3/14/2007

Tungsten: For More Than Just Lightbulbs

Interesting paper came out in JACS ASAP today. While the process of olefin metathesis is quite well-known, alkyne metathesis is also a well-studied process but is not nearly as widespread as the former. Several different types of alkyne metathesis catalyst systems exist, all of which involve a metal-alkylidyne moiety (that is, a carbon-metal triple bond), analagous to the metal-alkylidene moities present in the Grubbs and Schrock-type olefin metathesis systems. Alkyne metathesis is especially popular in organic materials synthesis as it is a simple method for the formation of poly-(p-phenyleneethynylene)s (PPEs), an important class of conjugated polymers.

Anyway, this JACS communication from the Johnson group from the University of Michigan is the first reported catalytic nitrile-alkyne cross-metathesis (NACM), using a tungsten-alkylidyne catalyst. The catalytic cycle is shown on the right, and it looks hella sick. However, as shown, nitrile-alkyne cross-metathesis cycle is pretty much the same as alkyne metathesis, except with a nitrogen triply bound to the metal instead of a carbon. This still (presumably) forms the ever-curious metallacyclobutadiene intermediates first proposed by Katz and characterized by Schrock. So Johnson's proposing it's basically the same cycle, twice over.

The most interesting thing about this reaction- and where its potential to be a real boon in materials synthesis- is its overall conversion. It utilizes a "sacrificial alkyne," the cheap 3-hexyne, in the route to cross-metathesis. Overall, you take two nitriles (in this case, 4-methoxybenzonitrile) and react them with 3-hexyne, and form a carbon-carbon triple bond across the two nitrile carbons, with propilonitrile as the byproduct. This could be especially useful in cases where nitrile groups are easier to deal with than alkynes, especially oxidation-prone terminal alkynes.


It's a very novel concept that I hope gets utilized in some new, materials-y fun ways. And while we're ont he subject, I'd like to see someone make some reactive, air-stable alkyne metathesis catalysts (aside from Mo(CO)6). Anyone up to the challenge? Anyone? Bueller?

11 comments:

Chemgeek said...

"...nitrile groups are easier to deal with than alkynes..."

And in many cases, the nitriles are easier to access. Thereby, expanding the utility of alkyne metathesis.

Behind the Desk said...

This is definitely nice chemistry, but don't be so quick to discount the terminal alkyne any time soon, especially with tricks like Haley's in situ silyl deprotection/Sonogashira-Hagihara coupling. Plus, 66% is far from a polymer grade reaction.

Excimer said...

Plus, 66% is far from a polymer grade reaction.

Depends. I'll take a 66% polymer yield with low PDI and few defects over a 99% yield with a PDI of 4 and lots of defects. This is especially true with conjugated polymers. (Not to say that this rxn is capable of control like that, but that's where my priorities lie.) I do like the Haley method though. It's clever.

Behind the desk said...

But if each coupling is at 66%, you get a nice 6.6% theoretical yield of the 11-mer, in addition to a broad distribution of other oligomers. That's why "real" polymer chemists stick to condensation and vinyl addition chemistry. Prospects are grim for the controlled synthesis of conjugated polymers (two points for the double entendre)

Excimer said...

I don't know how I feel about the term "'real' polymer chemists" aside from my opinion that "real" polymer chemists as you call them aren't actually chemists- they're engineers.

I mean, sure, this stuff isn't ready for industry yet, but it's an interesting starting point. This process would be more suited toward the synthesis of interesting small molecules anyway, but yes, it's far off from the practical stage. I'm thinking out loud here, really. Alkyne synthesis isn't going away anytime soon, if ever.

hmm... controlled synthesis of conjugated polymers... sounds like an NSF grant to me.

Mitch said...

Some recent alkyne papers.
http://pipes.yahoo.com/pipes/pipe.run?_id=vOrZbmnT2xGwr12mJhOy0Q&pubssearch=Alkyne&_render=rss

Mitch said...

Url was cut-off. Click my name to goto the url.

Mitch

behind the desk said...

"real" polymer chemists as you call them aren't actually chemists- they're engineers.

I challenge you to go up to some one who has spent the past week triply distilling a solvent and a new synthetic monomer (or multiple crystallizations of that monomer), and has just finished rinsing his/her flask with BuLi just to remove that last trace of water prior to doing a final vacuum transfer of solvent, all on a Schlenk line (for those not lucky to have the low t dry boxes) in order to make a monodisperse triblock copolymer (or some other well-defined macromolecule), and then tell them they are not a chemist.

Any joe schmoe can pop iodides on a monomer, push it through a column, throw in some Pd and call the resulting broad 40K material a "polymer" (myself included).

We do a disservice to our polymer bretheren by calling those who (due to the requirements for high MW polymers) restrict their reaction chemistries to those that work in 99.9+% yield per coupling any less of a chemist. If the person has an inspired design prior to setting up a synthetic reaction, they are a chemist. If they blindly follow preps, they are cooks. Regardless of the end target.

by the way, perhaps my double entendre was not entendu: for a neat controlled polymerization of conjugated polymers, look at McCullough's GRIM procedure.

Alexander said...

I was thinking whether to write about it or not.

Check it - with p-MeOC6H4CN the yields are 77% of ArCCAr, 15% of ArCCR and 8% unreacted SM.

I decided not to write - it's nice, but rxn still is not general and works bad on aliphatic nitriles. And.. they never tell if their compounds are air-stable or not! That's annoying.

...and I expected nitrogen to be a by-product.

Liquidcarbon said...

d'oh! it was me.

Excimer said...

Alexander- most tungsten-alkylidyne complexes aren't air-stable. I think some of the molybdenum ones even bind dinitrogen but I don't know anything about that. Ask Schrock.

The reaction is far from perfect, but I still liked it. I blog about things I like, not things that are useful. ;) I still see potential for it though.