A GENERAL ACID-CATALYZED ANION BREAKDOWN ASSOCIATED WITH AN E1CB REACTION IN THE HYDROLYSIS OF ARYL N-(SUBSTITUTED PHENYLSULFONYL)CARBAMATES

The hydrolysis of aryl N-(substituted phenylsulfonyl)carbamates at 50 degrees C in the pH range 0-13.5 leading to substituted benzenesulfona mides and phenols involves acyl group transfer. Reaction rates were me asured spectrophotometrically and are independent of the concentration of the catalyst for buffers of pK(BH) > 8. With more acidic buffers, a general acid catalysis is observed for the hydrolysis of phenyl N-(p henylsulfonyl)carbamate la (pK, = 2.85) leading to a Bronsted relation (alpha = 0.46) including hydronium ion while it does not occur for 4- cyanophenyl N-(phenylsulfonyl)carbamate 1d. In alkaline media, the val ues of the entropies of activation are positive and lie between 0.20 a nd 15.0 cal mol(-1) K-1. The Hammett rho(-) value for the expulsion ra te k(p) of phenolate ion from aryl N-(phenylsulfonyl)carbamates is pos itive (rho(-) = +2.93) while, for phenyl N-(substituted phenylsulfonyl )carbamates it is negative (rho = -0.66). An electrophilic intermediat e, phenylsulfonyl isocyanate, was trapped with piperidine to give N-(p iperidinocarbonyl)benzenesulfonamide. These results are consistent wit h an elimination-addition mechanism A(rh)D(H) + D-N (E1cB). In HCl sol utions, solvent isotope effects k(obsd)(H2O)/k(obsd)(D2O) equal to 1.6 8 for 1a and 2.46 for Id have been measured. For molar concentrations of water lying between 40 and 55.5 M(-1), the rate of hydrolysis of la in 0.5 M HCl solutions increases in the presence of dioxan while it d ecreases with chloral. The Hammett rho values for the reactivity k,in acidic media of aryl N-(phenylsulfonyl)carbamates (rho = +0.87) and fo r phenyl N-(substituted phenylsulfonyl)carbamates (rho = +0.22) agree with the protonation of the oxygen atom of the leaving group. Trapping experiment for the hydrolysis of 1a and 1d in a chloroacetate buffer in the presence of 4-chloroaniline gave in both cases N-(((4-chlorophe nyl)amino)carbonyl)sulfo. Reaction carried out in (H2O)-O-18 with O-16 - containing substrate indicates that there is no exchange from water to the carbonyl group of the sulfonylcarbamate. These data support a g eneral acid catalysis of the breakdown of the anion which cuts in depe nding on the leaving group ability.