P-substituted metallacycle

Alternatively, if the propylene interacts with a metal-methylidene intermediate (for illustrative purposes, the deuterium label has been moved to the vinyl terminus in Scheme 10.1b), again two intermediate metallacycles are possible. The formation of a P-substituted metallacycle exchanges the termini of the olefin and degeneratively regenerates a methylidene. If, instead, an a-substituted metallacycle is formed, an alkylidene intermediate is generated upon cycloreversion, thereby productively moving the catalyst species back to the first set of pathways. 

The experiments that are most relevant to the present discussion involved the cross metathesis of mixtures of 1-pentene and l-pentene-catalyst system, Calderon and coworkers monitored isotopic scrambling to form the d2 and mass spectrometry). After only short reaction times, the mixture reached 70% of the theoretical scrambling limit, with almost no formation of any octene products. To explain pentene s proclivity toward generating itself rather than octene in the presence of metathesis catalysts, Calderon suggested that either the mono-P-substituted metallacycle was favored over the mono-a-substituted isomer, or the a,a -disubstituted metallacycle was preferred relative to the a,p-substituted one (Scheme 10.4). In other words, degenerate metathesis can be performed by either M=CH2 or M=CHR species. These pathways were indistinguishable, but the results proved that one of the two inequalities must be true [5]. 

Scheme 10.16 Degenerate metathesis via a methylidene proceeds through a p-substituted metallacycle.

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