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Oxidation of Alcohols to Carbonyl Compounds Catalyzed by Oxo-Bridged Dinuclear Cerium Complexes with Pentadentate Schiff-Base Ligands under a Dioxygen Atmosphere
ACS Catalysis ( IF 12.9 ) Pub Date : 2018-06-14 00:00:00 , DOI: 10.1021/acscatal.8b01718
Satoru Shirase 1 , Koichi Shinohara 1 , Hayato Tsurugi 1 , Kazushi Mashima 1
Affiliation  

Ionic mononuclear and neutral dinuclear complexes of cerium(III) 3-L1–3-L9 bearing a series of dianionic pentadentate Schiff-base ligands were synthesized, characterized, and used as catalysts for N-oxyl radical-free aerobic alcohol oxidation. Reactions of Ce(NO3)3·6H2O with o-tert-butyl-substituted sterically hindered ligands NH(CH2CH2N═CHC6H2-3-(tBu)-5-R2-2-OH)2 (for L1H2, R2 = tBu; for L2H2, R2 = OMe; and for L3H2, R2 = H) in the presence of triethylamine afforded the corresponding anionic cerium complexes [HNEt3][Ce(L1–3)(NO3)2] (3-L13-L3), whereas complexation with sterically less hindered ligands, such as NH(CH2CH2N═CHC6H2-3-R1-5-R2-2-OH)2 (for L4H2, R1 = OMe and R2 = H; for L5H2, R1 = H and R2 = tBu; for L6H2, R1 = H and R2 = OMe; for L7H2, R1 = H and R2 = H; for L8H2, R1 = H and R2 = NO2; and for L9H2, R1 = tBu and R2 = NO2), afforded neutral dinuclear complexes [Ce(L4–9)(NO3)]2 (3-L4–3-L9). Among these newly prepared complexes, complex 3-L1 was selected as the best catalyst for oxidizing primary and secondary alcohols under a dioxygen atmosphere without any N-oxyl radicals such as TEMPO to produce the corresponding carbonyl compounds, where the oxo-bridged dinuclear complex worked as a catalyst while maintaining its dinuclear skeleton during the catalytic cycle. In addition, an intramolecular redox process between the two cerium centers through the bridging oxygen atom played a key role in forming the ligand phenoxide radical-mediated TEMPO-free alcohol oxidation reaction.

中文翻译:

双氧铈席夫碱配体在氧气氛下氧桥联双核铈配合物催化醇氧化为羰基化合物。

合成,表征了铈(III)3-L 1 –3-L 9的离子单核和中性双核配合物,它们带有一系列双阴离子五齿席夫碱配体,并用作无N-氧自由基的好氧醇氧化的催化剂。的Ce(NO的反应33 ·6H 2 O运用ö -丁基取代的空间位阻配体NH(CH 2 CH 2 N═CHC 6 ħ 2 -3-(丁基)-5-R 2 -2- OH)2(对于L 1 H 2,R2 = t Bu; 对于L 2 H 2,R 2= OMe;在三乙胺存在下,对于L 3 H 2,R 2 = H)提供相应的阴离子铈配合物[HNEt 3 ] [Ce(L 1–3)(NO 32 ](3-L 13-L 3),而使用空间位阻少配体,例如NH(CH络合2 CH 2 N═CHC 6 ħ 2 -3-R 1 -5-R 2 -2-OH)2(对于L 4 H 2,R 1 = OMe和R 2 = H;对于L 5 H 2,R 1 = H和R 2 = t Bu;对于L 6 H 2,R 1 = H和R 2 = OMe ;对于L 7 H 2,R 1 = H和R 2 = H;对于L 8 H 2,R 1 = H和R 2 = NO 2;对于L 9H 2,R 1 = t Bu和R 2 = NO 2),得到中性双核配合物[Ce(L 4–9)(NO 3)] 23-L 4 –3-L 9)。在这些新制备的配合物中,配合物3-L 1被选为在没有任何N的双氧气氛下氧化伯醇和仲醇的最佳催化剂。-氧基自由基(例如TEMPO)产生相应的羰基化合物,其中羰基桥连的双核配合物充当催化剂,同时在催化循环中保持其双核骨架。另外,两个铈中心之间通过桥接氧原子的分子内氧化还原过程在形成配体酚盐自由基介导的无TEMPO的醇氧化反应中起着关键作用。
更新日期:2018-06-14
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