Kinetics and mechanism of the reactions of quinuclidines with ethyl S-arylthiolcarbonates

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 reactio ns asr? studied by following spectrophotometrically (400 nm) the release of the corresponding substituted benzenethiolate anion. Under quinuclidine ex cess, 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 ind ependent, with slope k(N). The Bronsted-type plots (log k(N) vs pK(a) of qu inuclidinium ions) are linear, with slope beta = 0.85 for NPTC, in agreemen t with a stepwise mechanism where the breakdown of a tetrahedral addition i ntermediate (T+/-) is rate determining, and beta = 0.54 and 0.47 for DNPTC and TNPTC, respectively, consistent with a concerted mechanism. By comparis on of the reactions under investigation among them and with similar aminoly ses, the following conclusions can be drawn: (i) Substitution of the 4-nitr obenzenethio group in T+/- by 2,4-dinitrobenzenetbio or 2,4,6-trinitrobenze nethio destabilizes the tetrahedral intermediate. (ii) Quinuclidines destab ilize 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 qu inuclidines. (iii) Quinuclidines are more reactive toward the carbonyl grou p of phenyl 4-nitrophenyl carbonate than that of NPTC.