Toward Green and Sustainable Chemical Glycosylation: Enhanced Lewis Acidity of Recyclable Solid Super Acid Catalyst, SO₄/ZrO₂ by CaCl₂ Doping

Abstract

The simple doping of calcium chloride allowed highly improved yields in the glycosylation promoted by sulfated zirconia (SZ) without calcination. It was revealed by means of per-O-benzylated galactose trichloroacetimidate as a model glycosyl donor that this effect depends greatly on the cationic ion radius of used metal chlorides. Temperature-programmed desorption of ammonia (NH₃ TPD) and the pyridine IR method showed clearly that coordination of calcium ion provides the SZ surface with newly formed Lewis acidic sites while the Brønsted acid site disappeared, indicating that the enhanced catalytic potency of SZ is due to the increased Lewis acid sites by doping the optimal calcium ions. The present findings might give insight into the relationship between a catalytic mechanism and superacidity of SZ, which is crucial for the design of novel superacid–based catalysts and environmentally benign glycosylation processes.

Keywords:

• Glycosylation
• Green chemistry
• Recyclable solid superacid
• Lewis acid site
• Calcium chloride

ACKNOWLEDGEMENT

This work was supported partly by a grant for “Innovation COE Program for Future Drug Discovery and Medical Care” from the Ministry of Education, Culture, Science, and Technology, Japan.

Notes

^aTo each metal salt was added 0.4 equiv for 1.

^bIsolated yield.

^aMole ratio with 1.

^bIsolated yield.

^aIsolated yield.

^aEquivalents were calculated by the ratio with 1.

^bRelative value.