Direct Transformations of Amines to Urethanes
Section 22.214.171.124 presents nonphosgene routes to isocyanates by reactions of amines with dimethyl carbonates. In that process, isocyanate is created without phosgene but rather obtained by reversion of urethane to an isocyanate and an alcohol. In that method, the point is to obtain an isocyanate without employing the dangerous
FIGURE 12.8 Conversion of amine to carbamate by reaction with C02 and subsequent reaction with an alcohol. Polymers can be formed when the amine is a polyamine and the alcohol is a polyol. Box indicates the urethane product.
phosgene reagent. Work along these lines has also produced urethane functionality without isocyanate, which in principle can be extended to produce polyurethanes via simplified reaction and reagent choices.
One such route converts amines, alcohols, and C02 directly to urethanes [25-27]. The reaction is catalyzed and performed at elevated temperature and pressure (Fig. 12.8). Given the instability of the carbamic acid and the relative reaction rates, it is possible that this reaction actually proceeds through an isocyanate intermediate before forming the urethane. The reaction has been optimized to produce high yields and selectivities, however; reaction times tend to be long. The use of mixed poly-amines and polyols offers the potential to produce polymers with hard segments.
A related mechanism employs an onium salt (in the illustration case of Figure 12.9 an ammonium salt) with an alkyl halide to produce a urethane group . As before, the use of mixed polyamines and alternative alkyl halides offers the potential for polymerization and design flexibility.
Beginning with the same first step, urethane has also been produced with good yield and milder conditions using Mitsunobu's reagent [29, 30]. The relatively easier reaction conditions are partially offset by consumption of the reagent, making the reaction potentially expensive (Fig. 12.10). In fact, all reactions transforming amines to urethanes by reaction with C02, and a source of organic moiety to reduce the carbamic acid to the urethane, involve the loss of reagents and the need to separate the
FIGURE 12.9 Reaction of an amine with CO2 to make urethane via an omum salt complex. Polymer can be formed when a polyamine is employed and R' is capable of transurethanification.
FIGURE 12.10 Reaction of amine with CO2 to form urethane using Mitsunobu's reagent. A polyurethane can be produced when a polyamine is employed.
desired urethane from these compounds. Like the reactions of cyclic carbonates and amines to form urethanes discussed in the preceding section, conversion of amines and C02 to urethanes employs reagents with potentially troublesome safety profiles, but in this case, the materials are consumed in the industrial manufacturing step and would not provide a possible route of amine exposure to consumers or unregulated contractors.