金属热还原

醛、酮能被多种金属还原剂还原成醇。这些金属有锂、钠、钾碱金属; 铝、铁、锌等。他们和特定的试剂配合成常用的还原体系。如锂、钠、钾－氨、钠、钾－醇、铝－汞、铁-乙酸、锌-乙醇等。如钙拮抗剂盐酸马尼地平(Manidipine)中间体二苯甲醇的的制备，通过锌-乙醇还原二苯甲酮的收率达到90％～92％。

Na-NH3体系还原酮示例

A 5-L,three-necked, round-bottomed flask is equipped with a variable speed Hershberg stirrer and a 500-mL Dewar condenser filled with dryice–acetoneand connected by its upper joint to a mineral oil bubbler through which passesdry nitrogengas. The flask is immersed in a dry ice–acetone bath,flushed well with nitrogenand 3000 mL of ammoniais condensed into the flask via a glass tube passed through a rubber septum in the remaining neck of the flask. Clean sodium, 125 g (5.43 g-atom), is added slowly to the ammoniawith slow stirring. Then 250.3 g (0.906 mol) of 5-methyl-2-[1-methyl-1-(phenylmethylthio)ethyl]cyclohexanoneand 72.5 mL (1.8 mol) of methanolin 625 mL of anhydrous ether are added dropwise via a pressure-equalized addition funnel over 5 hr to thevigorously stirred (ca. 500 rpm) solution. Stirring is continued an additional30 min following which 150 mL of methanolis added over 2.5 hr dropwise (to avoid a violent eruption). The solution isallowed to warm slowly and the additionfunnel and the condenser are removed to allow the ammoniato evaporate overnight. The reaction flask is immersed in an ice bath and 700 mL of water is added cautiously over anhour to the yellow solid left by evaporation of the ammonia.The solution is transferred to a 2-L separatory funneland extracted with two 200-mL portions of ether,which are discarded. The aqueous layer is poured into a mixture of 500 mL of concentrated hydrochloric acidand 1000 g of ice, transferred to a 4-L separatory funnel,and extracted with four 200-mL portions of ether.The combined etherextracts are washed with 200 mL of water and 200 mL ofsaturated aqueous sodium chloride,dried over magnesium sulfate,and concentrated by rotary evaporation at aspirator vacuum. The residual liquidis placed under reduced pressure (0.2 mm) for 1 hr to remove the remainingsolvent to give 137–140 g (80–82%)of an orange oil that is a diastereomeric mixture of which the major componentconstitutes 80%, as indicated by 13CNMR.

【OrganicSyntheses, Coll. Vol. 8, p.302; Vol. 65, p.215】

Na-EtOH体系还原酮示例

In a 3-l. round-bottomed flask, fitted with an efficient Liebig condenser (100 by 1 cm.), 228 g. (2 moles) of methyl n-amylketoneis dissolved in a mixture of 600 cc. of95 per cent alcoholand 200 cc. of water. One hundred thirtygrams (5.6 gram atoms) of sodiumin the form of wire is gradually added through the condenser. During theaddition of the sodiumthe flask is cooled with running water so that the reaction does not becomeunduly violent.

When the sodiumhas dissolved, 2 l. of water is added and the mixture is cooled to 15℃. Theupper oily layer is then separated, washed with 50 cc. of1:1 hydrochloric acidand then with 50 cc. of water, dried over 20 g. ofanhydrous sodium sulfate,and distilled with a fractionating column. After asmall fore-run of low-boiling liquid, the pure heptanoldistils at 155–157.5℃. The yield is 145–150 g. (62–65 per cent ofthe theoretical amount).

【Organic Syntheses, Coll. Vol. 1, p.554; Vol. 7,p.88】

Na （Hg）体系还原酮示例

An amalgamprepared from 9.0 g. (0.39 atom) of sodium and 750 g. (55 cc.) of mercury is warmed to about 50℃in a 500-cc. Pyrex round-bottomed flask. To it is then addeda cold suspension of 25 g. (0.13 mole) of xanthone in 175 cc. of 95 percent ethyl alcohol.The flask is at once stoppered, held in a cloth, and vigorously shaken, the stopper being raised from time to time to release anypressure. The temperature rises rapidly to 60–70℃. The solid xanthonerapidly goes into solution, a very faint and transient blue color beingdeveloped. At the end of about five minutes the alcoholic solution is clear andpractically colorless.

Aftera further ten-minute shaking the mercuryis separated and washed with 10–15 cc. of alcohol.The alcoholic solution is filtered while warm and slowly poured with stirringinto 2 l. of cold distilled water. The precipitated xanthydrolis filtered with suction, washed with water until free of alkali, and dried at40–50℃to constant weight. The crystalline product, which weighs 23–24 g. (91–95 per cent ofthe theoretical amount), melts at 121–123℃.

【Organic Syntheses, Coll. Vol. 2, p.317; Vol. 10,p.60】

Fe-HOAc体系还原酮示例

The combined product ismixed with 1 l. of 20 per cent sodiumhydroxide solution and stirred on the steam bath for four hours, inorder to hydrolyze a small proportion of heptyl acetate.The oil is then separated and distilled, and the portion boiling at 172–176℃iscollected. The residue in the flask is mixed with about 100 cc. of water anddistilled, whereupon a further small quantity of oil passes over with thesteam. This distillate and the forerun are freed of the bulk of the water in a separatory funnel and distilled from a smaller flask. In this way the total yield of n-heptylalcohol boiling at 172–176℃(uncorr.) is 350–370 g. (75–81per cent of the theoretical amount). The alcohol may be redistilled underreduced pressure, when it passes over almost without loss at 71–72℃/12 mm.

In a 12-l. flask fitted with a mechanical stirrer and a shortreflux condenser are placed 1800 g. (32.2 atoms) ofiron filings,3 l. (52.5 moles) of glacial acetic acid,3 l. of water, and 450 g. (3.95 moles) of heptaldehyde.The mixture is heated on the steam bath, withstirring, for six to seven hours. The flask is then fitted to an apparatus forsteam distillation and the mixturedistilled in a current of steam until nomore oil passes over (7–8 l. of distillate). The oil is then separated, and theaqueous part distilled to recover a small quantity of dissolved or suspended heptylalcohol.

【Organic Syntheses, Coll. Vol. 1, p.304; Vol. 6, p.52】

Zn-EtOH体系还原酮示例

In a 3-l. round-bottomed flask fitted with a mechanical stirrer are placed 200g. of technical flake sodium hydroxide,200 g. of benzophenone,2 l. of 95 per cent alcohol,and 200 g. of technical zinc dust. The stirrer is started and the mixture slowlywarms to about 70° spontaneously. After two to three hours the mixture, whichhas started to cool, is filtered with suction, and the residue is washed twicewith 100-cc. portions of hot alcohol

The filtrate is poured into five volumes of ice wateracidified with about 425 cc. of concentrated commercial hydrochloricacid. The benzohydrol separates as awhite crystalline mass and is filtered by suction. The yield of crude air-driedproduct melting at 65℃is 194–196g. (96–97 per cent of the theoreticalamount). From 200 g. of crude product in 200 cc. of hot alcoholthere is obtained, after cooling in an ice-salt mixture, filtering, and drying,140–145 g. of product melting at 68℃. The benzohydrolremaining in the mother liquors may be precipitated with water.

【Organic Syntheses, Coll. Vol. 1, p.90; Vol. 8, p.24】

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