Kinetics and mechanism of the reactions of quinuclidines with ethyl S-aryl thiolcarbonates

The reactions of quinuclidines with ethyl S-(4-nitrophenyl) thiolcarbonate (NPTC), ethyl S-(2,4-dinitrophenyl) thiolcarbonate (DNPTC), and ethyl S-(2,4,6-trinitrophenyl) thiolcarbonate (TNPTC) are subjected to a kinetic study in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCl). The reactions asr? studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anion. Under quinuclidine excess, pseudo-first-order rate coefficients (k(obsd)) are found. Plots of k(obsd) VS [N] (N is the free substituted quinuclidine) are linear and pH independent, with slope k(N). The Bronsted-type plots (log k(N) vs pK(a) of quinuclidinium ions) are linear, with slope beta = 0.85 for NPTC, in agreement with a stepwise mechanism where the breakdown of a tetrahedral addition intermediate (T+/-) is rate determining, and beta = 0.54 and 0.47 for DNPTC and TNPTC, respectively, consistent with a concerted mechanism. By comparison of the reactions under investigation among them and with similar aminolyses, the following conclusions can be drawn: (i) Substitution of the 4-nitrobenzenethio group in T+/- by 2,4-dinitrobenzenetbio or 2,4,6-trinitrobenzenethio destabilizes the tetrahedral intermediate. (ii) Quinuclidines destabilize the tetrahedral intermediate relative to secondary alicyclic amines, anilines, and pyridines. The leaving abilities of isobasic amines from T+/- follow the sequence pyridines i anilines i secondary alicyclic amines i quinuclidines. (iii) Quinuclidines are more reactive toward the carbonyl group of phenyl 4-nitrophenyl carbonate than that of NPTC.