Synlett 2006(1): 0147-0149  
DOI: 10.1055/s-2005-922786
CLUSTER
© Georg Thieme Verlag Stuttgart · New York

Chiral Proton Catalysis: pKa Determination for a BAM-HX Brønsted Acid

Abram S. Hessa, Ryan A. Yodera,b, Jeffrey N. Johnston*a,b
a Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA
b Department of Chemistry, Indiana University, Bloomington, IN 47405-7102, USA
e-Mail: jeffrey.n.johnston@vanderbilt.edu;
Further Information

Publication History

Received 10 October 2005
Publication Date:
16 December 2005 (online)

Abstract

Perrin’s NMR titration procedure has been used to measure the pKa values for several Brønsted acids. These measurements allow a preliminary determination of the absolute pKa for these chiral acids.

    References and Notes

  • 1 Nugent BM. Yoder RA. Johnston JN. J. Am. Chem. Soc.  2004,  126:  3418 
  • 2 Alder RW. Bowman PS. Steele WRS. Winterman DR. J. Chem. Soc., Chem. Commun.  1968,  723 
  • 4 Perrin CL. Fabian MA. Anal. Chem.  1996,  68:  2127 
  • 5 Kolthoff IM. Chantooni MK. Bhowmik S. J. Am. Chem. Soc.  1968,  90:  231 
  • 6 Benoit RL. Lefebvre D. Fréchette M. Can. J. Chem.  1987,  65:  996 
  • 7 Perrin CL. Fabian MA. Rivero IA. Tetrahedron  1999,  55:  5773 
  • 8 Um I.-H. Lee E.-J. Jeon S.-E. J. Phys. Org. Chem.  2002,  15:  561 
  • 10 The absolute pKa measured for pyridinium and picolinium ion are to a single decimal place. The pKa for pyridinium was determined electrochemically using the perchlorate salt in DMSO. Details for the pKa measurement for picolinium remain unavailable: Bordwell FG. Acc. Chem. Res.  1988,  21:  456 
  • 11 In H2O, pyridinium (pKa = 5.23) is less acidic than quinolinium (pKa = 4.90): Alber A. Goldacre R. Phillips J. J. Chem. Soc.  1948,  2240 
  • 12 Charmant JPH. Lloyd-Jones GC. Peakman TM. Woodward RL. Tetrahedron Lett.  1998,  39:  4733 
3

Johnston, J. N.; Yoder, R. A. unpublished work.

9

The level of uncertainty in this value is greater than the others since it requires an estimation of the chemical shift that correlates to 2. In a chart of the movement of ligand chemical shift as a function of titer, there is a clear inflection point at one equivalent of acid relative to the ligand. This strongly suggests that the monosalt can be identified spectroscopically as a single entity, and the calculated differences in pKa between 2 and 10 is consistent with a >100:1 equilibrium favoring 2. The existence of an inflection point is common in all titrations we have performed to-date in which one would be anticipated.