Triphenylphosponium salt preparation from an electron rich alkyl halide

That is kind of a crappy SN2 electrophile. Things you can do:

• Add 10% potassium iodide or sodium iodide to refluxing acetone (DRY) as a catalyst
• Completely exchange the chloride out for a better leaving group using dry acetone using a Finkelstein reaction
• Move to a more polar, higher boiling solvent such as DMSO or HMPA. Degas using nitrogen and make sure it's dry to ensure your stuff is stable.

It's not the same molecule as benzyl chloride you can't compare them because there are several differences.  The oxygens are pulling on the chlroine bond in the way that the dipoles allign anti thus stabilizing the bond in the non polar solvent choices you chose.  You are trying to generate ions from neutral things in a non-polar solvent.  That means the reaction is up hill.  The substrate is labile, so that overtaking the SN2 reaction.  You can use another ylide equivalent.  For example ^-SMe will form a neutral desired product and a salt.  Stabilized ylides: RSO₂Ph(or just tosylate), HWE.  I would put it in a glyme solvent that is dried beforehand.