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I have been making ethyltriphenylphosphonium bromide (somewhat of a time crunch, would have used iodoethane instead of bromoethane if we didn't have to wait for it). All the syntheses I see of Wittig salts are in aromatic solvents and they take large amounts of time for what I would have expected to be a fairly quick reaction, usually at least a day, if not several. I have come across the use of iodide salts as a catalyst, some of which I had already added because I figured it wouldn't hurt.

$$\ce{PPh3 + EtBr ->[\text{aromatic}][\text{solvent}] [EtPPh3]Br}$$

Why are these conditions standard/most effective? More specifically, why is toluene (less often xylene, probably others but those are the two I've seen) used rather than some polar aprotic solvent? Also, besides increased temperature and the use of iodide catalysts, are there any common ways to decrease reaction time?

  • Is it simply that toluene boils at a relatively high temperature? Maybe the SN2 reaction is fairly easy once line up, but due to the large PPh3 being a bulky nucleophile, having the two reactive sites in close proximity is relatively rare, so increasing temperature is the most effect thing since random interaction is the driving force.

  • I can't imagine a primary alkylbromide reacts through SN1 rather than SN2. I guess maybe the higher temperature could allow for an SN1 reaction, though? I kinda doubt it, though...

  • I would think, even if the above was true, using THF and going through an SN2 mechanism at a lower temperature would be simpler than refluxing in toluene, and I still kind of doubt the SN1 on a primary bromide even at the elevated temperature.

  • Another thought is that the phosphonium salt crashes out of toluene, and therefore this equilibrium serves as a driving force for the reaction. Technically, this doesn't speed up the reaction, but it does significantly hinder the reverse reaction, so it leads to a net decrease in reaction time (and increases yield, I guess, if the reverse reaction is significant). I wouldn't really think the halide anions would be strong enough nucleophiles to be able to kick off a phosphine, though.

$$\ce{RX + PPh3}\xrightarrow{\text{toluene}}\ce{\underset{\small{(\sim none\ in\ sol'n)}}{[RPPh3]X_{$(s)$}} v}$$ $$\text{vs.}$$ $$\ce{RX + PPh3} \stackrel{\small{\text{THF}}}{\longrightleftharpoons} \ce{\underset{\small{(more\ than\ above)}}{[RPPh3]X_{$(org)$}}}$$

I realize THF isn't all that polar, but relative to toluene or xylene, I would really expect it to facilitate this reaction better.




P.S. This question isn't really an attempt to get help with my current situation (especially since I just finished working up my last batch, anyway) as much as it is just trying to get general information about the reaction.

I will mention for anyone who might be able to use it that I first tried to run mine in toluene at around 35 °C with catalytic KI (I've now seen LiI used instead of KI, not sure how much that matters). However, I was inspired by a literature report of a 7-day reaction at 30 °C (no iodide) to move the reaction in portions to pressure tubes and take it up to 130 °C. A paper reported 7 h (also no iodide) at this temp was effective, and I found that to be overkill. I didn't really see much difference between 2 h and 7 h (again, no iodide in their reaction, which may have caused the difference).

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  • $\begingroup$ Depending on the substrate, and the wanted $E/Z$-configuration of the alkene formed, an Arbuzov reaction (like 1.1 eq of triethylphosphite per 1 eq of alkylhalogenide; the former often deployed both reagent and solvent), refluxed for 1-3 h; telescoped / followed by a HWE with NaOEt could have been faster, easier to workup. $\endgroup$ – Buttonwood Mar 11 '17 at 15:07
  • $\begingroup$ @Buttonwood I'm having a hard time parsing your description there, but it sounds like it's definitely worth exploring. Ill have to draw it out, I think... I get everything up to how the NaOEt part works. Wouldn't I end up with the $\alpha,\beta$-unsaturated ethyl ester back from that? I also considered using diethyl ethylphosphonate as the ethylenation reagent. $\endgroup$ – SendersReagent Mar 11 '17 at 15:33
  • $\begingroup$ I was extrapolating from work done earlier, see in chat.stackexchange.com/rooms/55185/… $\endgroup$ – Buttonwood Mar 11 '17 at 16:07

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