[asa029]

Hi!

I'm currently working on production of tinn-compounds for a subsequent Stille-type coupling. On of the initial steps include the following reaction:

CH2Br2 + Me3SnCl (Me)3SnCH2Br2

The article that describe this, calls for usuage of LDA but I was wondering if anyone had experience using other bases for such a reaction. I was thinking along LAH og even SBH. Traditional aqeous bases cannot be used. I'm especially interested in bases that are easy to handle (which is why I want to avoid LDA which spontaneously combusted the last time I tried )

Thanks!

[Honclbrif]

Your LDA combusted? That stuff is supposed to be non-pyrophoric. Bottles of prepared reagent that have been sitting around for a long time do evolve LiH, maybe it was some of that which ignited on you. Or, if you generated it in situ from diisopropylamine and butyl lithium, you may have had excess butyl lithium left over.

As for your original question, I've made dichlorocarbene from chloroform and NaH (60% in mineral oil) in THF.

[asa029]

Your LDA combusted? That stuff is supposed to be non-pyrophoric. Bottles of prepared reagent that have been sitting around for a long time do evolve LiH, maybe it was some of that which ignited on you. Or, if you generated it in situ from diisopropylamine and butyl lithium, you may have had excess butyl lithium left over.

As for your original question, I've made dichlorocarbene from chloroform and NaH (60% in mineral oil) in THF.

Well, this was not a solution, this was just the compound (in dry form). I wanted to make the solution myself, as the quantities from Sigma tend to be inccurate and very different in color, which always make me suspicious. Even in nitrogen atmosphere it was pyrophoric and almost make a complete mess.

Thanks. People in my research group have used ordinary 50 % sodium hydroxide, so I guess the LAH should be more than basic enough. Only time will tell

[Dan]

CH2Br2 + Me3SnCl (Me)3SnCH2Br2

Me₃SnCH₂Br₂ has a pentavalent C. I assume it's Me₃SnCHBr₂?

Surely treating dibromomethane with LDA gives dibromomethyl lithium, which then displaces chloride nucleophilically? Where's the carbene?

Also lithium aluminium hydride can reduce alkyl bromides. So using a strong reducing agent like LAH as your base is probably not a good choice.

[asa029]

CH2Br2 + Me3SnCl (Me)3SnCH2Br2

Me₃SnCH₂Br₂ has a pentavalent C. I assume it's Me₃SnCHBr₂?

Surely treating dibromomethane with LDA gives dibromomethyl lithium, which then displaces chloride nucleophilically? Where's the carbene?

Also lithium aluminium hydride can reduce alkyl bromides. So using a strong reducing agent like LAH as your base is probably not a good choice.

It's supposed to be SnCHBr₂. Typo!

Well, isn't dibromobethyl lithium a carbene with lithium as counter-ion? the nuclophile is the carbene e.g. Li⁺(CHBr₂^-) which attacks the stannane and kicks out the chloride as an outgoing group.

Hmm, that's a good point. Maybe NaBH₄ can be used instead...

[Honclbrif]

"Well, isn't dibromobethyl lithium a carbene with lithium as counter-ion?"

No. Carbene is a neutral species.

You can probably find the droids answers you're looking for by looking up the pKa of your bromo species and the various bases you have at your disposal. There are numerous pKa tables available.

[Dan]

Well, isn't dibromobethyl lithium a carbene with lithium as counter-ion?

No, it is a carbanion with a Li counterion. Carbenes are divalent and uncharged, e.g. Cl₂C:

Hmm, that's a good point. Maybe NaBH₄ can be used instead...

Why are you choosing reducing agents? You are trying to deprotonate, so what you need is a strong base e.g. LDA, BuLi, KHMDS etc. etc.

[asa029]

Why are you choosing reducing agents? You are trying to deprotonate, so what you need is a strong base e.g. LDA, BuLi, KHMDS etc. etc.

LAH and SBH are also strong bases. My professor had a "hunch" (which is always bad news) that it would be benificial to use a reducing agent as a base. I, on the other hand, don't really see the point.

[Dan]

I, on the other hand, don't really see the point.

Me neither, I wonder what this hunch is about...

I agree LAH is a strong base, but NaBH₄ not so much - it's stable in isopropanol, and is almost always used in aqueous or alcoholic solutions.

[Sepelio]

Erm. You might be able to get it to work if you use carbon tetrabromide and BuLi instead. Li pulls off a Br and you get transient CLiBr3 then an elimination to Br2C: and LiBr. At least, thats what I've got in this set of notes infront of me for the Cl species.

NaH or KHDMS are probably the way to go though. That or use another species to generate the carbene from. Should work if you can get something more stabilising attached to the carbene C. Even if its something you need to cleave off later.