Chiral Sulfonic Acids

Sulfonic acids are a class of organic compounds in which a sulfo group is bonded to a hydrocarbon group (including an aryl group). The formula is R-SO₃H, wherein R represents a hydrocarbon group.

When the R group is a chiral group, it becomes a chiral sulfonic acid. The sulfonic acid group is a strong water-soluble strong acidic group, and the sulfonic acid is a water-soluble strongly acidic compound. The sulfonic acid group in the aromatic sulfonic acid molecule may be substituted with a hydroxyl group or a cyano group and is an intermediate for preparing a phenol or a nitrile. Due to the strong acidity of the chiral sulfonic acid and the diversity of substituents, it has been used in a variety of catalytic reactions.

Application:

• Catalytic oxidative addition reaction: Some researchers have synthesized a mixture of azacarbene and trifluoromethanesulfonate chiral rare earth metal salt, which shows good catalytic activity and stereoselection in the oxidative addition reaction of diene aldehyde with trifluoromethyl ketone.
• Catalytic Diels-Alder reaction: The Diels-Alder reaction is the simplest and the most common method of constructing a six-membered ring. Although the Diels-Alder reaction can be carried out at elevated temperatures, some Diels-Alder reactions are reversible and require a reduction in temperature to inhibit the occurrence of retro-Diels-Alder. The Diels-Alder reaction catalyzed by the chiral sulfonic acid metal salt can be carried out at a lower temperature with better regional and stereoselectivity. In recent years, researchers have successfully catalyzed asymmetric Diels-Alder reactions using RE(OTt)3 chiral trifluorosulfonic acid rare earth metal salt complexes as catalysts.

Figure 1. Chiral trifluorosulfonate catalyzes Diels-Alder reaction

• Catalytic Mannich reaction: The Mannish reaction is the most common method for the synthesis of β-aminoketones and β-aminoester derivatives. TiCl₄ has been reported as a catalyst to catalyze the reaction of the imine with enol silyl ether, but the reaction requires the addition of an equivalent catalyst. Chiral sulfonic acid metal salts as a catalyst to catalyze the reaction of the imine with enol silyl ether can greatly reduce the amount of catalyst.

Production:

• Sulfonation reaction of aromatic hydrocarbons
• The oxidation reaction of mercaptans
• Substitution of a halogen atom to obtain a sulfonate

References

1. Barroso S, Blay G et al. (2011). "Highly Enantioselective Nitrone Cycloadditions with 2-Alkenoyl Pyridine N-Oxides Catalyzed by Cu(II)-BOX Complexes". Org Lett, 13:402-405.
2. Dong L, Geng C W (2015). "Silyl Nitronate Cycloadditions Catalyzed by Cu(II)-Bisoxazoline." J Org Chem, 80:10992-11002.
3. Li Y N, Hu Y B, Li L J, et al (2016). "Copper-Catalyzed Enantioselective Hetero-Diels-Alder Reaction of Danishefsky's Diene with Glyoxals." J Org Chem, 81:2993-2999.