[Quimven]

Hi!

I need to do a wittig reaction and I'm considering deprotonate the alkylphosphonium salt (from TPP and propylbromide with a grignard but I didn't find any antecendent.
I find this suspect, is that for some reason is not convenient to do it this way? I would use DMSO o THF as solvent.
Thank you.

[tmartin]

Is there a reason you need/want to use a grignard reagent to deprotonate the alkylphospohnium salt?  I don't really see any advantages of using a grignard over something else, maybe complete formation of your ylide versus an equilibrium?  But I'm also not sure that you couldn't use one either.  I've seen HWE's done with all kinds of bases, from NaH to Ba(OH)2.

I've been working on a methylene wittig as of late, and generally use a 1M solution of tBuOK to deprotonate and form the ylide in THF at 0 C.  I suppose one nice thing about the tBuOK is i can weigh it out as a solid then make the solution, of course, you're probably using a grignard from a bottle with a known concentration and just taking out a syringe full.

Just thought of this, why not try it with some "dummy" aldehyde that's somewhat like your real system and see how it goes.  I don't imagine its hard to access the alkylphosponium salt, so you should have plenty to test with.

[Quimven]

Thank you Martin, sorry for the late reply. Well, I would prefer to work with a more weak base than BuLi and grignard. tBuOK seems a good base, but since  I will use acetone as electropile, I could think that it may form the enol and worst give condesation product if there is presence of base. I want to make react propyllphosphonium salt with acetona to give 2 methyl 2 pentene. what do you think?

[tmartin]

I perform a methylene Wittig with a ketone that is a lot more hindered than acetone using KOtBu.  Unfortunately as of late my yields have been lower (a little bit below 50%), but I'm thinking that has more to due with the presence of some adventitious water, or the hindered nature of my ketone.  I run the reaction with excess alkylphosphonium salt, which I pre-mix with KOtBu (but that doesn't mean all the base it gone).

It does seem you could run in to trouble there, but, acetone is pretty cheap and easy to acquire, so I think I would give it a shot just to see.   I think you might be able to make the salt-free Wittig: use NaH to deprotonate the alkyl phosponium and then filter that quickly under argon, concentrate, then make a solution with a known molarity, and try that as well, but it is a little bit more of a pain (making the ylide is at least an overnight affair).  I used to have the reference, I'll look for it around my desk.

I looked in Org. Syn. and unfortunately didn't see any preps for 2-methyl-2-pentene.  I'm guessing you'll want a large amount?  It seems like it is pretty expensive, hence the making it?  Would you plan to purify it by distillation under vacuum (just curious) or just a normal distillation?

[Quimven]

I looked in Org. Syn. and unfortunately didn't see any preps for 2-methyl-2-pentene.  I'm guessing you'll want a large amount?  It seems like it is pretty expensive, hence the making it?  Would you plan to purify it by distillation under vacuum (just curious) or just a normal distillation?

Yeah Tmartin, I haven't found a synthesis but I know that it can be made by reacting n-propilmagnesium bromide and acetone and dehydrating the alcohol in acid media, but it produces two isomers difficult to separate. I need at least 35 g , is very volatile bp 67 0c. What I think to do is make the witting in DMSO and add water, separate the two layers add toluene and distill normally, it seems easy. What I am thinking lately is add PhLi o BuLi in hexane to the solution of phosphonium salt in DMSO and under vacuum remove the hexane (the bp is similar to 2 methyl 2 pentene) and then add the acetone.

[nox]

What about doing it the other way around?

You can either make the isopropylphosphonium salt yourself, or you could buy it relatively cheap ($89.70 for 25g, CAS # 24470-78-8) from Sigma. Deprotonate to make the ylid, then react with propionaldehyde to get what you want. Since propionaldehyde is more reactive than acetone, this might give better yields. Of course it's also more electrophilic, so you'll want to be careful about how much base you use I suppose.