Friedrich Alexander, Eyselein Jonathan, Elsen Holger, Langer Jens, Pahl Jürgen, Wiesinger Michael, Harder Sjoerd
Inorganic and Organometallic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 1, 91058, Erlangen, Germany.
Chemistry. 2021 May 17;27(28):7756-7763. doi: 10.1002/chem.202100641. Epub 2021 May 2.
Strongly Lewis acidic cationic aluminium complexes, stabilized by β-diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C F ) salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the BDI ligand ( BDI=HC[C(R1)N(R2)] ) influence sterics and Lewis acidity. Ligand bulk increases along the row BDI< BDI≈ BDI< BDI; DIPP=2,6-C(H)Me -phenyl, DIPeP=2,6-C(H)Et -phenyl. The Gutmann-Beckett test showed acceptor numbers of: ( BDI)AlMe 85.6, ( BDI)AlMe 85.9, ( BDI)AlMe 89.7, ( BDI)AlMe 90.8, ( BDI)AlH 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation ( BDI)AlMe shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured.
通过β-二酮亚胺（BDI）配体稳定且不含路易斯碱的强路易斯酸性阳离子铝配合物已被制备为它们的B(CF)盐，并对其在亚胺氢化反应中的催化活性进行了研究。BDI配体（BDI=HC[C(R1)N(R2)]）上的主链（R1）和N（R2）取代基影响空间位阻和路易斯酸性。沿着BDI<BDI≈BDI<BDI的顺序配体体积增大；DIPP=2,6-C(H)Me -苯基，DIPeP=2,6-C(H)Et -苯基。古特曼-贝克特测试表明受体数为：(BDI)AlMe 85.6，(BDI)AlMe 85.9，(BDI)AlMe 89.7，(BDI)AlMe 90.8，(BDI)AlH 95.3。在亚胺氢化反应中，空间位阻和电子因素需要平衡。开放的、高路易斯酸性的阳离子会强烈配位亚胺，使其作为受阻路易斯对（FLP）无活性。体积最大的阳离子不配位亚胺，但它们的组合也不是活性催化剂。阳离子(BDI)AlMe对各种亚胺表现出最佳的催化活性，并且也是苯甲醛与苯甲酸苄酯的蒂申科反应的活性催化剂。对阳离子铝配合物催化亚胺氢化反应机理的密度泛函理论计算揭示了两个相互关联的催化循环协同运行。氢要么通过Al⋅⋅⋅亚胺对的FLP反应性被活化，要么在形成大量胺后通过与Al⋅⋅⋅胺对反应被活化。后一个自催化的Al⋅⋅⋅胺循环在能量上更有利。
