Metallacyclobutane complexes structure

The cycloaddition of alkenes with metal alkylidene complexes remains the most common entry into the metallacyclobutane structural class. Consistent with metallacyclobutane intermediacy in the olefin metathesis reaction, the [2+2] cycloaddition is generally reversible a propensity for cycloreversion (Section 2.12.6.2.4), however, can significantly limit the utility of metallacyclobutane complexes as intermediates in other synthetic transformations. 

During the course of metathesis-related investigations, a range of metallacyclobutane complexes of other metals have recently been isolated and characterized, at least transiently. The observed complexes are understandably poor catalysts, but the structural and mechanistic insight obtained from such systems has assisted in the development of improved catalysts. 

When the initiator has a doubly substituted carbene ligand the initially formed metallacyclobutane complex Xi is not detected. This is because it has both cisoid and transoid substituents at C-8, and cisoid structures are never detected. In this case, for the reaction of, say, norbomene, the first product seen in the H NMR spectrum is Pi, followed by the two transoid diastereomers of X2. When the initiator has a monosubstituted carbene ligand, and the norbomene carries a syn-1-methyl substituent, again the Xi species is not observed. One can expect from the structure of 7 that a syn-methyl group at C-7 will have the same sort of destabilizing... 

In some cases the metallacyclobutane complexes can be isolated and their crystal structure determined. 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

The first living metathesis polymerizations were established using metallacyclobutane complexes 3 and 4 as catalysts (Figure 20.11) and NB as the monomer. The latter complex has been employed for the synthesis of moderately regular (80 20 transicis) poly(anft-7-methylnorbornene) (poly(anti-7-MeNB)) that contains a small excess of syndiotactic structures. Trans double bonds are primarily associated with r dyads (r m = 75 25), whereas cis double bonds are predominantly associated with m dyads. The overall r m ratio of the hydrogenated derivative of the polymer (poly-H-(fl fi-7-MeNB), Figure 20.12) is 64 36. 

The stoichiometry of this conversion is in accordance with a carbene starting structure. An alternating alkyUdene/metallacyclobutane mechanism [102, 131-133], which has precedent in the ethylene polymerization catalyzed by a Ta(III) neopentilydene complex [134], has been proposed where the chro-miiun alkyUdenes undergo [2-1-2] cycloaddition to give chromacyclobutane intermediates (mechanism III in Scheme 7). 

Confirmation that the polymerizations proceed via metallacyclic intermediates was obtained by studying the ROMP of functionalized 7-oxanorbornadienes. These polymerize slower than their norbornene analogs, allowing NMR identification of the metallacyclobutane resonances and subsequent monitoring of ring opening to the first insertion product. In addition, the X-ray crystallographic structure of complex (212) has been reported.533... 

The formation of metallacyclobutane through a ir-allyl complex without involvement of carbene species [Eq. (12.19)] was also suggested as the initiation step for systems where carbene formation from the alkene is difficult to occur for structural reasons 72-74... 

The reactions of protic acids and other electrophiles at remote Lewis basic functionality have been investigated as a pathway to both transformations and interconversions of carbonyl-substituted metallacycles. Irida-, pallada-, and platinacyclobutanones react reversibly with protic acid to yield 73-2-hydroxyallyl complexes (Equation 36), modulating between metallacyclobutane and hydroxyallyl structures (and further discussed in Section 2.12.9.3.5) <1993CC1039, 1995JOM143, 19970M1159>. 

The current general understanding of the mechanism operating in cycloolefin metathesis polymerisation leads us towards the acceptance of the structure of active sites in systems with catalysts belonging to the aforesaid three major groups as one that alternates between metal carbene complexes and metallacyclobutanes. 

Besides rearrangements, ligand exchange, formation of alkanes, alkenes and other products, release of cyclopropanes is one of the most important reactions of metallacyclobutanes. Of course, this latter reaction is only useful in cyclopropane synthesis if the product is not identical with the starting material used to form the metallacyclobutane. Nevertheless, the discovery of a complex formed from hexachloroplatinic acid and cyclopropane and later structural elucidations have initiated intensive investigations on the conversion of cyclopropanes to metallacyclobutanes and release of cyclopropanes from the latter. These results have been thoroughly discussed in several reviews. " Therefore in this section only some general aspects of cyclopropane formation from metallacyclobutanes and selected synthetically useful methods are discussed. 

---