Pyridinium p-Toluenesulfonate(PPTS) is a salt of pyridine and p-toluenesulfonic acid. Pyridinium p-Toluenesulfonate is a colourless solid. Pyridinium p-Toluenesulfonate has been extensively explored as an organic catalyst for promoting chemical transformations such as synthesis and cleavage of acetals,and deprotection of silyl or tetrahydropyranyl groups.This pyridinium salt can be facilely prepared from pyridine and p-toluenesulfonic acid and has good solubility in organic solvents like methylene chloride, chloroform, acetone and ethanol. Pyridinium p-Toluenesulfonate is a mild acid which is especially usefulwhen a reaction substrate is unstable to strong acids. Pyridinium p-Toluenesulfonate is also a commonly used catalyst for the preparation of acetals and ketals from aldehydes and ketones[1].

Fig 1. Chemical structure formula and three-dimensional structure of Pyridinium p-Toluenesulfonate

As a co-catalyst with L-proline, Pyridinium p-Toluenesulfonate improves both yield and enantioselectivity of an asymmetric aldol condensation between dioxanones and aldehydes. Pyridinium p-Toluenesulfonate is an efficient catalyst for preparing tetrahydropyranyl ethers[2].

Pyridinium p-toluenesulfonate is a mild and efficient catalyst for the formation and cleavage of acetals. This method has been adapted in the completely symmetric synthesis of tetrahydrolipstatin. Pyridinium p-toluenesulfonate is also capable of selectively cleaving tert-butyldimethylsilyl (TBDMS) ethers in the presence of tert-butyldiphenlysilyl (TBDPS) ethers[3].

Pyridinium p-Toluenesulfate is used in the synthesis of a subdomain of the cytochrome P450 BM3 enzyme, for use in screening systems for drug candidates[4]. 

Pyridinium-p-Toluenesulfonate is used as a weakly acidic catalyst, providing an organic soluble source of pyridinium (C5H5NH+) ions. Pyridinium-p-Toluenesulfonate is also a commonly used catalyst for the preparation of acetals and ketals from aldehydes and ketones.

Aryl methyl ethers are deprotected in high yields using pyridinium p-toluenesulfonate under microwave irradiation and solvent-free conditions.

Pyridinium-p-Toluenesulfonate is soluble in water (partly), ethanol, acetone, dichloromethane, chloroform, methanol and dimethyl sulfoxide.

Pyridine p-toluenesulfonate is a widely used, mild, laboratory-ready acidic catalytic reagent for organic synthesis. Pyridine p-toluenesulfonate can be simply viewed as a substitute for p-toluenesulfonic acid, and all reactive Pyridine p-toluenesulfonate completed with p-toluenesulfonic acid can also be accomplished. However, Pyridine p-toluenesulfonate has more advantages than the use of p-toluenesulfonic acid. For example, p-toluenesulfonic acid is generally present and used in the form of monohydrate crystals, while Pyridine p-toluenesulfonate is anhydrous; p-toluenesulfonic acid is more acidic, The acidity of Pyridine p-toluenesulfonate does not have a significant effect on many acid-sensitive functional groups. The Pyridine p-toluenesulfonate catalyzed reaction usually has the characteristics of simple and mild reaction conditions, and the amount is generally within 30% by mole. The two most widely used reactions are the acetalization of hydroxyl groups and the silylation of hydroxyl groups, as well as the reverse reaction of these two reactions, so they are widely used in the protection and deprotection of hydroxyl groups.

References

[1] TOKYO CHEMICAL INDUSTRY CO., LTD. "Pyridinium p-Toluenesulfonate". Retrieved 2011-09-16.
[2] David A. Evans (2005-11-04). "Evans pKa Table" (PDF). Retrieved 2016-03-02.
[3] Synlett, 2387, (2006) 2. J. Am. Chem. Soc. 114, 5086, (1992) 3. J.
[4] Mild catalyst for the formation of THP derivatives of alcohols superior to p-toluenesulfonic acid, where acid-sensitive functionality is present. Similarly, the reagent has been used for deprotection of THP ethers in ethanol: J. Org. Chem., 42. 3772 (1977).