- Hydrolysis and aminolysis of alkyl xanthate esters and cellulose analogues
-
The hydrolysis and aminolysis of a series of S-substituted O-alkylxanthate esters was studied in 20% v/v aqueous methanol at 35°C. The pH-rate profiles of the hydrolyses showed water and hydroxide-ion-catalyzed reactions. The reaction of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) and p-nitrobenzyl cellulose xanthate (CelXNB) with polyalanine and lysozyme produced a covalent bond between the polypeptide and the cellulose matrix, as shown by solid-state 13C NMR. However, the nature of the bonding could not be identified. The reaction of nucleophiles (H2O, OH-, RNH2) and xanthic esters was consistent with an addition-elimination mechanism through a tetrahedral intermediate. Bronsted plots against the pKa of the nucleophile (βnu) or the nucleofuge of the substrate (βlg) were used to characterize the rate-determining step. The pKa values of the nucleophiles ranged between -1.74 and 15.74, and for the nucleofuges, they were in the range of 10.50-0.92. For nucleophiles with pKa values up to about 10, βlg was 0.10-0.15, and βnu changed from 0.48 to 0.35 for the strongest electron-withdrawing nucleofuge. It was concluded that the water-catalyzed hydrolyses, and also aminolyses with moderately basic amines, occur with rate-determining formation of the tetrahedral intermediate. For strong bases such as hydroxide ion, the disappearance of the intermediate becomes the slowest step. The reaction of cellulose xanthic esters with external nucleophiles as hydroxide ion and amines shows simple first-order kinetics and is slower than alkyl or sugar xanthates, probably due to the diffusion effect through the tight cybotactic region of cellulose.
- Humeres, Eduardo,Soldi, Valdir,Klug, Marilene,Nunes, Mauricea,Oliveira, Celia M.S.,Barrie, Patrick J.
-
p. 1050 - 1056
(2007/10/03)
-
- Electron Deficient Heteroaromatic Ammonioamidates. Part 26. N-(Quinazolin-3-io)amidates. Part 13. Phototransformations of an N-(Quinazolin-3-io)thioamidate and of a 10bH-1,3,4-thiadiazoloquinazoline, the Ring Isomer of an N-(Quinazolin-3-io)thioamidate, and the Photochemical Fo
-
Irradiation of the N-(quinazolin-3-io)thioamidate (10a) in ethanol and butylamine solution furnished, in addition to several other compounds, the 4,4'-biquinazolinyl (12a), the quinazolinyl alcohol (13a) and the quinazolinyl ketone (13b), respectively, the mesoionic triazoloquinazolinylium thiolate (18a) and the corresponding olate (18b) as novel type photolysis products.In contrast, irradiation of the 10bH-1,3,4-thiadiazoloquinazoline (11b), the ring isomer of the N-(quinazolin-3-io)thioamidate (10b) furnishes, with cleavage of either the quinazoline or the thiadiazole ring , a mixture of the thiadiazoles (21a,b), and the quinazoline derivatives (14a), (22a,b) in addition to 3,4-dimethoxybenzonitrile.Irradiation of various sulphur-containing quinazoline derivatives, including the quinazolinethiones (14a,e), the methylthioquinazoline (13f) and the diquinazolinyl disulphide (24a), leads to the formation of 4,4'-biquinazolinyls (12a) and (12b), respectively.Irradiation of the quinazoline (13c), which carries no sulphur-containing substituent, does not lead to the formation of the biquinazolinyl (12a); irradiation of a mixture of quinazoline (13e) and quinazolinethione (14a) gives rise to the formation of a mixture of biquinazolinyls (12a-c) with incorporation of the ring of the sulphur-free starting quinazoline (13e) into the products (12a) and (12b).
- Lempert-Sreter, Magda,Lempert, Karoly,Moeller, Joergen
-
p. 1143 - 1152
(2007/10/02)
-
- PYROTHIOCARBONATES I. AMINOLYSIS OF S-(ETHOXYCARBONYL) O-ETHYL DITHIOCARBONATE
-
The reaction of S-(ethoxycarbonyl) O-ethyl dithiocarbonate (1) with equimolar amounts of butylamine, benzylamine, diethylamine and piperidine in ethanol solution at 0 deg C is reported.The mole ratio of O-ethyl thiocarbamate (2) and O-ethyl carbamate (3) formed as main products is larger than unity during all the reaction.Bis(ethoxythiocarbonyl) sulfide (4) and bis(ethoxycarbonyl) sulfide (5) are also produced and their formation is explained in terms of the reaction of 1 with EtOCS2- and EtOCOS-, respectively, which are the leaving groups in the aminolysis of 1.Reactions 4 -> 2 and 5 -> 3 also take place.When 50percent of 1 has been consumed compound 4 is not detected and compound 5 is found at very low concentration, indicating that generation of 4 and 5 occurs mainly after aminolysis of 1.The fact that the ratio of 2 : 3 is larger than unity in this aminolysis step, and that S-methyl O-ethyldithiocarbonate is aminolyzed more readily than S-methyl O-ethylmonothiocarbonate under the same conditions as the aminolysis of 1, suggests that the thiocarbonyl group of 1 is more reactive than the carbonyl group.
- Palominos, Mario A.,Santos, Jose G.,Valderrama, Jaime A.,Vega, Juan C.
-
p. 245 - 252
(2007/10/02)
-