PROTON TRANSFERS OF SUBSTITUTED AMMONIUM-SALTS .14. 1,CIS-2,6-TRIMETHYLPIPERIDINIUM ION IN ANHYDROUS ACIDIC DIMETHYL-SULFOXIDE

The kinetics of nitrogen inversion of the title compound has been studied at 25¼ in acidic Me2SO using variable pH (from -0.3 to 7.7) and piperidine concentrations Co (0.2-0.5 M). The two isomeric piperidinium cations AH+ and BH+, where the TV-methyl is in equatorial or axial position, respectively, are observed either by DNMR using a solution of both isomers in equilibrium or by equilibration NMR using a solution initially containing one isomer (AH+) only. In both cases, the interconversion (i) is brought to within the appropriate time scale through protonation of the nitrogen atom according to the scheme in (ii). Three different laws are obtained: kNA + kNB = 120.4 X 10-7Co/[H+] or 3 X 10-4 or 3.5 X 10-4/[H+] (s-1) according to the pH range investigated (pH >2 or 0 < pH < 2 or pH < 0, respectively). They can be accounted for when the deprotonation and reprotonation rates are taken into consideration: kd = k3 [piperidine] + k1, and kp = f3[piperidinium ion] + where k-1 = 2.70 × 104 M-1 s-1,k1 = 3 × 104 and k-1 = 6.7 × 105 M-1 s-1 refer to the reactions Compared to previous data in aqueous solutions, the proton transfer and nitrogen inversion rates are respectively decreased and increased by four and three orders of magnitude. These results illustrate the importance of water as a small bridging molecule both allowing fast proton transfers within sterically hindered acid-base pairs and promoting the formation of associated species. A…H2O…AH+, which are not likely to undergo inversion. © 1979, American Chemical Society. All rights reserved.