Laboratory synthesis phosgene

Diisocyanates are usually synthesized by phosgenation of the corresponding diamines. A tabulation of diisocyanates synthesized up to 1972 with pertinent methods of synthesis is available (11). Instead of the toxic phosgene gas, the liquid trichloromethyl chloroformate (diphosgene) (12) or the solid bistrichloromethyl carbonate (triphosgene) (13) can be used in the laboratory (see PHOSGENE). Also, oligomeric t-butylcarbonates are used to convert diamines into diisocyanates (14). 

The toxicity of phosgene and thiocarbonyl chloride represents a drawback in the laboratory synthesis of isocyanates and isothiocyanates from amines the use of iminophosphoranes has been shown effectively to circumvent this problem. The key step is the mild reaction of CO2 or CS with the iminophosphoranes (50), which are readily available from the corresponding amines. The reaction appears to be general for both aromatic and aliphatic amines and yields are good (Scheme 60). 

Trichloromethyl chloroformate is useful in synthesis as a substitute for phosgene, which, owing to its high volatility and toxicity, presents a severe hazard in the laboratory. Although trichloromethyl chloroformate is toxic, it is a dense and less volatile liquid, b.p. 128°, d l 1.65, having a vapor pressure of only 10 mm. at 20°. Consequently it is more easily handled in a safe manner than phosgene. 

An extremely mild method for the synthesis of nitrate esters from easily oxidized or acid-sensitive alcohols involves the decomposition of a nitratocarbonate (29). The nitratocar-bonate is prepared in situ from metathesis between a chloroformate (reaction between phosgene and an alcohol) and silver nitrate in acetonitrile in the presence of pyridine at room temperature. Under these conditions the nitratocarbonate readily decomposes to yield the corresponding nitrate ester and carbon dioxide. Few examples of these reactions are available in the literature and they are limited to a laboratory scale. 

Thus, we have concentrated in this Chapter not only upon those reactions which constitute an efficient synthesis of phosgene or labelled phosgene, but also (owing to the exceptional toxicity of the compound) on those reactions which generate even comparatively low yields and which could be conceivably problematical in the chemical plant environment, and in both the organic and inorganic laboratory. 

Chemistry Phosgene can be prepared in the laboratory using a reaction similar to its commercial synthesis. In this gas-phase process, equimolar amounts of chlorine and carbon monoxide are passed over a bed of activated charcoal granules. 

In 1909 the German Chemist Fritz Haber discovered a catalyzed process [13], which allowed the synthesis of ammonia (NH3) from the elements hydrogen and nitrogen. He received the Nobel prize in chemistry for his discovery. The Nobel prize for Fritz Haber was a subject of controversy because Haber is also the inventor of war gas (phosgene COCI2), which killed hundreds of thousands of soldiers in World War I. Conscience-stricken, Haber s wife committed suicide. Carl Bosch succeeded to scale up Haber s synthesis from the laboratory scale to industrial production. After World War I other industrialized countries also introduced ammonia synthesis and therefore the consumption of hydrogen increased rapidly. 

On the industrial scale, carbon monoxide is used as a reducing agent in metallurgical operations, in the refining of metallic nickel, in the synthesis of phosgene, and in the synthesis of a wide variety of organic compounds. In the laboratory, it is used in the preparation of carbonyls and aromatic aldehydes. 

With a few exceptions, interfacial polycondensation has remained a laboratory method for the synthesis of polymers, since diacyl chlorides are too expensive for commercial production. The exceptions include the polycondensation of bisphenols with phosgene (see Section 26.5.1) and the synthesis of aromatic polyamides, from m-phenylene diamine, isophthaloyl chloride, and terephthaloyl chloride (Section 28.2.4). The method is also used to give wool a fluff-free finish by producing a polycondensate from sebacoyl chloride and hexamethylene diamine on the wool fiber. 

Diphenyl carbonate has been chosen as the most convenient phosgene equivalent for the laboratory-scale synthesis (up to 3 mol) of oxazolidin-2-one 711 starting from (IS,2J )-norephedrine 710 [501]. Direct fusion of a 3-amino-D-altritol derivative with diphenyl carbonate to furnish the corresponding oxazolidinone has also been reported [502],... 

Dimethyldithiocarbonate (DMDTC) 786 represents a mild and safely handled reagent structurally similar to phosgene, which is useful in the synthesis of ureas. DMDTC can be prepared from methanol, carbon disulfide, and dimethyl sulfate by a two-step sequence [575, 576[. Although dimethyl sulfate is a suspected human carcinogen, it is relatively non-volatile and with due care can be handled safely in the laboratory. 

Curtius degradation of [l- " C]acyl azides and [ " C]carbonylation of aliphatic and aromatic primary amines have become established methods for the synthesis of [ " C]isocyanates, key precursors for the preparation of labeled ureas, carbamates and thiocarbamates (see also Chapter 7, Section 7.4). Phosgene, classically the most common reagent for preparing such compounds in normal organic synthesis, is less attractive for use in carbon- 14-labeled form because it is difficult to prepare in the laboratory (see Section 5.2), it is costly to purchase. 

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