Metallacyclobutanes decomposition

Scheme 7.10 Mechanism of metallacyclobutane decomposition. Gibbs free energies (in kcaimoi" ) were computed at the PW91/DNP ievei of theory. Adapted from Ref [55a].

At this point it should be noted that this mechanism is unexpected. Simple platinum alkyls decompose by 3-elimination whenever possible and there are no well-established examples of a-elimination [10]. All previous studies have indicated that metallacyclobutanes decompose by 3-elimination, even for tantalum and titanium derivatives for which a-elimination is a frequent mechanism for decomposition of the simple alkyls [11, 12]. There is even a labelling study which appears to prove the 3-elimination mechanism for decomposition of platinacyclobutanes (equation 3) [13]. 

Metallacyclobutane complexes can undergo net reductive rearrangement to form alkene complexes or undergo decomposition with the liberation of free alkene. The rhodacyclobutane complex 112, for example, rearranges thermally to give propene complex 113 (Equation 33) <19980M4484>. 

Returning to the key metallacyclobutane intermediate above, we must now consider an alternative decomposition mechanism which may... 

Ethylene complexes arise from either the formation and bimolecular decomposition of an intermediate methylidene complex or the rearrangement of a met-allacyclobutane complex to propylene (Eq. (1.9)). The mechanistic details of the rearrangement of the unsubstituted metallacyclobutane complex to propylene are not... 

Many researchers put forward proposals to explain how metathesis could take place,but it was Herisson and Chauvin who proposed that in the catalyst the metal was 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 7.7). This metallacyclobutane intermediate then cleaves, yielding ethylene and a new metal alkylidene, which reacts with a new alkene substrate to yield another metallacyclobutane. On decomposition in the forward direction, this second intermediate yields the internal alkene product and regenerates the initial metal alkylidene. 

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