Evidence from the reactions of metallacyclobutane complexes

Titanacyclobutane d° complexes with two / -cyclopentadienyl ligands (Cp), of the kind shown in eqn. (15), were first prepared by Howard (1980a). 

The complex 17 reacts with diphenylacetylene 18 in CgDe at 40°C to give the titanacyclobutene complex 19 at a rate proportional to [17] but independent of [18], with an activation energy of 113 kJ mol (Lee, J.B. 1982). 

The reaction of 17 with McsCCHO or with McsSnCl proceeds at the same rate as reaction with 18. It is therefore postulated that all these reactions proceed via slow dissociation of 17 to Ti(=CH2)Cp2, followed by rapid reaction with PhC=CPh, MesCCHO or MesSnCl. These compounds thus behave as metal carbene traps. 

The reaction of 17 with 18 is essentially irreversible, but the reaction of 17 with acyclic olefins is reversible and leads to the expected metathesis reactions, for example, the cis/trans isomerization of HDC=CMePh (Lee, J.B. 1982). Many isotopic labelling and kinetic experiments have been carried out in an attempt to discover whether a titanium-carbene-olefin complex plays a significant kinetic role in these reactions. The general conclusion is that this is unlikely and it is thought that complete dissociation to Ti(=CH2)Cp2 must occur before reaction takes place with an olefin or acetylene (Gilliom 1986a Anslyn 1987 Hawkins 1988, see ref 4 therein). If such a complex does have a finite existence, it probably corresponds only to a very shallow minimum in the energy profile for the reaction. Stereochemical evidence for this conclusion comes from a study of the isomerization reaction (17). 

Cleavage of bonds 1—4 and 2—3, followed by rotation of the olefin in the titanium-carbene-olefin complex and re-formation of the titanacyclobutane complex, would have led only to cis-ll-A-d, while cleavage of bonds 1—2 and 3—4, followed by rotation of the olefin, would have led only to trans-l - i-d. In fact both cis and trans isomers of 21 and 22 are formed, as well as the cis isomer of 20 (Ikariya 1985). 

---