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 degrees C. The pa-rate profiles of the hydrolyses showed water and hydroxide-ion-catalyzed reacti ons. The reaction of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) and p-n itrobenzyl cellulose xanthate (CelXNB) with polyalanine and lysozyme produc ed a covalent bond between the polypeptide and the cellulose matrix, as sho wn by solid-state C-13 NMR. However, the nature of the bonding could not be identified. The reaction of nucleophiles (H2O, OH-, RNH2) and xanthic este rs was consistent with an addition-elimination mechanism through a tetrahed ral intermediate. Bronsted plots against the pK(a) of the nucleophile (beta (nu)) or the nucleofuge of the substrate (beta(lg)) were used to characteri ze the rate-determining step. The pK(a) values of the nucleophiles ranged b etween -1.74 and 15.74, and for the nucleofuges, they were in the range of 10.50-0.92. For nucleophiles with pK(a) values up to about 10, beta(lg) was 0.10-0.15, and beta(nu) changed from 0.48 to 0.35 for the strongest electr on-withdrawing nucleofuge. It was concluded that the water-catalyzed hydrol yses, and also aminolyses with moderately basic amines, occur with rate-det ermining formation of the tetrahedral intermediate. For strong bases such a s hydroxide ion, the disappearance of the intermediate becomes the slowest step. The reaction of cellulose xanthic esters with external nucleophiles a s 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.