Insights into the mechanism of the site-selective sequential palladium-catalyzed cross-coupling reactions of dibromothiophenes/dibromothiazoles and arylboronic acids. Synthesis of PPARβ/δ agonists
作者:Raquel Pereira、Audrey Furst、Beatriz Iglesias、Pierre Germain、Hinrich Gronemeyer、Angel R. de Lera
DOI:10.1039/b612235c
日期:——
A reactivity study, aided by NMR spectroscopy, allowed a mechanistic rationale to be postulated for the palladium-catalyzed regioselective coupling of arylboronic acid (and arylstannane where feasible) at the position next to the sulfur atom in functionalized dibromothiophenes and dibromothiazoles. The analysis of the NMR spectra (using 19F from the boronic acid CF3 group and 31P from the phosphine of the catalyst as probes) of the entire reaction starting from the dibromoheterocycles allowed the qualitative proposal that the transmetalation is the rate-limiting step for both sequential substitution processes. The extremely facile oxidative addition at the CâBr bond next to the sulfur atom of the heterocycle instead determines the positional selectivity. An additional Stille reaction then replaced the second halogen, providing the trisubstituted heterocyclic scaffolds of PPAR ligands, which displayed PPARβ/δ agonist activity, as revealed by reporter assays in living cells.
一项反应性研究,借助 NMR 光谱学,使得能够提出一种机制推断,用于钯催化的区域选择性偶联反应,即在功能化的二溴噻吩和二溴噻唑中,芳基硼酸(以及可行的芳基锡化合物)在硫原子相邻位置的偶联。对整个反应的 NMR 光谱(使用来自硼酸 CF3 组的 19F 和催化剂的膦的 31P 作为探针)进行分析表明,跨金属转移是两种顺序取代过程的速率限制步骤。异环次的 C–Br 键的极其容易的氧化加成决定了位置选择性。随后,通过额外的 Stille 反应替代第二卤素,提供了具有 PPAR 配体特征的三取代杂环支架,这些支架在活细胞中的报告酶测试中显示出 PPARβ/δ 激动剂活性。