Metal-alkylidenes metallacycles

These early results, along with a vast number of other data [45], establish that ring-opening metathesis polymerisation proceeds via a chain process [scheme (4) in Chapter 2] in which the structures of the active species fluctuate between metal alkylidenes (carbenes) and four-membered metallacycles (metallacyclo-butane)s, a concept that was first introduced by Herisson and Chauvin [46]. 

The stability of metal alkylidene (carbene) complexes and the corresponding metallacycles can be dependent on various factors, but it is worth noting that the kind of metal, the metal oxidation state and the ligands surrounding the metal are considered to be of essential significance. Although stable metal carbene complexes are usually obtained from W and Mo compounds whereas metallacycles are obtained from Ti compounds, systems have been found in which both the metal alkylidene complex and its precursor metallacyclobutane can be detected at lowered temperature by NMR spectroscopy [45]. 

As regards metal alkylidene and metallacycle active sites participating in metathesis polymerisation, it should be emphasised that either the alkylidene or the metallacyclobutane can be the resting state of the catalyst, depending on the catalyst used for the polymerisation [99]. 

A common theme in many reactions is the generation of an equivalent of CP2M through the reduction of a tetravalent precursor metallocene with a metal alkyl such as n-butyllithium. The chemistry of olefin complexes of CP2M is characterized by a facile interconversion between formally M(II) and M(IV) manifolds, as shown in equation 34. Another central motif of metallocene chemistry, especially that of titanocenes, is the accessibility of pathways connecting metallacycles and metal alkylidene complexes, eg in alkene metathesis (eq. 35). 

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... 

Insertion into metal carbon bonds for alkylidene [reaction (j)] and alkylidyne [reaction (k)] complexes that yield metallacycles will also be considered. 

In recent years, the use of metathesis catalysts to polymerize alkynes, instead of Ziegler-Natta catalysts, has increased. This is in part because they have been found to polymerize a wider range of monomers [8], and because the Schrock group has shown that well-defined metathesis catalysts allow some control of alkyne polymerizations (see below) [32, 67, 68]. Metathesis polymerizations differ from Ziegler-Natta polymerizations in that the active species is a metal-carbon double bond, or alkylidene . Alkynes add across this bond, in what may be thought of as a [2 2] cycloaddition [69] to form metallacycles. These in turn... 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

Neutron diffraetion eonfirmation of alkylidene interaetion with metal [31] Reaction of Ta alkylidene with olefins and metallacycle rearrangement [38] Synthesis of a titanaeyelobutene [45]... 

The author thanks the National Science Foundation for supporting research on multiple metal-carbon bonds and metallacycles, and the many coworkers who have contributed to high-oxidation state alkylidene chemistry. 

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. 

This method is specific for metallacyclopentanes. The alkene-coupling process is favored by metal reduction. A typical synthetic strategy is the in situ reduction of a metal halide precursor in the presence of the alkene see, for example, the synthesis of 79 in Scheme 34.1 An alkylidene precursor may also lead to a metallacycle with elimination of the car-bene ligand as in the synthesis of 81, representing a deactivation pathway for alkene metathesis catalysts. Ilie two alkenes may be generated in situ in the coordination sphere by rearrangement processes, such as intramolecular hydrogen transfer from an alkyl-vinyl precursor. I ... 

Computational studies have provided a valuable working hypothesis for the Z-selectivity exhibited by cyclometalated catalyst 6 [31, 71, 72], In contrast to the bottom-bound metallacycles observed with previous generaticMis of ruthenium metathesis catalysts (cf. Sect. 2.1), it is proposed that ruthenacycles derived from 4 and 6 adopt a side-bound conformation (7a). The ratimiale for the preferential formation of side-bound ruthenacycles is twofold (Fig. 1) first, there are significant steric interactions present between the developing metaUacylobutane and the adamantyl moiety in the bottom-bound conformation (7b) that are alleviated in the side-bound conformation. Moreover, the bottom-bound conformation is destabilized as it requires back-donation from the same ruthenium d-orbital that is back-donating into the NHC this competition is alleviated in the side-bound conformation, as two separate metal d-orbitals are now available for back-donation into both the NHC and alkylidene carbon p-orbitals (Fig. 1) [71]. 

---