Isocyanate generation from amines, carbon

Generation of Organic Isocyanates from Amines, Carbon Dioxide, and Electrophilic Dehydrating Agents... 

Heating A, Af -diphenylurea with amines (butylamine, cyclohexylamine, benzylamine, 2,6-diethylaniline or morpholine) in DMF in the presence of triethylamine yields unsym-metrical ureas, e.g. PhNHCONHBu from butylamine . High yields of isocyanates RNCO are obtained in the reactions of primary aliphatic amines, carbon dioxide and triethylamine with phosphorus oxychloride . Treatment of secondary amines with butylUthium, followed by carbon monoxide at —78 °C under atmospheric pressure, sulphur and an alkyl halide affords 5-alkyl thiocarbamates, e.g. Et2NC(0)SCH2Ph from diethylamine and benzyl bromide. The key step in the sequence is the generation of the lithium thiocarbamate shown in equation 112 . 

Generation of Urethanes and Isocyanates from Amines and Carbon Dioxide... 

In previous accounts the use of electrophilic dehydrating agents in the direct generation of isocyanates from amines and carbon dioxide has been discussed, equations 1-2 (1-3). 

One-part urethane adhesives have been used for many years as high performance sealants. In this capacity they provide a useful combination of strength, flexibility, and elastic recovery. As adhesives, these systems have limited use im-less formulated to overcome their inherent disadvantages. One-part polyurethane adhesives are typically moisture-cured and rely on a multistep reaction sequence as follows isocyanate reacts with water to form carbamic acid, the unstable car-bamic acid loses carbon dioxide and generates an amine, the amine reacts with additional isocyanate to form a urea, and the urea reacts with additional isocyanate to form a biuret, which includes a cross-link. Unless it diffuses out of the system, the CO2 can cause foaming. Formulators learn to minimize the isocyanate content (%NCO) of a system in order to balance cure speed with foam control. Cure speeds—and foaming rates—of these systems decrease from the outside in and vary with the amount of atmospheric moisture in the air, which changes hourly and seasonally. 

A very mild method for the preparation of isocyanates from primary amines (RNH2) and carbon dioxide (CO2) involves the use of a Mitsunobu zwitterion generated from either diisopropyl azodicarboxylate (DIAD) or di-tert-butyl azodicarboxylate and triphenylphosphine or tri-n.-butylphosphine. 

Compound 370 can also be obtained by direct cycloaddition of the isoquinoline to the pyrazole isocyanate (373), which is generated in situ from the corresponding amine 371 and (bischloromethyl)carbonate 372 (Scheme 134) (94UP1). 

It is the control of the reactivity of this carbamate anion which has allowed us to generate in quantitative yields either urethanes or isocyanates directly. By understanding those factors which promote the nucleophilicity of the carbamate oxygen, we have discovered how to generate urethanes directly from either primaiy or secondary amines (Activated Carbon Dioxide Chemistry-I (ACDC-I) (12, 13, 15-18). A key aspect of this chemistry was our discovery that carbamate anions can serve as excellent nucleophiles, when a suitable tertiary amine base is utilized to generate the carbamate anion in the reaction of carbon dioxide with a primary or secondary amine (equation 4). By proper choice of the tertiary amine co-base, which in general does not react with CO2 under the mild conditions employed. 

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