Alcoholysis of Urea Catalyzed by Palladium(II) Complexes

Abstract

The palladium(II) aqua complex cis-[Pd(en)(H(2)O)(2)](2+) catalyzes the alcoholysis of urea into alkyl carbamate and ammonia. The observed rate constants for the ester formation fall in the range from 1.8 x 10(-)(5) to 5.9 x 10(-)(1) min(-)(1) at 313 K and pH 3.3, depending on the alcohol. This catalyzed reaction is at least 10(5) times faster than the uncatalyzed alcoholysis of urea under the same conditions. This is the first example of catalytic, nonhydrolytic cleavage of the amide bond in urea. The following steps in the mechanism of the methanolysis reaction are studied quantitatively: binding of urea to the catalyst in the presence of various alcohols or various concentrations of water, direct methanolysis of O-bound and N-bound urea, formation of carbamic acid (NH(2)COOH) coordinated to palladium(II) via the nitrogen atom, methanolysis of this intermediate, and the fast dissociation resulting in free methyl carbamate. Ammonia, a product of alcoholysis, inhibits this reaction by binding to palladium(II). When, however, ammonia is sequestered by the silver(I) cation, alcoholysis becomes relatively fast, and catalytic turnover is achieved. Various alcohols are compared in their reactivity toward urea. The effects of nucleophilicity, steric bulk, size, and additional hydroxyl groups (in diols) are examined. The intramolecular alcoholysis in the 2,6-dithia-1,8-octanediol complex cis-[Pd(C(6)H(14)O(2)S(2))(H(2)O)(2)](2+) results in at least 100-fold rate enhancement relative to the intermolecular alcoholysis by cis-[Pd(en)(H(2)O)(2)](2+). Alkyl carbamates do not hydrolyze further into carbamic acid and alcohol. Aryl carbamates do hydrolyze further, and this reaction requires the palladium(II) aqua complex as a catalyst. Carbamic acid then spontaneously decomposes into carbon dioxide and ammonia. Observed rate constants for the appearance and disappearance of aryl carbamates agree with the relative nucleophilicities of aryl alcohols. This study of the catalysis by a metal complex may contribute to the understanding of the metalloenzyme urease. We propose a new method, alcoholysis, for cleaving amide bonds in peptides and proteins.

Urea methanolysis to dimethyl carbonate over ZnO–CeO2–MO (MO: La2O3, Y2O3, Co2O3, Ga2O3, and ZrO2) catalysts

“…In this process, the intermediate methyl carbamate (MC) is first formed and it is further converted to DMC by consecutive reaction with methanol, liberating ammonia gas at each step. It has been reported that ammonia formed during the reaction restricts the reaction shift toward MC to DMC due to the limitation of thermodynamic equilibrium [21,22]. Thus, an autoclave equipped with a condenser and a back pressure regulator has been used to remove ammonia gas during the reaction.…”

Ureas as new nucleophilic reagents for SNH amination and carbamoyl amination reactions in the 1,3,7-triazapyrene series

“…Ureas are normally rather inert towards alcohols, amines, and thiols: they require high temperatures, acidic or basic conditions, or metal catalysis, to undergo nucleophilic substitution reactions. [31][32][33][34] Ureas were used earlier as specific nitrogen nucleophiles in the palladium(II)-catalyzed amino carbonylation of unsaturated amines, 35 in intramolecular cyclisation of ureido acids and esters, 36 in the synthesis of a di-and triarylamines by interaction with unactivated aryl halides under palladium catalysis 37 and in Biginelli reactions. 38 In the course of this study, and taking into account the results of oxidative nucleophilic amidation of 1,3,7-triazapyrene, 28 we have tested the possibility of S N H carbamoyl amination of this heterocycle using urea as the nucleophilic reagent.…”

One-Pot Synthesis of Dialkyl Hexane-1,6-Dicarbamate from 1,6-Hexanediamine, Urea, and Alcohol over Zinc-Incorporated Berlinite (ZnAlPO4) Catalyst

“…The decrease in yield for 2 and the increase in selectivity for 1 at temperatures higher than 493 K may be due to the decomposition of 2 to 1, because 2 as a secondary product can been formed via the further reaction of 1, urea and butanol [17,20,27]. The selectivities to both 3 and 4 decreased slightly with increasing temperature, while that for 5 decreased very slightly over the temperature range 413 to 433 K, and rapidly at the temperature higher than 433 K. This could be due to the fact that a higher temperature accelerates the synthesis of 2 by the above reactions, including Reactions (5), (6) and (9) [17,20,27]. Although a higher conversion of HDA could be attained at high temperatures, a very high temperature reduced the yield of 2, possibly because of the partial decomposition of 2 to 1 [17,27].…”

“…However, a number of drawbacks may occur in the route, including difficult handling of the toxic phosgene and the seriously corrosive side product HCl [2]. Several phosgene-free routes for organic carbamates synthesis have also been reported [4][5][6][7][8][9], among which the synthesis from amines, urea and alcohols by a one-pot reaction [9] has received considerable attention. This route appears to be promising because, on the one hand, ammonia is obtained as a main byproduct and, on the other hand, urea can be prepared on a large scale by the dehydration of ammonia and CO 2 [10].…”

“…It is interesting that the main byproducts mixture (1, 3-5) mentioned above can also be directly employed as the raw materials in the synthesis of the desired product 2 by Reactions (5), (6), (8) and (9) [15,16,19,20,26,28].…”