Aromatic carbodiimides

At room temperature aliphatic and aromatic carbodiimides are liquid or solid. They usually can be purified by distillation under reduced pressure or by recrystallization to yield neutral products. On prolonged standing they polymerize to yield basic products. The liquid carbodiimides are less stable than the solid ones. The stability of carbodiimides increases with the degree of branching of the alkyl substituents attached to the nitrogen atom RCH2 < R2CH < R3C. For example, diethylcarbodiimide [14, 21] polymerizes in a few days, whereas diisopropyl- and dicyclohexylcarbodiimides [14] are stable for several months. Unsaturated substituents also cause a marked decrease in stability... 

In the preparation of aromatic carbodiimides the presence of substituents in the benzene ring affects the yields of the product and the by-product isothiocyanate [60]. 

Prolonged heating of free isocyanates in the absence of catalysts has been reported recently to give poor yields of carbodiimides unless a slow stream of nitrogen was passed through the boiling isocyanate [18]. In the presence of phosphorus catalysts [18, 19, 27-39], aromatic carbodiimides are obtained in high yield under mild conditions from isocyanates but not isothiocyanates [18]. Aliphatic isocyanates react more slowly, but improved yields are obtained in... 

Aliphatic and aromatic carbodiimides are liquids or solids at room temperature. The stability of substituted dialkylcarbodiimides increases as follows RCH2 < R2CH < R3C."" Dimethylcarbodiimide should be used freshly prepared, but it can be stored for several days below room temperature. Unsaturation in the aliphatic substituents decreases the stability of carbodiimides. Eor example, diallylcarbodiimide is unstable. 

In the aromatic carbodiimides, the solid products are more stable than the liquid products. N-alkyl-N -arylcarbodiimides are less stable than diarylcarbodiimides. The introduction of electron attracting groups into the aromatic substituents seems to increase the polymerization tendency of the resulting carbodiimide. In contrast, electron donating substituents on the aromatic ring of arylalkylcarbodiimides enhance their reactivity with carboxylates.""... 

The reaction of N,N -disubstituted thioureas 15 with phosgene (carbonyl chloride) affords aliphatic and aromatic carbodiimides 16 in good yields. For example, addition of phosgene... 

Also, 1,2,4-oxazetidines 151 afford aromatic carbodiimides 152 on thermolysis. 

When tetrafluoroboric acid is added to aromatic carbodiimides quinazolium salts 192 are obtained, which on hydrolysis afford 3-ary 1-2-arylamino-4-aryliminoquinazolines... 

Cycloaddition Reactions Across C—C Multiple Bonds. Cycloadducts derived from carbodiimides and olefins or allenes are not known. However, the [2+2] cycloaddition of ketenes, R2C=C=0, to carbodiimides affords 4-imino-2-azetidinones (/3-lactames) 239 in high yield. Aliphatic carbodiimides show higher reactivity in comparison to aromatic carbodiimides, and the reaction proceeds across the aliphatic C=N bond in N-alkyl-N -arylcarbodiimides. The cycloadducts obtained in this reaction are listed in Table 2.3. 

Anthranilic acid reacts with aromatic carbodiimides similarly to give 3-substituted 1,3-... 

Applications for the Stabaxol stabilizers include thermoplastic polyester urethanes, polyesteramide thermoplastic elastomers, castable polyester urethanes, polyester polyols, monofilament PET fibers, polycarbonates, polycarbonate/PETblends, EVA copolymers and poly(caprolactones). The thermal stabilization of poly(ethylene sulfide) is also accomplished with 4 % hexamethylenebis(t-butyl)carbodiimide and 2 % diphenylacetylene. Also, alternating carbon monoxide/ethylene copolymers are stabilized using aromatic carbodiimides. ... 

Aromatic carbodiimides (4) also dimerize however, they afford substituted quina-zolium salts (5), which on basification give 3-aryl-2-arylamino-4-aryliminoquinazo-lines (6). 

