Raman, IR and DFT studies of mechanism of sodium binding to urea catalyst

• Characterization of bis-camphorsulfonyl urea by Raman and IR.
• DFT study of the compound in free and metal bound complex form.
• Assignments of vibrational bands with the aid of DFT.
• Prediction of the mode of binding of Na+ with the sulfonyl urea through combined experimental and theoretical study.

Bis-camphorsulfonyl urea, a newly developed hydrogen bonding catalyst, was evaluated in an enantioselective Friedel–Crafts reaction. We observed that complexation of the sulfonyl urea with a sodium cation enhanced the selectivity of reactions in comparison to reactions performed with urea alone. To understand the role of sodium cation, we performed Infrared and Raman spectroscopic studies. The detailed band assignment of the molecule was made by calculating spectra using Density Functional theory. Our studies suggest that the binding of the cation takes place through the oxygen atoms of carbonyl and sulfonyl groups. Natural Bond Orbital (NBO) analysis shows the expected charge distribution after sodium binding. The changes in the geometrical parameter and charge distribution are in line with the experimentally observed spectral changes. Based on these studies, we conclude that binding of the sodium cation changes the conformation of the sulfonyl urea to bring the chiral camphor groups closer to the incipient chiral center.

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