Horner-Wadsworth-Emmons reaction mechanism

Fig. 11.13. The currently assumed mechanism for the Horner-Wadsworth-Emmons reaction.

Rein, T., Vares, L., Kawasaki, I., Pedersen, T. M., Norrby, P.-O., Brandt, P., Tanner, D. Asymmetric Horner-Wadsworth-Emmons reactions with meso-dialdehydes scope, mechanism, and synthetic applications. Phosphorus, Suifur Siiicon Reiat. Bern. 1999, 144-146, 169-172. 

An important modification (often called the Horner-Wadsworth-Emmons reaction) of fhe Wiffig reaction makes use of phosphonate esters, RP0(0R )2- It is highly stereoselective for the formation of E-alkenes.The reaction and mechanism are depicted below for the preparation of (E)-stilbene. Instead of using a phosphorus cation to stabilize the negative charge, as in the phosphonium ylide above, a phosphonate ester group is used to stabilize an adjacent carbanion. 

The mechanism of the Horner-Wadsworth-Emmons reaction of the lithium enolate derived from trimethyl phosphonoacetate with acetaldehyde has been investigated by ab initio calculations. Oxaphosphetane formation is rate determining, both in the gas phase and with one ether moleeule solvating. The transition state leading to trans-alkene is more stable than that giving the cis form. 

Propose a mechanism for the Horner-Wadsworth-Emmons reaction. The reaction of aldehydes with 10 provides 1,2-disubstituted olefins with high E-selectivity. Provide (or look up) a rationale for this observation. Then explain why the reaction of 3 with 10 shows little stereoselectivity. (CJH-2)... 

Maercker, A. The Wittig Reaction Organic Reactions 1965, 14, 270-490. Maryanoff, B. E. Reitz, A. B. The Wittig Olefmation Reaction and Modifications Involving Phosphoryl-Stabilized Carbanions. Stereochemistry, Mechanism, and Selected Synthetic Aspects Chemical Reviews 1989, 89, 863-927. Blanchette, M. A. Choy, W. Davis, J. T. Essenfeld, A. P. Masamune, S. Roush, W. R. Sakai, T. Horner-Wadsworth-Emmons Reaction Use of Lithium Chloride and an Amine for Base-Sensitive Compounds Tetrahedron Lett. 1984, 25, 2183-2186. 

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