Safe handling phosgene

Personnel protection and safe handling of phosgene 3.1.5.1 Safety procedures for chemical processing plants... 

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. 

All the precautions necessary for the safe handling, shipping and storage of any compressed gas cylinder must be observed with phosgene cylinders. 

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. 

DMC is classified as a flammable liquid, smells like methanol, and does not have irritating or mutagenic effects, either by contact or inhalation. Therefore, it can be handled safely without the special precautions required for the poisonous and mutagenic methyl halides and DMS, and extremely toxic phosgene. Some physicochemical properties of DMC are listed in Table 4.2. 

TBS-protected lactaldehyde 689 is readily prepared with high enantiomeric purity by a mild oxidation of the propanol 688 under Swem-type conditions using bis(trichloromethyl)car-bonate (triphosgene)-DMSO [203]. Triphosgene is a white crystalline solid that is easily handled, and it is a safe alternative for such other DMSO activators as phosgene or diphosgene dimer. 

Carbamates are generally prepared from isocyanates or chloroformates (see Scheme 4.2). Since isocyanates and chloroformates are usually prepared with phosgene, the corresponding synthetic routes are subject to the relevant safety considerations regarding the phosgene source (for a safe phosgene source, see Chapter 7). Moreover, the handling of several lower alkyl isocyanates is associated with a similar hazard level (toxicity), and therefore the search for a less hazardous procedure has been intense. 

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