Isocyanates formation reaction

Amides. Because amides are less basic, they chlorinate less rapidly than amines. A/-Halamides are converted to amines in basic solution via intermediate formation of an isocyanate (Hofmann reaction) (91). 

Since 1985, several thousands of publications have appeared on complexes that are active as catalysts in the addition of carbon monoxide in reactions such as carbonylation of alcohols, hydroformylation, isocyanate formation, polyketone formation, etc. It will therefore be impossible within the scope of this chapter to review all these reports. In many instances we will refer to recent review articles and discuss only the results of the last few years. Second, we will focus on those reports that have made use explicitly of coordination complexes, rather than in situ prepared catalysts. Work not containing identified complexes but related to publications discussing well-defined complexes is often mentioned by their reference only. Metal salts used as precursors on inorganic supports are often less well defined and most reports on these will not be mentioned. 

For this library, we chose to use three types of isocyanates (neutral, electron rich, and electron deficient) to demonstrate the broad utility of the urea-formation reactions. Employing the above strategy and using the split-and-pool approach, we synthesized a 27-membered urea library with purities ranging from 95 to 99%. All the compounds prepared were characterized by 1FI NMR and mass spectroscopy. Acetonitrile can also be used as a substitute for DCM, but lower yields and product purities are generally observed. Attempts to use other protic solvents, such as isopropyl and ethyl alcohol, were unsuccessful. The best results were achieved when a chlorinated solvent (DCM) was used. The structure identity of all products was confirmed by 1FI NMR and MS spectroscopy. Expected molecular ions (M + Na+) were observed for all the products, and in all cases as the base peak. The compounds and yields are listed in Appendix 3.1. 

Subsequent experiments on the same system aimed to determine the stability of the isocyanate species and to measure the reactivity of the Pd(lll) model catalyst for the CO + NO reaction.125 When exposing the sample to different CO/NO ratios (2 and 1.5) at room temperature, peaks were obtained which corresponded to threefold NO, atop NO, and threefold CO, with the higher CO/NO ratio leading to a greater amount of CO binding. When the samples were flashed to 650 K and cooled back to 300 K in the presence of the reaction mixtures, isocyanate was formed. However, as is apparent from Figure 10.25, an increase in the CO/NO ratio strongly favored isocyanate formation. 

A recent review on four-membered heterocycles formed from imino-phosphoranes concentrates on the preparation and the reactivity of 2,4-diimino-l,3-diazetidine and related compounds (93JPR305). As an example, the synthesis via bisiminophosphorane 85 is described in Scheme 42. The bisiminophosphorane has both a heteroaryl and a styryl site. From a mechanistic view, the reaction of the bisiminophosphorane proceeds with aryl isocyanate formation via an aza-Wittig mechanism. Intermediate car-bodiimide formation (86) occurs directly on the iminophosphorane moiety... 

For the catalysis of isocyanate-alcohol reactions in apolar solvents, several mechanisms have been proposed. However, the results of the kinetic measurements in DMF could not be explained with these mechanisms. So we concluded that, in the polar solvent DMF, the mechanism of the catalyzed urethane formation differs from the published mechanisms in apolar solvents. The behavior in DMF can be explained from a mechanism in which dibutyltin dilaurate dissociates into a catalytic active species. 

The toxicity of methyl isocyanate comes as a surprise if its high sensitivity to hydrolysis is considered. But actually, it may reach its site of action in the human body largely undecomposed. This is due to the reversible addition of another nucleophile to this heterocumulene. The tripeptide gluthathione, which is supposed to protect the body against oxidizing agents, adds to the C=N double bond of the isocyanate by means of its thiol group whereby the thio-carbamate is formed. When the latter decomposes in a reversal to its formation reaction, it releases the intact toxic methyl isocyanate. 

