Ureas alcoholysis

The intense patent activity in this area attests to the vast interest for implementing nonphosgene technologies to produce organic carbonates [12]. Hence, the state of the art in research for the three chemical routes based on C02 (i.e., transesterification, urea alcoholysis, and direct carbonation) is discussed in the following sections. 

A stimulating development of urea alcoholysis has been demonstrated very recently for better AE, in an innovative integrated process that incorporates fatty ester hydrolysis to co-amino-alkanoic acids [44], Within the scope of this chapter, the most interesting step of this process is the recycling of waste alcohol, formed by the hydrolysis step, for urea alcoholysis. Dialkyl carbonate is produced together with ammonia thereafter, the ammonia is engaged in the amination reaction to obtain the amino acids. The overall process avoids the storage of NH3 that is necessary for the amination route, and transforms a waste product-the alcohol-into the valuable dialkyl carbonate. 

Several metal oxides (either acidic or alkaline) have also been investigated for urea alcoholysis [228, 229], with PG finding PC product yields in excess of 90% for ZnO, PbO, and MgO. In such studies, the results obtained coupled with the results of thermal programmed desorption (TPD) and Fourier transform infrared (FTIR) analyses, indicated that catalysts with appropriate acid and base properties were required for the synthesis of CCs. These results confirmed the reports of Aresta et al. [94] and Ball et al. [39], who previously had investigated the reaction of primary and secondary alcohols with urea to form carbonate. These authors found the reaction to proceed in two steps, with a combination of a weak Lewis acid and a Lewis base improving the carbonate formation. 

Scheme 8. Urea alcoholysis mediated by 41 (101). (Solvent molecule as ligand = Solv.)...

It was indeed possible to detect the presence of triethylamine at the end of the reactions. With regard to the role of the triethylamine formed, several pieces of evidence, and data from the literature, make it clear that the carbonylation reaction proceeds through the intermediate formation of diphenylurea, which is only later alcoholyzed to carbamate, regenerating the aniline necessary for the reaction to proceed. Such alcoholysis is analogous to transesterification and is expected to be base-catalyzed. Indeed, a series of experiments on urea alcoholysis under typical catalytic reaction conditions showed that triethylamine does indeed accelerate urea alcoholysis. 

The reaction occurs even in the absence of a catalyst when the urea and an excess of the alcohol are heated together at temperatures higher than about 150 °C (for some examples see [18-22]). The reaction is usually complete for aromatic ureas, but is an equilibrium for aliphatic ones [20]. It can be catalysed by both acids [21] and bases [22], but is insensitive to the presence of a metal catalyst [20, 21]. In one paper [23], it has been reported that urea alcoholysis was accelerated by the presence of a palladium-based catalytic system. However, since the catalytic system employed in this paper also contained an excess of p-... 

The isocyanate formed in the previous step of the catalytic cycle reacts then in solution with aniline or methanol to afford respectively diphenylurea or methyl phenylcarbamate. The reaction with aniline is much fester then the one with methanol and, if both the reagents are present in solution, only urea is formed, unless the aniline/isocyanate ratio is lower than one. Since, however, the carbamate is the thermodinamically fevoured product, urea alcoholysis may then occur at a variable extent, depending on the reaction conditions. Under typical catalytic reaction conditions, where an aniline is added in large amount, urea is consequently inferred to be the exclusively primary product of the reaction of the isocyanate, although it may not be present at all at the end of the reaction. 

Some references cover direct preparation of the different crystal modifications of phthalocyanines in pigment form from both the nitrile—urea and phthahc anhydride—urea process (79—85). Metal-free phthalocyanine can be manufactured by reaction of o-phthalodinitrile with sodium amylate and alcoholysis of the resulting disodium phthalocyanine (1). The phthahc anhydride—urea process can also be used (86,87). Other sodium compounds or an electrochemical process have been described (88). Production of the different crystal modifications has also been discussed (88—93). 

Kostic et al. reported the use of various palladium(II) aqua complexes as catalysts for the hydration and alcoholysis of nitriles,435,456 decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.457 Labile aqua or other solvent ligands can be displaced by a substrate. In many cases, the coordinated substrate thus becomes activated toward nucleophilic addition of water or alcohols. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

Figure 36 Complexes catalyzing hydrolytic cleavage of peptides, hydrolysis and alcoholysis of nitriles, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various...

The complex m-[Pd(dtod)(sol)2]2+ also promotes the stoichiometric alcoholysis of urea by the hydroxyl groups within the ligand dtod.43... 

The addition of anilines to styrene oxide was reported to also proceed in the presence of 10mol% 37 affording the corresponding P-amino alcohols 1-5 in yields ranging from 75% to 92% (Scheme 6.37). Additionally, urea derivative 37 (20mol% loading) was found to catalyze the addition of aniline (2.0 equiv.) to ( )-stilbene oxide (92% yield 5.9 d 30°C), the addition of thiophenol (2.0 equiv.) to 2-methoxy styrene oxide (85% 20h rt), and the alcoholysis of 4-methoxy styrene oxide with benzyl alcohol (2.0 equiv.) affording the respective P-alkoxy alcohol (82% 20h rt). 

The alcoholysis of urea affords stepwise acyclic carbonates through the formation of an alkyl carbamate intermediate (Scheme 7.2). The reaction of urea with alcohols to carbamate is exothermic, whereas the subsequent reaction-carbamate to carbonate-is endothermic. Thus, the ideal gas free energy, AG, is positive for the latter step, which means that a low yield of carbonates would be expected [12],... 

Resolution of sec-amines. Reaction of sec-amines with (+)- or (- )-l results in two diasteriomeric ureas, R2NCONH(CH3)C6H5, which can usually be resolved by either chromatography or crystallization. The resolved sec-amine is obtained by alcoholysis of the optically pure urea. The method is particularly valuable where both optical isomers of the amine are desired see also (R)-( - )-(l-naphthyl)ethyl isocyanate (6, 416) for a similar reagent. 

The Pd(II) complex (41) promotes stoichiometric alcoholysis of urea according to Scheme 8, giving the carbamate esters of the ligand (101). For methylurea as the substrate, the major product is the one with R = H (75%), while the product with R = Me is the minor one (25%). This intramolecular alcoholysis is 240-380 times faster than the intermolecular alcoholysis involving external attack of free ethanol. The O-bound 1,3-dimethylurea does not undergo any detectable intramolecular or catalytic alcoholysis, since the N-bound isomer, which is the much more reactive one, is practically absent due to steric reasons. 

The alkoxylation process is easy to apply to PU foams having a low concentration of urethane and urea groups such as flexible and semiflexible foams, integral skin foams, PU elastomers and so on. Urea groups react in a similar way with urethane groups, with the formation of oxazolidones and amines by an intramolecular alcoholysis of urea groups (reaction 20.15). 

Likewise, hydrolysis, alcoholysis, and aminolysis of 6-substituted 1,3,5-oxadiazine-2,4(3/7)-diones (76) proceed readily to yield 77,77 -disubstituted ureas (77 = H, CO2 alkyl, CONR R ) <86CB669>. 

The role of the vanadium compound is not clear. The sulphurA 03 system also catalyses the oxidative carbonylation reaction of amines by CO/O2 [22 and references therein]. In this last case its main effect was to increase the proportion of carbamate in the product, with a corresponding decrease in the amount of the obtained urea, but the total amount of carbonylated products was little varied. It was also shown that the vanadium compound does not accelerate the alcoholysis of urea to carbamate (the reported data indicate that it even slows it down) thus it must act at another stage of the reaction, but it is not clear which one. 

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