[Letters]

http://img292.imageshack.us/img292/5292/grubbscatalysttq0.jpg

I have read up on the Grubbs catalyst and it says that it is a ring opening or ring closing catalyst however the question that im trying to answer: In a ring-closing metathesis reaction catalyzed by Grubbs' catalyst, which of the following products or intermediates are not possible? (N.B. - "Ru" is a ruthenium atom, just part of the whole Grubbs' catalyst molecule.)

Also includes intermediates which totally throws anything i assumed, however i still beleive that number 3 is not correct, can anybody help or guide me in some correct direction?

[Dan]

Look up the mechanism.

eg. http://www.organic-chemistry.org/namedreactions/olefin-metathesis.shtm

What are you basing your descision on? 2 would be an intermediate that forms 3. So if you think 2 is correct, why do you think 3 is not?

Draw the mechanism for the reaction (starting material to 3) out in full, the entire catalytic cycle, with all intermediates, and you should see which is the impossible intermediate.

[Letters]

After drawing it out im 90 percent certain that number 4 does not belong i base it on the follwoing judegement: first i was way off in my first estimate the end product cannot have the Ru group therefore that eliminates 3 so that leaves 1,2,4, 2 is correct like you stated it is the intermediate before getting number 3 its the step right before the Ru and the other cH group leave so that leaves number 1 and 4. 4 is incorrect because when the cyclobutane forms with the Ru the next natural step would be for it split away from the molecule however, since this has to be the first step after adding the catalyst that would be out of place so 1 would have to be right also because it is the first step in the progrection of the catalyst. I could be wrong but i just dont see it any other way, is it ok Dan

[Dan]

I'm not quite following everything you're saying, but

4 is incorrect because when the cyclobutane forms with the Ru the next natural step would be for it split away from the molecule however, since this has to be the first step after adding the catalyst that would be out of place so...

So, you're saying 4 does form but is unstable? So you're saying 4 is an intermediate ? The question asks for structures and intermediates that are not possible...

1 would have to be right also because it is the first step in the progrection of the catalyst.

No, count the carbons. An extra methyl group has appeared making the structure (1) impossible to form. It's a cheeky question, you have to spot a small change in the carbon skeleton rather than a mechanistic problem.

All the other structures/intermediates are possible ones.