Use of phosgene

The Germans were the first to use gas, against the Allies, at Ypres in Belgium [687,731,859a,1032,1711a,2155], The following account owes much to the descriptions of 

The reason why it was not entirely successful was probably that both the troops and the Command had become nervous as a result of the many abortive attempts, and also that by then there were no longer sufficient reserves available to consolidate the gains.  

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Attempts have been made to develop methods for the production of aromatic isocyanates without the use of phosgene. None of these processes is currently in commercial use. Processes based on the reaction of carbon monoxide with aromatic nitro compounds have been examined extensively (23,27,76). The reductive carbonylation of 2,4-dinitrotoluene [121 -14-2] to toluene 2,4-diaLkylcarbamates is reported to occur in high yield at reaction temperatures of 140—180°C under 6900 kPa (1000 psi) of carbon monoxide. The resultant carbamate product distribution is noted to be a strong function of the alcohol used. Mitsui-Toatsu and Arco have disclosed a two-step reductive carbonylation process based on a cost effective selenium catalyst (22,23). 

An important direct use of phosgene is in the preparation of polymers. Polycarbonate is the most significant and commercially valuable material (see Polycarbonates). However, the use of phosgene has been described for other polymer systems, eg, fiber-forming polymeric polyketones and polyureas (90,91). 

For environmental reasons there has been interest in methods for manufacturing isocyanates without the use of phosgene. One approach has been to produee diurethanes from diamines and then to thermal eleave the diurethanes into diisocyanates and alcohols. Although this method has been used for the production of aliphatic diisocyanates such as hexamethylene diisocyanate and isophorone diisocyanate, for economic reasons it has not been adopted for the major aromatic isocyanates MDI and TDI. 

Taking the manufacture of polycarbonates as an example, two manufacturing routes are shown in Scheme 3.1. In the first route phosgene is reacted with bisphenol A in dichloromethane. The main environmental concern with this process is the large-scale use of phosgene. In the second... 

Preparation of SC-PEG. Warning Synthesis of this reagent involves the use of phosgene, a highly toxic substance. All manipulations should therefore be performed in a well-ventilated hood. 

The formation of peptides from Leuchs anhydrides proceeds without the use of protecting groups, and no racemisation occurs. However, the use of phosgene cannot be considered a prebiotic process ... 

Figure 25.4 An SC derivative of mPEG was first prepared through the use of phosgene to form a chloroformate intermediate. Reaction with NHS gives the amine-reactive SC-mPEG.

The phase-transfer catalysed reaction of alkyl halides with potassium carbonate in dimethylacetamide, or a potassium carbonate/potassium hydrogen carbonate mixture in toluene, provides an excellent route to dialkyl carbonates without recourse to the use of phosgene [55, 56], An analogous reaction of acid chlorides with sodium hydrogen carbonate in benzene, or acetonitrile, produces anhydrides (3.3.29.B, >80%), although there is a tendency in acetonitrile for aliphatic acid chlorides to hydrolyse yielding the acids [57]. 

The traditional use of phosgene in Scheme 4 can be avoided by substituting it with the less toxic triphosgene, which gives comparable yields of the M-car-boxyanhydrides. Diphosgene may also be used to form the NCA, but the reaction requires the use of charcoal and is not as reliable. Free amino acids have also been converted to their corresponding NCAs by the use of benzyl chloroforma-te with thionyl chloride. 

Uses of phosgene include the manufacture of toluene diisocyanate (45%), methylene diphenyl diisocyanate and polymer diisocyanates (38%), and polycarbonate resins (12%). 

Daicel Chemical Industries in Japan patented a promising phosgene-free process involving the reaction of an aliphatic diamine with dimethyl carbonate (DMC) to produce carbamate esters, which are then thermally converted to the corresponding aliphatic diisocyanates [38] (Scheme 5.4). It is noteworthy that this process could be a total phosgene-free process since the reactant, DMC, can be made directly from methanol and carbon dioxide (or urea) and eliminates the use of phosgene [39]. 

Naumov. Isakova Kost Zakharov Zvolinskii Moiseikina Nikeryasova J. Org. Chem. USSR 1975, II. 362-,uFor the use of phosgene to carboxylate phenols, see Sartori Casnati Bigi Bonini Synthesis 1988, 763., uHalcs Jones Lindsey J. Chem. Soc. 1954, 3145. 

In this chapter the processes used for the synthesis of either acyclic or cyclic carbonates, as alternatives to the use of phosgene, will be considered. Details of both the rich patent and scientific literature that is appropriate to the subject will also be included. 

The use of phosgene in the absence of solvents or in the presence of an inert solvent has not been widely applied because of the undesirable lack of removal of the hydrogen chloride produced, but, in all the cases described, cyclic carbonates were obtained. 

Acylation of pentafluorobenzene to form ketones either with acid halides or with anhydrides has been achieved with excess SbFg [22]. Use of phosgene in place of acid chlorides results in the formation of pentafluorobenzoic acid in good yield [4]. 

Most of the conventional reagents for the synthesis of acid chlorides from carboxylic acids are unsatisfactory for the preparation of a-keto acid chlorides. For example, the reaction of pyruvic acid with phosphorus halides does not give pyruvoyl chloride whereas the use of phosgene or oxalyl chloride affords ether solutions of the acid chloride in low yield. Recently a useful preparation of pyruvoyl chloride from trimeth-ylsilyl pyruvate and oxalyl chloride has been described."... 

Cyclic carbonates are introduced through the use of phosgene or a phosgene equivalent and a base (pyridine). They are most stable to acid hydrolysis and are removed under rather strongly basic conditions. Interestingly, they can protect both cis- and frani-vicinal diols. 

This is the end of volume 1 mainly dedicated to the use of phosgene and its direct derivatives as building blocks providing the carbonyl group in organic molecules. 

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. 

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