Ethyltriphenylphosphonium iodide

For the first (6 g) and the second (60 g) campaign, utilizing the Zhao olefination procedure, also used by Smith and Marshall, we obtained the desired cis-vinyl iodide 4 in 20 to 31 % yield after chromatographic purification on silica gel. Only a small amount of the undesired trans isomer was detected cis trans - 10 1 to 15 1) and this could not be separated from the desired cis compound. This is fortunately not a problem as they can be separated after the next step in the sequence. We did not observe any rfes-iodo olefin 23, suggesting that the formation of the iodo ylide from ethyltriphenylphosphonium iodide, via ylide iodination (Scheme 4), was complete before it was added to aldehyde 18. 

Smith utilized iodine for the conversion of ethyltriphenylphosphonium iodide 19 into the iodo-ylide 20. During process optimization work for the third campaign (500 g), we found that TV-iodosuccinimide could be used to replace iodine without detriment. While this makes the reaction easier to handle and increased the reproducibility of the process, it did not contribute to an increase in yield. 

It was found, as reported, that the reaction of 2-iodo ethyltriphenylphosphonium iodide 20 with 18 afforded epoxide 25 as a mixture of isomers in addition to the desired 4 in a 1 1 ratio. Alternative approaches were investigated in an attempt to minimize this major byproduct, but they were unsuccessful. For example, employing a method described by Shen (where the initially formed betaine intermediate was deprotonated with a second equivalent of base and then iodinated) produced des-iodo olefin 23. Utilizing Hanessian s phosphonates in this process also resulted in only des-iodo olefin 23. 

Ethyltriphenylphosphonium iodide forms broad, colourless plates, M.pt. 164° to 165° C. The n-propylphosphonium iodide melts at 201-5° C. the isopropylphosphonium iodide yields prisms, M.pt. 191° C. the isobutylphosphonium iodide separates in plates or needles, M.pt. 176° to 177° C. and the isoamylphosphonium iodide crystallises in colourless prisms, M.pt. 147° C. 

Ethyltriphenylphosphonium iodide gave only a 30% yield of ethyldiphenylphosphine with even lower yields observed with longer alkyl chains. 

The customary generalization is that unstabilized ylides react with aldehydes to give predominantly Z-alkenes (cis), while stabilized ylides give predominantly E-alkenes trans) Unstabilized ylides include the typical alkylidenetriphenylphos-phoranes shown in entries 1-5 of Scheme 2.8. Their tendency to form Z-alkenes can be very high, especially when conditions are chosen so that lithium salts are not present in the reaction medium. Sodium amide (entry 3) and the sodium salt of hexamethyldisilazane (entry 5) are convenient strong bases for the preparation of salt-free solutions of ylides. When ethylidenetriphenylphosphorane is prepared from ethyltriphenylphosphonium iodide and n-butyllithium (entry 4), the high... 

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