Aryl isocyanate

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Asymmetric aryl isocyanate dimers, ia which the C=0 group of oae molecule reacts with the C=N group of another, have been postulated as labile iatermediates ia the formation of carbodiioiides (17) upoa heating isocyanates. 

Both alkyl and aryl isocyanates are found to trimerize upon heating or ia the preseace of catalysts to 1,3,5-trisubstituted hexahydro-j -triaziaetrioaes (18) (isocyanurates) (57). Only highly substituted isocyanates, such as tert-huty isocyanate [7188-38-7] and tert-octy isocyanate, fail to trimerize under these conditions. 

Carboxyhc acids react with aryl isocyanates, at elevated temperatures to yield anhydrides. The anhydrides subsequently evolve carbon dioxide to yield amines at elevated temperatures (70—72). The aromatic amines are further converted into amides by reaction with excess anhydride. Ortho diacids, such as phthahc acid [88-99-3J, react with aryl isocyanates to yield the corresponding A/-aryl phthalimides (73). Reactions with carboxyhc acids are irreversible and commercially used to prepare polyamides and polyimides, two classes of high performance polymers for high temperature appHcations where chemical resistance is important. Base catalysis is recommended to reduce the formation of substituted urea by-products (74). 

Addition of pyrazole to C—X double bonds is also common. Formaldehyde gives stable adducts (260) and (261) (69BSF2064), but in the addition to ketones, (262) is only observed at low temperatures (Section 4.04.1.3.3(i)). However, hexafluoroacetone forms a stable adduct (262 R = Cp3) that has been used as a chelating agent (Section 4.04.2.1.3(iv)). Addition of pyrazoles to aryl isocyanates affords (263) the addition is also reversible, but it requires high temperatures to dissociate the adduct (Section 4.04.1.5.1). 

Alkyl and aryl isocyanates react with anhydrous hydrogen fluonde to give carbamyl fluorides [J, 55], the more conveniently handled pyridinium poly(hydro-gen fluoride) reagent can also be used, although the yields tend to be poorer [9] (equation 10). 

Side chain fluonnaied aryl isocyanates are an important group of substances used in the manufacture of biologically active compounds The synthetic routes to the meta- and para trifluoromethylphenyl isocyanates permit the chlorine-fluonne exchange with hydrogen fluoride to proceed under moderate conditions and in a virtually quantitative manner (equation 22)... 

Treatment of resin-bound iminophosphorane 197 with an excess of solid CO2 and CS2 in a sealed glass tube, and with aryl isocyanates afforded I-OXO-, 1-thioxo- 198, and l-arylimino-l//-pyrido[l,2-c]pyrimidine-3-car-boxylates 199 (OlJMClOll). Reaction of iminophosphorane 200, derived from 2-(2-azidophenyl)pyridine with excess PPh3, yielded 6//-pyrido[l,2-c] quinazoline-6-thione (201). 

It is essential that reagent grade CHC13 is used in the synthesis as the trace of ethanol stabilizer removes the aryl isocyanate byproduct and prevents it... 

A (pentamethylcyclopentadienyl)iridium chelating guanidinate complex has been conveniently prepared by treatment of [Cp IrCl2]2 with N,N, N"-th-p-tolylguanidine and base in THF at room temperature followed by recrystallization of the green product from toluene and pentane (Scheme 154). Insertion reactions of the product with heterocumulenes (diaryl carbodiimides, aryl isocyanates) have been investigated. It was found that the complex serves as highly active catalyst for the metathesis of diaryl carbodiimides with each other and for the more difficult metathesis of diaryl carbodiimides with aryl isocyanates (cf. Section V.C). ... 

Glyphosate diarylphosphonate nitriles 31 reacted with aryl isocyanates to form the corresponding alicyclic ureas 102, which were thermally quite labile and easily reverted back... 

A useful method to synthesize ten and fourteen-membered ring imides 346 involved an initial condensation of macrocyclic -ketoestes 343 with alkyl or aryl isocyanates and carbodiimides, respectively, in the presence of a base [68]. After a nucleophilic attack of the enolate on the isocyanate C, the resultant amide N anion 344 induced a ring closure by addition to the keto group. Then, the intermediately formed four-membered ring 345 underwent a fragmentation... 

Matsuda et al. applied aryl isocyanates as acceptors in reductive couplings to methyl acrylate (Scheme 44) [77]. The cationic Rh complex [Rh(COD) P(OPh)3 2]OTf (1 mol%) and Et2MeSiH (200 mol%) catalyze the reaction in refluxing CH2C12 to provide products of hydrocarbamoylation,... 

