Phosgene as a reagent

The traditional synthesis of DMC used to require toxic, hazardous phosgene as a reagent. Nonphosgene alternative routes for DMC production basically have... 

The most widely utilized method for the synthesis of isocyanates, the phosgenation of amines, uses phosgene as a reagent. 

Finally, polymer-supported oxime 12 has served as a reagent for trapping primary amines after treatment with phosgene [22] to furnish intermediate oxime carbamates 13 (Scheme 5) [23], Thermolysis in the presence of amines results in release of ureas into solution. 

Phosgene is the acid chloride of carbonic acid. Although phosgene was used as a war gas in World War I, it is now used as a reagent for the synthesis of many useful products. Phosgene reacts like other acid chlorides, but it can react twice. 

Phosgene is widely used in organic chemistry as a building block providing the C = O such as in carbamates, carbonates, isocyanates, ureas, heterocycles etc., or as a reagent for chlorination, dehydration, alkylation, de-alkylation, protection and activation etc. 

In addition to those mentioned above, phosgene has been used as a reagent in making the following commercial dyes [825] Benzo Fast Orange, Benzo Fast Red, Benzo Fast Scarlet, Benzo Fast Yellow, Brilliant Sulphon Red B, Cotton Yellow, Ethyl Violet, Helindon Yellow, Chlorazol Fast Scarlet and Chlorazol Fast Orange. 

Because of the ease with which this cyclization reaction occurs, phosgene has been used as a reagent for the enantiomeric resolution of 1,2- and 1,3-diols [1151,1152] ... 

The iV-substituted carbamoyl chloride is considered to be the intermediate of this reaction [626]. W-methyl-2-amino acids [1152] and N-benzyloxy-2-amino acids [22] react simiiarly to give the corresponding oxazolidine-2,5-diones. Phosgene has been used as a reagent for the enantiomeric resolution of iV-methyl-2-amino acids by gas chromatography [1152],... 

Of course, unlabelled phosgene can be employed as a reagent in the chemically normal sense to prepare a radioactively labelled material, where the co-reactant itself is labelled. Thus, the antitumour agent, (10.46), was synthesised by the cyclocondensation of Me N CN with COClj in the presence of HCl (see Section 10.2.4.3), followed by condensation with methylamine, to give the molecule labelled at the positions shown. The reaction with COClj resulted in total labelling of the carbon atoms in the triazine [542]. 

Phosgene, unfortunately, retains the reputation of being a very unpleasant chemical, due to its early use as a war gas. Whilst undeniably toxic, like its precursors carbon monoxide and chlorine, it can be simply contained and easily converted into harmless added-value products, which contribute to our quality of life. Today we may consider it fully redeemed by its widespread use as an intermediate for producing the ubiquitous and useful polyurethanes, as well as contributing on a lesser scale as a reagent for s)mthesis of a number of valuable pharmaceuticals and crop protection agents. 

Oxazolidinones are commonly prepared from amino alcohols by incorporation of a carbonyl unit. Additions to the list of reagents that effect the transformation are bis(trichloromethyl)carbonate <89S875> and trichloromethyl chloroformate <89JOC323l>, which offer the advantages of easier handling and reduced risk of exposure compared to phosgene. As a further step in that direction,... 

Gyllenhaal, O. and J. Vessman. 1988. Phosgene as a derivatizing reagent prior to gas and liquid chromatography. /. Chromatogr. 435 259-269. 

Cyclic carbonates can be prepared by chemical fixation of CO2, which is a much more environmentally acceptable process than that using phosgene. The formation of propene carbonate has been achieved in CO2-IL systems, whereby CO2 serves as a reagent rather than a reaction medium [Eq. (9)] [49]. The production of cyclic carbonates has suffered in the past from serious disadvantages of separating the catalyst [50, 51], while the activity of heterogeneous catalysts is generally very poor [52]. 

The reactivity of DMC toward nucleophilic compounds is somewhat lower than that observed when analogous reactions are performed with phosgene and DMS. However, both carboxylation with phosgene and methylation with DMS generate stoichiometric quantities of inorganic salt as a byproduct because a base must be used as a reagent. The corresponding processes with DMC do not involve disposal problems since no salts are produced and the co-product methanol can be easily recycled in the DMC production plant (4). 

Phosgene (a toxic and corrosive gas) is inconvenient and sometimes hazardous to use as a reagent. Several phosgene substitutes have been developed. 

Phosgene is widely used as a reagent in commercially important reactions, such as formation of polycarbonate plastics (Figure 18.19 ) ... 

The DuPont process not only eliminates the use of phosgene as a starting material, but also avoids the production of large amounts of hydrochloric acid as an unwanted by-product. In this method, methyl-amine reacts with carbon monoxide to yield the corresponding aldehyde, which is then catalytically converted to isocyanate. This phosgene-free replacement synthesis also supports the trend in the chemical process industry to seek to reduce inventories on plant sites of hazardous synthetic reagents. Methyl isocyanate produced from this process is converted in situ to an agrochemical product. 

The procedure has the advantage of avoiding phosgene [513-517] as a reagent, and is preferable to that using CDI [518, 519], which only gives satisfactory yields with cysteine. 

The general advantage of using carbon tetrachloride or phosgene is that these compounds decompose at the reaction temperature to provide a uniform distribution of active carbon or carbon monoxide and chlorine at the reaction sites over the oxide surface. These reagents are, however, not as convenient to use as a carbon and chlorine mixture in large-scale operations. Besides, phosgene is poisonous. 

The reagent is prepared1 by reaction of phosgene with 3,5-dioxo-4-methyl-l,2,4-oxadiazolidine in refluxing toluene (82% yield). It serves as a coupling reagent for esterification and preparation of amides and carbamates. 

---