Catalysis carbodiimides

Schollenberger added 2% of a polycarbodiimide additive to the same poly(tetra-methylene adipate) urethane with the high level of acid (AN = 3.66). After 9 weeks of 70°C water immersion, the urethane was reported to retain 84% of its original strength. Carbodiimides react quickly with residual acid to form an acyl urea, removing the acid catalysis contributing to the hydrolysis. New carbodiimides have been developed to prevent hydrolysis of polyester thermoplastics. Carbodiimides are also reported to react with residual water, which may contribute to hydrolysis when the urethane is exposed to high temperatures in an extruder [90]. 

Carbodiimide functionality can be produced by reacting isocyanates at elevated temperature with proper catalysis (Scheme 4.15). Although carbodiimides undergo a variety of reactions,23 most commonly as dehydrating agents, in the presence of excess isocyanate they will form uretone imines. This not only increases the average functionality of the isocyanate product but also lowers its freezing point. For example, a liquefied (or modified) version of 4,4,-MDI can... 

The combination of carboxyl activation by a carbodiimide and catalysis by DMAP provides a useful method for in situ activation of carboxylic acids for reaction with alcohols.10 The reaction proceeds at room temperature. Carbodiimides are widely applied in the synthesis of polypeptides from amino acids. The proposed mechanism for this esterification reaction involves activation of the acid via isourea 28 followed by reaction with another acid molecule to form anhydride... 

Carbodiimides are known to react with primary amines to form guanidines.f In the case of amino acid esters, a subsequent elimination of alcohol is possible with formation of an imino-hydantoin species (Scheme 8). However, these reactions usually require higher temperatures or the occurrence of acid catalysis and therefore can be neglected when unprotonated amino acid derivatives are acylated using carbodiimides. On the other hand, this side reaction becomes more important when protonated amino acids (i.e., amino acid ester hydrochlorides) are coupledf and these byproducts may occur. 

Unsymmetrical carbodiimides. Isocyanides react with primary amines under catalysis of NiCh. Air is the oxidant. 

The titanium catalysts have been widely used in the catalysis of polymerization of olefins [335], the details of their applications will not be discussed here. Chiral titanium complexes have also been employed as the catalysts for asymmetric polymerization of a variety of carbodiimides by Novak [336]. As shown in Scheme 14.149, the polymerization of achiral carbodiimides in the presence of a chiral titanium complex derived from 3,3 -Br2BINOL and Ti(O Bu)4, gave the well-defined regio- and stereoregular helical polyguanidines with a relatively... 

Substituted (indol-2-yl)-ct-allenols show divergent patterns of reactivity under metal catalysis and an intramolecular 1,3-iodine migration has been reported (Scheme 208). Computational methods have addressed rearrangements of acyl, thioacyl, and imidoyl (thio)cyanates to iso(thio)cyanates, acyl iso(thio)cyanates to (thio)acyl isocyanates, and imidoyl iso(thio)cyanates to (thio)acyl carbodiimides, RCX-YCN RCX-NCY RCY-NCX RCY-XCN (X and Y = O, S, NR ). ... 

The cyclotrimerization of carbon cumulenes is usually initiated by heat or catalysis. Especially, the use of a catalyst assures that trimerization can be accomplished in quantitative yields. The base catalyzed cyclotrimerization reaction seems to be limited to ketenes, isocyanates, isothiocyanates and carbodiimides. In the trialkylphosphine catalyzed trimerization of methyl isocyanate an asymmetric trimer is obtained. 

---