Carbonyl diisocyanate phosgene

Treatment of trichloroisocyanuric acid (or sodium dichloroisocyanurate) with phosgene at 150-250 "C in 1,3,5-trichlorobenzene or 1,2-dichlorobenzene affords carbonyl diisocyanate, CO(NCO)j, in almost quantitative yield [616] ... 

A mixture of phosgene and carbonyl diisocyanate will undergo a comproportionation reaction at 180 C, to give a 56% yield of the mixed product [1018] ... 

This seems a reasonable mechanism, as the reaction of MCgSiCNCS) with phosgene gives CO(NCS)j [45a] and the reaction of tributyltin(IV) isocyanate with phosgene has been utilized for the synthesis of carbonyl diisocyanate [22a] ... 

Carbonyl chloride (phosgene), 14.11, is a highly toxic, colourless gas (bp 281 K) with a choking smell, and was used in World War I chemical warfare. It is manufactured by reaction 14.50, and is used industrially in the production of diisocyanates (for polyurethane pol5mers), polycarbonates and 1-naphthyl-A -methylcarbamate, 14.12 (for insecticides). 

Reductive carbonylation of nitro compounds (in particular aromatic dinitro compounds) is an important target in industry for making diisocyanates, one of the starting materials for polycarbamates. At present diisocyanates are made from diamines and phosgene. Direct synthesis of isocyanates from nitro compounds would avoid the reduction of nitro compounds to anilines, the... 

The above processes are only selected examples of a vast number of process options. In the case of carbonylation, the formation of by-products, primarily isocyanate oligomers, allophanates, and carbodiimides, is difficult to control and is found to greatly reduce the yield of the desired isocyanate. Thus a number of nonphosgene processes have been extensively evaluated in pilot-plant operations, but none have been scaled up to commercial production of diisocyanates primarily due to process economics with respect to the existing amine—phosgene route. Key factors preventing large-scale commercialization include the overall reaction rates and the problems associated with catalyst recovery and recycle. 

This process was elaborated as a heterogeneously catalyzed variation by Asahi Chemicals (Japan) in order to open a new route to diisocyanates, not depending on the use of phosgene [120, 134]. Ethyl phenylcarbamate, which in a first step is obtained by catalytic oxidative carbonylation of aniline, CO, oxygen, and ethanol (eq. (17)), is condensed with aqueous formaldehyde to yield methylene diphenyl diurethane. Thermal decomposition leads to methylene diphenyl diisocyanate (MDI), which is one of the most important intermediates for the industrial manufacture of polyurethanes (eq. (18)). The yields and selectivities of the last reaction step seem to be the main reasons why this process is still inferior to the existing ones. 

Because of the increased commercial interest in diisocyanates [85,91], new manufacturing routes without the costly phosgenation step (i.e, without total loss of chlorine as HC1) have been developed. Processes for the catalytic carbonylation of aromatic nitro-compounds or amines are the most likely to become commercially important. 

Phosgene produced by chlorinating carbon monoxide is used as a carbonylating agent to convert amines to isocyanates, as shown by reactions (8) and (9). Thus, the reaction of phosgene with diphenylmethane diamine results in the formation of methylene diphenyl diisocyanate (MDI), and with toluenediamine to form toluene diisocyanate (TDI). The isocyanates are used to produce polyurethanes for flexible and rigid foams, elastomers, coatings, and adhesives, for the construction and automotive industries. 

Many examples of phosgene-free processes, mainly concerning the carbonylation of aryl nitro derivatives, have been claimed. Some of the most representative, cited in part in the Kirk-Othmer Encyclopedia of Chemical Technology (4th ed., vol. 19), are illustrated belotv. 2,4-Dinitrotoluene undergoes reductive carbonylation with CO to form 2,4-toluene diisocyanate (TDI) in the presence of palladium catalysts [209-213]. A variation on this process involves capturing the isocyanate formed with methanol, followed by thermolysis of the bis(carbamate) 326 [212]. 

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