Verkade and co-workers have shown the usefulness of their phosphazanes in various stoichiometric as well as catalytic reactions <1999PS(144)101>. Compound 290 was used to promote the cyanohydration of benzaldehyde with trimethylsilyl cyanide (TMSCN). The cyanohydrin was isolated in 95% yield, but no enantioselectivity was noticed <2002JOM(646)161>. Compounds 291 and 292 were attached to dendrimers and shown to be effective in the catalysis of Michael reactions, nitroaldol reactions, and aryl isocyanate trimerizations <2004ASC1093>. 

An alternative strategy for the synthesis of these tricyclic compounds involves the reaction of the azidoalkenyl-functionalized furo[3,2-A]pyrrole 30, which reacts with triphenylphosphine to give the corresponding iminophos-phoranes 31 these upon reaction with aryl isocyanates give the pyrrolo[2, 3 4,5]furo[3,2-r-]pyridines 32, via the corresponding carbodiimides which are not isolated < 1994H(37) 1695, 1992M807> (Scheme 9). 

In contrast to thermal uncatalyzed reactions of /V-acylsulfinylamines with aryl isocyanates which give rise to azoarenes,188 the cobalt or iron carbonyl-catalyzed process gives additionally 3,5-dioxo-l,2,4-triphenyl-1,2,4-triazolidine (Scheme 124).189 The only possible restriction on this simple urazole synthesis would be the expectation that the substituents on the reactants must be the same to prohibit exchange. 

Another multicomponent synthesis giving N-3-substituted compounds 28 consisted of the sequential reaction of metallated phosphonoacetates, in one pot, with a nitrile then an aldehyde and finally an isocyanate. This was an extensive study of the scope and limitations of the different substituents on all the components. The most important feature was that, for good yields, the isocyanate should bear an electron-withdrawing group - tosyl was the most successful. However, an exchange reaction could be carried out by reaction of the tosyl products with aryl isocyanates under microwave irradiation, giving the N-3-aryl derivatives 29 <06CEJ7178>. 

Several approaches to the 1,2,3-triazole core have been published in 2000. Iodobenzene diacetate-mediated oxidation of hydrazones 152 led to fused 1,2,3-triazoloheterocycles 153 <00SC417>. Treatment of oxazolone 154 with iso-pentyl nitrite in the presence of acetic acid gave 1,2,3-triazole 155, a precursor to 3-(W-l,2,3-triazolyl)-substituted a,P-unsaturated a amino acid derivatives <00SC2863>. Aroyl-substituted ketene aminals 156 reacted with aryl azides to provide polysubstituted 1,23-triazoles 157 <00HC387>. 2-Aryl-2T/,4/f-imidazo[43-d][l,2,3]triazoles 159 were prepared from the reaction of triethyl AM-ethyl-2-methyl-4-nitro-l//-imidazol-5-yl phosphoramidate (158) with aryl isocyanates <00TL9889>. 

Generation of nitrile oxides by the Mukaiyama procedure, viz., dehydration of primary nitroalkanes with an aryl isocyanate, usually in the presence of Et3N as a base, is of high importance in nitrile oxide chemistry. Besides comprehensive monographs (4, 5), some data concerning the procedure and its use in organic synthesis can be found in References 61 and 62. 

The first reported synthesis of hydroxyurea (24) consists of the condensation of hy-droxylamine with potassium cyanate (Scheme 7.14) [87]. Condensation of hydroxy-lamine with ethyl carbamate also gives pure hydroxyurea in good yield after recrystallization (Scheme 7.14) [88]. Nitrogen-15 labeled hydroxyurea provides a useful tool for studying the NO-producing reactions of hydroxyurea and can be prepared by the condensation of N-15 labeled hydroxylamine with either potassium cyanate or trimethylsilyl isocyanate followed by silyl group removal (Scheme 7.14) [89, 90]. Addition of hydroxylamine to alkyl or aryl isocyanates yields alkyl or aryl N-hydroxyureas (Scheme 7.14) [91, 92]. The condensation of amines with aromatic N-hydroxy carbamates also produces N-substituted N-hydroxyureas (Scheme 7.14) [93]. 

Imidazoline 3-oxides 187 undergo regio- and diastereoselective 1,3-dipolar cyloaddidon with aryl isocyanates to give X-5,6,7,7a-tetrahydroimidazo[l,5- ][l,2,4]oxadiazoles 40 in good yields (Equation 80) <1997TL2299, 1997T13873>. 

The reaction of compounds 465 with an excess of aryl isocyanate in acetonitrile at reflux leads to the formation of imidazooxadiazolones 466, via a Beckmann fragmentation that affords a presumed cyclic nitrone intermediate 187 (Equation 107) <1999SC3889>. 

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