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The sodium ion effect in the Wittig reaction
Abstract
In an effort to understand better the curious effect imparted by sodium iodide on the stereochemical course of the Wittig reaction, a thorough investigation of the influence of various sodium salts has been carried out. It is known that, in general, nonstabilized triphenylphosphonium ylides produce predominantly cis alkenes in reactions with aldehydes, while stabilized ylides produce trans alkenes and semi-stabilized ylides produce mixtures of both cis and trans alkenes. Among other factors, the choice of solvent and the temperature can also influence the stereochemical outcome of Wittig reactions. To minimize the variables involved, reaction conditions have been restricted to the use of semi-stabilized ylides with THF as the solvent at relatively low temperatures. The observation that the presence of sodium iodide in the reaction mixture of the types of Wittig reactions specified above caused a higher cis/trans alkene ratio to result was used as a starting point for the development of highly cis selective Wittig reaction systems. The information obtained in the initial part of the study with relatively simple systems was subsequently applied to polymeric systems, and thus led to the development of cis selective polymer-bound Wittig reagents. The above studies also led to the investigation of Wittig reactions in which salt-free conditions were imposed or in which an insoluble metal halide was formed in the preparation of the ylide. This was a follow-up investigation to the recently proposed hypothesis of a spin-paired diradical mechanism of certain Wittig reactions by McEwen and Ward. Based on competition experiments, radical trapping experiments, and theoretical calculations, it seems that the Wittig reactions of unstabilized ylides involve an initial concerted $\pi$2s + $\pi$2s cycloaddition to form oxaphosphetanes, but that some degree of asynchronousness arises in the reactions of semi-stabilized and stabilized ylides.
Subject Area
Organic chemistry
Recommended Citation
Baughman, Lynda Lee, "The sodium ion effect in the Wittig reaction" (1991). Doctoral Dissertations Available from Proquest. AAI9207363.
https://scholarworks.umass.edu/dissertations/AAI9207363