Ebran Jean-Philippe, Hansen Anders L, Gøgsig Thomas M, Skrydstrup Troels
The Center for Insoluble Protein Structures inSPIN, Department of Chemistry, the Interdisciplinary Nanoscience Center, University of Aarhus, Langelandsgade 140, 8000 Aarhus, Denmark.
J Am Chem Soc. 2007 May 30;129(21):6931-42. doi: 10.1021/ja070321b. Epub 2007 May 3.
A catalyst system was identified which promotes the Heck coupling of nonactivated vinyl phosphates with electron deficient alkenes providing a new entry to diene products from simple and readily accessible starting materials. In contrast to our earlier work exploiting P(t-Bu)3 as the ligand in the presence of PdCl2(COD), the application of Buchwald's dialkylbiarylphosphines, X-Phos, effectively promoted the vinylic substitution with a wide range of alkenyl phosphates in the presence of 10 equiv of lithium chloride. Importantly, these reaction conditions suppressed 1,2-migration of the alkenyl palladium(II) intermediate. Further studies are also reported with the catalytic system which encourages isomerization in order to determine the range of vinyl phosphates that may participate in these coupling reactions. The extent of the 1,2-migration was dependent on the C1-substituent where best results were noted for substrates possessing a C1-alkyl quaternary carbon. Hence, with certain members of this class of alkenyl phosphates either the migrated or nonmigrated Heck products may be preferentially synthesized by selection of the phosphine ligand. Finally, competition experiments between an unactivated aryl chloride and a vinyl phosphate with a palladium catalyst possessing either X-Phos or P(t-Bu)3 as ligand demonstrated the ability to carry out Heck coupling reactions selectively with the aryl halide. Oxidative addition of the metal catalyst into the aryl chloride bond rather than the C-O bond of the alkenyl phosphate is therefore preferred.
已确定一种催化剂体系，该体系可促进未活化的乙烯基磷酸酯与缺电子烯烃的Heck偶联反应，从而从简单且易于获得的起始原料出发，为二烯产物提供了一条新的合成途径。与我们早期在PdCl2(COD)存在下利用三叔丁基膦作为配体的工作不同，在10当量氯化锂存在下，Buchwald的二烷基联芳基膦X-Phos的应用有效地促进了多种烯基磷酸酯的乙烯基取代反应。重要的是，这些反应条件抑制了烯基钯(II)中间体的1,2-迁移。还报道了对该催化体系的进一步研究，该体系促进异构化以确定可能参与这些偶联反应的乙烯基磷酸酯的范围。1,2-迁移的程度取决于C1取代基，对于具有C1-烷基季碳的底物，效果最佳。因此，对于这类烯基磷酸酯的某些成员，通过选择膦配体可以优先合成迁移的或未迁移的Heck产物。最后，在未活化的芳基氯与乙烯基磷酸酯之间使用以X-Phos或三叔丁基膦为配体的钯催化剂进行的竞争实验表明，能够与芳基卤化物选择性地进行Heck偶联反应。因此，金属催化剂优先氧化加成到芳基氯键而不是烯基磷酸酯的C-O键中。
