Isocyanates with aromatic rings

N-Substituted amides can be prepared by direct attack of isocyanates on aromatic rings.The R group may be alkyl or aryl, but if the latter, dimers and trimers are also obtained. Isothiocyanates similarly give thioamides. The reaction has been carried out intramolecularly both with aralkyl isothiocyanates and acyl isothiocyanates.In the latter case, the product is easily hydrolyzable to a dicarboxylic acid this is a way of putting a carboxyl group on a ring ortho to one already there (34 is... 

Poly(phenylene oxide)s undergo many substitution reactions (25). Reactions involving the aromatic rings and the methyl groups of DMPPO include bromination (26), displacement of the resultant bromine with phosphoms or amines (27), lithiation (28), and maleic anhydride grafting (29). Additional reactions at the open 3-position on the ring include nitration, alkylation (30), and amidation with isocyanates (31). 

This reaction is reported to proceed at a rapid rate, with over 25% conversion in less than 0.001 s [3]. It can also proceed at very low temperatures, as in the middle of winter. Most primary substituted urea linkages, referred to as urea bonds, are more thermally stable than urethane bonds, by 20-30°C, but not in all cases. Polyamines based on aromatic amines are normally somewhat slower, especially if there are additional electron withdrawing moieties on the aromatic ring, such as chlorine or ester linkages [4]. Use of aliphatic isocyanates, such as methylene bis-4,4 -(cyclohexylisocyanate) (HnMDI), in place of MDI, has been shown to slow the gelation rate to about 60 s, with an amine chain extender present. Sterically hindered secondary amine-terminated polyols, in conjunction with certain aliphatic isocyanates, are reported to have slower gelation times, in some cases as long as 24 h [4]. 

It is of note that hypoglycemic activity is maintained even when the aromatic ring is fused onto a carbocyclic ring. Chlor-sulfonation of hydrindan gives chloride, 206. Reaction of the sulfonamide (207) obtained from that intermediate with cyclohexyl isocyanate leads to glyhexamide (208). ... 

Aromatic A-oxides undergo 1,3-dipolar cycloaddition reactions. Phenyl isocyanate with pyridine 1-oxides involves a reaction sequence which in the end gives a 2-anilinopyridine. The initial product is the ring-fused 1,2-dihydropyridine 1-oxide (600), which rapidly undergoes a 1,5-sigmatropic shift to its more stable 2,3-dihydro counterpart (601) which is then aromatized by C02 expulsion (76JHC171,80H(14)19). 

Most of the commercially used isocyanates are diisocyanates and R is an aromatic ring. MIC is an exception its structure is H3C-N=C=0. The physicochemical properties of MIC differ from those of other isocyanates (Lowe, 1970 Tse and Pesce, 1978 Westcott, 1985 Worthy, 1985). Because of high chemical reactivity of MIC with alcohols, it serves as an intermediate in the production of the pesticide carbaryl. Diisocyanates are primarily used for the manufacture of polyurethanes. 

Trimerization of nitriles, isocyanates, isothiocyanates, imidates, and carbodiimides all lead to symmetrical 2,4,6-trisubstituted 1,3,5-triazines (see Section 6.12.9.5). The use of lanthanide trifluoromethanesulfonate and ammonia as cocatalysts is claimed as a big improvement. The trisaminal of 2,4,6-triformyl-l,3,5-triazine is also useful for further derivatization to unusual structures (see Section 6.12.7.1). Treatment of a 1 1 pyridine/conc. ammonia solution of an aromatic aldehyde with excess Fremy s salt is another development. Separation of the amide coproduct was claimed to be easy. The synthesis fails with aliphatic aldehydes (see Section 6.12.9.5.4). Aminolysis of 2,4,6-triaryl-1,3,5-oxadiazinium salts gives symmetrical 1,3,5-triazines but the reactions are limited in that electron-withdrawing groups in the aromatic rings lead to instability and difficulty in separation of products (see Section 6.12.10.4). 

In 1971, Bott showed for one case that alkenediazonium salts can be obtained by nitrosation, not of aminoalkenes, but from their isocyanate derivatives (2.278) with nitrosyl hexachloroantimonate under aprotic conditions (CH2CI2). By this method, 2,2-dichloroethenediazonium hexachloroantimonate (2.279) was synthesized in 40<7o yield (2-107). It may be mentioned that this nitrosation cannot be used for aromatic isocyanates nitrosyl cations attack phenyl isocyanate by nitrosation of the aromatic ring, as shown by Olah et al. (1966). 

Evidence of the condensation reaction of the isocyanate functions with the OH groups of cellulose was supported by FTIR (bands at 1712 and 1640 cm associated with CO and NH of the urethane, 1630-1500 cm associated with CO and NH of urea, and bands at 2850-2900 cm relative to aliphatic CH moieties of chain aliphatic). This evidence was also often corroborated by C CP-MAS NMR with the emergence of the typical peaks at 12-40 ppm assigned to methylene carbon of the anchored hydrocarbon chain, peaks at 125-136 ppm assigned to aromatic rings of MDI, and peaks at 157.46 ppm related to the carbamate functions. An example of C CP-MAS NMR of the is shown in Figure 17.15. 

RIM polyurethanes made with aromatic isocyanates (such as pure or polymeric MDI) have a tendency to darken as a result of the effect of UV light on the chemical ring structure of the MDI component. Soft white limestone or line carbon black is often used as filler to mask the effect of this color change. 

This makes it susceptible to attack by nucleophilic reagents, such as alcohols, acids, water, amines, and mercaptans. The electrophilicity of the carbon atom can be increased if R is an aromatic ring which can conjugate with the isocyanate group. Thus, aromatic diisocyanates are more reactive than aliphatic ones. 

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