Isocyanates catalytic conversion

Some examples of the catalytic conversion of isocyanates into carbodiimides are shown in Table I. 

Catalytic conversion of isocyanates into carbodiimides with the aid of l,3-dimethyl-l,3,2-diazaphospholidine 2-oxides and 1,3-dimethylhexahydro-1,3,2-diazaphosphorine 2-oxides [79]. 

By Catalytic Conversion. Symmetrically substituted carbodiimides are best obtained by the catalytic conversion of two equivalents of isocyanates into a carbodiimide and carbon dioxide. This is the most convenient method of synthesis of symmetrically substituted carbodiimides, because the byproduct is carbon dioxide gas, and the yields approach quantitative. A solvent is not necessary and numerous isocyanates are readily available as starting materials. ... 

All pentavalent P=0 compounds catalyze this reaction and the order of catalytic activity is phosphine oxide > phosphinate > phosphonate > phosphate. For example, addition of a catalytic amount of hexamethylphosphoric acid triamide inaeases the rate of conversion of phenyl isocyanate to diphenylcarbodiimide. The catalytic conversion of phenyl isocyanate to diphenylcarbodiimide, using isopropylmethylphosphonofluoridate,... 

The catalytic conversion of aryl and alkyl isocyanates 53, using 2-ethyl-1,3-dimethyl-... 

The reaction of sterically hindered aromatic diisocyanates with bases or phospholene oxide catalysts afford oligomeric carbodiimides having terminal isocyanate groups. If the catalytic conversion of 4,4 -diphenylmethane diisocyanate (MDI) is conducted in the... 

This reaction is utilized in the catalytic conversion of isocyanates into carbodiimides using phospholene oxides as the catalyst . When aliphatic diisocyanates were converted into carbodiimides using a phospholene oxide catalyst, the generated carbon dioxide was partially incorporated into six-membered ring [2+2+2] cycloadducts . ... 

This reaction was utilized in the catalytic conversion of isocyanates into carbodiimides. The order of catalytic activity is Fe(CO)4CNPh > Fe(CO)5 > Fe2(CO)9 > W(CO)6 >... 

Garbodiimide Formation. Carbodiimide formation has commercial significance in the manufacture of Hquid MDI. Heating of MDI in the presence of catalytic amounts of phosphine oxides or alkyl phosphates leads to partial conversion of isocyanate into carbodiimide (95). The carbodiimide (39) species reacts with excess isocyanate to form a 2 + 2cycloaddition product. The presence of this product in MDI leads to a melting point depression and thus a mixture which is Hquid at room temperature. 

The cycloaddition of isomiinchnones with acetylenic dipolarophiles followed by the extrusion of an alkyl or aryl isocyanate (RNCO) has proven to be an effective method for the synthesis of substituted furans. The Ibata group investigated the bimolecular 1,3-dipolar-cycloaddition of aryl-substituted isomiinchnones with a number of acetylenic dipolarophiles [50]. Aryl diazoimides of type 1 were heated in the presence of a catalytic amount of Cu(acac)2 and the appropriate acetylenic dipolarophile. Formation of the substituted furan was found to be temperature-dependent higher temperatures (ca. 120°C) were needed for complete conversion to the furan. It was reasoned that the extrusion of methyl isocyanate was not as facile as the loss of carbon dioxide from sydnones and miinchnones [50]. 

Several phosphorous compounds having a P=S bond are also catalysts for this reaction. For example, heating of phenyl isocyanate with triphenylphosphine sulfide at 160 °C affords diphenylcarbodiimide. Since triphenylphosphine sulfide was recovered unchanged, a different mechanism seems to be operative. A tricyclic P S compound, S=P[N(Me)CH2CH2l3N, also catalyzes the transformation of isocyanates to carbodiimides. Triphenylarsine oxide and triphenylantimony oxide also catalyze the conversion of isocyanates into carbodiimides. The catalytic activity of the oxides of phosphorous, arsenic and antimony are in agreement with the dipole moments of... 

Other catalysts effective in the conversion of isocyanates into carbodiimides include the naphthenates of Mn, Fe, Co, Cu and Pb, derivatives of metallic acetylacetonates, the alkoxides of titanium, zirconium and niobium and vanadium oxides or chlorides. Sterically hindered isocyanates are readily converted into carbodiimides upon heating in the presence of a catalytic amount of a strong base. For example, heating of... 

