Urethane/urea chemistry isocyanates

Figure 4.1 Representative urethane/urea chemistry reactions (a) between isocyanate and hydroxyl groups (to form a urethane bond) (b) between a cyclic carbonate activated carbonate, carbamate, or chloroformate derivative group and amino group (c) between isocyanate and amino groups (to form a urea bond) (d).

Can be synthetic polymers or natural-based polymers XH = -OH or -NH2 Figure 4.12 Application of urethane/urea chemistry in polymer functionalization and cross-linking (a) introduction of functional groups using functionalized monoisocyanates (b) introduction of isocyanates as functional groups (c) application of urethane/urea reaction as a cross-linking method. 

The preparation of acyl azides has received considerable attention due to the value of these compounds as synthetic intermediates. In the Gurtius rearrangement for example, acyl azides arc converted into isocyanates, urethans, ureas and amines and this aspect of the chemistry of acyl azides is considered in detail in a later chapter. The use of acyl azides in peptide synthesis has increased the scope of general... 

This section of the chapter will present the basic reactions found in the chemistry of polyurethane compounds, such as the reaction of isocyanates with polyols, water, and amines. The reactions of isocyanates with urethanes, ureas, and amides are also of significant importance in poljrurethane chemistry as they will lead to an increase in materials choice. 

In amino acid and peptide chemistry, the di-ferf-butyl dicarbonate (BocaO) is an extensively used reagent for the clean and rapid Boc-protection of amine functionalities.It is also an efficient ferf-butoxycarbonylating agent for alcohols, thiols, and carbon nucleophiles, and it has been used for the conversion of amines into isocyanates, carbamates, and urea derivatives.The reaction of amino acids with chloroformates to produce N-urethane-protected amino acids if not performed under optimal conditions is accompanied by the formation of N-protected oligomers this has been well documented in the case of and... 

The polyurethanes, although not used in large volumes, have specific applications of value for aerospace. Accordingly, a brief discussion of the chemistry and the material applications is in order. There are three chemical reactions of importance the reaction of hydroxyl groups with isocyanates to form urethanes (Reaction 1), the reaction of isocyanates with water to form amines (Reaction 2), and the reaction of amines with isocyanates to form ureas (Reaction 3). 

Biuret n (allophanamide, carbamylurea) NH2CONHCONH2 H2O. A white crystalline material derived from urea by heat or by reaction with an isocyanate. It is used primarily in analytical chemistry, but the biuret group is formed during some polymerization reactions, such as primary bonding in urethane elastomers. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. 

Even though the area of isocyanates chemistry becomes more and more wide, the largest majority of applications still remains based on the classic addition reactions leading in general to the appearance of urethane or urea groups. Numerous works were dedicated to the elucidation of the mechanism of these fundamental reactions. However, many fundamental problems are yet unclear so far. 

Preparation of Amines. Of the many ways of converting azides to amines, the direct hydrolysis of the intermediate isocyanates would appear to be the most efficient, since the reaction is much more rapid than the hydrolysis of either urethans or ureas. Nevertheless, this route to the amine has seen relatively little use, partly because isocyanates are converted by water quite easily to sym-ureas. It is wise not to risk valuable compounds, available in small amount only, with this method. No one method for converting azides to amines can be said to be the best, and the method must be chosen with due r ard for the chemistry of the other functional groups in the molecule. 

---