Analysis of heavy crude oil residues by carbon-13 Fourier transform nuclear magnetic resonance spectrometry

Alcoholysis of Urea Catalyzed by Palladium(II) Complexes

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.

Kinetics and Mechanism of Urea Hydrolysis Catalyzed by Palladium(II) Complexes

Four palladium(II) aqua complexes catalyze hydrolytic decomposition of urea into carbon dioxide and ammonia. The initial rates of carbon dioxide formation at 313 K and pH 3.3 fall in the range 6.7 x 10(-)(5) to 1.6 x 10(-)(4) M min(-)(1), depending on the catalyst. The pseudo-first-order rate constant for the formation of carbon dioxide is 1.7 x 10(-)(3) min(-)(1) in the presence of 0.30 M cis-[Pd(en)(H(2)O)(2)](2+) as the catalyst at 313 K and pH 3.3. This reaction is ca. 1 x 10(5) times faster than the uncatalyzed decomposition of urea. The reaction catalyzed by cis-[Pd(en)(H(2)O)(2)](2+) is monitored by (13)C and (15)N NMR spectroscopic methods. The following steps in the mechanism of this reaction are studied quantitatively: binding of urea to the catalyst, formation of carbamic acid (H(2)NCOOH) coordinated to palladium(II) via the nitrogen atom, and conversion of this intermediate into carbon dioxide and ammonia. These products are formed also by another pathway that does not involve carbamic acid. Kinetic effects of added acid and inhibition of the reaction by addition of thiourea and of bases are interpreted quantitatively. Ammonia inhibits the decomposition. When, however, this product is sequestered by metal cations, the reaction becomes relatively fast and catalytic turnover is achieved. The most effective of these sequestering agents is the silver(I) cation. Although the simple palladium(II) complexes are very different from the enzyme urease, which contains nickel(II) ions, the decomposition of urea catalyzed by both kinds of agents involves carbamic acid as the intermediate. Kinetic and mechanistic studies with metal complexes contribute to the understanding of the enzymatic mechanism.