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Ley-Griffith氧化

(2022-09-08 18:09:18) 下一个

Ley-Griffith氧化,又被称为TPAP氧化,利用催化量的TPAP(四丙基高钌酸铵)和当量的NMO(4-甲基吗啉-N-氧化物)做为氧化剂,将醇氧化为醛酮的反应。

TPAP(nPr4N+RuO4-;TetraPropylAmmonium Perruthenate)对空气、水分稳定,能溶于有机溶剂和水。TPAP相比起四氧化钌的氧化力要弱,底物中若含双键,不会被氧化断裂,伯醇仅氧化到醛,不会过度氧化为羧酸。实验步骤简单,反应进行的也非常快。另外,因反应条件也很温和(Reviews: Ley, Synthesis, 1994, 639),中性条件,反应为均相,常用于对复杂分子的氧化,其高度的化学选择性对许多其它官能团不产生影响,例如:烯键、炔键、内酯、环氧,甚至缩醛、硅醚、四氢吡喃醚等。

使用4-甲基吗啉-N-氧化物 (NMO) 或者其它氧化剂作为助氧化剂时,TPAP可以催化量地使用,反应一般在室温下进行,这样可以温和,高效,选择的氧化醇羟基生成醛和酮,它在氧化反应中具有不可取代的地位。对带有酸敏基团或者多官能团取代底物的氧化反应,选择TPAP/NMO组合试剂是非常合适的。PCCSwern氧化试剂在伯醇和仲醇氧化成相应醛和酮的过程中,TPAP/NMO完全可以两种试剂的功能,而且一般可以得到较好的结果。

 

General Procedure :
Between 0.02 and 0.15, typically 0.05 equiv. of TPAP are slowly added to a ca. 0.02–0.3 M solution of the alcohol in DCM, containing ca. 0.2–0.7 g of 4Å molecular sieves per mmol of alcohol and ca. 1.1 to 2.5, typically 1.5 equiv. of NMO. The resultant mixture is stirred at room temperature till most of the alcohol is consumed. 
work-up A:
The reaction mixture is filtered through a pad of Celite or silica gel and the resulting solution is concentrated, providing a residue that may need further purification. When the oxidation is performed in the presence of acetonitrile as solvent, as it tends to wash residual TPAP through the Celite or silica pad, it is advisable to evaporate the solvents and add some DCM before the filtering.
work-up B:
The reaction mixture is washed with a saturated Na2SO3 aqueous solution, a saturated CuSO4 aqueous solution and, optionally, with brine. Sometimes, it is advisable to add some organic solvent like DCM or EtOAc, in order to facilitate the washings. The organic phase is dried (MgSO4) and concentrated, giving a residue that may need further purification.

 TPAP能用(短硅胶柱)除掉。用Na2SO3还原NMO后,水洗就能从产物中除掉。大量反应时,较难控制反应平稳进行、所以加NMO?TPAP要非常注意。如果NMO在反应体系中已大量过量,反应却依然不进行时,继续加入NMO可能反应也不会进行了。反应溶剂一般是二氯甲烷(DCM)、但用乙腈作共溶剂能提高收率。

反应机理

钌(VII)和钌(VI)具有氧化活性。该反应机理相比起下图复杂,因为反应类似自催化反应 (反应一旦开始后会加速)。水的存在会阻碍此自催化过程,所以作反应要添加一些4A分子筛除水。NMO是钌的助氧化剤,这样,TPAP的量可以缩小到催化剂量。

Tetrapropylammonium perruthenate (16 mg, 0.0468mmol), N-methylmorpholine -N-oxide (0.16 g,1.4 mmol), and 3Å molecular sieves (0.47 g)were added to a solution of alcohol (400 mg, 0.94 mmol) in dichloromethane (10mL). The mixture was stirred at room temperature for 30 min then filteredthrough a Celite pad and the solvent evaporated in vacuo. Flash chromatographyof the residue using 10% diethylether - hexane as eluent afforded 2,8-Bis(tert-butyldimethylsilyloxy) undec-10-en-6-yn-5-one (385 mg, 98%) as colorless oil.

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