Phosgene reactions

Specialty Isocyanates. Acyl isocyanates, extensively used in synthetic appHcations, caimot be direcdy synthesized from amides and phosgene. Reactions of acid haUdes with cyanates have been suggested. However, the dominant commercial process utilizes the reaction of carboxamides with oxalyl chloride [79-37-8]. CycHc intermediates have been observed in these reactions which generally give a high yield of the desired products (86). 

Phosgenation. Reaction of phosgene with arylamines to form ureas, and with reactive aryl species to form substituted hen zophen ones, are special cases of acylation. They are dealt with separately siace a more specialized plant is required than for other acylations. Urea formation takes place readily with water-soluble arylamines by simply passiag phosgeae through a slightly alkaline solutioa. An important example is carbonyl-J-acid from J-acid. 

This arrangement permits the phosgene reaction to be carried out conveniently and without danger, provided that a good hood and an exhaust fan are available. A slight vacuum is maintained in the system. The excess phosgene is absorbed in 20 per cent sodium hydroxide solution, E. 

Thermal degradation in contact with flame or red hot surfaces will produce highly-toxic gases, e.g. acid chlorides and phosgene. Reaction with freshly-galvanized surfaces may produce dichloroacetylene, which is also highly toxic. 

Aromatic polycarbonates are currently manufactured either by the interfacial polycondensation of the sodium salt of diphenols such as bisphenol A with phosgene (Reaction 1, Scheme 22) or by transesterification of diphenyl carbonate (DPC) with diphenols in the presence of homogeneous catalysts (Reaction 2, Scheme 22). DPC is made by the oxidative carbonylation of dimethyl carbonate. If DPC can be made from cyclic carbonates by transesterification with solid catalysts, then an environmentally friendlier route to polycarbonates using C02 (instead of COCl2/CO) can be established. Transesterifications are catalyzed by a variety of materials K2C03, KOH, Mg-containing smectites, and oxides supported on silica (250). Recently, Ma et al. (251) reported the transesterification of dimethyl oxalate with phenol catalyzed by Sn-TS-1 samples calcined at various temperatures. The activity was related to the weak Lewis acidity of Sn-TS-1 (251). 

This is a modification of a synthetic method reported by Sahu. This method can be adapted for the synthesis of the isocyanides prepared in Section 6. A. Substitution of triphosgene for phosgene provides a safer alternative for the synthesis of isocyanides, in general. On the other hand, phosgene is less expensive and the phosgene reaction is easier to scale up. p-H2N(CgH2(CH3)2)NH2 is converted to the corresponding formamide by the procedure outlined in Section 6.A. 

The trichloromethylperoxyl radical adds to the iodide ion [reaction (39)] with subsequent decomposition into the trichloromethoxyl radical [reaction (40)] which is further reduced by iodide into trichloromethanol [reaction (41) Bonifacic et al. 1991]. Its decay is much faster [reaction (42), k > 8 x 104 s 1] than the subsequent hydrolysis of phosgene [reaction (43), k = 9 s 1 at 25 °C, / a = 53 kj mol1 Mertens et al. 1994]. 

Azine approach. DCC dehydration of the 3-oxoquinazoline-4-hydroxamic acid (602) gives an isocyanate (603) via a Lossen rearrangement addition of the AT-oxide oxygen to the isocyanate group effects the cyclization. The same product is formed by the phosgene reaction with 4-amino-2-methylquinazoline 3-oxide (76TL3615). 

The only example of an O-(chloroformyl) carbohydrate isolated from a base-catalyzed phosgene reaction was supplied by Freudenbei and his... 

Table 1-3 Examples of phosgene reactions to introduce the group >C=0.

We introduced these guanidinium salts in a 1985 patent (Ref. 6) on the conversion of carboxylic acids to acid chlorides with phosgene. In this process, only 0.02 mol. % of HBGCI was required, two orders of magnitude less than the quantities of other catalysts typically used. Many new other applications including phosgene reactions with phenols, thiols, aldehydes, epoxides or O-demethyla-tion methods have been developed later and are discussed in this book. 

Another example is the Friedel-Crafts phosgene reaction with 0.xylene giving 3,3, 4,4 -tetramethylbenzophenone in high yield and free of isomers (Ref. 14). This substituted benzophenone is easily oxidized into benzophenone tetra-carboxylic acid dianhydride (BTDA) widely used for the manufacture of polyimides [Scheme 18] ... 

Concerning phosgene reactions with active hydrogen substrates [Scheme 5] ... 

The photolysis of phosgene in the presence of ethene gives results which are quite different from those obtained by the photochlorination of ethene with molecular chlorine, in which the main reaction product is 1,2-dichloroethane. The proposed mechanism for the phosgene reaction is given below [2185] ... 

Although acid chlorides can be produced in a similar way from the reactions of sulfinyl chloride (SOClj) or phosphorus(V) chloride with RC(0)0H, the by-products obtained from the phosgene reaction are less troublesome and are more amenable to disposal. 

This reaction is much less efflcient than the analogous phosgene reaction (see Section 9.1.4.2), owing to the occurrence of the following side reactions [899,900] ... 

One of the most important uses of phosgene is the production of isocyanate for polyurethane. For example, toluene diisocyanate is produced by the reaction of diaminotoluene with phosgene [reaction (16)]. 

Polyureas (8). Many synthetic methods for preparation of polyureas are known. Examples of reactions that can be used for the preparation of a variety of polyurea structures include reaction of diamines with phosgene (Reaction 21), with urea (Reaction 22) or with diisocyanates (Reaction 23). 

Polycarbonates (. Polycarbonates may be prepared by a variety of synthetic methods. Procedures most commonly used include reaction of diols with phosgene (Reaction 2A), transesterification of diesters of carbonic acid with dihydroxy compounds (Reaction 25), and polymerization of cyclic carbonates (Reaction 26). 

A further variant can be used with strongly basic aliphatic amines and has proved especially valuable for diamines whose hydrochlorides do not react with phosgene first, dry carbon dioxide is led into a solution of the amine in a solvent suitable for the phosgene reaction, this giving a carbamic acid salt ... 

Carbon monoxide purity requirements vary considerably depending on end products. By-products formed during the production of phosgene can have a deleterious effect on downstream products formed by the phosgenation reaction. In general, the requirements for polycarbonates are more stringent then for isocyanates. The recommended CO feedstock specifications for each downstream product are given in the section for each product. 

An alternate synthesis was developed by Etablis-sements Clin-Byla which is based on the decarboxylation by heating of the aminoalkyl ester of the N-carboxy-phenothiazine, Specifically, 2-propionylphenothiazine is converted into its N-carbonyl chloride derivative by reacting with phosgene. Reaction of the chloride with 2-dimethylamino-l-propanol yields the ester hydrochloride, which is decarboxylated by heating (See Figure 7). 

The extension of this methodology to the synthesis of poly isocyanates of commercial interest has been done. Importantly, due to the extremely mild conditions under which this chemistry can be conducted, there are many potential substrates which can be converted to their isocyanates which could never survive the conditions present in a phosgenation reaction. Examples of poly isocyanates and unique isocyanates are shown below in Figure 3 all examples shown here have been prepared by the general methods and conditions documented in the examples shown in Table III and they have been prepared in >90% selectivities. 

---