Metal lacyclobutane

In the case of olefin metathesis, the selectivity in initiation products can be understood in terms of minimization of the steric interactions in the metal-lacyclobutane intermediates (vide supra), which are governed by the relative position of the substituents the metallacyclobutane with substituents in pos-... 

Puddephatt and co-workers (139, 140) also studied the title Pt(IV) metallacyclopentane derivatives (L = PMcaPh or PPhs) under a variety of conditions. Their results and mechanistic conclusions are summarized in Scheme 23 with two competing decay paths (A and B) for the excited molecules. No cyclobutane was formed in either case, in contrast to the production of cyclopropane upon irradiation of the corresponding metal-lacyclobutane complexes. However, as noted above, cyclopropane was a thermal rather than photochemical product. The gaseous photolysis products of the metallacyclopentane complexes were exclusively ethylene and... 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the... 

Alkyne polymerization can be initiated by metal-alkylidenes or metal-alkylidyne complexes. The mechanism involves metallacyclobutene or metallacyclobutadiene key intermediates, in the same way as alkene metathesis and ROM involve metal-lacyclobutane intermediates Katz mechanism, bottom of this page, an extension of the hauvin mechanism for alkyne metathesis. For instance, Schrock s catalyst W - u O - u 3, shown in section. as catalyst of disynunetrical alkyne metathesis, also initiates the polymerization of acetylene and terminal alkynes rather than metathesizing them. Indeed, the molecules of terminal alkynes successively insert into the W- bond of the metallacyclobutadiene intermediate... 

Scheme 32 Transformations of Metal lacyclobutanes and Metal lacyclopentanes...

Many researchers put forward proposals to explain how metathesis could take place, but it was Herisson and Chauvin who proposed the intervention of a metal bound to the carbon through a double bond (metal alkylidene). In the catalytic cycle of cross-metathesis, this active species first reacts with the olefin to form a four-membered ring (Scheme 7.5). This metalla-cyclobutane intermediate then cleaves, yielding ethylene and a new metal alkylidene, which reacts with a new alkene substrate to yield another metal-lacyclobutane. On decomposition in the forward direction, this second intermediate yields the internal alkene product and regenerates the initial metal alkylidene. ... 

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