Cyanides, addition onto

Urea. 50 g of potassium cyanide is heated in an iron cmcible under a large flame until it begins to fuse. 140 g of red lead is added in small portions, while the mixture is stirred with a rod. When the addition is complete and the frothing has stopped, the fused mass is poured onto an iron tray. When cold, the mass is separated from metallic lead, ground up, and digested with 200 cc of cold water for 1 hour. The filtrate from this mixture is treated with 25 g of ammonium sulphate and evaporated to dryness on a water bath. The residue is powdered well, transferred to a flask and is boiled with 3 installments of ethanol under reflux, to dissolve the urea from the potassium sulphate. Each of the 3 ethanol extracts is filtered, then combined, and evaporated to a small bulk, until crystals of urea separate on cooling and standing. Mp 132°. 

Initial preparative work with oxynitrilases in neutral aqueous solution [517, 518] was hampered by the fact that under these reaction conditions the enzymatic addition has to compete with a spontaneous chemical reaction which limits enantioselectivity. Major improvements in optical purity of cyanohydrins were achieved by conducting the addition under acidic conditions to suppress the uncatalyzed side reaction [519], or by switching to a water immiscible organic solvent as the reaction medium [520], preferably diisopropyl ether. For the latter case, the enzymes are readily immobilized by physical adsorption onto cellulose. A continuous process has been developed for chiral cyanohydrin synthesis using an enzyme membrane reactor [61]. Acetone cyanhydrin can replace the highly toxic hydrocyanic acid as the cyanide source [521], Inexpensive defatted almond meal has been found to be a convenient substitute for the purified (R)-oxynitrilase without sacrificing enantioselectivity [522-524], Similarly, lyophilized and powered Sorghum bicolor shoots have been successfully tested as an alternative source for the purified (S)-oxynitrilase [525],... 

The asymmetric addition of small molecules such as hydrogen cyanide onto C=0 bonds or water and ammonia across C=C bonds is typically catalyzed by lyases. During such a reaction one or (depending on the substitution pattern of the substrate) two chiral centers are created from a prochiral substrate. 

Hydroxynitrile lyase enzymes catalyze the asymmetric addition of hydrogen cyanide onto a carbonyl group of an aldehyde or a ketone thus forming a chiral cyanohydrin [1520-1524], a reaction which was used for the first time as long ago as 1908 [1525]. Cyanohydrins are rarely used as products per se, but they represent versatile starting materials for the synthesis of several types of compounds [1526] ... 

The catalyst precursors were prepared by deposition-precipitation of complex cyanides onto oxidic supports. Precipitation was effected by addition of a solution of a soluble complex cyanide to a suspension of the powdered support in a solution of a simple metal salt. 

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