer 4e has been determined to be (1S,2S) by X-ray structure analysis of a derivative.
过渡金属催化的环丙烷化反应,通过与烷基二氮酸酯的反应,能够平稳地获得由烃基醇或芳香族乙烯氟化物(由相应烯烃经过溴氟化和后续去溴化反应制得)的外消旋1 : 1混合物的顺/反凉氟化环丙烷羧酸酯。采用手性纯双(噁唑啉)配体和铜(I)三氟甲磺酸盐使得该反应具备顺式立体选择性和对映选择性。例如,α-氟苯乙烯(3a)与叔丁基二氮酸酯在以(S)-叔亮氨酸为基础的催化剂11b和CuOTf的存在下反应,得到4 : 1的反-2-氟-2-苯基环丙烷羧酸酯(4e),其对映体过量率(ee)为93%,以及相应的顺式异构体5e,其对映体过量率为89%。反式异构体4e的绝对构型已经通过衍生物的X射线结构分析确定为(1S,2S)。