Catalytic and substituent effects on cycloaddition reactions between isocyanates and aldehydes

The cycloaddition reactions of aldehydes and isocyanates leading to 1-oxo-3-azetidine-2-ones were studied theoretically. For the model reaction(HN=C=O + H2C=O) and the catalytic reaction(HN=C=O + H2C=O...BH3), the stationary points are located by HF/6-31G*, MP2/6-31G* and MP2/6-31+ +G** respectively. IRC calculation, at HF/6-31G* level, showed that this type of reaction is concerted. Four frontier orbitals are involved in these reactions, which are of '2 x [1+1]' type. The parent reaction has a 34.0 kcal/mol energy barrier at the MP2/6-31+ +G** level; while the BH3-catalyzed reaction has one at 18.1 kcal/mol. The presence of different substituted groups on isocyanates and/or on aldehydes will change energies as well as the structures of transition states, but all the studied reactions are believed to be concerted. The calculations also show that the substitution of the F or SO2Cl group on isocyanate and/or CF3 groups on aldehyde will favor the reaction. The reaction FNCO+PhCH=O is the easiest among our studied reactions. (C) 1999 Elsevier Science B.V. All rights reserved.