Both the singlet energy and electron transfer methods suffer from competing isocyanate formation. Energy and electron transfer in the triplet state avoids this rearrangement reaction which can result in higher yields of cycloaddition products. However, in both sensitization modes, side reactions of the triplet nitrene such as H-abstraction reactions have to be taken into account. 

This section will be limited to reactions involving the nitrogen atom, specifically N protonation and isocyanate formation. 

First-generation solventless polyurethane adhesives are one-component isocyanate terminated prepolymers formed by the reaction of MDI (4,4 methylene bis (phenyl isocyanate)), or other isocyanates with polyether and/ or polyester polyols. One-component 100% solids adhesives rely on moisture from the air or substrates or from induced moisture misting during the converting process, to cure the adhesive via an isocyanate/water reaction and subsequent polyurea-polyurethane polymer formation. Typically the high viscosity of the adhesive is such as to require adhesive delivery equipment and application rollers heated from 65-80 °C for use. They have a high level... 

Boltom H40 dendritic molecules were covalently linked in this work to make a network with aliphatic 1,6-hexamethylene diisocyanate (HDI). The molar NCO/OH ratio was varied for the reactants from 10 to 50% to prepare networks with different degrees of connectivity of dendritic units. The network samples are designated as H40/Z where Z stands for NCO/OH ratio expressed as a percentage. The network formation reaction of H40 with HDI was carried out in N,N-dimethylformamide (DMF) of 99.8% purity at 90 C. No catalyst was added. Isocyanates react readily with moisture to form urea linkages, therefore special precautionary measures were implemented to prevent moisture uptake either by HDI or DMF. Observation of the reaction vessel was maintained over the course of the reaction to monitor the viscosity of the solution. As viscosity of the solution increased to the desired level, suggesting that the gelation point was near, the solution was cast onto a glass plate which was immediately placed... 

One-shot polyether foams were studied, using a variety of catalysts. The formula contained 100 parts by weight of poly(oxypropylene)triol of 3000 M.W., 38 parts of 80 20-TDI, 2.9 of water, 0.3 of 4-dimethyl-aminopyridine, 0.5 of lV,iV-dimethylbenzylamine, varying amounts of metal catalysts, and 0.1 part of X-520 siloxaneoxyalkylene copolymer. All of the gas was evolved from these systems within 60 sec after mixing. Viscosity measurements were not satisfactory due to fracture of the polymeric phase. Analysis of the reaction mixture at the end of 55 sec reaction time indicated the relative rate of formation of various products, as indicated in Table 22. The importance of selecting the proper catalyst to avoid undesirable side reactions is readily apparent. The results shown in Table 22 indicate that both tin catalysts promote the isocyanate/water reaction more than the isocyanate/hydroxyl reaction in the system studied. This is unusual, since other reports, though often of dilute solution studies, have shown the tin catalysts to promote the isocyanate/ hydroxyl reaction more [145,147,196]. 

With use of appropriate catalysts, mixtures of dianhydrides and diisocyanates were directly converted to rigid foam products without use of foaming agents [105,106]. In situ formation of imide forms like the polyurethane process, was developed by Riccitiello et al. [107]. The system contains a unique furfuryl alcohol-acid mixture which generates heat to initiate the isocyanate-anhydride reaction. 

Both methods could be recognized as a new combination of a known gas-generation and a known polymer-formation reaction. Since then, a variety of isocyanate-based foams were developed as described below. New combinations of new gas-generation reactions and a known polymer formation lie outside of the prior art. This new area was studied by Ashida and his collaborators, and the work was reviewed (33). 

Because of the toxicity of isocyanates, there is interest in preparing blocked isocyanates and polyurethanes by a non-isocyanate process. Reaction of aliphatic diamines with cyclic carbonates (l,3-dioxolan-2-ones) results in the formation of /3-hydroxyalkyl carbamates. 

The formation of aryl or alkyl isocyanates by reaction of nitro derivatives with CO catalyzed by transition elements " is another example of the reductive capability of CO ... 

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