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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: katitava on April 12, 2011, 02:14:11 PM

Title: Need comparable Lewis Acid to Ti(OiPr)4
Post by: katitava on April 12, 2011, 02:14:11 PM

I need a Lewis acid that is similar to Ti(OiPr)4- but with less labile ligands. Its one of the few LA's that I've found to promote my reaction, but the isopropanol is reacting with my starting materials.   Halogens will kill my co-catalyst and stronger Lewis acids like scandium triflate and lanthanum triflate etc decompose the product and starting materials...  Any suggestions?  Does anyone know of a good place to compare the relative reactivity/acidity of common Lewis acids?   

Just buying the strem catalog willy-nilly isn't exactly an option....

Title: Re: Need comparable Lewis Acid to Ti(OiPr)4
Post by: sjb on April 12, 2011, 04:15:00 PM

How is the IPA reacting with your SM? Can you protect it somehow?

Title: Re: Need comparable Lewis Acid to Ti(OiPr)4
Post by: katitava on April 12, 2011, 06:39:24 PM

I guess I should ask if there's a a LA that's just a hair softer then Ti(OiPr)4 with less labile ligands...

There's a couple of different ways things could go wrong...but the short version is Ti is activating an acetal and then the isopropanol is adding in and my reactive site falls off.  The acetal in the starting material is less stable than the product for some reason even though it's relatively remote - because I don't see any decomp with the product. (At this point I'm hoping to avoid having to change the acetal... its key for the next step, which was supposed to be the hard one...).

Title: Re: Need comparable Lewis Acid to Ti(OiPr)4
Post by: Dan on April 12, 2011, 08:08:47 PM

Ti(O^tBu)₄ is commercial  (CAS: 3087-39-6) and ^tbutanol may not interfere so much. It's not cheap though.

Title: Re: Need comparable Lewis Acid to Ti(OiPr)4
Post by: Honclbrif on April 12, 2011, 10:49:26 PM

I don't know if a less labile ligand will help you; I think the Ti has to exchange one with your substrate for its Lewis acidic characteristics to really shine through. Have you tried substoichiometric Ti? Sometimes people dump excess catalyst into their reaction to get it to go fast when all it needs is a little time and love.

Furthermore: may be a stupid question on my part, but why not TiCl₄? If you're having trouble with the iPrOH messing with your acetal, I don't see the halide giving you much grief.

If you've got to switch metals, maybe something from the boron column?. They would have that vacant p-orbital available and might not require ligand exchange to do their magic.

Title: Re: Need comparable Lewis Acid to Ti(OiPr)4
Post by: Honclbrif on April 12, 2011, 10:50:59 PM

Never mind about the TiCl₄ question. Just re-read the original post.

What solvent is it in? Could you vac off the iPrOH as it forms, or use a methoxide or such to increase the volatility? Again, my concern is that ligand exchange may be unavoidable, but I am not an expert on transition metal catalysis.

Furthermore, I'm pretty sure you can get Ti(III) citrate. Is Ti(IV) citrate available? Might be a more innocuous ligand.

Title: Re: Need comparable Lewis Acid to Ti(OiPr)4
Post by: nox on April 13, 2011, 01:50:14 AM

What about Bi(OAc)₃? I think Bi(III) is comparable in strength to Ti(IV), if not slightly weaker. Not sure if those acetates will pose a problem for your substrates, but doesn't hurt to give it a shot. Relatively cheap too ($57.40 for 10g from Sigma).