Neopentyl ligands

The mechanism of this remarkable a-elimination reaction has been scrutinized by several research groups [17,49,51,396-404]. From the experimental data obtained this process is best described as an intramolecular deprotonation of one neopentyl ligand by another, the latter being released as neopentane (Figure 3.4). 

C, no solvent). The grafted catalyst proved to be highly active, with equilibrium reached in less than Ih. A TOP of 0.25molmoT s was attained, which corresponds to one of the best rates observed for a Re metathesis catalyst. Also observed during the metathesis reaction was the evolution of approximately 1 equivalent of a 1 3 mixture of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene, which arises from the cross-metathesis of the neopentyl ligand of the grafted complex and propene. 

A decrease in metal-carbon distances when going down a group has also been pointed out. However, if this observation can roughly be applied to Zr and Hf (owing to lanthanide contraction), it does not fit the data obtained for Ti (Table 11.1). In contrast, for tetracoordinated complexes of the third transition row, replacement of just one neopentyl ligand is reported only when silica was pretreated at 700°C [18, 19, 23, 24]. For the tantalum complex [Ta(=CHCMe3)Np3, its... 

The complex Ta(CH2But)5 has yet to be isolated initial attempts at its preparation led Schrock instead to a remarkable alkylidene (carbene) complex (Figure 4.28). Neopentane was amongst the side products of the reaction and this most likely arises from a concerted a-C-H abstraction by an adjacent neopentyl ligand. In other systems, there is evidence for transfer of the hydrogen to the metal centre and agostic alkyls (Figure 2.24) can be seen as a step en route to the a-M-H elimination transition... 

Another type of carbene complex is characterized by a high oxidation state of the central metal and an a-carbon atom that does not usually bear a hetero atom. It is called an alkylidene complex or Schrock-type complex, since R. R. Schrock first synthesized a tantalum complex of this type [14]. Formation of the tantalum carbon double bond is based on the a-elimination reaction of a neopentyl ligand as shown in eq.(2). 

Bis-alkylidene complexes can be made by a second a elimination from a monoalky-lidene complex, or directly from a tetraneopentyl complex or by what appears to be a Ta-mediated hydrogen transfer from a neopentyl ligand to a neopentylidyne ligand ... 

When a cyclohexane solution of thorium complex IV-16 (Scheme IV-15) containing neopentyl ligands is heated one of the ligands can be eliminated and cyclometalation of a remaining alkyl groups proceeds [25a], Metallacycle IV-17 thus formed metalates intermolecularly tetramethylsilane. The reaction does not stop when compound IV-18 is formed, and cyclometalated complex IV-19 can be obtained. The thorium complex also readily activates methane. It should be noted that formally thorium is not a low-valent ion in these reactions. 

At least two types of reactions are involved in the process of grafting at the surface addition of the surface hydroxyl groups to the carbynic bond and electrophilic cleavage of W-C or W-C1 bonds. Combination of these two reactions is possible, especially on the most hydroxylated surfaces. Finally, reductive elimination is possible for 1 species bearing at least two neopentyl ligands. 

Supported Ta-Neopentylidene Complexes A silica-supported Ta-alkylidene [(=SiO) Ta(=CHCMe3)(CH2CMe3)p j] catalyst has been synthesized and well characterized [76, 77]. This Ta-alkylidene catalyst possesses a neopentyl ligand and an alkylidene group, and was found to be active for propane metathesis (TON=33). Since the precursor is not a hydride and a proposed carbene hydride is known to be the intermediate, this implies that Ta-neopentyl and neopentylidene organometaUic species are initially transformed into this key species (Scheme 2.10) [78-83]. Different initiation steps have been suggested. One involves the initial addition of the alkane into the alkylidene moiety to form multiple Ta-alkyl species, which can then decompose via an a-H abstraction to produce a Ta-alkylidene (Scheme 2.10a) [84, 85]. 

With highly electrophilic early transition metal complexes, a-hydride elimination is a common reaction. If other alkyl groups are present on the metal center besides the group undergoing elimination, the facile nature of reductive elimination will lead to formation of an alkane. An example of this is given in Eq. 12.38. Whether a- or p-hydride elimination occurs, neopentane is the product that arises from reductive elimination of a neopentyl ligand with a hydride ligand. 

If two neopentyl ligands are located on a metal complex that has available valence d electrons, classic intramolecular oxidative addition of a C-H bond of a methyl group in y position with respect to the metal occurs, followed by reductive elimination of the intermediate hydride ligand with the other neopentyl ligand to form neopentane - ... 

The seminal work by Tobbin Marks group at Northwestern on rare-earth alkyl complexes also has examples of y-elimination in d complexes that parallel Schrock s reactions with d° Ta and Nb neopentyl complexes. A neopentyl ligand removes an H atom in y position of another neopentyl ligand to yield a dimethylmetallacyclobutane. Again the reaction proceeds by o-bond metathesis. 

Bergman et al. presented a reaction sequence, in which a neopentyl ligand at a tripod ruthenium(II) complex degraded stepwise to a trimethylenemethane ligand. The reaction sequence from 29 via 30 to 31 involves C,C- and C,H-activation steps (Scheme 10.11) [29]. 

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