Fischer Susanne M, Renner Simon, Boese A Daniel, Slugovc Christian
Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayrgasse 9, 8010 Graz, Austria.
Christian Doppler Laboratory for Organocatalysis in Polymerization, Stremayrgasse 9, 8010 Graz, Austria.
Beilstein J Org Chem. 2021 Jul 21;17:1689-1697. doi: 10.3762/bjoc.17.117. eCollection 2021.
Electron-rich triarylphosphines, namely 4-(methoxyphenyl)diphenylphosphine (MMTPP) and tris(4-trimethoxyphenyl)phosphine (TMTPP), outperform commonly used triphenylphosphine (TPP) in catalyzing oxa-Michael additions. A matrix consisting of three differently strong Michael acceptors and four alcohols of varying acidity was used to assess the activity of the three catalysts. All test reactions were performed with 1 mol % catalyst loading, under solvent-free conditions and at room temperature. The results reveal a decisive superiority of TMTPP for converting poor and intermediate Michael acceptors such as acrylamide and acrylonitrile and for converting less acidic alcohols like isopropanol. With stronger Michael acceptors and more acidic alcohols, the impact of the more electron-rich catalysts is less pronounced. The experimental activity trend was rationalized by calculating the Michael acceptor affinities of all phosphine-Michael acceptor combinations. Besides this parameter, the acidity of the alcohol has a strong impact on the reaction speed. The oxidation stability of the phosphines was also evaluated and the most electron-rich TMTPP was found to be only slightly more sensitive to oxidation than TPP. Finally, the catalysts were employed in the oxa-Michael polymerization of 2-hydroxyethyl acrylate. With TMTPP polymers characterized by number average molar masses of about 1200 g/mol at room temperature are accessible. Polymerizations carried out at 80 °C resulted in macromolecules containing a considerable share of Rauhut-Currier-type repeat units and consequently lower molar masses were obtained.
富电子三芳基膦，即4-（甲氧基苯基）二苯基膦（MMTPP）和三（4-三甲氧基苯基）膦（TMTPP），在催化氧杂-Michael加成反应中表现优于常用的三苯基膦（TPP）。使用由三种不同强度的Michael受体和四种酸度不同的醇组成的基质来评估这三种催化剂的活性。所有测试反应均在无溶剂条件下、室温下以1 mol%的催化剂负载量进行。结果表明，TMTPP在转化较差和中等强度的Michael受体（如丙烯酰胺和丙烯腈）以及转化酸性较弱的醇（如异丙醇）方面具有决定性优势。对于较强的Michael受体和酸性更强的醇，电子更丰富的催化剂的影响不太明显。通过计算所有膦-Michael受体组合的Michael受体亲和力，对实验活性趋势进行了合理化解释。除了这个参数外，醇的酸度对反应速度有很大影响。还评估了膦的氧化稳定性，发现电子最丰富的TMTPP对氧化的敏感性仅比TPP略高。最后，将这些催化剂用于丙烯酸2-羟乙酯的氧杂-Michael聚合反应。使用TMTPP在室温下可得到数均摩尔质量约为1200 g/mol的聚合物。在80℃下进行的聚合反应产生了含有相当比例的劳乌特-柯里尔型重复单元的大分子，因此得到的摩尔质量较低。
