Base-Free Transfer Hydrogenation of Ketones Using Arene Ruthenium(II) Complexes

Ruthenium derivatives of the type [RuCl(arene)(NN)][BPh4] (arene = benzene, p-cymene) have been synthesized with NN = 2-hydroxyphenylbis(pyrazol-1-yl)methane (bpzmArOH) or 2-hydroxyphenylbis(3,5-dimethylpyrazol-1-yl)methane (bpz*mArOH). In the p-cymene derivative containing bpzmArOH, activation of the hydroxy group is observed and a scorpionate complex is obtained with the ligand behaving in a tridentate manner, [Ru(bpzmArO-κ3-N,N,O)(p-cymene)][BPh4]. The structure of this derivative and that of [RuCl(p-cymene)(bpz*mArOH)][BPh4] were determined by X-ray diffraction. The new derivatives, along with other compounds previously described by us that contain similar ligands, were tested in the transfer hydrogenation of benzophenone and other carbonyl compounds under base-free conditions without any other additive to promote the reaction. The precursors were active in this process, and the p-cymene derivatives exhibited a higher activity than the benzene complexes. For the p-cymene complexes the activity was even higher than that found in the presence of base (KOH). The effect of the substituents on the methylene carbon was also studied. Mechanistic and kinetic studies were carried out, and hydride species were observed after a pretreatment of the precatalyst in 2-propanol. Taking into account all of the results, a proposal for the mechanism taking place during the hydrogenation of carbonyl groups under base-free conditions has been made.

一类通式为[RuCl(arene)(NN)][BPh4]的钌衍生物（其中芳烃(arene)可为苯、对伞花烃(p-cymene)）已成功合成，对应的NN配体分别为2-羟苯基双(吡唑-1-基)甲烷(2-hydroxyphenylbis(pyrazol-1-yl)methane，缩写bpzmArOH)与2-羟苯基双(3,5-二甲基吡唑-1-基)甲烷(2-hydroxyphenylbis(3,5-dimethylpyrazol-1-yl)methane，缩写bpz*mArOH)。在含bpzmArOH的对伞花烃衍生物中，观测到羟基活化现象，配体以三齿配位模式参与配位，获得蝎型配合物[Ru(bpzmArO-κ3-N,N,O)(p-cymene)][BPh4]。该衍生物与[RuCl(p-cymene)(bpz*mArOH)][BPh4]的结构均通过X射线衍射(X-ray diffraction)得以解析。将这批新型衍生物，连同本课题组此前报道的其他含类似配体的化合物，一同应用于无碱且无额外反应促进添加剂的二苯甲酮及其他羰基化合物的转移氢化反应测试中。实验结果表明，该类前驱体对该反应具有催化活性，且对伞花烃类配合物的催化活性高于苯基金属配合物。对于对伞花烃类配合物，其催化活性甚至高于添加氢氧化钾(KOH)作为碱时的反应效果。研究还考察了亚甲基碳上取代基的影响。通过机理与动力学研究，在异丙醇(2-propanol)中对预催化剂(precatalyst)进行预处理后，可观测到氢化物物种。综合所有实验结果，本文提出了无碱条件下羰基化合物转移氢化反应的可能机理。