Patents citing the use of other antioxidants include combinations of cyclic carbonate and secondary aromatic amine [64] PBT moulding grades stabilised with diamines such as diphenyl-p-phenylenediamine [65] polymeric diphenylamines made by reacting diphenylamine with dialkylalkenylbenzene or dihydroalkylbenzene [66] substituted phenothiazines [67] aliphatic sulfides such as R(OCH2CHR"CHR""SR"")n [68] and combinations of aromatic carbodiimides and quinones [69]. 

Carbodiimides (cyanamides) —N=C=N— 2140-2125 (s) 2150-2100 (vs) 1460 1150-1140 (vs) Asymmetric stretching of aliphatics Asymmetric stretching of aromatics two bands Symmetrical stretching of aliphatics Symmetric stretching of aryls... 

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

Wang recently reported [30] that thermolysis of carbodiimides 15 (Scheme 1.5) in aromatic solvents is an efficient route to indoloquinolines 18 used as precursors for synthesizing naturally occurring alkaloids [31], The cyclization is thought to occur through a two-step biradical Diels-Alder reaction that gives 17, which then tautomerizes to 18. 

Oxadiazoles are obtained by reaction of aromatic isocyanates or CS2 with N-acylaminoiminophosphoranes (151), as shown in Scheme 60. Upon treatment with isocyanate, an unstable carbodiimide 152 is generated, which cyclizes spontaneously in 80-84% yield. With CS2 as reagent, the corresponding isocyanate cyclizes without isolation of any intermediates to 2-mercapto-l,3,4-oxadiazole 154 in 72-90% yield [91PS(57)11]. 

An interesting variation of this quinoxaline synthesis is outlined by the synthesis of sydnoquinoxalines shown in Scheme 103. The starting material is phenylsydnone 288 with an iminophosphorane group in an o-position. With isocyanate or isothiocyanate carbodiimide intermediates 289 are formed by an electrophilic aromatic substitution at the sydnone ring (4 position), the 4-(arylamino)sydno[3,4-a]quinoxalines (290) are obtained (91S745). 

Diketene is of course a masked form of acetoacetic ester, and as such reacts in much the expected manner with a variety of mono- and di-nucleophiles. Aromatic and heteroaromatic amines, and phenols, for example, give acetoacetanilides and aryl acetoacetates the latter can be cyclized in excellent yield to coumarins while reaction of the former with excess of diketene followed by cyclization gives dioxopyridines (e.g. equation 164). Amidines, ureas, thioureas, S- alkylthioureas and carbodiimides also react with diketene to give pyrimidines e.g. equation 165), although in the case of amidines, S- alkylthioureas and carbodiimides the initially formed products are 1,3-oxazines which are converted into pyrimidines on subsequent treatment with acid or base. 

In the aromatic series, diphenylcarbodiimide [23] and di-p-iodophenyl-carbodiimide [23] polymerize in a few days, whereas di-p-tolylcarbodiimide when obtained in pure crystalline form is stable for several months [24, 25]. Di-p-dimethylaminophenylcarbodiimide is stable for three years [25],... 

In the case of aromatic isocyanates, electron-withdrawing groups increase the rate of carbodiimide formation in proportion to their electron-withdrawing power. Electron-releasing groups tend to inhibit the reaction. p-Nitrophenyl isocyanate reacts almost explosively to yield the carbodiimide when catalyzed [5]. o-Chlorophenyl isocyanate reacts seven times as fast as the o-methyl analog [33]. 

In general, most of published immunochemical methods on chloramphenicol assay are based on antibodies particularly specific for the aromatic ring and the propanediol moiety of chloramphenicol. This is due to the fact that the main chloramphenicol analogues such as thiamphenicol differ from chloramphenicol in one of these parts of the chloramphenicol molecule. To obtain such antibodies, chloramphenicol was linked to the carrier bovine serum albumin at the acyl chain of the molecule by mixed anhydride or carbodiimide reactions (49, 50). In both... 

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