Volume reaction + isocyanates

De Mello et al. have constructed a so-called pSYNTAS (miniaturized synthesis and total analysis system). The system was used to perform an Ugi-type reaction to form several a-aminoacetamides from amines, isocyanates and formaldehyde in the presence of water (Scheme 25) [56-58]. The reported system consists of a glass/silicon nanoreactor [59] in connection to a TOF-MS for the real-time online analysis of the reaction stream. Reactions were conducted in the 600 nl volume chip under continuous flow of 20-2 pl/min flow rate. Reduced flow rates resulted in increased outputs. The analyzed outlet flow showed high yields of the desired products with small quantities of starting materials and intermediates (no exact yields were reported). 

The two key isocyanates that are used in the greatest volumes for polyurethane polymers are toluene diisocyanate (TDl) and methylene diphenyl diisocyanate (MDl). Both isocyanates are produced first by nitration of aromatics (toluene and benzene, respectively), followed by hydrogenation of the nitro aromatics to provide aromatic amines. In the case of MDl, the aniline intermediate is then condensed with formaldehyde to produce methylene dianiline (MDA), which is a mixture of monomeric MDA and an oligomeric form that is typical of aniline/formaldehyde condensation products [2]. The subsequent reaction of phosgene with the aromatic amines provides the isocyanate products. Isocyanates can also be prepared by the reaction of aromatic amines with dimethylcarbonate [3, 4]. This technology has been tested at the industrial pilot scale, but is not believed to be practiced commercially at this time. 

It is well known that oxalyl chloride reacts with non-substituted amides to afford acyl isocyanates in high yields (Ref. 214). In contrast, phosgene acts as a dehydrating agent to give nitriles as shown in scheme 160. This interesting reaction, its mechanism and applications will be discussed in volume 2. 

The vinylcyclopropanes (1) and (5) react with chlorosulfonyl isocyanate (CSI) (1, 117-118 2, 70 3, 51-53 this volume) in different ways. Thus (1) reacts with CSI (CH2C12, 0") to give (9). In the case of (1) the cyclopropane ring is retained in the reaction of (5) the cyclopropane ring is opened. A possible explanation for the contrasting behavior of (1) and (5) has been suggested. 

The two types of continuous flow microreactors used are depicted in Figure 9.15. All isocyanate reactions were studied in two inlet chips (Figure 9.15a). A list of channel lengths (L), channel volumes (F) and respective residence times at flow rates ranging from 20 to 100 nLmin-1 is given in Table 9.3. 

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). 

No unfused 1,2,4-triazines seem to have been made by this atom combination. Mesomeric betaines containing the bicyclic pyrido[2,l-/][l,2,4]triazine system have been prepared by reaction of 2-alkoxycarbonyl pyridinium A -imines with phenyl isocyanate or isothiocyanate <93T3185>, but discussion of these reactions lies within the scope of another Volume. 

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