[dfodor]

I would have a question to the mechanism of olefin metathesis. There was a debate whether the mechanism is pairwise where two olefins form the transition state or non-pairwise where a the metallacyclobutane is formed by an olefin and the metal carbene. Grubbs proved the latter mechanism in 1975 (JACS, s. 9735) by reacting 1:1 1,7-octadiene and deuterated 1,7-octadiene. The assumption was that an ethylene (containing 0, 2 and 4 deuterium atoms) product ratio of 1:2:1 would prove the non-parirwise mechanism otherwise a product ratio of 1:1.6:1 would suggest a pairwise mechanism. What I do not understand is that actually the 1:2:1 product ratio of alkenes was observed in the 60's and 70's. So why was Grubbs' experiment new?

Thanks a lot.

[Woofuls]

Don't say prove. That word shouldn't be used outside of math.

In 1975, Grubbs conceded to earlier reports that it proceeded via a non-pairwise mechanism. Previously, he had suggested a pairwise mechanism with a five-membered metallocycle, see J. Am. Chem. Soc., 1972, 94, pp 2538–2540. The main significance of the '75 piece was that he used a new metathesis catalyst and used isotopically labeled protons instead of the various substituted olefins used by Chauvin and Katz. In fact, Katz's paper in 1975 was the first to provide unambiguous evidence for this mechanism. The Americans were pretty bad about noticing the good work by French chemists in the early 70s.

There is a good article on this in C&EN entitled, "Olefin Metathesis: The Early Days."

[azmanam]

also start here:

http://allthingsmetathesis.com
http://allthingsmetathesis.com/category/the-basics/

[dfodor]

The question was not answered.
Woofuls: thanks for the epistemological correction but you missed the point.

[Woofuls]

No, I didn't. He changed his mind about which was the correct mechanism. His experiment used a new catalyst that clearly suggested a single mechanism in addition to doing his work with isotopically labeled olefins. His experiment wasn't as important as Katz's.