The molecular imprinting method can be used to synthesize enantioselective solid materials for asymmetric organic synthesis. The first attempt to use a metal complex with an attached chiral ligand as a template was attempted by Lemaire [52]. The Rh complex, ((15,25)-V,V -dimethyl-l,2-diphenylethane diamine)-[Rh(CgHj2)Cl]2 coordinated with optically pure l-(5)-phenylethoxide or phenylethoxide (Rh 1-phenylethanolate) (template) was polymerized in the presence of isocyanate, and the polyurea-supported Rh complex is reacted with isopropanol to extract the template from the polymer backbone. They reported the influence of molecular imprinting on catalytic performance (conversion and enantiomeric excess) for the asymmetric transfer hydrogenation (Table 22.2). The imprinted polymer exhibited higher enantioselectivity compared to a nonimprinted... 

The production of aniline is a major international business, carried on in the US, Europe and Asia, mainly for the conversion, by reaction with formaldehyde under acid-catalyzed conditions, into diaminodiphenylmethanes 9a, 9b and 9c, and then into isocyanates, mainly 4,4/-methylenebis(phenylisocyanate) (MDI, also known as 4,4 -methylene-di-paraphenylene isocyanate, 4,4 -diphenylmethane diisocyanate, methylene diphenylene diisocyanate and diisocyanato diphenyl methane) (9d), from which polyurethanes are produced. This accounts for well over 60% of total demand (Figure 1). Aniline is also used in bulk for the production of antioxidants and vulcanization accelerators for rubber. Some 15.5 million lbs. of cyclohexylamine are made each year mainly by catalytic hydrogenation of aniline. Half the demand is for use as a boiler water additive. Other major uses include in the manufacture of herbicides, plasticizers, emulsifying agents, dyes, dry-cleaning soaps, acid gas absorbents and, in Asia, cyclamate sweeteners. Apart from India, the use of aniline for dyestuff manufacture represents about 10% of demand. 

The classical oxidation example is that of SO2 to SO3, which is described in more detail below. The second class is the partial oxidation of hydrocarbons in which the desired product selectivity is significantly sensitive to both temperature and the oxygen concentration. More specifically, yield losses to carbon oxides, which result from sequential or parallel side reactions, undermine the goal of achieving high product selectivity at a reasonable hydrocarbon conversion per pass. Most hydrocarbon partial oxidation reactions have this feature. Two illustrative commercial processes include the silver-catalyzed oxidation of methanol to formaldehyde and the catalytic oxidation of monomethylforma-mide to methyl isocyanate. 

As shown in Fig. 8.6, the manufacturing processes of fentrazamide involve the conversion of two inexpensive run-of-the mill anilines, 2ethyl aniline, into 2-chlorophenyl isocyanate and Nsodium azide in the presence of catalytic amounts of aluminum trichloride in dimethyFormamide [25]. The tetrazolinone reacted with N-cyclohexyl-N-ethylcarbamoyl chloride, in the presence of a catalytic amount of 4-dimethylaminopyridine (DMAP), to afford fentrazamide with no formation of its O-carbamoylated isomer [26]. 

The catalytic activity of these novel cationic lanthanide monoporphyrinates has also been studied (Wong et al. 1999). The ytterbiimi complex [Yb(TMPP)(H20)3]Cl, but not the yttrium and erbium analogs, can effectively catalyze the conversion of phenyl isocyanate into its cyclic trimer, l,3,5-triphenyl-Ar-triazine-2,4,6-trione. The complex appears to be a living catalyst as the resulting mixture can be re-used without losing any of its activity in the first three cycles. A plausible mechanism has been proposed as... 

An advantage of this catalytic reaction with respect to the direct carbonylation to give the isocyanates, is that in this case the products are more stable. They are not involved for example in polymerization reactions, which are the probable responsible for the dropping selectivity in the formation of the isocyanates at high conversions (4.2.5.). 

The chlorination of 4-nitrotoluene and subsequent catalytic reduction on sulfided Pd/active carbon catalysts yields 3-chloro-4-methylaniline, from which the herbicide chlortoluron is made by initial conversion into the isocyanate and subsequent reaction with dimethylamine. 

Stepwise degradation of polypeptides involving benzoylation, conversion to azides and treatment of the azides with benzyl alcohol this treatment yields, via rearrangement to isocyanates, carbobenzoxy compounds which undergo catalytic hydrogenation and hydrolysis to the amide of the degraded peptide ... 

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