A theoretical density functional study of association of Zn2+ with oxazolidine and its thio derivatives in the gas phase

We have performed density functional theory (DFT) calculations in order to study the gas‐phase interaction of oxo‐ and thio‐oxazolidine derivatives with Zn²⁺. The calculations were performed at B3LYP/6‐311+(2df,2p) level of theory. It has been found, in all cases, that the direct association of Zn²⁺ with the carbonyl and thiocarbonyl groups takes place at the heteroatom attached to position 2 irrespective of its nature. This preference has been attributed to the resonance effects caused by the nearest heteroatoms (oxygen and nitrogen). The most stable complexes correspond to structures with Zn²⁺ bridging between the heteroatom at position 2 or 4 of the 4‐ or 2‐enol (or the 4‐ or 2‐enethiol) tautomer and the dehydrogenated ring nitrogen atom, N3. Zn²⁺ association has a clear catalytic effect on the tautomerization processes which connect the oxo–thione forms with the enol–enethiol tautomers. Hence, although the enol–enethiol tautomers of oxazolidine and its thio derivatives should not be observed in the gas phase, the corresponding Zn²⁺ complexes are the most stable species and should be accessible, because the tautomerization barriers are smaller than the Zn²⁺ binding energies. Copyright © 2010 John Wiley & Sons, Ltd.