Nonphosgene Routes to TDI
Gas-phase phosgenation technology described in Section 220.127.116.11 results in industrial efficiency including reduced use of phosgene. Other pathways of TDI production have been explored to forgo phosgene consumption altogether. While these processes are in no way economically competitive with conventional phosgenation and even less so with gas-phase phosgenation (disadvantaged by about 25 and 35%, respectively), these costs do not reflect the incalculable potential costs associated with a phosgene accident. Three nonphosgene routes to TDI will be covered in the following. The first two have in common that isocyanate functionality is produced by making urethane and then thermally decomposing the urethane to isocyanate and alcohols. The differences are in how the initial urethane is achieved. The last, based on the Curtius reaction, is made by decomposition of an intermediate azide.
Thermolysis of Carbamic Acid,N,N'-(4-Methyl-l,3-Phenylene)Bis-,C,C'-Dimethyl Ester Made from the Reaction of Toluene Diamine with Methyl Carbonate The reaction of toluene diamine with DMC is known to produce the diurethane of TDI (Fig. 2.57) . DMC is a commodity chemical prepared from the oxidative carbonylation of methanol . The reaction proceeds via progressive heating of DMC and the toluene diamine up to about 170 °C. The diurethane product is subsequently heated to produce the diisocyanates and methanol, which is removed and recycled back to form DMC (Fig. 2.58). Yields of TDI are 90% based on input toluene diamine.
FIGURE 2.57 Preparation of TDI by reaction of toluene diamines with dimethyl carbonate followed by thermolysis to the diisocyanates.
FIGURE 2.59 Process for reductive carbonylation for non-phosgene production of TDI. Reprinted with permission from Ref. . © John Wiley & Sons, Inc.
Thermolysis of Carbamic Acid,N,N'-(4-Methyl-l,3-Phenylene)Bis-,C,C'-Dimethyl Ester Made from the Reductive Carbonylation of Dinitrotoluene Catalytic reductive decarbonylation of aromatic nitro compounds to isocyanates has been known since 1967 . Demonstration of this reaction stimulated a significant amount of research on catalysts and cocatalysts to provide ever greater activity and selectivity. It eventually became apparent that while conversion of dinitrotoluene to TDI could occur, it could not occur with sufficient efficiency to provide a commercial process. It was later observed  that with a [Pd(OAc)2 1,10-phenanthroline] catalyst, exceptionally efficient carbonylation of dinitrotoluene to dicarbamate could be effected in an alcohol solvent. The formation of the carbamate can be conceptualized as the formation of the isocyanate and subsequent stabilization via reaction with the alcohol solvent (Fig. 2.59). This concept has been validated by observation of urea formation when the reaction occurs in the presence of amine. The mechanism for such a large and complex transformation is clearly a matter of debate. One simplified
FIGURE 2.60 Simplified proposed mechanism for Pd catalyzed reductive carbonylation to carbamate product .
mechanism involves stepwise catalyst coordination, followed by insertion and elimination reactions, ending in the final carbamate product (Fig. 2.60) [135, 136]. Less expensive selenium catalysts have also demonstrated high conversion to carbamate. After the carbamate is formed, thermolysis is achieved at about 260 °C with methanol removed overhead. Despite high levels of optimization, this route has not achieved sufficient efficiency to make a viable industrial process.
Isocyanates by Thermal Decomposition of Acyl Azides: The CurtiusRearrangement Carboxylic acid chlorides can be converted to isocyanates via prior formation of the carbonyl chloride . The azide can be prepared by initial reaction of the acid chloride with a reagent such as sodium azide or by reaction of the acid hydrazide with nitrous acid. The azide can be converted to isocyanate by heating to about 100 °C (Fig. 2.61) . This reaction has been demonstrated using aliphatic, alicyclic, aromatic, and heterocyclic compounds. The explosion hazards of handling and heating azides makes this reaction unworkable from a practical perspective.
FIGURE 2.61 Non-phosgene preparation of isocyanates via the Curtius rearrangement.
FIGURE 2.62 Illustrative structures of MDI and pMDI. Reprinted with permission from Ref. . © John Wiley & Sons, Inc.