Carbodiimide reaction mechanism

Amidinate complexes of copper(I) and copper(II) have been found to catalyze the polymerization of carbodiimides. The copper catalysts are highly active even under air and oxygen. It was shown that the catalytic activity of an air-stable copper(II) amidinato complex is almost equal to that of reported air-sensitive titanium(IV) amidinate initiators. Scheme 225 illustrates the proposed reaction mechanism. ... 

R Rebek, D Feitler. Mechanism of the carbodiimide reaction, n. Peptide synthesis on the solid support. J Am Chem Soc 96, 1606, 1974. 

Moffatt, J. G. Sulfoxide-carbodiimide reactions. X. Mechanism of the oxidation reaction. J. Org. Chem. 1971, 36,1909-1913. 

Reaction mechanism Based uprm a series of stoichiometric reactions, and in contrast to the previously discussed hydrophosphinatiOTi reactions (vide supra), it was postulated that unsaturated homoleptic heavier alkaline-earth phosphides are unlikely to be long-lived intermediates in the catalytic hydrophosphination of carbodiimides conducted in non-coordinating solvents. Rather, it was proposed that the phosphaguanidine product was acting as a ligand for the Lewis acidic metal center [114]. 

Similarly, the use of a carbodiimide instead of an isocyanate afforded the corresponding iminopyridines 44 through the same reaction mechanism and under identical reaction conditions (Scheme 11.17). 

The nonlinearity of the correlation between log k/ko) and Hammett (Tp constant at the cyclization of compounds 3.971 suggests that there is a change in reaction mechanism of C -C cyclization of the enyne carbodiimide, possibly from carbene to polar intermediates. Electron donor substituents X and Y increase probability of formation of the high polar zwitterionic carbene intermediate (Scheme 3.147, structures B and C), whereas electron-withdrawing substituents lead to a less polar carbenes (Scheme 3.147, structure A). The mechanism of polar intermediate is supported by a noticeable increase of the reaction rate with increasing the donor properties of the solvent that was observed for carbodiimide 3.971a. However, more detailed study of conditions of the... 

Mechanism of amide formation by reaction of a carboxylic acid and an amine with dicyclohexyl-carbodiimide (DCC). 

A full development of the rate law for the bimolecular reaction of MDI to yield carbodiimide and CO indicates that the reaction should truly be 2nd-order in MDI. This would be observed experimentally under conditions in which MDI is at limiting concentrations. This is not the case for these experimements MDI is present in considerable excess (usually 5.5-6 g of MDI (4.7-5.1 ml) are used in an 8.8 ml vessel). So at least at the early stages of reaction, the carbon dioxide evolution would be expected to display pseudo-zero order kinetics. As the amount of MDI is depleted, then 2nd-order kinetics should be observed. In fact, the asymptotic portion of the 225 C Isotherm can be fitted to a 2nd-order rate law. This kinetic analysis is consistent with a more detailed mechanism for the decomposition, in which 2 molecules of MDI form a cyclic intermediate through a thermally allowed [2+2] cycloaddition, which is formed at steady state concentrations and may then decompose to carbodiimide and carbon dioxide. Isocyanates and other related compounds have been reported to participate in [2 + 2] and [4 + 2] cycloaddition reactions (8.91. 

Good yields of imidazoline derivatives have been obtained in the cocyclooligomerization of phenylacetylene with isocyanates and carbodiimides (Scheme 100).166 It has been demonstrated166 by labeling studies in the isocyanate reaction that the hydrogen shift is intramolecular and a mechanism accommodating this feature is illustrated in Scheme 101.166 The final step (85 - 86) in the proposed166 mechanism (Scheme 101) probably occurs via a coordinated acetylene complex and it is notable that related complexes... 

Triazines 347 were obtained from amidoximes 346 and ethyl orthoacetate (equation 151). The mechanism of formation of products 347 includes Beckmann rearrangement of amidoximes to carbodiimides, followed by reaction with amidoxime and orthoester. ... 

The mechanism of carbodiimide-mediated amide formation is outlined in Figure 13.4. The first intermediate formed during the reaction of a carbodiimide with a carboxylic acid (or an ammonium carboxylate [56]) is an O-acylisourea [1]. Under acidic conditions [57,58], this intermediate reacts with acids to yield anhydrides, and with alcohols, phenols, HOBt, or other compounds containing hydroxyl groups, to yield the corresponding esters. The carbodiimide is thereby transformed into a urea. 

Williams, A., and Ibrahim, I. A. (1981) A mechanism involving cyclic tautomers for the reaction with nucleophiles of the water-soluble peptide coupling reagent l-ethyl-3-(3-dimethyla-minopropyl) carbodiimide (EDC). J. Am. Chem. Soc. 103, 7090—7095. 

Palladium-mediated cydization based on the reactivity of o-alkynyl or alkenyl-phenyl isonitriles have been developed [105]. On the basis of their earlier studies on the three-component synthesis of allyl aryl cyanamides [106], Yamamoto and co-workers reported a palladium-catalyzed three-component coupling reaction of 2-alkynylisocyanobenzenes 122 with allyl methyl carbonate and trimethylsilylazide leading to N-cyanoindoles 125 [107]. One of the key steps of the proposed mechanism is the formation of 7i-allylpalladium carbodiimide 123 and its isomerization to rc-allylpalladium cyanamide complex 124 (Scheme 8.50). 

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... 

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