Molecular modeling of the Wittig olefination reaction
Abstract (Summary)The Wittig reaction has been investigated with the use of a molecular modeling approach utilizing empirical molecular mechanics calculations (MMX) and semiempirical molecular orbital methods (MNDO-PM3). The MNDO-PM3 method reproduces the geometric and thermodynamic parameters, previously calculated using a more elaborate ab initio molecular orbital approach, of the "mythical Wittig half-reaction". MNDO-PM3 calculations and subsequent analyses using bond orders and localized molecular orbitals indicates that the transition states of the mythical Wittig half-reaction and the Wittig half-reaction of unstabilized ylides with aldehydes have an important ionic character in which the C-C bond is about 30 to 40% formed, whereas there is no evidence of P-O bond formation. This type of transition state is best described as a 'syn' pseudo-betaine transition state, where the 'syn' geometry is a consequence of the strong attractive interaction of the positively charged phosphorus atom and the negatively charged oxygen atom. Therefore, a completely synchronous cycloaddition mechanism does not seem to be likely for the Wittig half-reaction of unstabilized ylides with aldehydes which affords the Z-oxaphosphetane as the major product. On the other hand, the reactions of stabilized and semistabilized ylides are less asynchronous, and evidence for partial P-O covalent bonding in the transition state is provided by the MNDO-PM3 calculations. In all cases of the present molecular modelling study, E-stereoselectivity is predicted for any 'syn' approach of the reactants. The only justification for the Z-stereoselectivity is attributable to an 'anti' approach of the reactants. The formation of an 'anti' transition state with the same characteristics as the 'syn' pseudo-betaine described with the use of the MNDO-PM3 SCF calculations, is conceivable for the reactions of unstabilized or semistabilized ylides in the presence of metal salts or possibly sufficiently polar solvents. Actually, two mechanistic variations can be envisioned to be operating in the Wittig reaction: In more nearly synchronous reactions, a 'syn' reaction path for the approach of the reactants is indicated, and in very asynchronous reactions an 'anti' reaction path for the approach of the reactants is favored. In some cases, these two mechanisms may operate in parallel, e.g. in the case of semistabilized ylides where mixtures of the Z and E alkenes are obtained. In other cases, one of the suggested mechanisms prevails over the other, such as in the case of the reaction of alkylidenetriphenylphosphoranes with aldehydes, where essentially only Z-alkenes are obtained, or, in the case of stabilized ylides, where only E-alkenes are obtained.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/1991