THEORETICAL-STUDY OF THE MECHANISM OF THE WITTIG REACTION - AB-INITIOAND MNDO-PM3 TREATMENT OF THE REACTION OF UNSTABILIZED, SEMISTABILIZED AND STABILIZED YLIDES WITH ACETALDEHYDE

In this study, we describe the results of ab initio (HF and MP2) and M NDO-PM3 calculations on the model reactions of unstabilized (Me3P=CH-C H3), semistabilized (Me3P=CH-C=CH), and stabilized (Me3P=CH-C=N) ylide s with acetaldehyde to form their respective Z and E olefins and trime thylphosphine oxide. These reactions occur in three stages: oxaphosphe tane formation, oxaphosphetane pseudorotation, and oxaphosphetane deco mposition. The calculated barriers for these processes vary considerab ly depending on the level of theory employed (ab initio vs. MNDO-PM3 o r HF vs. MP2 at the ab initio level). However, self-consistent geometr ies of reactants, intermediates, transition states and products are ob tained at all levels. Oxaphosphetane formation is best described as ve ry asynchronous cycloaddition (borderline two-step mechanisms). The ge ometries of the transition states are near planar with respect to P, C , C, and O atoms. Analysis of the bond indices of these reactions show s that the C-C bonds are between 44% (unstabilized case) and 60% (stab ilized case) formed whereas the corresponding P-O bonds have not been formed to any significant degree. Oxaphosphetane decomposition can be described as a very asynchronous retrocycloaddition where P-C bond bre akage runs ahead of C-O bond breakage. These results are compared with experimental findings for the Wittig reaction, and its relevance to t he overall mechanism of the olefination is discussed. (C) 1997 John Wi ley & Sons, Inc.