Isocyanate, formation

Sterically hindered amines often tend to form ureas with (BOC)20, because of isocyanate formation. The problem can be avoided by reacting the amine with NaHMDS and then with (BOC)20. The isocyanates can also be converted to the BOC group by heating with r-BuOH. When other alcohols are used, the corresponding carbamate is produced. ... 

Bergon M, NB Hamida, J-P Calmon (1985) Isocyanate formation in the decomposition of phenmedipham in aqueous media. J Agric Food Chem 33 577-583. 

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. 

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... 

Isocyanate formation through multiple bond metathesis of C02 with carbodiimide has been also demonstrated [112]. This transformation can be promoted by titanium isopropoxide, at 383 K, in THF as solvent. It is worth noting that the reverse process, which opens an entry into carbodiimide synthesis, is a well-known process that is catalyzed by several other systems, including trialkylphosphine oxides [113] or vanadium-oxo or -imido complexes [114]. 

Scheme 6.23 Proposed mechanism for isocyanate formation via [2+2] cycloaddition of C02...

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. 

The Addition of Isocyanates to Isocyanates (Formation of Carbodiimides) Alkylimino-de-oxo-bisubstitution... 

Eibler, E., Sauer, J. Contribution to isocyanate formation in the photolysis of acyl azides. Tetrahedron Lett. 1974, 2569-2572. 

Isocyanate formation from diamines is commercially quite important since they are polymer precursors (Khardin and Pershin, 1979). Under appropriate conditions, proximal diamines can be converted to cyclic structures, the preparation of the biotin skeleton being representative (Marx et al., 1977). 

A remarkably efficient FeCl2-catalyzed intermolecular amination of simple benzylic substrates has been described (Fig. 15) [76]. These same authors have also noted the ability of CuBr to operate in a similar capacity [77]. iV-Bromosucci-mide (NBS) is used as the oxidant together with either a carboxamide or sulfonamide starting material. The /V-brominated amide purportedly reacts with FeCF to generate an Fe nitrene species that is capable of oxidizing benzylic C-H bonds, though evidence for such a mechanism is absent from the discussion. If a nitrenoid pathway is indeed operative, one might expect isocyanate formation to compete... 

Figure 6. Infrared spectra of isocyanate formation of model Type A (R = CH,) via the sodium azide and the hydrazide pathways.

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