New CNN-Type Ruthenium Pincer NHC Complexes. Mild, Efficient Catalytic Hydrogenation of Esters
New CNN-Type Ruthenium Pincer NHC Complexes. Mild, Efficient Catalytic Hydrogenation of Esters
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DOI：
10.1021/om200367j
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发表时间：
2011-07-25
期刊：
影响因子：
2.8
通讯作者：
Milstein, David
中科院分区：
文献类型：
--
作者：
Fogler, Eran;Balaraman, Ekambaram;Milstein, David
合成并研究了基于新的联吡啶-NHC配体1的新型钳形钌配合物（2-6），在温和条件下，该配合物能有效地催化酯加氢合成相应的醇。配体1与RuH（Cl）CO（PPh（3））（3）反应，然后与一当量碱KHMDS反应，得到混合膦-NHC配合物2，其包含CH-活化的联吡啶配体。配合物2具有八面体结构，其含有两个彼此反式的磷原子、与NHC配体反式的氢化物和与联吡啶配体的CH-活性碳反式的CO。使用前体络合物Ru（p-cymene）Cl（2）（CO），与1反应，然后用1当量的KHMDS处理中间产物，导致形成二氯钳形络合物3a和3b，其处于平衡，如变温（1）H NMR所示。配合物3a是八面体的，中性的，对称的配合物，CO配体位于钳形配体的中心吡啶基团的反式，并且两个氯化物彼此反式，如单晶X-射线衍射所示。配合物3b是阳离子型的，具有外层氯化物。NHC配体1与LiHMDS在低温下反应，然后加入RuH（COCO（PPh（3））（3），得到混合膦-NHC络合物4，其具有八面体结构，含有与氢化物反式的磷、与NHC配体反式的CO和外层氯化物。氯代BAr（F-）得到的X-射线表征的配合物5。络合物4与KHMDS的去质子化导致形成脱芳构化的络合物6。原位制备的6（从配合物4和一当量的碱）是已知在温和条件下将未活化的酯氢化成相应醇的最好催化剂之一。
New pincer ruthenium complexes (2-6) based on the new bipyridine-NHC ligand 1 were prepared and studied, resulting in an efficient catalytic hydrogenation of esters to the corresponding alcohols under mild conditions. Reaction of the ligand 1 with RuH(Cl)CO(PPh(3))(3), followed by reaction with one equivalent of the base KHMDS, gave the mixed phosphine-NHC complex 2, incorporating a C H-activated bipyridine ligand. Complex 2 has an octahedral structure containing two phosphorus atoms trans to each other, a hydride trans to the NHC ligand, and CO trans to the C H-activated carbon of the bipyridine ligand. Using the precursor complex Ru(p-cymene)Cl(2)(CO), reaction with 1 followed by treatment of the intermediate product with one equivalent of KHMDS resulted in formation of the dichloride pincer complexes 3a and 3b, which are in equilibrium, as indicated by variable-temperature (1)H NMR. Complex 3a is an octahedral, neutral, and symmetric complex with the CO ligand positioned trans to the central pyridine group of the pincer ligand and the two chlorides trans to each other, as indicated by single-crystal X-ray diffraction. Complex 3b is cationic, with an outer-sphere chloride. Reaction of the NHC ligand 1 with LiHMDS at low temperature followed by addition of RuH(COCO(PPh(3))(3) resulted in the mixed phosphine-NHC complex 4, which has an octahedral structure containing phosphorus trans to the hydride, a CO trans to the NHC ligand, and an outer-sphere chloride. Chloride substitution by BAr(F-) gave the X-ray-characterized complex 5. Deprotonation of complex 4 with KHMDS resulted in formation of the dearomatized complex 6. The in situ prepared 6 (from complex 4 and an equivalent of base) is among the best catalysts known for the hydrogenation of nonactivated esters to the corresponding alcohols under mild conditions.