Chlorin phosgene equivalents, chlorinated

C = aliphatic primary, secondary or tertiary carbon atom, aryl or heteroaryl carbon atom carbonyl carbon atom CPE = chlorinated phosgene equivalent... 

The following section highlights some selected recent applications of the use of phosgene equivalents in the preparation of Vilsmeier-type chlorinated derivatives of amides and ureas. Thionyl chloride, carbonyl bromides, phosphorus oxychloride, phosgene, triphosgene, oxalyl chloride, and p-toluenesulfonyl chloride are all efficient oxophilic promoters capable of generating Vilsmeier-type chloro imi-nium ion intermediate 1770 by reaction with formamides, particularly dimethyl-formaraide, and ureas. 

The addition reaction of the phosgene equivalent 1,1-dichloromethyl methyl ether (Chloromyl ) to alkenes to afford yS-chlorinated aldehydes has been used for the preparation of pyrethroid intermediates [236, 237]. 

Both reagents have proved to be useful substitutes for phosgene in all its main applications. Indeed, they are sold commercially as the efficient equivalents of 2 and 3 phosgene molecules respectively in processes yielding chloro-formates, carbonates, carbamates, ureas and isocyanates, as well as in chlorinations, carboxylations and dehydrations (For a recent review of Triphosgene use in organic synthesis, see Ref. 33). 

The resuiting chloride is determined by titration with standardized siiver nitrate soiution, according to established methods (c/. Sections 3.2.1.1 and 3.2.4), to provide the total phosgene composition. In most cases, this method is sufficiently accurate, because free dichlorine and hydrogen chloride, which would interfere with the test are present in only low concentration. If, however, free dichlorine or HCl are present (see below), the dichlorine equivalent is deducted from the total chlorine and the difference calculated to the percentage value of phosgene. 

Therefore, diphosgene can be regarded as being equivalent to two molecules of phosgene, into which it is slowly decomposed when pure [58]. One of the internal trichloromethyl chlorine atoms probably plays the Nu role in a four-membered cyclic transition state. A continuous stream of phosgene is also emitted when trichloromethyl chloroformate is simply heated under reflux [55]. 

Dimethyl carbonate (DMG) is a Uquid equivalent of phosgene (mp 2-4 °G bp 90 °C). Reported toxicity and ecotoxicity data lead to the classification of DMC as both a non-toxic and environmentally benign chemical [116, 117] (see also Chapter 3). The areas in which DMC serves, or can serve, as an actual or potential phosgene substitute correspond to the main areas of phosgene industrial applications, e.g. the production of aromatic polycarbonate and isocyanates, leading the production of these important chemicals out of the chlorine cycle [117]. However, one major aspect has to be considered in this context, i.e. the production of DMC itself, since the traditional process for DMC production has involved phosgene as a raw material [118]. 

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